Increased expression of c-fos in the medial preoptic area after mating in male rats: Role of afferent inputs from the medial amygdala and midbrain central tegmental field
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Abstract
Immunocytochemical methods were used to localize the protein product of the immediate-early
gene, c-fos, in male rats after exposure to, or direct physical interaction with,
oestrous females. Increasing amounts of physical contact with a female, with resultant
olfactory-vomeronasal and/or genital-somatosensory inputs, caused corresponding increments
in c-fos expression in the medial preoptic area, the caudal part of the bed nucleus
of the stria terminalis, the medial amygdala, and the midbrain central tegmental field.
Males bearing unilateral electrothermal lesions of the olfactory peduncle showed a
significant reduction in c-fos expression in the ipsilateral medial amygdala, but
not in other structures, provided their coital interaction with oestrous females was
restricted to mount-thrust and occasional intromissive patterns due to repeated application
of lidocaine anaesthetic to the penis. No such lateralization of c-fos expression
occurred in other males with unilateral olfactory lesions which were allowed to intromit
and ejaculate with a female. These results suggest that olfactory inputs, possibly
of vomeronasal origin, contribute to the activation of c-fos in the medial amygdala.
However, lesion-induced deficits in this type of afferent input to the nervous system
appear to be readily compensated for by the genital somatosensory input derived from
repeated intromissions. Unilateral excitotoxic lesions of the medial preoptic area,
made by infusing quinolinic acid, failed to reduce c-fos expression in the ipsilateral
or contralateral medial amygdala or central tegmental field following ejaculation.
By contrast, combined, unilateral excitotoxic lesions of the medial amygdala and the
central tegmental field significantly reduced c-fos expression in the ipsilateral
bed nucleus of the stria terminalis and medial preoptic area after mating; no such
asymmetry in c-fos expression occurred when lesions were restricted to either the
medial amygdala or central tegmental field. This suggests that afferent inputs from
the central tegmental field (probably of genital-somatosensory origin) and from the
medial amygdala (probably of olfactory-vomeronasal origin) interact to promote cellular
activity, and the resultant induction of c-fos, in the ipsilateral bed nucleus of
the stria terminalis and medial preoptic area. The monitoring of neuronal c-fos expression
provides an effective means of studying the role of sensory factors in governing the
activity of integrated neural structures which control the expression of a complex
social behaviour.