drd2-cre:ribotagmouse line unravels the possible diversity of dopamine d2 receptor-expressing cells of the dorsal mouse hippocampus

Dopamine D1 receptors activation rescues hippocampal synaptic plasticity and cognitive impairments in the MK-801 neonatal schizophrenia model

Schizophrenia is a disorder with a higher cognitive decline in early adulthood, causing impaired retention of episodic memories. However, the physiological and behavioral functions that underlie cognitive deficits with a potential mechanism to ameliorate and improve cognitive performance are unknown. In this study, we used the MK-801 neurodevelopmental schizophrenia-like model. Rats were divided into two groups: one received MK-801, and the other received saline for five consecutive days (7-11 postnatal days, PND). We evaluated synaptic plasticity late-LTP and spatial memory consolidation in early adolescence and young adulthood using extracellular field recordings in acute hippocampal slices and the Barnes maze task. Next, we examined D1 receptor (D1R) activation as a mechanism to ameliorate cognitive impairments. Our results suggest that MK-801 neonatal treatment induces impairment in late-LTP expression and deficits in spatial memory retrieval in early adolescence that is maintained until young adulthood. Furthermore, we found that activation of dopamine D1R ameliorates the impairments and promotes a robust expression of late-LTP and an improved performance in the Barnes maze task, suggesting a novel and potential therapeutic role in treating cognitive impairments in schizophrenia.

Prolactin blood concentration relies on the scalability of the TIDA neurons' network efficiency in vivo

Our understanding and management of reproductive health and related disorders such as infertility, menstrual irregularities, and pituitary disorders depend on understanding the intricate sex-specific mechanisms governing prolactin secretion. Using ex vivo experiments in acute slices, in parallel with in vivo calcium imaging (GRIN lens technology), we found that dopamine neurons inhibiting PRL secretion (TIDA), organize as functional networks both in and ex vivo. We defined an index of efficiency of networking (Ieff) using the duration of calcium events and the ability to form plastic economic networks. It determined TIDA neurons' ability to inhibit PRL secretion in vivo. Ieff variations in both sexes demonstrated TIDA neurons' adaptability to physiological changes. A variation in the number of active neurons contributing to the network explains the sexual dimorphism in basal [PRL]blood secretion patterns. These sex-specific differences in neuronal activity and network organization contribute to the understanding of hormone regulation.

Serotonin modulates infraslow oscillation in the dentate gyrus during Non-REM sleep

Synchronous neuronal activity is organized into neuronal oscillations with various frequency and time domains across different brain areas and brain states. For example, hippocampal theta, gamma and sharp wave oscillations are critical for memory formation and communication between hippocampal subareas and the cortex. In this study, we investigated the neuronal activity of the dentate gyrus (DG) with electrophysiological and optical imaging tools during sleep-wake cycles. We found that the activity of major glutamatergic cell populations in the DG is organized into in-fraslow oscillations (0.01 - 0.03 Hz) during NREM sleep. Although the DG is considered a sparsely active network during wakefulness, we found that 50% of granule cells and about 25% of mossy cells exhibit increased activity during NREM sleep. Further experiments revealed that the infraslow oscillation in the DG is modulated by rhythmic serotonin release during sleep, which oscillates at the same frequency but in an opposite phase. Genetic manipulation of 5-HT receptors revealed that this neuromodulatory regulation is mediated by 5-HT1a receptors and the knockdown of these receptors leads to memory impairment. Together, our results provide novel mechanistic insights into how the 5-HT system can influence hippocampal activity patterns during sleep.

Expression of GCaMP6s in the dentate gyrus induces tonic-clonic seizures

GCaMP is a genetically encoded calcium indicator (GECI) widely used in neuroscience research. It measures intracellular Ca2+ level by fluorescence changes as it directly binds to Ca2+. In this process, the effect of this calcium buffer on the intracellular calcium signaling and cell physiology is often not taken into consideration. However, growing evidence from calcium imaging studies shows GCaMP expression under certain conditions can generate aberrant activity, such as seizures. In this study, we examined the effect of GCaMP6 expression in the dentate gyrus (DG) on epileptogenesis. We found that viral expression of GCaMP6s but not GCaMP6f in the DG induces tonic-clonic seizures several weeks after viral injection. Cell-type specific expression of GCaMP6s revealed the granule cells (GCs) as the key player in GCaMP6s-induced epilepsy. Finally, by using slice electrophysiology, we demonstrated that GCaMP6s expression increases neuronal excitability in the GCs. Together, this study highlights the ability of GCaMP6s in DG-associated epileptogenesis.

Dopamine D2 receptors in WFS1-neurons regulate food-seeking and avoidance behaviors

The selection and optimization of appropriate adaptive responses depends on interoceptive and exteroceptive stimuli as well as on the animal's ability to switch from one behavioral strategy to another. Although growing evidence indicate that dopamine D2R-mediated signaling events ensure the selection of the appropriate strategy for each specific situation, the underlying neural circuits through which they mediate these effects are poorly characterized. Here, we investigated the role of D2R signaling in a mesolimbic neuronal subpopulation expressing the Wolfram syndrome 1 (Wfs1) gene. This subpopulation is located within the nucleus accumbens, the central amygdala, the bed nucleus of the stria terminalis, and the tail of the striatum, all brain regions critical for the regulation of emotions and motivated behaviors. Using a mouse model carrying a temporally controlled deletion of D2R in WFS1-neurons, we demonstrate that intact D2R signaling in this neuronal population is necessary to regulate homeostasis-dependent food-seeking behaviors in both male and female mice. In addition, we found that reduced D2R signaling in WFS1-neurons impaired active avoidance learning and innate escape responses. Collectively, these findings identify a yet undocumented role for D2R signaling in WFS1-neurons as a novel effector through which dopamine optimizes appetitive behaviors and regulates defensive behaviors.

