On the possible central effects of calcium antagonists

Discriminative stimulus properties of isradipine: effect of other calcium channel blockers

This study constitutes the first report of a calcium channel blocker used as a drug capable of controlling differential responding in a drug-discrimination paradigm. Male Sprague-Dawley rats were trained to discriminate between intraperitoneally administered 10.0 mg/kg isradipine and its vehicle in a two-lever, food-motivated, operant task. Once trained, rats displayed a dose-related decrease in discriminative responding when tested with lower isradipine doses. An analysis of the dose-response curve indicated an ED50 = 5.71 mg/kg. As all training and dose-response testing occurred at 60 min postadministration, experiments were conducted with varying injection-to-test intervals ranging from 15-240 min. Results indicate that the optimum time for discriminative performance was at the time used in training, and that discrimination returned to nondrug (vehicle) levels 2 h postinjection. Administration of other L-type calcium channel blockers, viz., nifedipine (5-50 mg/kg), diltiazem (10-60 mg/kg), or nicardipine (0.5-3.0 mg/kg), as well as a novel antipsychotic that inhibits dopamine release (10-30 mg/kg of CGS 10746B), did not produce isradipine-like discriminative effects. Thus, there was no generalization from the training dose of 10 mg/kg isradipine to any of these other agents, and the results are discussed in light of the possible specificity of the isradipine discriminative stimulus cue as it is produced in the central nervous system.

Effects of flunarizine on ocular motor and postural compensation following peripheral vestibular deafferentation in the guinea pig

The aim of the present study was to determine if the calcium channel antagonist flunarizine would affect the time course of vestibular compensation for unilateral labyrinthectomy (UL) in guinea pigs. Animals received either a single IP injection of flunarizine 1 h pre-UL or a series of IP injections every 6 h for 24 h post-UL, starting at 6 h post-UL. Flunarizine was dissolved in 50-100% DMSO or suspended in 10% Tween-80 and administered at a dose of 10 mg/kg in the pre-UL condition and 10 or 20 mg/kg in the post-UL condition. All injections were 1 ml/kg in volume. Spontaneous nystagmus (SN), yaw head tilt (YHT), and roll head tilt (RHT) were measured using video analysis. When dissolved in DMSO and administered 1 h pre-UL, 10 mg/kg flunarizine had a small but significant effect on the rate of RHT compensation; otherwise, flunarizine had no significant effects on SN, YHT, or RHT when dissolved in DMSO. When suspended in Tween-80, 10 mg/kg flunarizine pre-UL resulted in a significant decrease in SN frequency and YHT relative to the control group, although the magnitude of the differences was small. When 20 mg/kg was given post-UL, both SN and YHT showed a small but significant change in the rate of compensation. No significant differences in RHT were observed. These results demonstrate that IP administration of flunarizine at a dose of 10-20 mg/kg IP has little effect on vestibular compensation compared to the effects obtained with low IM doses (0.8 mg/kg) of verapamil given 1 h pre-UL.

Neurochemical mechanisms of recovery from peripheral vestibular lesions (vestibular compensation)

This paper reviews the literature relating to the neurochemical basis of vestibular compensation, a process of behavioral recovery which occurs following the removal of afferent input from one labyrinth (unilateral labyrinthectomy, UL). Although vestibular compensation is known to be correlated with a return of resting activity to the vestibular nucleus (VN) ipsilateral to the UL (the deafferented VN), the neurochemical mechanisms by which this neuronal recovery occurs, are unknown. At present, there is little evidence to support the hypothesis that denervation supersensitivity of excitatory amino acid, dopamine, norepinephrine or acetylcholine receptors in the deafferented VN, is responsible for vestibular compensation: binding studies for glutamate or acetylcholine do not support an upregulation of these receptor types. However, changes in the affinity or efficacy of these receptor complexes cannot be ruled out. There are still many neurotransmitter systems, such as serotonergic and histaminergic systems, which have not been investigated in relation to vestibular compensation. In several species it has been shown that treatment with adrenocorticotropic hormone, fragment 4-10 (ACTH-(4-10], can accelerate vestibular compensation. It is not clear how these drugs exert their effects. In vitro electrophysiological studies have shown that VN neurons are capable of generating resting activity in the absence of their normal afferent inputs and it is possible that these neurons have pacemaker-like membrane characteristics which contribute to the regeneration of activity following UL. Recent biochemical studies have revealed changes in the phosphorylation patterns of a number of proteins during compensation. The possible relationship between these phosphorproteins and the synaptic or membrane changes which are responsible for vestibular compensation remains to be determined.

Hearing fluctuation during migraine attacks. Activity of propranolol-HCl in rats

Transitory hearing impairment has been well documented in humans during basilar migraine attacks. Ischemia induced by activation of central nervous system adrenoreceptors by cAMP is one of the mechanisms that has been implicated, as well as calcium metabolism alterations. Propranolol-HCl has proven to be an effective treatment. Massive local adrenaline release is regarded as the primum movens mechanism triggering the liberation of other beta-receptor activators resulting in sterile edema and exudate. This exudate can be visualized (via ophthalmoscope) and is viewed as pathognomonic of a migraine attack. We investigated both the action of adrenaline and Propranolol-HCl on brainstem auditory potentials in 3 groups of young rats. The first group was perfused intra-arterially with adrenaline. The second group received the same treatment plus a prophylactic daily dose of Propranolol-HCl. The third group served as the control and received Propranolol-HCl, but was perfused with a Ringer solution. Alterations in blood flow and hearing sensitivity (BERA) following perfusion occurred in the first group only. Propranolol seems to exhibit a protective action during experimental attempts to induce migraine-like attacks in rats.