Neurotransmitter-mediated changes in the electrophysiological properties of pituicytes.

Abstract Intracellular recordings were obtained from pituicytes in the neural lobe of the isolated rat pituitary. Like other glia, pituicytes lacked action potentials in response to depolarizing current injection, but they tended to have more positive resting membrane potentials and higher input resistances than astrocytes in other preparations. Dye-coupling typical of astrocytes was also demonstrated amongst pituicytes, and their morphologies were similar to those of pituicytes stained for glial fibrillary acidic protein. Action potentials, anode-break spikes or barium spikes were not observed in pituicytes, even under conditions that maximized the elicitation of Ca(2+)-dependent responses. This suggests that pituicytes either have no or a very low density of Ca(2+) channels or Ca(2+) currents that are too small to generate action potentials. Dynorphin A (1-13), a kappa-opioid agonist, produced long-lasting increases in pituicyte input resistance with no significant changes in resting membrane potential. Dynorphin's action was concentration-dependent and was blocked by the opioid antagonist naloxone. This is consistent with previous reports demonstrating kappa-opioid receptors on pituicytes in the neurohypophysis. The beta-adrenergic agonist isoproterenol (100 muM) reversed the increases in pituicyte input resistance produced by opioid application, with no significant changes in resting membrane potential. The fact that pituicytes responded to neurotransmitters suggests a functional link between pituicytes and neurosecretory nerve fibres.

Electrophysiological properties of reactive glial cells in the kainate-lesioned hippocampal slice.

Kainate-lesioned hippocampal slices provide an excellent system for examining the electrophysiological properties of non-cultured glial cells without interfering signals from surrounding neurons. Intracellular recordings in the intensely gliotic CA3 region indicated that the resting membrane potential and input resistance of these reactive glial cells were similar to those reported for non-reactive astrocytes. Dye-coupling typical of astrocytes was also demonstrated amongst these cells, and was considerably reduced by cytoplasmic acidification. Occasionally these cells demonstrated spontaneous, rhythmic oscillations of membrane potential associated with large changes in whole cell input resistance. The action potentials reported in cultured astrocytes were not observed in reactive glial cells, even under conditions that maximize the observation of Ca2(+)-dependent responses. This suggests that reactive glial cells in this preparation have either no voltage-activated Ca2+ channels or a very low density of such channels. These cells also lack a bicarbonate conductance, but they do appear to have an apamin-sensitive conductance, possibly a Ca2(+)-activated gK.

Membrane conductance oscillations in astrocytes induced by phorbol ester.

Glial cells in the central nervous systems (CNS) have complex functions which are difficult to decipher because of the intimate intertwining of glial cells with neurons. We have therefore developed an essentially neuron-free preparation of CNS astrocytes in the kainic acid lesioned hippocampal slice. With this preparation we have examined the effect of activating protein kinase C in astrocytes with a phorbol ester, TPA (12-O-tetradecanoyl-phorbol-13-acetate). In most cells, TPA induced rhythmic oscillations (0.1-3.0 Hz) of membrane potential which were typically 5-10 mV in amplitude and were associated with increases of up to eightfold in input resistance during the depolarizing phase. These large changes in membrane conductance are the first reported observations of endogenously generated conductance changes in astrocytes of the mammalian CNS and they could influence excitability of surrounding neurons, possibly by altering extracellular ion concentrations.

Altered sensitivity to arginine vasopressin (AVP) in area CA1 of the hippocampal slice following pretreatment of rats with AVP.

Extracellular recordings from cells in the CA1b region of the in vitro hippocampal slice preparation demonstrate that bath-applied AVP (10(-6)-10(-12) M) frequently results in a decrease in the orthodromically evoked population spike amplitude. This suggests that AVP inhibits CA1 pyramidal cell firing in response to an orthodromic volley. This effect appears to be receptor-mediated, since a potent antagonist of the AVP V1 (vasopressor) receptor and a mixed oxytocin/vasopressin antagonist prevented the decrease in population spike amplitude observed in response to bath application of AVP. Hippocampal slices prepared from rats injected two days earlier with 1.0 micrograms AVP (intracerebroventricular) display increased sensitivity to the depressant effects of AVP at lower doses compared to controls. These results suggest that pretreatment of rats with AVP may alter the sensitivity of hippocampal cells to the depressant effects of this neuropeptide.

