Regulatory Effects Of Noradrenaline On Substance P Release From Cultured Dorsal Root Ganglion Neurons

The noradrenergic system is subject to plastic changes that influence its antinociceptive efficacy after injury or inflammation. Noradrenaline (NA) is a key neurotransmitter that activates adrenoreceptors under physiological and pathophysiological conditions. Interestingly, both α1-and α2-adrenoreceptors are expressed in dorsal root ganglion (DRG) primary sensory neurons and regulate neurogenic inflammation and nociceptive responses. NA depresses heat-activated K+channel currents by activating α1-but not α2-adrenoreceptors in cultured rat DRG neurons. Substance P (SP), an11-amino acid neuropeptide found in sensory nerves innervating peripheral tissues is involved in inflammation and nociception. Primary sensory DRG neurons that respond to nociceptive stimuli are characterized by SP expression. SP release from the central processes activates second-order sensory neurons to transmit nociceptive signals into the central nervous system (CNS). Release of SP from peripheral endings causes a series of local inflammatory responses referred to as neurogenic inflammation. SP release and SP levels in primary sensory neurons are influenced by many factors. A previous study has shown that the α1-adrenoreceptor agonist phenylephrine causes the release of SP from terminals of capsaicin (CAP)-sensitive sensory neurons. Neurogenic inflammation and pain resulting from CAP injection respond to NA released from sympathetic efferents. It remains unknown whether SP expression in, and SP release from DRG neurons are influenced by the activation or inhibition of a-adrenoreceptors. The present study aimed to address this question by exposing DRG neurons to the a-adrenoreceptor agonist NA, NA plus the al-adrenoreceptor antagonist prazosin, or NA plus the a2-adrenoreceptor antagonist yohimbine in vitro, and assessing SP expression and release by reverse transcription-polymerase chain reaction (RT-PCR), Western blot assay, double fluorescent labeling, and enzyme-linked immunosorbent assay (ELISA). The results are as follows:(1) The a-adrenoreceptor agonist NA, the α1-adrenoreceptor antagonist prazosin, and the α2-adrenoreceptor antagonist yohimbine did not have significant effects on the expression of preprotachykinin (PPT) mRNA encoding for SP.(2) The a-adrenoreceptor agonist NA, the α1-adrenoreceptor antagonist prazosin, and the a2-adrenoreceptor antagonist yohimbine did not have significant effects on the peotein levels of SP.(3) The a-adrenoreceptor agonist NA, the α1-adrenoreceptor antagonist prazosin, and the a2-adrenoreceptor antagonist yohimbine did not have significant effects on the ratio of SP-expressing DRG neurons.(4) The a-adrenoreceptor agonist NA, the α1-adrenoreceptor antagonist prazosin, and the a2-adrenoreceptor antagonist yohimbine did not have significant effects on the basal SP release. Pretreatment with prazosin reduced the amount of CAP-evoked SP release compared to the NA alone, whereas yohimbine had no effect on the NA-induced increase of CAP-evoked SP release. In conclusion, NA, by acting on α1-but not a2-adrenoreceptors, increases CAP-evoked SP release from DRG neurons, indicating that it may be one of the pronociceptive noradrenergic mechanisms in the periphery. The results of the present study provide experiemtnal evidence for further studying the effects of NA on pain signaling transmission during neurogenic inflammation.