Potentiation of vasopressin analgesia in rats treated neonatally with monosodium glutamate

The analgesic response elicited by central administration of arginine vasopressin (AVP) appears to be dependent upon the integrity of the hypothalamic paraventricular nucleus (PVN), since lesions placed in the PVN eliminate AVP analgesia. A projection to the zona externa of the median eminence constitutes one of the VP-containing efferents of the PVN. Neonatal treatment with monosodium glutamate (MSG) destroys perikarya of the arcuate nucleus and median eminence. The present study examined whether AVP analgesia was affected in the MSG-treated rat and whether these alterations were accompanied by specific changes in VP immunoreactivity in the zona externa of the median eminence. Female rats, neonatally treated with either MSG or a saline control, were tested as adults on the tail-flick test following intracerebroventricular injections of 0, 75, 150 and 500 ng doses of AVP. After testing, selected animals were prepared for AVP and oxytocin immunocytochemistry of the median eminence. Significant potentiations in the magnitude of AVP analgesia were observed in MSG-treated rats. AVP and oxytocin immunoreactivity in the zona interna and oxytocin immunoreactivity in the zona externa of the median eminence were similar in MSG-treated and control rats. In contrast, AVP immunoreactivity in the zona externa of the median eminence was markedly reduced in the MSG-treated rat. These data suggest that VP analgesia may normally be inhibited by those medial-basal hypothalamic neurons affected by neonatal MSG treatment.     

Fos,RVLM-projecting neurons,and spinally projecting neurons in the PVN following hypertonic saline infusion

Hypertonic saline (HTS; 1.7 M) infused intravenously into conscious rats increases the production of Fos, a marker of cell activation, in the hypothalamic paraventricular nucleus (PVN). The parvocellular PVN contains subpopulations of neurons. However, which subpopulations are activated by HTS is unknown. We determined whether PVN neurons that innervate the rostral ventrolateral medulla (RVLM) or the spinal cord (important autonomic sites) expressed Fos following HTS. Experiments were performed 24-96 h after chronic implantation of an intravenous cannula. HTS significantly increased the number of Fos-positive cells. In the parvocellular PVN, the maximum number of Fos-positive cells occurred rostral of the anterior-posterior level at which the number of neurons that projected to the medulla or spinal cord peaked. Compared with controls, HTS did not significantly increase the number of double-labeled neurons. These findings demonstrate that an elevation in plasma osmolality activates PVN neurons but not the subgroups of PVN neurons with projections to the RVLM or to the spinal cord.     

Genomic Effects of Cold and Isolation Stress on Magnocellular Vasopressin mRNA-Containing Cells in the Hypothalamus of the Rat

We assessed the effects of cold and isolation stress on arginine vasopressin (AVP) mRNA in the paraventricular (PVN) and supraoptic (SON) nuclei of the hypothalamus. Vasopressin mRNA levels were determined by in situ hybridization histochemistry at the cellular level. In posterior magnocellular neurons of the PVN isolation stress for 7 or 14 days increased vasopressin mRNA levels 28 and 29%, respectively, compared to group-housed controls. No significant alterations in vasopressin gene expression were observed in the SON after 7 or 14 days of isolation stress. Scattered magnocellular AVP mRNA-expressing cells of the medial parvocellular PVN showed increases of 19 and 34% after 7 and 14 days of isolation, respectively. We also studied the effect of cold or combined cold and isolation stress on vasopressin gene expression in the PVN and SON. Cold stress for 3 h daily for 4 consecutive days increased AVP mRNA levels in the posterior magnocellular PVN by 15%. Cold-isolated animals showed an increase of 21%. No significant effect on AVP mRNA levels in the SON was observed. In contrast to the posterior magnocellular PVN, cold or cold-isolation stress increased AVP mRNA in magnocellular neurons of the medial parvocellular region of the PVN by 25 and 43%, respectively, relative to control rats. These results suggest that psychological and metabolic stress may be added to the list of stressors that activate the hypothalamo-neurohypophysial system.     

