Cable analysis with the whole-cell patch clamp. Theory and experiment

A theoretical analysis was undertaken of a Rall motoneuron under voltage clamp with a finite access resistance. This model is relevant to the conditions of the whole-cell patch clamp, which to date has been used very little for cable analysis. It was shown that the soma and cable charging currents can be distinguished, and that the soma is charged with a time constant approximately equal to the access resistance times the somatic capacitance. Thus, the charging time of the soma is similar to what it would be if the cell had no process. Simple formulas were derived that can be used to calculate the electrotonic length, the membrane time constant, and the soma-dendrite resistance ratio of a cell with a cylindrical process. With the aid of these formulas, reasonable estimates of parameter values were recovered from simulated transient currents. Tests of the Rall model were proposed to determine when there is an equivalent cylinder that is consistent with observed charging behavior. The analysis was extended to a cable with an open end and to a model in which the soma and dendrite have different membrane time constants. It was shown that with voltage-clamp data estimates of electrical parameters other than rho are relatively insensitive to differences between the membrane properties of the soma and dendrite. The methods of cable analysis introduced here were illustrated by application to charging transients recorded from a hippocampal pyramidal cell and from a neurohypophysial nerve ending. The Rall model provided a good description of the pyramidal cell current transient but was inconsistent with the charging behavior observed for the nerve ending. With the recent technical advance of patch clamp recording in brain slices, the analysis presented here should help neurophysiologists investigate cable properties in a wide variety of systems.

Ultrastructural distribution of glutamate immunoreactivity within neurosecretory endings and pituicytes of the rat neurohypophysis

An ultrastructural analysis of post-embedding glutamate immunocytochemistry within the neural lobe of the pituitary was used to explore the possible role of glutamate within the magnocellular neuroendocrine cells. Relative densities of a colloidal gold marker associated with various cellular and subcellular compartments of the neural lobe were quantified by computer analysis of electron micrographs. Robust glutamate immunoreactivity was observed in both pituicytes (cytoplasm, mitochondria and nucleus) and neurosecretory endings. Within the neurosecretory endings, glutamate staining was specifically localized to the microvesicles with no overlap into the neurosecretory granule population. Stimulation of the vasopressin/oxytocin neurosecretory system by water deprivation increased glutamate content in pituicytes and mitochondria within neurosecretory endings but had little influence on microvesicle glutamate content. The results are consistent with the existence of multiple functional pools of immunoreactive glutamate in both pituicytes and neurosecretory endings. Microvesicles within the neurosecretory endings exhibit many properties of secretory vesicles, appear to be functionally independent of the neurosecretory granules, and have sufficient glutamate immunoreactivity to suggest that this amino acid may be compartmentalized for release in the neural lobe.

Calcium currents and peptide release from neurohypophysial terminals are inhibited by ethanol

The effects of EtOH on peptide release and on high-threshold, voltage-activated calcium (Ca++) channels were examined in acutely dissociated rat neurohypophysial terminals. These terminals release the peptide hormones, arginine vasopressin (AVP) and oxytocin. Release of AVP from isolated intact neurohypophyses, induced by either electrical stimulation or elevated potassium, was inhibited by clinically relevant concentrations of EtOH. "Whole-cell" patch-clamp recording methods were used to study the effects of EtOH on voltage-activated Ca++ currents (ICa) in the peptidergic nerve terminals. Amplitudes of both fast-inactivating ICa and long-lasting ICa were reduced in EtOH, and the reduction in ICa did not result from a shift in its current-voltage or steady-state inactivation relationships. Only the fast-inactivating component recovered after removal of EtOH. The effects of EtOH on ICa could not be attributed to changes in osmolarity. In contrast to ICa, the fast, transient K+ current was insensitive to EtOH. These results suggest that EtOH-induced reduction of ICa in the peptidergic nerve terminals produces a decrease in AVP release, resulting in lowered plasma AVP levels.

