Contribution of different classes of glutamate receptors in the corticostriatal polysynaptic responses from striatal direct and indirect projection neurons
© Vizcarra-Chacón et al.; licensee BioMed Central Ltd. 2013
Received: 7 February 2013
Accepted: 7 June 2013
Published: 20 June 2013
Previous work showed differences in the polysynaptic activation of GABAergic synapses during corticostriatal suprathreshold responses in direct and indirect striatal projection neurons (dSPNs and iSPNs). Here, we now show differences and similarities in the polysynaptic activation of cortical glutamatergic synapses on the same responses. Corticostriatal contacts have been extensively studied. However, several questions remain unanswered, e.g.: what are the differences and similarities in the responses to glutamate in dSPNs and iSPNs? Does glutamatergic synaptic activation exhibits a distribution of latencies over time in vitro? That would be a strong suggestion of polysynaptic cortical convergence. What is the role of kainate receptors in corticostriatal transmission? Current-clamp recordings were used to answer these questions. One hypothesis was: if prolonged synaptic activation distributed along time was present, then it would be mainly generated from the cortex, and not from the striatum.
By isolating responses from AMPA-receptors out of the complex suprathreshold response of SPNs, it is shown that a single cortical stimulus induces early and late synaptic activation lasting hundreds of milliseconds. Prolonged responses depended on cortical stimulation because they could not be elicited using intrastriatal stimulation, even if GABAergic transmission was blocked. Thus, the results are not explained by differences in evoked inhibition. Moreover, inhibitory participation was larger after cortical than after intrastriatal stimulation. A strong activation of interneurons was obtained from the cortex, demonstrating that polysynaptic activation includes the striatum. Prolonged kainate (KA) receptor responses were also elicited from the cortex. Responses of dSPNs and iSPNs did not depend on the cortical area stimulated. In contrast to AMPA-receptors, responses from NMDA- and KA-receptors do not exhibit early and late responses, but generate slow responses that contribute to plateau depolarizations.
As it has been established in previous physiological studies in vivo, synaptic invasion over different latencies, spanning hundreds of milliseconds after a single stimulus strongly indicates convergent polysynaptic activation. Interconnected cortical neurons converging on the same SPNs may explain prolonged corticostriatal responses. Glutamate receptors participation in these responses is described as well as differences and similarities between dSPNs and iSPNs.
KeywordsSynaptic integration Glutamate NMDA AMPA KAINATE Striatum Corticostriatal inputs
Members of all three families of ligand-gated ionotropic receptors for glutamate, named after their selective agonists: α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or AMPA-receptors (GluR1-4), N-methyl D-aspartate or NMDA-receptors (NR1, 2A-D, 3A, B), and kainate or KA-receptors (GluK1-5), are present in the striatum [1–5], however, their distinct roles during corticostriatal synaptic activation have not been completely described, in particular, differences and similarities in the glutamatergic responses of striatal projection neurons from the direct and indirect pathways.
Striatal projection neurons (SPNs) receive monosynaptic glutamatergic inputs from diverse cortical areas (e.g. [6–12]). Monosynaptic contacts between pyramidal neurons and striatal neurons of the direct and indirect pathways (dSPNs and iSPNs), as well as with striatal interneurons, have been described based on the small variation in latency of the synaptic events [3, 13–22]. In contrast, polysynaptic corticostriatal contacts have received much less attention. Nevertheless, prolonged and late corticostriatal synaptic responses following single cortical stimulus that may last hundreds of milliseconds while sustaining repetitive discharge have been described in vivo and in vitro[23–27]. Here, we ask whether prolonged synaptic responses are in part due to glutamatergic inputs in vitro, suggesting polysynaptic and convergent corticostriatal synaptic activation. It is common knowledge that a main evidence of polysynaptic circuitry converging onto the same postsynaptic neurons is the wide time distribution of latencies, denoting different arrival times for the synaptic inputs (e.g. ). Late synaptic latencies spanning an amount of time more prolonged than that explained by monosynaptic events give rise to complex temporal and spatial synaptic integration as demonstrated by electromyography (e.g. ), current-clamp (e.g. [23, 30]) and voltage-clamp (e.g. [31, 32]) recordings in different neurons and circuits.