Selective contributions of the medial preoptic nucleus to testosterone-dependant regulation of the paraventricular nucleus of the hypothalamus and the HPA axis

Previous data have consistently demonstrated an inhibitory effect of androgens on stress-induced hypothalamic-pituitary-adrenal (HPA) responses. Several brain regions may influence androgen-mediated inhibition of the HPA axis, including the medial preoptic area. To test the role of the medial preoptic nucleus (MPN) specifically, we examined in high- and low-testosterone-replaced gonadectomized rats bearing discrete bilateral lesions of the MPN basal and stress-induced indexes of HPA function, and the relative levels of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) mRNA in the amygdala. High testosterone replacement decreased plasma adrenocorticotropin hormone (ACTH) and paraventricular nucleus (PVN) Fos responses to restraint exposure in sham- but not in MPN-lesioned animals. AVP-, but not CRH-immunoreactivity staining in the external zone of the median eminence was increased by testosterone in sham animals, and MPN lesions blocked this increment in AVP. A similar interaction between MPN lesions and testosterone occurred on AVP mRNA levels in the medial nucleus of the amygdala. These findings support an involvement of MPN projections in mediating the AVP response to testosterone in both the medial parvocellular PVN and medial amygdala. We conclude that the MPN forms part of an integral circuit that mediates the central effects of gonadal status on neuroendocrine and central stress responses.     

弧束核参与剌激下丘脑室旁核的镇痛作用

This study was undertaken to evaluate the analgesic effect of paraventricular nucleus (PVN) stimulation with tail stimulation-vocalization test. The mechanism of this analgesia was analysed with nuclear lesion and microinjection technique. The main results were as follows: (1) Electrical stimulation of the PVN could significantly enhance the pain threshold and increase the content of AVP in brainstem measured by radioimmunoassay. (2) Solitary tract nucleus (STN) lesion could eliminate the analgesic effect induced by PVN stimulation. (3) Intranuclear microinjection of AVP-antagonist and AVP-antiserum into the STN could block the analgesic effect of PVN stimulation. (4) Intranuclear microinjection of AVP into the STN could mimick the analgesic effect similar to that of PVN stimulation. These results suggest that electrical stimulation of the PVN could produce an analgesic effect. This effect might be mediated by the activation of VP-ergic neurons in PVN and upon releasing VP from the descending fibers, the activities of neurons in the STN are influenced.     

孤束核参与刺激下丘脑室旁核的镇痛作用

本实验用电刺激鼠尾-嘶叫法测痛,观察电刺激下丘脑室旁核的镇痛效应,并采用核团损毁和核团内微量注射药物等方法分析其镇痛通路。实验结果如下:(1)电刺激下丘脑室旁核能产生明显的镇痛效应。同时,放射免疫测定发现脑干加压素含量升高。(2)损毁孤束核能取消刺激下丘脑室旁核的镇痛效应,但对基础痛阈无影响。(3)孤束核内微量注射加压素拮抗剂[d(CH_2)_5 TYr(Me)-AVP]60ng/0.6μl 和加压素抗血清0.6μl 都可明显对抗刺激下丘脑室旁核的镇痛效应。(4)直接在孤束核内微量注射加压素60ng/0.6μl,能模拟刺激下丘脑室旁核的镇痛效应。实验结果表明:电刺激下丘脑室旁核能产生镇痛效应,其机理之一可能是兴奋了下丘脑室旁核中加压素能神经元胞体,后者通过下行投射纤维在孤束核中释放加压素,影响孤束核神经元的活动,从而产生镇痛。     

Effects of experimental hypothyroidism on the development of the hypothalamo-pituitary-adrenal axis in the rat

Hypothyroid pups were obtained by adding methimazole in the mother's drinking water from day 15 of gestation and sacrificed at 4, 8 or 15 days. Circulating corticosterone decreased at all ages, while CBG concentrations diminished at day 4, increased at day 8 and did not change at day 15 in hypothyroid rats. As opposed to controls, plasma ACTH concentrations decreased steadily with age while there was an accumulation of ACTH in the anterior pituitary of hypothyroid 15-day-old rats. Anterior pituitary POMC contents were unaffected by the treatment. In the hypothalamic PVN, CRF mRNA levels in the total population of CRF-synthesizing cells and in the CRF+/AVP+ subpopulation were below those of controls whatever the age considered while AVP mRNA in the CRF+/AVP+ subpopulation did not change at day 4 and decreased at day 8 and 15 in hypothyroid animals. Both the number of cell bodies expressing detectable levels of CRF mRNA and the percentage of CRF and AVP colocalization decreased at day 4 and were unchanged thereafter. CRF and AVP immunoreactivity in the zona externa of the median eminence increased with age but was not affected by methimazole treatment. The concentration of AVP mRNA in the magnocellular cell bodies of the PVN and the SON as well as AVP immunoreactivity in the zona interna of the median eminence were not changed by the treatment at days 4 and 8. In hypothyroid 15-day-old rats, SON AVP mRNA increased, AVP immunoreactivity decreased while plasma osmolality was enhanced. In conclusion, our data demonstrate that experimental hypothyroidism impairs specifically the maturation of hypothalamic parvocellular CRF and AVP gene expression during the stress hyporesponsive period. These observations suggest that the physiological peak in plasma thyroxine concentrations that occur between day 8-12 may participate in the maturation of hypothalamic CRF- and AVP-synthesizing cells.     

