Medial Preoptic Neurons Research Articles

Neurons of the median preoptic nucleus contribute to chronic angiotensin II-salt induced hypertension in the rat.

Experiments were designed to test the hypothesis that median preoptic (MnPO) neurons are necessary for the full hypertensive response to chronic angiotensin II (AngII) in rats consuming a high salt diet. The MnPO is implicated in many of the physiologic actions of AngII, primarily acting as a downstream nucleus to AngII binding at circumventricular organs such as the organum vasculosum of the lamina terminalis (OVLT). We have previously shown a prominent effect of lesion of the OVLT on the chronic hypertensive effects of AngII in rats consuming high salt. Additionally, we have shown that lesion of the MnPO attenuated the hypertensive response to chronic intravenous infusion of AngII in rats. However, whether MnPO neurons or fibers of passage contribute to this response is not clear. Male Sprague Dawley rats were randomly assigned to either sham (SHAM; n=8) or ibotenic acid lesion of the MnPO (MnPOx; n=6). In the MnPOx group, 200 nl of ibotenic acid in phosphate buffer saline (5 μg/μl) was injected into each of 3 predetermined coordinates targeted at the entire MnPO. After a week of recovery, rats were instrumented with radiotelemetric pressure transducers, provided 2.0% NaCl diet and distilled water ad libitum and given another week to recover. After 3 days of baseline measurements, osmotic minipumps were implanted subcutaneously in all rats for administration of AngII at a rate of 150 ng/kg/min. Blood pressure measurements were made for 14 days after minipump implantation. By day 7 of AngII treatment, blood pressure responses appeared to plateau in both groups while the hypertensive response was markedly attenuated in MnPOx rats (MnPOx, 122 ± 6 mmHg; SHAM, 143 ± 8 mmHg). These results support the hypothesis that neurons of the MnPO are involved in the central pathway mediating the chronic hypertensive effects of AngII in rats consuming a high salt diet.

Electrophysiological characterization of medial preoptic neurons in lactating rats and its modulation by hypocretin-1

The medial preoptic area (mPOA) undergoes through neuroanatomical changes across the postpartum period, during which its neurons play a critical role in the regulation of maternal behavior. In addition, this area is also crucial for sleep-wake regulation. We have previously shown that hypocretins (HCRT) within the mPOA facilitate active maternal behaviors in postpartum rats, while the blockade of endogenous HCRT in this area promotes nursing and sleep. To explore the mechanisms behind these HCRT actions, we aimed to evaluate the effects of juxta-cellular HCRT-1 administration on mPOA neurons in urethane-anesthetized postpartum and virgin female rats. We recorded mPOA single units and the electroencephalogram (EEG) and applied HCRT-1 juxta-cellular by pressure pulses. Our main results show that the electrophysiological characteristics of the mPOA neurons and their relationship with the EEG of postpartum rats did not differ from virgin rats. Additionally, neurons that respond to HCRT-1 had a slower firing rate than those that did not. In addition, administration of HCRT increased the activity in one group of neurons while decreasing it in another, both in postpartum and virgin rats. This study suggests that the mechanisms by which HCRT modulate functions controlled by the mPOA involve different cell populations.

The search for thermoregulatory neurons is heating up

Recent studies have shown that the median preoptic area contains a population of neurons expressing an array of fast neurotransmitters and receptors that collectively cause a fall in body temperature in response to environmental warming or depleted energy stores. In this issue of Cell Metabolism, Piñol etal. (2021) identify a separate population of median preoptic neurons that are responsible for cold defense and cause stress-related hyperthermia.

Caspase lesions of PVN-projecting MnPO neurons block the sustained component of CIH-induced hypertension in adult male rats.

