Activity of phasic neurosecretory cells during haemorrhage

Mechanism and function of phasic firing in vasopressin-releasing magnocellular neurosecretory cells

Magnocellular neurosecretory cells that release vasopressin (MNC^VP ) from axon terminals in the neurohypophysis display a unique pattern of action potential firing termed phasic firing. Under basal conditions, only a small proportion of MNC^VP display spontaneous phasic firing. However, acute and chronic conditions that stimulate vasopressin release, such as hemorrhage and dehydration, greatly enhance the number of MNC^VP that fire phasically. Phasic firing optimizes VP neurosecretion at axon terminals by allowing action potential broadening to promote calcium-dependent frequency-facilitation, at the same time as preventing the secretory fatigue caused by spike inactivation that occurs during prolonged continuous stimulation. This review provides an update on our mechanistic understanding of these processes and highlights important gaps in our knowledge that must be addressed in future experiments.

Electrophysiological properties of identified oxytocin and vasopressin neurones

To understand the contribution of intrinsic membrane properties to the different in vivo firing patterns of oxytocin (OT) and vasopressin (VP) neurones, in vitro studies are needed, where stable intracellular recordings can be made. Combining immunochemistry for OT and VP and intracellular dye injections allows characterisation of identified OT and VP neurones, and several differences between the two cell types have emerged. These include a greater transient K⁺ current that delays spiking to stimulus onset, and a higher Na⁺ current density leading to greater spike amplitude and a more stable spike threshold, in VP neurones. VP neurones also show a greater incidence of both fast and slow Ca²⁺ -dependent depolarising afterpotentials, the latter of which summate to plateau potentials and contribute to phasic bursting. By contrast, OT neurones exhibit a sustained outwardly rectifying potential (SOR), as well as a consequent depolarising rebound potential, not found in VP neurones. The SOR makes OT neurones more susceptible to spontaneous inhibitory synaptic inputs and correlates with a longer period of spike frequency adaptation in these neurones. Although both types exhibit prominent Ca²⁺ -dependent afterhyperpolarising potentials (AHPs) that limit firing rate and contribute to bursting patterns, Ca²⁺ -dependent AHPs in OT neurones selectively show significant increases during pregnancy and lactation. In OT neurones, but not VP neurones, AHPs are highly dependent on the constitutive presence of the second messenger, phosphatidylinositol 4,5-bisphosphate, which permissively gates N-type channels that contribute the Ca²⁺ during spike trains that activates the AHP. By contrast to the intrinsic properties supporting phasic bursting in VP neurones, the synchronous bursting of OT neurones has only been demonstrated in vitro in cultured hypothalamic explants and is completely dependent on synaptic transmission. Additional differences in Ca²⁺ channel expression between the two neurosecretory terminal types suggests these channels are also critical players in the differential release of OT and VP during repetitive spiking, in addition to their importance to the potentials controlling firing patterns.

Regulation of Neuronal Activity in Hypothalamic Vasopressin Neurons

Vasopressin is a peptide hormone secreted from the posterior pituitary gland in response to various physiological and/or pathological stimuli, including changes in body fluid volume and osmolality and stress exposure. Vasopressin secretion is controlled by the electrical activity of the vasopressinergic magnocellular neurosecretory cells located in the hypothalamic supraoptic nucleus and paraventricular nucleus. Vasopressin release can occur somatodendritically in the hypothalamus or at the level of pituitary axon terminals. The electrical activity of the vasopressin neurons assumes specific patterns of electrical discharge that are under the control of several factors, including the intrinsic properties of the neuronal membrane and synaptic and hormonal inputs. It is increasingly clear that glial cells perform critical signaling functions that contribute to signal transmission in neural circuits. Astrocytes contribute to neuronal signaling by regulating synaptic and extrasynaptic neurotransmission, as well as by mediating bidirectional neuronal-glial transmission. We recently discovered a novel form of neuronal-glial signaling that exploits the full spatial domain of astrocytes to transmit dendritic retrograde signals from vasopressin neurons to distal upstream neuronal targets. This retrograde trans-neuronal-glial transmission allows the vasopressin neurons to regulate their synaptic inputs by controlling upstream presynaptic neuron firing, thus providing a powerful means of controlling hormonal output.

Spike triggered hormone secretion in vasopressin cells; a model investigation of mechanism and heterogeneous population function

Vasopressin neurons generate distinctive phasic patterned spike activity in response to elevated extracellular osmotic pressure. These spikes are generated in the cell body and are conducted down the axon to the axonal terminals where they trigger Ca²⁺ entry and subsequent exocytosis of hormone-containing vesicles and secretion of vasopressin. This mechanism is highly non-linear, subject to both frequency facilitation and fatigue, such that the rate of secretion depends on both the rate and patterning of the spike activity. Here we used computational modelling to investigate this relationship and how it shapes the overall response of the neuronal population. We generated a concise single compartment model of the secretion mechanism, fitted to experimentally observed profiles of facilitation and fatigue, and based on representations of the hypothesised underlying mechanisms. These mechanisms include spike broadening, Ca²⁺ channel inactivation, a Ca²⁺ sensitive K⁺ current, and releasable and reserve pools of vesicles. We coupled the secretion model to an existing integrate-and-fire based spiking model in order to study the secretion response to increasing synaptic input, and compared phasic and non-phasic spiking models to assess the functional value of the phasic spiking pattern. The secretory response of individual phasic cells is very non-linear, but the response of a heterogeneous population of phasic cells shows a much more linear response to increasing input, matching the linear response we observe experimentally, though in this respect, phasic cells have no apparent advantage over non-phasic cells. Another challenge for the cells is maintaining this linear response during chronic stimulation, and we show that the activity-dependent fatigue mechanism has a potentially useful function in helping to maintain secretion despite depletion of stores. Without this mechanism, secretion in response to a steady stimulus declines as the stored content declines.

Vasopressin is a hormone that is secreted from specialised brain cells into the bloodstream; it acts at the kidneys to control water excretion, and thereby help to maintain a stable ‘osmotic pressure’. Specialised cells in the brain sense osmotic pressure, and generate electrical signals which the thousands of vasopressin neurons process and respond to by producing and secreting vasopressin. In response to these signals, vasopressin neurons generate complex “phasic” patterns of electrical activity, and this activity leads to vasopressin secretion in a complex way that depends on both the rate and pattern of this activity. We have now built a computational model that describes both how the vasopressin neurons generate electrical activity and also how that activity leads to secretion. The model, which gives a very close fit to experimental data, allows us to explore the adaptive advantages of particular features of the vasopressin neurons. This analysis reveals the importance of heterogeneity in the properties of vasopressin neurons, and shows how the vasopressin system is optimally designed to maintain a consistent hormonal output in conditions where its stores of releasable hormone are severely depleted.

G protein-coupled receptors in the hypothalamic paraventricular and supraoptic nuclei--serpentine gateways to neuroendocrine homeostasis

G protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in the mammalian genome. They are activated by a multitude of different ligands that elicit rapid intracellular responses to regulate cell function. Unsurprisingly, a large proportion of therapeutic agents target these receptors. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important mediators in homeostatic control. Many modulators of PVN/SON activity, including neurotransmitters and hormones act via GPCRs--in fact over 100 non-chemosensory GPCRs have been detected in either the PVN or SON. This review provides a comprehensive summary of the expression of GPCRs within the PVN/SON, including data from recent transcriptomic studies that potentially expand the repertoire of GPCRs that may have functional roles in these hypothalamic nuclei. We also present some aspects of the regulation and known roles of GPCRs in PVN/SON, which are likely complemented by the activity of 'orphan' GPCRs.

