Analysis of potentiation in the cerebral ganglion of Aplysia.

Quantal analysis was used to characterize synaptic transmission between A and B neurons in the cerebral ganglion of Aplysia in control and during slow developing potentiation, a form of synaptic plasticity exhibited by these synapses. Control values of mean quantal content (m) and quantal size (q) estimated by the method of coefficient of variation (CV) were m approximately 6, q approximately 56 microV in the solution with Ca2+/Mg2+ = 5/200 and m approximately 18, q approximately 41 microV in the solution with Ca2+/Mg2+ = 55/150. There was a good correlation between an increase in the amplitude of excitatory synaptic potential and an increase in calculated quantal content (m(cv)) during potentiation. A decrease of Ca2+/Mg2+ ratio in the bath solution allowed observation of transmission failures and in some cases regular peaks on excitatory postsynaptic potential amplitude histograms. The latter provided more direct estimate of the quantal size. Induction of the potentiation in this solution, however, became difficult. In cases of successful potentiation induction, probability of failures was less than in control; distances between histogram peaks, reflecting quantal size remained the same. The results obtained in this study support a hypothesis that potentiation of the synaptic transmission between A and B neurons of Aplysia is primarily due to an increase of transmitter release.