Determination of the source of 5-hydroxytryptaminergic neuronal projections to the neural and intermediate lobes of the rat pituitary gland through the use of electrical stimulation and lesioning experiments

5-Hydroxytryptamine (5-HT)-containing axons and terminals have been visualized in the neural and intermediate lobes of the rat pituitary gland, but the origin of these fibers remains in question. This study was designed to determine if 5-HT cell bodies in the brainstem or in the dorsomedial nucleus of the hypothalamus project to either of these pituitary lobes. Since lesions and electrical stimulation of 5-HT cell bodies decrease and increase, respectively, the rate of 5-HT synthesis in region innervated by these cells, these techniques were employed. The in vivo rate of 5-HT synthesis was determined by quantifying the rate of accumulation of the 5-HT precursor, 5-hydroxytryptophan (5-HTP), in the neural and intermediate lobes of the pituitary gland 30 min after the administration of a decarboxylase inhibitor (NSD 1015, 100 mg/kg, i.p.). The application of 30 min of stimulating current (monophasic cathodal pulses of 1 ms duration and 0.3 mA current delivered at a frequency of 10 Hz) to electrodes implanted in the dorsal and median raphe nuclei increased the rate of 5-HT synthesis in both the neural and intermediate lobes of the pituitary gland. 5,7-Dihydroxytryptamine lesions of these nuclei altered neither 5-HTP accumulation nor 5-HT concentrations in the neural and intermediate lobes, but similar lesions of the nuclei raphe pontis and raphe magnus decreased both the concentration of 5-HT and the accumulation of 5-HTP in these pituitary regions. Taken together these results suggest that electrical stimulation activated 5-HT fibers of passage which project to the neural and intermediate lobes and which originate in cell bodies caudal to the dorsal and median raphe nuclei. These perikarya may reside in the nucleus raphe pontis and/or raphe magnus. In contrast, neither electrical stimulation nor electrolytic lesioning of the dorsomedial nucleus of the hypothalamus altered 5-HTP accumulation in either the neural or intermediate lobe indicating that this nucleus does not contain the cell bodies of origin of 5-HT fibers in the pituitary gland.