Sustained increase of Ca+2 oscillations after chronic TRPV1 receptor activation with capsaicin in cultured spinal neurons.

Hyperalgesia and allodynia occur as a consequence of peripheral and central sensitization that follows sustained nociceptive activation. The cellular alterations associated to this state of nociceptive network hyperexcitability represent a form of neuronal plasticity, but they are not well understood because of its complexity in situ. In this study, after treating primary spinal neuron cultures with capsaicin (0.5-1 microM) for 48 h fluorimetric recordings were performed. The activation of TRPV1 receptors with capsaicin (0.5-1.0 microM) increased the frequency of calcium transients (0.03+/-0.002 Hz vs. 0.05+/-0.006 Hz, P<0.05), mediated by AMPAergic transmission, as well as the percent of neurons with activity (37+/-3% vs. 65+/-4%, P<0.05). The effect of capsaicin was long lasting and the neurons were found to be hyperfunctional and with increased levels of phosphorylated CREB (cAMP responsive element binding) even after 72 h of treatment with capsaicin (32+/-5% vs. 52+/-5%). The effect of capsaicin was blocked by capsazepine (1 microM), TTX (100 nM) and KN-62 (1 microM), but not by K252a (200 nM) or PD98059 (50 microM) indicating the involvement of TRPV1. The results suggest the participation of Ca2+, CaMKII and CREB on the prolonged enhancement of excitability following chronic exposure to capsaicin. Thus, it is likely that chronic TRPV1 activation is capable of inducing prolonged increases in neurotransmission mediated by glutamatergic receptors.