Organum Vasculosum Lamina Terminalis Neurons Research Articles

Synaptic control of rat magnocellular neurosecretory cells by warm-sensing neurons in the organum vasculosum lamina terminalis.

Increases in core body temperature cause secretion of vasopressin (vasopressin, antidiuretic hormone) to promote water reabsorption and blunt water losses incurred through homeostatic evaporative cooling. Subtypes of transient receptor potential vanilloid (Trpv) channels have been shown to contribute to the intrinsic regulation of vasopressin-releasing magnocellular neurosecretory cells (MNCs) in the supraoptic nucleus (SON) and paraventricular nucleus (PVN). However, MNCs in vivo can also be excited by local heating of the adjacent preoptic area, indicating they receive thermosensory information from other areas. Here, we investigated whether neurons in the organum vasculosum lamina terminalis (OVLT) contribute to this process using in vitro electrophysiological approaches in male rats. We found that the majority of OVLT neurons are thermosensitive in the physiological range (36-39°C) and that this property is retained under conditions blocking synaptic transmission. A subset of these neurons could be antidromically activated by electrical stimulation in the SON. Whole cell recordings from SON MNCs revealed that heating significantly increases the rate of spontaneous excitatory postsynaptic currents (sEPCSs), and that this response is abolished by lesions targeting the OVLT, but not by bilateral lesions placed in the adjacent preoptic area. Finally, local heating of the OVLT caused a significant excitation of MNCs in the absence of temperature changes in the SON, and this effect was blocked by inhibitors of ionotropic glutamate receptors. These findings indicate that the OVLT serves as an important thermosensory nucleus and contributes to the activation of MNCs during physiological heating.

Signal Transduction of Mineralocorticoid and Angiotensin II Receptors in the Central Control of Sodium Appetite: A Narrative Review.

Sodium appetite is an innate behavior occurring in response to sodium depletion that induces homeostatic responses such as the secretion of the mineralocorticoid hormone aldosterone from the zona glomerulosa of the adrenal cortex and the stimulation of the peptide hormone angiotensin II (ANG II). The synergistic action of these hormones signals to the brain the sodium appetite that represents the increased palatability for salt intake. This narrative review summarizes the main data dealing with the role of mineralocorticoid and ANG II receptors in the central control of sodium appetite. Appropriate keywords and MeSH terms were identified and searched in PubMed. References to original articles and reviews were examined, selected, and discussed. Several brain areas control sodium appetite, including the nucleus of the solitary tract, which contains aldosterone-sensitive HSD2 neurons, and the organum vasculosum lamina terminalis (OVLT) that contains ANG II-sensitive neurons. Furthermore, sodium appetite is under the control of signaling proteins such as mitogen-activated protein kinase (MAPK) and inositol 1,4,5-thriphosphate (IP3). ANG II stimulates salt intake via MAPK, while combined ANG II and aldosterone action induce sodium intake via the IP3 signaling pathway. Finally, aldosterone and ANG II stimulate OVLT neurons and suppress oxytocin secretion inhibiting the neuronal activity of the paraventricular nucleus, thus disinhibiting the OVLT activity to aldosterone and ANG II stimulation.

Sodium Is Detected by the OVLT to Regulate Sympathetic Tone

Hypernatremia is known to elicit a rise in sympathetic tone and blood pressure. In this issue of Neuron, Nomura etal. (2018) now show that this is mediated via the organum vasculosum laminae terminalis (OVLT). Na+ activates OVLT neurons via a paracrine mechanism involving sodium channel Nax expressed by astrocytes and the ependyma.

Na+] Increases in Body Fluids Sensed by Central Nax Induce Sympathetically Mediated Blood Pressure Elevations via H+-Dependent Activation of ASIC1a

Increases in sodium concentrations ([Na+]) in body fluids elevate blood pressure (BP) by enhancing sympathetic nerve activity (SNA). However, the mechanisms by which information on increased [Na+] is translated to SNA have not yet been elucidated. We herein reveal that sympathetic activation leading to BP increases is not induced by mandatory high salt intakes or the intraperitoneal/intracerebroventricular infusions of hypertonic NaCl solutions in Nax-knockout mice in contrast to wild-type mice. We identify Nax channels expressed in specific glial cells in the organum vasculosum lamina terminalis (OVLT) as the sensors detecting increases in [Na+] in body fluids and show that OVLT neurons projecting to the paraventricular nucleus (PVN) are activated via acid-sensing ion channel 1a (ASIC1a) by H+ ions exported from Nax-positive glial cells. The present results provide an insight into the neurogenic mechanisms responsible for salt-induced BP elevations.

