Functional specificity of ventral striatal compartments in appetitive behaviors.

The nucleus accumbens and its associated circuitry subserve behaviors linked to natural or biological rewards, such as feeding, drinking, sex, exploration, and appetitive learning. We have investigated the functional role of neurotransmitter and intracellular transduction mechanisms in behaviors subserved by the core and shell subsystems within the accumbens. Local infusion of the selective NMDA antagonist, AP-5, into the accumbens core, but not the shell, completely blocked acquisition of a bar-press response for food in hungry rats. This effect was apparent only when infused during the early stages of learning. We have also recently shown that infusion of certain protein kinase inhibitors into the core also impairs learning in the same paradigm. These results suggest that plasticity-related mechanisms within the accumbens core, involving glutamate-linked intracellular second messengers, are important for response-reinforcement learning. In contrast to the core, which primarily connects to somatic motor output systems, the shell is more intimately linked to viscero-endocrine effector systems. We have shown that both AMPA and GABA receptors within the medial shell (but not the core) are critically involved in controlling the brain's feeding pathways, via activation of the lateral hypothalamus (LH). This effect is blocked by local inhibition of the LH in double-cannulae experiments and also strongly and selectively activates Fos expression in the LH. These results provide a newly emerging picture of the differentiated functions of this forebrain region and suggest an integrated role in the elaboration of adaptive motor actions.