Identified GnRH neuron electrophysiology: a decade of study

Over the past decade, the existence of transgenic mouse models in which reporter genes are expressed under the control of the gonadotropin-releasing hormone (GnRH) promoter has made possible the electrophysiological study of these cells. Here, we review the intrinsic and synaptic properties of these cells that have been revealed by these approaches, with a particular regard to burst generation. Advances in our understanding of neuromodulation of GnRH neurons and synchronization of this network are also discussed.

Glutamatergic inputs contribute to phasic activity in vasopressin neurons

Many neurons in the CNS display rhythmic patterns of activity to optimize excitation-secretion coupling. However, the mechanisms of rhythmogenesis are only partially understood. Magnocellular vasopressin (VP) neurons in the hypothalamus display a phasic activity that consists of alternative bursts of action potentials and silent periods. Previous observations from acute slices of adult hypothalamus suggested that VP cell rhythmicity depends on intrinsic membrane properties. However, such activity in vivo is nonregenerative. Here, we studied the mechanisms of VP neuron rhythmicity in organotypic slice cultures that, unlike acute slices, preserve functional synaptic connections. Comparative analysis of phasic firing of VP neurons in vivo, in acute slices, and in the cultures revealed that, in the latter, the activity was closely related to that observed in vivo. It was synaptically driven, essentially from glutamatergic inputs, and did not rely on intrinsic membrane properties. The glutamatergic synaptic activity was sensitive to osmotic challenges and kappa-opioid receptor activation, physiological stimuli known to affect phasic activity. Together, our data thus strongly suggest that phasic activity in magnocellular VP neurons is controlled by glutamatergic synaptic inputs rather than by intrinsic properties.

Glutamatergic synaptic transmission in neuroendocrine cells: Basic principles and mechanisms of plasticity

Glutamate synapses drive the output of neuroendocrine cells in the hypothalamus, but until recently, relatively little was known about the fundamental properties of transmission at these synapses. Here we review recent advances in the understanding of glutamate signals in magnocellular neurosecretory cells (MNCs) in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus that serve as the last step in synaptic integration before neurohormone release. While these synapses exhibit many similarities with other glutamate synapses described throughout the brain, they also exhibit a number of unique properties that are particularly well suited to the physiology of this system and will be discussed here. In addition, a number of recent studies begin to provide insights into new forms of synaptic plasticity that may be common in other brain regions, but in these cells, may serve important adaptive roles.

Potassium accumulation as dynamic modulator of neurohypophysial excitability

Activity-dependent modulation of excitable responses from neurohypophysial axons and their secretory swellings has long been recognized as an important regulator of arginine vasopressin and oxytocin release during patterned stimulation. Various activity-dependent mechanisms, including action potential broadening, potassium accumulation, and autocrine or paracrine feedback, have been proposed as underlying mechanisms. However, the relevance of any specific mechanism on net excitability in the intact preparation, during different levels of overall activation, and during realistic stimulation with trains of action potentials has remained largely undetermined. Using high-speed optical recordings and potentiometric dyes, we have quantified the dynamics of global excitability under physiologically more realistic conditions, that is in the intact neurohypophysis during trains of stimuli at varying frequencies and levels of overall activity. Net excitability facilitated during stimulation at low frequencies or at low activity. During persistent high-intensity or high-frequency stimulation, net excitability became severely depressed. Depression of excitable responses was strongly affected by manipulations of extracellular potassium levels, including changes to resting [K(+)](out), increases of interstitial spaces with hypertonic solutions and inhibition of Na(+)/K(+) ATPase activity. Application of the GABA(A) receptor blocker bicuculline or manipulations of Ca(2+) influx showed little effect. Numerical simulation of K(+) accumulation on action potentials of individual axons reproduced optically recorded population responses, including the overall depression of action potential (AP) amplitudes, modest AP broadening and the prominent loss of hyperpolarizing undershoots. Hence, extracellular potassium accumulation dominates activity-dependent depression of neurohypophysial excitability under elevated stimulation conditions. The intricate dependence on the short-term stimulation history and its resulting feedback on neurohypophysial excitability renders [K(+)](out) accumulation a surprisingly complex mechanism for regulating axonal excitability and subsequent neuroendocrine release.

Metaplasticity of hypothalamic synapses following in vivo challenge

Neural networks that regulate an organism's internal environment must sense perturbations, respond appropriately, and then reset. These adaptations should be reflected as changes in the efficacy of the synapses that drive the final output of these homeostatic networks. Here we show that hemorrhage, an in vivo challenge to fluid homeostasis, induces LTD at glutamate synapses onto hypothalamic magnocellular neurosecretory cells (MNCs). LTD requires the activation of postsynaptic alpha2-adrenoceptors and the production of endocannabinoids that act in a retrograde fashion to inhibit glutamate release. In addition, both hemorrhage and noradrenaline downregulate presynaptic group III mGluRs. This loss of mGluR function allows high-frequency activity to potentiate these synapses from their depressed state. These findings demonstrate that noradrenaline controls a form of metaplasticity that may underlie the resetting of homeostatic networks following a successful response to an acute physiological challenge.

Interleukin-6 activates arginine vasopressin neurons in the supraoptic nucleus during immune challenge in rats

The increase of plasma arginin-vasopressin (AVP) release, which translates hypothalamic AVP neuron activation in response to immune challenge, appears to occur independently of plasma osmolality or blood pressure changes. Many studies have shown that major inflammatory mediators produced in response to peripheral inflammation, such as prostaglandin (PG)-E(2) and interleukin (IL)-1beta, excite AVP neurons. However, in vivo electrical activation of AVP neurons was still not assessed in relation to plasma AVP release, osmolality, or blood pressure or to the expression and role of inflammatory molecules like PG-E(2), IL-1beta, IL-6, and tumor necrosis factor-alpha (TNFalpha). This study aims at elucidating those factors that underlie the activation of AVP neurons in response to immune stimulation mimicked by an intraperitoneal injection of lipopolysaccharide (LPS) in male Wistar rats. LPS treatment concomittanlty decreased diuresis and increased plasma AVP as well as AVP neuron activity in vivo, and these effects occurred as early as 30 min. Activation was sustained for more than 6 h. Plasma osmolality did not change, whereas blood pressure only transiently increased during the first hour post-LPS. PG-E(2), IL-1beta, and TNFalpha mRNA expression were raised 3 h after LPS, whereas IL-6 mRNA level increased 30 min post-LPS. In vivo electrophysiological recordings showed that brain IL-6 injection increased AVP neuron activity similarly to peripheral LPS treatment. In contrast, brain injection of anti-IL-6 antibodies prevented the LPS induced-activation of AVP neurons. Taken together, these results suggest that the early activation of AVP neurons in response to LPS injection is induced by brain IL-6.

