SOME PHARMACOLOGICAL DIFFERENCES BETWEEN HIPPOCAMPAL EXCITATORY AND INHIBITORY SYNAPSES IN TRANSMITTER RELEASE - AN INVITRO STUDY

The effects of adenosine, carbachol, and baclofen on synaptic transmission between neurons in cultured rat hippocampal explants were studied using the tight-seal whole cell clamp technique. In the culture, stimulations of neurites cause postsynaptic currents (PSCs) in nearby neurons under voltage-clamp condition. In the presence of 20-mu-M bicuculline, most PSCs were considered as glutamatergic excitatory postsynaptic currents (EPSCs), because they were blocked by glutamate antagonist, kynurenate at 1 mM. In the presence of 1 mM kynurenate, PSCs seemed to be inhibitory postsynaptic currents mediated by gamma-aminobutyric acid (GABA), because they were blocked by GABA antagonist, bicuculline at 20-mu-M. Adenosine at 100-mu-M and carbachol at 10-mu-M suppressed these EPSCs to about 35% of control. However, adenosine and carbachol at the same concentration did not suppress the IPSCs. Baclofen at 10-mu-M suppressed both EPSCs and IPSCs significantly (EPSCs: to about 40% of control, IPSCs: to about 30% of control). In contrast, membrane currents elicited by ionophoretically applied glutamate and GABA were not suppressed by 100-mu-M adenosine, 10-mu-M carbachol, and 10-mu-M baclofen. From these results, it is suggested that the pharmacological sensitivities of transmitter release from presynaptic terminals are different between glutamatergic excitatory synapses and GABAergic inhibitory synapses in hippocampal cultures.