Increase of cardiodepressant activity in medium incubating the posterior pituitary lobe in situ during vagal nerve stimulation in rat

Previous studies have indicated that there is a cardiodepressant factor in the medium incubating the posterior pituitary lobe in situ. The cardiodepressant activity of the medium incubating the posterior pituitary lobe before and during stimulation of the vagus nerves was tested on isolated auricles of the right heart atrium of a two-day-old rat. It was found that the medium incubating the posterior pituitary lobe collected before stimulation decreased the contraction rate of the auricle by 34%, while that collected during the intermittent stimulation of the central ends of the cut vagus nerves caused a decrease of the auricle contractions frequency by 52%. The addition of cholinergic, serotoninergic, histaminergic receptor blockers or prostaglandin synthetase into Ringer-Lock's solution bathing the auricle has no effect on the changes of the contraction rate caused by the incubation medium.

Sodium inhibits hormone release and stimulates calcium efflux from isolated nerve endings of the rat neurohypophysis

1. We studied the effects of extracellular sodium on the secretion of vasopressin (VP) and oxytocin (OT) and the efflux of 45Ca from isolated, perfused nerve endings of the rat neurohypophysis (neurosecretosomes). 2. Upon removal of sodium from the perfusing medium, basal release of VP and OT increased by 3.95 +/- 0.23- and 3.71 +/- 0.22-fold, respectively, followed by a decline to about double the levels in normal (150 mM) sodium (P less than or equal to 0.1). 3. Compared to neurosecretosomes perfused in normal (150 mM) sodium, omission of sodium from the medium augmented ionomycin-induced VP and OT secretion by 66 +/- 5- and 20 +/- 3-fold, respectively, and A23187-induced secretion was increased 1.3 +/- 0.4- and 1.3 +/- 0.1-fold (P less than or equal to 0.01 for both ionophores). 4. The inhibition of ionomycin-induced secretion by sodium was concentration dependent (P less than or equal to 0.01 for sodium greater than or equal to 5 mM); the IC50 was about 10 mM sodium for both hormones, and the Hill slope was close to -1. 5. The rate of 45Ca efflux from neurosecretosomes showed 2.7 +/- 0.1-fold stimulation upon increasing sodium from 4.5 to 150 mM (P less than or equal to 0.01). 6. Our results suggest that sodium inhibits basal and stimulated secretion at the nerve terminal, possibly by reducing intraterminal calcium through sodium/calcium exchange.

Direct identification of individual vasopressin-containing nerve terminals of the rat neurohypophysis after 'whole-cell' patch-clamp recordings

The membrane currents in rat neurophypophysial nerve terminals, which contain either vasopressin or oxytocin, have been previously recorded using the 'whole-cell' patch-clamp technique. Interpretation of the electrophysiological data would be significantly strengthened by the ability to correlate them with knowledge of the peptide contents of the terminals being studied. Here, a novel method for detection of the peptide hormone, arginine vasopressin, in those individual isolated terminals is described. The unique aspect of this procedure is that the contents of the terminal are aspirated into the recording electrode after 'whole-cell' patch-clamp recording, and then a highly sensitive dot immunobinding assay allows identification of the peptide contents in the terminals.

Modulation of oxytocin and vasopressin release from rat neurosecretosomes: the roles of VIP oxytocin and GABA

The effects of vasoactive intestinal polypeptide (VIP), of a selective oxytocin antagonist and of GABA on basal and stimulated oxytocin and vasopressin release from isolated neurosecretory endings were investigated. Superfusion of the secretosomes with VIP (10(-7) M) induced an increased basal and stimulated release of both oxytocin and vasopressin. Addition of the oxytocin antagonist induced a decrease of the stimulated oxytocin release as compared to the control which indicated a positive feedback mechanism of oxytocin on oxytocin release. In presence of GABA (1 or 50 microM) no change in basal or stimulated oxytocin and vasopressin release was observed.

