Studies of antidromically identified neurosecretory cells of the hypothalamus by intracellular and extracellular recordings

Secretory cells of the supraoptic nucleus have central as well as neurohypophysial projections

Conventional neuroanatomical methods may fail to demonstrate the presence of axons that are finer than 1 microm in diameter because such processes are near or below the limit of resolution of the light microscope. The presence of such axons can, however, be readily demonstrated by recording. The most easily interpreted type of recording for this purpose is the demonstration of antidromic activation of the cell body following stimulation of the region through which the axon passes. We have exploited this technique in the hypothalamus and have demonstrated the presence of double axonal projections or axons branching very near the cell bodies of the secretory cells of the neurohypophysial system in the rat supraoptic nucleus. We found that a small proportion of supraoptic magnocellular cells could be antidromically activated both from the neural stalk and from elsewhere in the hypothalamus, including the suprachiasmatic nucleus (8 cells of a total of 182) and the antero-ventral third ventricular region (AV3V; 4 of 182 cells) near the organum vasculosum of the lamina terminalis (OVLT). Collision of antidromic and orthodromic spikes showed that the cells were clearly antidromically (rather than synaptically, or orthodromically) activated from both sites. A stimulus applied to one of the axons prevented propagation of a spike evoked by a pulse delivered to the other axon until sufficient time had elapsed after the first stimulus for the resultant spike to have propagated from the first stimulus site along one cell process (towards the cell body or branch point), and from this point along the other axonal branch to the second stimulus site (there was also a short additional delay period during which the axon at the site of the second stimulus recovered from its absolute refractory period). If the interval between the stimuli was progressively reduced, there came a point where the second spike failed. Such a clear demonstration of dual projections in a system where the cells were previously thought to have only a single axon raises the possibility that many nerve cells in the CNS have previously unsuspected projections.

Morphology and electrophysiology of neurons in dog paraventricular nucleus: in vitro study

The paraventricular nucleus (PVN) of the hypothalamus plays an important role in regulating gut motility. To date, there have been no intracellular electrophysiological studies of dog PVN neurons in vitro. The aims of this study were to: (1) adapt brain slice methods developed for studies of rodent CNS tissue to canine CNS tissue; and (2) study the electrophysiology and morphology of single neurons of the dog paraventricular nucleus (PVN). Coronal hypothalamic slice preparations (400 microm thick) of dog brain were used. Three groups of PVN neurons were classified based on their firing pattern. Continuous firing neurons (n=32) exhibited continuous ongoing action potentials (APs). Burst firing neurons generated bursts of APs (n=19). Intermittent firing neurons had only a few spontaneous APs. In contrast to continuous firing neurons, 14 of 19 burst firing neurons and 3 of 7 intermittent firing neurons responded to depolarizing current with a Ca2+-dependent low-threshold potential. Twenty-one PVN neurons studied electrophysiologically were filled with biocytin. Continuous firing neurons (n=12) had oval-shaped soma with two or three sparsely branched dendrites. Branched axons were found in two continuous firing neurons, in which one branch appeared to terminate locally. Burst firing neurons (n=8) generally had triangular soma with 2 to 5 branched dendrites. In summary, the brain slice technique was used to study the morphology and electrophysiology of single neurons of the dog brain. Electrophysiological and morphological properties of the three neuron groups were identified and discussed.

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.

Electrical stimulation of the central nucleus of the amygdala induces fos-like immunoreactivity in the hypothalamus of the rat: a quantitative study

The effect of electrical stimulation of an important forebrain autonomic structure, the central nucleus of the amygdala (CNA), on c-fos expression in three hypothalamic nuclei was studied in rat with immunocytochemistry to reveal the protein (Fos) encoded by the immediate early gene (IEG). Image analysis was used to quantify the Fos immunoreactive neurons within the supraoptic (SON), paraventricular (PVN), and arcuate (AN) nuclei. Stimulation for 60 min induced a statistically significant increase of the number of Fos immunoreactive neurons in all three nuclei ipsilateral to the CNA stimulation site. Double immunocytochemical staining (Fos and vasopressin or Fos and oxytocin) was employed to evaluate the participation of different subpopulations of neurons within the SON and PVN in response to CNA stimulation. In the SON, the increased number of Fos immunoreactive nuclei following the stimulation was observed in the vasopressin and oxytocin-secreting cells within this nucleus. In the PVN, the increase in the number of Fos immunoreactive neurons was predominantly within the parvocellular compartment. These studies demonstrate that IEG expression in hypothalamic neurons can be evoked as a result of afferent stimulation from the CNA. Activation of peptide- and hormone-containing neurons within the SON, PVN and AN, through mono- or multisynaptic pathways, may play a role in hormonal and autonomic responses.

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.

Activation of paraventricular neurosecretory cells by local osmotic stimulation of the median preoptic nucleus

Both electrical and local osmotic stimulation of the median preoptic nucleus (MnPO) predominantly produced excitation of paraventricular (PVN) neurosecretory cells in the rat. By contrast osmotic stimulation of the medial septal region was without effect, although electrical stimulation excited most cells. The results suggest that the MnPO is one of the osmosensitive sites controlling electrical activity of PVN neurosecretory cells.