Spatiomolecular Characterization of Dopamine D2 Receptors Cells in the Mouse External Globus Pallidus

The external globus pallidus (GPe) is part of the basal ganglia circuit and plays a key role in controlling the actions. Although, many evidence indicate that dopamine through its activation of dopamine D2 receptors (D2Rs) modulates the GPe neuronal activity, the precise spatiomolecular characterization of cell populations expressing D2Rs in the mouse GPe is still lacking. By combining single molecule in situ hybridization, cell type-specific imaging analyses, and electrophysiology slice recordings, we found that GPe D2R cells are neurons preferentially localized in the caudal portion of GPe. These neurons comprising pallido-striatal, pallido-nigral, and pallido-cortical neurons segregate into two distinct populations displaying molecular and electrophysiological features of GPe GABAergic PV/NKX2.1 and cholinergic neurons respectively. By clarifying the spatial molecular identity of GPe D2R neurons in the mouse, this work provides the basis for future studies aiming at disentangling the action of dopamine within the GPe.

Dopamine D2 receptors in hilar mossy cells regulate excitatory transmission and hippocampal function

Hilar mossy cells (MCs) are principal excitatory neurons of the dentate gyrus (DG) that play critical roles in hippocampal function and have been implicated in brain disorders such as anxiety and epilepsy. However, the mechanisms by which MCs contribute to DG function and disease are poorly understood. A defining feature of MCs is the promoter activity of the dopamine D2 receptor (D2R) gene (Drd2), and previous work indicates a key role for dopaminergic signaling in the DG. Additionally, the involvement of D2R signaling in cognition and neuropsychiatric conditions is well known. Surprisingly, though, the function of MC D2Rs remains largely unexplored. In this study, we show that selective and conditional removal of Drd2 from MCs of adult mice impaired spatial memory, promoted anxiety-like behavior, and was proconvulsant. To determine the subcellular expression of D2Rs in MCs, we used a D2R knockin mouse which revealed that D2Rs are enriched in the inner molecular layer of the DG, where MCs establish synaptic contacts with granule cells (GCs). D2R activation by exogenous and endogenous dopamine reduced MC to dentate GC synaptic transmission, most likely by a presynaptic mechanism. In contrast, exogenous dopamine had no significant impact on MC excitatory inputs and passive and active properties. Our findings support that MC D2Rs are essential for proper DG function by reducing MC excitatory drive onto GCs. Lastly, impairment of MC D2R signaling could promote anxiety and epilepsy, therefore highlighting a potential therapeutic target.

Deficiency of Cullin 3, a Protein Encoded by a Schizophrenia and Autism Risk Gene, Impairs Behaviors by Enhancing the Excitability of Ventral Tegmental Area (VTA) DA Neurons

The dopaminergic neuromodulator system is fundamental to brain functions. Abnormal dopamine (DA) pathway is implicated in psychiatric disorders, including schizophrenia (SZ) and autism spectrum disorder (ASD). Mutations in Cullin 3 (CUL3), a core component of the Cullin-RING ubiquitin E3 ligase complex, have been associated with SZ and ASD. However, little is known about the function and mechanism of CUL3 in the DA system. Here, we show that CUL3 is critical for the function of DA neurons and DA-relevant behaviors in male mice. CUL3-deficient mice exhibited hyperactive locomotion, deficits in working memory and sensorimotor gating, and increased sensitivity to psychostimulants. In addition, enhanced DA signaling and elevated excitability of the VTA DA neurons were observed in CUL3-deficient animals. Behavioral impairments were attenuated by dopamine D2 receptor antagonist haloperidol and chemogenetic inhibition of DA neurons. Furthermore, we identified HCN2, a hyperpolarization-activated and cyclic nucleotide-gated channel, as a potential target of CUL3 in DA neurons. Our study indicates that CUL3 controls DA neuronal activity by maintaining ion channel homeostasis and provides insight into the role of CUL3 in the pathogenesis of psychiatric disorders.SIGNIFICANCE STATEMENT This study provides evidence that Cullin 3 (CUL3), a core component of the Cullin-RING ubiquitin E3 ligase complex that has been associated with autism spectrum disorder and schizophrenia, controls the excitability of dopamine (DA) neurons in mice. Its DA-specific heterozygous deficiency increased spontaneous locomotion, impaired working memory and sensorimotor gating, and elevated response to psychostimulants. We showed that CUL3 deficiency increased the excitability of VTA DA neurons, and inhibiting D2 receptor or DA neuronal activity attenuated behavioral deficits of CUL3-deficient mice. We found HCN2, a hyperpolarization-activated channel, as a target of CUL3 in DA neurons. Our findings reveal CUL3's role in DA neurons and offer insights into the pathogenic mechanisms of autism spectrum disorder and schizophrenia.

Dopaminoceptive D1 and D2 neurons in ventral hippocampus arbitrate approach and avoidance in anxiety

The hippocampus 1-7, as well as dopamine circuits 8-11, coordinate decision-making in anxiety-eliciting situations. Yet, little is known about how dopamine modulates hippocampal representations of emotionally-salient stimuli to inform appropriate resolution of approach versus avoidance conflicts. We here study dopaminoceptive neurons in mouse ventral hippocampus (vHipp), molecularly distinguished by their expression of dopamine D1 or D2 receptors. We show that these neurons are transcriptionally distinct and topographically organized across vHipp subfields and cell types. In the ventral subiculum where they are enriched, both D1 and D2 neurons are recruited during anxiogenic exploration, yet with distinct profiles related to investigation and behavioral selection. In turn, they mediate opposite approach/avoidance responses, and are differentially modulated by dopaminergic transmission in that region. Together, these results suggest that vHipp dopamine dynamics gate exploratory behaviors under contextual uncertainty, implicating dopaminoception in the complex computation engaged in vHipp to govern emotional states.

