Inner capillary diameter of hypothalamic paraventricular nucleus of female rat increases during lactation
© Cortés-Sol et al.; licensee BioMed Central Ltd. 2013
Received: 15 August 2012
Accepted: 8 January 2013
Published: 10 January 2013
The role of the endothelial cell (EC) in blood flow regulation within the central nervous system has been little studied. Here, we explored EC participation in morphological changes of the anterior hypothalamic paraventricular nucleus (PVN) microvasculature of female rats at two reproductive stages with different metabolic demand (virginity and lactation). We measured the inner capillary diameter (ICD) of 800 capillaries from either the magnocellular or parvocellular regions. The space occupied by neural (somas, dendrites and axons) and glial, but excluding vascular elements of the neurovascular compartment was also measured in 100-μm2 sample fields of both PVN subdivisions.
The PVN of both groups of animals showed ICDs that ranged from 3 to 10 microns. The virgin group presented mostly capillaries with small ICD, whereas the lactating females exhibited a significant increment in the percentage of capillaries with larger ICD. The space occupied by the neural and glial elements of the neurovascular compartment did not show changes with lactation.
Our findings suggest that during lactation the microvasculature of the PVN of female rats undergoes dynamic, transitory changes in blood flow as represented by an increment in the ICD through a self-cytoplasmic volume modification reflected by EC changes. A model of this process is proposed.
KeywordsInner capillary diameter Endothelial cell Neurovascular compartment Hypothalamus Lactation
The blood brain barrier (BBB) in the central nervous system (CNS) regulates the passage of nutrients, essential components and metabolic constituents between the blood stream and the parenchymal tissue. Although local neural activity is known to promote local blood supply and is the basis of the so-called blood-oxygen level gradient/dependency (BOLD) [1–3], the regulation of local blood flow in the parenchyma of the CNS is not well understood. Contraction of pre-capillary arterioles has been suggested but so far without clear demonstration . Capillaries of the CNS do not have smooth musculature but contractility of pericytes associated with the capillaries of peripheral tissue such as the retina, cardiac and skeletal muscles has been observed [5–9], suggesting that contraction of these cells in the CNS can be a mechanism of blood supply regulation [10–12]. However, the finding that pericytes of the CNS lack the α-actin protein isoform found in contractile cells , appears to exclude their participation in CNS capillary contraction. Angiogenesis described during brain development  has also been suggested as a mechanism of blood flow increment, although without clear support in normal adulthood or non-pathological contexts [15–18]. The endothelial cells (EC), pericytes and basal lamina (anatomical constituents of the BBB), together with elements of the neurovascular compartment such as neurons, astrocytes, and other glial cells, adapt themselves to maintain homeostasis that promotes tissue survival [19, 20]. Although the endothelial cell can be considered as an element which plays an important role in blood flow regulation its participation has been little analyzed, at least partly due to the intrinsic difficulty of its individual staining in capillaries of the CNS [21–23].
However, in order to better understand the mechanisms that occur during local blood flow changes in the CNS parenchyma, the role of capillary morphology needs to be considered as an important issue. Electron microscopy (EM) has provided knowledge of the capillary wall constituents [24, 25], but the small spatial range of this method is a critical and important limitation of this powerful tool, e.g. the size of a capillary endothelial cell is still not known. Despite attempts to obtain markers enabling CNS capillary imaging or visualization of capillary networks through extracellular tracers, fluorescent markers, diamine benzidine reaction, immunocytochemistry as well as brain arterial injection with black ink or venous plastic infusion [1, 15, 22–24, 26], a complete view of the morphology of CNS capillaries still is lacking. In this respect, recently we proposed an alternative histological tool focused on detecting the relative space occupied by the capillary endothelial cell [27, 28]. This visualization through light microscopy is based on two facts: first, the optical effect that reflecting light produces in fixed and unstained capillaries, and second, evidence provided by EM [24, 25] showing that the internal wall of CNS capillaries is exclusively constituted by the endothelial cell. Using this procedure we found that changes in capillary luminal area can be detected indirectly via changes in the inner capillary diameter (ICD).
