TETRODOTOXIN BLOCKS NPY-INDUCED BUT NOT MUSCIMOL-INDUCED PHASE ADVANCES OF WHEEL-RUNNING ACTIVITY IN SYRIAN-HAMSTERS

During the middle of the subjective day, circadian activity rhythms in Syrian hamsters can be phase advanced by a variety of stimuli includi ng microinjection of neuropeptide Y (NPY) or muscimol into the suprach iasmatic nucleus (SCN). It is not known, however, if these treatments shift activity rhythms by acting directly on pacemaker cells within th e SCN. In the present study NPY and muscimol were microinjected with e ither tetrodotoxin or saline in order to determine whether classical s ynaptic transmission within the SCN is necessary for the phase advance s produced by NPY or muscimol. Blockade of sodium-dependent action pot entials within the SCN prevented NPY-but not muscimol-induced phase ad vances. These data, along with our previous finding that bicuculline b locks NPY-induced phase advances, suggest that NPY requires sodium-dep endent action potentials within GABAergic neurons in order to phase-sh ift the circadian pacemaker. (C) 1997 Elsevier Science B.V.