Burst initiation and termination in phasic vasopressin cells of the rat supraoptic nucleus: a combined mathematical, electrical, and calcium fluorescence study

Vasopressin secreting neurons of the rat hypothalamus discharge lengthy, repeating bursts of action potentials in response to physiological stress. Although many electrical currents and calcium-dependent processes have been isolated and analyzed in these cells, their interactions are less well fathomed. In particular, the mechanism of how each burst is triggered, sustained, and terminated is poorly understood. We present a mathematical model for the bursting mechanism, and we support our model with new simultaneous electrical recording and calcium imaging data. We show that bursts can be initiated by spike-dependent calcium influx, and we propose that the resulting elevation of bulk calcium inhibits a persistent potassium current. This inhibition depolarizes the cell above threshold and so triggers regenerative spiking and further calcium influx. We present imaging data to show that bulk calcium reaches a plateau within the first few seconds of the burst, and our model indicates that this plateau occurs when calcium influx is balanced by efflux and uptake into stores. We conjecture that the burst is terminated by a slow, progressive desensitization to calcium of the potassium leak current. Finally, we propose that the opioid dynorphin, which is known to be secreted from the somatodendritic region and has been shown previously to regulate burst length and phasic activity in these cells, is the autocrine messenger for this desensitization.

Phasic bursts in rat magnocellular neurosecretory cells are not intrinsically regenerative in vivo

Vasopressinergic hypothalamic magnocellular neurosecretory cells fire in phasic bursts. Burst initiation involves summation of postsynaptic potentials to generate action potentials. Action potentials are each followed by a nonsynaptic depolarizing after-potential that summates temporally to generate a plateau potential and so sustain activity throughout the burst. It is unknown whether this plateau potential exceeds spike threshold in vivo to cause intrinsic regenerative firing or simply approaches threshold to increase the probability that excitatory postsynaptic potentials will trigger further action potentials. Here we show that pharmacological blockade of ionotropic glutamatergic transmission by microdialysis application of kynurenic acid into the supraoptic nucleus of anaesthetized rats prevents spontaneous bursts and bursts (after-discharge) evoked by short trains of antidromically stimulated action potentials in magnocellular neurosecretory cells. Even during prolonged depolarization induced by 1 m NaCl infusion, kynurenic acid microdialysis application still blocked after-discharge. The ability of kynurenic acid to block after-discharge during osmotic stimulation was not caused by an unmasking of inhibitory postsynaptic potentials as kynurenic acid was equally effective in the presence of the ionotropic gamma-aminobutyric acid receptor antagonist, bicuculline, nor did it result from inhibition of plateau potential amplitude as this was unaffected by kynurenic acid and bicuculline in vitro, as was after-discharge evoked in vitro. We conclude that phasic bursts are nonregenerative in vivo but rather require continued excitatory synaptic input activity superimposed upon a subthreshold plateau potential to sustain burst activity.

Acute increases in arterial blood pressure do not reduce plasma vasopressin levels stimulated by angiotensin II or hyperosmolality in rats

The present study sought to determine whether an acute increase in arterial blood pressure (ABP) reduces plasma vasopressin (VP) levels stimulated by ANG II or hyperosmolality. During an intravenous infusion of ANG II (100 ng·kg−1·min−1), attenuation of the ANG II-evoked increase in ABP with diazoxide or minoxidil did not further enhance plasma VP levels in rats. When VP secretion was stimulated by an infusion of hypertonic saline, coinfusion of the α-adrenergic agonist phenylephrine (PE) significantly increased ABP but did not reduce plasma VP levels. In fact, plasma VP levels were enhanced. The enhancement of plasma VP levels cannot be explained by a direct stimulatory action of PE, as plasma VP levels of isosmotic rats did not change during a similar infusion of PE. An infusion of endothelin-1 in hyperosmotic rats significantly raised ABP but did not reduce plasma VP levels; rather, VP levels increased as observed with PE. In α-chloralose-anesthetized rats infused with hypertonic saline, inflation of an aortic cuff to increase ABP and stimulate arterial baroreceptors did not reduce plasma VP levels. In each experiment, plasma oxytocin levels paralleled plasma VP levels. Collectively, the present findings suggest that an acute increase in ABP does not inhibit VP secretion.

Autocrine feedback inhibition of plateau potentials terminates phasic bursts in magnocellular neurosecretory cells of the rat supraoptic nucleus

Phasic activity in magnocellular neurosecretory cells is characterized by alternating periods of activity (bursts) and silence. During phasic bursts, action potentials are superimposed on plateau potentials that are generated by summation of depolarizing after-potentials. Dynorphin is copackaged in vasopressin neurosecretory vesicles that are exocytosed from magnocellular neurosecretory cell dendrites and terminals, and both peptides have been implicated in the generation of phasic activity. Here we show that somato-dendritic dynorphin release terminates phasic bursts by autocrine inhibition of plateau potentials in magnocellular neurosecretory cells recorded intracellularly from hypothalamic explants using sharp electrodes. Conditioning spike trains caused an activity-dependent reduction of depolarizing after-potential amplitude that was partially reversed by alpha-latrotoxin (which depletes neurosecretory vesicles) and by nor-binaltorphimine (kappa-opioid receptor antagonist), but not by an oxytocin/vasopressin receptor antagonist or a micro-opioid receptor antagonist, indicating that activity-dependent inhibition of depolarizing after-potentials requires exocytosis of an endogenous kappa-opioid peptide. kappa-Opioid inhibition of depolarizing after-potentials was not mediated by actions on evoked after-hyperpolarizations since these were not affected by kappa-opioid receptor agonists or antagonists. Evoked bursts were prolonged by antagonism of kappa-opioid receptors with nor-binaltorphimine and by depletion of neurosecretory vesicles by alpha-latrotoxin, becoming everlasting in approximately 50% of cells. Finally, spontaneously active neurones exposed to nor-binaltorphimine switched from phasic to continuous firing as plateau potentials became non-inactivating. Thus, dynorphin coreleased with vasopressin generates phasic activity through activity-dependent feedback inhibition of plateau potentials in magnocellular neurosecretory cells.

Single-unit activity of paraventricular nucleus neurons in response to intero- and exteroceptive stressors in conscious, freely moving rats

Extracellular recordings of 114 neurons in the hypothalamic paraventricular nucleus (PVN) of conscious, freely moving male rats were performed using a movable electrode system. Single-unit activities were examined for their spontaneous firing patterns and responses to intero- and exteroceptive stressors, including disturbance in arterial blood pressure, water deprivation, air-jet stimulation, and systemic administration of cholecystokinin-8 (CCK). PVN neurons were assigned to one of two groups on the basis of their spontaneous firing patterns: phasic (n=29) and non-phasic (n=85). Intravenous (i.v.) administration of phenylephrine (8 microg/kg) resulted in the inhibition of a greater percentage of phasic-type (88.9%; 24/27) than non-phasic-type neurons (14.9%; 11/74). Most phasic-type neurons showed excitation in response to i.v. administration of sodium nitroprusside (20 microg/kg, 66.7%; 18/27) and water deprivation (15 h, 77.8%; 7/9) when compared to non-phasic-type neurons. Conversely, a greater number of non-phasic-type neurons showed excitation in response to air-jet stimulation (5 l/min, 10 s, 29.0%; 20/69) and to i.v. administration of CCK (5 microg/kg, 24.5%; 11/45) when compared to phasic-type neurons. However, most non-phasic-type neurons that demonstrated excitation in response to i.v. administration of CCK (88.9%; 8/9) did not respond to air-jet stimulation. The present study indicated that phasically firing neurons recorded from the PVN in conscious, freely moving rats are putative vasopressin-secreting neurons on the basis of their responses to intero- and exteroceptive stressors. These data contribute to our understanding of local neural mechanisms within the PVN that are responsible for stress responses in conscious rats.

