Enhancement of neuronal nicotinic receptor activity of rat chromaffin cells by a novel class of peptides

The N-terminal 1-7 fragment of the neuropeptide CGRP inhibits neuronal nicotinic acetylcholine receptors (nAChRs) of rat chromaffin cells. To identify the structural motif responsible for this action, we investigated the effects of shorter CGRP fragments on patch-clamped rat chromaffin cells in culture. CGRP(1-6) evoked no direct change in baseline current or input conductance, but it strongly potentiated inward currents induced by very fast, non-desensitizing applications of nicotine. Potentiation was use independent and present even when coapplied with nicotine. The action of CGRP(1-6) was voltage independent and agonist independent. Because equimolar concentrations of CGRP(1-6) and CGRP(1-7) left nicotine-induced submaximal currents unchanged, these peptides presumably acted via a similar site through which they generated opposite effects. This observation also suggests that a single amino acid deletion could transform a peptide antagonist into a potentiating one. Deleting one amino acid from the COO- end of the CGRP(1-6) sequence yielded CGRP(1-5), which retained smaller potentiating activity. Even the CGRP(1-4) fragment possessed slight potentiation, which was lost with CGRP(1-3). CGRP(1-6) preferentially potentiated small over large responses to nicotine. One possibility is that CGRP(1-6) interacted with nAChRs like an allosteric modulator (e.g., physostigmine). Coapplication of enhancing concentrations of physostigmine and CGRP(1-6) led to linear summation of the individual effects, while CGRP(1-6) could partly reverse the depression by a large concentration of physostigmine. These data indicate functionally distinct sites of action for CGRP(1-6) and physostigmine. Potentiation of nicotinic receptors by CGRP(1-6) and its derivatives suggests them to be a new class of molecules enhancing activity mediated by nAChRs.