The activation and distribution of GABA and L-glutamate receptors on goldfish Mauthner neurones: an analysis of dendritic remote inhibition.

1. The sensitivity of the membrane of the cell-body and lateral dendrite of the Mauthner neurone to iontophoretically applied GABA and L-glutamic acid has been investigated in goldfish. The cells were afterwards examined histologically. The results, taken together with physiological observations, help to explain some functional properties of dendritic synapses.2. GABA inhibits by increasing the membrane conductance, and has little effect on resting potential. L-Glutamate depolarizes, and its effects could sum with subthreshold excitatory post-synaptic potentials (e.p.s.p.s) to cause impulse firing. Neither substance was effective when released intracellularly, but only when applied to the outside of the membrane.3. Using a roving double-barrelled micropipette to apply brief iontophoretic pulses very close to the cell, and recording from the cell-body, the sensitivity to GABA was found to be uniform over a region extending from the axon-hillock to about half-way along the lateral dendrite; it then appeared to fall progressively as the drug was pipetted further and further out on the dendrite. The sensitivity to L-glutamate however seemed relatively unaffected by varying the site of application.4. In contrast to this, when the recording electrode was inserted into the same region of the cell where the drugs were applied, then the sensitivity to GABA, like that to L-glutamate, was found to be similar everywhere, including the distal part of the lateral dendrite.5. The antidromic spike, which does not actively invade the lateral dendrite, was used as a convenient test signal for detecting electrotonic decrement as well as changes in membrane conductance. The conductance changes due to small doses of GABA applied to the dendrite could be detected only over relatively short distances (tens of microns). But over these same distances very little decrement of potential change took place.6. The concept of dendritic remote inhibition has been put on a firm experimental basis. It occurs in the Mauthner cell, and is shown to be capable of providing a high spatial selectivity of action. The e.p.s.p.s most affected by it are those generated out on the same dendrite, and it follows that many of the e.p.s.p.s produced far out on other dendrites could well be influenced by it, though to a lesser extent. Least affected are large e.p.s.p.s generated in the cell-body and axon-hillock regions. The synaptic grouping provided by the distal parts of dendrites and their associated nerve endings is discussed.7. The results of a theoretical treatment (Appendix) of the effects of a localized membrane resistance change on electrotonic potentials recorded nearer to the source have been applied to results obtained on the Mauthner cell. The experimentally observed properties of the ;remote inhibition' caused by localized GABA application to the lateral dendrite, accord well with those predicted both for an infinitely long uniform cable and for a cable having a closed end at the region of the conductance change. The possible implications of these results for other central neurones are considered.8. Evidence is adduced showing that the synapses responsible for collateral inhibition in the Mauthner cell include at least part of the group of small nerve endings found on the axon-hillock, cell-body, proximal and distal lateral dendrite. (The latter distally-located endings cause dendritic remote inhibition.) All these regions, some of which certainly have excitatory synapses, thus receive an inhibitory input as well, and are sensitive to both GABA and L-glutamate.9. E.p.s.p.s recorded in the cell-body were reduced by about the same amount as was the antidromic spike when GABA was applied there, but considerably more than the spike when the GABA application was to the lateral dendrite near the origin of the e.p.s.p. Even when the recording was made from the affected region of the dendrite, the e.p.s.p. seemed more sensitive to GABA than was the electrotonically conducted spike. This, and other observations, suggests that GABA may act presynaptically on the excitatory endings as well as post-synaptically.10. The detection of effects, and the recognition of site of action, of drugs applied iontophoretically in the central nervous system is discussed on the basis of the present findings.