Ionotropic histamine receptors and H2 receptors modulate supraoptic oxytocin neuronal excitability and dye coupling

Histaminergic neurons of the tuberomammillary nucleus (TM) project monosynaptically to the supraoptic nucleus (SON). This projection remains intact in our hypothalamic slices and permits investigation of both brief synaptic responses and the effects of repetitively activating this pathway. SON oxytocin (OX) neurons respond to single TM stimuli with fast IPSPs, whose kinetics resemble those of GABA(A) or glycine receptors. IPSPs were blocked by the Cl(-) channel blocker picrotoxin, but not by bicuculline or strychnine, and by histamine H(2), but not by H(1) or H(3) receptor antagonists, suggesting the presence of an ionotropic histamine receptor and the possible nonspecificity of currently used H(2) antagonists. G-protein mediation of the IPSPs was ruled out using guanosine 5'-O-(2-thiodiphosphate) (GDP-betaS), pertussis toxin, and Rp-adenosine 3',5'-cyclic monophosphothioate triethylamine (Rp-cAMPs), none of which blocked evoked IPSPs. We also investigated the effects of synaptically released histamine on dye coupling and neuronal excitability. One hundred seventy-three OX neurons were Lucifer yellow-injected in horizontal slices. Repetitive TM stimulation (10 Hz, 5-10 min) reduced coupling, an effect blocked by H(2), but not by H(1) or H(3), receptor antagonists. Because H(2) receptors are linked to activation of adenylyl cyclase, TM-stimulated reduction in coupling was blocked by GDP-betaS, pertussis toxin, and Rp-cAMPs and was mimicked by 8-bromo-cAMP, 3-isobutyl-1-methylxanthine, and Sp-cAMP. Membrane potentials of OX and vasopressin neurons were hyperpolarized, accompanied by decreased conductances, in response to bath application of 8-bromo-cAMP but not the membrane-impermeable cAMP. These results suggest that synaptically released histamine, in addition to evoking fast IPSPs in OX cells, mediates a prolonged decrease in excitability and uncoupling of the neurons.

Rhythmic bursts of calcium transients in acute anterior pituitary slices

Endocrine cells isolated from the anterior pituitary fire intracellular Ca2+ ([Ca2+]i) transients due to voltage-gated Ca2+ entry. However, the patterns of [Ca2+]i transients within the glandular parenchyma of the anterior pituitary are unknown. Here we describe, using real-time confocal laser microscopy, several spontaneous patterns of calcium signaling in acute pituitary slices prepared from male as well as cycling and lactating female rats. Forty percent of the cells demonstrated a spontaneous bursting mode, consisting of an active period of [Ca2+]i transients firing at a constant frequency, followed by a rest period during which cells were either silent or randomly active. The remaining recordings from endocrine cells either demonstrated random [Ca2+]i transients or were silent. These rhythmic bursts of [Ca2+]i transients, which required extracellular calcium, were detected in lactotrophs, somatotrophs, and corticotrophs within the acute slices. Of significance was the finding that the bursting mode could be adjusted by hypothalamic factors. In slices prepared from lactating rats, TRH recruited more bursting cells and finely adjusted the average duty cycle of [Ca2+]i bursts such that cells fired patterned bursts for approximately 70% of the recording period. Eighty-six percent of these cells were lactotrophs. Thus, the rhythmic [Ca2+]i bursts and their tuning by secretagogues may provide timing information that could encode for one or more cellular functions (e.g. exocytosis and/or gene expression) critical for the release of hormones by endocrine cells in the intact gland.

Physiological pathways regulating the activity of magnocellular neurosecretory cells

Magnocellular oxytocin and vasopressin cells are among the most extensively studied neurons in the brain; their large size and high synthetic capacity, their discrete, homogeneous distribution and the anatomical separation of their terminals from their cell bodies, and the ability to determine their neuronal output readily by measurements of hormone concentration in the plasma, combine to make these systems amenable to a wide range of fundamental investigations. While vasopressin cells have intrinsic burst-generating properties, oxytocin cells are organized within local pattern-generating networks. In this review we consider the rôle played by particular afferent pathways in the regulation of the activity of oxytocin and vasopressin cells. For both cell types, the effects of changes in the activity of synaptic input can be complex.

kappa-opioid regulation of neuronal activity in the rat supraoptic nucleus in vivo

We investigated the influence of endogenous kappa-opioids on the activity of supraoptic neurons in vivo. Administration of the kappa-antagonist nor-binaltorphimine (200 micrograms/kg, i.v.), increased the activity of phasic (vasopressin), but not continuously active (oxytocin), supraoptic neurons by increasing burst duration (by 69 +/- 24%) and decreasing the interburst interval (by 19 +/- 11%). Similarly, retrodialysis of nor-binaltorphimine onto the supraoptic nucleus increased the burst duration (119 +/- 57% increase) of vasopressin cells but did not alter the firing rate of oxytocin cells (4 +/- 8% decrease). Thus, an endogenous kappa-agonist modulates vasopressin cell activity by an action within the supraoptic nucleus. To eliminate kappa-agonist actions within the supraoptic nucleus, we infused the kappa-agonist U50,488H (2.5 micrograms/hr at 0.5 micrograms/hr) into one supraoptic nucleus over 5 d to locally downregulate kappa-receptor function. Such infusions reduced the spontaneous activity of vasopressin but not oxytocin cells and reduced the proportion of cells displaying spontaneous phasic activity from 26% in vehicle-infused nuclei to 3% in U50, 488H-infused nuclei; this treatment also prevented acute inhibition of both vasopressin and oxytocin cells by U50,488H (1000 micrograms/kg, i.v.), confirming functional kappa-receptor downregulation. In U50, 488H-infused supraoptic nuclei, vasopressin cell firing rate was increased by nor-binaltorphimine (100 and 200 micrograms/kg, i.v.) but not to beyond that found in vehicle-treated nuclei, indicating that these cells were not U50,488H-dependent. Thus, normally functioning kappa-opioid mechanisms on vasopressin cells are essential for the expression of phasic firing.

Vasopressin regularizes the phasic firing pattern of rat hypothalamic magnocellular vasopressin neurons

Vasopressin (AVP) magnocellular neurons of hypothalamic nuclei express specific phasic firing (successive periods of activity and silence), which conditions the mode of neurohypophyseal vasopression release. In situations favoring plasmatic secretion of AVP, the hormone is also released at the somatodendritic level, at which it is believed to modulate the activity of AVP neurons. We investigated the nature of this autocontrol by testing the effects of juxtamembrane applications of AVP on the extracellular activity of presumed AVP neurons in paraventricular and supraoptic nuclei of anesthetized rats. AVP had three effects depending on the initial firing pattern: (1) excitation of faintly active neurons (periods of activity of <10 sec), which acquired or reinforced their phasic pattern; (2) inhibition of quasi-continuously active neurons (periods of silences of <10 sec), which became clearly phasic; and (3) no effect on neurons already showing an intermediate phasic pattern (active and silent periods of 10-30 sec). Consequently, AVP application resulted in a narrower range of activity patterns of the population of AVP neurons, with a Gaussian distribution centered around a mode of 57% of time in activity, indicating a homogenization of the firing pattern. The resulting phasic pattern had characteristics close to those established previously for optimal release of AVP from neurohypophyseal endings. These results suggest a new role for AVP as an optimizing factor that would foster the population of AVP neurons to discharge with a phasic pattern known to be most efficient for hormone release.

