Pathways for Memory, Cognition and Emotional Context: Hippocampal, Subgenual Area 25, and Amygdalar Axons Show Unique Interactions in the Primate Thalamic Reuniens Nucleus.

The reuniens nucleus (RE) is situated at the most ventral position of the midline thalamus. In rats and mice RE is distinguished by bidirectional connections with the hippocampus and medial prefrontal cortex (mPFC) and a role in memory and cognition. In primates, many foundational questions pertaining to RE remain unresolved. We addressed these issues by investigating the composition of the rhesus monkey RE in both sexes by labeling for GABA, a marker of inhibitory neurons, and for the calcium-binding proteins parvalbumin (PV), calbindin (CB), and calretinin (CR), which label thalamic excitatory neurons that project to cortex. As in rats and mice, the macaque RE was mostly populated by CB and CR neurons, characteristic of matrix-dominant nuclei, and had bidirectional connections with hippocampus and mPFC area 25 (A25). Unlike rodents, we found GABAergic neurons in the monkey RE and a sparser but consistent population of core-associated thalamocortical PV neurons. RE had stronger connections with the basal amygdalar complex than in rats or mice. Amygdalar terminations were enriched with mitochondria and frequently formed successive synapses with the same postsynaptic structures, suggesting an active and robust pathway to RE. Significantly, hippocampal pathways formed multisynaptic complexes that uniquely involved excitatory projection neurons and dendrites of local inhibitory neurons in RE, extending this synaptic principle beyond sensory to high-order thalamic nuclei. Convergent pathways from hippocampus, A25, and amygdala in RE position it to flexibly coordinate activity for memory, cognition, and emotional context, which are disrupted in several psychiatric and neurologic diseases in humans.SIGNIFICANCE STATEMENT The primate RE is a central node for memory and cognition through connections with the hippocampus and mPFC. As in rats or mice, the primate RE is a matrix-dominant thalamic nucleus, suggesting signal traffic to the upper cortical layers. Unlike rats or mice, the primate RE contains inhibitory neurons, synaptic specializations with the hippocampal pathway, and robust connections with the amygdala, suggesting unique adaptations. Convergence of hippocampal, mPFC, and amygdalar pathways in RE may help unravel a circuit basis for binding diverse signals for conscious flexible behaviors and the synthesis of memory with affective significance in primates, whereas disruption of distinct circuit nodes may occur in psychiatric disorders in humans.

Prefrontal-hippocampal pathways underlying inhibitory control over memory.

A key function of the prefrontal cortex is to support inhibitory control over behavior. It is widely believed that this function extends to stopping cognitive processes as well. Consistent with this, mounting evidence establishes the role of the right lateral prefrontal cortex in a clear case of cognitive control: retrieval suppression. Retrieval suppression refers to the ability to intentionally stop the retrieval process that arises when a reminder to a memory appears. Functional imaging data indicate that retrieval suppression involves top-down modulation of hippocampal activity by the dorsolateral prefrontal cortex, but the anatomical pathways supporting this inhibitory modulation remain unclear. Here we bridge this gap by integrating key findings about retrieval suppression observed through functional imaging with a detailed consideration of relevant anatomical pathways observed in non-human primates. Focusing selectively on the potential role of the anterior cingulate cortex, we develop two hypotheses about the pathways mediating interactions between lateral prefrontal cortex and the medial temporal lobes during suppression, and their cellular targets: the entorhinal gating hypothesis, and thalamo-hippocampal modulation via the nucleus reuniens. We hypothesize that whereas entorhinal gating is well situated to stop retrieval proactively, thalamo-hippocampal modulation may interrupt an ongoing act of retrieval reactively. Isolating the pathways that underlie retrieval suppression holds the potential to advance our understanding of a range of psychiatric disorders characterized by persistent intrusive thoughts. More broadly, an anatomical account of retrieval suppression would provide a key model system for understanding inhibitory control over cognition.

Correspondence of functional connectivity gradients across human isocortex, cerebellum, and hippocampus.

Gradient mapping is an important technique to summarize high dimensional biological features as low dimensional manifold representations in exploring brain structure-function relationships at various levels of the cerebral cortex. While recent studies have characterized the major gradients of functional connectivity in several brain structures using this technique, very few have systematically examined the correspondence of such gradients across structures under a common systems-level framework. Using resting-state functional magnetic resonance imaging, here we show that the organizing principles of the isocortex, and those of the cerebellum and hippocampus in relation to the isocortex, can be described using two common functional gradients. We suggest that the similarity in functional connectivity gradients across these structures can be meaningfully interpreted within a common computational framework based on the principles of predictive processing. The present results, and the specific hypotheses that they suggest, represent an important step toward an integrative account of brain function.