Dopamine D2 receptors in mossy cells reduce excitatory transmission and are essential for hippocampal function

Hilar mossy cells (MCs) are principal excitatory neurons of the dentate gyrus (DG) that play critical roles in hippocampal function and have been implicated in brain disorders such as anxiety and epilepsy. However, the mechanisms by which MCs contribute to DG function and disease are poorly understood. Expression from the dopamine D2 receptor (D2R) gene (Drd2) promoter is a defining feature of MCs, and previous work indicates a key role for dopaminergic signaling in the DG. Additionally, the involvement of D2R signaling in cognition and neuropsychiatric conditions is well-known. Surprisingly, though, the function of MC D2Rs remain largely unexplored. In this study, we show that selective and conditional removal of Drd2 from MCs of adult mice impaired spatial memory, promoted anxiety-like behavior and was proconvulsant. To determine the subcellular expression of D2Rs in MCs, we used a D2R knockin mouse which revealed that D2Rs are enriched in the inner molecular layer of the DG, where MCs establish synaptic contacts with granule cells. D2R activation by exogenous and endogenous dopamine reduced MC to dentate granule cells (GC) synaptic transmission, most likely by a presynaptic mechanism. In contrast, removing Drd2 from MCs had no significant impact on MC excitatory inputs and passive and active properties. Our findings support that MC D2Rs are essential for proper DG function by reducing MC excitatory drive onto GCs. Lastly, impairment of MC D2R signaling could promote anxiety and epilepsy, therefore highlighting a potential therapeutic target.

Psychostimulant Drugs Activate Cell-type Specific and Topographic cFos Expression in the Lumbar Spinal Cord

Psychostimulant drugs, such as cocaine, d-amphetamine and methylphenidate, alter a wide range of behaviors including locomotor activity and somatosensory perception. These altered behaviors are accompanied by the activation of specific neuronal populations within reward-, emotion- and locomotion-related circuits. However, whether such regulation occurs at the level of the spinal cord, a key node for neural circuits integrating and coordinating sensory and motor functions has never been addressed. By evaluating the temporal and spatial expression pattern of the phosphorylated form of the immediate early gene cFos at Ser32 (pS32-cFos), used as a proxy of neuronal activation, we demonstrate that, in adult male mice, d-amphetamine increases pS32-cFos expression in both inhibitory and excitatory neurons in dorsal and ventral horns at the lumbar spinal cord level. Interestingly, a fraction of neurons activated by a first exposure to d-amphetamine can be re-activated following d-amphetamine re-exposure. Similar expression patterns were observed in response to cocaine and methylphenidate, but not following morphine and dozilcipine administration. Finally, the blockade of dopamine reuptake was sufficient to recapitulate the increase in pS32-cFos expression induced by psychostimulant drugs. Our work provides evidence that cFos expression can be activated in lumbar spinal cord in response to acute psychostimulants administration.

Dopaminergic regulation of hippocampal plasticity, learning, and memory

The hippocampus is responsible for encoding behavioral episodes into short-term and long-term memory. The circuits that mediate these processes are subject to neuromodulation, which involves regulation of synaptic plasticity and local neuronal excitability. In this review, we present evidence to demonstrate the influence of dopaminergic neuromodulation on hippocampus-dependent memory, and we address the controversy surrounding the source of dopamine innervation. First, we summarize historical and recent retrograde and anterograde anatomical tracing studies of direct dopaminergic projections from the ventral tegmental area and discuss dopamine release from the adrenergic locus coeruleus. Then, we present evidence of dopaminergic modulation of synaptic plasticity in the hippocampus. Plasticity mechanisms are examined in brain slices and in recordings from in vivo neuronal populations in freely moving rodents. Finally, we review pharmacological, genetic, and circuitry research that demonstrates the importance of dopamine release for learning and memory tasks while dissociating anatomically distinct populations of direct dopaminergic inputs.

Mossy cells of the dentate gyrus: Drivers or inhibitors of epileptic seizures?

Mossy cells (MCs) are glutamatergic cells of the dentate gyrus with an important role in temporal lobe epilepsy. Under physiological conditions MCs can control both network excitations via direct synapses to granule cells and inhibition via connections to GABAergic interneurons innervating granule cells. In temporal lobe epilepsy mossy cell loss is one of the major hallmarks, but whether the surviving MCs drive or inhibit seizure initiation and generalization is still a debate. The aim of the present review is to summarize the latest findings on the role of mossy cells in healthy and overexcited hippocampus.

Cerebellar dopamine D2 receptors regulate social behaviors

The cerebellum, a primary brain structure involved in the control of sensorimotor tasks, also contributes to higher cognitive functions including reward, emotion and social interaction. Although the regulation of these behaviors has been largely ascribed to the monoaminergic system in limbic regions, the contribution of cerebellar dopamine signaling in the modulation of these functions remains largely unknown. By combining cell-type-specific transcriptomics, histological analyses, three-dimensional imaging and patch-clamp recordings, we demonstrate that cerebellar dopamine D2 receptors (D2Rs) in mice are preferentially expressed in Purkinje cells (PCs) and regulate synaptic efficacy onto PCs. Moreover, we found that changes in D2R levels in PCs of male mice during adulthood alter sociability and preference for social novelty without affecting motor functions. Altogether, these findings demonstrate novel roles for D2R in PC function and causally link cerebellar D2R levels of expression to social behaviors.

Atypical, but Not Typical, Antipsychotic Drugs Reduce Hypersynchronized Prefrontal-Hippocampal Circuits during Psychosis-Like States in Mice: Contribution of 5-HT2A and 5-HT1A Receptors

Neural synchrony and functional connectivity are disrupted in schizophrenia. We investigated changes in prefrontal-hippocampal neural dynamics during psychosis-like states induced by the NMDAR antagonist phencyclidine and subsequent rescue by two atypical antipsychotic drugs (AAPDs), risperidone and clozapine, and the classical APD haloperidol. The psychotomimetic effects of phencyclidine were associated with prefrontal hypersynchronization, hippocampal desynchronization, and disrupted circuit connectivity. Phencyclidine boosted prefrontal oscillatory power at atypical bands within delta, gamma, and high frequency ranges, while irregular cross-frequency and spike-LFP coupling emerged. In the hippocampus, phencyclidine enhanced delta rhythms but suppressed theta oscillations, theta-gamma coupling, and theta-beta spike-LFP coupling. Baseline interregional theta-gamma coupling, theta phase coherence, and hippocampus-to-cortex theta signals were redirected to delta frequencies. Risperidone and clozapine, but not haloperidol, reduced phencyclidine-induced prefrontal and cortical-hippocampal hypersynchrony. None of the substances restored hippocampal and circuit desynchronization. These results suggest that AAPDs, but not typical APDs, target prefrontal-hippocampal pathways to elicit antipsychotic action. We investigated whether the affinity of AAPDs for serotonin receptors could explain their distinct effects. Serotonin 5-HT2AR antagonism by M100907 and 5-HT1AR agonism by 8-OH-DPAT reduced prefrontal hypersynchronization. Our results point to fundamentally different neural mechanisms underlying the action of atypical versus typical APDs with selective contribution of serotonin receptors.

