CHARACTERIZATION AND SOLUBILIZATION OF THE FMRFAMIDE RECEPTOR OF SQUID

The optic lobe of squid (Loligo pealei) contains FMRFamide receptors t hat can bind an iodinated FMRFamide analog: [I-125]-desaminoTyr-Phe-no rLeu-Arg-Phe-amide ([I-125]-daYFnLRFa). Radioligand binding assays rev ealed that squid FMRFamide receptors are specific, saturable, high aff inity sites (K-d = 0.15 nM) densely concentrated in optic lobe membran es (B-max = 237 fmole/mg protein). The receptors appeared to be couple d to G(s) because guanine nucleotides inhibit receptor binding and the stimulation of adenylate cyclase by FMRFamide is GTP-dependent. Both the binding and cyclase data showed that FMRFamide, but not FMRF-OH, i nteracts at FMRFamide receptors; thus the C-terminal Arg-Phe-amide is critical for binding. The high binding affinity of FMRFamide (0.4 nM I C50) was specific for FMRFamide-like peptides. The structure-activity relations of many FMRFamide analogs were defined in detail and were ne arly identical for both the membrane-bound and detergent-solubilized r eceptors. We also found that squid optic lobe contains FMRFamide-like reactivity as measured with both a radioimmunoassay and a radiorecepto r assay. Moreover, we have sequenced a fragment of genomic DNA that en codes a FMRFamide precursor. Our findings in sum suggest that FMRFamid e is a neurotransmitter in squid optic lobe, and that this tissue is a good source from which to purify FMRFamide receptors.