Short‐term in vitro incubation of hypothalamo‐hypophyseal tissue from young rats was undertaken to discern more clearly the functional relationship between putative dopaminergic neural projections in the pars intermedia and the secretory activity of melanophore stimulating hormone (MSH). This explant consisted of a portion of the mediobasal hypothalamus containing the dopamine neurone cell bodies of interest, with the attached pituitary neuro‐intermediate lobe (n.i.l.). The n.i.l. was inserted into the end of a 1 mm diameter tube attached to a perfusion pump which allowed uninterrupted sampling of medium neighbouring the n.i.l. A 'real‐time' analysis of hormone secretion was obtained by immediately and continuously bioassaying for MSH. A bipolar stimulating electrode was placed on the ventral floor of the mediobasal hypothalamus either directly on the arcuate nucleus, median eminence or infundibular stalk. Electrical stimulation for 5 min (0.1‐20.0 Hz) caused a transient inhibition of basal MSH secretion, while continuous stimulation (0.1‐5.0 Hz) led to a much greater, long‐term, reversible inhibition. In the latter, the degree of inhibition was generally dependent on stimulation rate up to a maximum at 5 Hz. Application of the dopamine D2 receptor antagonist, 1‐sulpiride (0.001‐0.1 microM) to the perfusion medium not only completely and reversibly blocked the stimulus‐induced inhibition of MSH release but by itself, significantly increased the basal secretion rate. Applied to the isolated n.i.l., 1‐sulpiride did not alter release but did prevent the inhibitory response caused by exogenously applied dopamine (0.1 microM). The gamma‐aminobutyric acid receptor antagonist, bicuculline (0.01‐1.0 microM), had no effect on any of the parameters studied. In explants, cutting the infundibular stalk linking the mediobasal hypothalamus with the n.i.l., mimicked the effects of 1‐sulpiride by interrupting impulse flow to the gland. Thus, electrical stimulation of hypothalamic neurones in these explants apparently causes a release of dopamine from nerve terminals in the pars intermedia to inhibit MSH secretion and perhaps other pro‐opiomelanocortin‐derived peptides as well.
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