Somatostatin-positive interneurons in the dentate gyrus of mice provide local- and long-range septal synaptic inhibition

Chronic activation of the relaxin-3 receptor on GABA neurons in rat ventral hippocampus promotes anxiety and social avoidance

Anxiety disorders are highly prevalent in modern society and better treatments are required. Key brain areas and signaling systems underlying anxiety include prefrontal cortex, hippocampus, and amygdala, and monoaminergic and peptidergic systems, respectively. Hindbrain GABAergic projection neurons that express the peptide, relaxin-3, broadly innervate the forebrain, particularly the septum and hippocampus, and relaxin-3 acts via a G_i/o -protein-coupled receptor known as the relaxin-family peptide 3 receptor (RXFP3). Thus, relaxin-3/RXFP3 signaling is implicated in modulation of arousal, motivation, mood, memory, and anxiety. Ventral hippocampus (vHip) is central to affective and cognitive processing and displays a high density of relaxin-3-positive nerve fibers and RXFP3 binding sites, but the identity of target neurons and associated effects on behavior are unknown. Therefore, in adult, male rats, we assessed the neurochemical nature of hippocampal RXFP3 mRNA-expressing neurons and anxiety-like and social behavior following chronic RXFP3 activation in vHip by viral vector expression of an RXFP3-selective agonist peptide, R3/I5. RXFP3 mRNA detected by fluorescent in situ hybridization was topographically distributed across the hippocampus in somatostatin- and parvalbumin-mRNA expressing GABA neurons. Chronic RXFP3 activation in vHip increased anxiety-like behavior in the light-dark box and elevated-plus maze, but not the large open-field test, and reduced social interaction with a conspecific stranger. Our data reveal disruptive effects of persistent RXFP3 signaling on hippocampal GABA networks important in anxiety; and identify a potential therapeutic target for anxiety disorders that warrants further investigation in relevant preclinical models.

Complementary networks of cortical somatostatin interneurons enforce layer specific control

The neocortex is functionally organized into layers. Layer four receives the densest bottom up sensory inputs, while layers 2/3 and 5 receive top down inputs that may convey predictive information. A subset of cortical somatostatin (SST) neurons, the Martinotti cells, gate top down input by inhibiting the apical dendrites of pyramidal cells in layers 2/3 and 5, but it is unknown whether an analogous inhibitory mechanism controls activity in layer 4. Using high precision circuit mapping, in vivo optogenetic perturbations, and single cell transcriptional profiling, we reveal complementary circuits in the mouse barrel cortex involving genetically distinct SST subtypes that specifically and reciprocally interconnect with excitatory cells in different layers: Martinotti cells connect with layers 2/3 and 5, whereas non-Martinotti cells connect with layer 4. By enforcing layer-specific inhibition, these parallel SST subnetworks could independently regulate the balance between bottom up and top down input.

The α7 nicotinic acetylcholine receptors regulate hippocampal adult-neurogenesis in a sexually dimorphic fashion

Disruption in cholinergic signaling has been linked to many environmental and/or pathological conditions known to modify adult neurogenesis. The α7 nAChRs are in the family of cys-loop receptor channels which have been shown to be neuroprotective in adult neurons and are thought to be critical for survival and integration of immature neurons. However, in developing neurons, poor calcium buffering may cause α7 nAChR activation to be neurotoxic. To investigate whether the α7 nAChR regulates neurogenesis in the hippocampus, we used a combination of mouse genetics and imaging to quantify neural stem cell (NSC) densities located in the dentate gyrus of adult mice. In addition, we considered whether the loss of α7 nAChRs had functional consequences on a spatial discrimination task that is thought to rely on pattern separation mechanisms. We found that the loss of α7 nAChRs resulted in increased neurogenesis in male mice only, while female mice showed increased cell divisions and intermediate progenitors but no change in neurogenesis. Knocking out the α7 nAChR from nestin⁺ NSCs and their progeny showed signaling in these cells contributes to regulating neurogenesis. In addition, male, but not female, mice lacking α7 nAChRs performed significantly worse in the spatial discrimination task. This task was sexually dimorphic in wild-type mice, but not in the absence of α7 nAChRs. We conclude that α7 nAChRs regulate adult neurogenesis and impact spatial discrimination function in male, but not female mice, via a mechanism involving nestin⁺ NSCs and their progeny.

