DORSOCAUDAL SCN MICROKNIFE-CUTS DO NOT BLOCK SHORT-DAY RESPONSES IN SIBERIAN HAMSTERS GIVEN MELATONIN INFUSIONS

Siberian hamsters (Phodopus sungorus sungorus) undergo photoperiod-ind uced physiological and behavioral adaptations. These adaptations, incl uding changes in reproductive and metabolic status, are triggered by t he pineal gland through the nocturnal secretion of its principal hormo ne, melatonin. The possible CNS sites of melatonin action determined t hrough radiolabeled melatonin binding include the paraventricular and reuniens nuclei of the thalamus and the suprachiasmatic nucleus (SCN). However, we do not know the mechanisms and circuitry involved in the transmission of melatonin signals. Bilateral electrolytic lesions of t he SCN (SCNx) block the responses to short day-like (long duration) me latonin signals delivered daily via the timed infusion paradigm, sugge sting that the SCN receives and transmits short-day melatonin signals. The purpose of the present experiment was to answer the following que stion: are short-day melatonin signals transmitted to other brain stru ctures from the SCN through its dorsomedial/dorsocaudal fiber projecti ons? Pinealectomized adult male hamsters given horizontal knife cuts ( kc) just dorsocaudal to the SCN (SCN-kc), sham-kc, or SCNx were given daily subcutaneous short day-like melatonin infusions vie the timed in fusion paradigm for 6 weeks. Only the hamsters given SCNx exhibited lo ng day-like gonadal, epididymal fat pad, and body masses. Therefore, s hort day melatonin signals received by the SCN were not transmitted to other areas of the central nervous system through SCN efferents proje cting dorsomedially or dorsocaudally. (C) 1998 Elsevier Science Inc.