Worm Breeder's Gazette 16(1): 47 (October 1, 1999)

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.

Pharyngeal neuron I1 contributes to avermectin sensitivity

Leon Avery^1,2

At the Worm Meeting this spring, we reported that avermectin can act through any of three different glutamate-gated chloride channel a subunits, AVR-14, AVR-15, and GLC-1, to prevent growth (Dent et al, 1999 International Worm Meeting abstract 76). The effect mediated by AVR-14 requires the gap junction subunits UNC-7 and UNC-9. This observations led us to suggest that AVR-14 hyperpolarizes some non-essential cell, which hyperpolarizes an essential excitable cell to which it is connected by gap junctions. Finally, we discovered that avermectin prevents pharyngeal pumping in intact avr-15 glc-1 worms (in which killing is mediated entirely by AVR-14), but does not prevent pumping in pharynxes dissected from these worms. This suggests that AVR-14 may act on an extrapharyngeal cell that is coupled by gap junctions to a pharyngeal target. The extrapharyngeal and pharyngeal nervous systems are connected only by a bilaterally symmetric pair of extrapharyngeal neurons, the RIPs, which are gap-junctioned to the pharyngeal I1 neurons.

To test whether I1 contributes to AVR-14-mediated avermectin sensitivity, I killed I1 in newly hatched avr-15 glc-1 larvae with a laser, allowed them to grow for a day in the absence of drug (to allow time for I1 to die), then placed them on plates containing 10 ng/ml ivermectin. I then followed the worms for 21 days, noting whether they reached adulthood, and whether they produced progeny. The results were as follows:

I1^- avr-15 glc-1 worms were more resistant to ivermectin than intact avr-15 glc-1 worms. This difference is significant at P = 0.0036. Therefore, AVR-14 killing is mediated, at least in part, by the pharyngeal neuron I1.

This is a surprising result, because I1 itself is nonessential. In fact, under ordinary growth conditions killing I1 has almost no detectable effect. Why should hyperpolarization of I1 have a greater effect than killing it? I suspect that I1 may be connected by gap junctions to pharyngeal muscle. Normally neurons and muscles are not connected by gap junctions, since neurons are ectodermal and most muscles are mesodermal, and the two germ layers are separated by a basal lamina. Pharyngeal muscles, however, are myoepithelial cells, topologically part of the ectoderm. I1 is immediately adjacent to pharyngeal muscle membranes, and in fact I1 has free subcuticular endings of unknown function that are closely associated with muscle (Albertson and Thomson, 1976, Phil Trans 275B: 299). I hope to test directly whether I1 and pharyngeal muscle are connected by gap junctions.