The effects of dihydropyridine compounds in behavioural tests of dopaminergic activity

Psychostimulants, antidepressants and neurokinin-1 receptor antagonists ('motor disinhibitors') have overlapping, but distinct, effects on monoamine transmission: the involvement of L-type Ca2+ channels and implications for the treatment of ADHD

Both psychostimulants and antidepressants target monoamine transporters and, as a consequence, augment monoamine transmission. These two groups of drugs also increase motor activity in preclinical behavioural screens for antidepressants. Substance P-preferring receptor (NK1R) antagonists similarly increase both motor activity in these tests and monoamine transmission in the brain. In this article, the neurochemical and behavioural responses to these three groups of drugs are compared. It becomes evident that NK1R antagonists represent a distinct class of compounds ('motor disinhibitors') that differ substantially from both psychostimulants and antidepressants, especially during states of heightened arousal or stress. Also, all three groups of drugs influence the activation of voltage-gated Ca(v)1.2 and Ca(v)1.3 L-type channels (LTCCs) in the brain, albeit in different ways. This article discusses evidence that points to disruption of these functional interactions between NK1R and LTCCs as a contributing factor in the cognitive and behavioural abnormalities that are prominent features of Attention Deficit Hyperactivity Disorder (ADHD). Arising from this is the interesting possibility that the hyperactivity and impulsivity (as in ADHD) and psychomotor retardation (as in depression) reflect opposite poles of a behavioural continuum. A better understanding of this pharmacological network could help explain why psychostimulants augment motor behaviour during stress (e.g., in preclinical screens for antidepressants) and yet reduce locomotor activity and impulsivity in ADHD. This article is part of the Special Issue entitled 'CNS Stimulants'.

A controlled trial of amlodipine for cocaine dependence: a negative report

Preclinical models of cocaine dependence have shown favorable reductions in cocaine use using dihydropyridine calcium channel antagonists. This is one of the first reports testing the efficacy of the long-acting calcium channel antagonist, amlodipine, for the treatment of cocaine dependence. This was a 12-week, double-blind, randomized, placebo-controlled, parallel patient group trial of amlodipine vs. placebo for the treatment of cocaine dependence. One hundred and sixteen subjects participated in a 12-week medication trial in which 60 subjects received medication and 56 received placebo. Subjects in both groups received up to 12 standard manual-driven cognitive behavioral therapy sessions. Overall, drop-out rate for both groups was high, with only about 20% of subjects completing all 12 weeks of treatment. Both groups showed comparable levels of medication compliance and therapy attendance. In the end, amlodipine was no more effective than placebo in reducing craving or measured levels of cocaine use.

Effects of repeated-dose isradipine on the abuse liability of cocaine

Despite preclinical studies suggesting that isradipine may antagonize the abuse liability of cocaine, pretreatment with sustained-release isradipine did not reduce euphoric mood in cocaine-using volunteers. This double-blind, within-subject, crossover laboratory study determined whether maximal dose-loading with isradipine could antagonize effects of cocaine in 12 cocaine-dependent research volunteers administered intravenous cocaine doses (0, 0.325, and 0.65 mg/kg) on different days after 5 days of treatment with isradipine or placebo. Isradipine dose was 30 mg sustained release nightly plus 15 mg immediate release 2 hr before cocaine infusion. Cocaine produced dose-related increases in cocaine's subjective effects and a behavioral measure of reinforcement. Isradipine enhanced, rather than antagonized, subjective effects, indicating that isradipine does not antagonize cocaine's abuse liability in dependent research volunteers.

Amlodipine treatment of cocaine dependence

Preclinical studies indicate that dihydropyridine-type calcium channel antagonists modulate dopamine neurotransmitter function and can reduce cocaine-reinforced behaviors. Amlodipine, a long-acting dihydropyridine-type calcium channel antagonist related to isradipine and nifedipine, was administered in open label fashion for 12 weeks to 26 cocaine-dependent patients. In subjects expressing cocaine craving, craving significantly declined during the course of the 12 weeks. Five individuals reported flushing, headache, fatigue, nocturia, nausea, and lightheadedness. No conclusions regarding efficacy can be made due to the small number of subjects and the open-label design.

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.

Chronic nimodipine and yawning behavior in grouped or individually housed rats

1. The effects of a chronic administration (around 30 mg/ kg/day) of the dihydropyridine calcium antagonist nimodipine, on apomorphine induced yawning behaviour of grouped or individual housed rats, were studied. 2. Nimodipine treatment had no effect in grouped rats. 3. Individually housed animals gave a significant lower number of yawns in respect to grouped controls: this difference disappeared in isolated, nimodipine treated, group. 4. The results show the ability of nimodipine to restore a depressed behavioural performance.

