Enhanced synaptic transmission at identified synaptic connections in the cerebral ganglion of Aplysia

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.

Appearance and maturation of synaptic plasticity during juvenile development in Aplysia

In the present study we examine the developmental appearance and maturation of synaptic plasticity at the A-B neuron synapse in the cerebral ganglion of Aplysia. In the CNS of juvenile Aplysia 120 days after hatching, the excitatory synaptic connection between A and B cluster neurons is essentially the same as in the adult cerebral ganglion. No differences were observed between the amplitudes of the initial EPSPs in the cerebral ganglia of juveniles and adults. One form of plasticity, low frequency synaptic depression, is also present in juveniles. Another form of activity-dependent plasticity, slow developing potentiation (SDP) appears and matures during the late juvenile stage of development. At 120 days posthatching SDP, evoked by tetanic stimulation, is largely absent. Potentiated EPSPs have a significantly smaller amplitude than in adults. Over the next 80 days SDP undergoes a maturation process. The peak potentiation increases linearly with age from 135 +/- 12% at 125 days to 275 +/- 20% at 188 days. The duration of the potentiation, as measured by the time-constant its decay, also increases linearly from 12.7 +/- 3.4 min to 27.9 +/- 3.9 min. From 120-170 days, < 50% of the A neurons tested exhibited SDP. After 170 days, > 85% exhibited SDP. Changes in the rising phase of the A neuron action potential have been implicated in mediating SDP. At 120 days, the A neuron action potential has a significantly shorter duration (half-width) than in the adult. Between 120 and 200 days, both the duration and rise-time of the A neuron action potential increase linearly. These results confirm findings by other investigators, that different forms of synaptic plasticity develop independently, with depression appearing before potentiation.

Postsynaptic induction of long-term synaptic facilitation in snail central neurones

Long-term facilitation in molluscs is believed to be induced due to purely presynaptic activations. We recorded excitatory postsynaptic potentials (EPSPs) simultaneously from two identified neurones of snail parietal ganglia. We report a non-decrementing facilitation induced by intracellular tetanization with concomitant presynaptic activation. The mean EPSP amplitude measured in 10 neurones 30-50 min after tetanization was 17% greater than in the non-tetanized control neurones. Only short-lasting (5-10 min) postsynaptic changes were found (post-tetanic hyperpolarization and resistance decrease). The facilitation was especially prominent (34%, 10 min post-tetanus) but decreased within 15 min in preparations with rapid wash-out of the external media. The data suggest that induction of long-term enhancement in molluscs depends on postsynaptic events and, like in mammals, may involve increased postsynaptic Ca2+ and subsequent release of retrograde messengers.