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 109 M dose. Such a treatment had no effect on the expression of cytochrome P450 17alpha hydroxylase/17,20 lyase (cyp17) nor 3-b-hydroxysteroid dehydrogenase/D-5–4 isomerase (3b- 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 108 M dose. Differently from in vitro experiment, the in vivo administration of AEA reduced the expression of cyp17 and 3b-HSD. Whereas the effect on 3b-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 3b-HSD in frog testis travelling along the hypothalamus–pituitary axis.