Effects of intrathecal naloxone and atropine on the nociceptive suppression induced by norepinephrine and serotonin at the spinal level in rats

Involvement of spinal serotonin receptors in the regulation of intraspinal acetylcholine release

Stimulation of spinal serotonin (5-HT) receptors results in analgesia and release of acetylcholine. We investigated the involvement of 5-HT1, 5-HT2, and 5-HT3 receptor subtypes in the regulation of spinal acetylcholine release. A spinal microdialysis probe was placed dorsally at about the C5 level in anaesthetized rats. The selective serotonin reuptake inhibitor citalopram was found to increase acetylcholine release when infused via the microdialysis probe. Several doses of the 5-HT receptor agonists 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT, 5-HT1A), 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b]pyridin-5-one dihydrochloride (CP93129, 5-HT1B), alpha-methyl-5-hydroxytryptamine maleate (m5-HT, 5-HT2), 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI, 5-HT2C), and 1-(m-chlorophenyl)-biguanide (5-HT3) were subsequently infused via the microdialysis probe. Only 8-OH-DPAT, CP93129, and m5-HT increased acetylcholine release dose dependently. The 5-HT1A receptor selective antagonist (S)-N-tert-butyl-3-(4-(2-methoxyphenyl)piperazine-1-yl)-2-phenylpropanamide hydrochloride and the 5-HT2A receptor selective antagonist ketanserin tartrate inhibited the 8-OH-DPAT and the m5-HT induced acetylcholine release. The results suggest that 5-HT1A and the 5-HT2A receptors are involved in the regulation of acetylcholine release in the spinal cord.

Spinal muscarinic receptors are activated during low or high frequency TENS-induced antihyperalgesia in rats

Transcutaneous electrical nerve stimulation (TENS) is a non-pharmacological modality used clinically to relieve pain. Central involvement of serotonin and endogenous opioids are implicated in TENS-induced analgesia. Activation of spinal cholinergic receptors is antinociceptive and these receptors interact with opioid and serotonin receptors. In the current study, the possible involvement of spinal cholinergic receptors in TENS analgesia was investigated in rats. Hyperalgesia was induced by inflaming one knee joint with 3% kaolin-carrageenan and assessed by measuring paw withdrawal latency (PWL) to heat before and 4 h after injection. The non-selective nicotinic antagonist mecamylamine (50 microg), non-selective muscarinic antagonist atropine (30 microg) or one of the muscarinic subtype antagonists: pirenzepine (M1, 10 microg), methoctramine (M2, 10 microg), 4-DAMP (M3, 10 microg), or saline was administered intrathecally just prior to TENS treatment. Low or high frequency TENS was then applied to the inflamed knee and PWL was determined again. Atropine, pirenzepine and 4-DAMP significantly attenuated the antihyperalgesic effects of low and high frequency TENS while mecamylamine and methoctramine had no effects, compared to saline control. The results show that TENS-induced antihyperalgesia is mediated partially by activation of spinal muscarinic receptors but not spinal nicotinic receptors. Further, the results also indicate that spinal M1 and M3 muscarinic receptor subtypes mediate the muscarinic component of TENS antihyperalgesia.

Alpha-2 adrenoceptor mediating the facilitatory effect of norepinephrine on the glycine response in the spinal dorsal horn neuron of the rat

Effects of norepinephrine (NE) on the glycine-mediated inhibitory response were investigated in neurons acutely dissociated from the rat spinal dorsal horn, using nystatin perforated patch recording mode under voltage-clamp conditions. NE reversibly and concentration dependently facilitated Cl(-) current induced by 3 x 10(-5) M glycine. NE neither changed the reversal potential of the glycine response nor affected the affinity of glycine to its receptor. This effect could be mimicked by clonidine (10(-7) M) and blocked by yohimbine (10(-6) M), respectively. N-[2(methylamino)ethyl]-5-isoquinoline sulfonamide dihydrochloride (H-89), an inhibitor of protein kinase A, effectively mimicked the effect of NE on glycine response, whereas chelerythrine (an inhibitor of protein kinase C) failed. NE further enhanced glycine response even in the presence of chelerythrine or stearoylcarnitine chloride (another inhibitor of protein kinase C) or chelerythrine together with stearoylcarnitine chloride. The present results suggest that alpha2-adrenoceptor is involved in the potentiation of NE on glycine response in freshly isolated spinal dorsal horn neurons. Activation of alpha2-adrenoceptor down-regulates the activity of protein kinase A that results in the potentiation of the glycinergic inhibitory effects within the spinal dorsal horn.

Spinal cholinergic inhibition of the pressor response to skeletal muscle activation

The purpose of this study was to delineate the role of the cholinergic pathway within the spinal cord in the reflex cardiovascular responses to muscle activity. Based on dose-response experiments, we microdialyzed a 0.1 mM solution of the acetylcholinesterase inhibitor neostigmine into the L7 level of the dorsal horn of anesthetized cats to determine its effects on the mean arterial blood pressure (MAP) and heart rate (HR) responses to static muscle contraction or passive stretch. The peak responses to 1-min contractions and stretches were reduced from control levels after 1 h of drug administration. In four experiments, the cardiovascular responses returned to control levels after a 2-h recovery period. Perfusion of the cholinergic receptor antagonist atropine accentuated the peak MAP response to muscle contraction. By contrast, atropine administration had no effect on the peak MAP adjustment to passive muscle stretch. These data support the hypothesis that increased acetylcholine (ACh) concentrations in the spinal cord inhibit the reflex cardiovascular responses to static muscle contraction. Further, the results suggest that the spinal cholinergic system is activated by metabolic changes in skeletal muscle, but likely unaffected by mechanical muscle changes.