Dopamine D2 receptor regulates cortical synaptic pruning in rodents

Synaptic pruning during adolescence is important for appropriate neurodevelopment and synaptic plasticity. Aberrant synaptic pruning may underlie a variety of brain disorders such as schizophrenia, autism and anxiety. Dopamine D2 receptor (Drd2) is associated with several neuropsychiatric diseases and is the target of some antipsychotic drugs. Here we generate self-reporting Drd2 heterozygous (SR-Drd2+/-) rats to simultaneously visualize Drd2-positive neurons and downregulate Drd2 expression. Time course studies on the developing anterior cingulate cortex (ACC) from control and SR-Drd2+/- rats reveal important roles of Drd2 in regulating synaptic pruning rather than synapse formation. Drd2 also regulates LTD, a form of synaptic plasticity which includes some similar cellular/biochemical processes as synaptic pruning. We further demonstrate that Drd2 regulates synaptic pruning via cell-autonomous mechanisms involving activation of mTOR signaling. Deficits of Drd2-mediated synaptic pruning in the ACC during adolescence lead to hyper-glutamatergic function and anxiety-like behaviors in adulthood. Taken together, our results demonstrate important roles of Drd2 in cortical synaptic pruning.

Modeling intrahippocampal effects of anterior hippocampal hyperactivity relevant to schizophrenia using chemogenetic excitation of long axis-projecting mossy cells in the mouse dentate gyrus

Background

The anterior hippocampus of individuals with early psychosis or schizophrenia is hyperactive, as is the ventral hippocampus in many rodent models for schizophrenia risk. Mossy cells (MCs) of the ventral dentate gyrus (DG) densely project in the hippocampal long axis, targeting both dorsal DG granule cells and inhibitory interneurons. MCs are responsive to stimulation throughout hippocampal subfields and thus may be suited to detect hyperactivity in areas where it originates such as CA1. Here, we tested the hypothesis that hyperactivation of ventral MCs activates dorsal DG granule cells to influence dorsal hippocampal function.

Methods

In CD-1 mice, we targeted dorsal DG-projecting ventral MCs using an adeno-associated virus intersectional strategy. In vivo fiber photometry recording of ventral MCs was performed during exploratory behaviors. We used excitatory chemogenetic constructs to test the effects of ventral MC hyperactivation on long-term spatial memory during an object location memory task.

Results

Photometry revealed that ventral MCs were activated during exploratory rearing. Ventral MCs made functional monosynaptic inputs to dorsal DG granule cells, and chemogenetic activation of ventral MCs modestly increased activity of dorsal DG granule cells measured by c-Fos. Finally, chemogenetic activation of ventral MCs during the training phase of an object location memory task impaired test performance 24 hours later, without effects on locomotion or object exploration.

Conclusions

These data suggest that ventral MC activation can directly excite dorsal granule cells and interfere with dorsal DG function, supporting future study of their in vivo activity in animal models for schizophrenia featuring ventral hyperactivity.

Dorsal and ventral mossy cells differ in their axonal projections throughout the dentate gyrus of the mouse hippocampus

Glutamatergic hilar mossy cells (MCs) have axons that terminate both near and far from their cell body but stay within the DG, making synapses primarily in the molecular layer. The long-range axons are considered the primary projection, and extend throughout the DG ipsilateral to the soma, and project to the contralateral DG. The specificity of MC axons for the inner molecular layer (IML) has been considered to be a key characteristic of the DG. In the present study, we made the surprising finding that dorsal MC axons are an exception to this rule. We used two mouse lines that allow for Cre-dependent viral labeling of MCs and their axons: dopamine receptor D2 (Drd2-Cre) and calcitonin receptor-like receptor (Crlr-Cre). A single viral injection into the dorsal DG to label dorsal MCs resulted in labeling of MC axons in both the IML and middle molecular layer (MML). Interestingly, this broad termination of dorsal MC axons occurred throughout the septotemporal DG. In contrast, long-range axons of ventral MCs terminated in the IML, consistent with the literature. Taken together, these results suggest that dorsal and ventral MCs differ significantly in their axonal projections. Since MC projections in the ML are thought to terminate primarily on GCs, the results suggest a dorsal-ventral difference in MC activation of GCs. The surprising difference in dorsal and ventral MC projections should therefore be considered when evaluating dorsal-ventral differences in DG function.

Mossy Cells in the Dorsal and Ventral Dentate Gyrus Differ in Their Patterns of Axonal Projections

Mossy cells (MCs) of the dentate gyrus (DG) are a major group of excitatory hilar neurons that are important for regulating activity of dentate granule cells. MCs are particularly intriguing because of their extensive longitudinal connections within the DG. It has generally been assumed that MCs in the dorsal and ventral DG have similar patterns of termination in the inner one-third of the dentate molecular layer. Here, we demonstrate that axonal projections of MCs in these two regions are considerably different. MCs in dorsal and ventral regions were labeled selectively with Cre-dependent eYFP or mCherry, using two transgenic mouse lines (including both sexes) that express Cre-recombinase in MCs. At four to six weeks following unilateral labeling of MCs in the ventral DG, a dense band of fibers was present in the inner one-fourth of the molecular layer and extended bilaterally throughout the rostral-caudal extent of the DG, replicating the expected distribution of MC axons. In contrast, following labeling of MCs in the dorsal DG, the projections were more diffusely distributed. At the level of transfection, fibers were present in the inner molecular layer, but they progressively expanded into the middle molecular layer and, most ventrally, formed a distinct band in this region. Optical stimulation of these caudal fibers expressing ChR2 demonstrated robust EPSCs in ipsilateral granule cells and enhanced the effects of perforant path stimulation in the ventral DG. These findings suggest that MCs in the dorsal and ventral DG differ in the distribution of their axonal projections and possibly their function.SIGNIFICANCE STATEMENT Mossy cells (MCs), a major cell type in the hilus of the dentate gyrus (DG), are unique in providing extensive longitudinal and commissural projections throughout the DG. Although it has been assumed that all MCs have similar patterns of termination in the inner molecular layer of the DG, we discovered that the axonal projections of dorsal and ventral MCs differ. While ventral MC projections exhibit the classical pattern, with dense innervation in the inner molecular layer, dorsal MCs have a more diffuse distribution and expand into the middle molecular layer where they overlap and interact with innervation from the perforant path. These distinct locations and patterns of axonal projections suggest that dorsal and ventral MCs may have different functional roles.

