Age-related decline in brain stimulation reward: rejuvenation by amphetamine

A decline in a brain stimulation reward responding occurred in rats 15-20 months of age. These animals had platinum bipolar electrodes implanted in the lateral hypothalamus-medial forebrain bundle area of the brain at 6 months and were vigorous and reliable responders after implantation. The decline in behavior was demonstrated by a decrease in rate of lever-presses, increase in impedance to the stimulating current and failure to complete reward threshold determination procedures. Priming, increased current, food deprivation, and reshaping were generally ineffective individually or in combination. A single low dose (0.25-0.30 mg/kg) of d-amphetamine 5 to 30 minutes prior to session reinstated BSR behavior in the aged rats; although threshold and impedance were higher than 6-8 month values and priming and reshaping were required in some cases. Rate of response was unchanged. Unexpectedly, all rats showed improved performances in at least two subsequent sessions without further amphetamine. These data are discussed from the perspective of brain neurochemistry and possibility that such manipulation had a rejuvenating effect on declining neurotransmitter systems, particularly dopamine.