" the Neuroendocrine Control of Broodiness

The neuroendocrine control of broodiness was investigated in the bantam hen. The functional activity of hypothalamic monoaminergic neurones and the role of vasoactive intestinal polypeptide were considered in relation to the control of prolactin and luteinizing hormone (LH) secretion. The functional activity of hypothalamic noradrenergic neurones, based on an index of turnover, did not change after the onset of incubation. cv_ However, the functional activity of hypotha]£nic adrenergic neurones was inversely correlated with pls^na levels of LH and positively correlated with plasma levels of prolactin . This suggests that adrenaline-containing neurones may be, in part, responsible for the increase in plasma levels of prolactin and the decrease in plasma levels of LH at the onset of incubation. Similarly the functional activity of dopaminergic neurones in the anterior hypothalamus, but not the posterior hypothalamus, was inversely correlated with plasma LH and positively correlated with plasma prolactin. Putative dopamine receptors were demonstrated in both the hypothalamus and the pituitary gland. Hypothalamic putative dopamine receptors were the same in laying and incubating birds. The density of putative dopamine receptors was significantly lower in pituitary glands from incubating birds than in those of laying birds. This suggests that any direct inhibitory influence of dopaminergic neurones in the posterior hypothalamus on prolactin secretion in incubating birds is reduced by 'down-regulation' of dopamine receptors in the anterior pituitary gland. The distribution of 5-hydroxytryptamine (5HT) containing neurones in the brain-stem and hypothalamus was mapped using an immunohistochemical technique. Extensive 5HT fibres and terminals originating from cell bodies in the hind-brain and paraventricular organ were abundant throughout the hypothalamus with the highest densities occu^ng in the pre-optic area and basal hypothalamus. Putative 5HT receptors were demonstrated in the hypothalamus but were absent from the anterior pituitary gland. Within the anterior hypothalamus a unique population of putative 5HT receptors was found only in the laying bird. This population disappeared during the onset of broodiness. The disappearance of this receptor population may be in part responsible for an increase in the functional activity of 5HT neurones in the anterior hypothalamus which was observed after the onset of incubation. The functional activity of 5HT containing neurones in the posterior hypothalamus did not change after the onset of incubation. The possible role of hypothalamic 5HT-containing neurones in the stimulation of prolactin secretion was investigated by assessing the influence of various drugs known to alter 5HT function, on plasma levels of prolactin. Although the drug studies suggested a strong stimulatory influence of 5HT neurones on prolactin secretion when it was measured in a homologous chicken radioimmunoassay these findings were not confirmed when prolactin was measured in a heterologous radioimmunoassay. Separation of the proteins present in various in la. — vitro incubations of pituitary glands and hypoth^ni by polyacrylamide gel electrophoresis demonstrated that the homologous radioimmunoassay cross-reacted with proteins of hypothalamic origin as well as prolactin released from the pituitary gland. It was concluded that, although 5HT-containing neurones may be involved in the stimulation of a tr\ prolactin releasing factor from the hypothalamus, their role ^the neuroendocrine control of broodiness is limited. In the hypothalamus a vasoactive intestinal polypeptide-like substance was located immunohistochernically in neurones in the basal hypothalamus which had an anatomical distribution consistent with a role in the control of anterior pituitary gland function. Vasoactive intestinal polypeptide stimulated selectively prolactin secretion when administerd in vivo, and stimulated prolactin secretion from the isolated pituitary gland. These observations suggest that endogenous hypothaijmic vasoactive intestinal polypeptide is a specific prolactin-releasing factor in the bantam hen. It is also suggested that the stimulation of prolactin secretion associated with the hypothalamic monoamine neurotransmitters may be mediated in part by vasoactive intestinal polypeptide-containing neurones.