PROPERTIES OF INHIBITORY AND EXCITATORY SYNAPSES BETWEEN HIPPOCAMPAL-NEURONS IN VERY-LOW-DENSITY CULTURES

The whole cell patch clamp technique was used to examine the electroph ysiological properties of embryonic hippocampal neurons maintained in a very low density (VLD) culture preparation. The goal of these experi ments was to establish the viability of the VLD culture as a model sys tem in which to study regulation of neurotransmission at single monosy naptic connections, in the absence of polysynaptic innervation. Depola rization of neurons in the VLD culture revealed voltage-dependent sodi um, calcium, and potassium currents which were blocked with, respectiv ely, tetrodotoxin (TTX), cobalt, and tetraethylammonium and 4-aminopyr idine. When pairs of neurons were simultaneously recorded, action pote ntials evoked in presynaptic neurons elicited either excitatory or inh ibitory postsynaptic currents (EPSCs or IPSCs, respectively). The dual component EPSCs were due to the activation of both types of postsynap tic, ionotropic glutamate receptors: N-methyl-D-aspartate (NMDA) and n on-NMDA receptors. Evoked IPSCs were due to the activation of postsyna ptic gamma-aminobutyric acid (GABA) receptors. Both excitatory and inh ibitory synapses exhibited short term depression in response to high f requency stimulation, although IPSCs were routinely decreased to a muc h greater degree than EPSCs. Spontaneous miniature EPSCs and IPSCs wer e found to persist in TTX, were blocked by the same pharmacological an tagonists which blocked evoked responses, increased in frequency in re sponse to hyperosmotic solution, and were unaffected by changes in ext racellular calcium concentration, mIPSCS were found to occur at a sign ificantly lower frequency than mEPSCs. These experiments indicated tha t neurotransmission in the VLD cultures occurs in a manner consistent with the quantal hypothesis and, therefore, the VLD culture is a good model for studying excitatory and inhibitory neurotransmission between isolated pairs of neurons. In addition, these experiments, performed under comparable physiological conditions, demonstrated that there are fundamental differences underlying neurotransmitter release between e xcitatory and inhibitory neurons. (C) 1994 Wiley-Liss, Inc.