With the aim of investigating the participation of the capillary endothelial cell in the regulation of blood flow fluctuations that occur in response to changes in neuronal activity, the present study was directed to quantifying the internal diameter of capillaries in the anterior hypothalamic paraventricular nucleus (PVN) in female rats. We chose this nucleus since several of its physiological aspects but not its microvasculature have been intensely studied [29–35]. For example, it is well known that the magno- and parvocellular regions via a neuroendocrine reflex are involved in oxytocin (OT) production and release in response to mammary gland sensory stimulation [36, 37], and that the effect varies according to the reproductive phase of the female rat [33–35, 37]. Thus, we decided to measure and compare the capillary internal diameter during the diestrous phase of virgin rats and during the lactation period of mother rats. To ensure that the sensory information received by the two groups was significantly different, mammary glands of the mother rats were stimulated by their pups suckling for two weeks of lactation, while virgin rats had no stimulation.
General microvasculature characteristics
Inner capillary diameter (ICD)
PVN neurovascular compartment
Whereas the physiology and neuronal characteristics of the PVN have been widely investigated [29–35], its microvasculature has been much less so [29, 39, 40]. Morphological changes such as glial retraction, neuronal remodeling, c-Fos expression and electrophysiological activity increment during lactation have been reported [35, 41–43], and there is no doubt that greater synthesis and release of OT from both the PVN and supraoptic nucleus occurs [33–35]. This could reflect an increment in the local neural activity of these structures that in turn promotes an increment in local blood flow [1–4]. In the present study our results suggest that, at least in the PVN, local blood flow increment could be via an increment in the ICD rather by an increase in outer capillary diameter or angiogenesis. Our findings showing that the area occupied by the neural and glial elements of the neurovascular compartment did not change in virgin compared to lactating rats, suggest that angiogenesis is not present. In addition, it has been suggested that components of the capillary basal lamina such as laminin, cablin, fibronectin and type IV collagen among others [44–47] prevent the outer EC diameter from expanding.
In addition, it is currently thought that transport of water-soluble molecules across EC is accomplished by the continuous formation of plasmalemma vesicles followed by detachment and fusion to the membrane on the other side of the cell or by transport of cytoplasmic vesicles moving from one surface to the other without membrane fusion [47, 54, 55]. Either of these mechanisms of intracellular transport could be present in the capillary EC and depending on the luminal-abluminal distance the cytoplasmic transport time could be faster or slower. This could represent a dynamic and plastic mechanism by which constantly changing neuronal needs are rapidly met. As physiological changes associated with chronic dehydration, hemorrhage, stress condition as well as pregnancy and parturition [33, 56], are associated with the synthesis and release of OT or VP from the PVN and supraoptic nucleus, then, based on our results, we should not reject the idea that a similar dynamic mechanism of blood flow regulation in response to such stimuli might occur.
Finally, it is well-known that during lactation progesterone (P4) and prolactin (PRL) reach high levels in the blood stream . Since PRL has been implicated in OT release [57, 58], this hormone could be a potential candidate to participate in PVN blood flow regulation.
Our findings suggest that during lactation the microvasculature of the PVN of female rats undergoes dynamic, transitory changes in blood supply represented by an increment in the ICD through self-cytoplasmic volume modification reflected by EC changes. This is relevant to consider from the pharmacological point of view, since changes in cytoplasmic volume in the EC could help to increase blood flow and in this way facilitate the metabolic exchange between capillaries and the CNS parenchyma.
Animals, groups, and experimental rationale
All experimental procedures comply with the requirements of the Institutional Ethical Committee of the Universidad Veracruzana, which is in agreement with the official Mexican regulations (NOM-062-ZOO-1999). In addition the international guidelines for the production, care, and use of laboratory animals from The Society for Neuroscience, USA, were considered.