Late and variable arrival times of glutamatergic synaptic inputs have been shown to be generated when stimulation of groups of interconnected excitatory neurons converge onto the same postsynaptic cells (e.g. [23, 27, 30]). This happens because most interconnected neurons are in the vicinity of the first stimulated neurons and thus, polysynaptic convergence is the result of activating a coherent set of these neurons . Cortical stimulation, in vivo and in vitro, can initiate recurrent burst firing in both cortical and striatal neurons [23, 33] as well as polysynaptic activation of cortical and striatal circuits [23, 25, 27, 30]. In contrast, stimulation within the neostriatum is expected to activate sparsely extended cortical axons coming from distant cortical areas that would unlikely conform a connected circuit generating delayed activation times. Therefore, a main hypothesis of the present work is that a late prolonged activation of excitatory inputs would be elicited by a single cortical field stimulus but not by a neighboring single intrastriatal field stimulus of the same strength and within a few microns of distance. An additional hypothesis is that prolonged responses could be generated from any cortical area. Evidence for these hypotheses means that long-lasting synaptic responses may be derived from local cortical stimulus generating polysynaptic chains of neighboring connected neurons activated in sequence (e.g. ). This local polysynaptic activation of afferents should not be possible from the striatum, thus explaining the differences in the responses.
This work shows that the three families of glutamate receptors have different roles in the generation of corticostriatal and intrastriatal responses in dSPNs and iSPNs. In particular, the isolation of responses from AMPA-receptors showed the expected distribution of early and late latency inputs compatible with polysynaptic cortical convergence during corticostriatal responses. In contrast, intrastriatal stimulation was unable to elicit such prolonged responses with the same stimulus. An understanding of corticostriatal prolonged responses is important since they sustain recurrent burst discharge (up-states) [33, 34] that increases the probability of synchronization and correlated firing among neurons [35, 36]. These responses are a requirement for the generation of reverberant dynamics present in the striatal microcircuit [35, 37].
CNQX-sensitivity of prolonged glutamatergic corticostriatal responses
Distinct synaptic contributions of KA- and AMPA-receptors during corticostriatal responses in SPNs
Post-synaptic KA-receptors are present in striatal neurons [2, 39, 43]. Here, we asked whether some class of KA-receptors could be synaptically activated by cortical afferents during suprathreshold corticostriatal responses (i.e.: with endogenous glutamate release). The physiological response of native KA-receptors may have different kinetics than AMPA-receptors [2, 44, 45] and we wanted to observe if this is true for corticostriatal responses. ACET (see Methods) has been reported as a highly selective antagonist for GluK1 KA receptor subunits in expression systems [46–48] and there are GluK1 subunits in the striatum [5, 49] as reported by PCR techniques. GluK2, 3 and 5 subunits have also been detected [1, 43, 50, 51]. KA-receptor heteromers made of different subunits have been reported to be blocked by ACET and other antagonists at low micromolar concentrations [52–54]. Splice and edited variants of these subunits associated with auxiliary proteins composing native receptors have unknown affinities for available antagonists [46, 47, 53]. Due to these considerations, we decided to try ACET to investigate its actions on corticostriatal physiological responses (0.1-1 μM ACET; see Methods) .
Red trace in Figure 2A shows a suprathreshold corticostriatal response in a dSPN. Superimposed black trace shows a reduction of control response during application of ACET. A subsequent application of 10 μM CNQX reveals an additional reduction due to blockade of remaining AMPA-receptors or KA-receptors insensitive to ACET (pale grey trace). Figure 2B shows a similar experiment during intrastriatal suprathreshold stimulation in the same dSPN. Histogram in Figure 2C shows that these actions were significant: blockade of KA-receptors in dSPNs decreased the area under the corticostriatal response from 10,633 ± 340 mV · ms to 8,649 ± 231 mV · ms for a 19% reduction (Figure 2C; n = 6; **P < 0.01). The subsequent blockade of AMPA-receptors reduced the remaining response to 5,640 ± 56 mV · ms for a supplemental 35% reduction (Figure 2C; n = 6; ***P < 0.001). ACET plus CNQX reduced the corticostriatal response by about one half: 45 ± 4% (P < 0.001), comparable to percent reduction obtained by CNQX alone (see above). Blockade of KA-receptors in dSPNs also decreased the area under the much briefer response obtained after intrastriatal stimulation (Figure 2B) from 2,031 ± 102 mV · ms to 1,337 ± 67 mV · ms for a 34% reduction (Figure 2C; n = 6; **P < 0.01). A subsequent application of CNQX reduced the remaining response to 241 ± 9 mV · ms or 82% (Figure 2C; n = 6; ***P < 0.001). Whole reduction by ACET plus CNQX blocked most intrastriatal response: ca. 90% (P < 0.001); implying different percent compositions of these components after corticostriatal (ca. 45%) or intrastriatal stimulation (ca. 90%).
