Differential involvement of voltage-dependent calcium channels in apomorphine-induced hypermotility and stereotypy

Inhibition of evoked glutamate release by neurosteroid allopregnanolone via inhibition of L-type calcium channels in rat medial prefrontal cortex

Allopregnanolone is one of the most important neurosteroids in the brain. We studied the effect and mechanism of allopregnanolone on spontaneous and evoked glutamate release in the medial prefrontal cortex using electrophysiological and biochemical methods combined with pharmacological approaches. The results showed that allopregnanolone had no effects on the frequency of miniature excitatory postsynaptic current (mEPSCs), but inhibited the depolarizing agent veratridine-evoked increase in the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) and inhibited the first of the two responses evoked by a pair of electrical pulses more effectively than the second, resulting in increased paired-pulse facilitation (PPF) and thus suggesting a presynaptic inhibitory effect on electrical pulse-evoked glutamate release. A similar effect was also obtained for the effect of allopregnanolone on protein kinase A (PKA) activation, an upstream event of presynaptic glutamate release. Interestingly, allopregnanolone had none of these effects in the striatum. In the study of the upstream mechanism of the PKA inhibition by allopregnanolone, we found that allopregnanolone inhibited extracellular calcium influx-evoked PKA activation, but had no effects on intracellular calcium store release-evoked PKA activation; L-type calcium channel antagonists, but not N- and P/Q-type calcium channel antagonist, blocked the effect of allopregnanolone; allopregnanolone inhibited L-type calcium channel agonist-evoked increase in the PKA activity, intrasynaptosomal calcium concentration and frequency of sEPSCs. These results suggest that allopregnanolone inhibits evoked glutamate release via the inhibition of L-type calcium channels in the medial prefrontal cortex, but does not in the striatum.

L-type Ca(2+) channel blockers inhibit the development but not the expression of sensitization to morphine in mice

The relationship between opioid actions and L-type Ca(2+) channel blockers has been well documented. However, there is no report relevant to L-type Ca(2+) channel blockers and morphine sensitization, which is suggested to be an analog of behaviors that are characteristic of drug addiction. We now studied systematically the effects of three L-type Ca(2+) channel blockers, nimodipine, nifedipine and verapamil, on morphine-induced locomotor activity, the development and the expression of sensitization to morphine. The results showed that both nimodipine and verapamil attenuated, while nifedipine had only a tendency to decrease morphine-induced locomotor activity. All three drugs inhibited the development of sensitization to morphine. However, none of them showed any effects on the expression of morphine sensitization. These results indicate that blocking L-type Ca(2+) channel attenuates the locomotor-stimulating effects of morphine and inhibits the development but not the expression of morphine sensitization.

Effects of Ca2+ channel blockers on apomorphine, bromocriptine and morphine-induced locomotor activity in mice

The effects of L-type voltage-dependent Ca2+ channel blockers on apomorphine, bromocriptine and morphine-induced changes in locomotor activity were examined in mice. Apomorphine (4 mg/kg) and morphine (20 mg/kg) produced locomotor stimulation. Bromocriptine (8 mg/kg) produced a biphasic effect on motor behaviour, an early depressant phase, followed by locomotor stimulation. Amlodipine (2.5 mg/kg), nicardipine (10 mg/kg), diltiazem (10 mg/kg) and verapamil (10 mg/kg), which by itself did not affect locomotor activity, inhibited the stimulant phase of bromocriptine without altering the depressant phase, while they did not affect apomorphine- and morphine-induced locomotor stimulation. Apomorphine, bromocriptine and morphine-induced locomotor stimulation was decreased by SCH 23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7- ol) (0.05 mg/kg) or haloperidol (0.1 mg/kg). These results indicate that L-type voltage-dependent Ca2+ channels are involved in the motor stimulant effect of bromocriptine, but not in apomorphine- and morphine-induced locomotor stimulation. The effects of Ca2+ channel blockers on the dopaminergic system appears not to be directly related to dopamine receptor blockade.

Modification of effects of chronic electroconvulsive shock by voltage-dependent Ca2+ channel blockade with nifedipine

A single electroconvulsive shock produced analgesia (expressed as prolongation of hot-plate latency) in Wistar rats 45 min after the shock. The analgesic action was prevented by administration of nifedipine, 5 mg/kg i.p., 15 min before the electroconvulsive shock, while nifedipine injection after electroconvulsive shock did not affect the analgesia significantly. At the same time single electroconvulsive shock counteracted the reduction of [3H]nitrendipine binding to cortical and hippocampal membranes from rats pretreated with nifedipine. Chronic administration of electroconvulsive shock (once daily for 8 days) produced hyperalgesia, augmented locomotor responses to low doses of apomorphine and upregulation of cortical (but not hippocampal) voltage-dependent Ca2+ channels (assessed from [3H]nitrendipine binding). In rats receiving electroconvulsive shock chronically, always 15 min after nifedipine injection, neither behavioral hyperresponsiveness nor Ca2+ channel upregulation was observed. The results suggest that the primary event in post-electroconvulsive shock analgesia depends on Ca2+ influx into neurons through voltage-dependent Ca2+ channels, and that given under conditions of Ca2+ channel blockade electroconvulsive shock is unable to trigger changes leading to Ca2+ channel upregulation, and this is possibly the reason for prevention of development of hyperalgesia and increased responsiveness to dopaminergic stimulation.