Do dihydropyridine-sensitive calcium channels play a role in neurosecretion in the central nervous system?

High concentrations of KCl release noradrenaline from noradrenergic neurons in the rat anococcygeus muscle

The aim of the present study was to investigate the effects of high concentrations of KCl in releasing noradrenaline from sympathetic nerves and its actions on postsynaptic alpha-adrenoceptors. We measured the isotonic contractions induced by KCl in the isolated rat anococcygeus muscle under different experimental conditions. The contractile responses induced by KCl were inhibited by alpha-adrenoceptor antagonists in 2.5 mM Ca2+ solution. Prazosin reduced the maximum effect from 100 to 53.9 +/- 10.2% (P<0.05) while the pD2 values were not changed. The contractile responses induced by KCl were abolished by prazosin in Ca2+-free solution (P<0.05). Treatment of the rats with reserpine reduced the maximum effect induced by KCl as compared to the contractile responses induced by acetylcholine from 339.5 +/- 157.8 to 167.3 +/- 65.5% (P<0.05), and increased the pD2 from 1.57 +/- 0.01 to 1.65 +/- 0.006 (P<0.05), but abolished the inhibitory effect of prazosin (P<0.05). In contrast, L-NAME increased the contractile responses induced by 120 mM KCl by 6.2 +/- 2.3% (P<0.05), indicating that KCl could stimulate the neurons that release nitric oxide, an inhibitory component of the contractile response induced by KCl. Our results indicate that high concentrations of KCl induce the release of noradrenaline from noradrenergic neurons, which interacts with alpha1-adrenoceptors in smooth muscle cells, producing a contractile response in 2.5 mM Ca2+ (100%) and in Ca2+-free solution, part of which is due to a direct effect of KCl on the rat anococcygeus muscle.

Localization of P-type calcium channels in the central nervous system

The distribution of the P-type calcium channel in the mammalian central nervous system has been demonstrated immunohistochemically by using a polyclonal specific antibody. This antibody was generated after P-channel isolation via a fraction from funnel-web spider toxin (FTX) that blocks the voltage-gated P channels in cerebellar Purkinje cells. In the cerebellar cortex, immunolabeling to the antibody appeared throughout the molecular layer, while all the other regions were negative. Intensely labeled patches of reactivity were seen on Purkinje cell dendrites, especially at bifurcations; much weaker reactivity was present in the soma and stem segment. Electron microscopic localization revealed labeled patches of plasma membrane on the soma, main dendrites, spiny branchlets, and spines; portions of the smooth endoplasmic reticulum were also labeled. Strong labeling was present in the periglomerular cells of the olfactory bulb and scattered neurons in the deep layer of the entorhinal and pyriform cortices. Neurons in the brainstem, habenula, nucleus of the trapezoid body and inferior olive and along the floor of the fourth ventricle were also labeled intensely. Medium-intensity reactions were observed in layer II pyramidal cells of the frontal cortex, the CA1 cells of the hippocampus, the lateral nucleus of the substantia nigra, lateral reticular nucleus, and spinal fifth nucleus. Light labeling was seen in the neocortex, striatum, and in some brainstem neurons.

Depolarization-dependent actions of dihydropyridines on synaptic transmission in the in vitro rat hippocampus

Field potential and intracellular recordings were obtained in the in vitro hippocampal slice to study the effects on synaptic transmission of dihydropyridine (DHP) derivatives. Nimodipine or nifedipine by itself had little effect upon the postsynaptic response as determined by field potential analysis. However, facilitation became evident when DHP application was coupled with manipulations which induced a moderate degree of membrane depolarization. In accordance with the hydrophobic nature of these compounds, extensive washing in normal Krebs' solution failed to reverse the facilitation indicating that the DHP effects outlasted the induced depolarization. Nifedipine is photolabile and its actions were reversed when intense light was applied to the slice. Application of the DHP Bay K 8644, resulted in a similar depolarization-dependent increase in neuronal excitability which, upon washout and exposure to light, was at first attenuated and then reversed, resulting in a long-lasting depression of the EPSP that was sensitive to caffeine. This depressant action of Bay K 8644 appeared to be mediated at a site presynaptic to the pyramidal cell because the postsynaptic component of the field potential response to pulsed applications of glutamate was not altered. Intracellular recording from CA1 neurons supports a presynaptic locus for the depressant actions of Bay K 8644; spike threshold for synaptically evoked responses was greatly increased while spike threshold to direct depolarization of the soma was unchanged. These results indicate that DHPs can exert effects on synaptic transmission in hippocampal brain slice under conditions of moderate membrane depolarization.

Ca2+ and vasopressin release in isolated rat neurohypophysis: differential effects of four classes of Ca2+ channel ligands

In order to study the role of different types of voltage-sensitive Ca2+ channels (VSCC) in stimulus-secretion coupling in peptidergic neurons, effects of 4 major classes of pharmacological agents have been examined on evoked vasopressin release from isolated rat neurohypophyses. omega-Conotoxin GVIA (omega-CgTX), a potent blocker of N- and L-type Ca2+ channels, inhibited vasopressin release evoked electrically as well as by high K+. With maximal inhibition, release was decreased to 50% and 75% of control for electrical and 100 mM K+ stimulation, respectively. This stimulation mode-related difference in release sensitivity to omega-CgTX paralleled its stimulation mode-related sensitivity to tetrodotoxin, suggesting that the omega-CgTX-sensitive Ca2+ entry played a larger role when release was activated by action potentials invading nerve terminals. These data, and the characteristics of [125I]omega-CgTX binding to plasma membranes from bovine neurohypophyses, are consistent with N-type Ca2+ channels being responsible for the omega-CgTX-sensitive component of vasopressin release. Verapamil and diltiazem (phenylalkylamine and benzothiazepine, respectively) inhibited secretion in a pattern suggesting non-identical sets of action sites, and in a manner partly additive with inhibition by omega-CgTX. This inhibition by verapamil and diltiazem appeared at least in part to involve sites different from L channels. Several dihydropyridines known to act as agonists or antagonists at L channels did not affect vasopressin release (evoked either electrically or by high K+) in a specific manner. A significant component of neuropeptide release may depend on Ca2+ entry through omega-CgTX- and dihydropyridine-insensitive routes.