Regulation of the responses to gonadotropin-releasing hormone, muscarine and substance P in sympathetic neurons by changes in cellular constituents and intracellular application of peptide fragments of the substance P receptor.

The effects of alterations of intracellular constituents on the actions of three agonists [gonadotropin-releasing hormone, muscarine and substance P (SP)] on the M-type potassium current in bullfrog sympathetic neurons were examined. Application of maximal concentrations of each agonist resulted in inhibition of M-current followed by desensitization. Desensitization was greatest during SP application, less with gonadotropin-releasing hormone and least after muscarine. Recovery after agonist washout was greatest for SP, less for muscarine and least for gonadotropin-releasing hormone. The effects of varying intrapipette pH, [ATP], Ca buffer and free Ca on inhibition by each of the agonists, desensitization and recovery were tested. Comparison of the effects of different intracellular constituents showed that desensitization and recovery are distinct phenomena. Desensitization was greatest with 3 mM ATP in the pipette and was enhanced when pyruvate and glucose were added extracellularly. Two synthetic peptides, comprising amino acids 325-360 and 361-375, respectively, of the carboxyl tail of the rat SP receptor inhibited desensitization to SP, but not to the other agonists. A third peptide homologous to residues 376-407 and a peptide from the extracellular portion of the receptor (residues 168-179) did not affect desensitization. This suggests that the portion of the carboxyl tail of the SP receptor from amino acids 325-375 is involved in desensitization.