Activation of organum vasculosum neurons and water intake in mice by vasopressin neurons in the suprachiasmatic nucleus.

Previous studies have shown that mice housed under 12:12 h light-dark conditions display a pronounced increase in water intake during a 2-hour anticipatory period (AP) near the end of their active period (Zeitgeber Time ZT; ZT21.5-ZT23.5) compared to the preceding basal period (BP, ZT19.5-ZT21.5). This increased water intake during the AP is not associated with physiological stimuli for thirst, such as food intake, hyperosmolality, hyperthermia, or hypovolemia. Denying mice the water intake supplement during the AP causes them to be dehydrated at wake time. These observations suggest that this form of thirst may be driven by the circadian clock and serve to mitigate the dehydrating effect of absence of water intake during sleep. Here we review recent findings showing that this behavior is mediated by vasopressin (VP) containing neurons in the suprachiasmatic nucleus (SCN). SCN VP neurons project to the organum vasculosum lamina terminalis (OVLT) where the activity dependent release of VP causes excitation of thirst-promoting neurons. SCN VP neurons increase their electrical activity during the AP and the resultant release of VP causes an increase in the action potential firing rate of OVLT neurons. Experiments involving optogenetic control of VP release from the axon terminals of SCN neurons indicate that this network mechanism is necessary and sufficient to mediate pre-sleep water intake in mice. These findings provide insight into the output mechanisms that are used by the central clock to generate circadian rhythms, and reveal that the regulation of water intake contributes to osmoregulatory homeostasis during sleep. This article is protected by copyright. All rights reserved.

Firing Up in Anticipation

Firing Up in Anticipation

Clock-driven vasopressin neurotransmission mediates anticipatory thirst prior to sleep.

Circadian rhythms have evolved to anticipate and adapt animals to the constraints of the earth's 24-hour light cycle. Although the molecular processes that establish periodicity in clock neurons of the suprachiasmatic nucleus (SCN) are well understood, the mechanisms by which axonal projections from the central clock drive behavioural rhythms are unknown. Here we show that the sleep period in mice (Zeitgeber time, ZT0-12) is preceded by an increase in water intake promoted entirely by the central clock, and not motivated by physiological need. Mice denied this surge experienced significant dehydration near the end of the sleep period, indicating that this water intake contributes to the maintenance of overnight hydromineral balance. Furthermore, this effect relies specifically on the activity of SCN vasopressin (VP) neurons that project to thirst neurons in the OVLT (organum vasculosum lamina terminalis), where VP is released as a neurotransmitter. SCN VP neurons become electrically active during the anticipatory period (ZT21.5-23.5), and depolarize and excite OVLT neurons through the activation of postsynaptic VP V1a receptors and downstream non-selective cation channels. Optogenetic induction of VP release before the anticipatory period (basal period; ZT19.5-21.5) excited OVLT neurons and prompted a surge in water intake. Conversely, optogenetic inhibition of VP release during the anticipatory period inhibited the firing of OVLT neurons and prevented the corresponding increase in water intake. Our findings reveal the existence of anticipatory thirst, and demonstrate this behaviour to be driven by excitatory peptidergic neurotransmission mediated by VP release from central clock neurons.

Effects of Peritoneal Sepsis on Rat Central Osmoregulatory Neurons Mediating Thirst and Vasopressin Release.

Sepsis is a life-threatening condition caused by the systemic inflammatory response to bacterial infection. Although the early phase of sepsis features impaired thirst and enhanced vasopressin release, the basis for these defects is unknown. Here, we show that cecal ligation and puncture (CLP) in rats impairs the osmoresponsiveness of neurons in the organum vasculosum lamina terminalis (OVLT; which drives thirst) and attenuates that of neurosecretory neurons in the supraoptic nucleus (SON; which secrete oxytocin and vasopressin). Notably, we found that OVLT neurons are hyperpolarized and electrically silenced. In contrast, CLP increased the proportion of SON neurons displaying spontaneous electrical activity. Therefore, CLP affects the properties of osmoregulatory neurons in a manner that can affect systemic osmoregulation.

