L-GLUTAMATE AND SEROTONIN ARE ENDOGENOUS IN SQUID CHROMATOPHORE NERVES

Colour changes in cephalopods are controlled by complex organs termed chromatophores whose radial muscles are directly innervated from the b rain, In the squids Alloteuthis subulata and Loligo vulgaris, light mi croscopy of silver- or Methylene-Blue-stained preparations shows that each muscle is innervated by 2-6 nerves running along its length. An e lectron microscope (EM) study shows that most of these nerves contain 50 nm diameter electron-lucent vesicles organised into numerous synaps es along the muscle. Their size and appearance is consistent with thei r containing L-glutamate (L-Glu). Usually there is one nerve on each m uscle containing 95 nm diameter electron-dense vesicles that are not o rganised into synapses, Such vesicles, whose appearance is consistent with their containing serotonin (5-HT), are never found colocalised wi th the small, clear vesicles. Topically applied L-Glu causes the radia l muscles to contract (and the chromatophore to expand), even after ch ronic denervation; this effect is blocked by the glutamate antagonists CNQX and DNQX. In contrast, topically applied 5-HT (or its agonists 8 -OH-DOPAT and alpha-methyl 5-HT) induces relaxation of precontracted m uscle, Incubation with antibodies to L-Glu (Lg-A), using peroxidase an ti-peroxidase/diaminobenzidine visualisation, produces specific staini ng along the radial muscles like that seen with silver. Antibodies to 5-HT produce similar specific staining. When sections of skin that had stained positively with Lg-A in the light microscope are examined at the EM level, it is seen that such staining is confined to nerve axons . These results, showing that L-Glu and 5-HT are endogenous in the ner ves innervating squid chromatophores and that the radial muscles conta in receptors for both substances, suggest that L-Glu is an excitatory transmitter at squid chromatophore muscles. The way in which 5-HT acts to relax the muscles, however, remains to be established.