Excitation of hypothalamic paraventricular neurons by stimulation of the raphe nuclei

Extracellular recordings were made from 467 anti-dromically identified neurosecretory neurons and 148 non-neurosecretory neurons in the paraventricular nucleus of the hypothalamus of hemispherectomized cats under pentobarbital anesthesia. Stimulation of the dorsal, median, and pontine raphe nuclei excited 31%, 26%, and 12% of neurosecretory neurons tested, respectively, and inhibited 9%, 7%, and 8%. The excitatory responses in 13 of 14 neurons tested were blocked by either of two intravenously administered 5-HT2 antagonists, cyproheptadine or methysergide. The 5-HT1A antagonist, (-)pindolol, partially blocked the excitatory responses elicited by raphe stimulation in three of five neurons tested. The inhibitory responses to raphe stimulation were not affected by application of these antagonists. More non-neurosecretory neurons than neurosecretory neurons were excited in response to raphe stimulation and these excitatory responses were also blocked by these antagonists. We conclude that most electrically stimulated synaptic inputs from the midbrain raphe nuclei to the hypothalamic paraventricular nucleus are excitatory and are mainly mediated by 5-HT2 receptors.

Cytoarchitectonic and quantitative Golgi study of the hedgehog supraoptic nucleus

A cytoarchitectural study was made of the supraoptic nucleus (SON) of the hedgehog with special attention to the quantitative comparison of its main neuronal types. The main purposes were (1) to relate the characteristics of this nucleus in the hedgehog (a primitive mammalian insectivorous brain) with those in the SONs of more evolutionarily advanced species; (2) to identify quantitatively the dendritic fields of the main neuronal types in the hedgehog SON and to study their synaptic connectivity. From a descriptive standpoint, 3 neuronal types were found with respect to the number of dendritic stems arising from the neuronal soma: bipolar neurons (48%), multipolar neurons (45.5%) and monopolar neurons (6.5%). Within the multipolar type 2 subtypes could be distinguished, taking into account the number of dendritic spines: (a) with few spines (93%) and (b) very spiny (7%). These results indicate that the hedgehog SON is similar to that in other species except for the very spiny neurons, the significance of which is discussed. In order to characterise the main types more satisfactorily (bipolar and multipolars with few spines) we undertook a quantitative Golgi study of their dendritic fields. Although the patterns of the dendritic field are similar in both neuronal types, the differences in the location of their connectivity can reflect functional changes and alterations in relation to the synaptic afferences.

Intracellular study of calcium-related events in cat magnocellular neuroendocrine cells

1. Magnocellular neuroendocrine cells (MNCs) in the supraoptic nucleus (SON) of mammals synthesize vasopressin or oxytocin and release these hormones systemically from their neurohypophysial axon terminals. In the rat, release is facilitated by bursts of action potentials generated by the MNC. However MNC units in the intact cat discharge more slowly and do not display the repetitive bursts (phasic firing) that promote vasopressin secretion. The reasons why these cat endocrine neurones differ so dramatically in their firing behaviour from the rat model were examined using intracellular recording. 2. Cat and rat MNCs displayed similar mean resting potentials approximating -60 mV, and were usually linear in their voltage-current relationship in the hyperpolarizing direction. However cat MNCs displayed a higher mean cell input resistance (301 M omega; n = 56) than those of rat (150 M omega; n = 105). 3. Calcium influx to cat MNCs during firing appeared comparable to rat based on (a) the similar range of action potential broadening observed during a spike train, (b) the shoulder on the action potential's falling phase which was blocked in low-Ca2+ saline, and (c) the ability to evoke tetrodotoxin (TTX)-insensitive spiking and non-synaptic depolarizing potentials, both calcium-mediated events observed in the rat. 4. In cat MNCs, a depolarizing current pulse (100-500 ms; 0.1-0.3 nA) elicited a train of action potentials followed by a prominent after-hyperpolarization (AHP) several times the duration of its counterpart in the rat. The AHP reversed near the equilibrium potential for K+, was not voltage dependent and represented an increased membrane conductance. It was suppressed in low-Ca2+ saline and completely eliminated by the calcium-activated potassium current (IK(Ca)) blockers apamin (100 nM) or d-tubocurarine (50-200 microM). Both blockers decreased spike frequency adaptation but did not induce bursting. Therefore the cat AHP probably represents a Ca(2+)-activated K+ conductance with a similar blocker sensitivity to its briefer counterpart in the rat MNC. 5. The spike hyperpolarizing after-potentials (HAPs) in cat were more than twice the mean amplitude and several times the duration of HAPs in rat. Cat HAPs were qualitatively similar to their rat counterparts, remaining unaffected by apamin or tubocurarine. The intrinsic currents responsible for the AHP and HAP appear to generate the stronger activity-dependent inhibition displayed by cat MNCs. 6. Twenty-one of fifty-two cat MNCs displayed an inward rectification at membrane potentials more negative than -70 mV ([K+]o = 6.24 mM), causing a depolarizing 'sag' in the voltage trajectory lasting 100-200 ms which was TTX resistant.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane properties of rat magnocellular neuroendocrine cells in vivo

Membrane properties of rat magnocellular neuroendocrine cells (MNCs) were examined during intracellular recordings in vivo. Current-clamp experiments revealed sustained outward rectification positive to -55 mV and transient outward rectification of depolarizing responses elicited from negative holding potentials. Trains of impulses were associated with a progressive increase of spike duration. Such features, which were not observed in neighboring non-neuroendocrine cells, are similar to those of MNCs in slices or explants of rat hypothalamus. In contrast, however, cells recorded in vivo were characterized by intense synaptic inhibition and a lower specific impedance than commonly observed from MNCs impaled in vitro.

