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 HSNs Appear to be Serotonergic

C. Desai, S. McIntire R. Horvitz

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.