Electron microscopic and immunocytochemical study of rapidly frozen, freeze-substituted neural lobes of rats

Rapid freezing of freshly dissected or incubated neural lobes was explored as a means of obtaining ultrastructural preservation of the more natural state of this tissue. A quantitative assessment of the region of good fixation was made in order to determine the relative fractions occupied by axons, pituicytes and the extracellular space. The immunocytochemical distributions of neurophysins and the glycopeptide portion of the vasopressin precursor were evaluated using the immunogold technique in order to determine the relative numbers of oxytocin and vasopressin fibre types in the fixed region, and the subcellular localization of these antigens. The uncut surface of rat neural lobes was rapidly frozen against a highly polished copper plug and freeze-substituted in osmium-acetone either immediately after dissection (approximately 2 min), or after a 15 min incubation period in vitro in an oxygenated, balanced salt solution. Substituted neural lobes were prepared for either conventional electron microscopy, or for immunogold labelling of neurophysins and the glycopeptide precursor to vasopressin. Membranes, subcellular organelles and extracellular matrix were well preserved 10 microns deep to the contacted surface. The extracellular space accounted for approximately 30% of the cross-sectional area of the neuropil and could be divided into two domains: an extended perivascular space (28-29% of total area); and a narrow (approximately 24 nm; approximately 1% of total) space between closely apposed neurosecretory processes or between these processes and pituicytes. Pituicytes accounted for about 30% of the area and axons 20-25%. Pituicytes occupied close to 60% of the basal lamina at the neurohaemal contact zone, while axons occupied approximately 20%. There were no differences between neural lobes frozen immediately after dissection and those incubated for 15 min in any of these measures, suggesting minimal fluid redistribution. Gold particles were specifically localized over large (100-200 nm) dense core vesicles, and less frequently over multivesicular bodies and lysosomes. No etching of the plastic or reduction of osmium was necessary to achieve labelling. Specific labelling of one set of terminals and axons (about 80%) was observed with the monoclonal antibody previously shown to be specific for oxytocin-neurophysin, while in neighbouring sections the remaining 20% of the processes were labelled with the antiserum to the vasopressin precursor, or with non-specific antibodies to neurophysins. In conclusion, ultrarapid freezing preserves a large extracellular space in the neural lobe and provides for high resolution morphological and immunocytochemical studies of neurohypophysial structure.

A fast, transient K+ current in neurohypophysial nerve terminals of the rat

1. Nerve terminals of the rat posterior pituitary were acutely dissociated and identified using a combination of morphological and immunohistochemical techniques. Macroscopic terminal membrane currents and voltages were studied using the whole-cell patch clamp technique. 2. In physiological solutions, depolarizing voltage clamp steps, from a holding potential (-80 mV) similar to the normal terminal resting potential, elicited a fast, inward followed by a fast, transient, outward current. 3. The threshold of activation for the outward current was -60 mV. The outward current quickly reached a peak and then decayed more slowly. The decay was fitted by two exponentials with time constants of 21 +/- 2.9 and 143 +/- 36 ms. These decay constants did not show a dependence on voltage. The time to peak of the outward current decreased and the amplitude increased with increasingly depolarized potential steps. 4. The outward current was blocked by the substitution of K+ with Cs+ and its reversal potential was consistent with a potassium current. 5. The transient outward current showed steady-state inactivation at more depolarized (than -80 mV) holding potentials with 50% inactivation occurring at -47.9 mV. The time course of recovery from inactivation was complex with full recovery taking greater than 16 s. 6. 4-Aminopyridine (4-AP) blocked the transient outward current in a dose-dependent manner (approximately IC50 = 3 mM), while charybdotoxin (4 micrograms/ml) and tetraethylammonium (100 mM) had no effect on the current amplitude. 7. Lowering external [Ca2+] had no effect on the fast, transient outward current nor did the calcium channel blocker Cd2+ (2 mM). 8. The neurohypophysial outward current reported here corresponds most closely to IA, and not to the delayed rectifier or Ca2(+)-activated K+ currents. Neurohypophysial IA, however, appears to be different from the outward currents found in the cell bodies in the hypothalamus which project their axons to the posterior pituitary. 9. Under current clamp, evoked action potential duration increased (122%) upon application of 5 mM-4-AP, indicating that IA is involved in neurohypophysial spike repolarization. 10. The existence of this current could help explain why maximal peptide release only occurs in response to bursts of electrical activity invading the nerve terminals.

