Water deprivation induces regional expression of c-fos protein in the brain of inbred polydipsic mice

We studied the effects of water deprivation on the expression of c-fos protein (Fos) in the brain of inbred polydipsic mice, STR/N strain, that show extreme polydipsia without a lack of vasopressin in the body. Non-polydipsic mice, ICR strain, were used as controls. All male animals were deprived of water for 24 and 48 h. Fos-like immunoreactivity (Fos-LI) in the brain was studied by immunohistochemical techniques. In both groups of mice water deprivation induced a remarkable increase in Fos-LI in the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei, the median preoptic nucleus (MnPO), the organum vasculosum laminae terminalis (OVLT) and the subfornical organ (SFO). A far more increase, however, was seen in the MnPO, the SFO and the area postrema (AP) of the polydipsic mice compared to those of the non-polydipsic control mice. In the nucleus of the tractus solitarius (NTS) and in the anteroventral part of the PVN (avPVN), water deprivation caused a clear increase in Fos-LI in the polydipsic mice, while in the non-polydipsic mice the same treatment induced no Fos-LI in the NTS and no change in the avPVN. These results indicate that neurons in the circumventricular organs and the NTS are strongly activated by water deprivation in the polydipsic mice, suggesting that these brain structures play an important role in the polydipsia.     

Neuronal expression of Fos protein in the hypothalamus of rats with heart failure

We sought to identify the areas that have altered neuronal activity within the hypothalamus of rats with heart failure (HF) by mapping neuronal staining of c-Fos protein (Fos) 6-8 weeks following coronary artery ligation (HF group; n=17) or sham surgery (sham-operated control group, n=15). Fos-like immunoreactivity was observed in the paraventricular nucleus (PVN), supraoptic nucleus (SON), median preoptic nucleus (MnPO), anterior hypothalamus (AH) and posterior hypothalamus (PH) using a standard ABC immunocytochemical protocol. The rats in the HF group displayed infarcts averaging 34+/-2% of the outer circumference and 41+/-1% of the inner circumference of the left ventricular wall. Sham-operated control rats had no observable damage to the myocardium. Rats with chronic heart failure (n=5) but no manipulation (no surgery) had a similar number of Fos-staining cells in PVN SON, MnPO, AH and PH compared to sham-operated rats. Acute surgery for isolation of vagus nerves and anesthesia for 90 min increased the number of Fos positive cells in PVN, SON and MnPO of both sham-operated rats and rats with HF. Furthermore, rats with heart failure (n=5) had significantly higher number of Fos-staining cells in PVN (four times), SON (4.5 times) and MnPO (1.5 times) compared to sham-operated rats after acute surgery for isolation of the vagus. The number of Fos-staining cells remained unaltered in AH and PH in both groups of rats. However, in a third series of experiments vagotomy reduced the number of Fos-staining cells in the PVN, SON or MnPO of rats with HF (n=5) to those observed in sham-operated vagotomized rats. This study shows that: (1) there is augmented neuronal activity as indicated by increased number of Fos staining neurons in the PVN, SON and MnPO due to acute surgical stress in rats with HF, and (2) vagal afferents are responsible for the increased neuronal activity in PVN, SON and MnPO of rats with HF during acute surgical stress. These data support the conclusion that vasopressin producing neurons and autonomic areas within the hypothalamus influenced by vagal afferents are activated during HF and are sensitive to 'acute surgical stress' and may contribute to the elevated levels of vasopressin and sympatho-excitation commonly observed in heart failure.     

