Pimozide, a dopamine receptor blocker, was injected SC or implanted into the hypothalamus or adenohypophysis of ovariectomized rats to determine its effect on the release of LH and prolactin. Following SC injection of the drug, there was a gradual rise in plasma prolactin which became significant at 30 min after injection, and was maintained for 78 hr. This was accompanied by a gradual fall in LH which achieved significance at 2-4 hr after injection, and was maintained for54 hr. Implantation of Pimozide into the median eminence-arcuate region evoked a gradual consistent increase in prolactin which became significant at 1 hr, and was accompanied by a small decline in plasma LH at 4 hr. Much smaller increases in plasma prolactin followed implantation of the drug into the adenohypophysis and these implantations were not associated with any alterations in plasma LH. It is suggested that dopamine exerts an inhibitory control over prolactin release from the adenohypophysis by inhibiting the release of prolactin-inhibiting factor (PIF) from the median eminence and by acting directly on the pituitary to suppress prolactin release. Although the results with LH were not conclusive, they are consistent with a stimulatory dopaminergic control of LRF release.
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