Proceedings of The Physiological Society

University of Bristol (2001) J Physiol 536P, S106


Heterosynaptic facilitation in the feeding system of the snail Lymnaea by the octopaminergic OC interneurons

*Department of Biology, University of York, PO Box 373, York YO1 5YW, UK and †Balaton Limnological Institute of the Hungarian Academy of Sciences, PO Box 35, Tihany, H-8237 Hungary

The OC interneuron (Vehovszky et al. 1998) is an octopamine containing, modulatory neuron, intrinsic to the feeding system of the pond snail, Lymnaea stagnalis. Here we report that prestimulation of the OC interneuron enhances the strength of the cholinergic synapses made by the SO interneuron, a modulatory protraction phase interneuron (Elliott & Benjamin, 1985).

The isolated CNS was dissected out and neurons impaled as described recently (Vehovszky et al. 1998). The ganglia were bathed in a saline containing elevated magnesium and calcium to enhance monosynaptic pathways (Vehovszky et al. 2000). A series of constant current pulses were applied to the SO, with some pulses preceded by stimulation of the OC interneuron.

When the interval from OC to SO stimulation is 2 s, the postsynaptic response of N1M interneuron to SO stimulation is increased (Fig. 1); the mean increase in three preparations is from 7.4 to 13.5 mV (paired t test, N = 3, P < 0.001). An enhancement is seen when the interval from OC to SO stimuli is less than 6 s. Prestimulation of the OC interneuron also increases the excitability of the SO interneurons, lowering their threshold for action potentials and increasing the mean firing rate to constant current pulses (from 7.3 to 9.7 Hz, N = 4, paired t test, P < 0.002).The enhancement of postsynaptic responses is seen at all the outputs of the SO interneuron and at all those of a second protraction phase interneuron, N1L. The effect of OC prestimulation on SO synapses is selective, however, with no effect on the N3p interneuron, even when the same postsynaptic target is tested. The SO -N1M synapse and SO-B1 motoneuron synapse are also strengthened by application of 1-5 µM octopamine. The major effect is an increased excitability of the SO; the mean firing rate to constant current pulses increases from 6.6 to 10.8 mV (N = 8, paired t test, P < 0.01). The B1 motoneuron response to acetylcholine, the main transmitter of the SO, is unaffected. Heterosynaptic facilitation of the SO synapses by the OC interneurons is an important mechanism of aminergic modulation of the feeding system because strengthening the connections from the SO interneuron to the central pattern generator N1M interneurons results in stronger activation of the feeding network.We would like to thank the BBSRC for their support.

figure one
Figure 1. A, prestimulation of OC interneuron enhances the SO-N1M synaptic connection. B, SO evokes summated postsynaptic potentials on N1M interneuron (Bi ) which is increased after OC prestimulation (Bii). (Vertical scale: 30 mV.)
    Elliott, C.J.H. & Benjamin, P.R. (1985). J Neurophysiol. 54, 1412-1421.


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