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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 microM 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 microM. Adenosine at 100 microM and carbachol at 10 microM suppressed these EPSCs to about 35% of control. However, adenosine and carbachol at the same concentration did not suppress the IPSCs. Baclofen at 10 microM 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 microM adenosine, 10 microM carbachol, and 10 microM 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.

The probabilistic nature of excitatory postsynaptic potentials (EPSPs) induced monosynaptically in CA3 neurons by impulses of granule cells was studied in thin transverse sections of the guinea pig hippocampus. More than 600 EPSPs were recorded under several conditions, their amplitudes were measured, and histograms representing the EPSP amplitude distribution were constructed. Quantal parameters were estimated by the method of maximum likelihood. Of 9 neurons examined in the control solution, one neuron showed an exceptionally large number of transmission failures. The amplitude distribution of EPSPs recorded from this neuron could be described by Pascal statistics, but not by binomial or Poisson statistics. The EPSP amplitude distribution from the other neurons could be described by either binomial, Poisson, or Pascal predictions with a minor preference for the last statistic. When an apparently homogeneous group of data was divided into two subgroups and parameters were estimated separately, inconsistent values were obtained in some neurons with no failures. 2-Amino-4-phosphonobutyric acid (APB) suppressed the EPSPs reversibly at relatively low concentrations. Theoretical curves calculated according to the Pascal statistics fit quite well to the entire amplitude distribution of EPSPs recorded under the action of APB. The suppression of EPSPs by APB was accompanied by a marked decrease in mean quantal content (m) with no significant reduction in mean quantal amplitude (q). A quantum induced an increase in membrane conductance of about 150 pS. These results suggest that the release probability of the mossy fiber terminal fluctuates temporally according to a gamma distribution, and that APB reduces the liberation of the transmitter from mossy fiber terminals, thereby suppressing transmission between mossy fibers and CA3 neurons.

A technique has been devised to activate single granule cells in the hippocampus, and to record simultaneously spikes from the particular granule cell and excitatory postsynaptic potentials from a monosynaptically connected CA3 neuron. The unitary excitatory postsynaptic potentials (EPSPs) sustained for long observation periods, and increased in size with increases in stimulus frequency and in external Ca2+ concentration. This technique may be useful for quantal analysis of transmission through the synapse between mossy fibers and CA3 neurons.

The effects of some organic calcium channel blockers and a toxin on synaptic transmission were examined in hippocampal slices from the guinea pig. omega-Conotoxin fraction GVIA, a novel peptide that blocks N and L type voltage-sensitive calcium channels, blocked synaptic transmission from mossy fibers to CA3 neurons at very low concentration (100 nM). However, organic calcium channel blockers such as verapamil (100 microM) and nifedipine (10 microM), which block L type calcium channels, had little effect on the synaptic transmission. Phenytoin (100 microM, T type calcium channel blocker) was also ineffective to block the synaptic transmission. These results suggest that presynaptic calcium channels which cause transmitter release may be N type calcium channel.

Application of phorbol esters, selective activators of protein kinase C (PKC), causes a long-lasting enhancement of synaptic transmission in the hippocampus. We examined the relationship between this phorbol ester-induced enhancement of synaptic transmission and long-term potentiation (LTP) quantitatively in the mossy fiber-CA3 synapse of the guinea pig hippocampus in vitro. Potentiating action of 0.5 microM phorbol 12,13-diacetate (PDAc) on synaptic transmission was almost unaffected by preceding tetanic stimuli which had caused a full development of LTP. Tetanic stimulation induced LTP even after synaptic transmission had been enhanced by 0.5 microM PDAc. Quantitative estimations revealed that PDAc-induced synaptic enhancement and LTP were simply additive to each other. The relation between LTP and PKC activation is discussed.