We used adult female Wistar rats, bred and maintained in the vivarium at the Instituto de Neuroetología, Xalapa, Veracruz. They were kept on a 14:10 light/dark cycle with free access to rat chow (Harlan, Mexico) and water. Given that our aim was to investigate possible changes in the microvasculature of the PVN under different demands for OT secretion, we divided the animals randomly into two groups: virgin females and mother rats. The virgin females were housed in two collective cages (44 × 34 × 20 cm) with 5 animals in each. Their estrous cycle was determinated by vaginal smears and after two regular consecutive estrous cycles, 5 rats were chosen to be perfused during their diestrous-2 phase (n=5) (VIR group) (they were approximately 140 days old). Mother lactating rats (n=5) (LAC group) were kept with eight pups each in individual cages (37 × 27 × 17 cm). On the 14th day of lactation pups were separated from their mother and maintained in an incubator during 4 h, returned to their mother for a 30-min suckling period, after which the mother was perfused (also when approximately 140 days old). The pups were placed with a nursing dam to minimize animal sacrifice.
Under an i.p. overdose of sodium pentobarbital anesthesia (40 mg/kg), animals were transcardiacally perfused using 50 ml of a 0.9% saline solution and 300 ml of fixative at 4°C (4% paraformaldehyde, 1% glutaraldehyde, 0.002% calcium chloride and 3.2% sucrose, diluted in 0.1 M phosphate buffer pH 7.4). To minimize the effects of anoxic conditions and perfusion pressure on vessel diameters, the perfusion procedure was standardized in all animals (Masterflex pump 77200, 100 strokes-min, 0.25 ml/stroke calibration). Brains were rapidly removed into fresh fixative for 12 hr. They were then mounted on a vibratome (EMS OTS-4000) and 100-μm serial transversal sections were cut. Sections at the anterior hypothalamus level  were collected in 0.1 M phosphate buffer, mounted on gelatinized slides and dried at room temperature for immediate microscopy analysis (sections remained unstained) (Figures 2A and 3A). Afterwards, sections were stained using the Nissl technique (0.04% cresyl violet, pH 3.4) and prepared with a cover slip and permount for histological confirmation of the PVN borders as well as for identification of the magno- and parvocellular regions (Figure 3B).
Inner capillary diameter (ICD)
The unstained sections were examined at 20X and 40X magnification under a light microscope (Olympus CX31) equipped with a video camera (CoolSNAP-Pro, MediaCybernetics, Inc) for image storage. The condenser lenses were aligned to assure that Koehler illumination was optimal. Measurements were made using Image-Pro plus v.6 software (MediaCybernetics, Inc). The capillaries were clearly visualized, as well as their ICD (Figure 1 lower panel). When the ICD was larger than 10 μm, we considered it as a vessel instead of a capillary  and excluded it from our analysis. In the sections corresponding to the PVN middle area (see Figure 3), 20 capillaries from each magno- and each parvocellular subdivision from both sides were randomly selected and focused to measure their ICDs. A total of 80 measurements per animal were obtained and thus 400 measurements for each group.
Considering the criteria used in other studies where a morphometric analysis was done [60, 61] the percentage distribution of capillaries was organized according to individual ICDs for each PVN region (magno- or parvocellular) and for each group of rats. Also, in the present work the mean ICD percentage distribution was calculated and categorized according to bins, with an ICD range of 3-6 and 7-10 μm each.
PVN neurovascular compartment
After the analysis of ICDs, the area occupied by the neural and glial elements of the PVN neurovascular compartment in the magno- and parvocellular regions within each group (10 fields per group) was measured in a field of 10, 000 μm2 by painting them out so as to exclude the intravascular space (Figure 6).
The percentage of PVN capillaries distributed according to the ICD size and the space occupied by neural and glial elements of the neurovascular compartment were both analyzed using a one-way ANOVA. Tukey's HSD post-hoc test was applied when necessary and statistical significance set at P < 0.05. Data were analyzed using SPSS Statistics v.18 software (IBM). Descriptive statistics are expressed as means ± SEM.
