Tolerance to cocaine in brain stimulation reward following continuous cocaine infusions

Abstract

This study examined tolerance to cocaine's threshold-lowering effect in brain stimulation reward (BSR) following continuous cocaine infusions and secondly, used the nitric oxide synthase inhibitor Nω-nitro-L-arginine methyl ester (L-NAME) to determine NO's involvement in the development of cocaine tolerance. Animals were continuously infused with saline or cocaine (30mg/kg per day) via osmotic minipump for 14days and injected daily with saline or L-NAME (30mg/kg, i.p.) following BSR testing. Saline-treated animals continuously infused with saline showed stable BSR thresholds across the 14-day infusion period. Saline-treated animals continuously infused with cocaine showed markedly lowered BSR thresholds on Day 1 followed by a progressive increase in BSR thresholds across the infusion period – indicating the development of tolerance. L-NAME-treated animals continuously infused with cocaine showed stimulation thresholds that were not significantly different from saline-treated animals continuously infused with cocaine.A cocaine challenge injection (10mg/kg, i.p.) administered 3 and again at 10days following minipump removal revealed that saline-treated animals continuously infused with saline showed lowered BSR thresholds. Saline-treated animals continuously infused with cocaine displayed lowered BSR thresholds that were not significantly different from saline-infused animals. L-NAME treated animals continuously infused with cocaine showed higher BSR thresholds to a challenge 3days following pump removal. However, stimulation thresholds for this group failed to reach statistical significance on both days (i.e., Days 3 and 10) following pump removal.Results showed that animals continuously infused with cocaine develop robust tolerance to cocaine's threshold-lowering effect during the 14-day infusion period. Tolerance to cocaine's threshold-lowering effect was short-lived and dissipated soon after minipump removal. L-NAME treatment failed to significantly alter the development of tolerance to cocaine's threshold-lowering suggesting that NO does not have a primary role in the development of cocaine tolerance.