Rx 821002, an Alpha2 Adrenoceptor Antagonist, Enhances Acetylcholine Efflux in the Nucleus Accumbens of Freely Moving Rats Following Intra-Accumbal and Systemic Administration

Background The nucleus accumbens (NAc), a major terminal area of the mesolimbic dopaminergic system, contains cholinergic neurons (Dautan et al., 2014). Pharmaco-behavioral studies have shown that selective depletion of acetylcholine (ACh) in NAc induces cognitive deficits in rats (Laplante et al., 2013). The NAc also receives noradrenergic projections and expresses alpha adrenoceptor (-R) subtypes. We have previously reported that alpha1- but not alpha2-Rs in NAc regulate dopamine (DA) and noradrenaline (NA) release from accumbal nerve terminals (Saigusa et al., 2012). Additionally, intra-accumbal infusion of an alpha2-R agonist and antagonist decrease and increase accumbal ACh efflux in rats, respectively. These results suggest that alpha2-Rs in NAc may play inhibitory roles in regulating cholinergic but not dopaminergic or noradrenergic neural activity. Aims & Objectives Since alpha2-Rs are widely distributed in the brain, the effects of selective stimulation and blockade of these receptors on accumbal cholinergic neural activity remain unclear. Therefore, we analyzed the effects of systemic administration of a selective alpha2-R agonist (UK 14,304) and antagonist (RX 821002) on accumbal ACh efflux in freely moving rats using in vivo microdialysis. We also examined the effects of intra-accumbal infusion of these alpha2-R ligands on accumbal DA efflux. To detect extracellular ACh levels in dialysates collected via brain microdialysis, a small amount of the cholinesterase inhibitor physostigmine was added to the perfusate to reduce ACh degradation, which might alter the effects of alpha2-R ligands on accumbal DA efflux. Method Male Sprague-Dawley rats were used. ACh and DA levels in accumbal perfusates, collected every 15 or 20 minutes, were determined by HPLC-ECD. Doses of compounds infused intra-accumbally through the dialysis probe represent the total amounts (in mol) over a 30-minute infusion period. The modified Ringer's solution used as perfusate included a low concentration of physostigmine (50 nM). Results Intra-accumbal infusion of UK 14,304 (300 pmol) and RX 821002 (6 nmol) reduced and increased accumbal ACh levels, respectively, but neither altered accumbal DA levels. Co-infusion experiments using low doses of RX 821002 (0.6 pmol) and UK 14,304 (30 pmol), which did not alter basal ACh efflux, showed that the UK 14,304 (300 pmol)-induced decrease and RX 821002 (6 nmol)-induced increase in ACh levels were counteracted by RX 821002 and UK 14,304, respectively. Intraperitoneal injection of UK 14,304 (2.5 mg/kg) failed to alter accumbal ACh efflux, while RX 821002 (1.0 mg/kg) increased ACh efflux. The RX 821002-induced increase in ACh levels following intraperitoneal injection was suppressed by intra-accumbal infusion of UK 14,304 (30 pmol). Discussion & Conclusions The present study demonstrates that systemic administration of RX 821002 enhances cholinergic neural activity in NAc. Increase in accumbal ACh levels observed after systemic RX 821002 administration appears to be mediated by accumbal alpha2-R blockade. Given that depletion of accumbal ACh has been reported to impair cognitive functions, including memory, this study indicates the need to investigate whether systemic administration of RX 821002 can at least improve experimentally induced cognitive impairment in experimental animals.