Worm Breeder's Gazette 8(2): 26
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.
The two HSNs ('hermaphrodite-specific neurons') innervate the vulval muscles and are necessary for normal egg-laying by the C. elegans hermaphrodite. Wild-type hermaphrodites are stimulated to lay eggs by exogenous serotonin or imipramine. Hermaphrodites lacking the HSNs are egg-laying defective but are stimulated to lay eggs by exogenous serotonin (Trent et. al., Genetics 104: 619, 1983). In contrast, imipramine does not stimulate egg-laying by HSN-defective hermaphrodites. In other organisms, imipramine is believed to potentiate endogenous serotonin (specifically, imipramine is thought to inhibit the uptake of serotonin into presynaptic terminals and thus to increase the amount of serotonin in the synaptic cleft). The simplest interpretation of these data is that the HSNs are serotonergic: exogenous serotonin bypasses the need for input from the HSNs; imipramine fails to stimulate egg-laying by an HSN-defective animal because there is no serotonergic input from the HSNs to potentiate. Serotonin has been localized histochemically in C. elegans using the technique of formaldehyde-induced fluorescence (FIF) (Horvitz et al., Science 216: 1012, 1982). With this technique only the two pharyngeal NSMs neurosecretory-motor neurons') could be seen to be serotonergic. He have now used anti-serotonin antisera to attempt to confirm the serotonergic nature of the NSMs and to identify other serotonergic cells. After fixation in formaldehyde, animals are digested in elastase and/or collagenase to permeabilize the cuticle to antibodies. The primary rabbit anti-serotonin antibody is visualized using a rhodamine-conjugated goat anti-rabbit antibody. This antibody protocol has proved to be significantly more sensitive than FIF: NSM cell bodies as well as processes can be seen (FIF shows only the processes in animals that have not been exposed to exogenous serotonin) , and a variety of other cells are stained. These new, possibly serotonergic cells include about six cells in the head, two cells in the pharynx, six bright and four faint cells in the male ventral cord, four or five cells in the male tail, and the HSN cell bodies and processes in the hermaphrodite. We have isolated 45 mutants that appear to be functionally HSN- defective by the criteria described above (see Desai et al., C. elegans Newsletter 8: 26, 1983). Some of these mutants lack HSNs completely, some have variably absent or misplaced HSNs, and some have morphologically normal HSNs as visualized with Nomarski optics. He have now utilized the anti-serotonin antibody to subdivide this last category into two groups: n1126 and n1077 (which represent two genes) do not stain for serotonin, whereas n995, n1068 and n1082 (which represent three genes) have normally staining HSNs. Strikingly, the mutants that lack serotonin staining in the HSNs contain serotonin in at least most of the other putative serotonergic cells. We have also discovered that n997, one of the mutants previously described as lacking HSNs, displays a pair of serotonergic cells variably positioned between the vulva and the tail. This mutant appears likely to be defective in HSN migrations, which occur embryonically as these cells move from the tail to the vulva (Sulston et al., Devel. Biol. 100: 64, 1983). No other defects have been observed in this mutant, suggesting that it defines a gene that may be involved specifically in HSN migration. It is interesting that these displaced HSNs become serotonergic and send processes into the ventral cord to the nerve ring, although presumably functional synapses onto the vulval muscles are not formed.