Behavioural neuroscienceBlockade of mu-opioid receptors in the medial thalamus inhibits acquisition, but not expression, of morphine-induced conditioned place preference
Section snippets
Subjects
Male Sprague–Dawley rats (Charles River, Wilmington, MA, USA) weighing 225–250 g were housed in groups of two in transparent plastic cages with food and water ad libitum. Male rats were used in order to avoid the influence of estrogen on the rewarding effects of morphine (Carroll et al., 2004). Although a previous study found gender differences in morphine-induced CPP (Cicero et al., 2000), the differences did not appear at the doses that were used in the present study. The colony was
Results
Analysis of preconditioning data showed no significant difference in the percentage of time spent (mean±S.E.M.%) in the two distinct conditioning chambers across all animals (gray-wall chamber 48.75±0.91%, striped-wall chamber 51.25±0.91%, P=0.18, t=1.37, df=52), indicating no preconditioning bias for either specific chamber in the studied population of rats. Moreover, no significant differences in preference for either chamber were observed in experiment 1 (gray-wall chamber 48.62±1%,
Discussion
The major finding of this study is that administration of the mu-opioid receptor antagonist β-FNA into the medial thalamus prior to each morphine conditioning session completely blocked the acquisition of morphine-induced CPP (Fig. 1), whereas it had no effect when administered after morphine conditioning but before testing for place preference (Fig. 2). Our results indicate that mu-opioid receptors in the medial thalamus are required for the acquisition, but not expression, of morphine-induced
Conclusion
In summary, the current results show that inactivation of mu-opioid receptors in the medial thalamus blocks the acquisition, but not the expression, of morphine-induced CPP. Consistent with previous studies showing the involvement of the medial thalamus in reinforcement and reward (Gaffan and Murray 1990, Kawagoe et al 2007, McAlonan et al 1993, Mitchell and Dalrymple-Alford 2005, Oyoshi et al 1996), the present studies suggest that mu-opioid receptors in the medial thalamus contribute to the
Acknowledgments
This work was supported by HEF [2001-2007]-06 to R.E.H. and B.K.T. and by the VA and ONR (J.E.Z.).
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