Laser microbeam axotomy and conduction block show that electrical transmission at a central synapse is distributed at multiple contacts

Abstract

Touch (T) sensory neurons in the leech innervate defined regions of skin and synapse on other neurons, including other T cells, within the ganglionic neuropil. The cells' receptive fields in the periphery are comprised of a central region, innervated by thick axons, and adjoining regions (minor fields) innervated by thinner axons. Secondary branches, known to be sites of synapses, emerge from the thinner and thicker axons. Pairs of T cells appear to make up to 200 separate contacts distributed within the neuropil. When the T cell is hyperpolarized, as occurs during natural stimulation of the cell, action potentials generated in the minor field and travelling into the ganglion along the thin axons may fail to conduct at central branch points. Evidence is presented, using axon conduction block and laser axotomy of cells filled with 6-carboxy-fluorescein, that synapses between separate groups of branches can function independently. Thus, selective activation of branches of the thin anterior axon produced a synaptic potential 36 +/- 6% of control amplitude, which was consistent with counts of 39 +/- 6% of contacts made by these branches. Laser axotomy of postsynaptic neurons showed that the anterior contacts indeed made the principal or only contacts activated during anterior conduction block. The results show that conduction block can modulate transmission within the ganglion, and it operates by silencing particular contacts between cells.

A 3-Synapse Positive Feedback Loop Regulates the Excitability of an Interneuron Critical for Sensitization in the Leech

“…This has been demonstrated for sensory neurons in the leech (Muller and Scott, 1981;Macagno et al, 1987;Gu et al, 1989Gu et al, , 1991. The recording in the inexcitable soma of the coupling interneuron would therefore show the sum of activity in multiple branches contacting the S cell branches and perhaps the contralateral coupling interneuron.…”

Synaptic facilitation by reflected action potentials: Enhancement of transmission when nerve impulses reverse direction at axon branch points

“…Lucifer Yellow was injected into single P cells (14,17). Images of cells were rendered by combining 25 optical sections recorded from a Noran laserscanning confocal microscope by using a ϫ40 oil-immersion objective with a 1.4 numerical aperture, a 15-m slit, and filters optimized for Lucifer Yellow.…”

“…When measuring the P 3 AP synaptic potential, it was necessary to eliminate the contribution from P and T cells in the anterior ganglion that also synapsed on the posterior AP cell. To this end, the peripheral axons of these two neurons were cut with a laser microbeam as described (14). The cells were pressure injected with 0.17 M 6-carboxyfluorescein, pH 7.4, and their peripheral axons were cut by laser irradiation.…”

Progressive recovery of learning during regeneration of a single synapse in the medicinal leech

“…To address these questions, a laser microbeam was used to cut only the S-cell axon in vivo. The technique built on earlier work showing that single S-cell axons can be injected with dye and other substances in a live animal (Mason and Muller, 1996) and that an argon laser can selectively sever one dye-filled axon at any chosen point (Gu et al, 1989). The approach aimed to provide information both on the circuitry underlying sensitization and on the capacity of a regenerated synapse to restore learning.…”