Tubero-infundibular dopamine neurons and the secretion of luteinizing hormone and prolactin: extrahypothalamic influences, interaction with cholinergic systems and the effect of urethane anesthesia

Abstract

In ovariectomized, estrogen-progesterone-pretreated rats, the fluorescence intensity of the cell bodies of tubero-infundibular dopamine (DA) neurons — as detected by histochemical microfluorimetry — increased rapidly after unilateral stimulation (10 min) of arcuate nucleus, medial preoptic area, medial amygdaloid nucleus, nucleus of diagonal tract, bed nucleus of stria terminalis or ventromedial midbrain tegmentum. After catecholamine synthesis inhibition, ventral hippocampal stimulation (30 min) retarded the depletion of DA neurons and thus differed in its effect from preoptic stimulation. Atropine reduced the intensity response to stimulation in all the areas where its action was studied, except for the arcuate nucleus where the DA neurons are mainly localized. Its effect was dose-dependent (0.4–10 mg/kg). Methylatropine, which crosses the blood-brain barrier in much lower amounts, remained ineffective. An increase in the variance of the DA neuron population was noted after atropine, suggesting a certain functional heterogeneity. The changes in fluorescence intensity were correlated with the serum levels of luteinizing hormone (LH) and prolactin. Response patterns depended upon site of stimulation and treatment. With preoptic stimulation, the intensity response was accompanied by a rise in LH and no change in prolactin; atropine caused a reduction of intensity change and LH rise together with an increase in prolactin. With stimulation sites outside the preoptico-tuberal axis, such as medial amygdala, stimulation and atropine affected the intensity in a similar way, yet LH increased only after atropine while prolactin levels were elevated with or without atropine. Urethane anesthesia probably contributed to the differentiation of neuroendocrine responses, its effect being partly counteracted by atropine. Thus, short-term influences depending in part on cholinergic activity reach the tubero-infundibular DA neurons from various extrahypothalamic areas. The accompanying patterns of hormone secretion probably result from an interaction of these neurons with other systems projecting to the median eminence.