Prefrontal pathways target excitatory and inhibitory systems in memory-related medial temporal cortices.

The anterior cingulate cortex (ACC), situated in the caudal part of the medial prefrontal cortex, is involved in monitoring on-going behavior pertaining to memory of previously learned outcomes. How ACC information interacts with the medial temporal lobe (MTL) memory system is not well understood. The present study used a multitiered approach to address two questions on the interactions between the ACC and the parahippocampal cortices in the rhesus monkey: (1) What are the presynaptic characteristics of ACC projections to the parahippocampal cortices? (2) What are the postsynaptic targets of the pathway and are there laminar differences in innervation of local excitatory and inhibitory systems? Labeled ACC terminations were quantified in parahippocampal areas TH and TF and a cluster analysis showed that boutons varied in size, with a population of small (≤0.97 µm) and large (>0.97 µm) terminations that were nearly evenly distributed in the upper and deep layers. Exhaustive sampling as well as unbiased stereological techniques independently showed that small and large boutons were about evenly distributed within cortical layers in the parahippocampal cortex. Synaptic analysis of the pathway, performed at the electron microscope (EM), showed that while most of the ACC projections formed synapses with excitatory neurons, a significant proportion (23%) targeted presumed inhibitory classes with a preference for parvalbumin (PV+) inhibitory neurons. These findings suggest synaptic mechanisms that may help integrate signals associated with attention and memory.

Nucleus accumbens adenosine A2A receptors regulate exertion of effort by acting on the ventral striatopallidal pathway.

Goal-directed actions are sensitive to work-related response costs, and dopamine in nucleus accumbens is thought to modulate the exertion of effort in motivated behavior. Dopamine-rich striatal areas such as nucleus accumbens also contain high numbers of adenosine A(2A) receptors, and, for that reason, the behavioral and neurochemical effects of the adenosine A(2A) receptor agonist CGS 21680 [2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine] were investigated. Stimulation of accumbens adenosine A(2A) receptors disrupted performance of an instrumental task with high work demands (i.e., an interval lever-pressing schedule with a ratio requirement attached) but had little effect on a task with a lower work requirement. Immunohistochemical studies revealed that accumbens neurons that project to the ventral pallidum showed adenosine A(2A) receptors immunoreactivity. Moreover, activation of accumbens A(2A) receptors by local injections of CGS 21680 increased extracellular GABA levels in the ventral pallidum. Combined contralateral injections of CGS 21680 into the accumbens and the GABA(A) agonist muscimol into ventral pallidum (i.e., "disconnection" methods) also impaired response output, indicating that these structures are part of a common neural circuitry regulating the exertion of effort. Thus, accumbens adenosine A(2A) receptors appear to regulate behavioral activation and effort-related processes by modulating the activity of the ventral striatopallidal pathway. Research on the effort-related functions of these forebrain systems may lead to a greater understanding of pathological features of motivation, such as psychomotor slowing, anergia, and fatigue in depression.

The intercalated nuclear complex of the primate amygdala.

The organization of the inhibitory intercalated cell masses (IM) of the primate amygdala is largely unknown despite their key role in emotional processes. We studied the structural, topographic, neurochemical and intrinsic connectional features of IM neurons in the rhesus monkey brain. We found that the intercalated neurons are not confined to discrete cell clusters, but form a neuronal net that is interposed between the basal nuclei and extends to the dorsally located anterior, central, and medial nuclei of the amygdala. Unlike the IM in rodents, which are prominent in the anterior half of the amygdala, the primate inhibitory net stretched throughout the antero-posterior axis of the amygdala, and was most prominent in the central and posterior extent of the amygdala. There were two morphologic types of intercalated neurons: spiny and aspiny. Spiny neurons were the most abundant; their somata were small or medium size, round or elongated, and their dendritic trees were round or bipolar, depending on location. The aspiny neurons were on average slightly larger and had varicose dendrites with no spines. There were three non-overlapping neurochemical populations of IM neurons, in descending order of abundance: (1) Spiny neurons that were positive for the striatal associated dopamine- and cAMP-regulated phosphoprotein (DARPP-32+); (2) Aspiny neurons that expressed the calcium-binding protein calbindin (CB+); and (3) Aspiny neurons that expressed nitric oxide synthase (NOS+). The unique combinations of structural and neurochemical features of the three classes of IM neurons suggest different physiological properties and function. The three types of IM neurons were intermingled and likely interconnected in distinct ways, and were innervated by intrinsic neurons within the amygdala, or by external sources, in pathways that underlie fear conditioning and anxiety.

Intraseptal tacrine-induced disruptions of spatial memory performance.

