Glomerular Nucleus of the Weakly Electric Fish, Gymnotus sp.: Cytoarchitecture, Histochemistry, and Fiber Connections-Insights from Neuroanatomy to Evolution and Behavior

The present study provides a detailed description of morphological and hodological aspects of the glomerular nucleus in the weakly electric fish Gymnotus sp., and explores the evolutionary and functional implications flowing from this analysis. The glomerular nucleus of Gymnotus shows numerous morphological similarities with the glomerular nucleus of percomorph fish, although cytoarchitectonically simpler. In addition, congruence of the histochemical acetylcholinesterase (AChE) distribution with cytoarchitectonic data suggests that the glomerular nucleus, together with the ventromedial cell group of the medial subdivision of the preglomerular complex (PGm-vmc) rostrally, and the subglomerular nucleus (as identified by Maler et al. [1991] J Chem Neuroanat 4:1-38) caudally, may form a distinct longitudinally organized glomerular complex. Our results show that an important source of sensory afferents to the glomerular nucleus originates in the pretectal and electrosensorius nuclei. The glomerular nucleus in turn projects to the hypothalamus (inferior lobe and anterior hypothalamus), to the anterior tuberal nucleus, and to the medial region of the preglomerular nucleus (PGm). These data suggest that visual and electrosensory information reach the glomerular nucleus and are relayed to the hypothalamus and, via PGm, to the pallium. Such connections are similar to those of the glomerular nucleus in percomorphs and the posterior pretectal nucleus in osteoglossomorph, esocids, and salmonids, where they comprise one component of a visual processing pathway. In Gymnotiform fish, however, the pretectal region that projects to the glomerular nucleus is dominated by electrosensory input (visual input is minor), which is consistent with the dominant role of electroreception in these fish. J. Comp. Neurol. 519:1658-1676, 2011. (c) 2011 Wiley-Liss, Inc.

Activation of lateral parabrachial afferent pathways and endocrine responses during sodium appetite regulation

Modulation of salt appetite involves interactions between the circumventricular organs (CVOs) receptive areas and inhibitory hindbrain serotonergic circuits. Recent studies provide support to the idea that the serotonin action in the lateral parabrachial nucleus (LPBN) plays an important inhibitory role in the modulation of sodium appetite. The aim of the present work was to identify the specific groups of neurons projecting to the LPBN that are activated in the course of sodium appetite regulation, and to analyze the associated endocrine response, specifically oxytocin (OT) and atrial natriuretic peptide (ANP) plasma release, since both hormones have been implicated in the regulatory response to fluid reestablishment. For this purpose we combined the detection of a retrograde transported dye, Fluorogold (FG) injected into the LPBN with the analysis of the Fos immunocytochemistry brain pattern after sodium intake induced by sodium depletion. We analyzed the Fos-FG immunoreactivity after sodium ingestion induced by peritoneal dialysis (PD). We also determined OT and ANP plasma concentration by radioimmunoassay (RIE) before and after sodium intake stimulated by PD. The present study identifies specific groups of neurons along the paraventricular nucleus, central extended amygdala, insular cortex, dorsal raphe nucleus, nucleus of the solitary tract and the CVOs that are activated during the modulation of sodium appetite and have direct connections with the LPBN. It also shows that OT and ANP are released during the course of sodium satiety and fluid reestablishment. The result of this brain network activity may enable appropriate responses that re-establish the body fluid balance after induced sodium consumption. (C) 2009 Elsevier Inc. All rights reserved.

Fos expression in the NTS in response to peripheral chemoreflex activation in awake rats