Similar experiments were performed in iSPNs (Figure 2F-J), where green trace is the control and black trace shows the reduction produced by ACET. Subsequent addition of CNQX (pale grey trace) shows an additional reduction induced by blockade of remaining AMPA- or KA-receptors insensitive to ACET. Blockade of KA-receptors in iSPNs decreased the area under the corticostriatal synaptic response from 9,822 ± 330 mV · ms to 8,258 ± 430 mV · ms for a 16% reduction (Figure 2H; n = 6; *P < 0.05). A subsequent blockade of AMPA receptors reduced the remaining response to 4551 ± 397 mV · ms for a reduction of 45% (Figure 2H; n = 6; *P < 0.02). The combined action of both blockers in the original response amounted for about one half of the response: 46 ± 3% block (P < 0.01), not significantly different than the reduction obtained with CNQX alone (see above). ACET reduced the area under the response to intrastriatal stimulation from 2,203 ± 110 mV · ms to 1,842 ± 92 mV.ms for a 16% reduction (Figure 2H; n = 6; P < 0.1). Subsequent blockade of AMPA-receptors with CNQX reduced the remaining response to 254 ± 11 mV · ms for a larger block: 86 % reduction (Figure 2H; n = 6; ***P < 0.001).
Based on the above results we conclude that there are KA-receptors that respond to synaptic activation of cortical afferents (i.e.: endogenous glutamate release) in both dSPNs and iSPNs. Because the selectivity of ACET is still under debate (see above) we do not know whether the observed participation correspond to most or only some postsynaptic KA-receptors present in SPNs. Secondly, most glutamatergic actions during intrastriatal stimulation responses in dSPNs depend on AMPA/KA-receptors at −80 mV holding potential (Figure 2A, D, C). In contrast, response to ACET by iSPNs was more variable, although very clear in some cases (Figure 2G, J). But corticostriatal responses dissipate any doubt about ACET-sensitive responses in both dSPNs and iSPNs. However, in both neuronal classes only about half the corticostriatal response could be blocked by these antagonists. Therefore, cortical stimuli appear able to recruit a more complex response than intrastriatal stimuli, suggesting that the arrangement of cortical inputs is capable to activate more receptor classes [25, 42, 55]. In contrast, responses to intrastriatal stimulation go away almost completely after CNQX plus ACET, suggesting that recruited axons lack the necessary circuit arrangement for the response to build up.
In addition, subtractions of the KA- and CNQX-sensitive components (Figure 2D, E, I, J) disclose different time courses: the ACET-sensitive component is slowly rising and decaying and preferentially contributes during the late response. In contrast, the remaining CNQX-sensitive component participates in the fast early and the slow late response, suggesting that initial monosynaptic input is followed by a late polysynaptic barrage [30–32]. To conclude, a main difference between intrastriatal and corticostriatal responses is the duration and magnitude of the late response. More prolonged responses cannot be obtained stimulating within the striatum (even when GABAA-receptors are blocked, see below), suggesting a different arrangement or origin of afferents and synapses responding to the same stimulus at each site.
Because ACET may present some affinity problems (see above), we performed complementary experiments using the selective AMPA receptor antagonist, 25 μM GYKI 52466 (GYKI). The intention was to antagonize AMPA-receptors without affecting KA-receptors [1, 48, 56, 57]. However, saturating concentrations of GYKI do affect KA-receptors [46, 47, 54]. Therefore, we used non-saturating concentrations of GYKI since our main aim was to find out whether AMPA-receptors are the ones responsible for the early and late latency components during corticostriatal responses.
Figure 3F and G show similar experiments in an iSPN, green traces denote controls and black traces denote recordings after 25 μM GYKI. Figure 3H shows that GYKI-induced reduction of the response was significant. As in dSPNs, subtracted GYKI-sensitive components show separate early and late components in the corticostriatal response, but only early responses after intrastriatal stimulus.