Activity dependence and functional role of the apamin-sensitive K+ current in rat supraoptic neurones in vitro

1. Intracellular recordings were obtained from seventy-two magnocellular neurosecretory cells (MNCs) in superfused explants of rat hypothalamus. The current underlying the after-hyperpolarization (IAHP) following spike-evoked trains of action potentials was characterized using the hybrid-clamp technique. The activity-dependent requirements for the genesis of the AHP were determined. The functional role of the conductance was investigated using saturating concentrations (50-300 nM) of apamin, a selective blocker of the AHP in MNCs. 2. IAHP was reversibly abolished by the removal of extracellular Ca2+. The amplitude of IAHP varied linearly as a function of voltage and reversed at -100 +/- 3 mV in 3 mM external K+. Changes in the concentration of extracellular K+ resulted in shifts of the reversal potential consistent with Nernst equation predictions for a K+-selective conductance. 3. Action potentials triggered by brief depolarizing pulses elicited an AHP during trains evoked at frequencies > 1 Hz. Onset of the AHP progressed exponentially, reaching a maximum after the first fifteen to twenty impulses. The steady-state amplitude of the AHP increased logarithmically between 1 and 20 Hz. 4. Switching to voltage clamp during periods of continuous cell activity (firing rate > 4 Hz) confirmed the presence of an apamin-sensitive Ca2(+)-dependent K+ current. 5. Application of apamin produced a threefold increase in the mean firing rate of spontaneously active cells, but was without effect when applied to silent cells (firing rate < 0.5 Hz). 6. Apamin did not affect the ability of MNCs to fire in a phasic manner but caused a dramatic increase in the mean intraburst firing rate. Moreover, inhibition of IAHP by apamin strongly attenuated spike accommodation normally seen at the onset of phasic bursts. 7. While apamin did not enhance the amplitude of depolarizing after-potentials following single spikes, post-train plateau potentials and associated after-discharges were enhanced. 8. The possible consequences of IAHP modulation are discussed in the context of the regulation of firing rate and pattern in MNCs.

Median preoptic neurones projecting to the supraoptic nucleus are sensitive to haemodynamic changes as well as to rise in plasma osmolality in rats

Extracellular single unit activity was recorded from 73 neurones in the median preoptic nucleus (MnPO), identified by antidromic activation as projecting to the supraoptic nucleus (SON) area in urethane-anaesthetized male rats. Thirteen of 73 identified MnPO neurones were silent, and 44 of 60 spontaneously active MnPO neurones were tested for their responses to electrical stimulation of the nucleus tractus solitarius (NTS). The cells were divided into 4 groups according to their responses; those which were excited orthodromically (OD+; n = 15), those which were unresponsive (UN; n = 21), those which were inhibited orthodromically (OD-; n = 4), those which showed initial inhibition followed by excitation (OD-+ n = 4). Some of these neurones were further tested for their responses to haemorrhage and/or produced by intraperitoneal injection of 1.5 M NaCl. Six out of 10 OD+ cells were excited by haemorrhage, 6 out of 11 OD+ cells were inhibited by phenylephrine, and 5 out of 9 OD+ cells were excited by hypertonic saline. On the other hand the UN cells tended to be unresponsive to each type of stimulus. Three out of 7 OD+ cells were excited by both haemorrhage and hypertonic saline, and 3 out of 8 OD+ cells were inhibited by phenylephrine and excited by hypertonic saline. The results may suggest that MnPO neurones which receive afferent input from the NTS may be sensitive not only to haemodynamic change but also to change in plasma osmotic pressure and that such population of MnPO neurones may integrate a part of the haemodynamic and osmotic information and contribute to the control of neurohypophysial hormone release.

Extracellular GABA concentrations in rat supraoptic nucleus during lactation and following haemodynamic changes: an in vivo microdialysis study

Morphological and pharmacological evidence suggest that the dense GABAergic innervation of the supraoptic nucleus is important for regulating the electrical activity of vasopressin and oxytocin neurons. We have employed the technique of intracranial microdialysis to examine extracellular GABA concentrations in the supraoptic nucleus of the anaesthetized rat and questioned whether differences exist in the dynamics of GABA release between virgin and lactating rats, and if events during lactation or following blood pressure manipulation alter endogenous GABA levels in this nucleus. No significant differences were detected between virgin and lactating animals in either basal or 100 mM potassium ion-evoked GABA release. The inclusion of the GABA uptake blocker nipecotic acid (0.5 mM) into the dialysate resulted in a six- to eight-fold increase (P < 0.01) in GABA outflow in both groups of animals. In lactating rats, GABA outflow measured at 4 min intervals was not altered during a 60 min period of suckling by a full litter of pups and no significant change in GABA outflow was detected in relation to individual milk ejections. In virgin rats, removal of 1.5-2 ml of blood resulted in a 30-60 mmHg fall in blood pressure and a non-significant decline in GABA outflow. Replacement of blood resulted in an abrupt 50 mmHg increase in blood pressure and a significant 22% increase in GABA outflow (P < 0.01), but no change in aspartate or methionine concentrations. Repeated intravenous injections of the alpha-adrenoceptor agonist, metaraminol, similarly evoked approximately 50 mmHg increments in blood pressure and a 26% increase in GABA outflow (P < 0.05). Electrical stimulation of the diagonal band of Broca for 10 min produced a two-fold increase in GABA outflow from the supraoptic nucleus (P < 0.05). These results show that the overall profile of basal and potassium-stimulated GABA concentrations in the supraoptic nucleus is not substantially different between lactating and virgin rats. In lactating animals we have found that GABA levels are not altered in response to suckling or at the time of high-frequency firing by oxytocin neurons to induce milk ejection. In contrast, our data further support the hypothesis that GABA inputs to supraoptic neurons are part of a baroreceptor reflex, relaying through the diagonal band of Broca, to signal periods of acute hypertension and inhibit the firing of vasopressin neurons. Such observations suggest the physiological importance of GABA inputs to the supraoptic nuclei and indicate that GABA may be used in a stimulus-specific manner to influence the activity of magnocellular neurons.

