1 The effects of the dihydropyridine
calcium channel blocker nifédipine and
the activator Bay K 8644 were investigated in
different behavioural tests involving
dopaminergic systems. These were the
discriminative stimulus induced by amphetamine,
rotational behaviour in rats with unilateral
6-hydroxydopamine (6-OHDA) lesions and
apomorphine-induced yawning in rats.
2 The yawning induced by apomorphine
(40,ugkg-'s.c.) was significantly potentiated by
nifedipine (5-10mgkg-'i.p.). Bay K 8644
(0.05-0.5mgkg-'i.p.) dose-dependently inhibited
yawning induced by apomorphine (80,ugkg-'s.c.)
and, at 0.4mgkg-', inhibited the nifedipine
potentiation of apomorphine-induced yawning. In
contrast to their effects on apomorphine-induced
yawning, nifedipine and Bay K 8644 had no effect
on apomorphine-induced pende erection.
3 Bay K 8644 (0.06-0.5mgkg-'i-p.) and
nifedipine (5-20mgkg-'i.p.) had no dose-relate
effect on the discrimination performance of rats
trained to discriminate amphetamine from saline.
However, nifedipine dose-dependently reduced the
response rate of amphetamine-treated rats. Bay K
8644 had no effect on this measure except at
high doses that also caused disruption.
4 Neither nifedipine(5-10mgkg-'i.p.) nor Bay
K 8644(0.06-0.5mgkg-'i.p.)a ffected the turning
behaviour induced by amphetamine (1mgkg-'i.p.)
in rats with unilateral 6-OHDA lesion of the
medial forebrain bundle, and did not induce
turning themselves.
5 As the dihydropyridine compounds affected
apomorphine-induced yawning but not penile
erection, and did not affect amphetamine-induced
rotation or drug discrimination, it seems
unlikely that they are affecting dopamine
release in vivo.