Obstructive sleep apnea is characterized by interrupted breathing that leads to cardiovascular sequelae including chronic hypertension that can persist into the waking hours. Chronic intermittent hypoxia (CIH), which models the hypoxemia associated with sleep apnea, is sufficient to cause a sustained increase in blood pressure that involves the central nervous system. The median preoptic nucleus (MnPO) is an integrative forebrain region that contributes to blood pressure regulation and neurogenic hypertension. The MnPO projects to the paraventricular nucleus (PVN), a preautonomic region. We hypothesized that pathway-specific lesions of the projection from the MnPO to the PVN would attenuate the sustained component of chronic intermittent hypoxia-induced hypertension. Adult male Sprague-Dawley rats (250-300 g) were anesthetized with isoflurane and stereotaxically injected bilaterally in the PVN with a retrograde Cre-containing adeno-associated virus (AAV; AAV9.CMV.HI.eGFP-Cre.WPRE.SV40) and injected in the MnPO with caspase-3 (AAV5-flex-taCasp3-TEVp) or control virus (AAV5-hSyn-DIO-mCherry). Three weeks after the injections the rats were exposed to a 7-day intermittent hypoxia protocol. During chronic intermittent hypoxia, controls developed a diurnal hypertension that was blunted in rats with caspase lesions. Brain tissue processed for FosB immunohistochemistry showed decreased staining with caspase-induced lesions of MnPO and downstream autonomic-regulating nuclei. Chronic intermittent hypoxia significantly increased plasma levels of advanced oxidative protein products in controls, but this increase was blocked in caspase-lesioned rats. The results indicate that PVN-projecting MnPO neurons play a significant role in blood pressure regulation in the development of persistent chronic intermittent hypoxia hypertension.NEW & NOTEWORTHY Chronic intermittent hypoxia associated with obstructive sleep apnea increases oxidative stress and leads to chronic hypertension. Sustained hypertension may be mediated by angiotensin II-induced neural plasticity of excitatory median preoptic neurons in the forebrain that project to the paraventricular nucleus of the hypothalamus. Selective caspase lesions of these neurons interrupt the drive for sustained hypertension and cause a reduction in circulating oxidative protein products. This indicates that a functional connection between the forebrain and hypothalamus is necessary to drive diurnal hypertension associated with intermittent hypoxia. These results provide new information about central mechanisms that may contribute to neurogenic hypertension.

An innate circuit for object craving

Using a series of functional manipulation and in vivo recording tools, Park et al. identify a pathway from medial preoptic CaMKIIα-expressing neurons to the ventral periaqueductal gray that mediates object craving and prey hunting.

Neurons that drive and quench thirst.

Median preoptic neurons modulate water intake and the urge to drink

Contribution of Resting Conductance, GABAA-Receptor Mediated Miniature Synaptic Currents and Neurosteroid to Chloride Homeostasis in Central Neurons.

Maintenance of a low intraneuronal Cl– concentration, [Cl–]i, is critical for inhibition in the CNS. Here, the contribution of passive, conductive Cl– flux to recovery of [Cl–]i after a high load was analyzed in mature central neurons from rat. A novel method for quantifying the resting Cl– conductance, important for [Cl–]i recovery, was developed and the possible contribution of GABAA and glycine receptors and of ClC-2 channels to this conductance was analyzed. The hypothesis that spontaneous, action potential-independent release of GABA is important for [Cl–]i recovery was tested. [Cl–]i was examined by gramicidin-perforated patch recordings in medial preoptic neurons. Cells were loaded with Cl– by combining GABA or glycine application with a depolarized voltage, and the time course of [Cl–]i was followed by measurements of the Cl– equilibrium potential, as obtained from the current recorded during voltage ramps combined with GABA or glycine application. The results show that passive Cl– flux contributes significantly, in the same order of magnitude as does K+-Cl– cotransporter 2 (KCC2), to [Cl–]i recovery and that Cl– conductance accounts for ∼ 6% of the total resting conductance. A major fraction of this resting Cl– conductance is picrotoxin (PTX)-sensitive and likely due to open GABAA receptors, but ClC-2 channels do not contribute. The results also show that when the decay of GABAA receptor-mediated miniature postsynaptic currents (minis) is slowed by the neurosteroid allopregnanolone, such minis may significantly quicken [Cl–]i recovery, suggesting a possible steroid-regulated role for minis in the control of Cl– homeostasis.

Synaptic contact between median preoptic neurons and subfornical organ neurons projecting to the paraventricular hypothalamic nucleus.

