Interneurons of the hippocampus

The hippocampus, in particular the neocortex-hippocampus-neocortex circuit, is widely believed to be crucial in memory. Information flow in this circuit is strongly influenced by relatively sparse afferents derived from subcortical centres, such as the septum, involved in arousal, emotions and autonomic control. A powerful mechanism, by which numerically small inputs can produce profound effects, is feed-forward inhibition, that is, the activation of local inhibitory interneurons, which, in turn, control the activity of large populations of principal cells in the hippocampus. An example is the cholinergic input to the hippocampus from the septum, which is likely to be involved in feed-forward operations. Here, we demonstrate the existence of a circuit underlying another powerful mechanism of subcortical control of hippocampal information processing. We show that GABA-containing afferents originating in the septum innervate most of the GABA-containing interneurons in the hippocampus, making many synaptic contacts with each of them. Activation of the GABA-containing neurons in the septum is likely to lead to disinhibition of the principal neurons in the hippocampal formation and so this pathway is probably crucial in the induction of hippocampal electrical activity patterns, and may be involved in NMDA (N-methyl-D-aspartate) receptor-mediated functions, such as memory, in a permissive manner.

GABA-containing neurons in the septum control inhibitory interneurons in the hippocampus.

The medial prefrontal cortex (mPFC) has been associated with diverse functions including attentional processes, visceromotor activity, decision making, goal directed behavior, and working memory. Using retrograde tracing techniques, we examined, compared, and contrasted afferent projections to the four divisions of the mPFC in the rat: the medial (frontal) agranular (AGm), anterior cingulate (AC), prelimbic (PL), and infralimbic (IL) cortices. Each division of the mPFC receives a unique set of afferent projections. There is a shift dorsoventrally along the mPFC from predominantly sensorimotor input to the dorsal mPFC (AGm and dorsal AC) to primarily 'limbic' input to the ventral mPFC (PL and IL). The AGm and dorsal AC receive afferent projections from widespread areas of the cortex (and associated thalamic nuclei) representing all sensory modalities. This information is presumably integrated at, and utilized by, the dorsal mPFC in goal directed actions. In contrast with the dorsal mPFC, the ventral mPFC receives significantly less cortical input overall and afferents from limbic as opposed to sensorimotor regions of cortex. The main sources of afferent projections to PL/IL are from the orbitomedial prefrontal, agranular insular, perirhinal and entorhinal cortices, the hippocampus, the claustrum, the medial basal forebrain, the basal nuclei of amygdala, the midline thalamus and monoaminergic nuclei of the brainstem. With a few exceptions, there are few projections from the hypothalamus to the dorsal or ventral mPFC. Accordingly, subcortical limbic information mainly reaches the mPFC via the midline thalamus and basal nuclei of amygdala. As discussed herein, based on patterns of afferent (as well as efferent) projections, PL is positioned to serve a direct role in cognitive functions homologous to dorsolateral PFC of primates, whereas IL appears to represent a visceromotor center homologous to the orbitomedial PFC of primates.

Anatomical analysis of afferent projections to the medial prefrontal cortex in the rat

Functional organization of the extrinsic and intrinsic circuitry of the parahippocampal region

The medial prefrontal cortex in the rat: evidence for a dorso-ventral distinction based upon functional and anatomical characteristics.

Cortical projection patterns of magnocellular basal nucleus subdivisions as revealed by anterogradely transported Phaseolus vulgaris leucoagglutinin

https://pure.rug.nl/ws/files/6804224/1987BrainResBullNyakas.pdf

Detailed projection patterns of septal and diagonal band efferents to the hippocampus in the rat with emphasis on innervation of ca1 and dentate gyrus

https://pure.rug.nl/ws/files/15671928/1985NeurosciLettLuiten.pdf

The Pattern of Cortical Projections from the Intermediate Parts of the Magnocellular Nucleus basalis in the Rat Demonstrated by Tracing with Phaseolus vulgaris-Leucoagglutinin

D.G. Spencer

Papers

Cortical projection patterns of magnocellular basal nucleus subdivisions as revealed by anterogradely transported Phaseolus vulgaris leucoagglutinin

Detailed projection patterns of septal and diagonal band efferents to the hippocampus in the rat with emphasis on innervation of ca1 and dentate gyrus

The Pattern of Cortical Projections from the Intermediate Parts of the Magnocellular Nucleus basalis in the Rat Demonstrated by Tracing with Phaseolus vulgaris-Leucoagglutinin

Changes in septo-hippocampal projections after lateral entorhinal or combined entorhinal-raphé lesions as studied by anterograde tracing methods