Chemoreflex afferent fibers terminate in the nucleus tractus solitarii (NTS), but the specific location of the NTS neurons excited by peripheral chemoreflex activation remains to be characterized. Here, the topographic distribution of chemoreflex sensitive cells at the commissural NTS was evaluated. To reach this goal, Fos-immunoreactive neurons (Fos-ir) were accounted in rostro-caudal levels of the intermediate and caudal commissural NTS, after intermittent chemoreflex activation with intravenous injection of potassium cyanide [KCN (80 mu g/kg) or saline (0.9%, vehicle), one injection every 3 min during 30 min]. In response to intermittent intravenous injections of KCN, a significant increase in the number of Fos-ir neurons was observed specifically in the lateral intermediate commissural NTS [(LI)NTS (82 +/- 9 vs. 174 +/- 16, cell number mean per section)] and lateral caudal commissural NTS [(LI)NTS (71 +/- 9 vs. 199 +/- 18, cell number mean per section)]. To evaluate the influence of baroreceptor-mediated inputs following the increase in blood pressure during intermittent chemoreflex activation, we performed an intermittent activation of the arterial baroreflex by intravenous injection of phenylephrine [1.5 mu g/kg iv (one injection every 3 min during 30 min)]. This procedure induced no change in Fos-ir in (LI)NTS (64 +/- 6 vs. 62 +/- 12, cell number mean per section) or (LC)NTS (56 +/- 15 vs. 77 +/- 12, cell number mean per section). These data support the involvement of the commissural NTS in the processing of peripheral chemoreflex, and provide a detailed characterization of the topographical distribution of activated neurons within this brain region. (C) 2009 Elsevier B.V. All rights reserved.

HYPOTHALAMIC COCAINE- AND AMPHETAMINE-REGULATED TRANSCRIPT AND CORTICOTROPHIN RELEASING FACTOR NEURONS ARE STIMULATED BY EXTRACELLULAR VOLUME AND OSMOTIC CHANGES

Several studies suggest that hypothalamic cocaine- and amphetamine-regulated transcript (CART) may interact with the hypothalamic-pituitary-adrenal (HPA) axis in the control of neuroendocrine function and may also participate in cardiovascular regulation. Therefore, this study aimed to evaluate, in experimental models of isotonic (I-EVE) and hypertonic (H-EVE) extracellular volume expansion and water deprivation (WD), the activation of CART- and corticotrophin releasing factor (CRF)-immunoreactive neurons, as well as the relative expression of CART and CRF mRNAs in the paraventricular (PVN) and supraoptic (SON) nuclei of the hypothalamus. Both H-EVE (0.30M NaCl, 2mL/100g of body weight, in 1 minute) and 24 hours of WD significantly increased plasma sodium concentrations, producing, respectively, either an increase or a decrease in extracellular volume. I-EVE (0.15M NaCl, 2mL/100g of body weight, in 1 minute) evoked a significant increase in the circulating volume accompanied by unaltered plasma concentrations of sodium. CART-expressing neurons of both magnocellular and parvocellular hypothalamic divisions were activated to produce Fos in response to H-EVE but not in response to I-EVE. Furthermore, increased expression of CART mRNA was found in the PVN of H-EVE but not I-EVE rats. These data show for the first time that EVE not only activates hypothalamic CRF neurons but also increases CRF mRNA expression in the PVN. In contrast, WD increases the number of CART-immunoreactive neurons activated to produce Fos in the PVN and SON but does not change the number of neurons double labeled for Fos and CRF or expression of CRF mRNA in the PVN. These findings provided new insights into the participation of CART in diverse processes within the PVN and SON, including its possible involvement in activation of the HPA axis and cardiovascular regulation in response to changes in extracellular volume and osmolality. (C) 2011 IBRO. Published by Elsevier Ltd. All rights reserved.

Role of dorsal raphe nucleus 5-HT(1A) and 5-HT(2) receptors in tonic immobility modulation in guinea pigs

Tonic immobility (TI) is an innate defensive behavior characterized by a state of physical inactivity and diminished responsiveness to environmental stimuli. Behavioral adaptations to changes in the external and internal milieu involve complex neuronal network activity and a large number of chemical neurotransmitters. The TI response is thought to be influenced by serotonin (5-HT) activity in the central nervous system (CNS) of vertebrates, but the neuronal groups involved in the mechanisms underlying this behavior are poorly understood. Owing to its extensive afferents and efferents, the dorsal raphe nucleus (DRN) has been implicated in a great variety of physiological and behavioral functions. in the current study, we investigated the influence of serotonergic 5-HT(1A) and 5-HT(2) receptor activity within the DRN on the modulation of TI behavior in the guinea pig. Microinjection of a 5-HT(1A) receptor agonist (8-OH-DPAT, 0.01 and 0.1 mu g) decreased TI behavior, an effect blocked by pretreatment with WAY-100635 (0.033 mu g), a 5-HT(1A) antagonist. In contrast, activation of 5-HT(2) receptors within the DRN (alpha-methyl-5-HT, 0.5 mu g) increased the TI duration, and this effect could be reversed by pretreatment with an ineffective dose (0.01 mu g) of ketanserine. Since the 5-HT(1A) and 5-HT(2) agonists decreased and increased, respectively, the duration of TI, different serotonin receptor subtypes may play distinct roles in the modulation of TI in the guinea pig. (C) 2009 Elsevier B.V. All rights reserved.

