The administration of small doses of
apomorphine in rats produces a behavioural
syndrome characterized by decreased locomotor
activity, yawning, and penile erection. these
effects have been attributed to stimulation of
dopamine (DA) receptors in the CNS.
A great deal of evidence indicates that DA
and peptides interact at pituary and CNS levels.
Thus, peptides may coexist with DA in the same
neurons, peptides may influence DA synthesis and
release. Moreover, the release of different
peptides cause behavioural effects in common
with apomorphine. Thses observations led us to
study whether protein synthesis inhibition might
alter the behavioral effects of
apomorphine.
The present results indicate that
cycloheximide inhibits the yawning, penile
erection, and genital grooming induced by small
doses of apomorphine, but fails to alter the
stereotypy induced by higher doses of this drug.
[...]
Discussion :
The present results show that cycloheximide
prevents the yawning, penile erection and
genital grooming induced by a low dose of
apomorphine but fails to modify the stereotyped
behaviour induced by a relatively high dose of
this drug. Cycloheximide inhibition of
apomorphine effect was present from 1 to 6 h,
when brain protein synthesis was reduced by
about 50%, while 12 h after cycloheximide
treatment, both brain protein synthesis and the
behavioural effects of apomorphine returned to
normal values.
These observations suggest that the effect
of cycloheximide on apomorphine-induced
behavioural changes is secondary to its
inhibitory effect on brain protein synthesis,
rather than to its sedative effect. The latter
is still present at 12 h after treatment when
the antagonistic effect is no longer
evident.
Our results suggest that protein
synthesis is required for behavioural effects
induced by the low doses of apomorphine, but not
for the stereotyped behaviour induced by the
high doses of the drug.
There is considerable experimental evidence
suggesting the existence of two kinds of DA
receptors in the brain which mediate opposite
behavioural responses. Stimulation of motor
activity and stereotypy elicited by high doses
of apomorphine are considered to be mediated by
striatal or mesolimbic postsynaptic DA
receptors. On the contrary, it has been
suggested that the behavioural syndrome induced
by low doses of apomorphine is the behavioural
consequence of the inhibition
of dopaminergic transmission mediated by the
stimulation of DA autoreceptors.
The fact that active protein synthesis is
needed for such behavioural responses suggests
that the inhibition of DA transmission might
result in the release of some active peptides.
These might be identified with prolactin and/or
MSH, both of which are under tonic inhibition
control by DA and have been shown to produce
grooming in rats. Finally MSH shares with
apomorphine the ability to produce penile
erection and yawning.
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