Characteristics of ascorbic acid uptake by isolated ox neurohypophyseal nerve terminals and the influence of glucocorticoid and tri-iodothyronine on uptake

Isolated nerve endings (neurosecretosomes) from ox neurohypophyses took up L-[14C]ascorbic acid by a process or processes which showed energy dependence and which could be inhibited by unlabelled ascorbic acid in micromolar concentrations and by isoascorbic acid in millimolar concentrations, whereas dehydroascorbic acid only inhibited in concentrations of about 100 mM. The uptake showed saturation with increasing concentration of ascorbic acid and a Km value of 97 microM. Uptake was inhibited by increasing glucose concentration in the medium or by adding cytochalasin B, phloridzin, ethanol or probenecid to the medium. The uptake was inhibited by lowering the sodium concentration and by lack of calcium. These facts suggest the presence of both a glucose-dependent uptake and a sodium-dependent uptake. Cortisol and tri-iodothyronine inhibited uptake. This effect of cortisol, but not of tri-iodothyronine, was dependent on the presence of sodium in the medium. For both hormones it was still present when phloridzin or probenecid was added to the medium.

Chronic naltrexone infusion: effects on innervation of rat neurointermediate lobe

The rat pituitary neurointermediate lobe contains nerve fibers which are sensitive to the neurotoxic effects of 6-hydroxydopamine and to acute injections of the opiate antagonist, naltrexone. In the present study, naltrexone hydrochloride was administered to adult male Sprague-Dawley rats in a chronic infusion over a period of one week, using Alza osmotic pumps to determine whether a low (10(-5)M) dose could, over a longer period, also induce toxic effects on pituitary innervation. Electron microscopy of pituitary intermediate lobe fibers revealed swollen neurovesicles and membranous structures in nerve terminals, and significantly fewer normal-appearing nerve terminals, after drug infusion. Light microscopic immunostaining of paraffin sections of pituitary glands illustrated some diminution of serotonin (5-HT)-immunofluorescence in the intermediate lobe after naltrexone treatment. Immunostaining of sections from the same animals for tyrosine hydroxylase (TH) revealed intensified fiber staining in the intermediate lobe, with some regions of terminals having a swollen appearance. High pressure liquid chromatographic-electrochemical detection (HPLC-EC) analysis of neurotransmitters in selected brain areas and pituitary indicated a significant increase in norepinephrine levels in the hypothalamus. The observations suggest that intermediate lobe innervation is sensitive to low, continuously infused doses of naltrexone, which acts as a neurotoxin to produce degenerative changes in nerve terminals. The changes appear to be reflected in alterations in staining patterns of neurotransmitters, and may also affect transmitter uptake in damaged terminals.

Microvesicles of the neurohypophysis are biochemically related to small synaptic vesicles of presynaptic nerve terminals

Nerve endings of the posterior pituitary are densely populated by dense-core neurosecretory granules which are the storage sites for peptide neurohormones. In addition, they contain numerous clear microvesicles which are the same size as small synaptic vesicles of typical presynaptic nerve terminals. Several of the major proteins of small synaptic vesicles of presynaptic nerve terminals are present at high concentration in the posterior pituitary. We have now investigated the subcellular localization of such proteins. By immunogold electron microscopy carried out on bovine neurohypophysis we have found that three of these proteins, synapsin I, Protein III, and synaptophysin (protein p38) were concentrated on microvesicles but were not detectable in the membranes of neurosecretory granules. In addition, we have studied the distribution of the same proteins and of the synaptic vesicle protein p65 in subcellular fractions of bovine posterior pituitaries obtained by sucrose density centrifugation. We have found that the intrinsic membrane proteins synaptophysin and p65 had an identical distribution and were restricted to low density fractions of the gradient which contained numerous clear microvesicles with a size range the same as that of small synaptic vesicles. The peripheral membrane proteins synapsin I and Protein III exhibited a broader distribution extending into the denser part of the gradient. However, the amount of these proteins clearly declined in the fractions preceding the peak of neurosecretory granules. Our results suggest that microvesicles of the neurohypophysis are biochemically related to small synaptic vesicles of all other nerve terminals and argue against the hypothesis that such vesicles represent an endocytic byproduct of exocytosis of neurosecretory granules.

