Involvement of the noradrenergic afferents from the nucleus tractus solitarii to the supraoptic nucleus in oxytocin release after peripheral cholecystokinin octapeptide in the rat.

Activation of abdominal vagal afferents by peripheral injection of cholecystokinin octapeptide induces oxytocin release into the circulation. To test the hypothesis that cholecystokinin increases oxytocin release via activation of noradrenergic afferents from the brainstem, we injected rats with 5-amino-2,4-dihydroxy-alpha-methylphenylethylamine, a selective neurotoxin to noradrenergic fibres, into a lateral cerebral ventricle. The neurotoxin treatment reduced the noradrenaline content in the hypothalamus by 75% and reduced the oxytocin secretion in response to cholecystokinin by over 90%. In separate experiments, the neurotoxin was injected unilaterally in the vicinity of the supraoptic nucleus to test whether direct noradrenergic afferents to the supraoptic nucleus are involved in the response to cholecystokinin. The injection reduced the immunoreactivity for dopamine beta-hydroxylase in the supraoptic nucleus and significantly decreased the number of the supraoptic neurons expressing Fos-like protein after cholecystokinin but not after hypertonic saline. In further experiments, rhodamine-conjugated latex microspheres were injected into the supraoptic nucleus to retrogradely label afferent neurons, and the brains were processed with double-immunohistochemistry for tyrosine hydroxylase and Fos-like protein. In the C2/A2 but not the C1/A1 region of the brainstem, cholecystokinin increased the expression of Fos-like protein in the population of retrogradely-labelled catecholaminergic cells. In the C2/A2 region, the majority of retrogradely labelled cells expressing Fos-like protein after cholecystokinin were catecholaminergic. We conclude that noradrenergic afferents from the A2 but not from the A1 region of the brainstem to the hypothalamus mediate, at least in part, oxytocin release following cholecystokinin.