ABSTRACT
In old age there is evidence of waning motivation, and possibly lowering of mood. Physiologically there is a decline in the levels of brain neurotransmitters such as acetylcholine and dopamine, and loss of myelin. These changes might be expected to impair the functioning of brain circuitry for reinforcement, and to lead to impaired motivation. To evaluate the function of brain reinforcement mechanisms during aging we examined brain stimulation reward and its modulation by dopaminergic drugs in BN F344F1 rats aged from young adult (5 months) to old (37 months). Brain stimulation directly activates the neural circuitry for reinforcement, and the response rate-frequency tradeoff can be used to characterize the functioning of the system. Both young and old subjects readily learned to lever press for 0.6 s trains of 0.15 ms brain stimulation pulses, and there was no difference in the number of pulses per train required to maintain responding at 50% of the maximum rate (M50). Amphetamine (0.5 mg/kg) significantly reduced the M50, and the dopamine synthesis inhibitor alpha-methyl-p-tyrosine (100 mg/kg) increased the M50, but these effects were not influenced by the age of the subjects. The results suggest that in healthy animals dopaminergic modulation of reinforcement is functionally intact in old age.
Subject(s)
Aging/psychology , Brain/physiology , Conditioning, Operant/drug effects , Conditioning, Operant/physiology , Dopamine Agents/pharmacology , Reward , Animals , Brain Chemistry/drug effects , Dextroamphetamine/pharmacology , Dopamine Uptake Inhibitors/pharmacology , Dose-Response Relationship, Drug , Electric Stimulation , Enzyme Inhibitors/pharmacology , Rats , Rats, Inbred F344 , Reinforcement Schedule , alpha-Methyltyrosine/pharmacologyABSTRACT
The strength of a train of rewarding brain stimulation required to support a criterion level of operant performance declines hyperbolically as the duration is increased. This finding has been attributed to a process of leaky integration. However, the rate at which integration approaches asymptote has been shown to depend on stimulation strength, a finding that differs from the behavior of a simple leaky integrator. The authors replicate both findings and show that they are both well described by a new model that incorporates a hyperbolic strength-duration function, a logistic function mapping stimulation frequency onto reward intensity, and another logistic function mapping reward intensity onto performance.
