Septal connections with identified oxytocin and vasopressin neurones in the supraoptic nucleus of the rat. An electrophysiological investigation

Electrophysiological analysis of the morphofunctional organization of the limbic control of magnocellular neurosecretory nuclei in the rat hypothalamus

The effects of electrical stimulation of the ventral hippocampus, ventral subiculum, and corticomedial amygdala were used to obtain a general and comparative assessment of the organization of efferent outputs of limbic system structures to the magnocellular neurosecretory nuclei of the hypothalamus, i.e., antidromally identified neurosecretory cells and other groups of identified neurons in these nuclei and in the perinuclear zones. These studies showed that different efferent outputs of the hippocampal formation provide differential control of spike activity of neurosecretory cells in the supraoptic nucleus, with excitatory pathways from the ventral hippocampus and inhibitory pathways from the subiculum. The effects of the amygdala on neurosecretory cells of the paraventricular nucleus were shown to be excitatory, though they were less significant than the excitatory and inhibitory effects of the hippocampus. It was demonstrated that in general, the effect of limbic structures are addressed predominantly to cells which do not project to the posterior lobe of the hypophysis. Projections were mostly to interneurons, which, as convergence sites for excitatory influences both from limbic structures and neurosecretory cells, may thus be responsible for the involvement of the latter in integrative brain functions.

Electrophysiological and morphological characteristics of neurons in perinuclear zone of supraoptic nucleus

Electrophysiological and morphological characteristics of neurons in perinuclear zone of supraoptic nucleus. J. Neurophysiol. 78: 2427-2437, 1997. Neurons in the perinuclear zone (PZ) of the supraoptic nucleus (SON) are thought to serve as interneurons and may mediate changes in neurohypophysial hormone release in response to physiological changes in blood pressure. However, the morphology and electrophysiological characteristics of PZ neurons are unknown. In the present study, PZ neurons from male and female rats were recorded intracellularly to determine some membrane properties, then filled with biocytin or biotinamide for morphological analysis. In general, PZ neurons had faster spikes than magnocellular SON neurons, and the great majority were characterized by a subthreshold depolarizing hump when depolarized from a hyperpolarized (less than -80 mV) membrane potential. In most neurons, this hump was similar to low-threshold spikes described in other CNS regions. Near-threshold, fast action potentials were clustered near the onset of these depolarizations. Conspicuously absent in all PZ neurons was the strong transient and subthreshold outward rectification characteristic of vasopressin and oxytocin neurons of the SON. These results suggest that PZ neurons are electrophysiologically distinct from neurosecretory neurons of the SON. No differences were found between male and female rats in any of the basic properties examined, including input resistance, membrane time constant, spike height, spike width, spike threshold, and the size of the spike afterhyperpolarization. Morphologically, PZ neurons were diverse but were divided into spiny and aspiny groups. Three spiny neurons and one aspiny neuron contributed an axonal projection to the SON characterized by varicosities suggestive of terminals. In the case of the three spiny neurons, the SON projection was clearly a minor collateral projection. The axon arborized in the PZ, but one or more branches were cut at the edge of the explant, indicating a longer projection. In the remaining neurons, no axonal projection to the SON was detected and several had axons leaving the explant. Some portion of the dendritic tree penetrated the SON in several neurons. The morphology of PZ neurons was thus heterogeneous and suggests that, for some cells at least, the projection to the SON may be a minor collateral component of a much wider axonal projection.

Physiological mapping of local inhibitory inputs to the hypothalamic paraventricular nucleus

Local inhibitory synaptic inputs to neurons of the rat hypothalamic paraventricular nucleus (PVN) were studied by using glutamate microstimulation and conventional intracellular and whole-cell patch-clamp recording in coronal, horizontal, and parasagittal slices of rat hypothalamus. PVN cells were classified as magnocellular or parvocellular neurons on the basis of electrophysiological and post hoc immunohistochemical analyses; GABA-producing neurons were localized with in situ hybridization. Glutamate microstimulation of different sites around the PVN evoked volleys of postsynaptic potentials in 43% of the PVN neurons tested. Some responses to stimulation at each site were blocked by bicuculline, suggesting that they were mediated by the activation of presynaptic GABA neurons. In the coronal plane, presynaptic inhibitory sites were located lateral to the PVN and ventral to the fornix, corresponding to the lateral hypothalamic area and the posterior bed nucleus of the stria terminalis (BNST). In the horizontal plane, presynaptic inhibitory sites were found rostral, lateral, and caudal to the nucleus, corresponding to parts of the anterior hypothalamic area, the posterior BNST, the medial preoptic area, and the dorsomedial hypothalamus. In the parasagittal plane, presynaptic inhibitory neurons were revealed at sites rostral and caudal to the nucleus, corresponding to the medial preoptic area and the dorsomedial hypothalamus, and in a site dorsal to the optic chiasm that included the suprachiasmatic nucleus. These presynaptic sites each contained GABA-producing neurons based on in situ hybridization with a glutamic acid decarboxylase riboprobe and together formed a three-dimensional ring around the PVN. Unexpectedly, both magnocellular and parvocellular neurons received inhibitory synaptic inputs from common sites.

Putative excitatory amino acid projections to the suprachiasmatic nucleus in the rat

Retrograde axonal transport of the select neuronal tracer [3H]D-aspartate was used to demonstrate possible sources of excitatory input to the suprachiasmatic nucleus (SCN) in the albino rat. Following injection of [3H]D-aspartate into the SCN, neurons were retrogradely labeled in the infralimbic cortex, the lateral septal nucleus, the paraventricular thalamic nucleus, the medial preoptic area, the ventromedial, dorsomedial and posterior hypothalamic nuclei, the zona incerta, the intergeniculate leaflet and the ventral subiculum. Retinal ganglion cells, which project to the SCN and use glutamate as a neurotransmitter, were not labeled in our [3H]D-aspartate experiments, demonstrating a limitation of this method (i.e., false negatives). Our results show that the [3H]D-aspartate neuronal tracer labels a subset of areas known to project to the SCN, indicating these areas as likely sources of excitatory input to the SCN.

