Adenosine Suppresses The Response Of Neurons To γ-aminobutyric Acid In The Superficial Laminae Of The Rat Spinal Dorsal Horn

The superficial laminae (laminae 1 and II) of the spinal dorsal horn are the sites of the first synaptic relay in the nociceptive pathway. r-Aminobutyric acid (GABA) is a putative inhibitory neurotransmitter in the nervous system. Immunohistochemical studies have shown that GABA-like immunoreactive (GABA-LI) neurons, fibers and terminals and GABAa receptors are intensely located in the superficial laminae of the spinal dorsal horn, especially in lamina II (substantia gelatinosa, SG). Sufficient evidence suggests that GABA plays a key role in the modulation of sensory inputs in the spinal dorsal horn. Adenosine is an endogenous modulator of cellular activity with receptors expressed in a wide variety of tissues including both peripheral and central nervous systems. In the spinal cord, adenosine is known to modulate pain transmission as a neuromodulater. Autoradiographic studies have demonstrated that adenosine A1 and A2 receptors are expressed in the spinal dorsal horn, both on the post-synaptic SG neurons and the central terminals of the primary afferent neurons. Adenosine A1 receptors are selectively concentrated in the SG. A previous study has demonstrated that adenosine 5'-triphosphate (ATP) is co-released with the inhibitory neurotransmitter GABA. Adenosine. probably generated by extracellular metabolism of ATP, was found suppressing GABA currents in cultured neurons of the rat spinal dorsal horn. But no information is available about what the related intracellular signal transduction mechanism is and which adenosine receptor subtype gets involved. In order to clarify the matter and detect the related intracellular signal transduction, whole-cell patch-clamp recording technique was used to investigate the modulatory effect of adenosine on GABA activated whole-cell currents in the neurons acutely dissociated from the superficial laminae of the rat spinal dorsal horn.The results showed: (1) GABA acted on GABAA receptor and elicited inward C1- currents (IGABA) at a holding potential (VH) of-40 mV; (2) adenosine suppressed GABA-induced Cl- current with affecting neither the reversal potential of IGABA nor the apparent affinity of GABA to its receptor; (3) N6-cyc!o-hexyladenosine (CHA), a selective adenosine A1 receptor agonist, mimicked the suppressing effect of adenosine on IGABA, whereas 8-cyclopentyl-l,3-dipropylxanthine (DPCPX), a selective adenosine AI receptor antagonist, blocked the suppressing effect of adenosine; (4) chelerythrine, an inhibitor of protein kinase C, reduced the suppressing effect of adenosine on IGABA; (5) pretreatment with l,2-bis-(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxy-methyl) ester (BAPTA AM) , a Ca2+ chelator, did not affect adenosine-induced suppression of IGABA.The present results indicate that: (1) the suppression of adenosine on IGABA is mediated by adenosine A1 receptor and through a Ca2+-independent protein kinase C transduction pathway; (2) the interactions between adenosine and GABA might be involved in the modulation of nociceptive information transmission at spinal cord level.