Electrophysiology of limbic forebrain and paraventricular nucleus connections

The connections of forebrain structures with the hypothalamic paraventricular nucleus (PVN) were examined electrophysiologically in anaesthetized male rats. Single unit recordings from 336 neurons were made within the cingulate cortex (CC, n = 78), lateral septum (LS, 114), bed nucleus of the stria terminalis (BST, 27). bed nucleus proprioris commissurae (BCA, 27) and preoptic area (POA, 90). Following PVN stimulation, some cells in all regions were identified as projecting to the PVN. Antidromic (24%), orthodromic excitatory (44%) and inhibitory (22%) responses recorded from CC units demonstrated the presence of reciprocal pathways between PVN and CC. Thirty-eight percent of LS units were antidromically identified as projecting to the PVN and these appeared to show some discrete topographic organization. Fifty-seven percent of LS units responded orthodromically to stimulation of the PVN, the majority of such responses being excitatory. Within the bed nuclei, 24% of units were antidromically identified as projecting to PVN and 70% of cells in these regions responded orthodromically following PVN stimulation, excitatory responses predominating within the BST and inhibitory responses within the BCA. Within the POA, 38% of units were identified as projecting to the PVN and the remaining units were approximately divided between orthodromic excitatory and inhibitory responses. A small proportion of antidromically identified units (less than 20%) in all regions also exhibited responses suggesting the presence of reciprocal connections with the PVN.     

Central renin injections: effects on drinking and expression of immediate early genes

This study investigated the drinking response and the expression of Fos- and Egr-1-immunoreactivity (Fos-ir; Egr-1-ir) in the brain induced by endogenous angiotensin generated by intracerebroventricular (i.c.v.) injection of renin. Renin induced Fos-ir in the subfornical organ (SFO), median preoptic (MnPO), supraoptic and paraventricular nuclei (SON and PVN), area postrema (AP), nuclei of the solitary tract (NTS) and lateral parabrachial nuclei (LPBN). Renin-induced Egr-1-ir exhibited a similar pattern of distribution as that observed for Fos-ir. The dose of i.c.v. renin that induced expression of immediate early gene (IEG) product immunoreactivity also produced vigorous drinking. When renin-injected rats were pretreated with captopril, an angiotensin converting enzyme inhibitor, drinking was blocked. With the same captopril pretreatment, both Fos- and Egr-1-ir in the SFO, MnPO, SON, PVN, AP and LPBN were also significantly reduced.     

Involvement of hypothalamic paraventricular nucleus non-N-methyl-d-aspartate receptors in the pressor response to noradrenaline microinjected into the bed nucleus of the stria terminalis of unanesthetized rats

Microinjection of noradrenaline into the bed nucleus of the stria terminalis (BST) has been reported to cause a pressor response in unanesthetized rats, which was shown to be mediated by acute vasopressin release into the systemic circulation. In the present study we verified the involvement of magnocellular neurons of the hypothalamic paraventricular (PVN) or supraoptic (SON) nuclei and the local neurotransmitter involved in the pressor response to noradrenaline microinjection into the BST. The PVN pretreatment with the non-selective neurotransmission blocker CoCl₂ (1 nmol/100 nL) inhibited the noradrenaline-evoked pressor response. However, responses were not affected by SON treatment with CoCl₂. Further experiments were carried out to test if glutamatergic neurotransmission in the PVN mediates the pressor response evoked by noradrenaline microinjection into the BST. Pretreatment of the PVN with the selective N -methyl- d -aspartate (NMDA) receptor antagonist LY235959 (2 nmol/100 nL) did not affect the noradrenaline-evoked pressor response. However, PVN pretreatment with the selective non-NMDA receptor antagonist NBQX (2 nmol/100 nL) significantly reduced the pressor response to noradrenaline microinjection into the BST. In conclusion, our results suggest that pressor responses to noradrenaline microinjection into the BST are mediated by PVN magnocellular neurons without involvement of SON neurons. They also suggest that a glutamatergic neurotransmission through non-NMDA glutamate receptors in the PVN mediates the response.     