Oxytocin stimulates oxytocin release from isolated nerve terminals of rat neural lobes

Nerve endings from rat neural lobes isolated by homogenization were placed on a filter and constantly superfused. The effects of exogenous oxytocin and vasopressin (both added at 1 nM concentration) on basal and stimulated release of oxytocin and vasopressin were investigated. Stimulated release was evoked by 30 mM potassium and a simultaneous increase in osmolarity. A stimulatory effect of oxytocin on basal and evoked release of oxytocin was found while there was no effect on vasopressin release. The addition of vasopressin did not induce any change in the release of either hormone. The positive feedback mechanism observed with oxytocin might be active during the discharge of oxytocin which is known to occur in bursts.

Spike propagation and conduction failure in the rat neural lobe

1. Single units were recorded from the rat hypothalamo-neurohypophysial system in vivo to test the hypothesis that action potential conduction failure might contribute to the relative inefficiency of neurohypophysial hormone release at low frequencies of stimulation, and following prolonged stimulation. 2. Recordings were made from the cell bodies of supraoptic neurones which project to the neural lobe of the pituitary. Stimuli applied to the neural lobe evoked antidromic action potentials (in ten of forty cells) at times when the axonal membrane at the site of stimulation should have been refractory following the passage of a spontaneous, orthodromically conducted action potential. This observation suggests that failure of orthodromic action potentials may occur intermittently in the neural lobe. 3. Recordings from single units in the neural lobe showed similar spontaneous patterns of activity to those seen from cell bodies in the supraoptic nucleus. 4. Stimuli applied to the neural stalk evoked orthodromically conducted spikes in these single units: evoked spikes followed stimulation faithfully at 50-80 Hz for 1 s or at 20 Hz for 1 min. Such stimulation was accompanied by a reduction in spike height and a prolongation of latency. 5. Comparable changes were seen in the latency and amplitude of evoked potentials recorded from the neural lobe with low-resistance electrodes. 6. Stalk stimulation at 50 Hz for 1 s was accompanied by a reduction in the threshold for initiation of action potentials, suggesting an increase in the excitability of neural lobe axons. 7. We conclude that, during low-frequency activation, spike failure occurs intermittently in neurohypophysial axons, and that changes in the excitability of the axons during activation at high frequencies may contribute to the facilitation of neurohypophysial hormone release that occurs with increasing frequencies of stimulation.

Abundant GABAergic innervation of rat posterior pituitary revealed by inhibition of GABA-transaminase

An antibody against gamma-aminobutyric acid (GABA) was used to identify GABAergic elements immunocytochemically in the rat posterior pituitary. In order to increase the intracellular concentration of GABA, rats were treated with the GABA-transaminase inhibitor gamma-vinyl-GABA (GVG). Light-microscopic observations of Vibratome and semithin sections revealed the presence of numerous immunoreactive nerve fibers throughout the neural lobe; the mean number and length of these fibers increased by 90% after GVG treatment. Electron microscopy demonstrated the immunostained axons to be of small diameter. The reaction product was confined to small vesicles. No immunostaining occurred in pituicytes. The richness of the GABAergic innervation of the neural lobe contrasts with previous reports using antibodies against glutamate decarboxylase and supports the idea that GABA participates in the presynaptic control of neurosecretion.

Effect of electrical stimulation of the ventromedial nucleus and the dorsomedial nucleus on the activity of tuberoinfundibular dopaminergic neurons

