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Differences in biophysical properties of nucleus accumbens medium spiny neurons emerging from inactivation of inward rectifying potassium currents


Nucleus accumbens medium spiny neurons display a two state membrane potential controlled by active channels and synaptic input. Inward rectifying potassium (KIR) channels play a major role in maintaining one of the states, the hyperpolarized down state. The KIR currents in 60% of these neurons are non-inactivating whereas in the remaining, they inactivate [1]. The significance of this difference is unknown. We describe a computational study comparing the biophysical properties of medium spiny neurons possessing these two types of currents.


Two medium spiny neuron cells were modeled using NEURON, one equipped with non-inactivating KIR currents (henceforth, "Cell A") and the other with inactivating KIR currents (henceforth "Cell B") and their behaviors were compared in response to current injection inputs.


It was observed that these two kinds of cells were different in several notable ways. For instance, Cell B when compared with Cell A (i) had a resting potential higher by +0.6 mV; (ii) had a higher frequency of firing for the same injected current (Figure 1A); (iii) hyperpolarized more for the same injected negative current (Figure 1A); (iv) reached firing threshold with smaller injected currents; (v) had higher average inter-spike interval (by up to 15%) with the first spike occurring up to 32% earlier for injected currents matched for firing frequency; (vi) showed noticeable differences in strength-duration curves (Figure 1B), injected current vs spike frequency curves and voltage-current relationships.

Figure 1
figure 1

Comparison of Cell A with Cell B. (A) Membrane response to injected currents of 0.248 and -0.2 nA. (B) The strength-duration curves of the cells show a significant difference in trend.


These results show that clear biophysical differences in the properties of medium spiny neurons can emerge owing to the presence of inactivation in KIR channels and indicate that these differences can influence state transitions driven by cortical and hippocampal excitatory inputs. They also suggest that the two types of neurons expressing the different types of KIR channels may have computationally different functions.


  1. Mermelstein PG, Song W, Tkatch T, Yan Z, Surmeier DJ: Inwardly rectifying potassium (IRK) currents are correlated with IRK subunit expression in rat nucleus accumbens medium spiny neurons. J Neurosci. 1998, 18: 6650-6661.

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Correspondence to John Steephen.

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Open Access This article is published under license to BioMed Central Ltd. This is an Open Access article is distributed under the terms of the Creative Commons Attribution 2.0 International License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Steephen, J., Manchanda, R. Differences in biophysical properties of nucleus accumbens medium spiny neurons emerging from inactivation of inward rectifying potassium currents. BMC Neurosci 8 (Suppl 2), P116 (2007).

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  • Nucleus Accumbens
  • Biophysical Property
  • Excitatory Input
  • Spike Frequency
  • Medium Spiny Neuron