GYKI decreased the area under the corticostriatal response of dSPNs from 10,221 ± 292 mV · ms to 8,356 ± 313 mV · ms for an 18% reduction (Figure 3C; n = 7; **P < 0.01). In contrast, blockade of AMPA receptors in dSPNs during intrastriatal stimulation decreased the response from 3,314 ± 166 mV · ms to 1,500 ± 75 mV · ms for a 55% reduction (Figure 3B,C; n = 6; ***P< 0.001). GYKI decreased the area under the corticostriatal response of iSPNs from 6,815 ± 50 mV · ms to 5,374 ± 257 mV · ms for a 21% reduction (Figure 3H; n = 6; **P < 0.01). The action of GYKI on the responses to intrastriatal stimulation was from 2,306 ± 115 mV.ms to 1,486 ± 74 mV · ms (Figure 3H; n = 6; **P < 0.01) for about 36% reduction.
To conclude, the GYKI-sensitive fraction appears more important in the early latency component appearing after intrastriatal stimulation, while late latency components only appear clearly during corticostriatal responses. Taken together, the above data supports the idea that cortical stimulation activates cortical inputs in a way different than that employed during intrastriatal stimulus. One way to explain this difference has been posited in vivo: cortical stimulus activates a group of interconnected excitatory neurons that converge onto the same postsynaptic SPNs. The arrival of these inputs at different latencies would prolong the responses. It is also observed that while AMPA/KA receptors almost completely explain the synaptic responses to intrastriatal stimulation, in the case of corticostriatal entries AMPA/KA receptors only explain about half the response. To explain this behavior one evidence has been given: polysynaptic activation not only involves cortical neurons  but also striatal GABAergic inputs (interneurons, other SPNs) eliciting a mixed excitatory plus inhibitory polysynaptic response . Further experiments are needed to identify all neuronal classes participating. An additional component would be the contribution of NMDA-receptors.
NMDA-receptor contribution in corticostriatal and intrastriatal responses of SPNs
Figures 4F and G show a similar experiment in iSPNs. APV decreased corticostriatal and intrastrial dependent responses in various neurons, although decrease of responses to intrastriatal stimulation was more variable (Figure 4H). Interestingly, although subtracted APV-sensitive component in the corticostriatal response of iSPNs is of long duration (Figure 4I), the response is of lower amplitude (P < 0.001) than that of dSPNs and more similar to that found in other neurons . Two phenomena have been reported to explain these differences: first, shorter and fewer dendrites in iSPNs make them more excitable , so that iSPNs are more prone to fire autoregenerative spikes during suprathreshold synaptic responses . In turn, autoregenerative events trigger a stronger repolarization that reduces the amplitude of corticostriatal responses in iSPNs [58, 60]. In support to this more integrative explanation, stimulation of dendritic spines locally with uncaged glutamate produces similar dendritic plateau potentials in both dSPNs and iSPNs .
In a sample of dSPNs, the area under suprathreshold cortical response goes from 11,256 ± 436 mV · ms to 6,395 ± 300 mV · ms after APV for a 43% reduction (Figure 4C, n = 9; ***P < 0.001). The decrease of intrastriatal responses was from 3,099 ± 155 mV · ms to 2,334 ± 117 mV · ms for a 24% reduction (Figure 4C; n = 6; *P < 0.05). The decrease in the area under the corticostriatal response in iSPNs after APV goes from 6,921 ± 206 mV · ms to 5,456 ± 266 mV · ms for a reduction of about 21% (Figure 4H; n = 6; **P < 0.01), while during responses to intrastriatal stimulation the reduction was from 2,722 ± 136 mV · ms to 2,280 ± 114 mV · ms for an average 16% decrease (Figure 4G,H; n = 6; P < 0.1), that is, variability and small amplitude of the response precluded statistical significance with this sample size, underlying a main difference with corticostriatal responses. Nonetheless, one example with a clear APV-sensitive component is illustrated in Figure 4J.
It was concluded that each cell class configures its response to glutamate differently, not by possessing different assortments of glutamate receptor classes in their synaptic contacts, but by the different use they make of the prolonged time window conferred by long-lasting synaptic activation: dSPNs apparently maintain larger APV-sensitive components (43%) than iSPNs (21%). This difference between dSPNs and iSPNs in part explains a smaller area under the response in iSPNs. In addition, prolonged time windows for synaptic integration can only be generated when stimuli are delivered in the cerebral cortex; where polysynaptic activation is virtually inescapable . It cannot be generated when stimulus is delivered within the striatum: dSPNs (24%) and iSPNs (16%).
