Hypothalamic-pituitary responses to intracranial self-stimulation in the rat

The effects on pituitary and adrenal hormones of intracranial self-stimulation (ISS) and forced stimulation in rewarding (PS) and non-rewarding (NSS) sites were investigated in adult male rats. Growth hormone (GH) was suppressed during periods of ISS and PS and rose sharply to a peak within 15 min of cessation. The interval between GH secretory episodes was significantly shortened during all 3 types of stimulation when compared to the normal rhythmic discharge observed in freely behaving baseline (BL) sampled rats. ISS, PS and NSS resulted in a rapid rise in prolactin (Prl), which returned to normal by the end of each hourly period of stimulation. There was a significant reduction of Prl elevation in the second and third periods of stimulation, suggesting that the first exposure to ISS had a greater stimulatory effect than subsequent stimulations. There was a rapid and sustained release of corticosterone (CS) during ISS and PS. As with Prl, the initial period of either ISS or PS caused a greater effect that subsequent periods. These studies provide data to compare the relationship between ISS and neuroendocrine responses. The hormonal responses, with minor exceptions, were similar under all experimental conditions, and could not be clearly dissociated from previously described stress responses. Neural pathways and substrates involved in GH, Prl and CS secretion are discussed in relation to pathways activated by LH-MFB stimulation. Potential differential functions of monoamines and hypothalamic neuropeptides in behavioral and neuroendocrine regulation are hypothesized.