Differential behavioral effects following microinjection of an NMDA antagonist into nucleus accumbens subregions

Recent studies have demonstrated the existence of two distinct regions within the nucleus accumbens (N.Acc) known as "core" and "shell". In order to investigate whether the behavioral functions of excitatory amino acid receptors differed between these two subregions, rats were administered microinjections of 2-amino-5-phosphonovaleric acid (AP-5), a competitive NMDA antagonist (0, 0.05, 0.2, 0.5, 1.0 microgram/0.5 microliter) into selected central and medial regions of the accumbens. The central and medial sites were assumed to correspond approximately to core and shell subregions, respectively. The animals were tested in two exploratory tasks: the open field and a novel object test. In the open field test, AP-5 significantly decreased peripheral locomotion and center rearing frequency in the central but not the medial group. Locomotion and rearing were not affected by AP5 infusion into a control site, the anterior dorsal striatum (ADS). In the novel object test, animals were tested in the same open field, with prior habituation, and with several novel objects placed within it. In this test, infusions of AP-5 (0, 1.0 microgram/0.5 microliter) decreased the number and duration of contacts with the novel objects in the central but not the medial group. In addition, peripheral and center locomotion were decreased by AP-5 infusions into the central site, whether objects were present or not. In contrast, AP-5 infusions into the medial site elicited an increase in peripheral locomotion in both stimulus conditions. These findings provide behavioral-pharmacological evidence that the central and medial subregions of the nucleus accumbens can be differentiated.(ABSTRACT TRUNCATED AT 250 WORDS)