Tolerance to the memory disruptive effects of cannabinoids involves adaptation by hippocampal neurons

The effects of chronic exposure to cannabinoids on short-term memory in rats were assessed during repeated daily injections of an initially debilitating dose (3.75 mg/kg) of the potent CB1 cannabinoid receptor ligand, WIN 55,212-2. Delayed nonmatch to sample (DNMS) performance was assessed over a 35-day exposure period in which performance was initially disrupted during the first 21 days of exposure but recovered by day 30 and was stable at pre-drug levels for 5 days thereafter. Withdrawal was precipitated by injections of the CB1 receptor antagonist SR141716A and transiently reduced performance for 2 days but was restabilized to pre-drug levels within 3-4 days. Concomitant recording from identified CA1 and CA3 hippocampal neurons demonstrated a marked correspondence in the time course of suppression of peak firing in the sample and delay phases of the task to the drug-induced performance deficits over the same days of exposure. Hippocampal encoding of task-relevant events and performance levels "tracked" each other on a daily basis throughout the chronic cannabinoid treatment and withdrawal regimen. However, hippocampal neuronal activity in the nonmatch phase of the task was unaffected by the chronic cannabinoid treatment or withdrawal, suggesting that only a select population of hippocampal neurons and synapses are involved in cannabinoid-sensitive short-term memory processes.