Lever pressing for food reward and changes in dopamine turnover and uric acid in rat caudate and nucleus accumbens studied chronically by in vivo voltammetry

Abstract

The neurotransmitter dopamine (DA) is widely held to be involved in the maintenance of positively reinforced behaviour, on the basis of self-stimulation experiments and of the effects of drugs which alter DA transmission on self-stimulation, and on behaviours controlled by natural reinforcers. We have used the technique of linear sweep voltammetry at chronically implanted carbon paste electrodes to demonstrate that 30-min sessions of responding on a variable interval schedule for food pellet reward increases first uric acid and then homovanillic acid (HVA, the DA metabolite) in rat caudate. Ascorbic acid levels are reduced, but at later times. We have now established that very similar responses in magnitude and timing are seen in the nucleus accumbens. Furthermore, in individual animals, the response rates, the increases in dopamine turnover, and the changes in uric acid and ascorbic acid remain stable when studied periodically over a 3 month period. Our previous results are broadly consistent with those obtained by others using indirect single-point methods (ex vivo neurochemistry) or in vivo dialysis. However, our present extended observations made possible by the use of paste electrodes (rather than fibre electrodes or dialysis probes) open the possibilities of studying the evolution of neurochemical responses during learning, and the covariation of neurochemical and behavioural effects of drugs within individual animals.