Perikarya and terminals of tuberoinfundibular dopaminergic (TIDA) neurons are located in the arcuate nucleus (ARN) and in the median eminence (ME), respectively. Dopamine (DA) released from TIDA terminals in the ME inhibits prolactin secretion from the anterior pituitary. Anatomical studies have described the sources of afferents to ARN and ME, but not to TIDA neurons per se. The ventromedial nucleus (VMN) and the dorsomedial nucleus (DMN) of the hypothalamus project to ARN and ME and have a role in prolactin regulation. In the present study, VMN and DMN were investigated as possible sources of TIDA afferents. Alterations in the activity of TIDA neurons were estimated by measuring plasma concentrations of prolactin and the rates of DA synthesis (3,4-dihydroxyphenylalanine - DOPA - accumulation after administration of the decarboxylase inhibitor NSD 1015) and metabolism (concentrations of the DA metabolite 3,4-dihydroxyphenylacetic acid - DOPAC) in the ME following electrical stimulation of ARN, VMN, and DMN in ovariectomized female rats. Thirty minutes of bilateral stimulation of ARN or DMN increased DOPA accumulation in the ME; stimulation of the VMN had no effect. 5-Hydroxytryptamine synthesis in the ME was unaffected by stimulation of any region. Plasma prolactin levels declined during DMN stimulation, varying with the frequency and duration of the electrical stimulus. DA metabolism within TIDA neurons increased with DMN stimulation, as evidenced by increased DOPAC concentrations in the ME. In females whose basal TIDA activity has been increased by haloperidol treatment or decreased by bromocriptine treatment, DMN stimulation was still able to increase DOPA accumulation in the ME. The present data suggest the presence of stimulatory TIDA afferents originating from or passing through the DMN.

Morphology of the neural lobe of the hypophysis in rats treated with furosemide. I. Neurosecretory axons

The changes found in the neurosecretory axons of the neural lobe of the hypophysis were studied in rats treated with furosemide for three days: over the whole period of treatment, the animals, according to each group, were deprived or not of water. In the animals with free access to water the axons contained neurosecretory granules with scarce content and low electron density; only some of them had vacuoles and autophagic bodies. In the animals deprived of water axon morphology was variable and axons showing vacuoles and autophagic bodies were abundant, as well as those presenting lamellar and dense bodies and also those in which filaments prevailed. The most outstanding modifications in this last group of animals were related to the dehydration and were not found in those animals which were given access to water again and which were in consequence rehydrated.

Morphometric electron-microscopic investigation of the neuronal processes in the neurohypophysis in water-loaded, normal and water-deprived rats

The neuronal processes in the neurohypophysis of the rat were analyzed by electron microscopy and morphometry after the secretion of antidiuretic hormone had been fully suppressed by water load. The water was supplied through a catheter inserted in the external jugular vein for 1.5, 2.5 and 24 h, respectively. The neurohypophysis was also examined in normal rats and rats that had been water-deprived for 72 h. The rats were fixed through chronically implanted catheters, so that at the time of fixation the animal was uninfluenced by anaesthesia and surgery. Water load increased the density of the neurosecretory granules in the endings in the zone nearest the basal lamina of the pericapillary space. The interpretation of this is that water load fills up the readily releasable pool of the neurosecretory granules. Water-deprival increased the density of dispersed microvesicles in the endings, especially in the zone near the basal lamina, and it is suggested that the dispersed microvesicles are involved in membrane recapture.

Morphology of the neural lobe of the hypophysis in rats treated with furosemide. II. Pituicytes

Following three days of furosemide administration, the characteristics of the pituicytes in the neural lobe of the rat hypophysis were studied. The pituicytes of the animals which were treated with furosemide and had access to water during the treatment showed scarce cytoplasm organelles, conferring them with a clear appearance (light pituicytes), and a frequent finding was neurosecretory axons enclosed in their own cytoplasm. In the animals deprived of water during treatment, the pituicytes were dense (dark pituicytes) elongated, with a well-developed Golgi apparatus, numerous profiles of smooth endoplasmic reticulum and frequent annular gap junctions. It is concluded that the dehydration induced by the diuretic agent is the cause of the prevalence of the dark pituicytes.

Localisation of [3H] clonidine binding in rat neurohypophysis by means of electron-microscopic autoradiography

Electron-microscopic autoradiography of rat neurohypophyses treated with [3H] clonidine, an alpha 2-agonist, showed that binding apparently occurred preferentially at the neurosecretory endings and blood vessels rather than on the pituicytes. Since it is known that clonidine has a high affinity for plasma proteins, the distribution over the neurosecretory nerve endings would suggest the existence of presynaptic alpha 2-binding sites on neurosecretory neurones, which could indicate a regulatory function for catecholamines in neurohypophysial hormone release.

