Presynaptic inhibition in the crayfish brain

Summary

• 1. Presynaptic inhibition was examined at the synapses of identified interneurons in the crayfish brain.

• 2. Descending interneurons can be excited by sensory stimuli to the head, thorax and caudal appendages. EPSPs can be elicited in these interneurons by ascending cells arising in thoracic ganglia (Fig. 1B). Conditioning shocks to cephalic sensory roots produce a 40 to 60% amplitude reduction in the EPSPs (Figs. 2, 4).

• 3. The EPSP attenuation can be obtained under circumstances in which the conditioning shocks produce no observable postsynaptic action (Fig. 4).

• 4. The terminals of the presynaptic ascending interneurons are easily penetrated. They exhibit depolarizing compound IPSPs in response to cephalic root stimulation (Fig. 5).

• 5. The IPSPs exhibit reversal potentials (Fig. 6) and conductance changes (Fig. 7) similar to those reported in connection with primary afferent depolarization in crayfish abdominal ganglia.

• 6. Action potentials actively invade the terminal region (Table 2) and are only modestly reduced (3–10%) by concurrent terminal IPSPs. It is possible that action potential attenuation may be much greater at synaptic release sites if conduction is passive in the finer terminal branches.

• 7. Iontophoresis of GABA elicits depolarizing responses in ascending neuron terminals (Fig. 9). The GABA responses have properties similar to the synaptically elicited responses (Figs. 9, 10). Of particular note is the fact that GABA decreases the terminal input resistance by up to 70% but diminishes the presynaptic spike amplitude by less than 20%.

• 8. It is concluded that presynaptic inhibition probably acts by mechanisms other than or in addition to the attenuation of the presynaptic action potential.

• 9. It is also concluded that presynaptic inhibition in the brain prevents the summation of input from diverse sites and is therefore a mechanism for increasing the selectivity and temporal resolution of the large field sensory interneurons.