Differential regulation of TRP channels in a rat model of neuropathic pain

Neuropathic pain is a chronic disease resulting from dysfunction of the nervous system often due to peripheral nerve injury. Hypersensitivity to sensory Stimuli (mechanical, thermal or chemical) is a common source of pain in patients and ion channels involved in detecting these Stimuli are possible candidates for inducing and/or maintaining the pain. Transient receptor potential (TRP) channels expressed on nociceptors respond to different sensory stimuli and a few of them have been studied previously in the models of neuropathic pain. Using real-time PCR for quantification of all known TRP channels we identified several TRP channels, which have not been associated with nociception OF neuropathic pain before, to be expressed in the DRG and to be differentially regulated after spared nerve injury (SNI). Of all TRP channel members, TRPML3 showed the most dramatic change in animals exhibiting neuropathic pain behaviour compared to control animals. fit situ hybridisation showed a widespread increase of expression ill neurons of small, medium and large cell sizes, indicating expression ill multiple subtypes. Co-localisation of TRPML3 with CGRP, NF200 and IB4 staining confirmed a broad Subtype distribution. Expression studies during development showed that TRPML3 is all embryonic channel that is induced upon nerve injury in three different nerve injury models investigated. Thus. the current results link for the first time a re-expression of TRPML3 with the development of neuropathic pain conditions. In addition, decreased mRNA levels after SNI were seen for TRPM6, TRPM8, TRPV1, TRPA1, TRPC3, TRPC4 and TRPC5. (C) 2009 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.

GABA and nitric oxide in the PVN are involved in arterial pressure control but not in the chemoreflex responses in rats

GABAergic, nitrergic and glutamatergic mechanisms in the PVN on the baseline mean arterial pressure (MAP), heart rate (HR) and on the cardiovascular responses to chemoreflex activation in awake rat were evaluated. Chemoreflex was activated with KCN before and after microinjections into the PVN. Bicuculline into the PVN increased baseline MAP (94+/-3 vs 113+/-5 mmHg) and HR (350+/-9 vs 439+/-18 bpm) but had no effect on the pressor (49+/-5 vs 47+/-6 mmHg) or bradicardic (-213+/-23 vs -256+/-42 bpm) responses (n=7). Kynurenic acid into the PVN (n=6) produced no significant changes in the MAP (98+/-3 vs 100+/-3 mmHg), HR (330+/-5 vs 339+/-12 mmHg) or in the pressor (50+/-4 vs 42+/-4 mmHg) and bradicardic (-252+/-4 vs -285+/-16 bpm) responses to chemoreflex. L-NAME into the PVN (n=8) produced increase in the MAP (94+/-3 vs 113+/-5 mmHg) and HR (350+/-9 vs 439+/-18 bpm) but had no effect on the pressor (52+/-5 vs 47+/-6 mmHg) or bradicardic (-253+/-19 vs -320+/-25 bpm) responses to chemoreflex. We conclude that GABA(A) and nitric oxide in the PVN are involved in the maintenance of the baseline MAP but not in the modulation of the responses to chemoreflex. The results also show that Glutamate receptors in the PVN are not involved in maintenance of the baseline MAP, HR or in the cardiovascular responses to chemoreflex in awake rats. (C) 2008 Elsevier B.V. All rights reserved.

Intermittent hypoxia-induced sensitization of central chemoreceptors contributes to sympathetic nerve activity during late expiration in rats

