Nicotine differentially activates inhibitory and excitatory neurons in the dorsal spinal cord

The effects of chemical or surgical deafferentation on [3H]-acetylcholine release from rat spinal cord

Cholinergic modulation of nociceptive transmission through both nicotinic and muscarinic receptors in the spinal cord represents an important mechanism in pain signaling. However, what neuronal elements release acetylcholine and how release might change in response to deafferentation are unclear. The present studies demonstrated Ca++- and K+-dependent release of [3H]-acetylcholine from slices of regional areas of rat spinal cord. That [3H]-acetylcholine was synthesized from [3H]-choline was demonstrated by the lack of [3H]-acetylcholine release following incubation with either the choline uptake inhibitor hemicholinium or the choline acetyltransferase inhibitor bromoacetylcholine. Rats treated neonatally with capsaicin or with spinal nerve ligation as adults showed a significantly decreased K+-stimulated release of [3H]-acetylcholine from dorsal horn but not ventral horn lumbar spinal cord slices. In rats subjected to dorsal rhizotomy, while basal release from lumbar dorsal spinal cord slices was reduced, K+-stimulated [3H]-acetylcholine release, while decreased, was not significantly different compared with controls. The data presented here show that there are regional differences in the release of acetylcholine from spinal cord and that this release can be modulated by chemical or surgical deafferentation. These results also indicate that the source of acetylcholine in the dorsal cord originates mainly from resident somata and their collaterals, interneurons and/or descending terminals, with only very minor contributions coming from primary afferents. The present data help to further elucidate the role of acetylcholine in spinal signaling, particularly with respect to the effects of nerve injury and nociceptive neurotransmission.

Nicotinic modulation of GABAergic synaptic transmission in the spinal cord dorsal horn

While the mechanisms underlying nicotinic acetylcholine receptor (nAChR)-mediated analgesia remain unresolved, one process that is almost certainly involved is the recently-described nicotinic enhancement of inhibitory synaptic transmission in the spinal cord dorsal horn. Despite these observations, the prototypical nicotinic analgesic (epibatidine) has not yet been shown to modulate inhibitory transmission in the spinal cord. Furthermore, while nAChRs have been implicated in short-term modulation, no studies have investigated the role of nAChRs in the modulation of long-term synaptic plasticity of inhibitory transmission in dorsal horn. Whole-cell patch clamp recordings from dorsal horn neurons of neonatal rat spinal cord slices were therefore conducted to investigate the short- and long-term effects of nicotinic agonists on GABAergic transmission. GABAergic synaptic transmission was enhanced in 86% of neurons during applications of 1 microM nicotine (mean increased spontaneous GABAergic inhibitory postsynaptic current (sIPSC) frequency was approximately 500% of baseline). Epibatidine (100 nM) induced an increase to an average of approximately 3000% of baseline, and this effect was concentration dependent (EC50=43 nM). Nicotinic enhancement was inhibited by mecamylamine and DHbetaE, suggesting an important role for non-alpha7 nAChRs. Tetrodotoxin (TTX) did not alter the prevalence or magnitude of the effect of nicotine, but the responses had a shorter duration. Nicotine did not alter evoked GABAergic IPSC amplitude, yet the long-term depression (LTD) induced by strong stimulation of inhibitory inputs was reduced when paired with nicotine. These results provide support for a mechanism of nicotinic analgesia dependent on both short and long-term modulation of GABAergic synaptic transmission in the spinal cord dorsal horn.

The Role of Adrenergic and Cholinergic Transmission in Volatile Anesthetic-Induced Pain Enhancement

Volatile anesthetic drugs have a biphasic effect on pain transmission. At very small concentrations they enhance pain sensitivity whereas at larger subanesthetic concentrations they have an analgesic effect. Previous work has suggested that nicotinic inhibition could mediate the pronociceptive action of isoflurane. Furthermore, activation of nicotinic receptors facilitates the release of norepinephrine in the spinal cord. We hypothesize that nicotinic modulation of norepinephrine release in the spinal cord mediates isoflurane's pronociceptive action. We used hindpaw withdrawal latency as a measure of pain sensitivity after inhibition of adrenergic activity or treatment with nicotine in mice. Isoflurane's effect on pain is separable by concentration. The 50% effective concentration for pain enhancement is 0.16% isoflurane whereas the 50% effective concentration for the antinociceptive action of isoflurane is 0.8%. Depletion of systemic norepinephrine with the neurotoxin DSP-4 caused a reduction in baseline withdrawal latencies and prevented isoflurane pronociception. Baseline latency was also reduced by intrathecal yohimbine. After treatment with yohimbine, isoflurane had no additional pronociceptive effect. Nicotine administered through intracerebroventricular injection increased baseline latency but did not prevent isoflurane pronociception. Conversely, intrathecal applications of nicotine caused a slight reduction in baseline latency and prevented isoflurane's pronociceptive effect. We conclude that spinal noradrenergic transmission seems to be necessary for isoflurane pronociception to occur. Isoflurane may act by inhibiting tonically active nicotinic receptors that modulate the release of norepinephrine in the spinal cord.

Synaptic modulation in pain pathways

All higher organisms possess a sensory system that allows them to detect potentially tissue-damaging (or noxious) stimuli. The proper functioning of this system is essential to protect their bodies from tissue damage. However, under pathological conditions after severe tissue injury and in inflammatory or neuropathic diseases, this system can become sensitized, and pain can then turn into a disease. Such exaggerated pain sensation (or hyperalgesia) can arise at different levels of integration. It can originate from an increased responsiveness of primary nociceptors, specialized nerve cells, which sense noxious stimuli, or from changes in the central processing of nociceptive input. Like other sensory input, nociceptive signals are relayed in the central nervous system by neurons, which communicate with each other mainly through chemical synapses. Changes in the excitability of these neurons or in the strength of their synaptic coupling provide the cellular basis for many forms of pathological pain. This review focuses on the synaptic processing of pain-related signals in the spinal cord dorsal horn, the first site of synaptic integration in the pain pathway. Particular emphasis is paid to synaptic processes underlying the generation of pathological pain evoked by inflammation or neuropathic diseases.