Excitotoxic lesions of the basolateral amygdala impair the acquisition of cocaine-seeking behaviour under a second-order schedule of reinforcement.
In these experiments we sought to establish the intravenous (i.v.) self-administration of cocaine under a second-order schedule of reinforcement in order: (i) to obtain reliable, drug-free levels of responding with cocaine as a reinforcer, and (ii) to enable investigation of the neural mechanisms by which arbitrary cues gain motivational salience and, as conditioned reinforcers, control over drug-seeking behaviour. Initially, each infusion of cocaine was made contingent upon a response on one of two identical levers and was paired with a 20-s light conditioned stimulus (CS). Responses on the second lever were recorded, but had no programmed consequence. When rats acquired stable rates of self-administration, a second-order schedule of the type FRx(FRy:S) was introduced, with values of "x" being increased progressively to 10 and then "y" from 2 through 8. Priming (i.e. non-contingent) infusions of cocaine were never given. Once the first infusion was obtained under the second-order schedule, further infusions were made contingent on each response (to a maximum of ten infusions/day). Each stage was repeated daily until the first infusion of each session was achieved within a 5-min criterion. Rats with bilateral, excitotoxic lesions of the basolateral amygdala readily acquired the i.v. self-administration of cocaine under a continuous reinforcement schedule, initially administering more infusions and maintaining a slightly elevated level of self-administration than controls. Despite increased numbers of CS/drug pairings, basolateral amygdala-lesioned rats were severely impaired in the acquisition of the second-order schedule of i.v. cocaine reinforcement. Lesioned rats showed a cocaine dose-response function that was shifted upwards relative to control subjects. There was no significant difference between drug-naive amygdala-lesioned and control animals in the locomotor response to intraperitoneal injections of cocaine. These experiments indicate the feasibility and utility of second-order schedules in studying the neurobehavioural basis of cocaine-seeking behaviour. They suggest a dissociation in the neural mechanisms underlying cocaine-taking and cocaine seeking behaviour, and demonstrate the potential importance of the basolateral amygdala in the processes by which previously neutral stimuli gain control over drug-seeking behaviour.
Department of Experimental Psychology, University of Cambridge, UK.
R B
RB
Whitelaw
A
A
Markou
T W
TW
Robbins
B J
BJ
Everitt
D000679:Amygdala / Q000187:drug effects* / Q000473:pathology; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects*; D001927:Brain Diseases / Q000139:chemically induced / Q000473:pathology; D003042:Cocaine / Q000008:administration & dosage*; D003216:Conditioning, Operant; D008297:Male; D009294:Narcotics / Q000008:administration & dosage*; D017378:Quinolinic Acid; D051381:Rats; D012646:Self Administration
D009294:Narcotics; D017378:Quinolinic Acid; D003042:Cocaine