Direct innervation of GnRH neurons by encephalic photoreceptors in birds

In nonmammalian vertebrates, photic cues that regulate the timing of season al reproductive cyclicity are detected by nonretinal, nonpineal deep brain photoreceptors. It has long been assumed that the underlying mechanism invo lves the transmission of photic information from the photoreceptor to a cir cadian system, and thence to the reproductive axis. An alternative hypothes is is that there is direct communication between the brain photoreceptor an d the reproductive axis. In the present study, light and confocal microscop y reveal that gonadotropin releasing hormone (GnRH) neurons and processes a re scattered among photoreceptor cells (identified by their opsin-immunorea ctivity) in the lateral septum (SL). In the median eminence (ME), opsin and GnRH immunoreactive fibers overlap extensively. Single and double label ul trastructural immunocytochemistry indicate that in the SL and preoptic area (POA), opsin positive terminals form axo-dendritic synapses onto GnRH dend rites. In the ME, opsin and GnRH terminals lie adjacent to each other, make contact with tanycytes, or terminate on the hypophyseal portal capillaries . These results reveal that brain photoreceptors communicate directly with GnRH-neurons; this represents a means by which photoperiodic information re aches the reproductive axis.