FAST MULTISITE OPTICAL-RECORDING OF MONOSYNAPTIC AND POLYSYNAPTIC ACTIVITY IN THE HAMSTER SUPRACHIASMATIC NUCLEUS EVOKED BY RETINOHYPOTHALAMIC TRACT STIMULATION

Responses of the hamster suprachiasmatic nucleus (SCN) to retinohypoth alamic tract (RHT) stimulation were studied in horizontal hypothalamic slices using fast multisite optical recording techniques. A 124-eleme nt photodiode detector array provided high-speed monitoring (0.5 ms/fr ame) of evoked neural activity in the SCN, while a larger 464-element photodiode array yielded improved spatial imaging with some loss in te mporal resolution (1.6 ms/frame). Brief electrical stimulation of the optic nerves evoked a propagated compound action potential that was re corded optically as a single transient deplorization in many slice reg ions, including the SCN. Only within the SCN, however, was this optic tract signal followed by additional voltage-dependent optical response s which exhibited a fast and a slow depolarizing component. The initia l upstroke of the fast component was Ca2+-insensitive and is presumed to reflect activity in presynaptic RHT afferents. The remainder of the fast depolarization and the slow depolarization were Ca2+-sensitive. These responses were labeled the early population excitatory postsynap tic potential (Early P.E.P.S.P.) and the Late P.E.P.S.P. respectively. The Late P.E.P.S.P. was not enhanced by K+ channel blockade, suggesti ng that glial depolarization is not the primary source of this compone nt. Drugs known to suppress RHT-evoked SCN field potentials also suppr essed the Early and Late P.E.P.S.P.'s recorded optically in the SCN. U nexpectedly, the Early P.E.P.S.P. was also reduced by the GABA(A) anta gonist, bicuculline. Surface plots of normalized peak amplitudes showe d that both SCN components had similar spatial distributions within th e SCN, although the Early P.E.P.S.P. tended to be slightly more promin ent within the medial SCN in some preparations. It is suggested that t he Early P.E.P.S.P. represents firing of monosynaptically activated SC N neurons, while the Late P.E.P.S.P. reflects polysynaptic activity wi thin the intrinsic SCN neuronal network that may be involved in the li ght entrainment of the circadian oscillator. (C) 1994 Academic Press, Inc.