Arginine vasopressin-anti-idiotypic immunostaining of human brain cells

Abstract Polyclonal anti-idiotypic antibodies, generated against the IgG fraction of antisera to arginine vasopressin (AVP), were shown to recognize two proteins in rat brain and bovine pituitary associated with [(3) H]AVP binding. Immunochemical analyses with these antisera revealed reactivity in paraventricular and supraoptic nucleus neuronal elements and in terminals of the posterior pituitary in the human central nervous system. With the use of a dual immunocytochemical staining technique employing both the anti-idiotype and idiotype for AVP it was possible to demonstrate a pattern of AVP-anti-idiotypic-immunoreactivity on AVP neuronal elements which suggests the existence of autoreceptors.

Non-phasic discharge and osmoresponsiveness of cat magnocellular neuroendocrine cells

Abstract Repetitive bursting (phasic firing) generated endogenously by magnocellular neuroendocrine cells (MNCs) in the rat facilitates systemic release of vasopressin from axon terminals in the neurophypophysis. However, little is known of how MNCs function in other mammals. Using coronal slices of hypothalamus we studied the firing behaviour and intrinsic membrane properties of homologous neurons in the cat supraoptic nucleus where vasopressinergic MNCs outnumber oxytocinergic cells. Less than 1% of units recorded in cat supraoptic nuclei (2 of 270) spontaneously fired in a phasic mode compared to 39% in the rat (90 of 230). A discrete level of steady current across the extracellular recording micropipette promoted phasic firing in 66 of 152 non-phasic units tested in rat supraoptic nuclei, but no phasic activity in 189 units from the cat. One or several stimuli applied dorsal to supraoptic nuclei triggered a single burst (afterdischarge) in 115 of 180 MNC units from the rat, whereas none of 173 MNC units tested in the cat fired an afterdischarge. Intracellular recordings from 56 feline MNCs revealed that unlike the rat, spike depolarizing afterpotentials were absent in all cells. This explains both the absence of phasic firing and the inability to trigger regenerative bursts in the intact cat. The possible Osmoresponsiveness of cat MNCs was examined using unit recording. These units reversibly increased their firing rate as osmolality was elevated with mannitol or NaCl (10 to 100 mOsm/kg), comparable to rat units. However, in no case did hyperosmotic conditions elicit phasic firing. We conclude that cat MNCs lack a regenerative burst capability but that unit Osmoresponsiveness is comparable to rat MNC units. We hypothesize that since the kidney of the cat normally functions at high efficiency in terms of water resorption, there may be little need for the rapid and pronounced elevation in vasopressin release evoked by phasic firing.

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.

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)

Neuronal activity of the cat supraoptic nucleus is influenced by muscle small-diameter afferent (groups III and IV) receptors

In anesthetized cats, responses of single neurosecretory neurons of the supraoptic nucleus to activation of muscle receptors were investigated. Electrical stimulation (1-3 pulses at 200 Hz) of group III and IV pure muscle afferents (gastrocnemius nerve) evoked excitation of greater than 50% of supraoptic nucleus neurons (n = 50), whereas stimulation of group Ia or Ib fibers was ineffective. Baroreceptor stimulation inhibited 95% of these supraoptic nucleus neurons that responded to activation of muscle afferents. Excitation of receptors in the gastrocnemius muscle by intra-arterial injection of chemicals (NaCl, KCl, and bradykinin) increased firing rates of most (84%, 74%, and 80%, respectively) neurosecretary neurons. The magnitude of the excitatory response was dose dependent--bradykinin being the most effective. The response disappeared after muscle denervation. When the gastrocnemius muscle alone was contracted phasically by ventral root stimulation, discharges of the supraoptic nucleus neurons increased, whereas quick stretch of the muscle had no effect. We conclude that activation of muscle receptors by chemical or mechanical stimulus can directly excite neurosecretory neurons in the supraoptic nucleus and that afferent impulses are carried by polymodal fibers of small diameter but not by the largest afferents (group I) from the muscle. The results may relate to increased concentrations of plasma vasopressin during exercise.

A nonlinear amplifier for improving signal-to-noise ratio of single-unit recordings

A simple electronic circuit is described that suppresses the baseline noise and enhances the amplitude of extracellularly recorded action potentials. The circuit is useful for improving the signal-to-noise ratio of single-unit recordings, especially for potentials recorded by chronically electrodes.