Prevention of arginine-vasopressin-induced motor disturbances by a potent vasopressor antagonist.

The antivasopressor analog d(CH2)5Tyr(Me) arginine-vasopressin completely blocked the convulsive-like behavior and other severe motor disturbances which are normally observed following a second central arginine-vasopressin injection. This vasopressor antagonist appears to be selective for arginine-vasopressin-induced motor disturbances, in that the convulsive and motor effects of pentylenetetrazol and somatostatin were not altered significantly by pretreatment with the central antagonist. Results suggest that arginine-vasopressin-induced motor disturbances are mediated via central receptors. The classic antidiuretic (V2) type of arginine-vasopressin receptor does not appear to be involved, since the agonist 1-desamino-8-D-arginine-vasopressin did not elicit convulsive-like behavior or other severe motor disturbances 2 days following a first ('priming') injection of arginine-vasopressin.

Vasopressin-induced motor disturbances: localization of a sensitive forebrain site in the rat.

Arginine-vasopressin (AVP) microinjected into an area extending from the diagonal band of Broca to the anterior hypothalamus of the rat evokes severe motor disturbances, including barrel rotations and myoclonic/myotonic movements. These disturbances do not occur after administration of an artificial physiological solution or of oxytocin. Injection of this peptide into other areas of the brain does not cause these effects. This action of vasopressin is dose-related, can be prevented by the prior administration of an AVP receptor antagonist and involves a 'sensitization' process. It is possible that AVP, acting in this mediobasal region of the forebrain, might be involved as a causative agent in some convulsive disorders.

Brattleboro rats display increased sensitivity to arginine vasopressin-induced motor disturbances.

Motor disturbances observed in Brattleboro rats (homozygous for the diabetes insipidus (DI) trait) following a first intracerebroventricular injection of 1.0 microgram of arginine vasopressin (AVP) were not significantly different from those of the parent Long-Evans (LE) strain. These disturbances consisted of periods of staring and immobility, locomotor difficulties and discrete myoclonic jerks, often followed by scratching behavior. Following a second central injection of 10.0 ng AVP, however, the DI strain exhibited more pronounced motor disturbances than the LE strain. This increased sensitivity of the DI strain to the behavioral actions of AVP does not appear to be due to a generalized decrease of seizure threshold, since no significant differences were observed between the two strains in their susceptibility to convulse following pentylenetetrazol. As the behavioral and motor effects of AVP appear to be receptor-mediated, these findings suggest that homozygous DI rats possess central AVP receptors, which, in the absence of endogenous vasopressin, may have increased sensitivity to a second central injection of AVP.

Increased motor disturbances in response to arginine vasopressin following hemorrhage or hypertonic saline: evidence for central AVP release in rats.

The effects of hemorrhage and parenteral hypertonic saline on the behavioural responses to centrally-administered arginine vasopressin (AVP) were examined in rats. Both hemorrhage and hypertonic saline act as potent stimuli for neurohypophysial vasopressin release, and may serve as potential stimuli for cerebral AVP release. When administered into a lateral cerebral ventricle of the rat brain, AVP has a potent convulsant action; this effect increases in severity upon subsequent administration. Removal of 15% of the estimated blood volume from the conscious rat or infusion of 1.0 ml of 1.5 M sodium chloride solution into the peritoneal cavity can mimic the effect of a central injection of AVP in 'sensitizing' the brain to the behavioural effects of subsequent injections of AVP. This suggests that these stimuli which are known to activate posterior pituitary secretion of AVP also induce the release of AVP (or a closely related molecule), from neuronal fibres within the brain.