pituitrin and benoxathian

Conditioned fear or novel environmental stimuli suppress vasopressin (VP) and augment oxytocin (OT) and prolactin (PRL) release in rats. We examined the effects of intracerebroventricular (i.c.v.) injections of adrenoceptor antagonists on these neuroendocrine responses to conditioned fear or novel environmental stimuli in male rats. A beta1 antagonist, metoprolol, blocked the VP but not the OT or PRL response to conditioned fear stimuli, but did not abolish neuroendocrine responses to novel environmental stimuli. A beta2 antagonist, ICI118551, impaired the PRL but not the VP or OT response to fear or novel environmental stimuli. In rats injected with a alpha1 adrenoceptor antagonist, benoxathian, conditioned fear stimuli did not significantly induce the VP, OT or PRL responses. The effects of benoxathian were not due to a general reduction of arousal, since benoxathian did not prevent the VP, OT or PRL response to novel environmental stimuli. These data suggest that beta1 adrenoceptors play a selective role in the VP response to conditioned fear stimuli, as do beta2 adrenoceptors in the prolactin response to conditioned fear and novel environmental stimuli. We conclude that alpha1 adrenoceptors play a facilitative role in VP, OT, PRL responses to conditioned fear stimuli.

Topics: Adrenergic Antagonists; Animals; Conditioning, Psychological; Environment; Fear; Injections, Intraventricular; Male; Metoprolol; Oxathiins; Oxytocin; Prolactin; Propanolamines; Rats; Rats, Wistar; Receptors, Adrenergic; Receptors, Adrenergic, alpha-1; Receptors, Adrenergic, beta-1; Receptors, Adrenergic, beta-2; Stress, Physiological; Vasopressins

Noxious stimuli facilitate oxytocin release from the neurohypophysis. Oxytocin-secreting hypothalamic magnocellular neurosecretory neurons receive excitatory synaptic inputs from noradrenergic neurons in the medulla oblongata. The medulla oblongata includes the A2 noradrenergic and the A1 noradrenergic cells. Here we investigated whether medullary noradrenergic neurons mediate oxytocin release after noxious stimuli in male rats. 5-Amino-2,4-dihydroxy-alpha-methylphenylethylamine, a neurotoxin selective for noradrenergic fibers, was injected into the lateral cerebral ventricle or the medulla. Seven days after the injection, the hypothalamic content of noradrenaline was decreased. In the rats injected with the neurotoxin, the release of oxytocin but not vasopressin after footshocks was impaired. Surgical ablation by suction of the caudal dorsomedial medulla including the A2 cell region did not significantly affect oxytocin release after footshocks, though the surgery abolished oxytocin release after i.v. injection of cholecystokinin octapeptide. In the rats whose A2 cell region had been ablated, an i.c.v. injected alpha 1 adrenoreceptor antagonist, benoxathian, blocked oxytocin release after footshocks. These results demonstrate that brainstem noradrenergic neurons mediate oxytocin release following noxious stimuli in the rat and suggest that responsible noradrenergic neurons are the A1 cells in the caudal ventrolateral medulla.

Topics: Adrenergic alpha-1 Receptor Antagonists; Adrenergic alpha-Antagonists; Animals; Brain Stem; Electric Stimulation; Injections, Intraventricular; Male; Medulla Oblongata; Neurons; Neurotoxins; Norepinephrine; Oxathiins; Oxytocin; Phenethylamines; Pituitary Gland, Posterior; Rats; Rats, Wistar; Vasopressins