Roles of glutamate and FMRFamide-related peptides at the chromatophore neuromuscular junction in the cuttlefish, Sepia officinalis.

Abstract

Body patterning behavior, the expression of highly intricate patterns, is ubiquitous among all unshelled cephalopods. These body patterns are in part generated by the coordinated activity of millions of skin chromatophore organs, each of which is regulated by a set of chromatophore muscles directly innervated by centrally located chromatophore motoneurons. This study addresses the question of the identity and function of the transmitter(s) at the chromatophore neuromuscular junction (NMJ) in the European cuttlefish Sepia officinalis. Glutamate application causes a rapid contraction of the chromatophore muscles, resulting in chromatophore expansion. Pharmacological studies demonstrate that the chromatophore muscles contain receptors blocked by glutamate-specific antagonists. Glutamate-like immunoreactivity is also present in the somata of putative chromatophore motoneurons. These findings suggest that glutamate likely acts as a neurotransmitter at the chromatophore NMJ. Evidence is also presented suggesting that FMRFamide-related peptides (FaRPs) also function as neurotransmitters at the Sepia chromatophore NMJ. FMRFamide application causes contraction of chromatophore muscles; however, the FMRFamide effect is slower and longer lasting than that of glutamate. Pharmacological data show that FMRFamide acts directly on the chromatophore muscles. FMRFamide-immunopositive cells are present in the posterior chromatophore lobe, the putative location of the chromatophore motoneuron somata. A combination of immunocytochemistry and in situ hybridization shows that some putative chromatophore motoneurons express FaRP-like immunoreactivity and an FaRP-coding mRNA transcript. Many FMRFamide-immunopositive cells in the posterior chromatophore lobes also express glutamate-like immunoreactivity. We conclude that glutamate and FaRPs likely function as fast and slow transmitters, respectively, at the Sepia chromatophore NMJ.