Vasopressin in the brain of a desert hibernator, the jerboa (Jaculus orientalis): presence of sexual dimorphism and seasonal variation

The distribution of vasopressin innervation in the brain of the jerboa (Jaculus orientalis) was investigated, with special attention to sex differences and seasonal variations. Vasopressin perikarya were observed in the paraventricular and supraoptic nuclei, the suprachiasmatic nucleus, the periventricular nucleus, the medial preoptic area, the bed nucleus of the stria terminalis, and the medial amygdaloid nucleus. In addition, vasopressin cell bodies were observed in the ventral retrochiasmatic area. After treatment with colchicine, vasopressin perikarya were also observed around the organum vasculosum laminae terminalis, in the medial diagonal band of Broca, and in the dorsal medial preoptic nucleus. Vasopressin fibers were also found to be more widespread in the jerboa brain than in other rodents. Fibers were observed in the medial diagonal band of Broca, the stria medullaris, the tuber cinerum, the area postrema, the medial vestibular nucleus, and the dorsal motor nucleus of the vagus. Sexual dimorphism and seasonal variation in vasopressin immunoreactivity were observed in areas that not only showed a testosterone-dependent vasopressin innervation in other rodents but also in the paratenial and mediodorsal thalamic nuclei, the tuber cinerum, the supramammillary complex, the zona incerta, the interpeduncular complex, and the dorsal and medial raphe nuclei. A denser vasopressin innervation was observed in spring/summer (sexual active period) than in autumn. Numerous brain structures contained vasopressin receptors (cerebral cortex, hypothalamus, substantia nigra, dentate gyrus, thalamic nuclei, superior colliculus, dorsal cochlear nucleus, and cerebellum); no sex- or season-related differences were observed. These data indicate a high level of vasopressin in the jerboa brain, which may reflect an adaptation to its harsh bioclimatic environment.

Local origins of some GABAergic projections to the paraventricular and supraoptic nuclei of the hypothalamus in the rat

Axonal transport and immunohistochemical methods were used to characterize the organization of glutamic acid decarboxylase-immunoreactive (GAD-ir) projections to the paraventricular (PVH) and supraoptic (SO) nuclei of the hypothalamus in the rat. In line with prior reports, GAD-ir varicosities were found to be densely and quite uniformly distributed throughout the hypothalamus, including the PVH and the SO. Nonetheless, the periventricular part of the PVH was consistently found to contain a disproportionately high density of GAD-ir elements. Small crystalline implants of the retrograde tracer, true blue, into the PVH labeled GAD-ir cells in the anterior perifornical region, portions of the anterior hypothalamic area immediately ventral to the PVH, a region just dorsal to the rostral SO and extending caudomedially over the optic chiasm and tract, and within the anterior one-third of the PVH itself. Because possible uptake of retrograde tracer by local dendritic processes might have yielded false positive filling of nearby GAD-ir cells, anterograde transport, Phaseolus vulgaris-leucoagglutinin, and combined anterograde transport-immunohistochemical methods were used to attempt to confirm these four putative local sources of GAD-ir inputs. Tracer injections in each of the above mentioned regions labeled sparse to moderate axonal projections to the PVH, which ramified preferentially in the parvicellular division of the nucleus. Projections to the magnocellular division of the PVH and the SO were generally sparse and inconsistently observed in this material. A variable, and generally small, proportion of anterogradely labeled axons and terminals in the PVH also displayed GAD-ir. These results suggest that GABAergic projections to visceromotor cell types in the PVH and SO arise, at least in part, from several diffusely distributed local sources. The fact that these afferents were found to terminate preferentially in the parvicellular division of the PVH makes it likely that additional sources of GABAergic projections to the magnocellular neurosecretory system remain to be identified. Peri- and intranuclear GABAergic neurons could provide an intermediary by which documented (and generally inhibitory) limbic system influences on neuroendocrine function are exerted.

Electrical activity of neurons in the ventrolateral septum and bed nuclei of the stria terminalis in suckled rats: statistical analysis gives evidence for sensitivity to oxytocin and for relation to the milk-ejection reflex

Our previous results obtained by lesioning or stimulating the ventrolateral part of the lateral septum and the bed nuclei of the stria terminalis suggested that this area is involved in the control of milk ejection pattern in rats. The present study was undertaken with the aim of testing ventrolateral part of the lateral septum-bed nuclei of the stria terminalis neurons as a putative link of the neuronal network controlling the bursting activity of oxytocin neurons in suckled lactating rats (anaesthetized with urethane). Ventrolateral part of the lateral septum-bed nuclei of the stria terminalis neurons were recorded simultaneously with hypothalamic oxytocin neurons in either the paraventricular or supraoptic nucleus in rats with (n = 26) or without (n = 29) periodic milk ejections. Analysis of their firing pattern enabled differentiation of two subgroups: type I, characterized by numerous high frequency spikes, often grouped in clusters; and type II with very few or no high frequency clusters of spikes. The probability density function of the interspike intervals of both patterns could be modelled using a mixture of two log-normal distributions, the parameters of which differed significantly. The presence of absence of milk ejections did not influence the overall mean level of activity (2.0 +/- 0.5 and 1.9 +/- 0.4 spikes/s, respectively). However, the characteristics of the type I firing pattern were affected by the presence of the milk-ejection reflex. The average level of activity was not always constant and 16/55 ventrolateral part of the lateral septum-bed nuclei of the stria terminalis neurons displayed cyclical activity (from 0.6 +/- 0.2 to 4.0 +/- 0.5 spikes/s) both in the presence (n = 8) and absence (n = 8) of the milk-ejection reflex. In five of eight neurons recorded during milk-ejection reflex, the cycles in firing were clearly correlated with the bursting of oxytocin neurons. These five neurons exhibited the type I firing pattern. The three remaining neurons and the eight neurons recorded in the absence of milk-ejection reflex displayed the type II firing pattern. Oxytocin (1-2 ng = 0.45-0.9 mU) was injected into the third ventricle (i.c.v.) in order to examine the possible involvement of ventrolateral part of the lateral septum-bed nuclei of the stria terminalis neurons in the facilitatory effect of oxytocin on the reflex.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurohypophysial responses to emotional stress after deafferentation of sino-aortic baroreceptors in rats

In an attempt to test the possibility that sino-aortic baroreceptors may mediate the previously reported stress response in hypothalamic magnocellular neurosecretory cell activity in rats, effects of deafferentation of sino-aortic baroreceptors on plasma levels of vasopressin and oxytocin after fear-related emotional stress were studied in male rats 28-33 days after the surgery. An alpha 1-adrenergic receptor agonist, phenylephrine (1 mg/kg) injected i.p. under anesthesia increased arterial blood pressure in the rats that had received surgical operation of sino-aortic denervation (SAD) and in the rats of sham-operation control (SHAM). Reflex bradycardia after phenylephrine occurred in the SHAM but not in the SAD group. These results indicate that afferent signals originating from sino-aortic baroreceptors were effectively blocked by the SAD surgery. In the similarly prepared SAD group, plasma level of vasopressin was decreased and plasma level of oxytocin was increased significantly to the same extent as in the SHAM group after low-frequency shocks (0.05 Hz, 5 min) or environmental cue signals previously paired with shocks. It is therefore suggested that afferent neural signals originating from sino-aortic baroreceptors are not primarily involved in the suppressive vasopressin or the facilitatory oxytocin response to fear-related emotional stress in rats.