Monoamine and amino acid content in brain regions of Brattleboro rats

Monoamine and amino acid content were measured in brain regions from 12 week old male, homozygous Brattleboro (DI,n=12) and Long-Evans control (LE,n=12) rats. Norepinephrine (NE) content was significantly elevated (16–25%) in the spinal cord, pons-medulla and anterior hypothalamus of DI rats when compared to LE controls. NE content of the neurointermediate lobe of pituitary in DI rats was almost twice that of LE controls. Serotonin content was also significantly elevated in the spinal cord, pons-medulla, anterior hypothalamus and forebrain of DI rats relative to the LE controls. Taurine content in DI rats was increased (31–42%) above that of LE rats in the anterior hypothalamus, striatum and forebrain. Glutamine content was also greater in DI rats than LE in the spinal cord, pons-medulla, anterior hypothalamus, striatum, hippocampus and forebrain. The changes in monoamine and amino acid content were discussed in relation to the cardiovascular and osmoregulatory deficits that are present in DI rats due to arginine vasopressin (AVP) deficiency. The possible role of AVP in modulating NE turnover was also discussed. The increase in brain TAU content in DI rats may be a physiological response to hypernatremia.     

Central antinociceptive effects of lysine-vasopressin and an analogue

Vasopressin (VP) neurons project to extrahypothalamic sites involved in pain perception, including the substantia gelatinosa of the spinal cord as well as the trigeminal and vagus nerves. Previous studies have reported antinociceptive activity following intracerebroventricular (ICV) or subcutaneous (SC) VP injections (16–100 μg) on the tail-flick test while hyperalgesia has been observed in rats either genetically deficient in VP or treated with antisera to VP. The present study investigated whether nanogram (ng) doses of lysine-vasopressin (LVP) and a VP analogue with prolonged activity increased tail-flick latencies and flinch-jump thresholds following ICV or SC injections. LVP (150 and 500 ng, ICV) significantly increased tail-flick latencies while the analogue 1-deamino-(8-Lys-N^?-(Gly-Gly-Gly))-VP (500 ng, ICV) produced more powerful and prolonged analgesia. In contrast, latencies were not increased by SC injections of LVP (150–1500 ng). Further, flinch-jump thresholds were affected minimally by either ICV or SC LVP injections. These data suggest a role for VP in pain modulation and a central site of this action.     

Effect of intrathecal injection of pertussis toxin on substance P, norepinephrine and serotonin contents in various neural structures of arthritic rats.

The possible influence of spinal receptors coupled to Gi/Go regulatory proteins on chronic pain adaptive processes of neural tissues was investigated in normal and arthritic rats. Pain-suffering animals showed an enhanced immunoreactivity to substance P (ir-SP) in the lumbar spinal cord, pons-medulla oblongata region and thalamus. Norepinephrine (NE) levels were increased in the spinal cord, while serotonin (5-HT) was elevated in both spinal cord and midbrain. The intrathecal injection of 1 micrograms pertussis toxin 6 days before sacrifice of rats produced in these arthritic animals a pronounced reduction of ir-SP in the pons-medulla, midbrain and thalamus, but not in the spinal cord. The level of 5-HT was diminished in dorsal spinal cord and midbrain, whereas NE appeared unchanged. In contrast, the toxin only reduced ir-SP of normal rats in the midbrain, without altering the levels of NE or 5-HT, in all the areas analysed. These results suggest the involvement of certain spinal receptors coupled to Gi/Go transducer proteins in processes leading to the elevation of ir-SP and 5-HT in various neural structures of arthritic rats.     