Molkov YI, Zoccal DB, Moraes DJ, Paton JF, Machado BH, Rybak IA. Intermittent hypoxia-induced sensitization of central chemoreceptors contributes to sympathetic nerve activity during late expiration in rats. J Neurophysiol 105: 3080-3091, 2011. First published April 6, 2011; doi:10.1152/jn.00070.2011.-Hypertension elicited by chronic intermittent hypoxia (CIH) is associated with elevated activity of the thoracic sympathetic nerve (tSN) that exhibits an enhanced respiratory modulation reflecting a strengthened interaction between respiratory and sympathetic networks within the brain stem. Expiration is a passive process except for special metabolic conditions such as hypercapnia, when it becomes active through phasic excitation of abdominal motor nerves (AbN) in late expiration. An increase in CO(2) evokes late-expiratory (late-E) discharges phase-locked to phrenic bursts with the frequency increasing quantally as hypercapnia increases. In rats exposed to CIH, the late-E discharges synchronized in AbN and tSN emerge in normocapnia. To elucidate the possible neural mechanisms underlying these phenomena, we extended our computational model of the brain stem respiratory network by incorporating a population of presympathetic neurons in the rostral ventrolateral medulla that received inputs from the pons, medullary respiratory compartments, and retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG). Our simulations proposed that CIH conditioning increases the CO(2) sensitivity of RTN/pFRG neurons, causing a reduction in both the CO(2) threshold for emerging the late-E activity in AbN and tSN and the hypocapnic threshold for apnea. Using the in situ rat preparation, we have confirmed that CIH-conditioned rats under normal conditions exhibit synchronized late-E discharges in AbN and tSN similar to those observed in control rats during hypercapnia. Moreover, the hypocapnic threshold for apnea was significantly lowered in CIH-conditioned rats relative to that in control rats. We conclude that CIH may sensitize central chemoreception and that this significantly contributes to the neural impetus for generation of sympathetic activity and hypertension.

Lateral Parabrachial Afferent Areas and Serotonin Mechanisms Activated by Volume Expansion

Recent evidence has shown that the serotonergic mechanism of the lateral parabrachial nucleus (LPBN) participates in the regulation of renal and hormonal responses to isotonic blood volume expansion (BVE). We investigated the BVE-induced Fos activation along forebrain and hindbrain nuclei and particularly within the serotonergic clusters of the raphe system that directly project to the LPBN. We also examined whether there are changes in the concentration of serotonin (5HT) within the raphe nucleus in response to the same stimulus. With this purpose, we analyzed the cells doubly labeled for Fos and Fluorogold (FG) following BVE (NaCl 0.15 M, 2 ml/100 g b.w., 1 min) 7 days after FG injection into the LPBN. Compared with the control group, blood volume-expanded rats showed a significant greater number of Fos-FG double-labeled cells along the nucleus of the solitary tract, locus coeruleus, hypothalamic paraventricular nucleus, central extended amygdala complex, and dorsal raphe nucleus (DRN) cells. Our study also showed an increase in the number of serotonergic DRN neurons activated in response to isotonic BVE. We also observed decreased levels of 5HT and its metabolite 5-hydroxyindoleacetic acid (measured by high-pressure liquid chromatography) within the raphe nucleus 15 min after BVE. Given our previous evidence on the role of the serotonergic system in the LPBN after BVE, the present morphofunctional findings suggest the existence of a key pathway (DRN-LPBN) that may control BVE response through the modulation of 5HT release. (c) 2008 Wiley-Liss, Inc.

INTERMITTENT ACTIVATION OF PERIPHERAL CHEMORECEPTORS IN AWAKE RATS INDUCES Fos EXPRESSION IN ROSTRAL VENTROLATERAL MEDULLA-PROJECTING NEURONS IN THE PARAVENTRICULAR NUCLEUS OF THE HYPOTHALAMUS

Despite the well-established sympathoexcitation evoked by chemoreflex activation, the specific sub-regions of the CNS underlying such sympathetic responses remain to be fully characterized. In the present study we examined the effects of intermittent chemoreflex activation in awake rats on Fos-immunoreactivity (Fos-ir) in various subnuclei of the paraventricular nucleus of the hypothalamus (PVN), as well as in identified neurosecretory preautonomic PVN neurons. In response to intermittent chemoreflex activation, a significant increase in the number of Fos-ir cells was found in autonomic-related PVN subnuclei, including the posterior parvocellular, ventromedial parvocellular and dorsal-cap, but not in the neurosecretory magnocellular-containing lateral magnocellular subnucleus. No changes in Fos-ir following chemoreflex activation were observed in the anterior PVN subnucleus. Experiments combining Fos immunohistochemistry and neuronal tract tracing techniques showed a significant increase in Fos-ir in rostral ventrolateral medulla (RVLM)-projecting (PVN-RVLM), but not in nucleus of solitarii tract (NTS)-projecting PVN neurons. In summary, our results support the involvement of the PVN in the central neuronal circuitry activated in response to chemoreflex activation, and indicate that PVN-RVLM neurons constitute a neuronal substrate contributing to the sympathoexcitatory component of the chemoreflex. Published by Elsevier Ltd on behalf of IBRO.