Ventro-dorsal Hippocampal Pathway Gates Novelty-Induced Contextual Memory Formation

Novelty facilitates memory formation and is detected by both the dorsal and ventral hippocampus. Although dentate granule cells (GCs) in the dorsal hippocampus are known to mediate the formation of novelty-induced contextual memories, the required pathways and mechanisms remain unclear. Here we demonstrate that a powerful excitatory pathway from mossy cells (MCs) in the ventral hippocampus to dorsal GCs is necessary and sufficient for driving dorsal GC activation in novel environment exploration. In vivo Ca2+ imaging in freely moving mice indicated that this pathway relays environmental novelty. Furthermore, manipulation of ventral MC activity bidirectionally regulates novelty-induced contextual memory acquisition. Our results show that ventral MC activity gates contextual memory formation through an intra-hippocampal interaction activated by environmental novelty.

Translational changes induced by acute sleep deprivation uncovered by TRAP-Seq

Sleep deprivation is a global health problem adversely affecting health as well as causing decrements in learning and performance. Sleep deprivation induces significant changes in gene transcription in many brain regions, with the hippocampus particularly susceptible to acute sleep deprivation. However, less is known about the impacts of sleep deprivation on post-transcriptional gene regulation. To identify the effects of sleep deprivation on the translatome, we took advantage of the RiboTag mouse line to express HA-labeled Rpl22 in CaMKIIα neurons to selectively isolate and sequence mRNA transcripts associated with ribosomes in excitatory neurons. We found 198 differentially expressed genes in the ribosome-associated mRNA subset after sleep deprivation. In comparison with previously published data on gene expression in the hippocampus after sleep deprivation, we found that the subset of genes affected by sleep deprivation was considerably different in the translatome compared with the transcriptome, with only 49 genes regulated similarly. Interestingly, we found 478 genes differentially regulated by sleep deprivation in the transcriptome that were not significantly regulated in the translatome of excitatory neurons. Conversely, there were 149 genes differentially regulated by sleep deprivation in the translatome but not in the whole transcriptome. Pathway analysis revealed differences in the biological functions of genes exclusively regulated in the transcriptome or translatome, with protein deacetylase activity and small GTPase binding regulated in the transcriptome and unfolded protein binding, kinase inhibitor activity, neurotransmitter receptors and circadian rhythms regulated in the translatome. These results indicate that sleep deprivation induces significant changes affecting the pool of actively translated mRNAs.

High Sensitivity Mapping of Cortical Dopamine D2 Receptor Expressing Neurons

Cortical D2 dopamine receptor (Drd2) have mostly been examined in the context of cognitive function regulation and neurotransmission modulation of medial prefrontal cortex by principal neurons and parvalbumin positive, fast-spiking, interneurons in schizophrenia. Early studies suggested the presence of D2 receptors in several cortical areas, albeit with major technical limitations. We used combinations of transgenic reporter systems, recombinase activated viral vectors, quantitative translatome analysis, and high sensitivity in situ hybridization to identify D2 receptor expressing cells and establish a map of their respective projections. Our results identified previously uncharacterized clusters of D2 expressing neurons in limbic and sensory regions of the adult mouse brain cortex. Characterization of these clusters by translatome analysis and cell type specific labeling revealed highly heterogeneous expression of D2 receptors in principal neurons and various populations of interneurons across cortical areas. Transcript enrichment analysis also demonstrated variable levels of D2 receptor expression and several orphan G-protein-coupled receptors coexpression in different neuronal clusters, thus suggesting strategies for genetic and therapeutic targeting of D2 expressing neurons in specific cortical areas. These results pave the way for a thorough re-examination of cortical D2 receptor functions, which could provide information about neuronal circuits involved in psychotic and mood disorders.

Hippocampal mossy cell involvement in behavioral and neurogenic responses to chronic antidepressant treatment

Most antidepressants, including selective serotonin reuptake inhibitors (SSRIs), initiate their drug actions by rapid elevation of serotonin, but they take several weeks to achieve therapeutic onset. This therapeutic delay suggests slow adaptive changes in multiple neuronal subtypes and their neural circuits over prolonged periods of drug treatment. Mossy cells are excitatory neurons in the dentate hilus that regulate dentate gyrus activity and function. Here we show that neuronal activity of hippocampal mossy cells is enhanced by chronic, but not acute, SSRI administration. Behavioral and neurogenic effects of chronic treatment with the SSRI, fluoxetine, are abolished by mossy cell-specific knockout of p11 or Smarca3 or by an inhibition of the p11/AnxA2/SMARCA3 heterohexamer, an SSRI-inducible protein complex. Furthermore, simple chemogenetic activation of mossy cells using Gq-DREADD is sufficient to elevate the proliferation and survival of the neural stem cells. Conversely, acute chemogenetic inhibition of mossy cells using Gi-DREADD impairs behavioral and neurogenic responses to chronic administration of SSRI. The present data establish that mossy cells play a crucial role in mediating the effects of chronic antidepressant medication. Our results indicate that compounds that target mossy cell activity would be attractive candidates for the development of new antidepressant medications.