The patterns of [14C]2-deoxyglucose uptake in female rat brain produced by electrical stimulation of hypothalamic and limbic brain areas

The aim of this study was to investigate the pattern of [14C]2-deoxyglucose uptake in anaesthetized rat brain produced by electrical stimulation of brain areas implicated, by previous electrical stimulation studies, in the neural control of pituitary hormone and especially gonadotrophin secretion. Stimulation of the median eminence led to a significant increase in the relative metabolic activities of the arcuate, ventromedial hypothalamic, supraoptic and paraventricular nuclei and the preoptic area. Stimulation of the suprachiasmatic or paraventricular nuclei or the medial preoptic area, anterior hypothalamic area, the dorsal or ventral hippocampus or amygdala led to an increase in the relative metabolic activity of many brain regions known to have direct connections with these areas, but in addition produced increases in the relative metabolic activity of areas which have secondary connections. Hippocampal stimulation confirmed previous neuroanatomical findings of major intrinsic functional connections between different fields of the ipsilateral and contralateral hippocampus. Stimulation of the amygdala, unexpectedly, did not change the relative metabolic activity of the arcuate nucleus and medial preoptic area which have neuroanatomical connections with the amygdala. Similarly, stimulation of the medial preoptic area did not change significantly the relative metabolic activity of the mamillary body and dorsomedial thalamic area. The effect of preoptic area stimulation on the relative metabolic activity of several brain regions was changed by ovariectomy and by injection of oestradiol benzoate. Stimulation of the preoptic area and suprachiasmatic nuclei, but not the anterior hypothalamic area or other brain regions, increased significantly the plasma concentrations of luteinizing hormone. These results show that (i) electrical stimulation of brain areas concerned with the control of gonadotrophin and other pituitary hormone secretion changes the metabolic activity of nuclei and neural pathways extrinsic as well as intrinsic to the hypothalamic-pituitary system, (ii) the [14C]2-deoxyglucose method can detect changes in antidromic as well as orthodromic activity and in multi-synaptic pathways, (iii) neuroanatomical pathways are not always activated metabolically by electrical stimulation, and (iv) the preoptic-suprachiasmatic nucleus gonadotrophin control system is discrete and is little affected by increased metabolic activity of the hypothalamus produced by stimulation of the anterior hypothalamic area or other brain areas.

Cortisol alters firing rate and synaptic responses of limbic forebrain units

Single units antidromically identified as projecting to the hypothalamic paraventricular nucleus (PVN) from the lateral septum (LS) and preoptic area (POA) were recorded in adrenalectomized male rats. Eight of the 15 POA units identified as projecting to the PVN were affected by reciprocally projecting pathways also showing either orthodromic excitation or inhibition. A variety of responses from POA units following cortisol injection suggested a heterogeneity of PVN afferent projections arising from the POA and possibly involved in the regulation of adrenocortical secretion. Cortisol injection had variable effects on the reciprocal POA axon pathways demonstrated and also revealed the presence of reciprocal projections to four of the LS cells. Most of the 10 LS cells tested showed an increase in firing rate following the steroid injection. It is suggested that the reciprocal pathways demonstrated may provide a steroid-dependent, 'ultra-short loop' negative feedback circuit serving to regulate the neural control of adrenocortical secretion. Unidentified cells in both regions were also affected by cortisol injection, the majority of such cells tested being excited, some also showing steroid-sensitive synaptic effects following PVN stimulation. The latter results may be related to the involvement of the structures examined, and of adrenal glucocorticoids, in central mechanisms of arousal.

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.

Distribution and morphology of vasopressin-, neurophysin II-, and oxytocin-immunoreactive cell bodies in the forebrain of the cat

Experiments were done to provide a detailed map of the location and a description of morphological characteristics of vasopressin (AVP-IR)-, neurophysin II (NII-IR)- and oxytocin (OXY-IR)-immunoreactive neuronal perikarya in the forebrain of the cat. In addition, the location of cells in the forebrain retrogradely labeled following injections of tracers into the neurohypophysis was determined. The distribution of AVP-IR and NII-IR was similar in all cases studied. Most of the cells containing AVP-IR and OXY-IR were observed in the hypothalamic paraventricular (PVH) and supraoptic (SON) nuclei. In addition, AVP-IR and OXY-IR cell bodies were found in the regions of the nucleus of the diagonal band of Broca, the dorsal chiasmatic nucleus, the anterior hypothalamic-preoptic area, the periventricular area, the nucleus circularis, the perifornical area of the lateral hypothalamus, the accessory SON, the area of the tuber cinereum (Tca), and the medial nucleus of the amygdala. The density of AVP-IR cells was greater than that of OXY-IR cells in these regions. Several forebrain areas were also observed to contain only AVP-IR perikarya: the suprachiasmatic nucleus (Sc), the bed nucleus of the stria terminalis, and the region of the substantia innominata and ventral globus pallidus (SI/GP). In addition, the dorsomedial nucleus of the hypothalamus only contained OXY-IR perikarya. Most of the cells immunoreactive to AVP were multipolar and had spinelike processes over their somata and proximal dendrites. In addition, the majority of cells in the PVH and SON were round or oval, whereas those outside these nuclei were fusiform or triangular. The mean somal area of AVP-IR cells in the region of the SI/GP was significantly (P less than 0.05) larger than that of AVP-IR cells in all other regions examined, whereas the mean somal area of Sc AVP-IR cells was significantly (P less than 0.05) smaller than that of all other groups of AVP-IR cells examined. Most OXY-IR cells were similar morphologically to those immunoreactive to AVP, except that OXY-IR cell bodies and their appendages did not have spinelike processes. In addition, OXY-IR perikarya were generally of uniform size. OXY-IR cells in the PVH and accessory SON were significantly (P less than 0.05) larger than AVP-IR cells in the same regions, but were not different from AVP-IR cells in the lateral hypothalamus and SON.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Patterns of spontaneous unit preoptic neurosecretory cell discharges in the rainbow trout, Salmo gairdneri

