Significance: Although early studies suggested that cannabinoid CB₂receptors (CB₂Rs) are absent in the brain, this view has been challenged by recent findings of significant brain CB₂R involvement in several dopamine (DA)-related CNS disorders. The cellular mechanisms underlying these actions are unclear, however. Using multiple approaches, we found that CB₂R genes and receptors are expressed in midbrain DA neurons, and that activation of CB₂Rs inhibits DA neuronal firing and i.v. cocaine self-administration. These findings not only challenge the long-held view that brain CB₂Rs are not expressed in neurons, but also suggest that neuronal CB₂Rs modulate DA neuronal activity and DA-regulated behavior. Thus, brain CB₂Rs may constitute a new therapeutic target in medication development for treatment of a number of CNS disorders.

Nicotinic acetylcholine receptors (nAChRs) containing α7 subunits are thought to assemble as homomers. α7-nAChR function has been implicated in learning and memory, and alterations of α7-nAChR have been found in patients with Alzheimer's disease (AD). Here we report findings consistent with a novel, naturally occurring nAChR subtype in rodent, basal forebrain cholinergic neurons. In these cells, α7 subunits are coexpressed, colocalize, and coassemble with β2 subunit(s). Compared with homomeric α7-nAChRs from ventral tegmental area neurons, functional, presumably heteromeric α7β2-nAChRs on cholinergic neurons freshly dissociated from medial septum/diagonal band (MS/DB) exhibit relatively slow kinetics of whole-cell current responses to nicotinic agonists and are more sensitive to the β2 subunit-containing nAChR-selective antagonist, dihydro-β-erythroidine (DHβE). Interestingly, presumed, heteromeric α7β2-nAChRs are highly sensitive to functional inhibition by pathologically relevant concentrations of oligomeric, but not monomeric or fibrillar, forms of amyloid β_1–42(Aβ_1–42). Slow whole-cell current kinetics, sensitivity to DHβE, and specific antagonism by oligomeric Aβ_1–42also are characteristics of heteromeric α7β2-nAChRs, but not of homomeric α7-nAChRs, heterologously expressed inXenopusoocytes. Moreover, choline-induced currents have faster kinetics and less sensitivity to Aβ when elicited from MS/DB neurons derived from nAChR β2 subunit knock-out mice rather than from wild-type mice. The presence of novel, functional, heteromeric α7β2-nAChRs on basal forebrain cholinergic neurons and their high sensitivity to blockade by low concentrations of oligomeric Aβ_1–42suggests possible mechanisms for deficits in cholinergic signaling that could occur early in the etiopathogenesis of AD and might be targeted by disease therapies.

Background: Although evidence suggests that the prevalence of Parkinson’s disease (PD) is lower in smokers than in non-smokers, the mechanisms of nicotine-induced neuroprotection remain unclear. Stimulation of the α7 nicotinic acetylcholine receptor (α7-nAChR) seems to be a crucial mechanism underlying the anti-inflammatory potential of cholinergic agonists in immune cells, including astrocytes, and inhibition of astrocyte activation has been proposed as a novel strategy for the treatment of neurodegenerative disorders such as PD. The objective of the present study was to determine whether nicotine-induced neuroprotection in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model occurs via α7-nAChR-mediated inhibition of astrocytes. Methods: Both in vivo (MPTP) and in vitro (1-methyl-4-phenylpyridinium ion (MPP⁺) and lipopolysaccharide (LPS)) models of PD were used to investigate the role(s) of and possible mechanism(s) by which α7-nAChRs protect against dopaminergic neuron loss. Multiple experimental approaches, including behavioral tests, immunochemistry, and stereology experiments, astrocyte cell cultures, reverse transcriptase PCR, laser scanning confocal microscopy, tumor necrosis factor (TNF)-α assays, and western blotting, were used to elucidate the mechanisms of the α7-nAChR-mediated neuroprotection. Results: Systemic administration of nicotine alleviated MPTP-induced behavioral symptoms, improved motor coordination, and protected against dopaminergic neuron loss and the activation of astrocytes and microglia in the substantia nigra. The protective effects of nicotine were abolished by administration of the α7-nAChR-selective antagonist methyllycaconitine (MLA). In primary cultured mouse astrocytes, pretreatment with nicotine suppressed MPP⁺-induced or LPS-induced astrocyte activation, as evidenced by both decreased production of TNF-α and inhibition of extracellular regulated kinase1/2 (Erk1/2) and p38 activation in astrocytes, and these effects were also reversed by MLA. Conclusion: Taken together, our results suggest that α7-nAChR-mediated inhibition of astrocyte activation is an important mechanism underlying the protective effects of nicotine.

Systemic exposure to nicotine induces glutamatergic synaptic plasticity on dopamine (DA) neurons in the ventral tegmental area (VTA), but mechanisms are largely unknown. Here, we report that single, systemic exposure in rats to nicotine (0.17 mg/kg free base) increases the ratio of DA neuronal currents mediated by AMPA relative to NMDA receptors (AMPA/NMDA ratio) assessed 24 h later, based on slice-patch recording. The AMPA/NMDA ratio increase is evident within 1 h and lasts for at least 72 h after nicotine exposure (and up to 8 d after repeated nicotine administration). This effect cannot be prevented by systemic injection of either α7-nAChR (nicotinic ACh receptor)-selective [methyllycaconitine (MLA)] or β2*-nAChR-selective [mecamylamine (MEC)] antagonists but is prevented by coinjection of MLA and MEC. In either nAChR α7 or β2 subunit knock-out mice, systemic exposure to nicotine still increases the AMPA/NMDA ratio. Preinjection in rats of a NMDA receptor antagonist MK-801((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate), but neither DA receptor antagonists [SCH-23390 (R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) plus haloperidol] nor a calcineurin inhibitor (cyclosporine), prevents the nicotine-induced increase in AMPA/NMDA ratio. After systemic exposure to nicotine, glutamatergic (but not GABAergic) transmission onto rat VTA DA neuronal inputs is enhanced. Correspondingly, DA neuronal firing measured 24 h after nicotine exposure using extracellular single-unit recordingin vivois significantly faster, and there is conversion of silent to active DA neurons. Collectively, these findings demonstrate that systemic nicotine acting via either α7- or β2*-nAChRs increases presynaptic and postsynaptic glutamatergic function, and consequently initiates glutamatergic synaptic plasticity, which may be an important, early neuronal adaptation in nicotine reward and reinforcement.