Two observations are evident: first, blockade of GABAergic inputs with bicuculline decreases the late depolarizing part of both corticostriatal and intrastriatal responses in dSPNs, while it enhanced both corticostriatal and intrastriatal responses in iSPNs . Secondly, in both dSPNs and iSPNs activation of GABAergic polysynaptic inputs is larger for cortical than striatal stimulation  in absolute terms; although similar in percentage. In other words, dissimilarities in responses as a function of stimulation site could not be explained by differences in GABAergic activation.
Interneurons that exhibit persistent low threshold spikes (PLTS) also exhibit prolonged corticostriatal depolarizations lasting hundreds of milliseconds after a single cortical stimulus (Figure 7C) [63, 65, 66]. However, they exhibit little output in terms of action potentials at these holding potential. Instead, they may exhibit autoregenerative events (“low threshold spikes”, arrows in Figure 7C, D), confirming that their way of activation may not necessarily arise from the polarized membrane potentials used for the present comparison (ca. -80 mV) . Latency to subthreshold responses was 2.5 ± 0.3 ms (n = 6); not significantly different to that of SPNs. When action potentials are fired, initial frequency may reach 117 ± 17 Hz (Figure 7G).
Finally, tonically active neurons (TANs), known to be putative large aspiny cholinergic interneurons (Figure 7E) [65, 67–70] may respond with repetitive firing after a single cortical stimulus, with maximal frequencies of 19 ± 6 Hz (n = 9), higher than those reached with intracellular current injections: 6–15 Hz (Figure 7E, F, G), but lower than those attained by any other striatal cell class. Nevertheless, corticostriatal latency to subthreshold PSPs in these neurons was 1.2 ± 0.11 ms, briefer than that of SPNs (P < 0.001), although not significantly different than that of FS interneurons. Depolarizing responses in TANs may, on occasion, last several seconds and be lengthier than those from any other striatal neuron (inset in Figure 7F, Figure 7H).
Taken together, the present results show: first, that GABAergic inputs from interneurons and other SPNs onto postsynaptic SPNs are more efficiently activated from the cortex than from the striatum itself, confirming that different GABAergic entries cannot explain differences in duration between corticostriatal responses and intrastriatal stimulus. Secondly, by themselves, interneurons responses to corticostriatal single stimulus are a direct proof that polysynaptic activation occurs during corticostriatal responses, not only involving cortical cells [23, 30] but even striatal neurons [23, 61], explaining the GABAergic component . Finally, they show that not only corticostriatal responses between SPNs differ, but a comparison of areas under the responses as well as durations at half amplitude, as those seen in histograms of Figures 7G-I, is enough to confirm that each neostriatal neuron class exhibits a particular and distinct corticostriatal response, perhaps reflecting diverse combinations of cortical connections. This point has been shown for FS interneurons [13, 19, 62–64]. Further analysis of these differences is out of the scope of the present report.
Corticostriatal responses do not depend on the cortical area being stimulated
We conclude that differences in the responses were independent of the cortical area stimulated and on the assortment of receptors being activated, and more dependent on the different intrinsic properties that these neurons exhibit . In fact, more differences have been documented after intrasomatic stimulation [42, 60, 71].
Here, we present evidence of two previously underestimated phenomena in in vitro corticostriatal slice preparations: First, that prolonged corticostriatal responses, evoked by stimulating cortical afferents and terminals with a single stimulus, are sensitive to ionotropic glutamate receptors antagonists, and therefore, may be the result of activating cortical neurons in sequence, in the vicinity of the stimulating electrode . The activation of these cortical ensembles explains the continuous arrival of cortical inputs following a single stimulus, that is, they generate a polysynaptic activation that explains the prolonged late corticostriatal response. In support of this explanation, prolonged late components cannot be evoked by stimulating within the striatum (with or without the cortex or thalamus), but can be evoked by stimulating in any cortical area (with or without the thalamus). Secondly, this polysynaptic activation may in turn activate several striatal projection neurons and interneurons, thus explaining the GABAergic component of the complex corticostriatal response . Third, KA-receptors contribute to these responses since they are activated by glutamate released endogenously by stimulated excitatory afferents. Fourth, the differences in the magnitude of the corticostriatal response between dSPNs and iSPNs is not due to different assortments of the main classes of glutamate ionotropic receptors, but by different integrative mechanisms that, among other things, produce larger APV-sensitive components in dSPNs.
