Agonistas -adrenérgicos modulam canais iônicos nociceptivos em neurônios dos gânglios da raiz dorsal

Abstract

Noradrenalin and the circulating adrenaline, through the activation of -and -adrenergic receptors are involved in the intrinsic control of pain. In the peripheral nervous system, sympathetic stimulation can excite sensory neurons in animals with inflamed tissues or after peripheral nerve injury. Furthermore, models of neuropathic pain are usually associated with an increased sympathetic tone of the fibers innervating the Dorsal Root Ganglia of (DRG), which suggests the existence of interactions between the activity of the sympathetic nervous system (SNS) and primary afferent neurons under painful condition. Changes in sensitivity to noxious stimuli may be caused by changes in the properties of ionic channels of primary afferent sensory neurons. For example, noradrenaline increases the firing rate of nociceptors neurons via activation of 1-adrenoceptors (Zhang et al., 2011). However, known about the effects of activation of -adrenergic receptors on molecules that initiate the transduction of the stimulus in primary nociceptors. To verify that, we tested whether activation of adrenergic receptors modulates ion channels activated by ligands expressed in sensory neurons. In addition, we seek changes in parameters of Action Potential (PA) that can determine the sensitivity of neurons to noxious stimuli. Thus, the aim of this study is to identify the existence of interactions between -adrenergic agonists and ion channels that transform noxious stimuli into electrical signals to be propagated in DRG neurons, channels such as TRPV1 receptors (Transient Receptor Potential Vanilloid type 1), P2X (Purinergic channels, activated by ATP) and ASICs (Acid-Sensing Ion Channels). In this work, we used the DRG neurons dissociated from male Wistar rats weighing 220-280g. The dissociated cells were treated with adrenergic agonists, phenylephrine (1; 1M) or clonidine (2; 10M), and the respective control groups received the vehicle dilution of agonists. Electrophysiological recordings were performed between 24-96 hours after culture. Electrophysiological recordings were performed using the patch clamp technique in whole-cell mode: voltage clamp to record ionic currents and current clamp to record action potentials and excitability thresholds of neurons. To elicit the ionic currents, fast application (2 seconds) of capsaicin (3M), ATP (50M), 5 pH = 7 and pH 6 were made. These solutions activate ionic currents via TRPV1, ASICs and P2X receptors, respectively. Phenylephrine increased P2Xnão3 current density from -21 ± 6 pA/pF (mean ± SEM, n=22) in control to -51 ± 11 pA/pF, n=24 (p < 0.05). Furthermore, phenylephrine increased by 95% (p < 0.05) neuronal excitability threshold of neurons with ASIC currents triggered by pH6s solution. Clonidine reduced capsaicin-evoked current from -218 ± 224 pA/pF (n=53) to -146 ± 26 pA/pF, n=44 (p < 0.05). No significant effect of clonidine and phenylephrine were observed in acid activated currents (pH = 7.0 or pH = 6.0) or the proportion of cells responsive to different stimuli (ATP, capsaicin and low pH), or the proportion of cells with characteristics tonic versus phasic. Our results suggest that activation of -adrenoceptors modulates two important transducing nociceptive molecules. This effect was evidenced by the increased P2Xnão3 current density in the presence of phenylephrine (-1 adrenergic agonist) and the reduction of the current density mediated by TRPV1 receptors in the presence of clonidine (-2 adrenergic agonist).