It is known that the median preoptic nucleus (POMe) sends dense projections to the subfornical organ (SFO). However, the functional significance of these projections have not been well discussed. In this electron microscopic study, we investigated the presence of synapses between POMe-derived axon terminals and SFO neurons that project to the paraventricular hypothalamic nucleus (PVN). After injection of a retrograde tracer, wheat germ agglutinin-conjugated horseradish peroxidase-colloidal gold complex, into the PVN, many labeled neurons were found in the SFO. In contrast, after injection of an anterograde tracer, biotinylated dextran amine, in the POMe, abundant labeled axon varicosities were observed in the SFO. Using electron microscopy, synapses were identified between retrogradely labeled dendrites and cell bodies, and anterogradely labeled axon terminals, indicating that POMe neurons innervate SFO neurons projecting to the PVN. The possibility that POMe neurons play multiple roles in the neuronal circuit responsible for vasopressin release and/or cardiovascular regulation is also discussed.

Reciprocal Control of Drinking Behavior by Median Preoptic Neurons in Mice.

Stimulation of glutamatergic neurons in the subfornical organ drives drinking behavior, but the brain targets that mediate this response are not known. The densest target of subfornical axons is the anterior tip of the third ventricle, containing the median preoptic nucleus (MnPO) and organum vasculosum of the lamina terminalis (OVLT), a region that has also been implicated in fluid and electrolyte management. The neurochemical composition of this region is complex, containing both GABAergic and glutamatergic neurons, but the possible roles of these neurons in drinking responses have not been addressed. In mice, we show that optogenetic stimulation of glutamatergic neurons in MnPO/OVLT drives voracious water consumption, and that optogenetic stimulation of GABAergic neurons in the same region selectively reduces water consumption. Both populations of neurons have extensive projections to overlapping regions of the thalamus, hypothalamus, and hindbrain that are much more extensive than those from the subfornical organ, suggesting that the MnPO/OVLT serves as a key link in regulating drinking responses. Neurons in the median preoptic nucleus (MnPO) and organum vasculosum of the lamina terminalis (OVLT) are known to regulate fluid/electrolyte homeostasis, but few studies have examined this issue with an appreciation for the neurochemical heterogeneity of these nuclei. Using Cre-Lox genetic targeting of Channelrhodospin-2 in transgenic mice, we demonstrate that glutamate and GABA neurons in the MnPO/OVLT reciprocally regulate water consumption. Stimulating glutamatergic MnPO/OVLT neurons induced water consumption, whereas stimulating GABAergic MnPO neurons caused a sustained and specific reduction in water consumption in dehydrated mice, the latter highlighting a heretofore unappreciated role of GABAergic MnPO neurons in thirst regulation. These observations represent an important advance in our understanding of the neural circuits involved in the regulation of fluid/electrolyte homeostasis.

Activation of hypothalamic gono-like neurons in female rats during estrus.

In mammals, gonadal function is controlled by the activity of hypothalamic gonadotropin-releasing hormone neurons, which control the secretion of adenohypophyseal and gonadal hormones. However, there are a number of unanswered questions in relation to gonadal function. It is currently unknown how erotogenic stimulation of the genitals influences the subpopulation of hypothalamic medial preoptic area neurons, antidromically identified as projecting to the median eminence at different periods of the estrous cycle. Additionally, the distinctiveness of hypothalamic medial preoptic area neurons, with respect to methods of feedback control by exogenous hormones, is also unknown. In this study, spontaneous discharges from individual neurons encountered within the medial preoptic area, gono-like neurons, were recorded extracellularly using glass microelectrodes. To confirm the cellular and histochemical properties of the recording units, antidromic stimulation was performed using a side-by-side bipolar stimulating electrode placed into the median eminence, alongside microiontophoretic injections of the conventional tracer, horseradish peroxidase. In addition, further immunohistochemical analyses were performed. Results showed that elevated gono-neuron activity was accompanied by increased background activity and greater responses to erotogenic stimuli during estrus. Application of clitoral traction stimulation resulted in increased activation of the gono-like neurons. This neuronal activity was noticeably inhibited by β-estradiol administration. Immunohistochemical analyses revealed the presence of gonadotropin-releasing hormone-reactive protein in hypothalamic cells in which electrophysiological recordings were taken. Thus, medial preoptic area neurons represent the subset of hypothalamic gonadotropin-releasing hormone neurons described from brain slices in vitro, and might serve as a useful physiological model to form the basis of future in vivo studies.