Hypertonicity Sensing in Organum Vasculosum Lamina Terminalis Neurons: A Mechanical Process InvolvingTRPV1But NotTRPV4

Primary osmosensory neurons in the mouse organum vasculosum lamina terminalis (OVLT) transduce hypertonicity via the activation of nonselective cation channels that cause membrane depolarization and increased action potential discharge, and this effect is absent in mice lacking expression of the transient receptor potential vanilloid 1 (Trpv1) gene (Ciura and Bourque, 2006). However other experiments have indicated that channels encoded by Trpv4 also contribute to central osmosensation in mice (Liedtke and Friedman, 2003; Mizuno et al., 2003). At present, the mechanism by which hypertonicity modulates cation channels in OVLT neurons is unknown, and it remains unclear whether Trpv1 and Trpv4 both contribute to this process. Here, we show that physical shrinking is necessary and sufficient to mediate hypertonicity sensing in OVLT neurons isolated from adult mice. Steps coupling progressive decreases in cell volume to increased neuronal activity were quantitatively equivalent whether shrinking was evoked by osmotic pressure or mechanical aspiration. Finally, modulation of OVLT neurons by tonicity or mechanical stimulation was unaffected by deletion of trpv4 but was abolished in cells lacking Trpv1 or wild-type neurons treated with the TRPV1 antagonist SB366791. Thus, hypertonicity sensing is a mechanical process requiring Trpv1, but not Trpv4.

Hyperosmotic activation of CNS sympathetic drive: implications for cardiovascular disease

Evidence now indicates that exaggerated sympathetic nerve activity (SNA) significantly contributes to salt-sensitive cardiovascular diseases. Although CNS mechanisms that support the elevation of SNA in various cardiovascular disease models have been intensively studied, many mechanistic details remain unknown. In recent years, studies have shown that SNA can rise as a result of both acute and chronic increases of body fluid osmolality. These findings have raised the possibility that salt-sensitive cardiovascular diseases could result, at least in part, from direct osmosensory activation of CNS sympathetic drive. In this brief review we emphasize recent findings from several laboratories, including our own, which demonstrate that neurons of the forebrain organum vasculosum laminae terminalis (OVLT) play a pivotal role in triggering hyperosmotic activation of SNA by recruiting neurons in specific regions of the hypothalamus, brainstem and spinal cord. Although OVLT neurons are intrinsically osmosensitive and shrink when exposed to extracellular hypertonicity, it is not yet clear if these processes are functionally linked. Whereas acute hypertonic activation of OVLT neurons critically depends on TRPV1 channels, studies in TRPV1(-/-) mice suggest that acute and long-term osmoregulatory responses remain largely intact. Therefore, acute and chronic osmosensory transduction by OVLT neurons may be mediated by distinct mechanisms. We speculate that organic osmolytes such as taurine and possibly novel processes such as extracellular acidification could contribute to long-term osmosensory transduction by OVLT neurons and might therefore participate in the elevation of SNA in salt-sensitive cardiovascular diseases.

Contribution of TRPV channels to osmosensory transduction, thirst, and vasopressin release

Systemic osmoregulation is an integrated physiological process through which water intake and excretion are continuously balanced against salt intake and excretion to maintain the osmolality of the extracellular fluid near an optimal 'set-point' value. The behaviors (that is, thirst and sodium appetite) and renal responses (diuresis and natriuresis) that are modulated to mediate osmoregulatory homeostasis are mainly controlled by the nervous system. Appropriate regulation of these parameters depends in large part on specialized osmosensitive neurons, termed osmoreceptors, which convert changes in plasma osmolality into electrical signals that ultimately modulate effector functions to achieve homeostasis. Previous work has shown that mechanosensitive cation channels expressed in osmoreceptor neurons play a key role in the process of osmosensory transduction. Although the molecular identity of these channels remains unknown, a growing body of evidence, reviewed here, indicates that members of the transient receptor potential vanilloid family of ion channels may contribute to osmosensory transduction and to homeostatic responses implicated in the control of water balance.

Organum vasculosum laminae terminalis contributes to increased sympathetic nerve activity induced by central hyperosmolality

