Patterns of overlap and segregation between insular cortical, intermediodorsal thalamic and basal amygdaloid afferents in the nucleus accumbens of the rat

Single midline thalamic neurons projecting to both the ventral striatum and the prefrontal cortex in the rat

The midline thalamic nuclei have been known to send projection fibres to the ventral striatum and the autonomic/limbic-associated areas of the prefrontal cortex. In the present study, we sought to determine whether or not single midline thalamic neurons project both to the ventral striatum and to the cerebral cortical areas. Experiments were performed on chloral hydrate-anaesthetized male Sprague Dawley rats; two fluorescent retrograde tracers were centred on the medial or lateral part of the nucleus accumbens--the major part of the ventral striatum--and the medial or lateral prefrontal viscerolimbic cortex. Our retrograde double-labelling study revealed that a subset of midline thalamic neurons send projection fibres to both the nucleus accumbens and the cerebral cortex. Such neurons projecting to both targets were principally identified in the paraventricular thalamic nucleus. The majority of the dually-labelled neurons in the paraventricular thalamic nucleus projected to the lateral part of the nucleus accumbens and the medial wall of the prefrontal cortex. Dually-labelled neurons were additionally found in other midline nuclei, including the paratenial, intermediodorsal, rhomboid, and reuniens nuclei, as well as in the medial part of the parafascicular thalamic nucleus. Dually-projecting neurons identified in the present study may represent a potential link between the limbic striatum and the viscerolimbic-associated cortex, thus suggesting that non-discriminative information relayed to the prefrontal cortex might exert an influence through the same neurons on the nucleus accumbens implicated in affective behaviour.

Efferent projections of the nucleus accumbens in the rat with special reference to subdivision of the nucleus: biotinylated dextran amine study

The nucleus accumbens (Acb) of the rat has been divided immunohistochemically into shell and core, and further, it was subdivided into several portions in relation to functional significance. In this report, the efferent projection of each subdivision of the Acb was examined using biotinylated dextran amine as an anterograde tracer. In rostral Acb, the dorsomedial shell mainly projected to the dorsomedial ventral pallidum (VP), lateral hypothalamus (LH) and substantia nigra pars compacta (SNc), while the ventromedial shell projected to the ventromedial VP, lateral preoptic area, LH and ventral tegmental area (VTA). The dorsal core of rostral Acb projected to the caudate putamen, dorsolateral VP, globus pallidus (GP), LH, and substantia nigra pars reticulata (SNr). In the middle to caudal Acb, the dorsomedial shell mainly projected to the dorsomedial VP, LH and VTA, the ventromedial shell projected to the ventromedial VP, substantia innominata, VTA, SNc and retrorubral area, and the ventrolateral shell projected to the ventrolateral VP and SNc. Furthermore, the ventromedial shell projected to the parabrachial nucleus (PB). The dorsomedial core projected to the dorsal VP, LH, SNc and SNr, and the ventral and lateral core sent axons to the dorsolateral VP, GP and SNc. From the point of view of projection patterns, shell and core are distinct throughout the rostro-caudal extent of the Acb. The ventrolateral shell at the caudal Acb was clearly differentiated. A direct projection from the ventromedial shell of the Acb to PB was also recognised.

Cortical pathways to the mammalian amygdala

The amygdaloid nuclear complex is critical for producing appropriate emotional and behavioral responses to biologically relevant sensory stimuli. It constitutes an essential link between sensory and limbic areas of the cerebral cortex and subcortical brain regions, such as the hypothalamus, brainstem, and striatum, that are responsible for eliciting emotional and motivational responses. This review summarizes the anatomy and physiology of the cortical pathways to the amygdala in the rat, cat and monkey. Although the basic anatomy of these systems in the cat and monkey was largely delineated in studies conducted during the 1970s and 1980s, detailed information regarding the cortico-amygdalar pathways in the rat was only obtained in the past several years. The purpose of this review is to describe the results of recent studies in the rat and to compare the organization of cortico-amygdalar projections in this species with that seen in the cat and monkey. In all three species visual, auditory, and somatosensory information is transmitted to the amygdala by a series of modality-specific cortico-cortical pathways ("cascades") that originate in the primary sensory cortices and flow toward higher order association areas. The cortical areas in the more distal portions of these cascades have stronger and more extensive projections to the amygdala than the more proximal areas. In all three species olfactory and gustatory/visceral information has access to the amygdala at an earlier stage of cortical processing than visual, auditory and somatosensory information. There are also important polysensory cortical inputs to the mammalian amygdala from the prefrontal and hippocampal regions. Whereas the overall organization of cortical pathways is basically similar in all mammalian species, there is anatomical evidence which suggests that there are important differences in the extent of convergence of cortical projections in the primate versus the nonprimate amygdala.

The anatomical relationships of the prefrontal cortex with limbic structures and the basal ganglia

This paper briefly discusses the anatomical criteria that have been used to delineate the prefrontal cortex (PFC) from the (pre)motor cortical areas in the frontal lobe. Single anatomical criteria, such as cytoarchitecture, connectivity with the mediodorsal thalamic nucleus or a dopaminergic innervation, are insufficient to unequivocally define the PFC. It is argued that, with respect to a number of structural aspects, the prefrontal and the (pre)motor cortical areas must be viewed as a continuum, whereas a (functional) differentiation is based on the type of information that is being processed in different parts of the frontal lobe. The involvement of the PFC, like the premotor cortex, in a number of basal ganglia-thalamocortical circuits may be interpreted in the same way. The paper also summarizes the organization of the inputs from midline/intralaminar thalamic nuclei, the basal amygdaloid complex and the hippocampus into the PFC-ventral striatal system. The results of tracing studies in rats indicate that these thalamic and limbic inputs both at the level of the PFC and the ventral striatum show various patterns of convergence and segregation. This leads to the conclusion that the PFC-ventral striatal system consists of a number of smaller modules.