Effect of alpha 7 nicotinic receptor antagonism of morphine conditioned place preference in mice.

VL Wright1, CP Bailey1, DJ Heal2, S Wonnacott1. 1University of Bath, Bath, UK, 2Renasci, Nottiningham, UK

Addictions are complex relapsing brain disorders, and the development of addiction causes long-lasting neuronal changes that persist long after drug taking has stopped. Nicotinic acetylcholine receptors (nAChRs) are implicated in nicotine addiction but also more recently, in modulating responses to other drugs of abuse (1,2). To explore the roles of α7 nAChRs in morphine-related learning, conditioned place preference (CPP) was used as a model. The effect of methyllycaconitine (MLA), an α7 selective nAChR antagonist, on the acquisition, reconsolidation and reinstatement of morphine-CPP was examined. A previous study (2) has shown that MLA inhibits reinstatement to morphine CPP but the effects of MLA on acquisition and reconsolidation of morphine CPP have not been studied to date.

Adult male C57BL/6J mice (7-8weeks) were used in an unbiased balanced CPP protocol. Three separate experiments were performed. 1. Acquisition: animals were randomly allocated into one of two groups, receiving either MLA (4mg/kg, s.c.) or saline (10ml/kg, s.c.) 20 mins prior to morphine (10mg/kg, i.p.) or saline conditioning dose. 2. Reconsolidation: all animals were conditioned to morphine (10mg/kg, i.p.). 5 days after conditioning, animals were randomly allocated one of two groups, both received one further conditioning trial (morphine, 10mg/kg, i.p.) and immediately after received either MLA (4mg/kg, s.c.) or saline (10ml/kg), followed by a CPP trial 1 day later. 3. Reinstatement: all animals were conditioned to morphine (10mg/kg, i.p.) then CPP was extinguished by repeated saline injections. Twenty mins prior to morphine reinstatement (5mg/kg, i.p.) animals received either saline (10ml/kg s.c.) or MLA (4mg/kg, s.c.). Time spent in drug-paired environment (15 minute test session) was measured. Data were analysed using In-vivo stat with a 2-way repeated measures mixed model ANOVA and adjusted with Bonferroni’s correction for multiple comparisons.

Morphine produced a robust CPP and MLA had no effect on the acquisition (control: 532.1±17.3, MLA: 539.9±22.9 s, p=0.69 n=16/treatment) or reconsolidation (control: 548.6±32.6, MLA: 549.4±25.6 s, p=0.981, n=12/treatment) of morphine-CPP. However, while the control morphine-primed group showed significant reinstatement (571±39 s post-reinstatement vs. 468±14 s at extinction, p=0.005, n=19), the MLA-treated group did not significantly reinstate (528±37 s post-reinstatement vs. 465±12 s at extinction, p=0.125, n=20). Comparison of time spent in the drug-paired environment after saline and MLA were not significantly different.

The absence of significant reinstatement in the presence of MLA suggests that α7 nAChRs may make a contribution to controlling reinstatement to morphine-CPP, and this is consistent with the similar but greater effect reported in Balb/c mice (2). This contrasts with the clear absence of an effect by MLA on acquisition and reconsolidation. Experiments to determine the effect of activation of the alpha7 nAChR are on going, as well as biochemical experiments to investigate the mechanism.

(1) Rezayof A et al,Behav Brain Res 166:281, 2006.

(2) Feng B et al, Behav Brain Res, 220:100, 2011.