Dose-related neuroprotective effects of chronic nicotine in 6-hydroxydopamine treated rats, and loss of neuroprotection in alpha4 nicotinic receptor subunit knockout mice.

The present study examined the effect of a range of doses of chronic nicotine (0.75, 1.5, 3.0 and 30.0 mg kg(-1) day(-1), s.c., 14 days) upon striatal dopaminergic nerve terminal survival following 6-hydroxydopamine (6-OHDA; 10 microg intrastriatal unilaterally) in rats; and the effects of acute nicotine (1 mg kg(-1), s.c.) pretreatment upon striatal neurodegeneration induced by methamphetamine (5 mg kg(-1), i.p., three doses at 2 h intervals) in wild-type and alpha4 nicotinic receptor (nAChR) subunit knockout mice. In both models of Parkinsonian-like damage, loss of striatal dopaminergic nerve terminals was assessed by [(3)H]-mazindol autoradiography. In rats, chronic nicotine infusion delivered by osmotic minipump implanted subcutaneously 7 days prior to intrastriatal 6-OHDA injection produced significant and dose-related protection against 6-OHDA-induced neurodegeneration. Low (0.75 and 1.5 mg kg(-1) day(-1)) but not high (3.0 and 30.0 mg kg(-1) day(-1)) nicotine doses significantly inhibited 6-OHDA-induced degeneration. In wild-type mice, acute nicotine treatment produced significant inhibition of methamphetamine-induced neurodegeneration. In alpha4 nAChR subunit knockout mice, acute nicotine treatment failed to inhibit methamphetamine-induced neurodegeneration. Nicotine is capable of protecting dopaminergic neurons against Parkinsonian-like neurodegeneration in vivo. In rats, this neuroprotective effect is critically dependent upon nicotine dose and is consistent with the activation of nAChRs, as high, desensitizing doses of nicotine fail to be neuroprotective. Further, neuroprotection is absent in alpha4 nAChR subunit knockout mice. The current results therefore suggest that activation of alpha4 subunit containing nAChRs constitutes a major component of the neuroprotective effect of nicotine upon Parkinsonian-like damage in vivo.