NMDA receptor-mediated rhythmic bursting activity in rat supraoptic nucleus neurones in vitro

1. Intracellular recordings were obtained from 112 supraoptic nucleus magnocellular neurosecretory cells (MNCs) in superfused explants of rat hypothalamus maintained in vitro. The effects of glutamate receptor agonists and antagonists were examined at 32-34 degrees C. 2. In control solutions, spontaneously active (> 5 Hz) phasic or continuous neurones showed interspike interval distributions slightly skewed toward short intervals, but did not feature pauses in the 0.4-2 s range. Current injection to alter the rate of cell discharge shifted the histograms according to the mean firing rate, but failed to induce intermittent pauses in the 0.4-2 s range. 3. Application of N-methyl-D-aspartate (NMDA) induced a mode of firing in which bimodal interspike interval distributions reflected a high incidence of clusters of short interspike intervals (0.5-1.5 s) recurring every 1-3 s. In contrast, firing evoked by application of D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionic acid (AMPA) was not associated with a clustering of impulse discharge. 4. The putative endogenous excitatory amino acid transmitters L-glutamate, L-aspartate and quinolinate all mimicked the effects of NMDA. Clustered spiking responses to these agents were reversibly blocked by D,L-2-amino-5-phosphono-valerate (APV), but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In contrast, the non-NMDA receptor ligands kainate and quisqualate caused CNQX-sensitive increases in firing rate, but these responses were not associated with the appearance of clustered activity. 5. When applied to cells showing negative resting potentials (< -70 mV), or to neurones hyperpolarized by current injection, responses to NMDA consisted of rhythmic (approximately 1 Hz) voltage oscillations associated with bursts of spike discharge. In the presence of TTX, NMDA could induce subthreshold voltage oscillations in the absence of action potentials. 6. Application of a voltage clamp to potentials between -75 and -55 mV during rhythmic bursting responses failed to reveal any rhythmic oscillation of the membrane current. In all cases, rhythmic bursting activity resumed upon returning to the current-clamp mode. 7. Rhythmic bursting responses to NMDA application were abolished in Mg(2+)-free solutions, suggesting that the voltage dependence of NMDA channels served to promote regenerative voltage changes throughout the cycle. The NMDA-induced current itself, however, did not appear to decrease with time, suggesting that a distinct, outward current, was necessary to initiate the repolarizing phase of each cycle.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypotension preferentially induces c-fos immunoreactivity in supraoptic vasopressin neurons

Immunoreactivity to Fos protein (Fos-IR) was detected in rat hypothalamic neurons within 1 h of onset of hemorrhage by withdrawing 4-5 ml of blood, which lowered the arterial blood pressure to 50-70 mm Hg. About 70% of vasopressin (AVP)-containing neurons in the supraoptic nucleus (SON) and 20% in the paraventricular nucleus (PVN) expressed Fos-IR. In contrast, 5% of oxytocin (OXY)-containing neurons in the SON and < 1% in PVN were Fos-IR. Intravenous infusion of the vasodilating agent, nitroprusside, which lowered the blood pressure to levels comparable to that attained by hemorrhage, induced Fos-IR in greater than 65% of AVP-containing neurons in the SON, while relatively few AVP neurons in the PVN were Fos positive. These results suggest that hemorrhage or hypotension preferentially induces c-fos expression in supraoptic AVP-containing neurons.

Characterization of the parabrachial nucleus input to the hypothalamic paraventricular nucleus in the rat

The brainstem parabrachial nucleus (PBN) is viewed as an increasingly important site for the transfer of autonomic-related information to more rostral structures in the forebrain including the hypothalamus. In this study, we examined electrophysiologically in vivo and anatomically the nature of PBN input to the hypothalamic paraventricular nucleus (PVN) and particularly to the vasopressin-and oxytocin-secreting magnocellular neurosecretory cells within this nucleus. In urethane-anaesthetized rats, extracellular recordings from 108 antidromically identified neurosecretory PVN cells revealed an excitatory (37/43 cells) and less frequently an inhibitory (6/43 cells) response consequent to electrical stimulation in the PBN. Both vasopressin (12/37 cells)-and oxytocin (9/37 cells)-secreting neurons appear to respond to the PBN stimulus. Four cells projecting to the neurohypophysis could also be antidromically activated from PBN, and this observation may be indicative of collateral branching in some PVN neurosecretory neurons. In addition, recordings from 60 non-magnocellular (i.e. non-neurohypophysially-projecting) PVN cells revealed a facilitatory response (43/60 cells) following PBN stimulation, Iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) were made within the rat lateral PBN and brains prepared for immunocytochemical examination of projections to the PVN region. PHA-L-labelled fibres and terminals were visualized within both the parvocellular and magnocellular divisions of the PVN. In addition, labelled fibres were also seen in a region immediately dorsal to the PVN. PHA-L-labelled fibres with axonal varicosities and boutons were visualized over immunocyto-chemically-identified vasopressin and oxytocin neurons within the magnocellular PVN. These convergent electrophysiological and anatomical data provide evidence for a PBN projection to the PVN that is predominantly excitatory to both magnocellular neurosecretory and non-magnocellular cells. Moreover, with respect to vasopressin-and oxytocin-secreting cells, the PBN input appears to be directed at both populations of peptidergic neurons.

Interactions between emotional stress due to fear and hypovolemic stimuli in the control of vasopressin secretion in rats

Interactions between emotional stress due to fear and hypovolemic stimuli on vasopressin secretion were studied in rats. Intraperitoneally injected dextran did not significantly change plasma osmolality and arterial blood pressure but increased blood hemoglobin and plasma vasopressin level. An i.v. infused physiological solution reversed these changes. Emotional stress due to fear acquired by learning suppressed plasma vasopressin level in dextran-injected rats. Emotional stress due to fear produced by low-frequency footshocks also suppressed the increased plasma vasopressin level. These results suggest that emotional stress due to fear interacts with afferent neural signals originating from cardio-vascular volume receptors in the control of vasopressin secretion.

Resolution of central sites involved in picrotoxin-induced vagal activation and vasopressin release

This investigation tested the hypotheses that picrotoxin, a drug which blocks the inhibitory effect of gamma-aminobutyric acid (GABA), would, in spinal cord-transected rats, (1) suppress the cardiac vagus when localized to the forebrain and stimulate the cardiac vagus by acting in the brainstem and (2) stimulate the release of vasopressin into the systemic circulation through separate forebrain and brainstem GABAergic mechanisms. An intra-arterial infusion technique allowed for delivery of picrotoxin selectively to either forebrain or brainstem areas. Administration of picrotoxin via the vertebral artery decreased sinus rate and increased circulating levels of vasopressin. Infusion of picrotoxin into the internal carotid artery caused increases in sinus rate, blood pressure and plasma vasopressin. These data support the hypothesis that GABAergic mechanisms at different levels of the neuraxis exert opposite effects on cardiac vagal activity, and that GABAergic mechanisms in both the brainstem and forebrain inhibit the release of AVP into the systemic circulation.