Vasopressin analgesia: specificity of action and non-opioid effects

Recent neuroanatomical and behavioral evidence has indicated that vasopressin (VP) increases pain thresholds. In the present study intracerebroventricular (ICV) administration of both arginine VP (AVP: 75-500 ng) and 1-deamino-8-D-arginine vasopressin (DDAVP: 150-500 ng) elevated tail flick latencies. Oxytocin (OXY, ICV), also elevated tail-flick latencies (150-1000 ng); however this increase was accompanied by "barrel-roll" seizure activity. VP analgesia was eliminated by pretreatment with 1-deamino-penicillamine-2(O-methyl)tyrosine-AVP (dPTyr(me)AVP: 500 ng, ICV), a VP antagonist, but not naloxone (1 or 10 micrograms, ICV), suggesting that VP modulates nonciceptive thresholds through its own binding sites. Conversely, pretreatment with naloxone (1 micrograms, ICV) but not dPTyr(me)AVP (1 microgram, ICV) attenuated the analgesic efficacy of systemic morphine (10 mg/kg), further dissociating VP and central opiate analgesic processes. Finally, systemic pretreatment with dexamethasone potentiated VP analgesia. These data support the notion that VP is a specific non-opioid pain inhibitor.     

Estradiol potentiates hypothalamic vasopressin and oxytocin neuron activation and hormonal secretion induced by hypovolemic shock

Estrogen receptors are located in important brain areas that integrate cardiovascular and hydroelectrolytic responses, including the subfornical organ (SFO) and supraoptic (SON) and paraventricular (PVN) nuclei. The aim of this study was to evaluate the influence of estradiol on cardiovascular and neuroendocrine changes induced by hemorrhagic shock in ovariectomized rats. Female Wistar rats (220-280 g) were ovariectomized and treated for 7 days with vehicle or estradiol cypionate (EC, 10 or 40 μg/kg, sc). On the 8th day, animals were subjected to hemorrhage (1.5 ml/100 g for 1 min). Hemorrhage induced acute hypotension and bradycardia in the ovariectomized-oil group, but EC treatment inhibited these responses. We observed increases in plasma angiotensin II concentrations and decreases in plasma atrial natriuretic peptide levels after hemorrhage; EC treatment produced no effects on these responses. There were also increases in plasma vasopressin (AVP), oxytocin (OT), and prolactin levels after the induction of hemorrhage in all groups, and these responses were potentiated by EC administration. SFO neurons and parvocellular and magnocellular AVP and OT neurons in the PVN and SON were activated by hemorrhagic shock. EC treatment enhanced the activation of SFO neurons and AVP and OT magnocellular neurons in the PVN and SON and AVP neurons in the medial parvocellular region of the PVN. These results suggest that estradiol modulates the cardiovascular responses induced by hemorrhage, and this effect is likely mediated by an enhancement of AVP and OT neuron activity in the SON and PVN.     

Upregulation of CRH gene expression and downregulation of arginine vasopressin gene expression in the hypothalamus of bilateral nephrectomized rats

The expression of the corticotropin-releasing hormone (CRH) gene and the arginine vasopressin (AVP) gene in the hypothalamus examined in bilateral nephrectomized rats by in situ hybridization histochemistry. The expression of the CRH gene was significantly increased in the parvocellular part of the paraventricular nucleus (PVN) 12 and 20 h after bilateral nephrectomy in comparison with that after sham operation. The plasma concentration of adrenocorticotropic hormone (ACTH) in nephrectomized rats was significantly higher than that in sham operated rats 20 h after surgery. In contrast, the expression of the AVP gene in both the parvocellular and magnocellular parts of the PVN and throughout the supraoptic nucleus (SON) was significantly decreased 20 h after bilateral nephrectomy in comparison with that after sham operation. These results suggest that nephrectomy-induced upregulation of the CRH gene with elevation of plasma ACTH may be due to the activation of the hypothalamo-pituitary adrenal (HPA) axis.     

Glucocorticoids regulate peptidyl-glycine alpha-amidating monooxygenase gene expression in the rat hypothalamic paraventricular nucleus

Peptidyl-glycine alpha-amidating monooxygenase (PAM) is a posttranslational processing enzyme which catalyzes the formation of biologically active alpha-amidated peptides. The two major neuropeptides involved in the regulation of ACTH secretion [CRF and arginine vasopressin (AVP)], synthesized in the parvocellular part of the hypothalamic paraventricular nucleus (PVN), are amidated, and their synthesis and/or release is negatively regulated by glucocorticoids. In this study, using in situ hybridization, we have shown that PAM mRNA is abundantly expressed in the hypothalamic paraventricular and supraoptic nucleus. Surgical adrenalectomy (ADX) induced increases in PAM, CRF, and AVP mRNA in the parvocellular part of the PVN, while corticosterone treatment normalized these values. PAM and AVP gene expression were not changed in the magnocellular part of the PVN or in the supraoptic nucleus. These observations suggest that in addition to stimulation of CRF and AVP synthesis, ADX induces an increase in PAM synthesis in the PVN and, thus, support the hypothesis of increased secretion of both CRF and AVP after ADX.     

