Immunocytochemical demonstration of CB1 cannabinoid receptors in the anterior lobe of the pituitary gland

Both exogenous and endogenous cannabinoids can influence hormone secretion from the anterior pituitary gland. A large body of information proves that the primary target of these effects is the neuroendocrine hypothalamus, How ever, recent studies using cannabinoid (CB) receptor autoradiography, messe nger RNA in-situ hybridization and in-vitro analysis, indicate direct effec ts of cannabinoids at the level of the anterior pituitary gland itself. In the present paper, the immunocytochemical distribution of CB in the adult r at anterior pituitary was studied using specific polyclonal antibodies agai nst CB1 (central) and CB2 (peripheral) receptors, Due to its resolution, th is method allowed identification of individual anterior pituitary cells pos sessing cannabinoid receptors, The specific hormone immunoreactive cells wi th receptor-like immunoreactivity were compared on adjacent sections. CB1-l ike immunoreactivity (CB(2)ir) was found in the lactotroph cells as well as in luteinizing hormone (LH) secreting gonadotrophs, The CB(1)ir positive m aterial present in the cytoplasm of these cells was less homogeneous than t he hormone immunorective material, and it was also seen at the periphery of the cells, presumably on the cell membrane. No CB(1)ir was found in growth hormone (GH) secreting cells and it was hardly seen in the corticotrophs. No CB(1)ir was detected in the posterior pituitary, CB(2)ir was not observe d in any part of the pituitary gland. The results support the view that the site of action of cannabinoids on neuroendocrine regulatory mechanisms may be both at pituitary and hypothalamic levels. We suggest that at least the direct effect of cannabinoids on the regulation of LH and prolactin secret ion is mediated via CB1 cannabinoid receptors in the anterior pituitary.