Apomorphine-induced yawning in the rat: influence of fasting and time of day

Yawning behavior is an experimental tool to study physiological responses, to elucidate the mechanisms of action of some drugs and hormones, and it is also a paradigm for some diseases and for dopamine (DA) agonists' clinical use. In this study, the effects of 24- and 48-h fasting as well as the influence of the light-dark cycle on apomorphine (APO)-induced yawning were evaluated. Initially, control and 48-h-fasted adult male rats were tested for yawning induced by APO (50, 100, 150 micrograms/kg, SC). The most effective dose tested was 100 micrograms/kg. Fasting significantly lowered yawning in all doses tested. Comparison between 24- and 48-h-fasted rats for APO (100 micrograms/kg)-induced yawning showed no significant difference between groups. Ad lib-fed groups were tested for APO (100 micrograms/kg)-induced yawning in both the light and in the dark phases of the cycle. Total number of yawnings increased significantly in the dark period. The present data show that fasting reduces and dark period increases APO-induced yawning in rats, suggesting that these conditions modulate the expression of this behavior.

Pharmacological mechanisms in cocaine's cardiovascular effects

The squirrel monkey is a reliable model for the cardiovascular effects of cocaine in that it mimics the human response to cocaine; low to moderate doses of cocaine produce a sustained pressor effect and tachycardia. Pretreatment experiments have indicated the importance of alpha-1 and beta-1 adrenoceptor mechanisms in mediating the pressor and tachycardiac effects of cocaine, respectively. Little support for a role of dopaminergic mechanisms in the hemodynamic effects of cocaine has been found. Toxicity to cocaine is often observed hours after its administration, pointing to a potential role of the cocaine metabolites. Studies on the direct effects of a variety of cocaine metabolites indicate that their cardiovascular effects do not necessarily mimic those produced by cocaine, and therefore these differing effects of the metabolites should be considered when evaluating the cardiovascular toxicity of cocaine. Further, as these metabolites are present in the body for long periods of time, these results suggest a role of the metabolites in producing toxicity long after cocaine administration. Finally, studies using both dopaminergic and calcium channel antagonists indicate that the pharmacological mechanisms involved in the cardiovascular effects of cocaine are not the same as those involved in its behavioral effects.

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

The involvement of the voltage-dependent calcium channel in behavioral effects of apomorphine was tested in naive rats and in animals which were morphine-abstinent or were subjected to chronic electroconvulsive treatment (ECS). In naive rats a calcium channel blocker, nifedipine, which by itself does not affect locomotor activity, inhibited the locomotor stimulation induced by apomorphine, while it facilitated stereotyped behavior. Morphine-abstinent and ECS-treated rats displayed elevated responsiveness to apomorphine, reflected by hypermotility and stereotyped behavior after a dose of 1 mg/kg IP that does not produce overt behavioral effects in naive animals. Nifedipine, 5 mg/kg IP, significantly reduced hypermotility produced by apomorphine in morphine abstinent or ECS-treated rats. The calcium channel blocker did not, however, antagonize enhanced stereotyped behavior. The results indicate that apomorphine hypermotility is controlled by dihydropyridine calcium channels and that enhancement of calcium channel density produced by morphine abstinence and by chronic ECS potentiates the hypermotility response. Calcium channels seem to be differently involved in control of apomorphine-induced hypermotility and stereotypy.

The Ca2+ channel agonist Bay K 8644 induces central respiratory depression in cats, an effect blocked by naloxone

We analyzed the respiratory effects induced by the Ca2+ channel agonist Bay K 8644 and its enantiomers, Bay R 5417 and Bay R 4407, applied to the ventral medullary surface of cats. Bay K 8644 (10 to 100 micrograms) and Bay R 5417 (50 to 200 micrograms) elicited a dose-dependent respiratory depression. Naloxone (0.1 mg/kg), but not D-naloxone, reversed these effects, indicating that an endogenous opioid mechanism was involved. Bay R 4407 (100 micrograms) was ineffective. The respiratory depressant effects induced by Bay K 8644 and its (-) enantiomer were a consequence of their agonist properties on the L-type Ca2+ channel, since (1) the activity of Bay K 8644 was stereospecific, and (2) nimodipine prevented the effect. We suggest that potent activation of Ca2+ channels or other mechanisms by high doses of Ca2+ agonists elicits the release of endogenous opioid peptides in medullary respiration-related structures.

