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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 brain. In the squids Alloteuthis subulata and Loligo vulgaris, light microscopy of silver- or Methylene-Blue-stained preparations shows that each muscle is innervated by 2-6 nerves running along its length. An electron microscope (EM) study shows that most of these nerves contain 50 nm diameter electron-lucent vesicles organised into numerous synapses along the muscle. Their size and appearance is consistent with their containing l-glutamate (l-Glu). Usually there is one nerve on each muscle containing 95 nm diameter electron-dense vesicles that are not organised into synapses. Such vesicles, whose appearance is consistent with their containing serotonin (5-HT), are never found co-localised with the small, clear vesicles. Topically applied l-Glu causes the radial muscles to contract (and the chromatophore to expand), even after chronic 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 -methyl 5-HT) induces relaxation of precontracted muscle. Incubation with antibodies to l-Glu (Lg-A), using peroxidase anti-peroxidase/diaminobenzidine visualisation, produces specific staining 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 nerves innervating squid chromatophores and that the radial muscles contain 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.