Which mechanisms account for the sensory neuron blocking action of capsaicin on primary afferents in the rat urinary bladder?

In the rat isolated bladder, capsaicin produced a concentration-dependent contraction, shown previously to depend upon transmitter release from peripheral endings of primary afferents. When using low concentrations (30-300 nM) of capsaicin, exposure to a second and third dose of capsaicin produced smaller responses than the first application, although a subsequent challenge with 10 microM capsaicin still elicited a contraction which was not reduced as compared to the response produced by the first exposure to a low dose of capsaicin. Capsaicin also evoked a prompt outflow of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI), taken as a marker for sensory nerve activation. A second or third application of a submaximal concentration of the drug was ineffective, although a subsequent challenge with 1 microM capsaicin was effective. These findings indicate that neuropeptide depletion does not necessarily account for the early stage of capsaicin 'desensitization' of primary afferents.