Facilitation of shuttle-box avoidance behaviour in mice treated with nifedipine in combination with amphetamine

The dihydropyridine calcium channel antagonist nifedipine, tested in mice of CD-1, C57BL/6 and DBA/2 strains, at doses of 2.5, 5 and 10 mg/kg IP, had no significant effect on shuttle-box avoidance acquisition. Nifedipine also failed to affect performance retention in CD-1 mice subjected to a one-trial passive avoidance task (step-through). While ineffective alone, nifedipine strongly enhanced the shuttle-box avoidance facilitating action of amphetamine (1 and 2 mg/kg IP) in low performing CD-1 mice. The results indicate that although calcium channel blockers do not affect learning in avoidance paradigms in normal animals, they can interfere with the effects of other centrally acting drugs. Calcium antagonists might interfere with neuronal changes induced by amphetamine, but at present it is difficult to explain the strong avoidance facilitation produced by combinations of nifedipine and amphetamine. A possibility that the action of nifedipine on cerebral circulation is involved in the amphetamine-nifedipine interaction cannot be excluded.

Behavioural evidence that different neurochemical mechanisms underly stretching-yawning and penile erection induced in male rats by SND 919, a new selective D2 dopamine receptor agonist

The behavioural effects induced in male Wistar rats by SND 919, a new drug reputed to have selective agonistic activity at D2 dopamine (DA) receptors, were studied. The following aspects of behaviour were considered: motor activity, stretching-yawning (SY), penile erection (PE) and stereotyped behaviour (SB). Intraperitoneal injection (IP) of the drug (0.01-20 mg/kg) induced an SY syndrome in the form of a bell-shaped dose-response curve, the effect being maximal at the dose of 0.1 mg/kg and disappearing completely at 10 mg/kg. SND 919 also potently elicited PE; this latter effect, however, was not coincident with SY induction, being maximal at 1 mg/kg and persisting at 10 and 20 mg/kg. SND 919-induced SY was potently antagonized by pretreatment not only with the D2 antagonist, L-sulpiride (20 mg/kg), but also with the alpha 2 antagonist, yohimbine (1, 3 mg/kg), and the more selective alpha 2 antagonist, idazoxan (1, 2 and 5 mg/kg). While sulpiride also decreased SND 919-induced PE, idazoxan at all doses and yohimbine at 1 mg/kg did not affect this behaviour. Inhibition of motor activity was induced by the D2 agonist at low doses (0.05, 0.1 mg/kg), while at high doses (1, 10 and 20 mg/kg), it was actually replaced by a form of SB characterized by downward sniffing and licking. When, for comparison, the D2 agonist, RU 24213 (0.1-20 mg/kg IP), was tested for PE, SY, motor activity and SB, it displayed a behavioural pattern very similar to that obtained with SND 919. Idazoxan (2 mg/kg), administered before RU 24213 (10 mg/kg), significantly antagonized the drug-induced SY, but not PE.(ABSTRACT TRUNCATED AT 250 WORDS)

Yawning induced by apomorphine, physostigmine or pilocarpine is potentiated by dihydropyridine calcium channel blockers

Previous studies have shown that dihydropyridine (DHP) calcium channel blockers can potentiate yawning induced by apomorphine in rats. The present study was undertaken to examine whether or not this interaction was seen with other compounds that induce yawning or if it represented a specific interaction with dopaminergic mechanisms. Yawning induced by apomorphine (40 micrograms/kg SC), physostigmine (50 micrograms/kg SC) or pilocarpine (1 mg/kg SC) was dose-dependently potentiated by the DHP calcium channel blocker nifedipine (1.25-10 mg/kg IP). Nimodipine (1.25-5 mg/kg IP) and nitrendipine (1.25-5 mg/kg IP) also significantly increased the yawning response. The DHP calcium channel blockers alone induced only a low incidence of yawning. The effects of nifedipine on physostigmine-induced yawning were reversed by the DHP calcium channel activator BAY K 8644 which also inhibited yawning induced by physostigmine (100 micrograms/kg SC) and pilocarpine (2 mg/kg SC). In contrast to the DHP compounds, diltiazem (2.5-10 mg/kg IP) and verapamil (2.5-10 mg/kg IP) failed to potentiate yawning. Sulpiride (10 mg/kg SC) antagonised the nifedipine potentiation of apomorphine-induced yawning but not that of physostigmine-induced yawning; atropine (2.5 mg/kg SC) antagonised both effects. These results support the hypothesis that this effect of dihydropyridine compounds is not dependent on, nor mediated through, dopaminergic mechanisms.