Blood brain barrier
Central nervous system
Inner capillary diameter
Hypothalamic paraventricular nucleus
We gratefully acknowledge Dr. Hans Distel (Institut für Medizinische Psychologie Ludwig-Maximilians-Universität München Goethestr. 31 80336 München, Germany), for his critical comments and thank Carolina Rojas for technical assistance. This work was supported by Spanish grant from the “Ministerio de Ciencia e Innovacion” SAF2009-10689 to PB and by a CONACyT (México) fellowship to ACS 201991.
- Villringer A, Dirnagl U: Coupling of brain activity and cerebral blood flow: basis of functional neuroimaging. Cerebrovasc Brain Metab Rev. 1995, 7: 240-276.PubMed
- Thompson JK, Peterson MR, Freeman RD: Single-neuron activity and tissue oxygenation in the cerebral cortex. Science. 2003, 299: 1070-1071. 10.1126/science.1079220.View ArticlePubMed
- Pillai JJ: Insights into adult postlesional language cortical plasticity provided by cerebral blood oxygen level-dependent functional MR imaging. AJNR Am J Neuroradiol. 2010, 31: 990-996. 10.3174/ajnr.A1896.View ArticlePubMed
- Malonek D, Dirnagl U, Lindauer U, Yamada K, Kanno I, Grinvald A: Vascular imprints of neuronal activity: relationships between the dynamics of cortical blood flow, oxygenation, and volume changes following sensory stimulation. Proc Natl Acad Sci USA. 1997, 94: 14826-14831. 10.1073/pnas.94.26.14826.PubMed CentralView ArticlePubMed
- Herman IM, D'Amore PA: Microvascular pericytes contain muscle and non-muscle actins. J Cell Biol. 1985, 101: 43-52. 10.1083/jcb.101.1.43.View ArticlePubMed
- Tilton RG: Capillary pericytes: perspectives and future trends. J Electron Microsc Tech. 1991, 19: 327-344. 10.1002/jemt.1060190308.View ArticlePubMed
- Metea MR, Newman EA: Glial cells dilate and constrict blood vessels: a mechanism of neurovascular coupling. J Neurosci. 2006, 26: 2862-2870. 10.1523/JNEUROSCI.4048-05.2006.PubMed CentralView ArticlePubMed
- Peppiatt CM, Howarth C, Mobbs P, Attwell D: Bidirectional control of CNS capillary diameter by pericytes. Nature. 2006, 443: 700-704. 10.1038/nature05193.PubMed CentralView ArticlePubMed
- Fernández-Klett F, Offenhauser N, Dirnagl U, Priller J, Lindauer U: Pericytes in capillaries are contractile in vivo, but arterioles mediate functional hyperemia in the mouse brain. Proc Natl Acad Sci USA. 2010, 107: 22290-22295. 10.1073/pnas.1011321108.PubMed CentralView ArticlePubMed
- Hamilton NB, Attwell D, Hall CN: Pericyte-mediated regulation of capillary diameter: a component of neurovascular coupling in health and disease. Front Neuroenergetics. 2010, 2: 5-18.PubMed CentralView ArticlePubMed
- Bonkowski D, Katyshev V, Balanov RD, Borisov A, Dore-Duffy P: The CNS microvascular pericyte: pericyte-astrocyte crosstalk in the regulation of tissue survival. Fluids Barriers CNS. 2011, 8: 8-10.1186/2045-8118-8-8.PubMed CentralView ArticlePubMed
- Dalkara T, Gursoy-Ozdemir Y, Yemisci M: Brain microvascular pericytes in health and disease. Acta Neuropathol. 2011, 122: 1-9. 10.1007/s00401-011-0847-6.View ArticlePubMed
- Nehls V, Drenckhahn D: Heterogeneity of microvascular pericytes for smooth muscle type alpha-actin. J Cell Biol. 1991, 113: 147-154. 10.1083/jcb.113.1.147.View ArticlePubMed
- Argandoña EG, Lafuente JV: Effects of dark-rearing on the vascularization of the developmental rat visual cortex. Brain Res. 1996, 732: 43-51. 10.1016/0006-8993(96)00485-4.View ArticlePubMed
- Black JE, Issacs KR, Anderson BJ, Alcantara AA, Greenough WT: Learning causes synaptogenesis whereas motor activity causes angiogenesis, in cerebellar cortex of adult rats. Proc Natl Acad Sci USA. 1990, 87: 5568-5572. 10.1073/pnas.87.14.5568.PubMed CentralView ArticlePubMed
- LaManna JC, Chavez JC, Pichiule P: Structural and functional adaptation to hypoxia in the rat brain. J Exp Biol. 2004, 207: 3163-3169. 10.1242/jeb.00976.View ArticlePubMed
- Alonso G, Galibert E, Duvoid-Guillou A, Vincent A: Hyperosmotic stimulus induces reversible angiogenesis within the hypothalamic magnocellular nuclei of the adult rat: a potential role for neuronal vascular endothelial growth factor. BMC Neurosci. 2005, 24: 6-20.