The median preoptic nucleus participates in the control of paraventricular vasopressin neurons by the subfornical organ in the rat

Extracellular single-unit activity was recorded from phasically firing neurohypophyseal neurons (n = 41) in the hypothalamic paraventricular nucleus (PVN) of urethane-anesthetized male rats. Electrical stimulation of the subfornical organ (SFO) produced orthodromic long-duration (n = 18) or short-duration (n = 10) excitation or inhibition (n = 8) of the activity of PVN neurons. The long-duration excitatory response of about half (n = 7) the neurons (n = 15) tested was reversibly abolished by microinjection of the local anesthetic lidocaine into the median preoptic nucleus (MnPO), whereas neither the short-duration excitatory (n = 7) nor inhibitory (n = 6) responses were affected. These results suggest that the SFO efferents through the MnPO to the PVN may transmit the neuromodulatory signals which evoke long-duration increases in the excitability of putative vasopressin (VP)-secreting neurons in the PVN.     

Noradrenergic excitatory inputs to median preoptic neurones in rats.

Extracellular single-unit activity was recorded from 21 median preoptic nucleus (MnPO) neurones, antidromically identified as projecting to the hypothalamic paraventricular nucleus (PVN), in urethane-anaesthetized male rats. Of these identified MnPO neurones, 14 displayed an excitatory response in neuronal excitability following electrical stimulation (5 Hz, 600 microA) of the A1 noradrenergic region of the ventrolateral medulla, while the remaining neurones were unresponsive. The excitatory response of MnPO neurones was blocked by microiontophoretically applied phentolamine, an alpha-adrenoceptor antagonist, but not by timolol, a beta-adrenoceptor antagonist. These results suggest that the A1 region acts to enhance the activity of MnPO neurones projecting to the PVN via an alpha-adrenoceptor mechanism.     

Sleep-active neurons in the preoptic area project to the hypothalamic paraventricular nucleus and perifornical lateral hypothalamus

The lamina terminalis consists of the organum vasculosum of the lamina terminalis (OVLT), median preoptic nucleus (MnPO) and subfornical organ. The MnPO and ventrolateral preoptic area (vlPOA) are known to contain high densities of neurons that are sleep active. The prevalence of sleep-active neurons in the OVLT and subfornical organ is unknown. The vlPOA and subdivisions of the lamina terminalis project to hypothalamic regions involved in the control of behavioral, electrographic or autonomic arousal, including the lateral hypothalamic area (LHA) and paraventricular nucleus (PVN). The extent to which projection neurons are active during sleep is unknown. We quantified c-Fos protein immunoreactivity (IR) in the lamina terminalis and vlPOA in sleeping and awake rats that received injections of retrograde tracer into either the LHA or PVN. Fos IR was also examined in lamina terminalis neurons following tracer injections into the vlPOA. Significantly more projection neurons from the MnPO, OVLT and vlPOA to the LHA were Fos-immunoreactive in sleeping vs. awake animals. Waking Fos IR was more prevalent in lamina terminalis neurons projecting to the PVN although a subset of MnPO projection neurons in sleeping rats was Fos-immunoreactive. Almost 50% of vlPOA-PVN projection neurons expressed Fos IR during sleep, compared with 3% during waking. Significantly more neurons in the OVLT and MnPO projecting to the vlPOA were Fos-immunoreactive in sleeping vs. awake rats. Inhibition of LHA and PVN neurons arising from OVLT, MnPO and vlPOA neurons may contribute to suppression of behavioral, electroencephalographic and sympathetic nervous system activation during sleep.     

