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.

Developmental Changes in the Neural Circuitry for Touch Sensitivity

M. Chalfie, J. White, J. Sulston

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) 
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.