Activity of preoptic area neurons supports thermal effector activation during fever

Prostaglandin E2 (PGE2) in the preoptic area (POA) mimics fever by stimulating thermogenesis in brown adipose tissue (BAT), skeletal muscle via shivering and the heart via tachycardia, as well as by reducing heat loss via skin vasoconstriction. PGE2‐evoked stimulations of BAT, shivering EMGs and heart rate (HR) are postulated to arise from disinhibition of thermogenesis‐promoting neurons in the dorsomedial hypothalamus (DMH), and possibly in the rostral raphe pallidus (rRPa), due to the EP3 receptor (EP3‐R)‐mediated inhibition of GABAergic, warm‐sensitive neurons in the POA that project to inhibit DMH (and possibly rRPa) neurons. We tested the hypothesis that neurons in the medial preoptic (MPO) provide the necessary excitatory drive to downstream, thermogenesis‐promoting neurons for febrile heat production. Bilateral muscimol injection in the MPO completely inhibited the shivering EMGs and reversed the increases in BAT temperature and HR following injection of PGE2 into the POA. We conclude that the MPO contains neurons whose activity is necessary to support thermogenesis during fever. The data are consistent with a model in which a population of MPO neurons provides a tonically‐active, excitatory (glutamatergic) input to DMH (and rRPa?) neurons that drives thermogenesis upon disinhibition of DMH neurons during EP3‐R activation in the POA and potentially during cold exposure. Support NIH grant NS40987.

Induction of prolonged, continuous slow‐wave sleep by blocking cerebral H1 histamine receptors in rats

Classic H(1) histamine receptor (H(1)R) antagonists are non-selective for H(1)R and known to produce drowsiness. Modern antihistamines are more selective for H(1)R, and are 'non-drowsy' presumably due to reduced permeability through the blood-brain barrier. To characterize both histaminergic sleep regulation and the central actions of antihistamines, in the present study we analysed the effect of classic and modern antihistamines on rats' sleep using continuous i.c.v. infusions. Effects of classic (d-chlorpheniramine; d-CPA) and second-generation (cetirizine) antihistamines on sleep were compared after i.p. injections or continuous i.c.v. infusions into rats. Fluorescent cetirizine/DBD-pz was synthesized to trace the approximate distribution of cerebral cetirizine. Furthermore, the effects of H(1) R antagonists on cultured preoptic neurons were examined using calcium imaging. d-CPA 4 mg·kg(-1) i.p. increased non-rapid eye movement (REM) sleep whereas 10-40 mg·kg(-1) d-CPA decreased non-REM sleep at dark onset time. Nocturnal i.c.v. infusions of d-CPA (10 µmol·100 µL(-1)·10 h(-1)) increased drowsiness but not non-REM sleep, whereas the same i.c.v. infusions of cetirizine significantly increased non-REM sleep, abolished REM sleep, and decreased wakefulness for more than 10 h. The medial preoptic area contained the greatest fluorescent labelling after i.c.v. cetirizine/DBD-pz infusions. Histamine-induced Ca(2+) increases in medial preoptic neurons were blocked by d-CPA or cetirizine, whereas d-CPA, but not cetirizine, increased Ca(2+) irrespective of antihistaminergic activity at ≥ 100 µM. The excitatory action of d-CPA may explain the seemingly inconsistent actions of d-CPA on sleep. Cerebral H(1)R inhibition by cetirizine induces synchronization of cerebral activity and prolonged, continuous slow-wave sleep.

