Hypothalamus-pituitary axis: an obligatory target for endocannabinoids to inhibit steroidogenesis in frog testis.

Endocannabinoids - primarily anandamide (AEA) and 2-arachidonoylglycerol (2-AG) - are lipophilic molecules that bind to cannabinoid receptors (CB1 and CB2). They affect neuroendocrine activity inhibiting gonadotropin releasing hormone (GnRH) secretion and testosterone production in rodents, through a molecular mechanism supposed to be hypothalamus dependent. In order to investigate such a role, we choose the seasonal breeder, the anuran amphibian Rana esculenta, an experimental model in which components of the endocannabinoid system have been characterized. In February, at the onset of a new spermatogenetic wave, we carried out in vitro incubations of frog testis with AEA, at 10(-9)M dose. Such a treatment had no effect on the expression of cytochrome P450 17alpha hydroxylase/17,20 lyase (cyp17) nor 3-β-hydroxysteroid dehydrogenase/Δ-5-4 isomerase (3β-HSD), key enzymes of steroidogenesis. To understand whether or not the functionality of the hypothalamus-pituitary axis could be essential to support the role of endocannabinoids in steroidogenesis, frogs were injected with AEA, at 10(-8)M dose. Differently from in vitro experiment, the in vivo administration of AEA reduced the expression of cyp17 and 3β-HSD. Whereas the effect on 3β-HSD was counteracted by SR141716A (Rimonabant) - a selective antagonist of CB1, thus indicating a CB1 dependent modulation - the effect on cyp17 was not, suggesting a possible involvement of receptors other than CB1, probably the type-1 vanilloid receptor (TRPV1), since AEA works as an endocannabinoid and an endovanilloid as well. In conclusion our results indicate that endocannabinoids, via CB1, inhibit the expression of 3β-HSD in frog testis travelling along the hypothalamus-pituitary axis.