Extracellular antidromic potentials recorded from the neurosecretory cell body were characterized by the following criteria: constant latency, the ability to follow a high frequency rate of stimulation and the collision test. The latency of the antidromic potentials ranged from 12 to 24 ms (17.46 +/- 3.10 SD) which gave a mean conduction velocity of 0.19 m/s, typical of unmyelinated nerve fibers. Two components could be clearly distinguished in the antidromic potential. A small "A" spike which showed constant latency and a large "B" spike with a variable latency and amplitude. A delay of 6.5 ms between the two spikes could occur and sometimes the "B" spike was blocked leaving only the "A" spike. Four patterns of spontaneous activity seem to emerge: Type I (26% of units, M +/- SD = 0.77 +/- 0.32 sp/s) corresponds to a slow and irregular pattern of activity; Type II (28% of units, M = 1.58 +/- 0.47 sp/s) is hard to classify and may be related to an irregular bursting pattern of activity; Type III (28% of units, M = 2.59 +/- 1.19 sp/s) corresponds to a continuous pattern of activity; Type IV (18% of units) represents a rhythmic pattern of activity with an active phase of about 3 min (M = 2.42 +/- 0.90 min), a silent phase of about 4 min (M = 3.89 +/- 3.02 min) and a maximal frequency of unit discharge in the range 2-18 sp/s. No statistical differences exist for the mean dorsal aortic pressure (DAP) between the four types of neurosecretory cell activity.

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.

Functional development of the prenatal brain. I. Recording of extracellular action potentials from the magnocellular system of the 18-day-old chicken embryo

For the full understanding of the ontogeny of the electrical activity in the brain it is essential to record single unit activity of the fetus. However, investigations of the functional development of neuronal properties in mammals are largely limited by the inaccessibility of the prenatal brain. Therefore, we have designed a new method to record extracellular single unit activities of identified magnocellular neurones in the Nucleus paraventricularis of the chicken embryo after 18 days of incubation. One hundred and four magnocellular neurones were identified by antidromic stimulation from the neural lobe. In a high percentage of the neurones an A-B inflexion of the action potential could be observed similar to that frequently encountered in mammalian magnocellular neurones. The mean duration of the action potential was 2.8 ms with a range between less than 1 ms and 7 ms. This large range is probably due to developmental processes of the cell membrane and subsequent changes in the extra- und intracellular ion concentration. Fourty-six percent of the neurones generated spontaneous action potentials with a slow irregular firing pattern. The mean discharge frequency was estimated as approx. 1 Hz. In further 13% of the cells orthodromic action potentials could be observed only after the occurrence of several antidromic spikes. The data presented are the first recordings of single unit activity in the magnocellular system in the prenatal brain. They demonstrate that the chicken embryo may offer a suitable model to study the ontogeny of neuroendocrine systems in the fetal brain in-vivo.

Properties of antidromically identified neurons in the enkephalinergic magnocellular dorsal nucleus of the guinea pig hypothalamus

In the guinea pig, immunocytochemical and neuroanatomical studies have demonstrated that enkephalin-containing neurons in the hypothalamic magnocellular dorsal nucleus (MDN) terminate in the lateral septum (LS). In the present investigation, 114 MDN neurons, studied with extracellular recording techniques, were identified by antidromic activation from the LS. Latencies of responses from ipsilateral and contralateral LS were 13.5 and 18.78 ms, respectively, corresponding to an axonal conduction velocity of 0.1 m/s. By using the reciprocal collision test, evidence is presented for bilateral projection of individual MDN neurons to the LS. Fifty-one (44.73%) MDN-LS neurons discharged in a slow irregular pattern. Interspike time histograms were very similar and had a mode of about 280 ms. Peristimulus time histograms were compiled from 15 active MDN-LS neurons. Stimulation which elicited antidromic spikes resulted in a brief silent period in the spontaneous activity which was related to the normal interspike interval pattern of the firing. Prolonged silent periods as well as silent period occurring after subthreshold stimulus and increasing with the stimulus intensity were attributed to inhibitory synaptic effects. On the other hand, some MDN-LS neurons displayed orthodromic excitatory responses following LS stimulation. These observations provide electrophysiological evidence of a direct MDN-LS pathway, in all likelihood of enkephalinergic nature, and indicate that some MDN-LS neurons receive inhibitory and excitatory afferents from the LS.

4-aminobutyrate: 2-oxoglutarate transaminase-containing neurons in the perinuclear zone of the rat supraoptic nucleus

A pharmaco-histochemical method for demonstrating the enzyme 4-aminobutyrate: 2-oxoglutarate transaminase was applied to the sections of the rat supraoptic nucleus region. The reactions of GABAergic interneurons and their relationship to neurosecretory neurons were studied. Medium-sized neurons heavily stained for transaminase were detected in the perinuclear zone just dorsal to the supraoptic nucleus. Neurons within the supraoptic nucleus were not stained. However, the perikarya of some neurosecretory neurons in the dorsal region of the supraoptic nucleus, as well as in discrete groups scattered throughout the nucleus; were surrounded by the granular reaction products. The results strongly suggest that these strongly positive neurons in the perinuclear zone send axons to the supraoptic nucleus, where they richly divide into many branches, which synapse on the perikarya of some vasopressin and oxytocin cells.