Both olfactory epithelial and vomeronasal inputs are essential for activation of the medial amygdala and preoptic neurons of male rats

Chemosensory inputs signaling volatile and nonvolatile molecules play a pivotal role in sexual and social behavior in rodents. We have demonstrated that olfactory preference in male rats, that is, attraction to receptive female odors, is regulated by the medial amygdala (MeA), the cortical amygdala (CoA), and the preoptic area (POA). In this paper, we investigated the involvement of two chemosensory organs, the olfactory epithelium (OE) and the vomeronasal organ (VNO), in olfactory preference and copulatory behavior in male rats. We found that olfactory preferences were impaired by zinc sulfate lesion of the OE but not surgical removal of the VNO. Copulatory behaviors, especially intromission frequency and ejaculation, were also suppressed by zinc sulfate treatment. Neuronal activation in the accessory olfactory bulb (AOB), the MeA, the CoA, and the POA was analyzed after stimulation by airborne odors or soiled bedding of estrous females using cFos immunohistochemistry. Although the OE and VNO belong to different neural systems, the main and accessory olfactory systems, respectively, both OE lesion and VNO removal almost equally suppressed the number of cFos-immunoreactive cells in those areas that regulate olfactory preference. These results suggest that signals received by the OE and VNO interact and converge in the early stage of olfactory processing, in the AOB and its targets, although they have distinct roles in the regulation of social behaviors.

Serum levels of FSH, LH and estradiol-17 beta in female rats around the time of puberty onset.

Groups of female rats were autopsied at daily intervals from 27 days of age through the first vaginal cycle and the serum FSH and LH and plasma estradiol-17 beta (E2) concentrations were estimated. Fluctuating hormone concentrations were recorded between days 27 and 31. Both gonadotropins and E2 dropped to a very low level on days 32 and 33 and showed then a progressive increase that commenced on day 34 and terminated for LH and E2 on day 39, i.e. in proestrus, whereas FSH declined from day 37 to day 39. The circulating gonadotropin level was lower during the first postpubertal diestrus than during most of the prepubertal phase. The findings suggest that a high sensitivity to the gonadotropin -inhibiting effect of estrogen prevented completion of an ovarian cycle up to day 33. After that, desensitization to the negative estrogen feedback as indicated by the simultaneous rise of E2 and gonadotropins makes final maturation of ovarian follicles and the first ovulation possible. In part of the rats aged 27-38 days cell nuclear volumes of medial preoptic neurons were evaluated, because recent results suggest that estrogen induces the desensitization process by an inhibitory action on the medial preoptic area. In accordance with this assumption, an inverse relationship between nuclear size and the E2 level in the blood could be revealed.

Cytokine‐induced Suppression of Medial Preoptic Neurons

We have shown that the medial preoptic area (MPO) in the hypothalamus is a major site where interferon (IFN)-alpha acts to induce suppression of splenic natural killer (NK) cell activity through an activation of sympathetic nervous system (SNS) in rats. Here, we discuss the hypothalamic mechanisms of the cytokine action using in vivo and in vitro preparations in rats. Lesion of the MPO activated the SNS and suppressed splenic NK cell activity in anesthetized rats, suggesting that the MPO had an inhibitory influence on nerve activity. Since both IFN-alpha and interleukin (IL)-1beta are known to suppress MPO neuron activity, it is suggested that the suppression/loss of the MPO caused by cytokine actions/lesions disinhibits the hypothalamic-sympathetic pathway, thereby resulting in an increase in the splenic SNS and reduction of NK activity. To explore the cellular mechanisms of the suppression of MPO neurons, the effects of Prostaglandin E2 (PGE2), one of the major mediators of cytokine action in the brain, on the glutamate-induced membrane currents were examined using the perforated patch-clamp method in mechanically dissociated MPO neurons. Patch-clamp analysis revealed that PGE2 potentiated the Ca2+-dependent K+ current (KCa) stimulated by Ca2+ entry through N-methyl-D-aspartate channels. We suggest that the cytokine-induced decrease in the firing rates of MPO neurons may be a result of an increase in interspike intervals caused by PGE(2)-induced enhancement of KCa in the presence of glutamatergic inputs.

