Electrophysiological features of habenular neurons of the guinea pig: High efficacy of synaptic transmission and variable configuration of unitary spikes

Abstract

Properties of single cell discharges recorded extracellularly in the medial habenular nucleus (MHN) were examined in guinea pig MHN slices. MHN neurons fired spontaneously as well as in response to stimulation of the stria medullaris. Positive deflections representing excitatory postsynaptic potentials (EPSPs) were recorded in isolation from a neuron. In most neurons, however, EPSPs differentiable from the base-line triggered action potentials consistently, and were recorded in isolation only during administration of 6-cyano-7-nitroquinoxaline-2,3-dione. Increases in spontaneous discharge rate during administration of nicotinic agonists were not accompanied by expected decreases in latency of synaptically-evoked spikes. Action potentials were recorded as biphasic potential changes consisting of a smaller positive phase and a subsequent negative phase. The amplitude of the negative phase in synaptically-evoked spikes fluctuated from trace to trace and was larger than in spontaneous spikes. The above observations suggest high efficacy of synaptic transmission from the stria medullaris to MHN neurons and variable areas of the soma-dentritic region invaded by action potentials generated in the initial segment.