Neuronal organization of the avian paraventricular nucleus: intrinsic, afferent, and efferent connections

The heterogeneous paraventricular nucleus (PVN) of birds offers favorable conditions for the analysis of intrinsic, afferent, and efferent connections of neuroendocrine systems. Paraventricular neurons are successfully impregnated with the Golgi-technique. The findings indicate a direct influence of the cerebrospinal fluid (CSF) on the magnocellular neurons that, via their axon terminals in the neural lobe of the pituitary, are also exposed to the hemal milieu. The magnocellular neurons are intermingled with parvocellular elements which may represent local interneurons. A group of parvocellular nerve cells is identified as CSF-contacting neurons. This type of cell forms a basic morphologic component of the avian neuroendocrine apparatus. Immunocytochemical and ultrastructural studies further support the concept of neuronal interactions between parvocellular and magnocellular elements. Moreover, these findings speak in favor of the existence of recurrent collaterals of the magnocellular neurons. Nerve cells giving rise to afferent connections to the PVN are located in the limbic system and autonomic areas of the upper and lower brainstem. Further afferents may originate from the subfornical organ, the organon vasculosum laminae terminalis, the ventral tegmentum, and the area postrema. Via efferent projections, the PVN is connected to the nucleus accumbens, lateral septum, several hypothalamic nuclei, the neural lobe of the pituitary, the organon vasculosum laminae terminalis, the subfornical organ, the pineal organ, the area postrema, the lateral habenular complex, and various autonomic areas of the reticular formation in the upper and lower brainstem and the spinal cord. In conclusion, the PVN may be regarded as an integral component of the neuroendocrine apparatus reciprocally coupled to the limbic system, several circumventricular organs, and various autonomic centers of the brain.     

Regulatory mechanism of the arginine vasopressin-enhanced green fluorescent protein fusion gene expression in acute and chronic stress

Various kinds of stress cause neuroendocrine responses such as corticotropin-releasing hormone (CRH) or arginine vasopressin (AVP) release from parvocellular division of the paraventricular nucleus (PVN) and activation of the hypothalamo-pituitary adrenal (HPA) axis. We examined the effects of acute and chronic stress on the expression of the AVP-enhanced green fluorescent protein (eGFP) fusion gene in the hypothalamus, using chronic salt loading as an osmotic stimulation, intraperitoneal administration of lipopolysaccharide (LPS) as acute inflammatory stress and adjuvant arthritis (AA) as chronic inflammatory/nociceptive stress. Salt loading caused a marked increase in the eGFP gene expression and eGFP fluorescence in the supraoptic nucleus, magnocellular division of the PVN and internal layer of the median eminence (ME). Administration of LPS caused increased fluorescence in parvocellular division of the PVN and external layer of the ME. AA rats revealed an increased expression of the eGFP gene and eGFP fluorescence in both magnocellular and parvocellular divisions of the PVN and both internal and external layers of the ME. On the other hand, the levels of the CRH gene expression in parvocellular division of the PVN were significantly decreased as AA developed, though plasma concentrations of corticosterone were significantly increased. These results indicate that AVP-eGFP transgenic rats enable the detection of changes in AVP expression more easily than by using procedures such as immunohistochemistry. We propose that AVP-eGFP transgenic rats represent a useful animal model for further understanding of the physiology of AVP expression in the hypothalamo-pituitary system under various physiological conditions, including various kinds of stress.     