The medial septal nucleus regulates the physiology and emergent functions (e.g., memory formation) of the hippocampal formation. This nucleus is particularly rich in cholinergic receptors and is a putative target for the development of cholinomimetic cognitive enhancing drugs. Several studies have examined the direct effects of intraseptal cholinomimetic treatments and the results have been somewhat conflicting with both promnestic and amnestic effects. Several variables (e.g., age, task difficulty, timing of drug administration) may influence treatment outcome. The present study examined the effects of intraseptal infusion of the acetylcholinesterase inhibitor tacrine (0-25 microg/0.5 microl) on spatial memory performance. Tacrine was infused into the medial septum just prior to testing. Tacrine infusions did not significantly affect the number of correct choices in the first eight entries, or the number of correct choices until an error. This treatment did not alter the angle of arm entries, or impair the animals' ability to complete the task (enter all baited arms). However, tacrine produced a linear dose-dependent increase in errors, doubling (12.5 microg) and tripling (25.0 microg) the number of errors made before rats completed the task. The deficit demonstrates that activation of intraseptal cholinergic receptors can disrupt spatial memory performance. These findings are discussed with regards to septohippocampal-dependent memory processes and the development of therapeutic strategies in the treatment of age-related memory disorders.

Theta variation and spatiotemporal scaling along the septotemporal axis of the hippocampus.

Hippocampal theta has been related to locomotor speed, attention, anxiety, sensorimotor integration and memory among other emergent phenomena. One difficulty in understanding the function of theta is that the hippocampus (HPC) modulates voluntary behavior at the same time that it processes sensory input. Both functions are correlated with characteristic changes in theta indices. The current review highlights a series of studies examining theta local field potential (LFP) signals across the septotemporal or longitudinal axis of the HPC. While the theta signal is coherent throughout the entirety of the HPC, the amplitude, but not the frequency, of theta varies significantly across its three-dimensional expanse. We suggest that the theta signal offers a rich vein of information about how distributed neuronal ensembles support emergent function. Further, we speculate that emergent function across the long axis varies with respect to spatiotemporal scale. Thus, septal HPC processes details of the proximal spatiotemporal environment while more temporal aspects process larger spaces and wider time-scales. The degree to which emergent functions are supported by the synchronization of theta across the septotemporal axis is an open question. Our working model is that theta synchrony serves to bind ensembles representing varying resolutions of spatiotemporal information at interdependent septotemporal areas of the HPC. Such synchrony and cooperative interactions along the septotemporal axis likely support memory formation and subsequent consolidation and retrieval.

Within-subject memory decline in middle-aged rats: effects of intraseptal tacrine.

A longitudinal design was used to examine spatial working memory performance in aging Long-Evans rats on a 12-arm, delayed-non-match-to-sample radial maze task. Compared to performance at 12-13 months of age, the same rats exhibited a significant performance deficit at 15-16 months of age across all retention intervals (1.5-10h). All rats exhibited some degree of decline, and no rat performed as well as they had 3 months earlier. This early onset deficit may relate to the degree of difficulty required to perform accurately in a task that maximizes both spatial information processing and flexible working memory representations. Following our observation, rats were implanted with a chronic cannula aimed at the medial septal nucleus. Acute intraseptal tacrine treatments (0.0-25 micrograms/0.5 microl) did not significantly affect any index of performance. Rats exhibited further memory decline over the course of testing (up to 20 months of age). Detection of early onset dysfunction could allow for experimental analysis of underlying mechanisms and therapeutic strategies early in the course of age-related changes.

Intraseptal tacrine can enhance memory in cognitively impaired young rats.

The medial septum is rich in cholinergic receptors and is a target for the development of cognitive enhancers. Intraseptal cholinomimetics have produced both promnesic and amnesic effects. Several variables (e.g. age, task difficulty) may influence treatment outcome. The present study examined the effects of intraseptal tacrine in a group of young cognitively impaired rats. These rats had been culled from a difficult radial maze task because they could not achieve criterion performance. Tacrine (0-12.5 microg/0.5 microl) enhanced radial maze performance in these animals. This effect contrasts with findings that intraseptal choliomimetics often have no effect or disrupt performance in young rats. Understanding the conditions in which cholinomimetics are promnesic is important for the further development of cognitive enhancers.

Parallel prefrontal pathways reach distinct excitatory and inhibitory systems in memory-related rhinal cortices.

