Elsevier

Neuroscience

Volume 105, Issue 4, 22 August 2001, Pages 863-870
Neuroscience

Nucleus accumbens dopamine depletions make animals highly sensitive to high fixed ratio requirements but do not impair primary food reinforcement

https://doi.org/10.1016/S0306-4522(01)00249-4Get rights and content

Abstract

It has been suggested that dopamine in nucleus accumbens is involved in the process of enabling organisms to overcome work-related response costs. One way of controlling work costs with operant schedules is to use fixed ratio schedules with different ratio requirements. In the present study, the effects of nucleus accumbens dopamine depletions were investigated using six schedules: fixed ratio 5, 20, 50, 100, 200, and 300. In the first three schedules the food reinforcement consisted of one 45 mg food pellet per ratio completed. In the remaining schedules the food reinforcement per ratio completed was increased to two pellets for fixed ratio 100, four pellets for fixed ratio 200, and six pellets for fixed ratio 300. All rats were trained extensively prior to surgery, and rats were able to maintain high levels of responding on all schedules up to the fixed ratio 300. After training, rats were injected with either ascorbate vehicle or 6-hydroxydopamine into the nucleus accumbens. Rats were tested post-surgically on each of the schedules, with 3 days of testing per schedule.

Rats with nucleus accumbens dopamine depletions exhibited behavioral deficits that were highly dependent upon the ratio value. There were small and transient effects of dopamine depletion on fixed ratio 5 lever pressing, but as the ratio value got larger the impairment became greater. On the fixed ratio 20 and 50 schedules, response rates were partially reduced in dopamine-depleted rats. Responding on the fixed ratio 200 and 300 schedules was severely impaired, and on the last day of fixed ratio 300 testing no dopamine-depleted rats obtained a single reinforcer. These data are consistent with previous reports that accumbens dopamine depletions enhance ‘ratio strain’, making rats more sensitive to high ratio values. The induction of ratio strain by dopamine depletions does not appear to be related to a loss of appetite, and seems to be relatively independent of the baseline rate of responding and the overall density of food reinforcement across the session.

We conclude that dopamine in nucleus accumbens may be important for enabling rats to overcome behavioral constraints such as work-related response costs, and may be critical for the behavioral organization and conditioning processes that enable animals to emit large numbers of responses in the absence of primary reinforcement.

Section snippets

Subjects

A total of 10 adult male Sprague–Dawley rats (Harlan Sprague–Dawley, Indianapolis, IN, USA) were used in this experiment. Rats were housed in a colony maintained at a constant temperature (23°C) with a 12-h light/dark cycle (lights on at 07:00 h). All rats weighed between 290 and 330 g at the beginning of the study. Animals were food-deprived to 85% of their free feeding body weight, but were allowed a modest growth (up to 95% of original body weight) over the course of the study. Water was

Results

Figure 1 displays the total number of lever presses per day for 6-OHDA and vehicle-treated rats across all six lever pressing schedules. For the FR5 schedule, there was not a significant overall effect of 6-OHDA (F(1,8)=3.45, P<0.1), but there was a significant effect of days (F(2,16)=62.7, P<0.01) and a significant treatment group×day interaction (F(2,16)=18.43, P<0.01). Analysis of simple effects to determine the source of the interaction demonstrated that 6-OHDA-treated rats differed from

Discussion

The present results clearly demonstrate that accumbens DA depletions produce effects on lever pressing that are highly dependent upon the ratio requirement of the schedule. Performance on the FR5 schedule was only slightly suppressed by accumbens DA depletions, but the DA-depleted rats recovered rapidly. This is consistent with previous reports of recovery of response rate on FR4 or FR5 schedules after accumbens DA depletions (Salamone et al., 1993b, Aberman and Salamone, 1999). Responding on

Conclusions

In summary, it does not appear as though accumbens DA depletions have substantial effects on operant lever pressing on some schedules because of reduced appetite, or because of sensitivity to differences in molar reinforcement density between different schedules, or impairments in the perception of reinforcement magnitude. Nevertheless, across a broad range of experiments that include the present data, three generalizations can be made. First, on some schedules, such as FR1 or variable-interval

Acknowledgements

This work was supported by a grant to J.D.S. from the United States National Science Foundation, and by a grant to M.C. from Fundacio Caixa Castello-Bancaixa-UJI, Spain.

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