Further Evidence that the Septum is not part of the Main Pathway of the Milk Ejection Reflex in the Rat

Abstract The possible role of the medial septum in the control of oxytocin release and of the milk ejection reflex induced by suckling was investigated in lactating rats by using electrical stimulations and lesions. In anaesthetized animals, brief electrical stimulation of the medial septum at 5 to 50 Hz elicited a single brief milk ejection similar to natural reflex milk ejections, whereas prolonged low frequency stimulations (5 to 10 Hz) induced a prolonged inhibition of the reflex. In acute experiments under anaesthesia, lesions of the medial septum did not impair the amplitude and pattern of reflex milk ejections. In chronic experiments, lesions of the medial septum resulted first in a loss of body weight of the mothers and a parallel reduction in growth of the litters. After a few days, the litters gained weight normally, and the pattern of milk ejections was normal. Thus, the pathways which pass through or originate from the medial septum and which are excitatory for oxytocin release appear not to be involved in the regulation of the milk ejection reflex. In view of these results and those from our previous study on the lateral septum, we conclude that the whole septum is not essential to the milk ejection reflex. However, the effects of septal stimulation suggest that the medial and lateral septum may be involved in a secondary neural circuitry that can inhibit the reflex.

Oxytocin in the bed nucleus of the stria terminalis and lateral septum facilitates bursting of hypothalamic oxytocin neurons in suckled rats

Abstract Several regions of the forebrain possess high densities of oxytocin (OT)-binding sites including the bed nucleus of the stria terminalis (BST) and lateral septum (LS). In order to examine whether these regions participate in the central facilitation of the milk ejection reflex by OT, microinjections of OT (1 ng in 100 nl containing Janus Green dye) were made into the BST (13 tests) or LS (9 tests) of anaesthetized, suckled rats, while recording the electrical activity of OT neurons in the contralateral supraoptic nucleus. Histological localization of injection sites using Janus Green demonstrated that all BST injections were close to the anterior commissure, and LS injections were all located in the ventral division of the LS. Film autoradiographic visualization of OT-binding sites (in 7 tests using [(125)I]OT antagonist) confirmed that the BST and LS injections were located within regions of high OT binding. Injections into both regions facilitated the milk ejection reflex by increasing either the frequency and/or amplitude of OT neuron bursts, or by triggering bursts in animals that previously had shown no milk ejection responses; the mean number of milk ejections in the 30 min before and after injection increasing from 1.6.0.5 to 3.6.0.5 for BST and from 1.5.0.6 to 3.9.0.4 for LS. The OT microinjections had a more variable effect on background activity of OT neurons, increasing firing in some cases and not in others. This facilitatory effect was similar to that induced by microinjections into the lateral ventricle, but was smaller and delayed compared to that induced by injection into the third ventricle (9 tests), possibly due to unilateral activation of target sites. The facilitatory effect was unlikely to have been due to diffusion of OT into the ventricle, since injections into control sites (striatum and thalamus) at similar distances from the ventricle (9 tests) had no facilitatory effect (number of bursts during 30 min before and after injection; 2.2.0.5 and 1.8.0.5, respectively). These data suggest that limbic structures (BST and LS) participate in the action of central OT on the pattern of milk ejections in the suckled rat.

Efferent projections from the lateral septal nucleus to the anterior hypothalamus in the rat: a study combining Phaseolus vulgaris-leucoagglutinin tracing with vasopressin immunocytochemistry

The tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected into the lateral septum of the rat at different rostrocaudal locations to study the efferent septal projections to the anterior hypothalamus. For spatial correlation of these septofugal elements with the vasopressinergic system a dual immunocytochemical technique was used (i) to demonstrate nerve fibers and their corresponding bouton-like structures labeled with the tracer, and (ii) to identify vasopressin in the same section. The hypothalamic paraventricular and supraoptic nuclei, the accessory hypothalamic magnocellular system, and the suprachiasmatic nucleus are recipients of PHA-L-labeled fibers from all parts of the lateral septum. Close appositions between (i) these axons and their varicosities, and (ii) vasopressin-immunoreactive perikarya and their processes, putatively indicating functional interrelationships, were observed in all these nuclear areas, especially in their neuropil formations.

Electrophysiological investigation of the hippocampal projections to the neurosecretory cells of the supraoptic nucleus of the rat hypothalamus

The impulse activity of antidromically identified neurosecretory cells of the supraoptic nucleus of the hypothalamus of rats in response to stimulation of the ventral hippocampus was investigated. Short-latency phasic excitation reactions were identified, and inhibition reactions were not found. The presence of excitatory synaptic inputs from the hippocampus to other neurons of the nucleus and of the perinuclear zone, which are predominant by comparison with analogous projections to the neurosecretory cells, was demonstrated. The features of limbic-hypothalamic relationships are discussed in the context of afferent control of the activity of the neurosecretory cells.

Evidence for a functional and anatomical relationship between the lateral septum and the hypothalamus in the control of flank marking behavior in Golden hamsters

Golden hamsters with established dominant/subordinate relationships communicate their social status by rubbing pheromone-producing flank glands against objects in the environment. This behavior, called flank marking, is controlled by vasopressin-sensitive neurons localized to the anterior hypothalamus. Vasopressinergic magnocellular neurons in the nucleus circularis and medial aspect of the supraoptic nucleus are thought to be a source of neurotransmitter for the initiation of flank marking. The present study was undertaken to examine the extrahypothalamic control of flank marking. The anatomical and functional connections between the lateral septum and the vasopressin-containing nuclear groups in and around the anterior hypothalamus were examined by: (1) tracing afferent and efferent connections following microinjection of horseradish peroxidase and Phaseolus vulgaris-leucoagglutinin into the lateral septum, and (2) recording odor-induced flank marking prior to and following ibotenate lesions in the septum. The greatest number of perikarya retrogradely labeled with horseradish peroxidase were found lateral to the anterior hypothalamus and ventral to the fornix in the area of the lateral hypothalamus. The vasopressin-containing nuclear groups, e.g., paraventricular, supraoptic, suprachiasmatic nuclei, and the nucleus circularis, were devoid of labeled perikarya. Nerve terminals anterogradely labeled with Phaseolus vulgaris-leucoagglutinin were primarily localized to the anterior hypothalamus, in and around the nucleus circularis, and the medial aspect of the supraoptic nucleus. The lateral aspect of the supraoptic nucleus was devoid of nerve terminals as were the paraventricular and suprachiasmatic nuclei. The anatomical connections between the lateral septum and the hypothalamus appear to be necessary for the control of flank marking, since the microinjection of ibotenate into this limbic site significantly reduced odor-induced flank marking as compared to control microinjections of 0.9% NaCl.

Afferent connections of the rat's supraoptic nucleus

Neurons projecting to the supraoptic nucleus (SON) have been identified following stereotaxic injections of either horseradish peroxidase or fast blue into the SON region of adult rats. The subfornical organ, median preoptic nucleus, organum vasculosum of the lamina terminalis and medial septal nucleus were the source of the largest numbers of supraoptic-projecting neurons. Several smaller projections also originate from the ipsilateral locus coeruleus, preoptic area, lateral parolfactorial area, dorsomedial nucleus of the hypothalamus, lateral parabrachial nucleus and ventrolateral medulla. Several other areas appeared to project only to the region immediately dorsal to the SON: lateral septal nucleus, diagonal band of Broca, ventral tegmental nucleus, and the supramamillary nucleus. These areas may influence SON neurosecretory function by way of interneurons found immediately dorsal to SON. Additional areas were identified with retrograde fluorescent label only, and these projected to the area immediately dorsal to SON and/or to SON itself.

