Nucleus reuniens of the midline thalamus: link between the medial prefrontal cortex and the hippocampus

Abstract

The medial prefrontal cortex and the hippocampus serve well recognized roles in memory processing. The hippocampus projects densely to, and exerts strong excitatory actions on, the medial prefrontal cortex. Interestingly, the medial prefrontal cortex, in rats and other species, has no direct return projections to the hippocampus, and few projections to parahippocampal structures including the entorhinal cortex. It is well established that the nucleus reuniens of the midline thalamus is the major source of thalamic afferents to the hippocampus. Since the medial prefrontal cortex also distributes to nucleus reuniens, we examined medial prefrontal connections with populations of nucleus reuniens neurons projecting to hippocampus. We used a combined anterograde and retrograde tracing procedure at the light and electron microscopic levels. Specifically, we made Phaseolus vulgaris-leuccoagglutinin (PHA-L) injections into the medial prefrontal cortex and Fluorogold injections into the hippocampus (CA1/subiculum) and examined termination patterns of anterogradely PHA-L labeled fibers on retrogradely FG labeled cells of nucleus reuniens. At the light microscopic level, we showed that fibers from the medial prefrontal cortex form multiple putative synaptic contacts with dendrites of hippocampally projecting neurons throughout the extent of nucleus reuniens. At ultrastructural level, we showed that medial prefrontal cortical fibers form asymmetric contacts predominantly with dendritic shafts of hippocampally projecting reuniens cells. These findings indicate that nucleus reuniens represents a critical link between the medial prefrontal cortex and the hippocampus. We discuss the possibility that nucleus reuniens gates the flow of information between the medial prefrontal cortex and hippocampus dependent upon attentive/arousal states of the organism.

Figure 1

(a–d) Series of low magnification rostrocaudally aligned photomicrographs through the thalamus showing: (1) the distribution of PHA-L labeled fibers (brown) in the paratenial (PT), paraventricular (PV) and reuniens (RE) nuclei of the midline thalamus produced by a PHA-L injection in the medial prefrontal cortex (mPFC, see Fig. 2a), and (2) retrogradely-labeled cells (black) in RE produced by a Fluorogold injection in the hippocampus (Fig. 2b). Note the significant overlap of PHA-L labeled fibers and retrogradely labeled cells at three rostrocaudal levels of RE, but no overlap in PT and PV due to the absence of retrogradely labeled cells in PT/PV. Scale bar, 300 µm. FX, fornix, MT, mammillothalamic tract, PT, paratenial nucleus of thalamus; PV, paraventricular nucleus of thalamus; RE, nucleus reuniens of thalamus, V3, third ventricle.

Figure 2

(a) Low magnification photomicrograph of a PHA-L injection in the medial prefrontal cortex, virtually confined to the prelimbic cortex. (b) Low magnification photomicrograph of a FG injection in the ventral hippocampus, with the core of the injection (arrow) centered in stratum lacunosum-moleculare of ventral CA1. (c) Low magnification photomicrograph showing pronounced overlap in RE (dorsolateral to the third ventricle) of PHA-L labeled fibers (black) and FG labeled cells (brown). (d) High magnification photomicrograph showing putative synaptic contacts of a PHA-L labeled fiber (arrow) on a dendrite of a FG labeled cell. (e,f) High magnification photomicrographs showing putative synaptic contacts (arrows) of PHA-L labeled fibers on proximal dendrites FG labeled cells (f, higher magnification of d). Scale bars, a,b, 1200 µm; for c, 60 µm; for d, 25 µm; for e,f, 15 µm. AC, anterior cingulate cortex; ACC, nucleus accumbens; AGl, lateral agranular (frontal) cortex; CA1, CA1 field of Ammon’s horn of hippocampal formation; EC, entorhinal cortex; IL, infralimbic cortex, OC, occipital cortex; PL, prelimbic cortex; SUBv, ventral subiculum of hippocampal formation.

Figure 3

(a–d) Series of consecutive electron micrographs within nucleus reuniens showing asymmetric contacts of a single PHA-L labeled (F) fiber (from the mPFC) on a labeled dendritic shaft, identified by the presence of numerous silver intensified gold deposits (arrows in D), of a RE cell retrogradely from the hippocampus. Note also the presence of asymmetric contacts of an unlabeled fiber (A) on the same labeled dendrite (D). Scale bar, 1 um.

Figure 4

(a) An electron micrograph showing a PHA-L labeled fiber (F, arrow head) from the mPFC forming an asymmetric contact with an unlabeled dendrite within RE and several (four) unlabeled asymmetric contacts (A, arrow heads) with the same dendritic process. (b) A PHA-L labeled fiber (F) from the mPFC in close proximity but without synaptic contact on a labeled RE cell body (C) and nucleus (N). (c) An unlabeled symmetric contact (A, arrow head) with a labeled RE cell body (C). Scale bar, 1 µm.