Prolonged corticostriatal responses
Here, we show in vitro, that a single stimulus in the cortex evokes a complex and prolonged suprathreshold response, separate from the monosynaptic response, lasting hundreds of milliseconds, and sensitive to ionotropic glutamate receptors antagonists. Besides projection neurons (SPNs), several classes of striatal neurons including FS-, PLTS-, and TAN-interneurons could powerfully be activated by cortical stimuli, supporting and extending direct evidence of polysynaptic activation that includes not only cortical but also striatal neuron pools. It is important to demonstrate in vitro, that a long-lasting arrival of inputs may underlie prolonged corticostriatal responses: microcircuit dynamics demonstrated in the corticostriatal slice [35, 36] is sustained by long-lasting up-states and recurrent burst firing  that allows neuronal synchronization. We infer that an up-state may be the result of the activity of a cortical ensemble. This information allows future comparisons and interpretations of changes in dynamics after decortication, or different forms of deafferentation, in control and Parkinsonian subjects.
Prolonged activation of excitatory inputs has been explained by the activation of neighboring cells by the first stimulated cortical neurons [12, 23]. Sequentially activated cortical neurons  would then converge onto the same postsynaptic SPNs at different times, thus shaping up a response that may last hundreds of milliseconds setting a window for synaptic integration and correlated or synchronous firing . Since striatal neurons are also activated by the stimulus, both cortical and striatal inputs together shape up the striatal output conveyed by SPNs.
An obvious question is why intrastriatal stimuli could not induce these prolonged responses. Our working hypothesis, confirmed so far, is that intracortical connections cannot be stimulated in these conditions, instead, stimulation within the neostriatum activates sparsely extended cortical axons coming from distant cortical areas that would unlikely be interconnected. Antidromic activation is not dense enough to stimulate neurons that are in the vicinity of the first stimulated cells . The alternative explanation, that intrastriatal stimuli better activate inhibition, was discarded in the present work: it was shown that polysynaptic activation of inhibitory entries is larger when the stimulus is within the cortex and not within the striatum.
To conclude: accepting that cortical stimulation generates recurrent bursting and correlated firing due to the sequential activation of cortical neuron sets [30, 33, 72–75], and that these neurons sets may converge in target postsynaptic sets of SPNs, then, prolonged SPNs responses reflect circuitry processing among cortical and striatal neuron ensembles . Striatal ensembles can also be activated in sequence , perhaps, encoding motor programs and procedures. Still an improbable alternative is that some thalamic inputs may be stimulated in a non equal manner by current diffusion. However, in vivo experiments point right toward the opposite phenomenon . Finally, responses obtained in vitro are not equal to those evoked in vivo: a long-lasting disinhibition due to intracortical connections activating nets of inhibitory cortical interneurons [12, 22, 23, 27] appears to be small or lacking in the present preparation.
Roles of glutamate receptors during corticostriatal responses
Prolonged corticostriatal responses displayed GYKI-, ACET-, CNQX- and APV-sensitive components in both dSPNs and iSPNs. In particular, the contribution of GYKI- and CNQX-sensitive components in both neuron classes exhibited early and late components. We conclude that both projection neuron classes have qualitatively similar assortments of glutamate receptors. Therefore, differences between the magnitudes of orthodromically activated responses (e.g., areas under the response) are not due to differences in expressed glutamate receptors as experiments with uncaged glutamate confirm , but by different ways of integration due to anatomical and intrinsic differences (e.g., different sets of G-protein coupled receptors and modulation - unpublished). Taken all together, these results show that up-states appear not to be a simple spatial or temporal summation of otherwise uncoordinated synaptic inputs. They are the manifestation of ensembles processing and circuitry modulation.
The ACET-sensitive component suggests either that glutamatergic synapses in SPNs have GluK1 subunits [5, 49], or that heteromers are sensitive to ACET [46, 53]. The postsynaptic presence of an ACET-sensitive component with a physiological stimulus (release of endogenous glutamate) confirms previous suggestions about their potential postsynaptic importance [2, 51].