Diagonal band projection towards the hypothalamic supraoptic nucleus: light and electron microscopic observations in the rat

Supraoptic nucleus (SON) neurons receive a prominent gamma-amino-butyric acid (GABA) input. This study evaluated the hypothesis, partly on the basis of recent electrophysiological data, that this innervation might arise from GABAergic neurons located in the ventral diagonal band of Broca area. For retrograde transport studies, pentobarbital-anesthetized male Long-Evans rats received 0.03-0.20-microliter injections of a suspension of rhodamine tagged latex microspheres into the SON. In two cases where such injections were confined to the SON, less than 60 retrogradely labeled neurons were detected in the ipsilateral diagonal band. In three animals where injections extended into the perinuclear zone around the SON, more than 2,000 retrogradely labeled cells were counted in the ipsilateral diagonal band. For anterograde transport studies, another group of animals received either 30% horseradish peroxidase (HRP) in 0.5% poly-L-ornithine (0.05-0.10 microliter injections) or Phaseolus vulgarus (iontophoresed from a 2% solution) into the diagonal band. After survivals of 18-24 hours (HRP) or 5 days (PHAL-L) labeled axon terminals invested the perinuclear zone above the SON. The presence of just a single fiber within the nucleus indicated a minor projection to the SON itself. The HRP-injected material was processed for ultrastructural examination and revealed dense HRP-labeled axon terminals in this perinuclear zone, most often (98%) forming axodendritic appositions. A postembedding colloidal gold technique to visualize GABA-synthesizing terminals revealed that fewer than 5% of these perinuclear HRP-labeled terminals also exhibited GABA-like immunoreactivity. Within the SON, where GABAergic axon terminals are abundant, few (less than 5%) GABAergic terminals contained HRP.(ABSTRACT TRUNCATED AT 250 WORDS)

Supraoptic nucleus afferents from the main olfactory bulb--II. Intracellularly recorded responses to lateral olfactory tract stimulation in rat brain slices

To establish the functional nature of the anatomically demonstrated main olfactory bulb inputs to the supraoptic nucleus, electrophysiological responses of intracellularly recorded supraoptic neurons to lateral olfactory tract stimulation were recorded in horizontal slices of basal forebrain and hypothalamus. A total of 71 synaptically influenced neurons were studied in slices from adult rats of both sexes. Of these, 60 cells (84%) were monosynaptically activated by olfactory tract stimulation; seven cells (10%) were activated via polysynaptic pathways; and four cells (6%) were characterized by long latency inhibitory responses. Lucifer Yellow was injected into 64 cells and subsequent immunocytochemical identification of 44 of these neurons showed that both oxytocin and vasopressin cells, in approximately equal numbers, were excited by olfactory stimulation. Polysynaptically mediated excitation, however, was only associated with oxytocin cells (six of the six identified cells). These results corroborate anatomical tract tracing data showing main olfactory bulb efferents to both supraotic neurons and to neurons of the perinuclear zone. Also supported are earlier speculations of olfactory participation in release of oxytocin and vasopressin during various physiological states.

Oxytocin release evoked by electrical stimulation of the medial forebrain in the rat: analysis of stimulus parameters and supraoptic neuronal activity

The role of the medial forebrain area (vertical limb of the diagonal band, medial septum and medial nucleus accumbens) in the control of oxytocin secretion in lactating rats was investigated. Electrical stimulation of the medial forebrain evoked a reproducible rise in intramammary pressure, equivalent to that caused by i.v. injection of 1 mU oxytocin. No pressor effect accompanied this response. Radioimmunoassay of plasma samples showed that stimulation caused a significant rise in the concentration of circulating oxytocin. The effects of changing the parameters of stimulation to the medial forebrain were compared with those evoked by stimulation of the neural stalk. The optimal frequency for stimulation of the forebrain was found to be four-fold lower (10-20 Hz) than that for stimulation of the neural stalk (50 Hz). During continuous prolonged stimulation of the forebrain (20 Hz; 2 min) only a single transient response was obtained, whereas a protracted response was obtained as a result of prolonged stimulation of the stalk. Recordings were made from antidromically identified neurosecretory cells in the supraoptic nucleus. Electrophysiological responses to electrical stimulation of the medial forebrain were characterized by two main features. (1) Single-pulse stimulation produced only a small excitation (one or two action potentials), while high-frequency trains produced a profound facilitation of this response, with each pulse evoking short-duration 'bursting' behaviour in the supraoptic neurons. (2) During long trains of stimulation this frequency-dependent facilitation declined and could only be renewed after a period of rest.(ABSTRACT TRUNCATED AT 250 WORDS)

Coordinate hormonal and synaptic regulation of vasopressin messenger RNA

Previous studies have shown that adrenalectomy augments arginine vasopressin (AVP) messenger RNA levels in the adult paraventricular nucleus. It is now demonstrated that unilateral lesions in the lateral septal nucleus enhance the adrenalectomy-induced expression of AVP mRNA. This effect was entirely ipsilateral to the lesion and most prominent in the rostral paraventricular nucleus and related nuclei. Moreover, AVP and AVP mRNA were found to be colocalized with oxytocin in a few neurons. These results indicate that mRNA expression is modulated by synaptic influences and raise the possibility that synaptically mediated selection of neuronal phenotypes is a dynamic feature of the mature central nervous system.

A study of ionic conductances involved in plateau potential activity in putative vasopressinergic neurons in primary cell culture

Vasopressin neurons in hypothalamic cell culture display regenerative calcium-dependent plateau potentials. The present study was undertaken to investigate the mechanisms underlying their time course and periodicity. Intracellular recordings showed that the duration of the plateaux was controlled by a progressive activation of a voltage- and calcium-dependent potassium conductance, but not by a progressive inactivation of calcium conductances. The refractory period appeared to be due to a calcium-dependent potassium conductance activated by membrane potential depolarization.

Oxytocinergic innervation of the brain of the garden dormouse (Eliomys quercinus L.)

The oxytocinergic innervation of the brain of the garden dormouse (Eliomys quercinus L.) was studied by means of immunocytochemistry. In contrast to the sparse oxytocin innervation of the rat forebrain, dense fibre networks in various cortical and limbic brain areas were demonstrated in this animal. These include, e.g., the prefrontal cortex, the claustrum, the septum, and the hippocampus. A very dense innervation was also seen in the caudal regions of the garden dormouse brain; these regions are already known to have a relatively dense oxytocin fibre network in the rat. A dense innervation of oxytocin fibres is seen in several brain regions which, in the rat, have oxytocin binding sites but no visible oxytocin innervation. This discrepancy suggests that the differences in the oxytocinergic innervation of these two rodent brains may be due to an oxytocin system in the rat brain that is more difficult to detect immunocytochemically.