The contribution of the organum vasculosum laminae terminalis (OVLT) in mediating central hyperosmolality-induced increases of sympathetic nerve activity (SNA) and arterial blood pressure (ABP) was assessed in anesthetized rats. Solutions of graded NaCl concentration (150, 375, and 750 mM) were injected (150 mul) into the forebrain vascular supply via an internal carotid artery (ICA). Time-control experiments (n = 6) established that ICA NaCl injections produced short-latency, transient increases of renal SNA (RSNA) and mean ABP (MAP) (P < 0.05-0.001). Responses were graded, highly reproducible, and unaltered by systemic blockade of vasopressin V1 receptors (n = 4). In subsequent studies, stimulus-triggered averaging of RSNA was used to accurately locate the OVLT. Involvement of OVLT in responses to ICA NaCl was assessed by recording RSNA and MAP responses before and 15 min after electrolytic lesion of the OVLT (n = 6). Before lesion, NaCl injections increased RSNA and MAP (P < 0.05-0.001), similar to time control experiments. After lesion, RSNA responses were significantly reduced (P < 0.05-0.001), but MAP responses were unaltered. To exclude a role for fibers of passage, the inhibitory GABA-A receptor agonist muscimol was microinjected into the OVLT (50 pmol in 50 nl) (n = 6). Before muscimol, hypertonic NaCl increased RSNA, lumbar SNA (LSNA), and MAP (P < 0.05-0.001). After muscimol, both RSNA and LSNA were significantly reduced in response to 375 and 750 mM NaCl (P < 0.05). MAP responses were again unaffected. Injections of vehicle (saline) into OVLT (n = 6) and muscimol lateral to OVLT (n = 5) each failed to alter responses to ICA NaCl. We conclude that OVLT neurons contribute to sympathoexcitation by central hyperosmolality.

Transient Receptor Potential Vanilloid 1 Is Required for Intrinsic Osmoreception in Organum Vasculosum Lamina Terminalis Neurons and for Normal Thirst Responses to Systemic Hyperosmolality

Recent studies have indicated that members of the transient receptor potential vanilloid (TRPV) family of cation channels are required for the generation of normal osmoregulatory responses, yet the mechanism of osmosensory transduction in primary osmoreceptor neurons of the CNS remains to be defined. Indeed, despite ample evidence suggesting that the organum vasculosum lamina terminalis (OVLT) serves as the primary locus of the brain for the detection of osmotic stimuli, evidence that neurons in the OVLT are intrinsically osmosensitive has remained elusive. Here we show that murine OVLT neurons are intrinsically sensitive to increases in the osmolality of the extracellular fluid. Hypertonic conditions provoked increases in membrane cation conductance that resulted in the generation of an inward current, depolarizing osmoreceptor potentials, and enhanced action potential discharge. Moreover, we found that this osmosensory signal transduction cascade was absent in OVLT neurons from TRPV1 knock-out (TRPV1-/-) mice and that responses of wild type (WT) OVLT neurons could be blocked by ruthenium red, an inhibitor of TRPV channels. Finally, TRPV1-/- mice showed significantly attenuated water intake in response to systemic hypertonicity compared with WT controls. These findings indicate that OVLT neurons act as primary osmoreceptors and that a product of the trpv1 gene is required for osmosensory transduction.

A rat brain slice preserving synaptic connections between neurons of the suprachiasmatic nucleus, organum vasculosum lamina terminalis and supraoptic nucleus

The organum vasculosum lamina terminalis (OVLT), the suprachiasmatic nucleus (SCN) and the supraoptic nucleus (SON) are three hypothalamic structures involved in the osmotic and circadian control of neurohypophysial secretion. Recent experiments have suggested that interactions between osmotic and circadian factors may be important for homeostasis. The existence of an in vitro slice preparation retaining these nuclei and their interconnections would therefore be useful for the analysis of synaptic interactions. In the rat, the OVLT, SCN and SON are found at increasingly ventral and lateral positions along the rostro-caudal axis, such that conventional 400 μm slices taken in the pure coronal or horizontal planes do not retain all three nuclei. Here we show that horizontal slices cut at angles of 38–42° relative to the dorsal surface of the cortex retain large fractions of the three nuclei. Intracellular recordings revealed membrane properties consistent with those previously published for OVLT, SCN and SON neurons. Moreover, antidromic and synaptic responses evoked by electrical stimulation revealed that extensive axonal projections are retained between these nuclei. Finally, chemical and osmotic stimulation of the OVLT exerted powerful influences on the rate of spontaneous synaptic events in SON neurons. We therefore conclude that angled horizontal hypothalamic slices represent a useful preparation for the analysis of physiological interactions between the OVLT, SCN and SON.

Axonal projections from the organum vasculosum lamina terminalis to the supraoptic nucleus: functional analysis and presynaptic modulation.

SUMMARY1. The rat organum vasculosum lamina terminalis (OVLT) contains GABA‐ and glutamate‐releasing neurons that project directly to magnocellular neurosecretory cells (MNC) in the supraoptic nucleus.2. Changes in osmolality over the OVLT in hypothalamic explants cause proportional changes in firing in MNC through corresponding changes in the frequency of spontaneous glutamatergic excitatory post‐synaptic potentials without affecting GABAergic inhibitory post‐synaptic potentials.3. Exogenously applied atrial natriuretic peptide inhibits the osmotic control of MNC by causing a decrease in the amount of glutamate released provoked by action potentials originating from OVLT neurons.