Neuronal expression of fos protein in the forebrain of diabetic rats

We sought to identify the areas that have altered neuronal activity within the hypothalamus of diabetic rats by mapping neuronal expression of c-fos protein (Fos) and Fos-related antigens. After a standard PAP immunocytochemical protocol, Fos-like immunoreactivity was observed in the paraventricular nucleus (PVN), supraoptic nucleus (SON), median preoptic area (MnPO), anterior hypothalamus (AH) and posterior hypothalamus (PH) of control (vehicle; n=6) and diabetic rats (Sprague-Dawley rats injected with STZ 65 mg/kg/ip 4 weeks prior to the experiment; n=6). Blood glucose levels were significantly elevated in the diabetic group (370+/-8 mg/dl) compared to control group (104+/-3 mg/dl). Diabetic rats had a significantly higher number of Fos-positive cells in PVN (2.5x), SON (7x) and MnPO (2x) compared to the control rats. However, diabetic rats had significantly fewer Fos-positive cells in the AH (0.3x) and no difference was observed in the PH between the diabetic and control rats. Despite the elevated number of Fos-positive cells in the diabetic rats, dehydration (water withdrawal for 24 h) or hypertonic challenge (1.5 ml of 0.1 M NaCl i.p. injection) produced a further increase in the number of Fos-positive cells in the PVN, SON and MnPO. Dehydration did not alter the number of Fos-positive cells in the AH or PH, but hypertonic challenge produced a significant increase in the Fos-positive cells in both the AH and PH of diabetic rats. This study demonstrates that: (1) there is increased basal neuronal activity in the PVN, SON and MnPO, a decrease in neuronal activity in the AH and no change in neuronal activity in the PH as indicated by Fos staining in diabetic rats; and (2) dehydration or hypertonic challenge produces a further increase in the number of Fos-positive cells in the PVN, SON, and MnPO which is comparable to control rats. These data support the conclusion that vasopressin producing neurons in the PVN and SON and autonomic areas within the lamina terminalis and hypothalamus are activated during diabetes and may contribute to the elevated levels of vasopressin and autonomic dysfunction during diabetes.     

Increased norepinephrine turnover in the median preoptic nucleus following reduced extracellular fluid volume

The role of noradrenergic input to fluid balance regulatory systems in the anterior hypothalamus was studied by examination of norepinephrine (NE) turnover during reduction of systemic extracellular fluid volume. Extracellular fluid volume was decreased iso-osmotically by subcutaneous polyethylene glycol (PEG), known to increase thirst and vasopressin secretion. NE turnover was assessed by measuring the decline of NE concentration in brain micropunches after administration of alpha-methyl tyrosine in PEG- or sham-treated groups. Several hypothalamic areas were investigated, including the median preoptic area (MnPO), preoptic area (POA), paraventricular nucleus, supraoptic nucleus (SON), subfornical organ, ventromedial hypothalamus, and posterior hypothalamus. Volume-depleted animals showed significantly increased NE turnover in the MnPO, an important area for integration of fluid balance information. The POA and the SON also showed trends toward increased NE turnover. All other areas showed no difference in NE turnover between volume-depleted and normal animals. These results are consistent with previous findings that NE innervation to the MnPO is important in the control of fluid balance and also support the hypothesis that basal forebrain NE projections facilitate thirst and vasopressin secretion.     

Effect of lesions of forebrain circumventricular organs on c-fos expression in the central nervous system to plasma hypernatremia

Experiments were carried out on conscious adult male Wistar rats to investigate the effect of selective ablation of the subfornical organ (SFO), and/or the anteroventral third ventricular (AV3V) region on the induction of Fos in central structures in response to plasma hypernatremia. Fos induction, detected immunohistochemically, was used as a marker for neuronal activation. Intravenous infusions of hypertonic saline resulted in dense Fos-like immunoreactivity in several forebrain (paraventricular nucleus of the hypothalamus (PVH), supraoptic nucleus (SON), median preoptic nucleus (MnPO), medial preoptic nucleus, organum vasculosum of the laminae terminalis and SFO) and brainstem (nucleus of the solitary tract, ventrolateral medulla, and parabrachial nucleus) structures. Intravenous infusions of the hypertonic saline solution into animals with lesions of either the SFO, the AV3V or both resulted in a decreased number of Fos-like immunoreactive neurons in the MnPO, PVH and SON. In addition, the number of Fos-labeled neurons in the SON after lesions of both the SFO and the AV3V was significantly greater than that observed in isotonic saline infused controls. Finally, lesions of the forebrain circumventricular structures did not alter the Fos labeling in brainstem structures as a result of the infusion of the hypertonic solution. These data suggest that changes in plasma osmolality and/or concentration of sodium alter the activity of SON and brainstem neurons in the absence of afferent inputs from the SFO and AV3V.     

