Lesions of the nucleus accumbens core modulate development of matching behavior

Abstract

Background: The development of choice is a crucial determinant in the performance of appetitive responses. Given two options with different reinforcement rates, animals match their relative rate of responding to the relative rates of reinforcement (i.e., matching behavior). A previous study has shown that the nucleus accumbens core (AcbC) is involved in the performance of matching behavior in trained animals. However, the role of the AcbC in the acquisition of matching behavior has not been addressed.

Results: We conducted a series of experimental sessions to examine the role of the AcbC on the development of matching behavior. Instrumental responding was measured in rats with excitotoxic lesions of the AcbC. Rats were given two options that differed in the relative rate of reinforcement under concurrent variable-interval schedules. The locations of the more frequently reinforced option and the alternative option were randomly switched between sessions. Lesions of the AcbC accelerated the development of matching behavior compared to the sham-operated group. The AcbC-lesioned rats exhibited closer conformity to the matching law than shams when the options were in the same positions as in the previous session (the same condition), but not when the option locations had been switched (the different condition). The AcbC rats showed smaller probabilities of switching behavior between alternatives than shams. Post-reinforcement pausing was not affected by the AcbC lesion. Neither numbers of rewards obtained nor number of lever presses were different between the AcbC-lesioned rats and shams over session blocks.

Conclusions: Our results suggest that the AcbC plays a regulatory role in the development of matching behavior through switching probabilities rather than perception of reward magnitude. The differential effect of AcbC lesions on the matching behavior between the same and different conditions suggests influence of the spontaneous recovery, that is, reversion to a previously reinforced choice at the beginning of the next session, on the development of matching behavior in the AcbC-lesioned rats.

Figure 1

Lesions of the AcbC. (A and B) Photographs of coronal sections through the nucleus accumbens immunostained for NeuN from an AcbC-lesioned (A), and a sham-operated (B) rat. The black dotted lines delineate the AcbC. The solid lines outline the boundary of the anterior commissure to facilitate its distinction from the area of lesion. Scale bar in B applies to all photographs and represents 0.5 mm. Abbreviation: ac, anterior commissure. (C) Schematic diagram of a series of coronal sections of the rat brain illustrating the extent of bilateral AcbC lesions (darkness represents coincidence of lesions from different animals). The numbers refer to millimeters anterior to bregma according to the standard atlas [39].

Figure 2

Effect of the AcbC lesion on development of matching behavior. (A) Data are expressed as mean (±SEM) of deviation from matching in the session block. Asterisks indicate a significant simple main effect of group (p < 0.01). (B) Each panel shows data from a particular session block (indicated at the top left of each panel). Data are expressed as mean (±SEM) of deviation from matching in the 10-min bins. The simple main effect of time (p < 0.001) is as follows: block 2, F(2, 44) = 42.469, ϵ = 1.0; block 3, F(1.7, 37.8) = 31.003, ϵ = 0.858; block 4, F(1.3, 29.4) = 34.219, ϵ = 0.667; block 5, F(1.4, 31.8) = 46.863, ϵ = 0.724; and block 6, F(1.5, 33.0) = 51.772, ϵ = 0.748. (C) Effect of the AcbC lesion on development of matching behavior across session blocks under the same and the different conditions. Data are expressed as mean (±SEM) of deviation from matching in each of the session blocks. *p < 0.05.

Figure 3

Effect of the AcbC lesion on lever-press switching, performance, and pausing. (A) Mean (±SEM) switching probabilities of lever presses between the AcbC-lesioned rats and shams over session blocks. Asterisks indicate a significant simple main effect of group (p < 0.01). (B) Comparison of switching between the same and different conditions. (C) Mean (±SEM) number of lever presses performed by the AcbC-lesioned and the shams over session blocks. (D) Number of pellets earned by each group in each session. (E) Post-reinforcement pausing between the AcbC group and shams. Data are expressed as mean (±SEM) pausing time in each session block.