Dorsocaudal SCN microknife-cuts do not block short day responses in Siberian hamsters given melatonin infusions

Siberian hamsters (Phodopus sungorus sungorus) undergo photoperiod-induced physiological and behavioral adaptations. These adaptations, including changes in reproductive and metabolic status, are triggered by the pineal gland through the nocturnal secretion of its principal hormone, melatonin. The possible CNS sites of melatonin action determined through 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 the SCN (SCNx) block the responses to short day-like (long duration) melatonin signals delivered daily via the timed infusion paradigm, suggesting that the SCN receives and transmits short-day melatonin signals. The purpose of the present experiment was to answer the following question: are short-day melatonin signals transmitted to other brain structures from the SCN through its dorsomedial/dorsocaudal fiber projections? 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 via the timed infusion paradigm for 6 weeks. Only the hamsters given SCNx exhibited long day-like gonadal, epididymal fat pad, and body masses. Therefore, short day melatonin signals received by the SCN were not transmitted to other areas of the central nervous system through SCN efferents projecting dorsomedially or dorsocaudally.