Fos expression variances in mouse hypothalamus upon physical and osmotic stimuli: co-staining with vasopressin, oxytocin, and tyrosine hydroxylase

Fos expression in the hypothalamus and its quantification in vasopressinergic (AVP), oxytocinergic (OXY) and tyrosine hydroxylase (TH) immunoreactive cells in the hypothalamic paraventricular (PVN), supraoptic (SON), suprachiasmatic (SCh), and arcuate (Arc) nuclei was performed in response to physiologically two different, i.e. osmotic (i.p. hypertonic saline, HS) and immobilization (IMO), stimuli in mouse using a dual Fos-neuropeptide immunohistochemistry. Both 60 min of HS and 120 min of IMO evoked Fos induction in many hypothalamic structures, whereas, HS evoked more extensive Fos labeling than IMO in the SON, ventromedial (VMN) and dorsomedial (NDM) hypothalamic nuclei and the retrochiasmatic area (RCh). Other hypothalamic structures including the anterior hypothalamic area (AHA), the latero-anterior hypothalamic nucleus (LA), the Arc, the perifornical nucleus (PeF), and the lateral hypothalamic area (LH) showed similar Fos incidence after both HS and IMO. However, after both stimuli explicitly most extensive Fos expression was observed in the PVN. In addition, in the PVN substantially more Fos-AVP (62-67% versus 10-15%) and Fos-OXY (38-45% versus 4-8%) perikarya were observed after HS than IMO, respectively. Incidence of TH-immunoreactive Fos labeled cells in the PVN was also more frequent after HS. In the SON, HS activated more than 50% of AVP and OXY neurons while IMO less than 4%. The number of TH activated neurons in Arc was also higher after HS (11%) than IMO (4%). Lowest number of colocalizations was revealed in the SCh where both HS and IMO activated around 2% of AVP neurons. The present data demonstrate that both HS and IMO are powerful stimuli for the majority of hypothalamic structures displaying considerable topographic similarity in Fos expression suggesting their multifunctional involvement. The quantity and phenotypic differences of activated hypothalamic neurons may speak out for functional dissimilarities in response to HS and IMO.     

Progesterone receptor expression in the pregnant and parturient rat hypothalamus and brainstem

Oxytocin is synthesized by magnocellular neurons in the supraoptic and paraventricular nuclei (SON and PVN) and during pregnancy progesterone prevents premature activation of oxytocin neurons. Progesterone receptors (PR) are not detectable in SON oxytocin neurons of non-pregnant rats, so we sought to determine whether they are expressed during pregnancy and parturition. In addition, we examined PR expression in brainstem and hypothalamic regions that have known direct projections to the SON. Neuronal immunoreactive PR (irPR)-labeled nuclei were counted in sections from proestrous virgin, late pregnant (day 21) and parturient rats (90 min from birth onset). IrPR nuclei were not evident in the SON at any stage but irPR expression in the medial preoptic nucleus (MPA) significantly increased in pregnancy and parturition (159% and 189% of proestrous controls, respectively). Other hypothalamic areas did not exhibit a significant change in irPR expression. In the nucleus tractus solitarius (NTS) in the brainstem, there was no significant change in irPR in late pregnancy, but there was a significant reduction in irPR expression at parturition (22% of proestrous controls). Very few NTS neurons immunoreactive for tyrosine hydroxylase (irTH), and thus putatively noradrenergic, contained irPR. These findings taken with evidence that brainstem irTH neurons projecting to the SON are stimulated at parturition, whereas MPA cells projecting to the SON are not, suggest that any direct actions of progesterone or progesterone withdrawal on NTS or SON neurons are not mediated through the classical PR. Upregulation of PR expression in the MPA during pregnancy and parturition may relate to the onset of maternal behavior and/or regulation of GnRH neuronal activity.     

