Myogenic activity and serotonergic inhibition in the chromatophore network of the squids, Dosidicus gigas (Family Ommastrephidae) and Doryteuthis opalescens (Family Loliginidae)

Abstract

Seemingly chaotic waves of spontaneous chromatophore activity occur in the ommastrephid squid, Dosidicus gigas, in the living state and immediately after surgical disruption of all known inputs from the central nervous system. Similar activity is apparent in the loliginid, Doryteuthis opalescens, but only after chronic denervation of chromatophores for 5-7 days. Electrically-stimulated, neurally-driven activity in intact individuals of both species is blocked by tetrodotoxin (TTX), but TTX has no effect on spontaneous wave-activity in either Dosidicus or denervated D. opalescens. Spontaneous, TTX-resistant activity of this sort is therefore likely myogenic, and such activity is eliminated in both preparations by 5-HT, a known inhibitor of chromatophore activity. Immunohistochemical techniques reveal that individual axons containing L-glutamate (L-Glu) or 5-HT (and possibly both in a minority of processes) are associated with radial muscle fibers of chromatophores in intact individuals of both species, although the area of contact between both types of axons and muscle fibers is much smaller in Dosidicus. Glutamatergic and serotonergic axons degenerate completely following denervation in D. opalescens. Spontaneous waves of chromatophore activity in both species are thus associated with reduced (or no) serotonergic input in comparison to the situation in intact D. opalescens. Such differences in the level of serotonergic inhibition are consistent with natural chromogenic behaviours in these species. Our findings also suggest that such activity might propagate via the branching distal ends of radial muscle fibers.