Antidromic responses in the paraventricular magnocellular neurons of the rat hypothalamus: latency variations correlated with the firing rate

Magnocellular neurosecretory cells were antidromically identified in the hypothalamic paraventricular nucleus (PVN) of urethane-anesthetized, ovariectomized female rats following electrical stimulation of the neurohypophysis. Seventy-one cells with a tonic pattern of spontaneous discharge were distinguished and used to examine the relationships between the measures of antidromic spike latency, activation threshold and discharge rate. The discharge rate was artificially modulated by either microiontophoresis of glutamate or antidromic stimulation of the neurohypophysis. In all the PVN cells with tonic activity, the latency lengthened and the threshold increased as a function of the discharge rate. Activation of individual cells by microiontophoresis of glutamate was effective, as was simultaneous activation of many PVN cells by antidromic stimulus. Similar relationships between the discharge rate and the parameters of antidromic activation were seen in 3 cells, when their rates varied spontaneously over a wide range without manipulation. These data suggest that the excitability of axons of presumed oxytocinergic cells in the PVN-neurohypophyseal system are influenced by their prior activity, probably through metabolic changes in individual axons.

Facilitatory effect of antidromic stimulation on milk ejection-related activation of oxytocin neurons during suckling in the rat

Oxytocin neurons of the paraventricular nucleus were identified by their antidromic response to stimulation of the neurohypophysis and a short high-frequency burst of spikes displayed before reflex milk ejection in the urethane-anesthetized lactating rat. During suckling, the milk ejection-related bursts recurred at regular intervals. Stimulation of the neurohypophysis at 50-300 pulses/s for 0.5-6 s (current 1 mA) evoked additional burst(s) with the amplitude higher than those of spontaneously occurring ones. Stimulation with subthreshold current for antidromic response or constant-collision test could also facilitate the neurosecretory bursts. Possibly, oxytocin released within the magnocellular nuclei is responsible for these effects.

Effects of electrical and chemical stimulation of the paraventricular nucleus on neurons in the subfornical organ of cats

Single unit recordings were made from neurons of the subfornical organ (SFO) in hemispherectomized cats following electrical stimulation in the paraventricular nucleus (PVN) of the hypothalamus. Of 132 neurons tested, 48 (36%) were inhibited, 3 (2.3%) excited and two (1.5%) antidromically activated. In separate experiments the wall of the third ventricle near the PVN was locally perfused with glutamate solution using a concentric push-pull cannula. Of 35 SFO neurons tested, 6 were inhibited, 5 were facilitated and 24 unaffected. The results indicate that one-third of SFO neurons receive synaptic inputs from cells in or near the PVN.

Intrinsic inhibition in magnocellular neuroendocrine cells of rat hypothalamus

Endogenous mechanisms of inhibition in magnocellular neuroendocrine cells were studied with intracellular recordings in the rat hypothalamic slice preparation. Hyperpolarizing after-potentials (duration up to 125 ms) followed single action potentials and after-hyperpolarizations (a.h.p.s) lasting hundreds of milliseconds followed brief evoked spike trains. The amplitude and duration of the a.h.p. increased after spike trains of longer duration or higher frequency. The a.h.p. appears endogenous, rather than synaptically mediated from recurrent inhibition, because it persisted after pharmacological blockade of axonal conduction or of chemical synaptic transmission. The reversal potential of the a.h.p. was at least 20 mV more negative than that of inhibitory post-synaptic potentials. Cl- ionophoresis did not alter the a.h.p. Chelation of intracellular Ca2+ with EGTA injection eliminated the a.h.p. A Ca2+-activated K+ conductance, rather than recurrent synaptic inhibition, apparently causes the a.h.p. and is at least partly responsible for the inhibition after single spikes in magnocellular neurones. During hormone release, this endogenous mechanism may contribute to the post-burst silent period in putative oxytocinergic cells and to the interburst interval in phasic neurones, which are known to fire repetitive bursts associated with vasopressin release.

Complex action potential waveform recorded from supraoptic and paraventricular neurones of the rat: evidence for sodium and calcium spike components at different membrane sites

Magnocellular neurones of the supraoptic and paraventricular nuclei display a complex waveform when recorded extracellularly. The present work has examined the possible reasons for the complexity of this waveform by making extracellular recordings from antidromically identified neurones in the anaesthetized rat and extra- and intracellular recordings from similar neurones in the hypothalamic slice preparation, where external solutions can be changed. In extracellularly recorded units, action potentials had two positive going components. The first of these was abolished by tetrodotoxin. The second, slower component was abolished when external Ca++ concentration was lowered, but enhanced in magnitude and duration when Ba++ was placed in the external medium. The second component was sensitive to electrode movement and was not observed to the same degree when intracellular recordings were made. In light of these observations and the known morphology of these neurones, it is suggested that the magnocellular neurone action potential is comprised of a fast Na+-dependent component and a slower component dependent on Ca++ entry which may originate from a part of the cell other than the soma. The most likely site for this Ca++ component to occur is at the cell dendrite.

