Dopamine–melatonin neurons in the avian hypothalamus and their role as photoperiodic clocks

A timing mechanism in the brain governs reproduction in seasonally breeding temperate zone birds by triggering gonad development in response to long days in the spring. The neural mechanism(s) responsible for the timing and induction of reproductive activity by this clock are unknown. Utilizing in situ hybridization, immunocytochemistry and reverse transcriptase-polymerase chain reaction techniques, a group of dopamine (DA) neurons in the premammillary nucleus (PMM) of the caudal turkey hypothalamus that synthesize and colocalize both DA and melatonin (MEL) were identified. In addition, these neurons are found to express clock genes and the circadian photoreceptor melanopsin. DA–MEL neurons reach threshold activation (c-fos expression) when a light pulse is given during the photosensitive phase. This is associated with increases in the number of gonadotropin releasing hormone-I (GnRH-I) neurones activated, as well as an up-regulation of GnRH-I mRNA expression. The expression of tyrosine hydroxylase (TH; the rate limiting enzyme in DA biosynthesis) and tryptophan hydroxylase 1, (TPH1; the first enzyme in MEL biosynthesis) and consequently DAergic–MELergic activities are associated with the daily light-dark cycle. TPH1 mRNA expression shows low levels during the light phase and high levels during the dark phase of the light/dark illumination cycle and is 180° out of phase with the rhythm of TH mRNA expression. Hypothalamic DA–MEL neurons may constitute a critical cellular process involved in the generation and expression of seasonal reproductive rhythms and suggests a previously undescribed mechanism(s) by which light signals gain access to neural targets.