Effects of extended-access self-administration and deprivation on breakpoints maintained by cocaine in rats.
BACKGROUND
Animal models that identify the effects of self-administration histories on subsequent patterns, levels of intake, and other aspects of reinforcement will help clarify the controlling variables of human drug use.


OBJECTIVE
Identify the effects of extended-access to cocaine and 1 or 7 days of deprivation on cocaine-maintained breakpoints on a progressive ratio (PR) schedule of reinforcement.


METHODS
Male, Sprague-Dawley rats were trained to self-administer intravenous cocaine (expt 1: 1.5 mg/kg per infusion; expt 2: 0.75 mg/kg per infusion), and then given various histories of self-administration and deprivation. Breakpoints, the number of infusions self-administered on a PR schedule, were assessed following the deprivation period.


RESULTS
Rates of cocaine intake increased when access to cocaine was extended to 6 h/day. From day 1 to day 14, daily intake increased from 92 (+/-2.5) to 101 (+/-2.8) mg/kg in expt 1, and from 55 (+/-4) to 78 (+/-2.2) mg/kg in expt 2. Total intake across this 2-week period was approximately 1260 and 970 mg/kg in expts 1 and 2. Breakpoints were not different following this escalation period. The introduction of a 7-day deprivation period failed to alter breakpoints.


CONCLUSIONS
There is dissociation between changes in rate of cocaine intake (or consumption) and breakpoints maintained on a PR schedule. Extended-access to cocaine produced increases in rate of intake without altering breakpoints. Depending on the experimental question, extended-access conditions may prove useful for studying changes in certain aspects of reinforcement, such as consumption, but not others, such as the strength of a drug as a reinforcer.
Liu
Yu
Y
Neuroscience Program, Wake Forest University School of Medicine, Winston-Salem, NC 27157, USA.
Roberts
David C S
DC
Morgan
Drake
D
D000818:Animals; D003042:Cocaine / Q000008:administration & dosage*; D003216:Conditioning, Operant / Q000187:drug effects* / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D012055:Reinforcement Schedule; D012646:Self Administration; D013997:Time Factors
D003042:Cocaine