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 KM 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 y-aminobutyric acid (GABA), because they were blocked by GABA antagonist, bicuculline at 20 pM. Adenosine at 100 p. M and carbachol at 10 pM suppressed these EPSCs to about 35% of control. However, adenosine and carbachol at the same concentration did not suppress the IPSCs. Baclofen at 10 pM 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 pM adenosine, 10 pM carbachol, and 10 pM 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.
MATERIALS AND METHODS Culture
Hippocampi of rat fetus on approximately embryonic day 18 were cut into small pieces (about 0.3 mm in diameter). Tissue pieces from 10 to 12 embryos were collected in 5 ml of culture medium and gently dissoci-