ELECTROPHYSIOLOGICAL PROPERTIES OF RAT PONTINE NUCLEI NEURONS IN-VITRO II - POSTSYNAPTIC POTENTIALS

We investigated the postsynaptic responses of neurons of the rat ponti ne nuclei (PN) by performing intracellular recordings in parasagittal slices of the pontine brain stem. Postsynaptic potentials (PSPs) were evoked by brief (0.1 ms) negative current pulses (10-250 mu A) applied to either the cerebral peduncle or the pontine tegmentum. First, exci tatory postsynaptic potentials (EPSPs) could be evoked readily from pe duncular stimulation sites. These EPSPs exhibited short latencies, a n onlinear increment in response to increased stimulation currents, and an unconventional dependency on the somatic membrane potential. Pharma cological blockade of the synaptic transmission using 6,7-dinitroquino xaline-2,3-dione and D,L-2-amino-5-phosphonovaleric acid, selective an tagonists of the ha-amino-3-hydroxy-5-methyl-4-isoxazilepropionate- (A MPA) and the N-methyl-D-aspartate (NMDA)-type glutamate receptors, sho wed that these EPSPs were mediated exclusively by excitatory amino aci ds via both AMPA and NMDA receptors. Moreover, the pharmacological exp eriments indicated the existence of voltage-sensitive but NMDA recepto r-independent amplification of EPSPs. Second, stimulations at peduncul ar and tegmental sites also elicited inhibitory postsynaptic potential s (IPSPs) in a substantial proportion of pontine neurons. The short la tencies of all IPSPs argued against the participation of inhibitory in terneurons. Their sensitivity to bicuculline and reversal potentials a round -70 mV suggested that they were mediated by gamma-aminobutyric a cid-A (GABA(A)) receptors. In addition to single PSPs, sequences consi sting of two to four distinct EPSPs could be recorded after stimulatio n of the cerebral peduncle. Most remarkably, the onset latencies of th e following EPSPs were multiples of the first one indicating the invol vement of intercalated synapses. Finally, we used the classic paired-p ulse paradigm to study whether the temporal structure of inputs influe nces the synaptic transmission onto pontine neurons. Pairs of electric al stimuli applied to the cerebral peduncle resulted in a marked enhan cement of the amplitude of the second EPSP for interstimulus intervals of 10-100 ms. Delays >200 ms left the EPSP amplitude unaltered. These data provide evidence for a complex synaptic integration and an intri nsic connectivity within the PN too elaborate to support the previous notion that the PN are simply a relay station.