Subfornical organ and hypothalamic paraventricular nucleus connections with median preoptic nucleus neurons: an electrophysiological study in the rat

The role of pathways from the subfornical organ (SFO) to the hypothalamic paraventricular nucleus (PVN) through the median preoptic nucleus (MnPO) in regulating the activity of putative vasopressin (VP)-secreting neurons in the PVN was examined in urethane-anesthetized male rats. The activity of the majority (79%) of SFO neurons antidromically identified as projecting to the MnPO was excited by microiontophoretically (MIPh) applied angiotensin II (ANG II) and the effect was blocked by MIPh-applied saralasin (Sar), an ANG II antagonist. Identified SFO neurons that were excited by MIPh-applied ANG II were also excited by intravenously administered ANG II. Electrical stimulation of the SFO produced orthodromic excitation (48%) or inhibition (24%) of the activity of MnPO neurons antidromically identified as projecting to the PVN. Identified MnPO neurons that were excited by SFO stimulation were also excited by MIPh-applied ANG II, while the remaining neurons were not affected. The excitatory responses to SFO stimulation and to MIPh-applied ANG II were both blocked by MIPh-applied Sar, whereas the inhibitory responses to SFO stimulation were not affected. ANG II injected into the region of the SFO produced either an excitation (55%) or no effect (45%) on the activity of identified MnPO neurons. Electrical stimulation of the MnPO produced orthodromic excitation (27%) or inhibition (23%) of the activity of putative VP-secreting PVN neurons. ANG II injected into the region of the MnPO produced either an excitation (31%) or no effect (69%) on the activity of putative VP-secreting PVN neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

The ventral septal area: electrophysiological evidence for putative arginine vasopressin projections onto thermoresponsive neurons

The present experiments were conducted to identify thermoresponsive neurons in the ventral septal area and to characterize such units with respect to their connectivity to potential sources of arginine vasopressin in this area (the paraventricular nucleus and the bed nucleus of the stria terminalis) and to other brain regions (fornix and amygdala). Single unit in vivo microelectrode techniques were used to classify warm responsive, cold responsive, dynamic, biphasic and phasically active thermoresponsive neurons in the ventral septal area which altered their spontaneous activity in response to thermal stimulation of the scrotal skin. The fornix provided a large number of primarily inhibitory afferents to ventral septal thermoresponsive neurons while the amygdala projection consisted of approximately equal excitatory and inhibitory inputs. Electrical stimulation of the paraventricular nucleus resulted in orthodromic inhibition in 9 of 12 thermoresponsive units while projections from the bed nucleus of the stria terminalis appeared to preferentially inhibit warm responsive neurons in this area. These findings implicate the ventral septal area in thermoregulatory pathways by identifying neurons in this area responsive to peripheral thermal stimulation. Further, evidence in support of arginine vasopressin acting in this area to influence thermoregulation is provided by the identification of the paraventricular nucleus and bed nucleus of the stria terminalis as possible sources of afferents to the ventral septal area, and the characterization of the influence of this afferent input on thermoresponsive neurons in this region.

Calcium-dependent spike after-current induces burst firing in magnocellular neurosecretory cells

Magnocellular neurosecretory cells (MNCs) were impaled in perfused explants of rat hypothalamus. Application of a voltage clamp after 1-5 current-evoked spikes revealed a tetrodotoxin-resistant, but Cd2+-sensitive inward current. From a threshold near -85 mV, the amplitude of this current increased as post-spike commands were made to more positive potentials. Following its activation, the current-voltage relation of the cell displayed a region of negative resistance which crossed the spike threshold. This Ca2+-dependent spike after-current can therefore induce and sustain burst firing in MNCs.

Electrophysiological evidence that circulating angiotensin II sensitive neurons in the subfornical organ alter the activity of hypothalamic paraventricular neurohypophyseal neurons in the rat

Thirteen neurons in the subfornical organ (SFO) were antidromically activated by electrical stimulation of the paraventricular nucleus (PVN) in the rat. The activity of these identified SFO neurons was excited by intravenous injection of angiotensin II (AII). Electrical stimulation of the SFO produced orthodromic excitation (40%) and inhibition (40%) of the activity of putative vasopressin (VP)-secreting PVN neurons. These results suggest that circulating AII sensitive SFO neurons with efferent projections to the PVN have both excitatory and inhibitory influences on the activity of putative VP-secreting neurons in the PVN.

Inputs from the A1 noradrenergic region to hypothalamic paraventricular neurons in the rat

Electrical stimulation of the rat A1 noradrenergic region produced excitation (77%) of the activity of putative vasopressin (VP)-secreting neurons in the paraventricular nucleus (PVN) and produced excitation (4%), inhibition (26%) and excitation-inhibition (11%) of the activity of PVN neurons that were not antidromically identified by neurohypophysial stimulation. The excitatory response of putative VP-secreting neurons was blocked by microiontophoretically applied phentolamine, an alpha-adrenoceptor antagonist, but not by timolol, a beta-adrenoceptor antagonist. The inhibitory response of unidentified PVN neurons, on the other hand, was blocked by timolol, but not by phentolamine.

Response of phasically firing paraventricular neurons to A1 noradrenergic region stimulation and its attenuation by adrenoceptor antagonists

In urethane-anesthetized rats electrical stimulation of the A1 noradrenergic region in the ventrolateral medulla produced excitatory (27%), excitatory-inhibitory (24%), or inhibitory (34%) responses of spontaneous activity of phasically firing units in the paraventricular nucleus (PVN) of the hypothalamus. The latency of excitatory responses was significantly shorter in those followed by inhibition, suggesting the existence of two distinct excitatory pathways. The latency of inhibitory responses, however, was not different between primary and postexcitatory inhibitory responses. Effects of alpha- and beta-adrenoceptor antagonists on responses of phasically firing PVN units to stimulation of the A1 region were tested. The excitatory responses that were not followed by inhibition were significantly attenuated by intraperitoneal phenoxybenzamine, whereas the other responses were not affected. Both primary and postexcitatory inhibitory responses were significantly attenuated by intravenous propranolol, whereas neither type of excitatory response was affected. These results suggest that the medullary (A1 region) inputs to phasically firing PVN units are mediated by alpha-adrenergic (excitatory), beta-adrenergic (inhibitory), and nonaminergic (excitatory) pathways.

Noxious inputs to supraoptic neurosecretory cells in the rat

The effects of noxious stimuli were studied on discharge activity of the neurosecretory cells identified in the supraoptic nucleus by antidromic excitation after pituitary stimulation, in anaesthetized rats. Tail pinching excited 24% and inhibited spontaneous discharge of 6% of the 91 cells tested. Noxious heat stimuli (44-63 degrees C) applied to the hindlimb paw produced a transient excitation in 26% of the 23 cells tested. Electric stimulation of either the sciatic or cutaneous nerve with 20-Hz pulses for 1 s, at an intensity 5 times stronger than the threshold for evoking the changes in respiratory movements and blood pressure similar to those after tail pinching or noxious heat stimuli, excited about 30% of the cells tested. The excitation produced by these noxious stimuli preceded, on some occasions, the respiratory movement and blood pressure decrease which occurred concomitantly. Peristimulus time histograms of spontaneous discharges constructed during stimulation of either nerve at 1 Hz, revealed the presence of excitatory synaptic inputs in about 35% of the neurosecretory cells tested. These data indicate the existence of direct neural pathways which mediate excitatory synaptic inputs originating from nociceptors to supraoptic neurosecretory cells. Since 9 of the 22 cells which were excited by tail pinching exhibited a "phasic" pattern of spontaneous discharge which is known to characterize certain vasopressin-secreting neurones in rats, it is suggested that these excited cells were, at least in part, vasopressinergic.

The action of the A1 noradrenergic region on phasically firing neurons in the rat paraventricular nucleus

Electrical stimulation of the rat A1 noradrenergic region produced orthodromic inhibition (34%), excitation (23%) or excitation-inhibition sequence (25%) of the spontaneous activity of phasically firing units in the paraventricular nucleus of the hypothalamus (PVH). Excitatory responses could be distinguished into two types on the basis of their latency and duration. Both the primary and post-excitatory inhibitory responses were significantly attenuated by intravenous injection of propranolol, a beta-adrenoceptor antagonist, whereas neither type of excitatory response was affected.