Nitric oxide modulates the firing rate of the rat supraoptic magnocellular neurons

In vitro, nitric oxide (NO) inhibits the firing rate of magnocellular neurosecretory cells (MNCs) of hypothalamic supraoptic and paraventricular nuclei and this effect has been attributed to GABAergic activation. However, little is known about the direct effects of NO in MNCs. We used the patch-clamp technique to verify the effect Of L-arginine, a precursor for NO synthesis, and N-omega-nitro-L-arginine methyl ester hydrochloride (L-NAME), an inhibitor of NOS, on spontaneous electrical activity of MNCs after glutamatergic and GABAergic blockade in Wistar rat brain slices. 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 mu M) and DL-2-amino-5-phosphonovaleric acid (DL-AP5) (30 mu M) were used to block postsynaptic glutamatergic currents, and picrotoxin (30 mu M) and saclofen (30 mu M) to block ionotropic and metabotropic postsynaptic GABAergic currents. Under these conditions, 500 mu M L-arginine decreased the firing rate from 3.7 +/- 0.6 Hz to 1.3 +/- 0.3 Hz. Conversely, 100 mu M L-NAME increased the firing rate from 3.0 +/- 0.3 Hz to 5.8 +/- 0.4 Hz. All points histogram analysis showed changes in resting potential from -58.1 +/- 0.8 mV to -62.2 +/- 1.1 mV in the presence of L-arginine and from -59.8 +/- 0.7 mV to -56.9 +/- 0.8 mV by L-NAME. Despite the nitrergic modulator effect on firing rate, some MNCs had no significant changes in their resting potential. In those neurons, hyperpolarizing after-potential (HAP) amplitude increased from 12.4 +/- 1.2 mV to 16.8 +/- 0.7 mV by L-arginine, but without significant changes by L-NAME treatment. To our knowledge, this is the first demonstration that NO can inhibit MNCs independent of GABAergic inputs. Further, our results point to HAP as a potential site for nitrergic modulation. (C) 2008 IBRO. Published by Elsevier Ltd. All rights reserved.

Increased sympathetic outflow in juvenile rats submitted to chronic intermittent hypoxia correlates with enhanced expiratory activity

Chronic intermittent hypoxia (CIH) in rats produces changes in the central regulation of cardiovascular and respiratory systems by unknown mechanisms. We hypothesized that CIH (6% O(2) for 40 s, every 9 min, 8 h day(-1)) for 10 days alters the central respiratory modulation of sympathetic activity. After CIH, awake rats (n = 14) exhibited higher levels of mean arterial pressure than controls (101 +/- 3 versus 89 +/- 3 mmHg, n = 15, P < 0.01). Recordings of phrenic, thoracic sympathetic, cervical vagus and abdominal nerves were performed in the in situ working heart-brainstem preparations of control and CIH juvenile rats. The data obtained in CIH rats revealed that: (i) abdominal (Abd) nerves exhibited an additional burst discharge in late expiration; (ii) thoracic sympathetic nerve activity (tSNA) was greater during late expiration than in controls (52 +/- 5 versus 40 +/- 3%; n = 11, P < 0.05; values expressed according to the maximal activity observed during inspiration and the noise level recorded at the end of each experiment), which was not dependent on peripheral chemoreceptors; (iii) the additional late expiratory activity in the Abd nerve correlated with the increased tSNA; (iv) the enhanced late expiratory activity in the Abd nerve unique to CIH rats was accompanied by reduced post-inspiratory activity in cervical vagus nerve compared to controls. The data indicate that CIH rats present an altered pattern of central sympathetic-respiratory coupling, with increased tSNA that correlates with enhanced late expiratory discharge in the Abd nerve. Thus, CIH alters the coupling between the central respiratory generator and sympathetic networks that may contribute to the induced hypertension in this experimental model.

