Age-dependent effects of dopamine receptor inactivation on cocaine-induced behaviors in male rats: Evidence of dorsal striatal D2 receptor supersensitivity

Abstract

N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), which irreversibly inactivates dopamine (DA) receptors, causes pronounced age-dependent behavioral effects in rats. For example, EEDQ either augments or does not affect the DA agonist-induced locomotor activity of preweanling rats while attenuating the locomotion of adolescent and adult rats. The twofold purpose of this study was to determine whether EEDQ would: (a) potentiate or attenuate the cocaine-induced locomotor activity of preweanling, adolescent, and adult rats; and (b) alter the sensitivity of surviving D2 receptors. Rats were treated with vehicle or EEDQ (2.5 or 7.5 mg/kg) on postnatal day (PD) 17, PD 39, and PD 84. In the behavioral experiments, saline- or cocaine-induced locomotion was assessed 24 hr later. In the biochemical experiments, dorsal striatal samples were taken 24 hr after vehicle or EEDQ treatment and later assayed for NPA-stimulated GTPγS receptor binding, G protein-coupled receptor kinase 6 (GRK6), and β-arrestin-2 (ARRB2). GTPγS binding is a direct measure of ligand-induced G protein activation, while GRK6 and ARRB2 modulate the internalization and desensitization of D2 receptors. Results showed that EEDQ potentiated the locomotor activity of preweanling rats, while attenuating the locomotion of older rats. NPA-stimulated GTPγS binding was elevated in EEDQ-treated preweanling rats, relative to adults, indicating enhanced functional coupling between the G protein and receptor. EEDQ also reduced ARRB2 levels in all age groups, which is indicative of increased D2 receptor sensitivity. In sum, the present results support the hypothesis that D2 receptor supersensitivity is a critical factor mediating the locomotor potentiating effects of EEDQ in cocaine-treated preweanling rats.

Keywords: G protein-coupled receptor kinase 6 (GRK6); GTPγS receptor binding; N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ); ontogeny; β-arrestin-2 (ARRB2).

FIGURE 1

Mean (±SD) distance traveled scores of preweanling, adolescent, and adult rats (n = 8 rats per group) injected with saline or cocaine (15 mg/kg, i.p.) immediately before testing. On the pretreatment day, which occurred 24 h earlier, rats had been injected with vehicle or EEDQ (2.5 or 7.5 mg/kg, i.p.). ‘a’ Significant difference between same-age rats receiving vehicle and 7.5 mg/kg EEDQ. ‘b’ Significant difference between same-age rats receiving vehicle and 2.5 mg/kg EEDQ. ‘c’ Significant difference between older (adolescents and adults) and preweanling rats in the control groups (i.e., Vehicle-Saline or Vehicle-Cocaine groups). ‘d’ Significant difference between preweanling and adolescent rats in the 7.5 mg/kg EEDQ-Cocaine group. ‘e’ Significant difference between preweanling and adult rats in the 7.5 mg/kg EEDQ-Cocaine group.

FIGURE 2

Mean (±SD) distance traveled scores converted to percent of vehicle controls at each age (these are the same rats as described in Figure 1). ‘a’ Significantly different from same-age rats in their vehicle control group (represented by the dashed line). ‘b’ Significantly different from same-age rats in their vehicle control group (represented by the dashed line). ‘c’ Significantly different from same-age rats in the 2.5 mg/kg EEDQ-Vehicle group.

FIGURE 3

Mean (±SD) distance traveled scores of preweanling rats (n = 8 rats per group) injected with saline or cocaine (5 or 10 mg/kg, i.p.) immediately before testing. On the pretreatment day, which occurred 24 h earlier, rats had been injected with vehicle or EEDQ (7.5 mg/kg, i.p.). ‘a’ Significant difference between the Vehicle-Saline group and rats receiving cocaine alone (i.e., the Vehicle-5 mg/kg Cocaine and Vehicle-10 mg/kg Cocaine groups). ‘b’ Significant difference between the vehicle- and EEDQ-treated rats tested with 5 mg/kg cocaine. ‘c’ Significant difference between the vehicle- and EEDQ-treated rats tested with 10 mg/kg cocaine.

FIGURE 4

Mean (±SD) distance traveled scores of preweanling rats converted to percent of vehicle controls (these are the same rats as described in Figure 3). ‘a’ Significantly different from the Vehicle-Saline group (represented by the dashed line). ‘b’ Significantly different from the Vehicle-Cocaine (5 mg/kg) group. ‘c’ Significantly different from the Vehicle-Cocaine (10 mg/kg) group.

FIGURE 5

Mean efficacy (E_max) of NPA-stimulated [³⁵S]GTPγS specific binding in the dorsal striatum of preweanling, adolescent, and adult rats (n = 7 per group). On the pretreatment day, which occurred 24 h earlier, rats had been injected with vehicle or EEDQ (2.5 or 7.5 mg/kg, i.p.). E_max is expressed as the percentage of stimulation above basal binding. “V+E+E” refers to mean E_max values collapsed across the vehicle and two EEDQ groups. ‘a’ Significantly different from adult rats. ‘b’ Significantly different from vehicle-treated preweanling rats. ‘c’ Significantly different from EEDQ-treated adult rats.

FIGURE 6

Mean β-arrestin levels (pg/mg protein) in the dorsal striatum of preweanling, adolescent, and adult rats (n = 10–12 per group). On the pretreatment day, which occurred 24 h earlier, rats had been injected with vehicle or EEDQ (2.5 or 7.5 mg/kg, i.p.). “V+E+E” refers to mean β-arrestin values collapsed across the vehicle and two EEDQ groups. ‘a’ Significantly different from adult rats. ‘b’ Significantly different from adolescent rats.

FIGURE 7

Mean GRK6 levels (pg/mg protein) in the dorsal striatum of preweanling, adolescent, and adult rats (n = 10–12 per group). On the pretreatment day, which occurred 24 h earlier, rats had been injected with vehicle or EEDQ (2.5 or 7.5 mg/kg, i.p.). “V+E+E” refers to mean GRK6 values collapsed across the vehicle and two EEDQ groups. ‘a’ Significantly different from adult rats. ‘b’ Significantly different from adolescent rats.