A particular interest of the KA-receptors mediated responses is that their slow duration may ignite intracellular signaling cascades that cross-talk with other modulatory signaling cascades (e.g., dopaminergic; cholinergic). Further research is needed to observe this possibility as well as its interactions with voltage-gated currents.
In previous works we showed the participation of GABAergic and intrinsic components in the complex and prolonged corticostriatal suprathreshold response [24, 25, 33]. In the present work, we demonstrate the participation of the three different classes of ionotropic glutamatergic receptors in the same response. Further, by isolating responses from GYKI- and CNQX-sensitive components out of the complex suprathreshold response of SPNs, a single cortical stimulus revealed that prolonged responses are due to the polysynaptic and sequential activation of cortical and striatal microcircuits converging on the same SPN; from either direct or indirect pathways. In fact, prolonged responses could not be evoked with intrastriatal stimulation, where apparently, the necessary arrangement of cortical connections to set the sequential and convergent activation cannot be activated with a single stimulus. Prolonged responses from APV- and ACET-sensitive components were shown to contribute to plateau depolarizations. However, the APV-sensitive component had lower amplitudes in iSPNs. GABAergic polysynaptic inputs were also shown to be recruited from the cortex.
In summary, more physiological responses (up-states) would be composed by sequences of cortical stimulus expected to trigger voltage transitions that last over a second. Therefore, up-states represent the product of polysynaptic convergence and are expected to contain all these components. Because up-states represent the convergence that allows the synchronization of SPNs [35, 37], then, synchronization of SPNs represents the interaction among cortical and striatal assemblies. Prolonged NMDA- and KA-receptors contributions may allow the time to cross-talk with modulatory signaling purveyed by G-protein coupled receptors. Further work is needed to see whether metabotropic synaptic components are a part of these responses as well as their similarities and differences between dSPNs and iSPNs.
All experiments were carried out in accordance with the National Institutes of Health Guide for Care and Use of Laboratory Animals and were approved by the Institutional Animal Care Committee of the Universidad Nacional Autónoma de México. D1 and D2 dopamine receptor-eGFP BAC transgenic mice, between postnatal days 30–60 (PD30-60; FVB background, developed by the GENSAT project) were used. Adult Wistar rats, wild mice, and non-fluorescent cells of BAC-mice were also recorded to detect possible inconsistencies due to transgenes expression . In all the present cases we obtained consistent and similar results. The number of animals employed in the experimental samples was near the minimal possible to attain robust reproducible results and/or statistical significance. Animals were anesthetized with ketamine/xylazine. Their brains were quickly removed and placed into ice cold (4°C) bath saline containing (in mM): 126 NaCl, 3 KCl, 25 NaHCO3, 1 MgCl2, 2 CaCl2, 11 glucose, 300 mOsm/L, pH = 7.4 with 95% O2 and 5% CO2. Hemispheres were separated at this stage. Para sagittal (or horizontal in about 20% of cases) neostriatal slices (250–300 μm thick) were cut using a vibratome and stored in oxygenated bath saline at room temperature for at least 1 h before recording.
Intracellular recordings were carried out using sharp microelectrodes (80–120 MΩ) filled with 1% biocytin and 3M potassium acetate fabricated from borosilicate-glass (FHC) and pulled on a Flaming-Brown puller (P-97; Sutter Instruments). Recordings were obtained with a high input impedance electrometer (Neurodata, New York, NY, USA) with an active bridge circuit. Because responses from either dSPNs or iSPNs did not differ as a function of site of the cortex stimulated or slice orientation (parasagittal or horizontal; see Results), most recordings (80%) were performed in parasagittal slices. Slices were submerged in the bath solution and superfused with the same saline at 2 ml/min (34–36°C). Cell membrane potential was ca. -85 mV and input resistance obtained near the resting membrane potential was 50–100 MΏ. Recordings were digitized and stored with the aid of software designed in the laboratory in the Lab View environment (National Ins., Austin, TX, USA). Drugs were dissolved in the bath saline from stock solutions made daily. Some recordings, were carried out using whole-cell patch pipettes filled with (in mM): 115 KH2PO4, 2 MgCl2, 10 HEPES, 0.5 EGTA, 0.2 Na2ATP, 0.2 Na3GTP and 1% biocytin. Recordings using this internal solution and patch pipettes did not apparently differ from those using sharp electrodes and corresponding solution. After recording, neurons were injected with biocytin as previously described for their identification .
