| Abstract No.: | A-G1197 |
| Country: | Canada |
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| Title: | AT WHAT STAGE OF PROCESSING DOES COCAINE INFLUENCE PERFORMANCE FOR BRAIN STIMULATION REWARD? |
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| Authors/Affiliations: | 1 Giovanni Hernandez*; 1 Yannick-André Breton; 1 Peter Shizgal;
1 Concordia University, Montreal, QC, Canada.
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| Content: | Intracranial self-stimulation (ICSS) is modulated by changes in DA neurotransmission, but it is not known which psychological processes are responsible. In some accounts, DA neurons determine the reward threshold whereas in others, DA neurons determine the proclivity of the subject to invest effort in working for rewards. We have developed a model relating ICSS to the cost and strength of the electrical stimulation. A spatiotemporal integrator transforms the aggregate rate of firing of the directly stimulated neurons into a signal representing reward intensity; the peak of the reward-intensity signal is recorded in memory. Payoff is determined by the ratio of the peak-intensity signal to the product of two costs: the exertion entailed in operant performance and the opportunity cost of eschewing competing activities. An allocation function derived from Hernnstein’s matching law translates payoff into behaviour. Thus, performance can be represented in a 3D space defined by time allocation to ICSS, pulse frequency and opportunity cost (time required to harvest a reward). The 3D structure is shifted along the frequency axis by variables, such as the current or train duration that act prior to the integrator output. Changing the maximum integrator output, the required exertion or the value of competing activities shifts the 3D structure along the cost axis.
Objective: to pinpoint the stage of processing at which DA influence performance for ICSS (prior to or beyond the output of the integrator). We blocked DA reuptake by continuous subcutaneous infusion of cocaine in a novel manner that avoids tissue damage.
Materials and Methods:
Seven Long-Evans male rats (350-400 grams) underwent surgical implantation of porous tubing under the skin of the dorsum and monopolar electrodes aimed at the lateral hypothalamus. After recovery the rats were trained to hold down a lever to earn rewarding stimulation. The “price” of the reward was defined as the cumulative time that the rat had to depress the lever in order to trigger a stimulation train. Once performance stabilized, lever-holding was measured while the strength and price of the rewarding stimulation was varied and saline or cocaine (1.75 or 3.5 mg/kg/hr) was infused continuously.
Results:
The 3D structure always shifted rightward along the cost axis under the influence of cocaine but rarely shifted along the frequency axis. These effects are orthogonal to those produced by varying the current or train duration.
Conclusions:
These results imply that the leftward shifts seen in “rate-frequency” studies of the effect of cocaine on ICSS are misleading: In the light of our findings, the prior results would appear to be due to displacement of the diagonally oriented face of the 3D structure along the cost axis, which shifts the 2D silhouette leftward in the performance-frequency plane and creates the illusion of a cocaine-induced decrease in the reward threshold. The observed displacement along the cost axis could reflect decreased exertion cost, devaluation of competing activities, or amplification of integrator output. |
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