Doing Protein Kinase C: Membrane Estrogen Receptor Signaling in a Neural Circuit

Female sexual behavior has been an exquisitely valuable model for developing an understanding of mechanisms of steroid action on nerve cells (1,2). The report by Dewing et al. (3) in this issue continues this grand tradition by identifying a set of biochemical pathways that mediate intracellular signaling stemming from interactions of estradiol with membrane binding sites. The work is of note from two points of view. The first is the inclusion of these novel signaling pathways coupled to membrane estrogen receptors with the more conventional intracellular mechanisms of estrogen action on reproduction. The second is the incorporation of these findings into a functional neuronal circuit mediating reproduction. For those new to this field, the acceptance of multiple forms of the estrogen receptor and even the existence of membrane estrogen receptors seems commonplace. Lost in today’s calm sea are the stormy history of dogmatic fervor and the battles defending each fundamentalist view of how estrogen exerts its biological actions. Jensen’s reviews of this history (4,5) are very revealing, tracing the transitions from the early beliefs that estrogen had an enzymatic-like action to the discovery and defense of the existence of intracellular receptors. The acceptance of intracellular receptors for estrogen (and other steroids of course) led to other debates about activated and inactivated receptor states, including the relocation of receptors in these states to different subcellular compartments (e.g. Refs. 6 and 7). This acceptance of intracellular receptors grew largely from the development of methodological approaches to visualize the receptors themselves using labeling of specific receptor antibodies (4) or by autoradiographic means to see the incorporation of estrogen into cells (8,9). In contrast, the existence of membrane steroid receptors was not so fondly embraced. These receptors are much like the dark side of the moon in that despite the weight of decades of physiological evidence (10), the inability to see the receptors has contributed to continued skepticism (and mystery). The appeal of the scientific approach taken in the research leading to this report is that the authors have taken a set of molecular processes identified in individual hippocampal neurons (11) and applied them to solving a fundamental question within a hypothalamic circuit. The model from hippocampal neurons suggests that estradiol-17β binds to conventional estrogen receptors that have been inserted into the cell membrane. The estrogen receptors in turn couple (directly or indirectly) with metabotropic glutamate receptors (mGluR; in this case mGluR1a), triggering signaling cascades associated with the glutamate receptor. Dewing et al. (3) took a systematic approach to investigating the role of protein kinase C (PKC) mediating the effects of estradiol and glutamate. The signaling pathway studied came from a combination of previous research and from the authors’ microarray analysis of estradiol-induced proteins in the arcuate nucleus. The arcuate nucleus sends projections to the medial preoptic area, and this circuit is part of a clearly larger neural pathway known to regulate the expression of female sexual behavior (termed lordosis in rodents). The functional endpoints included lordosis itself as well as internalization of the μ-opioid receptor in medial preoptic neurons. The rationale for this latter measure comes from the authors’ previous work showing that opioid release in the medial preoptic area is a key synaptic event regulating the expression of lordosis (12). Thus, μ-opioid receptor internalization serves as a proxy for increased synaptic opioid release, which in turn is a proxy for neural activation necessary for lordosis (when lordosis itself was not measured). Indeed, the results of the study supported the proposed model. Administration of either estradiol or an mGluR1a agonist produced opioid receptor internalization. Blocking PKC antagonized the effect mediated by the stimulation of either receptor. Finally, PKC activation mimicked the effects of estradiol or mGluR1a on opioid receptor internalization or female sexual behavior. The linchpin in all of this is the demonstration that estrogen receptors co-immunoprecipitated with mGluR1 in membrane fractions from the arcuate nucleus. Two key developments in this field loom on the horizon. The first is of course the visualization of membrane estrogen receptors. Here, Jensen’s (5) lesson of alternative approaches derived from the identification of intracellular receptors will be applicable, because little progress has been made in this arena. The other development will be the molecular details of the nature of the membrane interactions between estrogen receptors and other protein partners. Like the basic research and clinical impact of such a simple discovery as a method to immunostain the estrogen receptor, a similar discovery for membrane estrogen receptors is eagerly awaited.

Synaptic contacts between median preoptic neurons and subfornical organ neurons projecting to the paraventricular hypothalamic nucleus

Synaptic contacts between median preoptic neurons and subfornical organ neurons projecting to the paraventricular hypothalamic nucleus

Vagal afferent input alters the discharge of osmotic and ANG II-responsive median preoptic neurons projecting to the hypothalamic paraventricular nucleus