Presynaptic GABA B receptors modulate organum vasculosum lamina terminalis-evoked postsynaptic currents in rat hypothalamic supraoptic neurons

Whole-cell patch-clamp recordings obtained from 36 hypothalamic supraoptic nucleus neurons in explant preparations evaluated a role for GABA B receptors in modulating postsynaptic inhibitory and excitatory currents evoked by electrical stimulation in the organum vasculosum of the lamina terminalis. At a holding current of −65 mV, application of baclofen (1–10 μM) induced a dose-dependent reduction in the amplitude of pharmacologically isolated inhibitory and excitatory postsynaptic currents, converted paired-pulse depression in inhibitory postsynaptic currents to paired-pulse facilitation, and enhanced paired-pulse ratios for excitatory postsynaptic currents. In media containing 2-hydroxysaclofen (200–400 μM), baclofen-associated events were blocked and paired-pulse depression in evoked inhibitory postsynaptic currents was abolished. In addition, a progressive increase in the amplitude of inhibitory postsynaptic currents implied that GABA was endogenously active at presynaptic GABA B receptors. In contrast, no paired-pulse depression was observed for inhibitory postsynaptic currents evoked in six non-magnocellular neurons. Neither baclofen nor 2-hydroxysaclofen altered holding currents or input resistances in supraoptic neurons, or altered the kinetics of the evoked responses. These observations imply that the terminals of both inhibitory (GABAergic) and excitatory (glutamatergic) afferents to supraoptic nucleus neurons from organum vasculosum lamina terminalis neurons are subject to modulation by presynaptic GABA B receptors, and that this modulation is preferentially directed to the inhibitory inputs.

Sodium depletion induces Fos immunoreactivity in circumventricular organs of the lamina terminalis

Acute sodium depletion by peritoneal dialysis (PD) induces c- fos expression in the subfornical organ (SFO) and organum vasculosum laminae terminalis (OVLT), in conscious rats. Fos immunoreactive (Fos-ir) neurons detected by immunohistochemistry first appeared in these nuclei 60 min after PD, increased gradually in the next 4 h and remained high for 27 h following PD. Fos-ir cells were distributed throughout the body of SFO, being the core of the posterior sections preferentially activated, whereas Fos-ir neurons occurred around the periphery of OVLT (annular disposition). When rats were allowed to drink sodium salt (1.8% NaCl) 24 h after PD, there was a marked reversion of the c- fos expression in the OVLT and a comparatively smaller effect in the SFO. Intracerebroventricular infusion of hypertonic CSF (170 mM NaCl) from 30 min before and during 4 h after PD, significantly inhibited the c- fos expression in both nuclei. These results demonstrate that an acute body sodium deficit induces c- fos activity in SFO and OVLT neurons, indicating the special role of these structures in sodium balance regulation. They also show that the sodium-depletion-induced production of Fos in neurons of the lamina terminalis can be modulated by central or systemic reposition of sodium.

Characterization of vasopressin receptors in cultured cells derived from the region of rat brain circumventricular organs

