Relationships between CB1 cannabinoid receptors and pituitary endocrine cells in Xenopus laevis: an immunohistochemical study.

The distribution of the cannabinoid CB1 receptor and its relationships with individual endocrine cell types were investigated by immunohistochemistry in the anterior lobe of the Xenopus adenohypophysis. By use of a specific primary antibody raised in rabbit against the amino terminus of the rat CB1, we have found numerous CB1-like-immunoreactive cells distributed throughout all of the pituitary anterior lobe with the exception of the ventrocranial area adjacent to the median eminence of the neurohypophysis. Aided by both double-immunostaining on consecutive serial sections and double-simultaneous immunofluorescence on the same section of the gland, the CB1-like immunoreactivity was compared to some specific hormone immunoreactive cells. CB1 labelings were mainly codistributed, and even colocalized, with lactotrophs and thyrotrophs. Gonadotrophs containing CB1 receptors were also observed. In contrast, corticotrophs, which are located mainly in the ventrocranial pole of the anterior lobe, were generally devoid of CB1. Since nerve terminals immunoreactive to the CB1 antibody were observed within the vascular zone of the median eminence, the possibility that endocannabinoids are involved in the control of some secretory activities of Xenopus pituitary, either indirectly via hypothalamic neurosecretory mechanisms or directly on the pituitary cells, was envisaged. In particular, the present study suggests the occurrence of a direct cannabinergic modulation of the prolactin, gonadotrophin, and thyrotrophin secretions through the CB1 receptor.