The goal of the present study was to determine the effect of activating vagal afferent fibers on the discharge of median preoptic (MnPO) neurons responsive to peripheral angiotensin II (ANG II) and osmotic inputs. Vagal afferents were activated by electrical stimulation of the proximal end of the transected cervical vagus nerve (3 pulses, 100 Hz, 1 ms, 100–500 μA). Of 21 MnPO neurons, 19 were antidromically activated from the hypothalamic paraventricular nucleus (PVH) (latency: 10.3 ± 1.3 ms, threshold: 278 ± 25 μA). MnPO-PVH cells had an average spontaneous discharge of 2.1 ± 0.4 Hz. Injection of ANG II (150 ng) and/or hypertonic NaCl (1.5 Osm/L, 100 μl) through the internal carotid artery significantly ( P < 0.01) increased the firing rate of most MnPO-PVH neurons (16/19, 84%). Vagus nerve stimulation significantly ( P < 0.01) decreased discharge (− 73 ± 9%) in 10 of 16 (63%) neurons with an average onset latency of 108 ± 19 ms. Among the remaining 6 MnPO-PVH neurons vagal activation either increased discharge (177 ± 100%) with a latency of 115 ± 15 ms ( n = 2) or had no effect ( n = 4). Pharmacological activation of chemosensitive vagal afferents with phenyl biguanide produced an increase ( n = 3), decrease ( n = 2), or no change ( n = 6) in discharge. These observations indicate that a significant proportion of ANG II- and/or osmo-sensitive MnPO neurons receive convergent vagal input. Although the sensory modalities transmitted by the vagal afferents to MnPO-PVH neurons are not presently known, the presence of inhibitory and excitatory vagal-evoked responses indicates that synaptic processing by these cells integrates humoral and visceral information to subserve potentially important cardiovascular and body fluid homeostatic functions.

Lipopolysaccharide Increases γ‐Aminobutyric Acid Synthesis in Medial Preoptic Neurones in Association with Inhibition of Steroid‐Induced Luteinising Hormone Surge in Female Rats

Lipopolysaccharide (LPS), an endotoxin produced by gram-negative bacteria, inhibits gonadotrophin secretion and ovulation in female mammals. The present study was undertaken to examine whether gamma-aminobutyric acid (GABA)-synthesising neurones in the medial preoptic area (mPOA) may be involved in the inhibition by LPS of progesterone-induced luteinising hormone (LH) surge in ovariectomised oestrogen-primed rats. An intravenous injection of LPS (10 microg) at noon significantly increased the content of glutamic acid decarboxylase 67 (GAD67), a GABA-synthesising enzyme, in brain tissues including the mPOA. LPS treatment also significantly increased the number of detectable GAD67-immunoreactive (ir) as well as GABA-ir cells in the mPOA, compared to control rats. The same LPS challenge completely eliminated the steroid-induced LH surge, which was normally induced in control rats in the afternoon. A local injection of muscimol (100 ng), a GABA(A) receptor agonist, into the mPOA also significantly attenuated the LH surge, mimicking the effect of LPS. These results suggest that LPS stimulates GABA synthesis in preoptic neurones, which in turn inhibit the LH surge in female rats. The results also support the hypothesis that diminution of the GABAergic suppressive activity in the mPOA permits the LH surge to be induced.

Unmasking of α 1 adrenoceptor induced hypnogenic response from medial preoptic area

Earlier studies had indicated the possibility of the involvement of the α 1 adrenergic receptors on the medial preoptic (mPOA) neurons in hypnogenesis. Microinjection of α 1 agonist methoxamine (0.05 μg) into the mPOA of the rats, maintained at an ambient temperature ( T amb) of 24 °C, did not produce any significant change in sleep–wakefulness ( S– W), except for an arousal of short duration, which coincided with the steep fall in body temperature ( T b). It was probably not possible to elicit sleep, because of hypothermia simultaneously produced by this drug. To test this hypothesis, experiments were conducted in an environment where the effects of hypothermia could be partially compensated for. When the rats were allowed to select their T amb, they moved over to a compartment having a higher T amb of 30 °C, on administration of methoxamine into the mPOA. Simultaneously, there was induction of sleep, which lasted for 60 min. Thus, it was possible to demonstrate the hypnogenic action of methoxamine, which was masked by hypothermic action of the drug. The findings indicate that the α 1 adrenergic receptors in the mPOA are involved in the induction of sleep.