Activation of subfornical organ efferents stimulates oxytocin secretion in the rat

The effects of activation of subfornical organ (SFO) efferents on plasma oxytocin concentrations were examined in conscious freely moving male Sprague-Dawley rats. Blood samples were obtained through chronically implanted atrial catheters and SFO efferents were activated electrically using chronically implanted bipolar stimulating electrodes. Hormone concentrations were measured by radioimmunoassay, and experimental animals were assigned to one of 3 experimental groups according to histologically verified anatomical locations of stimulating electrodes in either the SFO, the hippocampal commissure (HC), or the medial septum (MS). Electrical stimulation in the SFO resulted in increased plasma concentrations of oxytocin from control values of 2.54 +/- 0.9 pg/ml, to a post-stimulation level of 65.6 +/- 27.0 pg/ml. In contrast, stimulation in immediately adjacent structures including HC and MS was found to be without effect on plasma concentrations of oxytocin. These studies provide the first definitive evidence that SFO efferents may play a significant role in controlling the secretion of oxytocin from the posterior pituitary.

Effects of septal and hippocampal stimuli on paraventricular nucleus neurons

The electrical activity of 125 neurons within the hypothalamic paraventricular nucleus was recorded in urethan-anaesthetized male rats. Spontaneous activity of the cells and their responses following electrical stimuli delivered to the ipsilateral lateral septum and dorsal hippocampus were recorded. The mean firing rate of all the cells recorded was 3.5 +/- 0.4 Hz and the majority were located within the dorsal and medial components of the paraventricular nucleus. Forty-six percent of the cells were inhibited following stimulation of the lateral septum (onset, 22.8 +/- 6.7 ms; offset, 195.1 +/- 28.5 ms). Inhibitory responses to dorsal hippocampus stimulation were recorded from 44% of all cells (onset, 28.1 +/- 4.7 ms; offset, 180.7 +/- 28.7 ms). Stimulation of both sites caused excitation of equal proportions (26%) of the cells tested (lateral septum onset, 47.7 +/- 4.5 ms; offset, 64.8 +/- 6.6 ms; dorsal hippocampus onset, 48.7 +/- 5.6 ms; offset, 72.3 +/- 8.8 ms). Of the sub-population of cells identified as projecting to the median eminence, inhibition was recorded from 50% following lateral septum stimulation and 43% following dorsal hippocampus stimulation, excitatory responses being recorded from only 9% of cells tested. The excitatory responses were only recorded from phasically firing, vasopressin-secreting cells identified as projecting to the median eminence, and also to the neurohypophysis. Following stimulation of either site, more phasic cells were excited whilst only few were inhibited. Continuously active cells, identified as projecting to the neurohypophysis, showed more mixed responses following stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurophysiology of a central baroreceptor pathway projecting to hypothalamic vasopressin neurons

Controversy exists as to the neural network whereby peripheral arterial baroreceptor information is transmitted to vasopressin (VP)-secreting neurons of the hypothalamic supraoptic nucleus (s.o.n.). In vivo electrophysiological studies in the rat were undertaken to characterize the selective depression of VP cell activity consequent to activation of peripheral baroreceptors. Electrical stimulation of the diagonal band of Broca (DB) in the rat evoked a similar selective inhibition of vasopressinergic neurons of the s.o.n. Local application of bicuculline, a GABA antagonist, abolished both the DB-evoked and baroreceptor-induced inhibition of VP-secreting neurons. In addition, recordings from DB neurons antidromically activated from the s.o.n. displayed an increase in firing consequent to baroreceptor activation, coinciding with the suppression of firing in s.o.n. VP neurons. These observations collectively indicate that an intrinsic GABA projection arising in the DB cell group selectively inhibits vasopressinergic neurons of the s.o.n. and that this pathway mediates peripheral arterial baroreceptor activity that influences the release of VP in the neurohypophysis. These data may be of critical importance in our understanding the etiology of those forms of experimental hypertension where abnormalities in central baroreceptor pathways have been implicated but not proven.

A gamma-aminobutyric-acid-mediated baroreceptor input to supraoptic vasopressin neurones in the rat

1. Extracellular recordings in pentobarbitone anaesthetized male Long-Evans rats examined the influence of electrical stimulation in the diagonal band of Broca on the excitability of 113 putative vasopressin-secreting and 22 putative oxytocin-secreting neurosecretory neurones in the hypothalamic supraoptic nucleus. 2. Single pulse or repetitive (5-20 Hz) stimulation in the ventral part of the diagonal band evoked a prominent reduction in the excitability of 83% of vasopressin-secreting neurones with no effect on the remainder. Amongst oxytocin-secreting neurones, 59% were unresponsive, 27% responded with an increase in activity while only 14% revealed an inhibitory pattern similar to vasopressin-secreting neurones. 3. Diagonal band stimulation-evoked inhibitions were reversibly abolished by local pressure applications of bicuculline methiodide (100 microM) to twenty out of twenty vasopressin secreting cells tested, whereas strychnine sulphate (100 microM) was without effect on four out of four cells tested. 4. In five out of five vasopressin-secreting cells tested, bicuculline applications reversibly abolished the reduction in their activity that follows peripheral baro-receptor activation. Failure to alter baroreflex-evoked depressions in firing during similar trials with prazosin hydrochloride (10 microM, six cells tested), timolol maleate (20 microM, six cells tested) or strychnine sulphate (100 microM, three cells tested) indicated the specificity of bicuculline's action. 5. These findings suggest that a GABAergic pathway from the diagonal band of Broca preferentially innervates vasopressin-secreting neurosecretory supraoptic nucleus (s.o.n.) neurones, and support the view that the baroreflex-induced depression in firing of s.o.n. vasopressin-secreting neurones is mediated in large part through this input.

Electrophysiology of limbic forebrain and paraventricular nucleus connections

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

Septal release of vasopressin in response to osmotic, hypovolemic and electrical stimulation in rats

The central release of vasopressin was studied in anesthetized rats using push-pull perfusions and radioimmunoassay of the hormone. A basal release was observed in the lateral septum and in the lateral ventricle, whereas no vasopressin was detected in the perfusates from the caudate nucleus. Under osmotic stimulation, vasopressin release increased up to 12 and 60 times basal levels following i.p. injections of 5 ml and 10 ml/kg b.wt. of 2 M NaCl, respectively. This increase was blocked by using a calcium-free perfusion medium containing 0.1 mM EGTA. In the lateral ventricle, osmotic stimulation (5 ml/kg of 2 M NaCl i.p.) had the same effect as in the septum. In the caudate nucleus, no release was observed. Hemorrhage also increased the septal release of vasopressin in 5 out of 6 animals tested. Electrical stimulation of the pituitary stalk and of the supraoptic nucleus was used to evoke the release of vasopressin into the bloodstream. Septal release slightly decreased during pituitary stalk stimulation, whereas it did increase during stimulation of the supraoptic region. Our results show that systemic stimuli for vasopressin release evoke both a peripheral and a septal release of the hormone. The dissociation of the effects of electrical stimulation of the pituitary stalk and of the supraoptic nucleus suggests, however, that the vasopressinergic neurones responsible for septal release are distinct from those which project to the neurohypophysis.

