3 DAILY MELATONIN INFUSIONS ALTER GONADAL DEVELOPMENT BUT NOT GNRH NEURON NUMBER IN THE MEDIAL PREOPTIC AREA OR DIAGONAL BAND OF BROCA IN SIBERIAN HAMSTERS

Among juvenile Siberian hamsters reproductive development is associate d with an increased number of unipolar GnRH-immunoreactive neurons in the diagonal band of Broca and medial preoptic area. In the present st udy, GnRH neuron morphology was assessed in male juvenile hamsters whi ch were treated with three daily melatonin (MEL) infusions to initiate or delay gonadal development. Hamsters gestated in short days were tr ansferred to constant light (LL) at day 14 of age and infused on days 18-20 with MEL for 6 h/day. This treatment stimulated testis developme nt compared to that in saline-infused controls. By contrast, testis gr owth was suppressed by three 12 h MEL infusions in long-day gestated h amsters in LL from day 14. The number of unipolar and bipolar GnRH neu ron subtypes was the same irrespective of MEL infusion or preweaning p hotoperiod. Thus, gonadal response to three MEL infusions is independe nt of changes in GnRH neuron number.