Worm Breeder's Gazette 7(2): 51
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
We have recently extended the analysis of the C. elegans touch circuitry (see WBG 5 no. 2). The previous experiments had shown that the touch (microtubule) cells in the tail mediated touch sensitivity through the gamma interneurons (now called the PVC cells) and the touch cells in the head through both the delta (AVD) and beta (AVB) interneurons. The development of the anterior touch circuit is somewhat complicated. At hatching only the two lateral microtubule cells (ALMR and ALML) are found in the head (see Chalfie and Sulston, Dev. Biol. 82: 358 (1981) for a description of the cells). These cells form gap junctions with the AVD cells, but do not synapse onto each other or the AVB cells. Later, a third anterior touch cell (AVM) arises from divisions of the Q2 lineage. AVM forms gap junctions not only with the AVD cells but also with the other two touch cells. In addition it forms chemical synapses onto the AVB cells. AVM appears to serve two functions: 1) to link up with the other two touch cells and form a complicated nerve net of touch receptors (this may be required in C. elegans because the cells don't have intricate branching patterns) and 2) to provide this nerve net with synapses onto the AVB interneurons. More subtle changes occur in the anterior touch circuitry. Specifically the lateral touch cells appear to lose the gap junctions with the AVD interneurons in the adult. Nerve ring synapses have been examined in two series: JSH (mid L4) and N2U (gravid adult). In the younger animal all three touch cells form gap junctions with the AVD cells; in the older animal only AVM does. This loss has been confirmed in laser ablation studies in which Q2 has been killed in newly hatched L1s. These animals, which lack AVM, are touch sensitive in the head until three-four days after the L4 molt. At this time they have reduced touch sensitivity when compared with unlasered controls. The loss is quite stochastic: in nine experimental animals some were virtually touch insensitive, others were almost wild type. These experiments suggest that ALM-AVD gap junctions are lost in older animals. This loss is quite specific. Comparing other synapses in the JSH and N2U series. we find that the touch cells retain gap junctions to other cells and the AVD cells not only retain gap junctions to other neurons, but also form new gap junctions with additional cells as the ALM-AVD connections are being eliminated. Thus, the development of touch cell synapses is quite involved with the formation of a complex nerve net of similar receptor cells and the subsequent loss of specific gap junctions. The function of these changes remains obscure.