- Weber B, Keller AL, Reichold J, Logothetis NK: The microvascular system of the striate and extrastriate visual cortex of the macaque. Cereb Cortex. 2008, 18: 2318-2330. 10.1093/cercor/bhm259.View ArticlePubMed
- Balabanov R, Dore-Duffy P: Role of the CNS microvascular pericyte in the blood-brain barrier. J Neurosci Res. 1998, 53: 637-644. 10.1002/(SICI)1097-4547(19980915)53:6<637::AID-JNR1>3.0.CO;2-6.View ArticlePubMed
- del Zoppo GJ: The neurovascular unit in the setting of stroke. J Intern Med. 2010, 267: 156-171. 10.1111/j.1365-2796.2009.02199.x.PubMed CentralView ArticlePubMed
- Bell MA, Scarrow WG: Staining for microvascular alkaline phosphatase in thick celloidin sections of nervous tissue: morphometric and pathological application. Microvasc Res. 1984, 27: 189-203. 10.1016/0026-2862(84)90053-0.View ArticlePubMed
- Harrison R, Harel N, Panesar J, Mount R: Blood capillary distribution correlates with hemodynamic-based functional imaging in cerebral cortex. Cereb Cortex. 2002, 12: 125-233.View Article
- Chilingaryan A, Chilingaryan AM, Martin GG: The three-dimensional direction of microvasculature bed in the brain of white Rattus norvergicus by a Ca2+-ATPase method. Brain Res. 2006, 1070: 131-138. 10.1016/j.brainres.2005.11.059.View ArticlePubMed
- Resse TS, Karnovsky MJ: Fine structural localization of a blood-brain-barrier to exogenous peroxidase. J Cell Biol. 1967, 34: 207-217. 10.1083/jcb.34.1.207.View Article
- Peters A, Palay S, Webster H: The fine structure of the nervous system. 1991, Oxford: Oxford University Press
- Tata DA, Anderson BJ: A new method for the investigation of capillary structure. J Neurosc Methods. 2002, 113: 199-206. 10.1016/S0165-0270(01)00494-0.View Article
- Pacheco P, González-Bernabé G, Alvarado M, Camacho M, Cuevas E, Carrillo P: Brain microvasculature of the rat: Its integrative organization [abstract]. Soc Neurosci SfN. 2005, 189: 8.
- Cortés-Sol A, Domínguez A, Alvarado M, Lara-García M, Camacho M, Pacheco P: Capillary distribution in magnocellular and parvocellular area of paraventricular nucleus of virgin and lactating rats [abstract]. Soc Neurosci SfN. 2008, 395: 15.