Inhibitory network interactions shape the auditory processing of natural communication signals in the songbird auditory forebrain

The role of GABA in the central processing of complex auditory signals is not fully understood. We have studied the involvement of GABA(A)-mediated inhibition in the processing of birdsong, a learned vocal communication signal requiring intact hearing for its development and maintenance. We focused on caudomedial nidopallium (NCM), an area analogous to parts of the mammalian auditory cortex with selective responses to birdsong. We present evidence that GABA(A)-mediated inhibition plays a pronounced role in NCM`s auditory processing of birdsong. Using immunocytochemistry, we show that approximately half of NCM`s neurons are GABAergic. Whole cell patch-clamp recordings in a slice preparation demonstrate that, at rest, spontaneously active GABAergic synapses inhibit excitatory inputs onto NCM neurons via GABA(A) receptors. Multi-electrode electrophysiological recordings in awake birds show that local blockade of GABA(A)-mediated inhibition in NCM markedly affects the temporal pattern of song-evoked responses in NCM without modifications in frequency tuning. Surprisingly, this blockade increases the phasic and largely suppresses the tonic response component, reflecting dynamic relationships of inhibitory networks that could include disinhibition. Thus processing of learned natural communication sounds in songbirds, and possibly other vocal learners, may depend on complex interactions of inhibitory networks.

Plasma hormonal profiles and dendritic spine density and morphology in the hippocampal CA1 stratum radiatum, evidenced by light microscopy, of virgin and postpartum female rats

Successful reproduction requires that changes in plasma follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin (PRL), oxytocin (OT), estrogen (E-2) and progesterone (P-4) occur together with the display of maternal behaviors. Ovarian steroids and environmental stimuli can affect the dendritic spines in the rat hippocampus. Here, studying Wistar rats, it is described: (a) the sequential and concomitant changes in the hormonal profile of females at postpartum days (PP) 4, 8, 12, 16, 20 and 24, comparing to estrous cycle referential values; (b) the dendritic spine density in the stratum radiatum of CA1 (CA1-SR) Golgi-impregnated neurons in virgin females across the estrous cycle and in multiparous age-matched ones; and (c) the proportion of different types of spines in the CAI-SR of virgin and postpartum females, both in diestrus. Plasma levels of gonadotrophins and ovarian hormones remained low along PP while LH increased and PRL decreased near the end of the lactating period. The lowest dendritic spine density was found in virgin females in estrus when compared to diestrus and proestrus phases or to postpartum females in diestrus (p < 0.03). Other comparisons among groups were not statistically significant (p > 0.4). There were no differences in the proportions of the different spine types in nulliparous and postpartum females (p > 0.2). Results suggest that medium layer CA1-SR spines undergo rapid modifications in Wistar females across the estrous cycle (not quite comparable to Sprague-Dawley data or to hormonal substitutive therapy following ovariectomy), but persistent effects of motherhood on dendritic spine density and morphology were not found in this area. (c) 2008 Elsevier Ireland Ltd. All rights reserved.

Hypothalamic supraoptic but not paraventricular nucleus is involved in cardiovascular responses to carbachol microinjected into the bed nucleus of stria terminalis of unanesthetized rats

Microinjection of the cholinergic agonist carbachol into the bed nucleus of the stria terminalis (BST) has been reported to cause pressor response in unanesthetized rats, which was shown to be mediated by an acute release of vasopressin into the systemic circulation and followed by baroreflex-mediated bradycardia. In the present study, we tested the possible involvement of the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei in the pressor response evoked by carbachol microinjection into the BST of unanesthetized rats. For this, cardiovascular responses following carbachol (1 nmol/100 nL) microinjection into the BST were studied before and after PVN or SON pretreatment, either ipsilateral or contralateral in relation to BST microinjection site, with the nonselective neurotransmission blocker cobalt chloride (CoCl(2), 1 mM/100 nL). Carbachol microinjection into the BST evoked pressor response. Moreover, BST treatment with carbachol significantly increased plasma vasopressin levels, thus confirming previous evidences that carbachol microinjection into the BST evokes pressor response due to vasopressin release into the circulation. SON pretreatment with CoCl(2), either ipsilateral or contralateral in relation to BST microinjection site, inhibited the pressor response to carbachol microinjection into the BST. However, CoCl(2) microinjection into the ipsilateral or contralateral PVN did not affect carbachol-evoked pressor response. In conclusion, our results suggest that pressor response to carbachol microinjection into the BST is mediated by SON magnocellular neurons, without significant involvement of those in the PVN. The results also indicate that responses to carbachol microinjection into the BST are mediated by a neural pathway that depends on the activation of both ipsilateral and contralateral SON. (C) 2011 Elsevier B.V. All rights reserved.