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Received February 10, 2005
Revised March 7, 2005
Accepted after revision March 7, 2005
1 Universita 'La Sapienza'
2 Universita "La Sapienza"
* To whom correspondence should be addressed. E-mail: sergio.fucile{at}uniroma1.it.
Ca2+ entry through neuronal nicotinic ACh receptors (nAChRs) modulates many biological processes in the nervous tissue. In order to study the functional role of nAChRs in peripheral sensory signalling, we measured their Ca2+ permeability in rat dorsal root ganglion (DRG) neurones, and analysed the effects of nAChR-mediated Ca2+ influx on the function of the vanilloid receptor TRPV1. The fractional Ca2+ current (Pf; i.e. the percentage of current carried by Ca2+ ions) flowing through nAChR-channels was measured by Ca2+ imaging fluorescence microscopy in combination with the patch-clamp technique. Functional nAChRs were expressed in a subset of adult DRG neurones (about the twenty-four percent of the cells), typically with small to medium size, as measured by their capacitance (40±3 pF). In most cells, ACh evoked slowly desensitising currents, insensitive to methyllicaconitine (MLA, 10 nM), a potent antagonist of homomeric nAChRs. Fast decaying currents, likely mediated by a7*-nAChRs, were observed in the fifteen percent of ACh-responsive cells, in which slowly decaying currents, mediated by heteromeric nAChRs, were simultaneously present. The nAChRs of adult DRG neurones exhibited a Pf value of 2.2±0.6 % in the presence of MLA, and of 1.9±0.6 % (P>0.1) in its absence of MLA, indicating that homomeric MLA-sensitive nAChRs do not contribute to Ca2+ entry into adult DRG neurones. Conversely, the ten percent of neonatal DRG neurones showed ACh-evoked currents completely blocked by MLA. In these neurones, nAChRs showed a larger Pf value (9.5±1.5 %), indicating the expression of bona fide a7*-nAChRs. Finally, we report that Ca2+ influx through nAChRs in adult DRG neurones negatively modulated the TRPV1-mediated responses, representing a possible mechanism underlying the analgesic properties of nicotinic agonists on sensory neurones.
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