- Van den Pol AN: The magnocellular and parvocellular paraventricular nucleus of the rat: intrinsic organization. J Comp Neurol. 1982, 206: 317-345. 10.1002/cne.902060402.View ArticlePubMed
- Swanson LW, Sawchenko PE: Hypothalamic integration: organization of the paraventricular and supraoptic nuclei. Annu Rev Neurosci. 1983, 6: 269-324. 10.1146/annurev.ne.06.030183.001413.View ArticlePubMed
- Armstrong WE: Hypothalamic supraoptic and paraventricular nuclei. The rat nervous system. Edited by: Paxinos G. 1985, Sydney: Academic Press, 119-128.
- Moos F, Richard P: Paraventricular and supraoptic bursting oxytocin cells in rat are locally regulated by oxytocin and functionally related. J Physiol. 1989, 408: 1-18.PubMed CentralView ArticlePubMed
- Wakerley JB: Milk ejection and its control. Physiology of reproduction. Edited by: Knobil E, Neill JD. 2006, New York: Academic Press, Elsevier, 3129-3176. 3
- Newman I, Russell JA, Landgraf R: Oxytocin and vasopressin release within the supraoptic and paraventricular nuclei of pregnant, parturient and lactating rats: a microdialysis study. Neuroscience. 1993, 53: 65-75. 10.1016/0306-4522(93)90285-N.View Article
- Jiang QB, Wakerley JB: Analysis of bursting responses of oxytocin neurons in the rat in late pregnancy, lactation and after weaning. J Physiol. 1995, 486: 237-248.PubMed CentralView ArticlePubMed
- Mena F, Pacheco P, Aguayo D, Clapp C, Grosvenor CE: A rise in intramammary pressure follows electrical stimulation of mammary nerve in anesthetized rats. Endocrinology. 1978, 103: 1929-1936. 10.1210/endo-103-5-1929.View ArticlePubMed
- Sutherland RC, Aizlewood ES, Wakerley JB: Changing characteristics of the milk ejection reflex during pregnancy, lactation and after weaning in the rat. J Reprod Fertil. 1986, 76: 123-130. 10.1530/jrf.0.0760123.View ArticlePubMed
- Paxinos G, Watson C: The rat brain in stereotaxic coordinates. 2007, London: Academic Press, Elsevier
- Ambach G, Palkovits M: Blood supply of the rat hypothalamus. II. Nucleus paraventricularis. Acta Morphol Acad Sci Hung. 1974, 22: 311-320.PubMed
- Sposito NM, Gross PM: Morphometry of individual capillary beds in the hypothalamus-neurohypophysial system of rats. Brain Res. 1987, 403: 375-379. 10.1016/0006-8993(87)90079-5.View ArticlePubMed
- Catheline G, Touquet B, Lombard MC, Poulain DA, Theodosis DT: A study of the role of neuro-glial remodeling in the oxytocin system at lactation. Neuroscience. 2006, 137: 309-316. 10.1016/j.neuroscience.2005.08.042.View ArticlePubMed
- Theodosis DT, Trailin A, Poulain DA: Remodeling of astrocytes, a prerequisite for synapse turnover in the adult brain? Insights from the oxytocin system of the hypothalamus. Am J Physiol Regul Integr Comp Physiol. 2006, 290: 1175-1182.View Article
- Lin SH, Miyata S, Matsunaga W, Kawarabayashi T, Nakashima T, Kiyohara T: Metabolic mapping of the brain in pregnant, parturient and lactating rats using fos immunohistochemistry. Brain Res. 1998, 787: 226-236. 10.1016/S0006-8993(97)01484-4.View ArticlePubMed
- Laurie GW, Leblond CP, Martin GR: Localization of type IV collagen, laminin, heparin sulfate proteoglycan, and fibronectin to the basal lamina of basement membranes. J Cell Biol. 1982, 95: 340-344. 10.1083/jcb.95.1.340.View ArticlePubMed
- Charron AJ, Xu W, Bacallao RL, Wandinger-Ness A: Cablin: a novel protein of the capillary basal lamina. Am J Physiol Heart Circ Physiol. 1999, 277: 1985-1996.