Dopamine D2-Like Receptors Modulate Intrinsic Properties and Synaptic Transmission of Parvalbumin Interneurons in the Mouse Primary Motor Cortex

Dopamine (DA) plays a crucial role in the control of motor and higher cognitive functions such as learning, working memory, and decision making. The primary motor cortex (M1), which is essential for motor control and the acquisition of motor skills, receives dopaminergic inputs in its superficial and deep layers from the midbrain. However, the precise action of DA and DA receptor subtypes on the cortical microcircuits of M1 remains poorly understood. The aim of this work was to investigate in mice how DA, through the activation of D2-like receptors (D2Rs), modulates the cellular and synaptic activity of M1 parvalbumin-expressing interneurons (PVINs) which are crucial to regulate the spike output of pyramidal neurons (PNs). By combining immunofluorescence, ex vivo electrophysiology, pharmacology and optogenetics approaches, we show that D2R activation increases neuronal excitability of PVINs and GABAergic synaptic transmission between PVINs and PNs in Layer V of M1. Our data reveal how cortical DA modulates M1 microcircuitry, which could be important in the acquisition of motor skills.

Functional and molecular heterogeneity of D2R neurons along dorsal ventral axis in the striatum

Action control is a key brain function determining the survival of animals in their environment. In mammals, neurons expressing dopamine D2 receptors (D2R) in the dorsal striatum (DS) and the nucleus accumbens (Acb) jointly but differentially contribute to the fine regulation of movement. However, their region-specific molecular features are presently unknown. By combining RNAseq of striatal D2R neurons and histological analyses, we identified hundreds of novel region-specific molecular markers, which may serve as tools to target selective subpopulations. As a proof of concept, we characterized the molecular identity of a subcircuit defined by WFS1 neurons and evaluated multiple behavioral tasks after its temporally-controlled deletion of D2R. Consequently, conditional D2R knockout mice displayed a significant reduction in digging behavior and an exacerbated hyperlocomotor response to amphetamine. Thus, targeted molecular analyses reveal an unforeseen heterogeneity in D2R-expressing striatal neuronal populations, underlying specific D2R's functional features in the control of specific motor behaviors.

A Toolbox of Criteria for Distinguishing Cajal-Retzius Cells from Other Neuronal Types in the Postnatal Mouse Hippocampus

The study of brain circuits depends on a clear understanding of the role played by different neuronal populations. Therefore, the unambiguous identification of different cell types is essential for the correct interpretation of experimental data. Here, we emphasize to the broader neuroscience community the importance of recognizing the persistent presence of Cajal-Retzius cells in the molecular layers of the postnatal hippocampus, and then we suggest a variety of criteria for distinguishing Cajal-Retzius cells from other neurons of the hippocampal molecular layers, such as GABAergic interneurons and semilunar granule cells. The toolbox of criteria that we have investigated (in male and female mice) can be useful both for anatomical and functional experiments, and relies on the quantitative study of neuronal somatic/nuclear morphology, location and developmental profile, expression of specific molecular markers (GAD67, reelin, COUP-TFII, calretinin, and p73), single cell anatomy, and electrophysiological properties. We conclude that Cajal-Retzius cells are small, non-GABAergic neurons that are tightly associated with the hippocampal fissure (HF), and that, within this area of interest, selectively express the proteins p73 and calretinin. We highlight the dangers of using markers such as reelin or COUP-TFII to identify Cajal-Retzius cells or GABAergic interneurons because of their poor specificity. Lastly, we examine neurons of the postnatal hippocampal molecular layers and show cell type-specific differences in their dendritic/axonal morphologies and density distributions, as well as in their membrane properties and spontaneous synaptic inputs. These parameters can be used to distinguish biocytin-filled and/or electrophysiologically recorded neurons and should be considered to avoid interpretational mistakes.

Gradient of Expression of Dopamine D2 Receptors Along the Dorso-Ventral Axis of the Hippocampus

Dopamine D2-like receptors (D2R) play an important role in the regulation of hippocampal neuronal excitability and contribute to the regulation of synaptic plasticity, the encoding of hippocampus-dependent memories and the regulation of affective state. In line with this, D2R are targeted in the treatment of psychosis and affective disorders. It has been proposed that the dorso-ventral axis of the hippocampus can be functionally delineated into the dorsal pole that predominantly processes spatial information and the ventral pole that mainly addresses hippocampal processing of emotional and affective state. Although dopaminergic control of hippocampal information processing has been the focus of a multitude of studies, very little is known about the precise distribution of D2R both within anatomically defined sublayers of the hippocampus and along its dorsoventral axis, that could in turn yield insights as to the functional significance of this receptor in supporting hippocampal processing of spatial and affective information. Here, we used an immunohistochemical approach to precisely scrutinize the protein expression of D2R both within the cellular and dendritic layers of the hippocampal subfields, and along the dorso-ventral hippocampal axis. In general, we detected significantly higher levels of protein expression of D2R in the ventral, compared to the dorsal poles with regard to the CA1, CA2, CA3 and dentate gyrus (DG) regions. Effects were very consistent: the molecular layer, granule cell layer and polymorphic layer of the DG exhibited higher D2R levels in the ventral compared to dorsal hippocampus. D2R levels were also significantly higher in the ventral Stratum oriens, Stratum radiatum, and Stratum lacunosum-moleculare layers of the CA1 and CA3 regions. The apical dendrites of the ventral CA2 region also exhibited higher D2R expression compared to the dorsal pole. Taken together, our study suggests that the higher D2R expression levels of the ventral hippocampus may contribute to reported gradients in the degree of expression of synaptic plasticity along the dorso-ventral hippocampal axis, and may support behavioral information processing by the ventral hippocampus.

