Short- and long-term enhancement of excitatory transmission in the spinal cord dorsal horn by nicotinic acetylcholine receptors

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 alpha7 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 alpha7 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 alpha7 nAChRs and N-methyl-D-aspartate glutamate receptors. Our findings demonstrate that alpha7 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.