Intragranular colocalization of immunoreactive methionine-enkephalin and oxytocin within the nerve terminals in the posterior pituitary

To determine differential tissue antigens in the same section immunocytochemically using the electron microscope, the neurohypophysis was examined following the application of a freeze-drying tissue preparation and staining with the protein A-colloidal gold-antibody complex method (Hisano S, Adachi T, Daikoku S: J Histochem Cytochem 32:705, 1984). At the light microscopic level, colocalized immunostaining for methionine-enkephalin (ENK) and oxytocin (OXT) was found in the rat neurohypophysis under different physiological states. Small pieces of the neurohypophysial tissue were frozen and dried. The dried tissue was fixed with paraformaldehyde vapor and embedded. The ultrathin sections were stained with the antibody for ENK coupled with protein A-small colloidal gold, and antibody for OXT or vasopressin (VP) conjugated with protein A-large colloidal gold. The ultrastructures of the nerve terminals were well preserved and showed many membrane-limited secretory granules. It was possible to identify both OXT- and VP-containing nerve terminals as their secretory granules were differentially labeled with protein A-colloidal gold anti-OXT or anti-VP complex, respectively. The secretory granules, which were labeled with large gold particles for OXT, also carry small gold particles. It is evident that ENK coexists with OXT in the same granules.

Central and peripheral catecholamine innervation of the rat intermediate and posterior pituitary lobes

Catecholamines were measured in the separated intermediate and posterior lobes of the rat after stalk section and bilateral superior cervical ganglionectomy. In the intermediate lobe, after stalk section, dopamine decreases 96%, norepinephrine decreases 80%, and epinephrine is undetectable. In the posterior lobe, dopamine and epinephrine are absent, and norepinephrine decreases by 70% after the operation. In contrast, bilateral superior cervical ganglionectomy produces only a 40% decrease in norepinephrine in the posterior lobe, and no changes in catecholamines in the intermediate lobe. Our results demonstrate that most of the intermediate and posterior pituitary catecholamines are present in nerve fibers of brain origin, and that the posterior lobe has a dual norepinephrine innervation, partly originating in the superior cervical ganglia.

Subfornical organ efferents influence the excitability of neurohypophyseal and tuberoinfundibular paraventricular nucleus neurons in the rat

Electrical stimulation in the subfornical organ (SFO) alters the excitability of antidromically identified paraventricular nucleus neurons. Extracellular recordings demonstrate that the dominant effect of single stimuli delivered to the SFO on neurohypophyseal oxytocin and vasopressin containing neurons is an increase in excitability. In 35% of cells tested, this excitation showed a long latency (44.3 +/- 3.4 ms) prolonged duration (208.7 +/- 23.5 ms), while in 16% of the neurons the excitation observed may be described as short latency (24.7 +/- 1.8 ms) short duration (11.6 +/- 1.4 ms). Of the remaining cells antidromically identified as projecting to the posterior pituitary, 12% showed initial decreases in excitability following SFO stimulation while the remaining 37% were unaffected. Evidence is presented demonstrating that stimulation in the region of the SFO results in short latency (27.9 +/- 2.4 ms) short duration (7.8 +/- 0.7 ms) increases in excitability in 22% of antidromically identified PVN tuberoinfundibular neurons tested. These data provide electrophysiological evidence in support of the proposed role of the subfornical organ in the control of posterior and anterior pituitary function.

Properties of the GABA receptors located on spinal primary afferent neurones and hypophyseal neuroendocrine cells of the rat

Electrophysiological techniques have been used to study the pharmacological characteristics of GABA receptors in two in vitro preparations likely to provide the ionic basis for GABAergic inhibition of excitation-secretion coupling. The shortening of Ca2+ spikes duration by GABAB receptors was shown to occur in slow conducting dorsal root ganglion cells, independently of marked depression of inward calcium currents. Ion-selective electrodes (K+ or Ca2+) were used to show the presence of both GABAA and GABAB receptors on the neurosecretory terminals and gland cells from hypophyseal neuro-intermediate lobe (NIL). In this latter preparation, potentiation of hormone release was observed under GABAA receptor activation, whilst inhibition was seen with GABAB agonists.

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.