Spinal administration of nicotinic agonists can produce both hyperalgesic and analgesic effects in vivo. The cellular mechanisms underlying these behavioral phenomena are not understood. As a possible explanation for nicotinic hyperalgesia, we tested whether nicotinic acetylcholine receptors (nAChRs) could enhance excitatory transmission onto spinal cord dorsal horn neurons. Whole-cell patch-clamp recordings were performed in neonatal rat spinal cord slices. Activation of nAChRs enhanced glutamatergic synaptic transmission in 59% of dorsal horn neurons tested, and this effect was blocked by methyllycaconitine (10 nM), suggesting a key role for ␣7 nAChRs. Inhibition of acetylcholinesterase with methamidophos also enhanced transmission, demonstrating a similar effect of endogenous acetylcholine. nAChR activation also enhanced transmission by dorsal root entry zone stimulation, suggesting that ␣7 nAChRs on the central terminals of DRG afferents mediate this effect. Paired pre-and postsynaptic stimulation induced long-term potentiation of excitatory inputs to some of the dorsal horn neurons. Long-term potentiation induction was much more prevalent when nicotine was applied during stimulation. This effect also depended on both ␣7 nAChRs and N-methyl-D-aspartate glutamate receptors. Our findings demonstrate that ␣7 nAChRs can contribute to both short-and long-term enhancement of glutamatergic synaptic transmission in the spinal cord dorsal horn and provide a possible mechanism for nicotinic hyperalgesia.E ffective pain management is a major challenge for modern healthcare. Most pain medications fall under two classes of compounds, opioids and nonsteroidal antiinflammatory drugs. Recently, structural analogs of nicotine have also demonstrated strong analgesic potency (1, 2). Although these agents may provide alternate pain therapies, a number of issues remain to be resolved, such as addictive profiles and side effects, including paradoxical hyperalgesia. Considerable research has focused on the analgesic effects of nicotine and its analogs (2-7), yet the mechanisms underlying nicotinic hyperalgesia are not well understood.Hyperalgesia and͞or irritation have been observed after central injections of nicotinic agonists (8-10), systemic injections of subanalgesic doses (11), and intrathecal applications (12,13). This intrathecal effect is associated with significant increases in excitatory transmitter release (14), and both nicotinic and glutamate receptor antagonists can decrease this hyperalgesia (15, 16). Although dorsal horn neurons are excited by nicotine (17), it is not known which subtypes of nicotinic acetylcholine receptors (nAChRs) are expressed by dorsal horn neurons or on presynaptic afferent terminals. For example, nicotinic enhancement of synaptic activity has been noted in the spinal cord dorsal horn (SCDH) in mice that lack the ␣4 nAChR subunit, but the nAChR subtype(s) that mediate this effect is unknown (5).Neuronal nAChRs are pentameric ligand-gated ion channels, and molecular cloning has identified nin...