- del Zoppo GJ, Milner R: Integrin-matrix interactions in the cerebral microvasculature. Arterioscler Thromb Vasc Biol. 2006, 26: 1966-1975. 10.1161/01.ATV.0000232525.65682.a2.View ArticlePubMed
- Alberts B, Bray D, Lewis J, Raff M, Roberts K, Watson J: Molecular biology of the cell. 2007, New York: Garland Science, Taylor & Francis Group
- Sanders AG, Ebert RH, Florey HW: The mechanism of capillary contraction. Q J Exp Physiol Cogn Med Sci. 1940, 30: 281-287.
- Rosso L, Peteri-Brunbäck B, Poujeol P, Hussy N, Mienville JM: Vasopressin-induced taurine efflux from rat pituicytes: a potential negative feedback for hormone secretion. J Physiol. 2004, 554: 731-742.PubMed CentralView ArticlePubMed
- Avella M, Ducoudret O, Pisani DF, Poujeol P: Swelling-activated transport of taurine in cultured gill cells of sea bass: physiological adaptation and pavement cell plasticity. Am J Physiol Regul Integr Comp Physiol. 2009, 296: 1149-1160. 10.1152/ajpregu.90615.2008.View Article
- Lang F: Mechanisms and significance of cell volume regulation. J Am Coll Nutr. 2007, 26: 613-623.View Article
- Yool AJ: Aquaporins: multiple roles in the central nervous system. Neuroscientist. 2007, 13: 470-485. 10.1177/1073858407303081.View ArticlePubMed
- Freeman LR, Keller JN: Oxidative stress and cerebral endothelial cells: regulation of the blood-brain-barrier and antioxidant based interventions. Biochim Biophys Acta. 2012, 1822: 822-829. 10.1016/j.bbadis.2011.12.009.PubMed CentralView ArticlePubMed
- Simionescu N, Simionescu M, Palade GE: Differentiated microdomains on the luminal surface of the capillary endothelium. I. Preferential distribution of anionic sites. J Cell Biol. 1981, 90: 605-613.PubMed
- Tuma PL, Hubbard AL: Transcytosis: crossing cellular barriers. Physiol Rev. 2003, 83: 871-932.View ArticlePubMed
- Landgraf R, Neumann ID: Vasopressin and oxytocin release within the brain: a dynamic concept of multiple and variable modes of neuropeptide communication. Front Neuroendocrinol. 2004, 25: 150-76. 10.1016/j.yfrne.2004.05.001.View ArticlePubMed
- Parker SL, Armstrong WE, Sladek CD, Grosvenor CE, Crowley WR: Prolactin stimulated the release of oxytocin in lactating rats: evidence for a physiological role via an action at the neural lobe. Neuroendocrinology. 1991, 53: 503-510. 10.1159/000125764.View ArticlePubMed
- Vega C, Moreno-Carranza B, Zamorano V, Quintanar-Stéphano A, Méndez I, Thebault S, Martínez De La Escalera G, Clapp C: Prolactin promotes oxytocin and vasopressin release by activating neuronal nitric oxide synthase in the supraoptic and paraventricular nuclei. Am J Physiol Regul Integr Comp Physiol. 2010, 299: 1762-1770.View Article
- Ivanov KP, Kalinina MK, Levkovich Yu I: Blood flow velocity in capillaries of brain and muscles and its physiological significance. Microvasc Res. 1981, 22: 143-155. 10.1016/0026-2862(81)90084-4.View ArticlePubMed
- Nnodim JO: Quantitative study of the effects of denervation and castration on the levator ani muscle of the rat. Anat Rec. 1999, 255: 324-33. 10.1002/(SICI)1097-0185(19990701)255:3<324::AID-AR8>3.0.CO;2-1.View ArticlePubMed
- Alvarado M, Cuevas E, Lara-García M, Camacho M, Carrillo P, Hudson R, Pacheco P: Effect of gonadal hormones on the cross-sectional area of pubococcygeus muscle fibers in male rat. Anat Rec. 2008, 291: 586-92. 10.1002/ar.20694.View Article
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