Serotonin 5-HT1A, 5-HT2A and dopamine D2 receptors strongly influence prefronto-hippocampal neural networks in alert mice: Contribution to the actions of risperidone

Atypical antipsychotic drugs (APDs) used to treat positive and negative symptoms in schizophrenia block serotonin receptors 5-HT_2AR and dopamine receptors D₂R and stimulate 5-HT_1AR directly or indirectly. However, the exact cellular mechanisms mediating their therapeutic actions remain unresolved. We recorded neural activity in the prefrontal cortex (PFC) and hippocampus (HPC) of freely-moving mice before and after acute administration of 5-HT_1AR, 5-HT_2AR and D₂R selective agonists and antagonists and atypical APD risperidone. We then investigated the contribution of the three receptors to the actions of risperidone on brain activity via statistical modeling and pharmacological reversal (risperidone + 5-HT_1AR antagonist WAY-100635, risperidone + 5-HT_2A/2CR agonist DOI, risperidone + D₂R agonist quinpirole). Risperidone, 5-HT_1AR agonism with 8-OH-DPAT, 5-HT_2AR antagonism with M100907, and D₂R antagonism with haloperidol reduced locomotor activity of mice that correlated with a suppression of neural spiking, power of theta and gamma oscillations in PFC and HPC, and reduction of PFC-HPC theta phase synchronization. By contrast, activation of 5-HT_2AR with DOI enhanced high-gamma oscillations in PFC and PFC-HPC high gamma functional connectivity, likely related to its hallucinogenic effects. Together, power changes, regression modeling and pharmacological reversals suggest an important role of 5-HT_1AR agonism and 5-HT_2AR antagonism in risperidone-induced alterations of delta, beta and gamma oscillations, while D₂R antagonism may contribute to risperidone-mediated changes in delta oscillations. This study provides novel insight into the neural mechanisms for widely prescribed psychiatric medication targeting the serotonin and dopamine systems in two regions involved in the pathophysiology of schizophrenia.

Hypothalamic dopamine signaling regulates brown fat thermogenesis

Dopamine signaling is a crucial part of the brain reward system and can affect feeding behavior. Dopamine receptors are also expressed in the hypothalamus, which is known to control energy metabolism in peripheral tissues. Here we show that pharmacological or chemogenetic stimulation of dopamine receptor 2 (D2R) expressing cells in the lateral hypothalamic area (LHA) and the zona incerta (ZI) decreases body weight and stimulates brown fat activity in rodents in a feeding-independent manner. LHA/ZI D2R stimulation requires an intact sympathetic nervous system and orexin system to exert its action and involves inhibition of PI3K in the LHA/ZI. We further demonstrate that, as early as 3 months after onset of treatment, patients treated with the D2R agonist cabergoline experience an increase in energy expenditure that persists for one year, leading to total body weight and fat loss through a prolactin-independent mechanism. Our results may provide a mechanistic explanation for how clinically used D2R agonists act in the CNS to regulate energy balance.

Dentate granule and mossy cells exhibit distinct spatiotemporal responses to local change in a one-dimensional landscape of visual-tactile cues

The dentate gyrus (DG) is critical for detecting changes in environments; however, how granule cells (GCs) and mossy cells (MCs), the two excitatory cell types of the DG, respond to small changes in the object layout is unclear. Here, we recorded GCs and MCs, identified by spike feature and optogenetic tagging, as mice ran on a treadmill belt enriched with visual-tactile cues. We observed that fixing a new cue on the belt induced a reconfiguration of GC and MC spatial representations via the emergence, extinction and rate alteration of firing fields. For both GCs and MCs, the response was maximal near the cue and spread over the entire belt. However, compared to the GC response, the MC response was stronger and more immediate, peaked at a slightly earlier belt position, and exhibited a transient component reminiscent of neuromodulatory activity. A competitive neural network model reproduced the GC response contingent on both the introduction of new object-vector inputs and the reconfiguration of MC activity, the former being critical for spreading the GC response in locations distant from the cue. These findings suggest that GCs operate as a competitive network and that MCs precede GCs in detecting changes and help expand the range of GC pattern separation.

Adult Hippocampal Neurogenesis Can Be Enhanced by Cold Challenge Independently From Beigeing Effects

In this study, we investigated the effects of cold challenge on adult hippocampal neurogenesis (AHN) and hippocampal gene expression and whether these are mediated by beigeing of peripheral fat tissues. Cold challenge (6 ± 2°C) for 1 and 4 weeks was found to induce beigeing effects in inguinal white adipose tissue based on hematoxylin and eosin staining as well as uncoupled protein-1 immunohistochemical staining. In the hippocampus, cold challenge for 1 or 4 weeks increased dentate gyrus neurogenesis and expression of genes related to AHN, including notch signaling, G protein-coupled receptor signaling, and adrenergic beta receptor-1. However, this enhancement of neurogenesis and gene expression by cold challenge was not shown by administration of CL 316,243, which induces peripheral beigeing similar to cold challenge but does not cross the blood-brain barrier. These results suggest that cold challenge promotes AHN and central expression of AHN-related, signaling, and β1-adrenergic receptors genes, and that peripheral beigeing by itself is not sufficient to mediate these effects. Considering the increase in AHN and gene expression changes, cold challenge may offer a novel approach to hippocampal modulation.

Genetic labeling reveals temporal and spatial expression pattern of D2 dopamine receptor in rat forebrain

The D2 dopamine receptor (Drd2) is implicated in several brain disorders such as schizophrenia, Parkinson’s disease, and drug addiction. Drd2 is also the primary target of both antipsychotics and Parkinson’s disease medications. Although the expression pattern of Drd2 is relatively well known in mouse brain, the temporal and spatial distribution of Drd2 is lesser clear in rat brain due to the lack of Drd2 reporter rat lines. Here, we used CRISPR/Cas9 techniques to generate two knockin rat lines: Drd2::Cre and Rosa26::loxp-stop-loxp-tdTomato. By crossing these two lines, we produced Drd2 reporter rats expressing the fluorescence protein tdTomato under the control of the endogenous Drd2 promoter. Using fluorescence imaging and unbiased stereology, we revealed the cellular expression pattern of Drd2 in adult and postnatal rat forebrain. Strikingly, the Drd2 expression pattern differs between Drd2 reporter rats and Drd2 reporter mice generated by BAC transgene in prefrontal cortex and hippocampus. These results provide fundamental information needed for the study of Drd2 function in rat forebrain. The Drd2::Cre rats generated here may represent a useful tool to study the function of neuronal populations expressing Drd2.

