Effect of methamphetamine neurotoxicity on learning-induced Arc mRNA expression in identified striatal efferent neurons

Abstract

Methamphetamine abuse results in lasting, partial depletions of striatal dopamine and cognitive dysfunction. However, the effect of partial dopamine depletions on the expression of an effector immediate early gene, Arc (activity regulated, cytoskeletal-associated protein), known to be involved in synaptic modifications underlying learning and memory, has heretofore not been examined. Male Sprague-Dawley rats were pretreated with a neurotoxic regimen of methamphetamine or saline. Seven weeks later, rats were trained in a motor-response task on a T-maze for five days, and then underwent reversal training on day five. Rats were sacrificed 5 min after reaching criterion on the reversal task, and the brains were removed and processed using double-label fluorescent in situ hybridization for Arc and preproenkephalin (PPE) mRNA expression in the dorsomedial striatum. Rats pretreated with methamphetamine had an average (+/-SEM) 54.4+/-7.9% loss of dopamine in dorsomedial striatum. Interestingly, there was no difference in reversal trials to criterion in methamphetamine- vs. saline-pretreated rats. However, the expression of Arc mRNA in dorsomedial striatum was attenuated in methamphetamine-pretreated animals, particularly in PPE-negative neurons. Furthermore, the correlation between Arc mRNA expression in dorsomedial striatum and learning was abolished in methamphetamine-pretreated animals. These data suggest that methamphetamine-induced partial monoamine loss is associated with disrupted induction of the effector immediate early gene Arc during a behavioral task, particularly in PPE-negative (presumed striatonigral) neurons, as well as with disruption of the relation between Arc mRNA expression in dorsomedial striatum and reversal learning.

Fig. 1

Dopamine (DA) tissue content (ng/mg protein; mean ± SEM) in the dorsomedial (DM) striata of rats pretreated with saline (4 × 1 mL at 2-hr intervals, n=7) or a neurotoxic regimen of methamphetamine (METH; 4 × 10 mg/kg, sc at 2-hr intervals, n=7) seven weeks prior to sacrifice.

Fig. 2

Number of trials required on a motor response learning task on a T-maze to reach criterion (90% correct responses over 10 trials) on the first day of acquisition training and on the reversal training on the final day of testing. Saline-pretreated rats (SALINE) received 4 injections of saline (1 mL) and methamphetamine (METH)-pretreated rats received 4 injections of METH (10 mg/kg, s.c.) at 2-hr intervals six weeks prior to the start of the motor response training. Data are mean ± SEM, n=7 / group.

Fig. 3

Confocal images showing Arc mRNA (cy-3/red), preproenkephalin mRNA (PPE, cy-5/blue) and nuclear (SYTOX Green) staining in the dorsomedial striatum of a saline- (4 × 1 mL/kg, s.c. at 2-hr intervals; A) and a methamphetamine- (METH; 4 × 10 mg/kg, s.c. at 2-hr intervals; B) pretreated rat. The average number (± SEM, n= 4 for caged controls and n=7 for both saline- and METH-pretreated groups) of PPE− and PPE+ neurons expressing Arc mRNA in the cytoplasm in a 0.7 mm × 0.7 mm field of the dorsomedial striatum (C). * Significantly different from respective phenotype in caged controls, p<0.05. Scale bar, 10 µm.

Fig. 4

Correlations between the numbers of preproenkephalin-negative (PPE−; A, B; open symbols) and preproenkephalin-positive (PPE+; C, D; closed symbols) neurons expressing Arc mRNA (Arc+) in a 0.7 mm × 0.7 mm field of dorsomedial striatum and the number of trials to reach 9/10 correct responses during reversal learning in a motor response task on a T-maze. Values are from rats pretreated with saline (4 × 1 mL/kg; A, C, squares) or a neurotoxic regimen of methamphetamine (METH; 4 × 10 mg/kg, s.c. at 2-hr intervals; B, D, circles) approximately seven weeks prior to testing. * Significant correlation.