Central neurochemical effects of dihydropyridine calcium antagonists

Endothelin modulates dihydropyridine receptor decreased binding in hippocampal slices from normotensive and spontaneously hypertensive rats

Previous studies in rats have demonstrated both the link between voltage-operated calcium channels and endothelin and their cerebral involvement in the pathophysiology of spontaneous hypertension. In the present study, the interaction of endothelin with specific dihydropyridine (DHP) binding sites was investigated using the brain slices model. In rat hippocampal slices, pre-incubation with Bay K 8644 decreased [3H] (+) PN 200-110 binding. There was no difference in agonist-induced decrease of DHP binding in normotensive and spontaneously hypertensive (SH) rats. The effect of Bay K 8644 was partially inhibited by endothelin but not by angiotensin in both normotensive and hypertensive rats. These data compared to those of other studies suggest that DHP binding sites which are regulated by endothelin are post-synaptic. We conclude that brain slices provide a good in vitro model to study DHP receptor regulation and to explore endothelin interactions with DHP-sensitive Ca2+ channels.

On the usefulness of Fura-2 measurements of intrasynaptosomal calcium levels in rat cortical synaptosomes to study mechanisms of presynaptic function

Levels of [Ca2+]i in rat cortex synaptosomes were measured using the Ca2+ indicator Fura-2. Ca2+ influx was induced by veratridine in a concentration-dependent manner (1-10 microM). The resulting increase in [Ca2+]i was inhibited by tetrodotoxin (TTX). K+ (18 mM) increased the [Ca2+]i which was not influenced by TTX. K(+)-channel blockers such as 4-aminopyridine, alpha- and delta-dendrotoxin pre se were ineffective. The veratridine-induced Ca2+ influx in synaptosomes was reduced by L-type Ca(2+)-channel blockers, such as felodipine, nifedipine and PN-200-110, verapamil and diltiazem. omega-Conotoxin, and N-type Ca(2+)-channel blocker, did not inhibit the veratridine-stimulated [Ca2+]i increase. Bay K 8644, and L-channel agonist, stimulated an increase of [Ca2+]i in synaptosomes which was not sensitive to TTX. R-N6-Phenyl-isopropyl-adenosine (R-PIA) and clonidine, agonists at adenosine A1-receptors and alpha 2-adrenoceptors, respectively, did not influence the veratridine-stimulated [Ca2+]i increase. R-PIA did not interact with Bay K 8644-stimulated [Ca2+]i increase in synaptosomes. The results for all the substances used show major differences between the effects on Ca2+ influx in synaptosomes and on the electrically evoked neurotransmitter release in slice preparations. Thus, the synaptosome preparation is not a generally applicable experimental model for the study of Ca2+ mechanisms of presynaptic neuromodulation.

Concurrent nimodipine attenuates the withdrawal signs and the increase of cerebral dihydropyridine binding after chronic morphine treatment in rats

The effect of chronic administration of dihydropyridine calcium channel antagonist nimodipine (1 mg/kg/day) given concurrently with morphine on the signs of morphine withdrawal and on the [3H]nitrendipine binding in the rat brain has been investigated. Chronic morphine administration in increasing daily doses from 20 mg/kg to 70 mg/kg for 24 days and consequent withdrawal for 24 h induced loss of body weight, wet dog shakes, episodes of writhing and yawning behaviour. The density of [3H]nitrendipine binding was elevated in the cortex and limbic structures but not in the striatum after chronic morphine treatment. Chronic concurrent administration of nimodipine prevented the loss of body weight and reduced the scores of wet dog shakes and writhing, but did not affect yawning behaviour at 24 h after morphine withdrawal. The concurrent nimodipine treatment also prevented the rise in the density of central dihydropyridine binding sites which occurred upon chronic morphine treatment. These results suggest that chronic nimodipine treatment attenuates the development of the withdrawal signs which occur upon the termination of chronic morphine treatment by preventing the up-regulation of the central dihydropyridine-sensitive binding sites.

An examination of the anticonvulsant properties of voltage-sensitive calcium channel inhibitors in amygdala kindled seizures

Voltage-sensitive calcium (VSC) channels may contribute to epileptogenesis. A systematic examination of the anticonvulsant efficacy of different classes of VSC channel inhibitors, however, is lacking in chronic seizure models. The present study evaluated representatives from three different classes of VSC channel inhibitors for their protection against amygdala kindled seizures. Adult male rats (n = 12) were kindled to stage 5 seizures (GS), and a threshold intensity required to evoke a GS was determined. The Ca(++)-channel inhibitors (verapamil 0, 10, 20, 40 mg/kg; nimodipine 0, 5, 25, 50 mg/kg; nitrendipine 0, 25, 50, 100 mg/kg and flunarizine 0, 20, 40, 80 mg/kg) were administered 60-90 min prior to amygdala stimulation at the established threshold. None of the drugs altered threshold for inducing a seizure. Verapamil, a phenylalkylamine, and the dihydropyridines nimodipine and nitrendipine were without effect on kindled seizures. The diphenylalkylamine, flunarizine, was found to be the most efficacious, reducing AD duration and duration of clonic seizure activity by more than 60% in most animals. Flunarizine also decreased the severity of behavioral seizures, with 40% of the animals displaying Stage 1-2 seizures only. It is concluded that some VSC Ca(++)-channel inhibitors do possess anticonvulsant potential. Thus influx of extracellular calcium through VSC channels may contribute to the expression of kindled seizures.