Recordings were carried out in the dorsal striatum. Stimulation was performed with concentric bipolar electrodes (tip = 50 μm) to stimulate locally and avoid charge diffusion between cortex and striatum or vice versa. In our experience non-concentric wide open bipolar electrodes frequently produce charge diffusion. The distance between recording and stimulating electrode was around 1 mm for both cortical and intrastriatal stimulations. In many occasions, stimulating electrodes located in the cortex or the striatum were a few microns (500–1000 μm) away of each other and in many cases both sites were stimulated in both locations with the same electrode. In case of current diffusion we may have seen prolonged responses while recording in the striatum. Synaptic responses were evoked by a single square pulse of 0.1 ms. Stimulation was delivered with a stimulator (S-8800; Grass, West Warwick, RI) using an isolation unit. The cell membrane potential was held at −80 mV. A series of current pulses of increasing intensities were used to elicit suprathreshold responses, with or without the firing of repetitive action potentials. The same series of stimulus intensities were used for both intracortical or intrastriatal stimulus. Response magnitudes were measured as areas under the synaptic responses by numerical integration . Responses obtained with suprathreshold stimulus strength (2X threshold) are compared. Statistical values in histograms and text are presented as mean ± SEM. Digital subtraction was used to obtain time courses and the components sensitive to different glutamate receptor antagonists. Normality tests allowed the comparison of paired samples with two tailed Student´s t tests. ANOVA and a post hoc Bonferroni test were used when the sample was subject to more than one comparison. Statistical significance was fixed at P < 0.05. In most cases, we used a standard statistical table and report the nearest significant larger value for the corresponding degrees of freedom, so that symbols in the histograms were made homogeneous through different figures.
After recordings, neurons were injected with biocytin. eGFP-positive visualization was observed on a confocal microscope as previously described . Current-clamp data were obtained to observe the most physiological response. A voltage-clamp recording is shown (inset in Figure 1) evoked with low intensity strength to illustrate the barrage of synaptic inputs lasting hundreds of milliseconds after the first monosynaptic response. However, voltage-clamp responses at higher stimulus strengths involve escape currents due to the absence of space-clamp in complex dendritic arbors and the partial filtering introduced by point voltage-clamp that makes physiologically complex suprathreshold responses non-interpretable. eGFP-positive and negative neurons from D1 and D2 eGFP animals are compared. Dynamic voltage-clamp is out of the scope of the present report.
The following drugs: the N-methyl-D-aspartate (NMDA) receptor antagonist 5-Phosphono-DL-norvaline DL-2-Amino-5-phosphonovaleric acid (AP-5 or APV), the α-amino-3-hydroxy-5-methyl-isoxazole-4-propionate AMPA and Kainate receptor antagonist 6-Cyano-7-nitroquinoxaline-2,3-dione disodium salt hydrate (CNQX) and bicuculline were obtained from Sigma-RBI. The AMPA receptor antagonist 4-(8-Methyl-9H-1,3-dioxolo[4,5-h][2, 3] benzodiazepin-5-yl)-benzenamine dihydrochloride (GYKI 52466) and the kainate receptor antagonist (S)-1-(2-Amino-2-carboxyethyl)-3-(2-carboxy-5-phenylthiophene-3-yl-methyl)-5-methylpyrimidine-2,4-dione (ACET) were obtained from TOCRIS.
We thank Antonio Laville, Gabriela X Ayala and Adriana Hernández for technical support and advice and to Dr. Claudia Rivera for animal care. This work was supported by Programa de Investigación Multidisciplinaria de Proyectos Universitarios de Liderazgo y Superación Académica (IMPULSA)-Universidad Nacional Autónoma de México (UNAM).Consejo Nacional de Ciencia y Tecnología (México) grants 98004 and 154131, and by grants from Dirección General de Asuntos del Personal Académico. Universidad Nacional Autónoma de México: IN206010 and IN205610 to EG and JB, respectively. Bianca Vizcarra-Chacón had a CONACyT doctoral fellowship and data in this work are part of her doctoral dissertation in the Posgrado en Ciencias Biomédicas de la Universidad Nacional Autónoma de México.
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