AMPA Receptors in the Nucleus Accumbens Core are Involved in Long Term Sensitization to Cocaine after a Single Restraint Stress

GARCÍA KELLER, C; ESPARZA, A.; MARTÍNEZ, S.; CANCELA, L.M.

Córdoba, Argentina.

Congreso; Primera reunión internacional de Ciencias Farmacéuticas; 2010

AMPA Receptors in the Nucleus Accumbens Core are Involved in Long Term Sensitization to Cocaine after a Single Restraint Stress   García Keller, C*; Esparza, A.**; Martínez, S.; Cancela, L.M#.   IFEC CONICET, Departamento de Farmacología, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Haya de la Torre esq. Medina Allende, Ciudad de Córdoba, CP: 5016.   Introduction Several evidences indicate that cocaine is able to induce behavioural and molecular sensitization in both, the dopaminergic (DA) and glutamatergic (Glu) mesocorticolimbic systems. It has been shown that a single restraint stress induces sensitization to behavioural and neurochemical effects of psychostimulants (Pacchioni et al., 2007). The aim of this work was to study whether or not AMPA receptors (AMPARs) participate in the behavioural manifestation of long-term cross-sensitization between stress and cocaine (21 days after a single stress session) and to investigate the participation of nucleus accumbens core (NAc Core) in this cross-sensitization.     Material and Methods Male Wistar rats (250-350 g) were restrained for two hours (acute­­-stress group) while control animals were left undisturbed in their cages. Twenty-one days after this single stress episode, all animals were assigned to one of the following two experiments: I) Locomotor activity in response to saline or cocaine (15 mg/kg i.p.); and locomotor activity followed by intra-NAc Core microinfusions of increasing doses of AMPA (1 nM, 10 nM and 100 nM). II) AMPA receptor expression in NAc Core after i.p. injection of saline or cocaine (15 mg/kg i.p.). The animals were decapitated 45 minutes after the injection and the NAc Core were dissected. Brain slices were incubated for one hour in ice-cold sulfo-NHS-LC-biotin. Samples were then centrifuged to pellet the insoluble fraction. Biotinylated surface proteins present in the remaining supernatant were immunoprecipitated with avidin-agarose beads for 2 h at 4°C. Beads were pelleted, and in the supernatant the surface fraction was subjected to quantitative immunoblotting for AMPARs using anti-GluR1 as a primary antibody.   Results Ours results demonstrate that the expression of sensitization can be induced with AMPA in NAcc Core twenty-one days after an acute stress episode, as compared with the remaining control groups. AMPA produced a dose-related increase in horizontal photocell counts (1). After the highest dose of AMPA (0.1 ug/ul) in NAc Core, the number of horizontal photocell counts was greater in the sensitized than in the control animals. These results are discussed in the framework of an increase in the surface cellular expression of GluR1 (the AMPA receptors subunit), in NAc Core, in stressed animals following a long period after the stress episode (21 days), independently of saline or cocaine injection. There are significant differences between stress and non-stress groups.   Conclusions In conclusion, the present behavioural and neurochemical findings reveal that AMPA receptors are critical to underlie the expression of behavioural sensitization to cocaine following a single restraint stress. These results extend and confirm our hypothesis of a common molecular mechanism underlying the drug- and stress-induced sensitization to cocaine´s behavioural effects (Pierce et al., 1996; Boudreau et al. 2005, 2007; Kourrich et al., 2007).   Acknowledgements This work was supported by grants from FONCyT, CONICET, SECyT and Agencia Córdoba Ciencia (Argentina). The authors are grateful to Estela Salde for her laboratory technical assistance.   Bibliography -Pacchioni AM, Cador, M, Bregonzio, C, Cancela LM (2007). A glutamate-dopamine interaction in the persistent enhanced response to amphetamine in nucleus accumbens core but not shell follwing a single restraint stress. Neuropsychospharmacology 32: 682-692. -Pierce, RC, Bell K, Duffy P, Kalivas PW (1996). Repeated cocaine augments excitatory amino acid transmission in the nucleus accumbens only in rats having developed behavioral sensitization, J. Neurosci. 16: 1550-1560. -Boudreau AC, Wolf ME (2005). Behavioral sensitization to cocaine is associated with increased AMPA receptor surface expression in the nucleus accumbens. J Neurosci 25: 9144–9151.