The role of cardiovascular and muscle afferent systems in control of body water balance

Influences of afferent inputs from cardiovascular and muscle receptors on the activities of neurosecretory neurons in the hypothalamus, which secrete vasopressin (ADH) were studied. Recordings were made from identified neurosecretory neurons in the supraoptic (SON) and paraventricular nuclei (PVN) of cats and rats. Activation of baroreceptors in the carotid sinus and aortic arch and atrial receptors inhibited SON and PVN neuron activities, while activation of chemoreceptors in the carotid sinus excited them. Repetitive electrical stimulation of the carotid sinus and aortic nerves showed that weak stimulation produced excitation and stronger stimulation produced inhibition of SON and PVN neurons. Electrical stimulation of these nerves and the nucleus tractus solitarius (NTS) by a single or short train of pulses showed that 'fast' and 'slow' pathways between the NTS and the SON existed, while these two types of pathways were not observed between the NTS and the PVN. Evidence of direct connections from the NTS to the PVN was found by means of antidromic stimulation of the PVN. Electrical stimulations of group I afferent fibers from the gastrocnemius muscle did not change SON neuron discharges, while activation of group III and IV afferent fibers excited them. Injection of chemicals (NaCl, KCl, bradykinin) into arteries supplying the muscle excited SON neurons. The excitation disappeared after section of the muscle nerves. The results indicated that activation of small afferents from the muscle excites the SON neurons, leading to an increase in vasopressin secretion. All these studies show that afferent inputs from receptors in the cardiovascular system and in the muscle have modulatory effects on neurosecretory neurons, and participate in control of body water balance by regulating vasopressin secretion from the neurohypophysis.

Possible projections from regions of paraventricular and supraoptic nuclei to the spinal cord: electrophysiological studies

The existence of monosynaptic connections between neurons in the paraventricular nucleus (PVN) of the hypothalamus and the intermediolateral cell column (ILC) of the spinal cord was studied by electrophysiological techniques in chloralose-anesthetized cats. Sympathetic preganglionic discharges (recorded from the 2nd or 3rd thoracic white ramus) were evoked by microstimulation of certain regions in or near the PVN with short train of pulses and below 50 microA current. By recording responses of 'identified' and 'non-identified' neurosecretory cells in the PVN and supraoptic nucleus (SON) to stimulation of the ILC of the thoracic cord, it was possible to identify antidromically evoked action potentials in 9 out of 297 neurons tested. Among them, 2 neurons were also antidromically excited by the pituitary stalk stimulation, 5 were orthodromically excited by the same stimulus and the remaining 2 were not excited by the stalk stimulation. Our results indicate that some PVN neurons, though small in number, send axons directly to the ILC of the cord, and that a very few neurons among these also send their axons to the pituitary gland.

Correlations between the firing of supraoptic neurones in slices of rat brain

These experiments were an attempt to study the possible interactions between cells of the supraoptic nucleus. In isolated brain slices pairs of supraoptic neurones were recorded simultaneously either with a single electrode or with two electrodes and cross-correlograms produced. Correlations were demonstrated in 22 of the 82 pairs studied and were found to be more common between closely neighbouring pairs of cells. Ten of the correlations indicated that the spikes of one cell followed spikes in the other cell. The correlations of another 10 pairs indicated that the cells were coactivated. In only 2 pairs was there a correlation indicative of an inhibitory connexion. That these correlations could result either from synaptic connexions within the nucleus, or from coactivation of cells from an extranuclear site is discussed.

Supraoptic neurosecretory cells: synaptic inputs from the nucleus accumbens in the rat

Synaptic inputs from the nucleus accumbens (ACB) to neurosecretory cells of the supraoptic nucleus (SON) were studied in the rat. One hundred and twenty SON neurones responded antidromically to pituitary stalk stimulation and were identified as neurosecretory cells. Sixty-three of these cells were identified as vasopressin-secreting cells and 45 as oxytocin-secreting cells by their spontaneous firing patterns. About one half of the vasopressin-cells and two thirds of the oxytocin-cells were responsive to stimulation of the basal forebrain including the ACB. More vasopressin-cells were excited than were inhibited, and oxytocin-cells were mainly inhibited. Depth profile of effective stimulation sites in the basal forebrain revealed that ACB stimulation selectively produced the responses. Most of those SON neurones responsive to ACB stimulation also responded to septal stimulation. A positive correlation was observed between responses to ACB and septal stimulation in each unit. After septal lesion, the number of SON neurones which were responsive to ACB stimulation was significantly decreased. In two rats, a single SON unit was tested for ACB stimulation both before and after septal lesion, and the previously observed synaptic inputs were not seen after the lesion. Fifty septal neurones projecting to the area including the SON were antidromically identified after SON stimulation. About one half of these neurones were excited by ACB stimulation. These results demonstrate the existence of a neural pathway from the ACB to the SON and suggest that the pathway is mediated by septal neurones.

