Effect of environmental enrichment on escalation of cocaine self-administration in rats

Abstract

Background: Previous studies found that environmental enrichment protects against the initiation of stimulant self-administration in rats, but it is unclear if enrichment also protects against the escalation of stimulant use with long-term exposure.

Objective: The current study examined the effects of environmental enrichment on escalation of cocaine self-administration using an extended access procedure.

Methods: Rats were raised from 21 days in an enriched condition (EC) with social cohorts and novel objects, a social condition with only social cohorts (SC), a novelty condition (NC) with novel objects in isolated cages, or an isolated condition (IC) without social cohorts or novel objects. In young adulthood, EC, SC, NC, and IC rats were separated into short access (ShA) or long access (LgA) groups that received either 1 or 6 h, respectively, of daily cocaine self-administration (0.1 mg/kg/infusion) for 14 days. In a second experiment, EC and IC rats were used to assess differences in acquisition and escalation of cocaine self-administration at a 0.5 mg/kg/infusion unit dose.

Results: With ShA sessions, EC rats acquired cocaine self-administration at a slower rate than IC rats at both unit doses; however, with extended training, both groups eventually reached similar rates. At the 0.1 mg/kg/infusion dose, only NC and IC rats escalated in amount of intake when switched to the LgA sessions. At the 0.5 mg/kg/infusion dose, rates of cocaine self-administration escalated in LgA groups over 14 days regardless of EC or IC rearing condition; however, EC rats escalated at a faster rate, eventually reaching the same level of intake observed in IC rats.

Conclusions: Although environmental enrichment protects against escalation of a low unit dose of cocaine, it may not protect against escalation with a higher unit dose. In addition, at a lower unit dose, this protective mechanism appears to be due to the presence of social cohorts rather than novel objects.

Figure 1

Number of responses (mean ± SEM) on active and inactive levers for (a) EC and IC, (b) SC, and (c) NC rats during the 7 sessions of acquisition of cocaine self-administration at the 0.1 mg/kg/infusion unit dose (* p < .001 compared to inactive responses).

Figure 2

Total number of cocaine infusions (mean ± SEM) during the extended access phase at the 0.1 mg/kg/infusion unit dose in (a) EC and IC ShA and LgA, (b) SC ShA and LgA, and (c) NC ShA and LgA groups. (* p < .001 compared to session 8; # p < .001 as linear trend across all sessions).

Figure 3

Number of cocaine infusions (mean ± SEM) on first hr sessions 8 and 21 (the first and last session of extended access phase) in (a) EC and IC LgA, (b) SC LgA, and (c) NC LgA groups at the 0.1 mg/kg/infusion unit dose across 5-min time intervals. (* p < .001 compared to session 8).

Figure 4

Number of responses (mean ± SEM) on active and inactive levers for (a) EC and (b) IC rats during the 7 sessions of acquisition of cocaine self-administration at the 0.5 mg/kg/infusion unit dose (* p < .001 compared to IC inactive responses).

Figure 5

Total number of cocaine infusions (mean ± SEM) in (a) EC and (b) IC groups during the extended access phase at the 0.5 mg/kg/infusion unit dose (* p < .001 compared to session 8; # p < .001 as linear trend across all sessions).

Figure 6

Number of cocaine infusions (mean ± SEM) on first hr of sessions 8 and 21 (first and last session of extended access phase) in (a) EC and (b) IC LgA groups at the 0.5 mg/kg/infusion unit dose across 5-min time intervals (* p < .001 compared to session 8).