Electrophysiological evidence for facilitatory control of oxytocin neurones by oxytocin during suckling in the rat

Antidromically identified paraventricular neurones were recorded simultaneously with intramammary pressure in urethane (1.2 g/kg) anaesthetized rats during suckling. The correlation of the firing pattern of these neurones with milk ejection enabled distinction between oxytocin and vasopressin neurones. Oxytocin neurones displayed a short (2-6 s) characteristic high-frequency burst of spikes. This activation probably occurred simultaneously in all oxytocin neurones 12-18 s before milk ejection and was regular in both frequency and amplitude (total number of spikes). The role of neurohypophysial peptides and analogues in the control of these characteristics was studied. Injecting 10 pg, 100 pg and 1 ng of oxytocin into the 3rd ventricle increased background activity of slow-firing oxytocin neurones (less than 3 spikes/s) and had a strong dose-dependent facilitatory effect on the milk ejection reflex, increasing both the amplitude and frequency of neurosecretory bursts. No effect was observed on non-neurosecretory neurones. Such injection also triggered the milk ejection reflex when it had not appeared an hour after suckling began. Oxytocin did not itself induce neurosecretory activation, which only appeared if the young rats were sucking. Injecting oxytocin into the lateral ventricle was less effective than into the 3rd ventricle. No effect was observed after injection into the venous blood or into the 4th ventricle, which suggested that oxytocin acts in the hypothalamus. Injecting mesotocin or isotocin into the 3rd ventricle had a facilitatory effect similar to that of oxytocin but vasopressin, vasotocin, MIF I (pro-leu-gly-NH2, terminal triplet oxytocin) or bovine neurophysins I and II did not modify neurosecretory activation or the milk ejection pattern. Injecting an oxytocin antagonist, ([1(beta-mercapto-beta, beta cyclopentamethylene propionic acid), 8-ornithine] vasotocin, d(CH2)5OVT) into the 3rd ventricle decreased milk ejection frequency and considerably delayed the reappearance of the first milk ejection. This resulted from a decrease in both frequency and amplitude of neurosecretory bursts, which were too small to induce detectable oxytocin release. Moreover, d(CH2)5OVT suppressed the facilitatory effect of exogenous oxytocin. Under normal conditions, endogenous oxytocin seemed to be involved in the control of neurosecretory activation. Injecting 1 ng oxytocin or 1 or 10 ng vasopressin into the 3rd ventricle did not modify the firing pattern of vasopressin neurones whether activated by hyperosmotic stimulation (1 ml NaCl, 9% solution (w/v) I.P.) or not.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential changes in substance P and somatostatin in brain nuclei of rats exposed to hemorrhagic shock

Concentrations of immunoreactive substance P ( SPir ) and immunoreactive somatostatin ( SSir ) were determined in discrete brain nuclei of normal rats and rats exposed to acute hypovolemic hypotension (8 ml/300 g body weight). SPir in the hypothalamic paraventricular nucleus (PVN) and the caudal and rostral parts of the nucleus of the tractus solitarius (NTS) were significantly reduced 2 h after hemorrhage, but no changes were found in any of the brain nuclei when examined 5 min after the same bleeding. SSir in the NTS and nucleus ambiguus (NA) were reduced by 50-60% 2 h after the hemorrhage, as compared either to intact rats or rats examined 5 min after bleeding. In the forebrain, significantly lower concentrations of SSir were found in the nucleus supraopticus at both 5 min and 2 h after shock and in the PVN of rats 2 h after shock. These data suggest a role for SPir and SSir in central adaptation to hypovolemic hypotension and further indicate that functions regulated by these neuropeptides might be substantially affected by shock states.

Altered baroreceptor inputs to the supraoptic nucleus of the Brattleboro rat

In Brattleboro rats, many more supraoptic neurones are baroresponsive than in normal rats. This may reflect the more widespread noradrenergic innervation of the supraoptic nucleus in Brattleboro rats. If so, the present data suggest that baroreceptor influences on vasopressin secretion are mediated by a noradrenergic pathway and that the altered responsiveness reflects the altered innervation.

The organization of noradrenergic pathways from the brainstem to the paraventricular and supraoptic nuclei in the rat

Axonal transport and immunohistochemical methods have been used to clarify the organization of pathways from noradrenergic and adrenergic cell groups in the brainstem to the paraventricular (PVH) and supraoptic (SO) nuclei of the hypothalamus. First, the location of such cells was determined with a combined retrograde tracer-immunofluorescence method. The fluorescent tracer, True Blue, was injected into the PVH or the SO, and sections through the brainstem were stained with anti-(rat) DBH, a specific marker for noradrenergic and adrenergic neurons. It was found that, after injections in the PVH, doubly labeled neurons were confined almost exclusively to 3 cell groups, the A1 region of the ventral medulla, which contained a majority of such cells, the A2 region in the dorsal vagal complex, and the locus coeruleus (A6 region). After injections centered in the SO an even greater proportion of doubly labeled cells were found in the A1 region, although some were also found in the A2 and A6 regions. The topography of doubly labeled cells indicates that these projections arise primarily from noradrenergic neurons, although adrenergic cells in both the C1 and the C2 groups probably contribute as well. Because well over 80% of the retrogradely labeled cells in these three regions were also DBH-positive, we next placed injections of [3H]amino acids into each of them in different groups of animals, and traced the course and distribution of the ascending (presumably DBH-positive) projections to the PVH and SO in the resulting autoradiograms. Injections centered in the A1 region labeled a substantial projection to most parts of the parvocellular division of the PVH, and was most dense in the dorsal and medial parts. In addition, terminal fields were labeled on those parts of the magnocellular division of the PVH, and of the SO, in which vasopressinergic cell bodies are concentrated. Injections centered in the A2 region also labeled a projection to the parvocellular division of the PVH that was topographically similar, but less dense, than that from the A1 region. In contrast, [3H]amino acid injections centered in the locus coeruleus labeled a moderately dense projection to the PVH that was limited to the medialmost part of the parvocellular division. Neither the A2 nor the A6 cell groups project to the magnocellular parts of PVH, or to the SO. The autoradiographic material, and additional double-labeling experiments, were used to identify and to characterize projections that interconnect the A1, A2 and A6 regions, as well as possible projections from these cell groups to the spinal cord. These results may be summarized as follows: a substantial projection from the nucleus of the solitary tract to the A1 region was identified, but this pathway does not arise from catecholaminergic neurons in the A2 cell group. DBH-stained cells in the A1 region project back to the dorsal vagal complex, as well as quite massively to the locus coeruleus (A6 region)...

Oxytocin, vasopressin, and estrogen-stimulated neurophysin: daily patterns of concentration in cerebrospinal fluid

The concentrations of oxytocin, arginine vasopressin, and estrogen stimulated neurophysin in cerebrospinal fluid of monkeys showed a daily fluctuation with high concentrations occurring during the light period. The patterns of oxytocin and estrogen-stimulated neurophysin in the cerebrospinal fluid were not observed in the plasma nor were they altered after the administration of a dose of estradiol that increased concentrations of estrogen-stimulated neurophysin in plasma. The disassociation between these cerebrospinal fluid and plasma patterns and values suggests that the secretory activity of neurons that release estrogen-stimulated neurophysin and oxytocin into the cerebrospinal fluid is controlled by mechanisms different from those that control their release into the plasma.