The aim of the present study was to characterize vasopressin receptors within the two circumventricular organs located in the lamina terminalis of the rat brain, namely the organum vasculosum of the lamina terminalis and the subfornical organ. Cells derived from both structures were isolated, cultured and intracellular Ca2+ concentrations were measured in single fura-2 loaded neurons and astrocytes after application of vasopressin and various vasopressin analogues. Subsequent to Ca2+ measurements, the identification of neurons and astrocytes was verified using immunocytochemistry with cell type-specific antibodies. High proportions of subfornical organ (34%) and organum vasculosum laminae terminalis (28%) neurons exhibited increased intracellular Ca2+ concentration after exposure to 1-1000 nM vasopressin. Within single cells, the response was dose-dependent. Similar results were obtained in subfornical organ (62%) and organum vasculosum laminae terminalis (38%) astrocytes with minor differences in the transient amplitude and pattern distribution when compared with neurons. Since omission of extracellular Ca2+ preserved vasopressin responsiveness, it is likely that intracellular stores were the main source of mobilized Ca2+. The preincubation of neurons and astrocytes with the V1 receptor-specific antagonist d(CH2)5[Tyr(Me)2]8-arginine vasopressin (10-100 nM) selectively and reversibly blocked the vasopressin-mediated response. Oxytocin-induced Ca2+ transients (0.32-1000 nM), which were observed in 32% (63%) or organum vasculosum laminae terminalis and in 54% (42%) of subfornical organ neurons (astrocytes), were not affected by the V1-specific antagonist. These data indicate the presence of a V1-like vasopressin receptor and an oxytocin receptor in cultured neurons and astrocytes from both circumventricular organ structures. In addition, the exposure to the highly selective V2 receptor agonist, 1-desamino,8-D-arginine vasopressin, evoked Ca2+ transients almost exclusively in organum vasculosum laminae terminalis neurons (eight of 18 tested). Only 1 (n = 14) subfornical organ neuron and none of the astrocytes tested (n = 26) responded to 1-desamino,8-D-arginine vasopressin. Since 1-desamino,8-D-arginine vasopressin acting via "classical" V2 receptors is not expected to affect the intracellular Ca2+ concentration, these data indicate the tissue and cell type-specific expression of a 1-desamino,8-D-arginine vasopressin-sensitive vasopressin receptor in neurons of the organum vasculosum laminae terminalis. In summary, the results indicate a heterogeneity of neurohypophyseal peptide receptor subtypes in the primary cell culture of both circumventricular structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Osmoreceptors, Osmoreception, and Osmoregulation

Mammals have evolved sophisticated behavioral and physiological responses to oppose changes in the osmolality of their extracellular fluid. The behavioral approach consists of regulating the intake of salt and water through changes in sodium appetite and thirst. The physiological approach comprises adjustments of renal excretion of water and sodium which are achieved through changes in the release of antidiuretic and natriuretic hormones. Individually, these osmoregulatory responses are controlled by "osmoreceptors": groups of specialized nerve cells capable of transducing changes in external osmotic pressure into meaningful electrical signals. Some of these sensors are located in the region of the hepatic portal vein, a strategic site allowing early detection of the osmotic impact of ingested foods and fluids. Changes in systemic osmolality, however, are detected centrally, within regions that include the medial preoptic area, the median preoptic nucleus, the organum vasculosum lamina terminalis (OVLT), the subfornical organ, and the supraoptic nucleus (SON). While studies have indicated that these central and peripheral osmoreceptors participate in the control of osmoregulatory responses, little is known of the mechanisms by which this is achieved. One notable exception, however, consists of the osmotic control of electrical activity in SON neurons which, in the rat, contributes to the regulation of natriuresis and diuresis through effects on the secretion of oxytocin and vasopressin. Previous studies have shown that these cells are respectively excited and inhibited by hypertonic and hypotonic conditions. Experiments in vitro indicate that these responses result from both the endogenous osmosensitivity of these cells and changes in synaptic drive. Patch-clamp analysis has revealed that SON neurons are respectively depolarized and hyperpolarized by increases and decreases in external osmolality and that these intrinsic responses result from changes in the activity of mechanosensitive cationic channels. Moreover, intracellular recordings in hypothalamic explants have shown that changes in electrical activity are associated with proportional changes in the frequency of glutamatergic excitatory postsynaptic potentials derived from osmosensitive OVLT neurons. Both of these mechanisms, therefore, may participate in the osmotic regulation of neurohypophysial hormone release in situ.

Membrane properties of organum vasculosum lamina terminalis neurons recorded in vitro

Intracellular recordings of organum vasculosum lamina terminalis (OVLT) neurons were obtained from superfused explants of rat hypothalamus. Most (32 of 34) OVLT neurons displayed a low threshold spike response during depolarizing pulses applied from holding membrane potentials negative to -70 mV. In 17 of 34 cells, electrical stimulation of the supraoptic nucleus area evoked antidromic responses. In 20 of the 34 cells, 8 of which were antidromically driven, identical stimuli also revealed either excitatory (n = 12) or inhibitory (n = 5) or mixed (n = 3) postsynaptic potentials. Axonal projections to the ipsilateral supraoptic nucleus were confirmed afterwards using reconstruction of Lucifer yellow-filled cells. A 10-40 mosmol/kgH2O increase in the osmolality of the superfusion media by addition of NaCl or mannitol prompted a membrane depolarization of 2-10 mV in each of nine OVLT neurons tested. These results indicate that OVLT neurons project to the supraoptic nucleus and possess intrinsic properties capable of influencing their excitability. Because neurons in OVLT depolarize consequent to elevations in media osmolality, the OVLT may provide a means by which hyperosmotic stimuli influence neuroendocrine function.