Electrophysiological study of paraventricular nucleus neurons projecting to the dorsomedial medulla and their response to baroreceptor stimulation in rats

In male rats anesthetized with urethane, extracellular recordings were made from 415 neurons in the paraventricular nucleus (PVN) and adjacent areas. Of these neurons 64 were excited antidromically by stimulation of the dorsomedial medulla but not by stimulation of the pituitary stalk (first group). Seventy-three neurons were antidromically excited by stimulation of the pituitary stalk but not of the dorsomedial medulla (second group, neurosecretory cells). The other 2 neurons were antidromically excited by stimulation of both the dorsomedial medulla and the pituitary stalk (third group). Latencies of antidromically evoked action potentials by stimulation of the dorsomedial medulla and of the pituitary stalk ranged between 8 and 60 ms (mean +/- S.D., 38.5 +/- 9.8, n = 66) and from 7 to 24 ms (mean +/- S.D., 13.0 +/- 3.6, n = 75), respectively, suggesting unmyelinated fiber projections in both instances. PVN neurons of these 3 groups were found to be dispersed throughout the PVN and no difference in specific locations between the neuron groups existed. Their characteristics, however, were different. The first group of neurons discharged at a slower rate and showed no phasic pattern of firing, while 28% of the second group of neurons ('identified' neurosecretory cells) showed phasic patterns of firing and their rates of discharge were higher than those of the first group of neurons. The two neurons belonging to the third group showed irregular spontaneous discharges. The areas within the dorsomedial medulla stimulation of which evoked antidromic excitation of PVN neurons were located within and adjacent to the nucleus of the tractus solitarius (NTS) and the dorsal motor nucleus of the vagus (DMV). Among PVN neurons which were antidromically excited by stimulation of dorsomedial medulla, 51 cells were examined for their responses to excitation of baroreceptors. An increase in pressure of the 'isolated' carotid sinus excited 2 neurons, and inhibited 7 (14%). On the other hand, 27% (11 out of 41) of neurosecretory cells (second group) were inhibited by baroreceptor stimulation. From these results, it was concluded that essentially separate populations of PVN neurons project to the neurohypophysis and to the NTS, DMV and their vicinities, and that some of the caudally-projecting PVN neurons receive synaptic input from carotid baroreceptor reflex pathway, suggesting the possible involvement of these PVN neurons in central cardiovascular regulation.

Histochemical studies on the distribution of thiamine pyrophosphatase and enzymes related to carbohydrate metabolism in the intercalated neurons of the rat supraoptic nucleus

Histochemical studies have been conducted by applying hexokinase (HK), aldolase (AD), glyceraldehyde-3-phosphate dehydrogenase (G3), succinate dehydrogenase (SDH), glucose-6-phosphate dehydrogenase (G6PD), and thiamine pyrophosphatase (TPPase) methods, as well as Nissl staining and Gomori's chrome-alum-hematoxylin-phloxine (CHP) methods to intercalated neurons of the supraoptic nucleus (SO) on Wistar strain rats. Intercalated neurons reacted weakly to the AD, G3, G6PD, and SDH tests, indicating that they belong to the category of ordinary neurons with low carbohydrate metabolism. Many fibrous astrocytes showing strong HK reactions surround neurosecretory neurons. However, they do not surround intercalated neurons with mild HK activity. These results indicate that the latter receive a poor supply of energy from glucose in the circulating blood in contrast to the former. Intercalated neurons are very rich in Nissl substance but lack CHP-positive material. They may have a high potential for synthesizing protein. The principal morphological features of the TPPase-positive Golgi material are peculiar and heterogeneous shape and poor development. These findings together with mild G6PD activity suggest that intercalated neurons are very likely to have poor synthesizing activity.

Phasically firing neurons in the supraoptic nucleus of the rat hypothalamus: immunocytochemical and electrophysiological studies

Relations between firing patterns and peptides in supraoptic neurons of rat hypothalamic slice preparations were studied by electrophysiology, intracellular fluorescent dye-marking and immunocytochemistry. Seven out of 10 magnocellular neurons which showed phasically firing patterns were identified by injections of Lucifer Yellow-CH (LY); these were also stained with an anti-vasopressin serum. This report presents direct evidence that most of the phasically firing neurosecretory neurons in the supraoptic nucleus contain vasopressin. This study demonstrates the feasibility of combining immunocytochemical and electrophysiological techniques to study the peptides contents of single mammalian neurons.

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.

A perfused in vitro preparation of hypothalamus for electrophysiological studies on neurosecretory neurons

A technique for the preparation and recording of mammalian supraoptic nucleus neurosecretory neurons maintained in vitro through perfusion of the anterior cerebral artery is described. A slice of basal diencephalon measuring approximately 8 X 8 X 2 mm that includes the pituitary stalk and medial preoptic-anterior periventricular area permits stable extracellular and intracellular recordings of antidromic and orthodromic activity from identified supraoptic neurosecretory cells. The preparation as described remains viable for 10-12 h, permits tests on osmotic sensitivity of neurosecretory cells, the possibility of synaptic isolation through addition of excess magnesium and tests on neuropharmacology of SON neurons during the addition of putative neurotransmitter substances. The principle advantage of this perfused preparation over in vitro hypothalamic slice recordings is the maintenance of connections that permit identification of neurosecretory neurons and study of certain synaptic connections with neurophysiological and pharmacological techniques.

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.

Immunohistochemical identification of Lucifer Yellow-labeled neurons in the rat supraoptic nucleus

An attempt was made to identify Lucifer Yellow-labeled neurons in the rat supraoptic nucleus as vasopressin-containing neurons, by means of a combination of immunoperoxidase histochemistry and iontophoretic single cell-injection. We came to the conclusion that the fluorescent dye does not diminish the immunoreactivity of vasopressin in the magnocellular neurons. This newly developed method, along with its modifications, should prove to be quite useful for electrophysiological and morphological studies on the neuropeptide-releasing neurons in the mammalian neuroendocrine system.