Presynaptic inhibition in the crayfish brain

Summary

• 1. Ascending interneurons were functionally identified and injected with Lucifer Yellow and horseradish peroxidase. The distribution of their axon terminals in the supraesophageal ganglion was examined with light and electron microscopy

• 2. The terminals of all cells examined invade the ipsilateral parolfactory neuropil. Most of the cells also project processes to the ipsilateral antennal and/or optic neuropil and a few cells terminate in the optic ganglia. Thus ascending interneurons may distribute their actions to all three neuromeres of the brain and to distal elements of the visual pathway (Figs. 1–3).

• 3. The axons and secondary processes of ascending interneurons are distinguished by the presence of microtubules. Both processes exhibit protuberances (Fig. 5) which in the electron microscope have all the features of subsynaptic zones (Fig. 6).

• 4. HRP labeled processes in the synaptic neuropil region contain small round vesicles and large dense core vesicles (Fig. 7). The two vesicle types are segregated. Only the small round vesicles are found at synaptic release sites.

• 5. Presynaptic sites containing small round vesicles (Fig. 8) are identifiable along the secondary and tertiary process of the HRP labeled ascending axon terminals. We infer that these synapses are the basis of the excitatory functional connection between ascending and descending interneurons.

• 6. Vesicle-containing neural processes in the antennal and antennule neuropils make axo-axonic contacts with the HRP labeled ascending cell terminals (Figs. 6 and 9). A preponderance of these synapses contain elongated vesicles which imply an inhibitory function. We infer that these axoaxonic synapses are the structural correlates of the functional inhibitory inputs to ascending interneuron terminals.

• 7. The axo-axonal synapses are principally located on the terminal arborization between the axon proper and the release sites of the ascending interneuron. This location is consistent with inhibitory action on events intermediate between the axonal action potential and the transmitter release mechanism.