Effect of environmental enrichment on escalation of cocaine self-administration in rats.
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.
Center for Drug Abuse Research Translation (CDART), University of Kentucky, 741 S. Limestone, Lexington, KY 40536-0509, USA.
Cassandra D
CD
Gipson
Joshua S
JS
Beckmann
Shady
S
El-Maraghi
Julie A
JA
Marusich
Michael T
MT
Bardo
D000704:Analysis of Variance; D000818:Animals; D016739:Behavior, Addictive; D001522:Behavior, Animal / Q000187:drug effects*; D000697:Central Nervous System Stimulants / Q000008:administration & dosage*; D003042:Cocaine / Q000008:administration & dosage*; D019970:Cocaine-Related Disorders / Q000517:prevention & control / Q000523:psychology*; D003213:Conditioning, Psychological; D003463:Cues; D004305:Dose-Response Relationship, Drug; D004777:Environment; D005106:Exploratory Behavior / Q000187:drug effects; D007262:Infusions, Intravenous; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D012054:Reinforcement, Psychology; D012646:Self Administration; D012919:Social Behavior; D013997:Time Factors
D000697:Central Nervous System Stimulants; D003042:Cocaine