Immunocytochemical analysis of the GABAergic innervation of oxytocin- and vasopressin-secreting neurons in the rat supraoptic nucleus

Antisera specific for gamma-aminobutyric acid (GABA) or its biosynthetic enzyme, glutamate decarboxylase, were used in pre- and postembedding immunocytochemical techniques at the light and electron microscopic levels, to visualize the GABAergic innervation of the hypothalamic supraoptic nucleus. Immunostaining for glutamate decarboxylase or gamma-aminobutyric acid were also combined with oxytocin and vasopressin immunolocalization, thereby permitting evaluation of the contribution of the innervation onto each type of neuron in this nucleus. Light microscopy of semithin plastic sections or vibratome slices stained for glutamate decarboxylase or gamma-aminobutyric acid, with peroxidase-antiperoxidase as immunolabel, revealed an extensive punctate labeling in the supraoptic nucleus and its immediate surroundings. Quantitative analysis of glutamate decarboxylase immunostaining in semithin sections indicated a comparable density of immunopositive punctae at the anterior and posterior levels of the nucleus (14-27 X 10(6) per mm3 tissue). Glutamate decarboxylase- or gamma-aminobutyric acid-immunoreactive cell bodies were never observed within the nucleus although they were detected in the hypothalamus immediately dorsolateral to the nucleus. Electron microscopy of vibratome slices treated with antiglutamate decarboxylase or antigamma-aminobutyric acid and peroxidase-antiperoxidase, or of ultrathin sections stained directly with antigamma-aminobutyric acid and immunoglobulin-coupled colloidal gold, showed that the immuno-reactive punctae represented, in the main, axonal terminals. They invariably contained small, rounded clear vesicles and, at times, one or two larger, dense cored vesicles; they all formed symmetrical synapses onto magnocellular cell bodies and dendrites. Oxytocin and vasopressin neurons were contacted in a similar fashion by glutamate decarboxylase- or gamma-aminobutyric acid-positive boutons in semithin sections of the nucleus stained simultaneously for glutamate decarboxylase and oxytocin and in ultrathin sections stained for glutamate decarboxylase or gamma-aminobutyric acid and oxytocin or vasopressin. Glutamate decarboxylase- or gamma-aminobutyric acid-positive terminals often formed synapses onto two postsynaptic elements in the same plane of section ("double" synapses), a synaptic configuration usually encountered in supraoptic nuclei of lactating animals. In such cases, the postsynaptic somata were oxytocinergic.(ABSTRACT TRUNCATED AT 400 WORDS)

Extra-hypothalamic afferent inputs to the supraoptic nucleus area of the rat as determined by retrograde and anterograde tracing techniques

To detect neuronal cell bodies whose axon projects to the hypothalamic supraoptic nucleus, small volumes (10-50 nl) of 30% horseradish peroxidase or 2% fast blue solutions were pressure-injected into the area of one supraoptic nucleus of rats. Both dorsal and ventral approaches to the nucleus were used. In animals where the injection site extended beyond the limits of the supraoptic nucleus, retrogradely labelled cell bodies were found in many areas of the brain, mainly in the septum, the nucleus of the diagonal band of Broca and ventral subiculum in the limbic system; the dorsal raphe nucleus, the locus coeruleus, the nucleus of the dorsal tegmentum, the dorsal parabrachial nucleus, the nucleus of the solitary tract and the catecholaminergic A1 region in the brain stem; in the subfornical organ and the organum vasculosum of the lamina terminalis, as well as in the median preoptic nucleus. In contrast, when the site of injection was apparently restricted to the supraoptic nucleus, labelling was only clearcut in the two circumventricular organs, the median preoptic nucleus, the nucleus of the solitary tract and the A1 region. Injections of wheat germ agglutinin coupled with horseradish peroxidase (60-80 nl of a 2.5% solution) made in the septum and in the ventral subiculum anterogradely labelled fibers coursing in an area immediately adjacent to the supraoptic nucleus but not within it. In contrast, labelling within the nucleus was found following anterograde transport of tracer deposited in the A1 region and in an area that includes the nucleus of the solitary tract. Neurones located in the perinuclear area were densely labelled by small injections into the supraoptic nucleus; they may represent a relay station for some afferent inputs to the supraoptic nucleus. These results suggest that the supraoptic nucleus is influenced by the same brain areas which project to its companion within the magnocellular system, the paraventricular nucleus.

Influence of lateral septum and amygdala stimulation on the excitability of hypothalamic supraoptic neurons. An electrophysiological study in the rat

Extracellular recordings were obtained from 116 phasically-active (putative vasopressinergic) and 113 continuously-active (putative oxytocinergic) neurosecretory neurons in the hypothalamic supraoptic nucleus of urethane or pentobarbital anesthetized male Sprague-Dawley rats. Single 1 Hz pulse stimulation in most regions of the amygdala and the ipsilateral lateral septum was followed by a transient (20-140 ms) reduction in the excitability of more than 90% of responsive cells; one third displayed a reduction in excitability to both amygdala and lateral septum stimulation. Amygdala or lateral septum stimuli delivered in brief trains of 20-100 pulses at 5-20 Hz during ongoing phasic discharges could induce silent periods lasting up to 30 or more seconds beyond the time of application. The same stimuli also reduced ongoing activity among continuously-firing SON cells but their response lasted only as long as the duration of the applied stimulus. These data suggest that neurons in both the ipsilateral septum and the various amygdaloid nuclei exert a predominantly inhibitory influence on the excitability of both vasopressinergic and oxytocinergic SON neurons in the rat.

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

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

The effects of transverse cuts caudal to the preoptic recess on the hypothalamo-neurohypophyseal neurosecretory system

Electrolytic lesions of tissue surrounding the preoptic recess (AV3V region) appear to cause loss of stimulatory input to the supraoptic nuclei from angiotensin receptors and osmoreceptors. To investigate the pathways affected by AV3V lesions, we observed the ultrastructural effects of coronal cuts in a plane caudal to the organum vasculosum lamina terminalis upon supraoptic nuclei and neural lobes of rats. Like AV3V lesions, these cuts caused degeneration of axons and terminals in the supraoptic nuclei. Degenerating terminals lay in axodendritic synapses and in axosomatic synapses on neurosecretory cells. Unlike AV3V lesions, the cuts did not result in an appearance of decreased secretory activity in the supraoptic nuclei or decreased release of hormone from the neural lobe. On the contrary, terminals in the neural lobe tended to be depleted of neurosecretory material, and glial cell processes tended to be withdrawn from the secretory interface at the basal lamina surrounding fenestrated capillaries; both are changes which have been associated with enhanced hormone release. We suggest that inhibitory input to the supraoptic nuclei is lost as a result of these cuts.

