The role of the lateral cortico-cortical prefrontal pathway in self-stimulation of the medial prefrontal cortex in the rat

Organization of afferents to the orbitofrontal cortex in the rat

The prefrontal cortex (PFC) is usually defined as the frontal cortical area receiving a mediodorsal thalamic (MD) innervation. Certain areas in the medial wall of the rat frontal area receive a MD innervation. A second frontal area that is the target of MD projections is located dorsal to the rhinal sulcus and often referred to as the orbitofrontal cortex (OFC). Both the medial PFC and OFC are comprised of a large number of cytoarchitectonic regions. We assessed the afferent innervation of the different areas of the OFC, with a focus on projections arising from the mediodorsal thalamic nucleus, the basolateral nucleus of the amygdala, and the midbrain dopamine neurons. Although there are specific inputs to various OFC areas, a simplified organizational scheme could be defined, with the medial areas of the OFC receiving thalamic inputs, the lateral areas of the OFC being the recipient of amygdala afferents, and a central zone that was the target of midbrain dopamine neurons. Anterograde tracer data were consistent with this organization of afferents, and revealed that the OFC inputs from these three subcortical sites were largely spatially segregated. This spatial segregation suggests that the central portion of the OFC (pregenual agranular insular cortex) is the only OFC region that is a prefrontal cortical area, analogous to the prelimbic cortex in the medial prefrontal cortex. These findings highlight the heterogeneity of the OFC, and suggest possible functional attributes of the three different OFC areas.

Aging, prefrontal cortex, and amino acid neurotransmitters: differential effects produced by electrical stimulation

The purpose of the present study was to investigate the effects of electrical stimulation on the in vivo release of amino acid neurotransmitters in the prefrontal cortex of young (3-4 months) and aged (27-30 months) rats. In vivo push-pull perfusions were performed in the conscious animal. The levels of aspartate, glutamate, and glycine were analyzed by HPLC-fluorimetric detection. Electrical stimulation of the medial prefrontal cortex at the intensity of 0.3 mA produced a significant increase in the levels of aspartate, glutamate, and glycine in young but not aged rats. However, at the intensity of 0.6 mA, a significant delayed increase of these same amino acids was produced in the aged rats. These results support the existence of a differential effect of electrical stimulation on amino acid neurotransmitters in the prefrontal cortex of young and aged rats.