Hypothalamic cholinergic regulation of body temperature and water intake in rats.

Without disturbing the behavior of unanesthetized rats, the perfusion of neostigmine through microdialysis probe into the anterior hypothalamus (AH), paraventricular nucleus (PVN) and lateral ventricle (LV) decreased body temperature and increased water intake. On the other hand, the perfusion into the supraoptic nucleus (SON) increased the body temperature. The perfusion of neostigmine increased the extracellular concentration of acetylcholine in the perfusion sites except LV. Changes, both decrease and increase, in body temperature and increase in water intake were correlated with increases in c-fos-like immunoreactivity (Fos-IR) in the hypothalamus, pons and medulla. Distinct Fos-IR was found in the PVN, SON, median preoptic nucleus (MnPO), locus coeruleus (LC), area postrema and nucleus of the solitary tract (NTS). Co-administration of atropine with neostigmine completely suppressed the changes in the body temperature, water intake and Fos-IR, all of which were induced by the neostigmine perfusion into AH, PVN and SON. In the LV-perfused rats, on the other hand, co-administration of atropine and neostigmine only partially prevented body temperature reduction and still induced significant hypothermia. These results suggest that muscarinic receptor activation in specific regions of the hypothalamus and the activation of LC and NTS are implicated in the regulation of body temperature and water intake. Other receptor processes are involved in the LV-induced changes.     

A combined electron microscopic HRP and immunocytochemical study of the limbic projections to rat hypothalamic nuclei containing vasopressin and oxytocin neurons

Light microscopic studies in our laboratory have indicated that the lateral septum, amygdala, and ventral subiculum project in a perinuclear fashion to the paraventricular (PVN), supraoptic (SON), and suprachiasmatic (SCN) nuclei (Oldfield et al., '82; Silverman and Oldfield, '84). In the present paper a combined anterograde HRP and immunocytochemical procedure has been used to determine the connectivity between these limbic efferents and peptide-containing processes emanating from the above mentioned hypothalamic nuclei. Synaptic associations were found to exist between efferents from (1) the septum and both vasopressin (VP)- and oxytocin (OX)-positive dendrites derived from cells in the PVN and SON, (2) the septum and VP dendrites dorsal to the SCN, (3) the ventral subiculum and both VP and OX dendrites arising from the PVN and SON, and (iv) the amygdala and VP dendrites from the PVN. These observations help clarify an apparent discrepancy between electrophysiological data, in which limbic efferents have been shown to influence the activity of VP and OX neurons in the PVN and SON, and anatomical evidence which indicates only a perinuclear innervation from these sites not encroaching on the hypothalamic nuclei themselves. In each case the synaptic connections are made on dendrites external to the nucleus: those lateral and ventrolateral to the PVN, dorsal to the SON, and dorsal or dorsolateral to the SCN.     

大鼠延髓内脏带与下丘脑室旁核、视上核往返投射通路参与高渗调节反应的研究

目的探讨延髓内脏带(MVZ)与下丘脑室旁核(PVN)和视上核(SON)之间是否存在往返渗透压投射通路。方法通过给予大鼠饮用3%氯化钠的方法制作高渗刺激模型,并用WGA-HRP逆行追踪、抗Fos、抗酪氨酸羟化酶(TH)或加压素(VP)及胶质纤维酸性蛋白(GFAP)免疫组织化学相结合的四重标记方法,观察MVZ、PVN和SON中WGA-HRP、Fos、TH、VP和GFAP阳性分布及表达状况。结果高渗刺激后MVZ、PVN和SON内Fos阳性细胞明显增多;GFAP阳性结构也明显增多,其分布与Fos阳性细胞分布基本一致,表现为胞体肥大、突起粗长。星形胶质细胞(AST)紧密包绕在神经元周围形成神经元-AST复合体(N-ASC)。结论神经元和AST以N-ASC的形式共同参与渗透压调节反应,体内存在MVZ和SON或PVN之间往返的渗透压调节通路。     

