The administration of small doses of
apomorphine or other dopaminomimetic drugs in
rats produces a behavioural syndrome
characterized by hypomotility, yawning, penile
erection and genital grooming.
These effects are considered to be the
behavioural consequence of the inhibition of
dopaminergic transmission mediatedby the
stimulation of dopamine (DA) autoreceptors in
the CNS. Indeed, doses of DA agonists producing
these behavioural effects are within the dose
range needed to activate DA autoreceptors, but
much lower than those needed to stimulate
postsynaptic DA receptors and to produce
stereotypy and stimulation of motor
activity.
We have recently shown that the inhibition
of protein synthesis prevents
apomorphine-induced yawning, penile erection and
genital grooming, suggesting that the inhibition
of DA transmission might result in the release
of some newly synthetized peptides.
The fact that MSH-producing cells in the
intermediate lobe of the pituitary are under the
inhibitory control of dopaminergic neurons and
that ACTH and MSH share with apomorphine the
ability to produce penile erection and yawning
suggests that these peptides might be possible
candidates for mediating yawning, penile
erection and genital grooming induced by low
doses of apomorphine.
On the basis of these considerations, we
might suggest that inhibition of dopaminergic
activity results in the release of ACTH or MSH
peptides from pituitary or from peptidergic
neurons in the brain.
The present study was carried out to
investigate the role of pituitary on the
behavioural effects induced by low doses of
apomorphine. The present results indicate that
hypophysectomy inhibits yawning, penile
erection and genital grooming but not the
hypomotility induced by small doses of
apomorphine. [...]
The present findings indicate that
hypophysectomy abolishe some behavioural effects
of small doses of apomorphine, namely yawning,
penile rection and genital grooming, but fails
to prevent apomorphine-induced hypomotility.
Since, at the time of the experiments, there
was a significant difference in body weight
between control and hypophysectomized animals,
the possible influence of the debilitating
effect of hypophysectomy in the inhibition of
apomorphine-induced yawning, penile erection and
genital grooming cannot be entirely excluded.
However, an unspecific effect is unlikely since
hypophysectomy specifically antagonizes some
apomorphine effects but does not influence
apomorphine-induced hypomotility.
These results support the hypothesis that the
former behavioural effects of apomorphine are
mediated by some pituitary hormones. These
hormones might be
identified with ACTH and MSH
peptides, since they are the only
ones capable of producing yawning and penile
erection. Moreover, MSH-producing cells in the
pars intermedia of the pituitary are under the
inhibitory control of dopaminergic neurons
originating from the arcuate nucleus in the
hypothalamus. In might be suggested that
stimulation of DA autoreceptors present on these
neurons results in the removal of the inhibitory
control of DA on MSH release. MSH released from
pituitary might reach the target areas in the
brain via a retrograde portal flow.
An alternative interpretation of our results
might be that the lack of some pituitary hormone
alters the sensitivity of DA receptors
responsible for yawning, penile erection and
genital grooming, but not those responsible for
the reduction of motor activity.
Finally, the suppressant effect of
hypophysectomy on apomorphine-induced yawning,
penile erection and genital grooming might be
indirect, secondary to the involution of the
gonads or adrenals. In fact, it has been
reported that castration reduces
apomorphine-induced yawning.
The fact that apomorphine-induced
hypomotility is not altered by hypophysectomy
suggests that, contrary to that observed for the
other behavioural responses, pituitary peptides
are neither directly nor indirectly involved in
the sedative effect of the drug.
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