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The ultrastructural effects of drugs enhancing, by different mechanisms, cholinergic transmission in the crop-sac (the target for prolactin secretion in birds) and the anterior pituitary lactotrophs, were studied in pigeons (Columba livia). The systemic or intraventricular administration of physostigmine, carbachol and muscarine produced maximal crop-sac stimulation with milk-like secretion, as demonstrated by the observation of ultrastructural changes in the lactiferous areas through scanning and transmission electron microscopy of the crop-sac mucosa. A marked activation was also observed in anterior pituitary lactotrophs. Crop-sac and anterior pituitary lactotrophs stimulatory effects were prevented by an atropine pretreatment, but not by mecamylamine and pempidine pretreatments. The present results suggest that muscarinic receptors at the hypothalamic and/or anterior pituitary level are involved in avian species in the control of prolactin secretion.

The effects of dermorphin, a new potent opioid peptide, on structural and ultrastructural changes in the crop sac and in pituitary lactotrophs were assessed in pigeons (Columba livia). A single administration of dermorphin into the third cerebral ventricle (1 microgram) and the same dose repeated for 3 consecutive days both produced maximal crop sac stimulation, as documented by the presence of milk-like material as well as by scanning and transmission electron microscopy of crop sac mucosa. In addition, marked stimulatory effects were also observed in pituitary lactotrophs, as shown by stimulation of Golgi apparatus, enlargement of rough endoplasmic reticulum, increase in mitochondria size, and the presence of lysosomes and electron-opaque granules. The stimulatory effects of dermorphin on the crop sac and lactotrophs were completely prevented by naloxone, an antagonist at opiate receptors. Since dermorphin is a natural constituent of the avian brain, it has been suggested that this peptide may represent the still-obscure hypothalamic prolactin stimulatory factor.

On an absolute basis, the intradermal pigeon crop-sac bioassay (PCA) gave results that were 20.5% higher than the radioimmunoassay (RIA) in rat anterior pituitary (AP) preparations. A highly significant correlation (r = 0.87) was obtained between RIA (in microgram) and PCA (in Reece-Turner units) when 58 medium samples obtained by culturing rat APs in vitro were assayed for prolactin (PRL) content.