Epithelial Na⁺ sodium channels in magnocellular cells of the rat supraoptic and paraventricular nuclei

The epithelial Na? channels (ENaCs) are present in kidney and contribute to Na? and water homeostasis. All three ENaC subunits (α, β, and γ) were demonstrated in the cardiovascular regulatory centers of the rat brain, including the magnocellular neurons (MNCs) in the supraoptic nucleus (SON) and the paraventricular nucleus (PVN). However, the functional significance of ENaCs in vasopressin (VP) and oxytocin (OT) synthesizing MNCs is completely unknown. In this study, we show with immunocytochemical double-labeling that the α-ENaC is colocalized with either VP or OT in MNCs in the SON and PVN. In addition, parvocellular neurons in the dorsal, ventrolateral, and posterior subregions of the PVN (not immunoreactive to VP or OT) are also immunoreactive for α-ENaC. In contrast, immunoreactivity to β- and γ-ENaC is colocalized with VP alone within the MNCs. Furthermore, immunoreactivity for a known target for ENaC expression, the mineralcorticoid receptor (MR), is colocalized with both VP and OT in MNCs. Using single-cell RT-PCR, we detected mRNA for all three ENaC subunits and MR in cDNA libraries derived from single MNCs. In whole cell voltage clamp recordings, application of the ENaC blocker benzamil reversibly reduced a steady-state inward current and decreased cell membrane conductance approximately twofold. Finally, benzamil caused membrane hyperpolarization in a majority of VP and about one-half of OT neurons in both spontaneously firing and quiet cells. These results strongly suggest the presence of functional ENaCs that may affect the firing patterns of MNCs, which ultimately control the secretion of VP and OT.     

Evidence for antipyretic vasopressinergic pathways and their modulation by noradrenergic afferents.

The experimental evidence for the antipyretic action of arginine vasopressin (AVP) in guinea-pigs can be summarized as follows: The febrile response to a bacterial pyrogen can be reduced by a microinfusions of exogenous AVP into the ventral septal area of the limbic system. Immunohistochemical studies indicate increased activity of AVP terminals in the ventral septal area (VSA) and in parvocellular AVP neurones of the hypothalamic paraventricular nucleus (PVN) in several stressful situations accompanied by reduced febrile responses (late stage of pregnancy, immobilization, cold adaptation, osmotic stimulation). Also the peripheral and/or central release of AVP measured in some of these situations is increased. Electrical stimulation of the PVN suppresses fever, this suppression can, at least partly, be cancelled by simultaneous intraseptal application of the vasopressinergic V1 receptor antagonist. The documented AVP pathways from the PVN to the septum receive noradrenergic afferents from the lower brainstem. Chronic destruction of these afferents by microinjections of 6-hydroxydopamine (6-OHDA) significantly reduced the fever responses to pyrogen application, while microinfusion of noradrenaline (NA) enhances the fever reaction.     

Water deprivation increases Fos immunoreactivity in PVN autonomic neurons with projections to the spinal cord and rostral ventrolateral medulla

The present study sought to determine whether water deprivation increases Fos immunoreactivity, a neuronal marker related to synaptic activation, in sympathetic-regulatory neurons of the hypothalamic paraventricular nucleus (PVN). Fluorogold (4%, 50 nl) and cholera toxin subunit B (0.25%, 20-30 nl) were microinjected into the spinal cord (T1-T3) and rostral ventrolateral medulla (RVLM), respectively. Rats were then deprived of water but not food for 48 h. Water deprivation significantly increased the number of Fos-positive nuclei throughout the dorsal, ventrolateral, and lateral parvocellular divisions of the PVN (water deprived, 215 +/- 23 cells; control, 45 +/- 7 cells, P < 0.01). Moreover, a significantly greater number of Fos-positive nuclei were localized in spinally projecting (11 +/- 3 vs. 2 +/- 1 cells, P < 0.025) and RVLM-projecting (45 +/- 7 vs. 7 +/- 1 cells, P < 0.025) neurons of the PVN in water-deprived vs. control rats, respectively. The majority of these double-labeled neurons was found in the ventrolateral and lateral parvocellular divisions of the ipsilateral PVN. Interestingly, a significantly greater percentage of RVLM-projecting PVN neurons were Fos positive compared with spinally projecting PVN neurons in the ventrolateral (25.8 +/- 0.7 vs. 8.0 +/- 1.5%, respectively, P < 0.01) and lateral (23.4 +/- 2.1 vs. 5.0 +/- 0.9%, respectively, P > 0.01) parvocellular divisions. In addition, we analyzed spinally projecting neurons of the RVLM and found a significantly greater percentage were Fos positive in water-deprived rats than in control rats (26 +/- 3 vs. 3 +/- 1%, respectively; P < 0.001). Collectively, the present findings indicate that water deprivation evokes a distinct cellular response in sympathetic-regulatory neurons of the PVN and RVLM.