| ORL1 (opioid receptor-like 1) and cannabinoid receptors are two newly discovered G-protein-coupled receptors (Mollereau et al., 1994; Lachowicz et al., 1995; Pertwee, 1997, 1998). With the cloning of these receptors, the endogenous ligands for them are also discovered, which are nociceptin (also named orphanin/FQ) and anandamide (N-arachidonoylethanolamide), respectively. There was evidence showing that these two kinds of receptors have very similar actions. For instance, they are negatively coupled to adenylyl cyclase. Activation of these two receptors results hi opening inwardly-rectifying K"1" channel in different types of neurons (eg. PAG, hippocampus etc.) and inhibiting Ca2+ currents hi rat hippocampal and dorsal root ganglion neurons. Immunohistochemical and in situ hybridization histochemical studies have demonstrated ORL1 receptor, cannabinoid receptorand their ligands are densely distributed in the superficial doral hom of the spinal cord. According to behavioral studies in adult rats, intrathecal administration of nociceptin, anandamide and WIN-55,212 (WIN-2) (cannabinoid receptor agonists) produces an antinoceptive effect in versatile pain models including acute, tonic and chronic pain models (see Mogil & Pasternak, 2001; Pertwee, 2001 for review). Thus, it could be concluded that nociceptin-ORLl receptor and anandamide-cannabinoid receptor systems constitute an important part of endogenous antinociceptive system. However, cellular mechanisms underlying the actions of ORL1 and cannabinoid receptors on nociceptive information transmission in the spinal cord are still unknown. There was much evidence showing that substantia gelatinosa (SG) of the spinal dorsal horn plays an important role in nociceptive information transmission from the periphery to central nervous system. Thus, the present study firstly investigated the effect of nociceptin on membrane current of spinal dorsal hom substantia gelatinosa (SG) neurons by use of the whole-cell patch-clamp technique. Secondly, we examined the actions of nociceptin on excitatory and inhibitory synaptic transmission to SG neurons. We finally observed the effect of anandamide on membrane current of SG neurons and on excitatory synaptic transmission to SG neurons. The comparison of anandamide and capsaicin actions was also analyzed. The contents of this paper are as follows:1. The effect of nociceptin on membrane current of SG neurons in the adult rat spinal dorsal horn under voltage clampSuperfusion of nociceptin could induce an outward current on SG neurons of spinal dorsal hom at a holding potential of -70 mV; this action was dose-dependent with an EC$o value of 0.23 ^M and Hill coefficient of 1.5. The outward current induced by 1 jaM of nociceptin averaged to be 26 ?5 pA (n = 68). The nociceptin-induced current reversed its polarity at a potential which was close to the equilibrium potential for K+ (n = 4). Nociceptin-induced currentcould be inhibited by pretreatment of BaCl2 (100 jiM; n=4; by 56 ?8%) but not by 4-aminopyridine (4-AP; 1 mM; n = 4) and Tetraethylammonium (TEA; 5 mM; n = 4). The nociceptin-induced current was not affected by tetrodotoxin (TTX; 1 uM; n = 4) and also by a non-specific opioid receptor antagonist, naloxone (1 uM; n = 4). When examined using some inhibitors with respect to ORLl receptor, the nociceptin-induced current was depressed in amplitude by nocistatin (a putative nociceptive precursor product) and also by CompB (a non-peptidyl ORLl receptor antagonist) to a different extent without a change in holding currents of SG neurons. The inhibitory rate was 18 ?4% by nocistatin (1 nM; n = 6) and 64 ?10% by CompB (1 uM; n = 7). On the other hand, a putative ORLl receptor antagonist, [PheV(CH2-NH)Gly2]nociceptin-(l-13)-NH2 (1 (iM, which is a derivative of nociceptin), by itself induced an outward current, during which the nociceptin current was suppressed in amplitude by 56 ?8% (n = 8).From the above mentioned, we conclude that nociceptin activates in SG neurons a K+ channel exhibiting a mild inwardly rectif...
|
PDF Full Text Request