Treating Brain Disorders by Targeting Adult Neural Stem Cells

Adult neurogenesis, a developmental process of generating functionally integrated neurons from neural stem cells, occurs throughout life in the hippocampus of the mammalian brain and highlights the plastic nature of the mature central nervous system. Substantial evidence suggests that new neurons participate in cognitive and affective brain functions and aberrant adult neurogenesis contributes to various brain disorders. Focusing on adult hippocampal neurogenesis, we review recent findings that advance our understanding of the key properties and potential functions of adult neural stem cells. We further discuss the key evidence demonstrating the causal role of aberrant hippocampal neurogenesis and various brain disorders. Finally, we propose strategies aimed at simultaneously correcting stem cells and their niche for treating brain disorders.

Morphological diversity and connectivity of hippocampal interneurons

The mammalian forebrain is constructed from ensembles of neurons that form local microcircuits giving rise to the exquisite cognitive tasks the mammalian brain can perform. Hippocampal neuronal circuits comprise populations of relatively homogenous excitatory neurons, principal cells and exceedingly heterogeneous inhibitory neurons, the interneurons. Interneurons release GABA from their axon terminals and are capable of controlling excitability in every cellular compartment of principal cells and interneurons alike; thus, they provide a brake on excess activity, control the timing of neuronal discharge and provide modulation of synaptic transmission. The dendritic and axonal morphology of interneurons, as well as their afferent and efferent connections within hippocampal circuits, is central to their ability to differentially control excitability, in a cell-type- and compartment-specific manner. This review aims to provide an up-to-date compendium of described hippocampal interneuron subtypes, with respect to their morphology, connectivity, neurochemistry and physiology, a full understanding of which will in time help to explain the rich diversity of neuronal function.

GABAergic interneurons: The orchestra or the conductor in fear learning and memory?

Fear conditioning is a form of associative learning that is fundamental to survival and involves potentiation of activity in excitatory projection neurons (PNs). Current models stipulate that the mechanisms underlying this process involve plasticity of PN synapses, which exhibit strengthening in response to fear conditioning. However, excitatory PNs are extensively modulated by a diverse array of GABAergic interneurons whose contributions to acquisition, storage, and expression of fear memory remain poorly understood. Here we review emerging evidence that genetically-defined interneurons play important subtype-specific roles in processing of fear-related stimuli and that these dynamics shape PN firing through both inhibition and disinhibition. Furthermore, interneurons exhibit structural, molecular, and electrophysiological evidence of fear learning-induced synaptic plasticity. These studies warrant discarding the notion of interneurons as passive bystanders in long-term memory.

Controlling learning and epilepsy together

Silencing dentate gyrus mossy cells affects spatial learning as well as seizures

GABAergic Neurons in the Rat Medial Septal Complex Express Relaxin-3 Receptor (RXFP3) mRNA

The medial septum (MS) complex modulates hippocampal function and related behaviors. Septohippocampal projections promote and control different forms of hippocampal synchronization. Specifically, GABAergic and cholinergic projections targeting the hippocampal formation from the MS provide bursting discharges to promote theta rhythm, or tonic activity to promote gamma oscillations. In turn, the MS is targeted by ascending projections from the hypothalamus and brainstem. One of these projections arises from the nucleus incertus in the pontine tegmentum, which contains GABA neurons that co-express the neuropeptide relaxin-3 (Rln3). Both stimulation of the nucleus incertus and septal infusion of Rln3 receptor agonist peptides promotes hippocampal theta rhythm. The G_i/o-protein-coupled receptor, relaxin-family peptide receptor 3 (RXFP3), is the cognate receptor for Rln3 and identification of the transmitter phenotype of neurons expressing RXFP3 in the septohippocampal system can provide further insights into the role of Rln3 transmission in the promotion of septohippocampal theta rhythm. Therefore, we used RNAscope multiplex in situ hybridization to characterize the septal neurons expressing Rxfp3 mRNA in the rat. Our results demonstrate that Rxfp3 mRNA is abundantly expressed in vesicular GABA transporter (vGAT) mRNA- and parvalbumin (PV) mRNA-positive GABA neurons in MS, whereas ChAT mRNA-positive acetylcholine neurons lack Rxfp3 mRNA. Approximately 75% of Rxfp3 mRNA-positive neurons expressed vGAT mRNA (and 22% were PV mRNA-positive), while the remaining 25% expressed Rxfp3 mRNA only, consistent with a potential glutamatergic phenotype. Similar proportions were observed in the posterior septum. The occurrence of RXFP3 in PV-positive GABAergic neurons gives support to a role for the Rln3-RXFP3 system in septohippocampal theta rhythm.