REENTRAINMENT OF MOTOR-ACTIVITY AND SPONTANEOUS NEURONAL-ACTIVITY IN THE SUPRACHIASMATIC NUCLEUS OF DJUNGARIAN HAMSTERS

Neurons in the suprachiasmatic nucleus (SCN) of the hypothalamus exhib it a daily rhythm in spontaneous electrical activity. Essentially vivo methods have been employed to record this circadian rhythm: (1) an in vitro brain slice technique and (2) in vivo multiunit recordings, Ree ntrainment of a circadian output to a shifted light:dark cycle commonl y takes several cycles (depending on the amount of shift) until comple ted. Such a resetting kinetic has also been shown to be valid for SCN electrical activity if recorded in vivo, In an in vitro slice preparat ion, however, pharmacologically induced resetting is much faster and l acks transients; that is, a shift is completed within one cycle. This study was designed to probe for the presence of transients in the neur onal activity of the SCN in a brain slice preparation. The authors exp osed Djungarian hamsters to an 8-h advanced or delayed light:dark cycl e and monitored wheel-running activity during reentrainment. Additiona l groups of identically treated hamsters were used to record the patte rn of spontaneous neuronal activity within the SCN using the brain sli ce preparation. Neuronal activity exhibited the usual rhythm with high firing rates during the projected day and low firing rates during the projected night. However, following 1 day of exposure to the 8-h adva nced light:dark cycle, this rhythm disappeared in 6 of 7 slices. Rhyth micity was still absent following 3 days of exposure to the advanced l ight:dark cycle (n = 4). By contrast, 3 of 7 slices prepared from hams ters exposed to a delayed light:dark cycle for 3 clays exhibited a dai ly rhythm in electrical activity. Although pharmacological agents rese t the in vitro SCN neuronal activity almost instantaneously and in in vivo studies a stable phase relationship to a shifted light:dark cycle occurs gradually over several cycles, the authors did not detect eith er of these patterns. Such differences in resetting kinetics (e.g., ra pid resetting, gradual reentrainment, temporary lack of measurable rhy thmicity) may be due to (a) application of a resetting stimulus in viv o versus in vitro, (b) duration of the resetting stimulus, (c) the nat ure of the resetting stimulus, or (d) the recording technique employed .