Activity patterns and osmosensitivity of rat supraoptic neurones in perfused hypothalamic explants

Extracellular recordings were obtained in vitro from supraoptic neurones in an explant of rat hypothalamus maintained viable through intravascular perfusion with artificial media. The spontaneous activity observed from 64% of cells included continuously active neurones (mean frequency 3.2 +/- 0.8 Hz) located throughout the nucleus, and phasically active neurones (8.6 +/- 0.6 Hz) located predominantly in the caudal half of the nucleus. Supraoptic neurones displayed antidromic activation (latency range 6.5-18.2 ms) following stimulation of the pituitary stalk and orthodromic excitatory or inhibitory responses following stimulation in the anteroventral third ventricular (AV3V) region. Synaptic responses were reversibly abolished during perfusion with media containing 12 mM-Mg2+. A 15-40 mosmol/l increase in the osmotic pressure of the perfusion media, through addition of NaCl, sucrose or mannitol, prompted nineteen of twenty-three cells to increase their discharge frequency. The patterns of response included the induction or simple increase in continuous firing frequency, the induction or enhancement of phasic or bursting activity and a change from continuous to phasic activity. Similar responses to an osmotic stimulus were obtained from thirteen of twenty-one supraoptic neurones, including two phasic neurones, where synaptic activity had been abolished during perfusion with 12 mM-Mg2+. Osmosensitivity appeared to be selective for supraoptic cells; no significant change in firing frequency was observed from any of six cells recorded in the lateral hypothalamus or thirteen cells recorded in the medial hypothalamus during exposure to a change in osmotic pressure of +20 to +40 mosmol/l, using either control media (seven cells) or media containing 12 mM-Mg2+ (twelve cells). These observations indicate that supraoptic neurones maintained in vitro can display spontaneous, antidromie , orthodromic and osmotically induced activity patterns identical to those observed with in vivo recordings. The persistence of a reduced osmosensitivity among supraoptic neurones in the absence of synaptic transmission indicates that although these cells can function as osmoreceptors, their osmosensitivity may be enhanced through synaptic input from adjacent neurones, possibly located in the AV3V area. The presence of phasic activity among supraoptic neurones maintained in media where synaptic transmission has been abolished suggests that the mechanisms responsible for such activity patterns are endogenous membrane properties of a subpopulation of supraoptic neurones.

The afferent pathway for carotid body chemoreceptor input to the hypothalamic supraoptic nucleus in the rat

The pathway for chemoreceptor input to hypothalamic supraoptic nuclei has been examined in anaesthetised lactating and non-lactating rats. In lactating rats, the increase in intramammary pressure following bilateral carotid occlusion, which is probably mainly due to vasopressin, was abolished by lesions in the septum, but not by lesions in more caudal regions of the hypothalamus. In non-lactating rats, electrophysiological experiments demonstrated that the input from carotid body chemoreceptors to phasically-discharging supraoptic neurones is ipsilateral only. The effects of chemoreceptor stimulation on the neurones can be mimicked by electrical stimulation within the medial preoptic area and anterior hypothalamus in a region medial and rostral to the supraoptic nuclei. Lesions within this region abolish the chemoreceptor input to the supraoptic nuclei, but leave the baroreceptor input intact. It is proposed that chemoreceptor afferents to the supraoptic nuclei pass in the lateral hypothalamus to the region of the septum where they turn medially and descent through the medial part of the rostral hypothalamus. The results are discussed in terms of the general role of the chemoreceptor reflex and, more specifically, with respect to the possible significance of vasopressin in the control of arterial blood pressure.

Synaptic input to vasopressin neurons of the paraventricular nucleus (PVN)

Following injections of horseradish peroxidase into the PVN, retrogradely filled cells were found in regions of the limbic system known to contain glucocorticoid concentrating neurons [4, 31, 44]. To determine if these regions which include the lateral septum, medial amygdala and ventral subiculum have a monosynaptic input to vasopressin neurons we developed a double label ultrastructural technique [20] to simultaneously visualize immunoreactive neuropeptide and anterogradely transported HRP. Following injections of tracer into all three of these regions, HRP labeled fibers were seen at the light microscopic level to form a halo in the perinuclear, cell poor zone around the PVN. Ultrastructural examination of this area resulted in the discovery of a small number of limbic system synapses on vasopressin dendrites. These synapses were most numerous in the ventral and medial portion of the cell poor zone. A similar pattern of innervation was seen for the supraoptic and suprachiasmatic nucleic which also contain vasopressin cells whose dendrites extend beyond the nuclear boundaries. In a similar fashion we were interested in determining the distribution of noradrenergic terminals on vasopressin neurons in the various subnuclei of the PVN. We have combined immunocytochemistry for vasopressin with radioautography for 3H-norepinephrine (NE) at the ultrastructural level. NE terminals were numerous in the periventricular zone, innervating both vasopressin containing dendrite and non-immunoreactive dendrites and cell bodies. The vasopressin dendrites could originate from cells either resident in the periventricular zone or from cells situated in more lateral subnuclei. In the main, lateral magnocellular region, noradrenergic terminals were very few in number and innervated almost exclusively non-vasopressin containing structures. These studies demonstrate the need for ultrastructural analysis of synaptic input to neurosecretory cells.

Effects of morphine and D-Ala, D-Leu enkephalin on the electrical activity of supraoptic neurosecretory cells in vitro

Experiments were performed to investigate the effects of morphine and [D-Ala2, D-Leu5]enkephalin on supraoptic cells in hypothalamic slices in vitro. To ensure the presence of a steady background activity, the cells were recorded with glutamate-filled glass microelectrodes and the level of activity was controlled by selecting a suitable retaining current (0.1-9.8 nA). Under these conditions, supraoptic cells showed either the non-phasic (65%) or phasic (35%) firing pattern previously associated with oxytocin or vasopressin cells, respectively. During perifusion of the slice with morphine (10 microM), 10 out of 17 non-phasic supraoptic cells were profoundly inhibited, five cells showed no response and the remaining 2 cells were excited. Similarly with [D-Ala2, D-Leu5]enkephalin (10 microM), 11 out of 15 non-phasic cells were inhibited, 3 cells showed no response and 1 cell was excited. The inhibition produced by morphine or [D-Ala2, D-Leu5]enkephalin could be reversed by concomitant application of naloxone (10 microM). In contrast to the profound effects seen in the non-phasic cells, only 1 out of 13 phasic cells tested with either morphine or [D-Ala2, D-Leu5]enkephalin was inhibited. The remaining 12 phasic cells showed no change in either their overall firing rate or pattern of activity during opiate perifusion. These results provide further evidence that, in addition to their inhibitory effects within the posterior pituitary, opiates can directly suppress the electrical activity of magnocellular neurosecretory cells at the level of the hypothalamus. However, the absence of an opiate effect on the phasic cells might suggest that the action of opioid peptides within the hypothalamus would be exerted predominantly on the oxytocin, rather than the vasopressin cells.