Median preoptic neurons projecting to the hypothalamic paraventricular nucleus are sensitive to blood pressure changes

Twenty-one neurons in the median preoptic nucleus (MnPO) were antidromically activated by electrical stimulation of the hypothalamic paraventricular nucleus (PVN) in male rats under urethane anesthesia. The activity of these identified neurons was tested for a response to activation of peripheral baroreceptors, achieved by rising arterial blood pressure with an intraveous administration of the α-agonist metaraminol. Of the neurons tested, 14 displayed a reduction and 2 exhibited an increase in neuronal excitability that accompanied a 30- to 50-mmHg elevation in mean arterial pressure, while 5 were unresponsive. The results show that efferent pathways from the MnPO to the PVN may receive neural inputs from the peripheral baroreceptors, suggesting the involvement of the pathways in the control of cardiovascular function.     

Cyclic nucleotide dynamics in the rat hypothalamus during osmotic stimulation: in vivo and in vitro studies

The molecular mechanisms which regulate expression of vasopressin (AVP)- and oxytocin (OT)-encoding genes are unknown. We have investigated the regulatory role of one class of second messenger, the cyclic nucleotides, by examining levels of both adenosine 3',5'-monophosphate (cAMP) and guanosine 3',5'-monophosphate (cGMP) in hypothalamic nuclei of rats during osmotic stimulation. In vivo studies, in which rats were given 2% saline to drink for different periods (salt loading), demonstrated elevated levels of cAMP in the supraoptic nucleus (SON) after 2 days. Raised levels were also evident at 3 and 7 days. A similar (less marked) pattern was observed in the paraventricular nucleus (PVN) but not in the suprachiasmatic nucleus (SCN). cGMP was present at much lower levels than cAMP and did not exhibit parallel dynamics during salt loading; however, significant changes in cGMP levels were found in the SON and PVN. In vitro studies, in which explant cultures of punched hypothalamic nuclei were challenged with hypertonic media, demonstrated that increasing medium osmolality from 290 to 310 mOsm/kg doubled the level of cAMP in the SON but did not change levels in the PVN or SCN. A greater stimulus, 325 mOsm/kg, caused a 4-fold increase in SON cAMP, and small cAMP responses in the PVN and SCN. Marked cGMP responses were also observed in the SON following stimulation at 310 and 325 mOsm/kg, smaller responses being found in the PVN and SCN. These results are consistent with previous demonstrations of SON neuron osmosensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Intracerebroventricular injection of a nitric oxide donor attenuates Fos expression in the paraventricular and supraoptic nuclei of lactating rats

The exact nature of how nitric oxide (NO) acts in the regulation of milk ejection during lactation is not clearly understood at the moment. In this study, we have examined the effect of drugs which spontaneously release NO (sodium nitroprusside, SNP) or inhibit the NO synthase (NOS) enzyme (N^ω-nitro- -arginine, -NA) on the activity of some hypothalamic and functionally associated nuclei using Fos expression as an index of neuronal activation. Lactating rats received intracerebroventricular injection of SNP, -NA or vehicle (saline) just before they were reunited with their pups after a 12-h period of separation and allowed to suckle for 2 h. The difference in the total pup body weight before and after the period of suckling was used as a functional end-point of milk transfer. Central injection of SNP in conscious rats significantly inhibited Fos expression in the paraventricular nucleus (PVN), supraoptic nucleus (SON), periventricular and preoptic nuclei and also decreased pup body weight compared with saline- or -NA-injected rats. Urethane-anesthetized animals, compared with their conscious counterparts, showed increased Fos expression in the PVN and SON. However, Fos expression in the PVN of the anesthetized animals was attenuated by -NA injection compared with SNP and saline injection. Taken together with an earlier finding that SNP disrupts the milk ejection burst of oxytocinergic neurons, these observations suggest that NO may act within the neuron(s) possibly to alter the mechanism(s) regulating the periodic neuronal burst activity during lactation.