To investigate how prefrontal cortices impinge on medial temporal cortices we labeled pathways from the anterior cingulate cortex (ACC) and posterior orbitofrontal cortex (pOFC) in rhesus monkeys to compare their relationship with excitatory and inhibitory systems in rhinal cortices. The ACC pathway terminated mostly in areas 28 and 35 with a high proportion of large terminals, whereas the pOFC pathway terminated mostly through small terminals in area 36 and sparsely in areas 28 and 35. Both pathways terminated in all layers. Simultaneous labeling of pathways and distinct neurochemical classes of inhibitory neurons, followed by analyses of appositions of presynaptic and postsynaptic fluorescent signal, or synapses, showed overall predominant association with spines of putative excitatory neurons, but also significant interactions with presumed inhibitory neurons labeled for calretinin, calbindin, or parvalbumin. In the upper layers of areas 28 and 35 the ACC pathway was associated with dendrites of neurons labeled with calretinin, which are thought to disinhibit neighboring excitatory neurons, suggesting facilitated hippocampal access. In contrast, in area 36 pOFC axons were associated with dendrites of calbindin neurons, which are poised to reduce noise and enhance signal. In the deep layers, both pathways innervated mostly dendrites of parvalbumin neurons, which strongly inhibit neighboring excitatory neurons, suggesting gating of hippocampal output to other cortices. These findings suggest that the ACC, associated with attention and context, and the pOFC, associated with emotional valuation, have distinct contributions to memory in rhinal cortices, in processes that are disrupted in psychiatric diseases.

Theta and gamma coherence along the septotemporal axis of the hippocampus.

Theta and gamma rhythms synchronize neurons within and across brain structures. Both rhythms are widespread within the hippocampus during exploratory behavior and rapid-eye-movement (REM) sleep. How synchronous are these rhythms throughout the hippocampus? The present study examined theta and gamma coherence along the septotemporal (long) axis of the hippocampus in rats during REM sleep, a behavioral state during which theta signals are unaffected by external sensory input or ongoing behavior. Unilateral entorhinal cortical inputs are thought to play a prominent role in the current generation of theta, whereas current generation of gamma is primarily due to local GABAergic neurons. The septal 50% (4-5 mm) of the dentate gyrus (DG) receives a highly divergent, unilateral projection from any focal point along a lateral band of entorhinal neurons near the rhinal sulcus. We hypothesized that theta coherence in the target zone (septal DG) of this divergent entorhinal input would not vary, while gamma coherence would significantly decline with distance in this zone. However, both theta and gamma coherence decreased significantly along the long axis in the septal 50% of the hippocampus across both DG and CA1 electrode sites. In contrast, theta coherence between homotypic (e.g., DG to DG) sites in the contralateral hemisphere ( approximately 3-5 mm distant) were quite high ( approximately 0.7-0.9), much greater than theta coherence between homotypic sites 3-5 mm distant ( approximately 0.4-0.6) along the long axis. These findings define anatomic variation in both rhythms along the longitudinal axis of the hippocampus, indicate the bilateral CA3/mossy cell projections are the major determinant of theta coherence during REM, and demonstrate that theta coherence varies as a function of anatomical connectivity rather than physical distance. We suggest CA3 and entorhinal inputs interact dynamically to generate theta field potentials and advance the utility of theta and gamma coherence as indicators of hippocampal dynamics.

Intraseptal infusion of the cholinergic agonist carbachol impairs delayed-non-match-to-sample radial arm maze performance in the rat.

The medial septal nucleus regulates the physiology and emergent functions (e.g., memory formation) of the hippocampal formation. This nucleus is particularly rich in cholinergic receptors and is a putative target for the development of cholinomimetic cognitive enhancing drugs. A large number of studies have demonstrated that direct intraseptal drug infusions can produce amnestic or promnestic effects. While a few studies have examined the effects of direct intraseptal infusion of cholinomimetics on spatial memory performance (with drug "on-board" at the time of testing), the effects of post-acquisition infusions have not been assessed. We hypothesized that post-acquisition intraseptal infusion of cholinomimetics, by promoting hippocampal theta and suppressing the occurrence of hippocampal sharp waves, may disrupt the long-term retention and consolidation of memory. The present study examined the effects of intraseptal infusion of the cholinergic agonist carbachol in a delayed-non-match-to-sample radial maze task. Treatments were administered immediately following (within 1 min) the sample session with a retention session 2 h later. Carbachol infusions (12.5-125 ng in 0.5 microl) produced a linear dose-dependent decrease in correct entries and increase in retroactive errors, without any change in proactive errors or latency-per-choice. These findings suggest that post-acquisition intraseptal cholinergic treatments can produce amnesia. These findings are discussed with regard to multi-stage models of hippocampal-dependent memory formation and the further development of therapeutic strategies in the treatment of mild cognitive impairment as well as age-related cognitive decline and Alzheimer's dementia.