The role of the septum in the control of the milk ejection reflex in the rat: effects of lesions and electrical stimulation

Experiments were undertaken to determine the role of the septum on the afferent control of the milk ejection reflex in lactating rats. Massive septal lesions were produced by passing radio-frequency current through lesioning electrodes. Intramammary pressure recordings during suckling showed no significant alterations either in the frequency of milk ejections or in their amplitude and time course. Electrophysiological recordings of identified oxytocin-secreting neurones in supraoptic nuclei of septal-lesioned rats engaged in suckling showed that the pattern of background electrical activity and of the high frequency discharges at milk ejection were normal. The weight of litters from rats lesioned on the third day post-partum increased in a way parallel to that of control litters up to the fifteenth post-natal day. Electrical stimulation was applied bilaterally to the lateral septum in trains of long duration (20-55 min) at varying frequencies. Frequencies of 5 and 10 Hz interrupted the reflex during the period of stimulation. At 1 Hz, milk ejections were not interrupted but the intervals between successive milk ejections were significantly increased in comparison to the intervals before stimulation. Electrical stimulation applied to the septum in short trains of 1 or 3 min at 5 and 10 Hz significantly delayed the appearance of the subsequent milk ejection. At 1 Hz, no effect was observed. Septal stimulation at 1 Hz for 20 min or more did not significantly alter the electrocorticogram during the period of stimulation. Stimulation at 5 Hz for the same period of time always desynchronized the e.e.g. for several minutes after the cessation of stimulation. It is concluded that the septum is not essential for the onset and the maintenance of reflex milk ejections during lactation. The results suggest, however, that in the normal non-anaesthetized animal, septal activation could modulate the frequency of milk ejections.

Influences of the limbic system on hypothalamo-neurohypophysial system

(1) Effects of stimulations of various limbic structures (the olfactory bulb, olfactory tubercle, prepyriform cortex, endopyriform nucleus and various parts of amygdaloid nuclei) on the neurosecretory neurons in the supraoptic (SON) and paraventricular nuclei (PVN) of the hypothalamus were studied. All regions stimulated received strong inputs from the olfactory bulb. (2) Out of 195 "identified' neurosecretory neurons tested one-half or more (49-74%, depending on the structures stimulated) were inhibited by stimuli consisting of 1-3 short pulses. The inhibition occurred immediately after the stimulus in approximately one-fifty of all inhibited neurons, in the remaining four-fifths inhibition occurred after more than 20 ms latency. Inhibition of neurosecretory neuron activity lasted for several hundred milliseconds, often followed by clear post-inhibitory excitation or rebound. (3) In 23 neurons, a distinct "evoked' response of brief duration occurred with a 30 ms latency following stimulation of the lateral and medical amygdala, olfactory tubercle and prepyriform cortex. In another 17 neurons, a general increase in background activity with a longer latency (50-100 ms) occurred following stimulation of nearly all amygdaloid nuclei, olfactory tubercle and the pyriform cortex: lateral amygdala stimulation caused an excitation of the largest proportion of neurosecretory cells (30%) while none was excited by stimulation of the olfactory bulb and endopyriform cortex, except those occurring as post-inhibitory excitation. (4) There was a convergence of afferent impulses on single neurosecretory cells. A large proportion (42%) of the neurons received inputs from 2 to 4 limbic regions. (5) Neurosecretory cells which were influenced by limbic stimuli were also inhibited by baroreceptor activation and excited by osmotic stimulation. "Unidentified' neurons within SON and PVN and "atypical neurosecretory cells' (those responding to pituitary stalk stimulation with varying latencies) were also affected by the forebrain stimulation; some of these were also affected by an osmotic stimulus. A part of this group may send their axons to the median eminence.

Electrical properties of paraventricular neurosecretory neurons with and without recurrent inhibition

Twelve out of 32 neurosecretory neurons in the paraventricular nucleus of rats showed a silent phase following subthreshold stimulation to the posterior pituitary gland. After suprathreshold stimulation, the duration of the silent phase was significantly longer than that of the remaining 20 neurons, which did not show the silent phase at subthreshold stimulation. The latency and threshold in the former neurons were significantly longer and higher than those of the latter neurons. These data indicate a relationship between the recurrent inhibitory system and other electrical properties in the paraventricular neurons.

Neurohypophysial hormones and brain function: the neurophysiological effects of oxytocin and vasopressin

An increasing body of evidence suggests that the neurohypophysial hormones, in addition to their classical actions, may also act as neurotransmitters. They have a widespread but discontinuous distribution in the CNS; apart from their presence in the magnocellular nuclei they may be found in the hippocampus, amygdala, septum, substantia nigra, brainstem and spinal cord. They exert profound effects on behavior, particularly on memory, a function frequently ascribed to the hippocampus, amygdala and septum; on memory consolidation, internal reward and self stimulation functions frequently ascribed to brainstem and diencephalic aminergic systems including the substantia nigra and on sensory and autonomic responses which involve the medulla and spinal cord. When applied to the CNS they alter multiple unit activity in certain regions, particularly the hippocampus and cells which contain neural lobe hormones appear to be able to drive other cells synaptically. Finally application of the hormones can profoundly affect the activity of single nerve cells in just those parts of the CNS where, on the basis of their behavioral actions, they might be expected to act.

Lateral septum-medial hypothalamic connections: an electrophysiological study in the rat

Participation of both the lateral septum and the medial hypothalamus in similar behavioral and homeostatic functions, combined with anatomical data indicating a modest descending lateral septum projection to medial hypothalamic areas, prompted an investigation of the electrophysiology of lateral septum--medial hypothalamic connections in pentobarbital-anesthetized male Sprague-Dawley rats. Field potentials evoked by lateral septum stimulation were prominent within the hypothalamic ventromedial nucleus: their distribution closely overlapped with amygdala-evoked field potentials. Lateral septum stimulation evoked orthodromic responses from 67% of 800 medial hypothalamic and preoptic neurons (55.5% excitatory, latency 17.5 +/- 0.5 ms; 11.5% inhibitory, latency 19.1 +/- 1.4 ms), including 15 of 31 tuberoinfundibular neurons activated antidromically from the median eminence, 37.4% of cells responded orthodromically to both lateral septum and amygdala stimulation, whereas only 5.8% of cells demonstrated orthodromic responses to both lateral septum and midbrain periaqueductal gray stimulation. These observations suggest that lateral septum neurons influence the excitability of many medial preoptic and hypothalamic neurons, and indicate a convergence of lateral septum and amygdala influences onto 37% of medial hypothalamic cells. In the rat, the lateral septum's influence on adenohypophyseal hormone secretion appears to be mediated indirectly, i.e. through monosynaptic connections with medial hypothalamic tuberoinfundibular neurons.