IN SYRIAN-HAMSTERS, 5-HT FIBERS WITHIN THE SUPRACHIASMATIC NUCLEI ARENECESSARY FOR THE EXPRESSION OF 8-OH-DPAT INDUCED PHASE-ADVANCE OF LOCOMOTOR-ACTIVITY RHYTHM

8-Hydroxy-2-[di-n-propylamino]-tetralin) (8-OH-DPAT), a 5-HT1A/7 recep tor agonist, has a chronobiological effect on the circadian system. To identify how the 8-OH-DPAT exerts this effect, we specifically destro yed the serotonergic (5-HT) fibres connecting the median raphe nuclei (RN) to the suprachiasmatic nuclei (SCN) of the hypothalamus by using microinjections of a neurotoxin 5,7-dihydroxytryptamine into the SCN. After administration of 8-OH-DPAT(0.1 mi, 5 mg/kg) at circadian time 7 , the control and the 'partially-lesioned' animals showed a large phas e-advance whereas in the 'well-lesioned' hamsters the phase-advances w ere significantly reduced or absent. The present study demonstrates th at, in the Syrian hamster, the 5-HT fibres connecting the RN to the SC N are essential for the phase-shifting action of peripheral 8-OH-DPAT injections, and that the drug does very probably not exert its chronob iological effect directly onto SCN neurons but through receptors local ized on median raphe nucleus neurons. (C) 1998 Elsevier Science Irelan d Ltd. All rights reserved.