A fresh look at cortical dopamine D2 receptor expressing neurons

The dopamine D2 receptor (DRD2) remains the principal target of antipsychotic drugs used for the management of schizophrenia and other psychotic disorders. This receptor is highly expressed within the basal ganglia, more specifically the striatal caudate nucleus and the nucleus accumbens. The general functions, signaling and behavioral contributions of striatal DRD2 are well understood. However, the study of cortical DRD2 expression and functions has for the most part been restricted to a subset of pyramidal neurons and interneurons (e.g. parvalbumine positive) of the pre frontal cortex where DRD2 regulated local circuits are believed to contribute to the regulation of emotional and cognitive functions. The further investigations of cortical DRD2 functions have been hindered by relatively low receptor expression and the sensitivity of detection methods. Here we report recent findings by our group using high sensitivity approaches to map cortical DRD2 expression. Results from these investigations revealed different scales of heterogeneity within DRD2 expressing neurons. These variations affected the types of neurons expressing DRD2 as well as the co-expression of DRD2 with other receptors across several cortical regions. Furthermore several cortical regions showing higher clusters of DRD2 expressing neurons are involved in the regulation of emotional, cognitive and sensory functions that can be involved in the expression of psychotic symptoms. These findings underscore the need for a reexamination of cortical DRD2 mediated synaptic plasticity in the context of schizophrenia and other psychotic disorders.

Mossy Cells Control Adult Neural Stem Cell Quiescence and Maintenance through a Dynamic Balance between Direct and Indirect Pathways

Mossy cells (MCs) represent a major population of excitatory neurons in the adult dentate gyrus, a brain region where new neurons are generated from radial neural stem cells (rNSCs) throughout life. Little is known about the role of MCs in regulating rNSCs. Here we demonstrate that MC commissural projections structurally and functionally interact with rNSCs through both the direct glutamatergic MC-rNSC pathway and the indirect GABAergic MC-local interneuron-rNSC pathway. Specifically, moderate MC activation increases rNSC quiescence through the dominant indirect pathway, while high MC activation increases rNSC activation through the dominant direct pathway. In contrast, MC inhibition or ablation leads to a transient increase of rNSC activation, but rNSC depletion only occurs after chronic ablation of MCs. Together, our study identifies MCs as a critical stem cell niche component that dynamically controls adult NSC quiescence and maintenance under various MC activity states through a balance of direct glutamatergic and indirect GABAergic signaling onto rNSCs.

Linking neuronal structure to function in rodent hippocampus: a methodological prospective

Since the discovery of place cells, hippocampus-dependent spatial navigation has proven to be an ideal model system for resolving the relationship between neural coding and behavior. Electrical recordings from the hippocampal formation in freely moving animals have revealed a rich repertoire of spatial firing patterns and have enormously advanced our understanding of the neural principles of spatial representation. However, limited progress has been achieved in resolving the underlying cellular mechanisms. This is partially attributable to the inability of standard recording techniques to link neuronal structure to function directly. In this review, we summarize recent efforts aimed at filling this gap. We also highlight the development of methodologies that allow functional measurements from identified neuronal elements in behaving rodents. Recent progress in the dentate gyrus serves as a showcase to reveal the potential of such methodologies and the necessity of resolving structure-function relationships in order to access the cellular mechanisms of hippocampal circuit computations.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

Physiological Properties and Behavioral Correlates of Hippocampal Granule Cells and Mossy Cells

The hippocampal dentate gyrus is often viewed as a segregator of upstream information. Physiological support for such function has been hampered by a lack of well-defined characteristics that can identify granule cells and mossy cells. We developed an electrophysiology-based classification of dentate granule cells and mossy cells in mice that we validated by optogenetic tagging of mossy cells. Granule cells exhibited sparse firing, had a single place field, and showed only modest changes when the mouse was tested in different mazes in the same room. In contrast, mossy cells were more active, had multiple place fields and showed stronger remapping of place fields under the same conditions. Although the granule cell-mossy cell synapse was strong and facilitating, mossy cells rarely "inherited" place fields from single granule cells. Our findings suggest that the granule cells and mossy cells could be modulated separately and their joint action may be critical for pattern separation.

Transcriptional dysregulation causes altered modulation of inhibition by haloperidol

Many neuropsychiatric and neurodevelopmental disorders such as schizophrenia and autism involve interneuron transcriptional dysregulation. The transcriptional coactivator PGC-1α regulates gene expression in GABAergic interneurons, which are important for regulating hippocampal network activity. Genetic deletion of PGC-1α causes a decrease in parvalbumin expression, similar to what is observed in schizophrenia postmortem tissue. Our lab has previously shown that PGC-1α-/- mice have enhanced GABAergic inhibition onto CA1 pyramidal cells, which increases the inhibition/excitation (I/E) ratio, alters hippocampal circuit function, and impairs hippocampal dependent behavior. The typical antipsychotic haloperidol, a dopamine receptor antagonist with selectivity for D2-like receptors, has previously been shown to increase excitation in the CA1 region of hippocampus. We therefore tested whether haloperidol could normalize the I/E balance in CA1 of PGC-1α-/- mice, potentially improving circuit function and behavior. Surprisingly, we discovered instead that interneuron transcriptional dysregulation caused by loss of PGC-1α alters the effects of haloperidol on hippocampal synaptic transmission and circuit function. Acute administration of haloperidol causes disinhibition in CA1 and decreases the I/E ratio onto CA1 pyramidal cells in slices from PGC-1α+/+ mice, but not PGC-1α-/- mice. The spread of activity in CA1, assessed by voltage sensitive dye imaging, is increased by haloperidol in slices from PGC-1α+/+ mice; however haloperidol decreases the spread of activity in slices from PGC-1α-/- mice. Haloperidol increased the power of hippocampal gamma oscillation in slices from PGC-1α+/+ mice but reduced the power of gamma oscillations in slices from PGC-1α-/- mice. Nest construction, an innate hippocampal-dependent behavior, is inhibited by haloperidol in PGC-1α+/+ mice, but not in PGC-1α-/- mice, which already have impaired nest building. The effects of haloperidol are mimicked and occluded by a D2 receptor antagonist in slices from PGC-1α+/+ mice, and the effects of blocking D2 receptors are lost in slices from PGC-1α-/- mice, although there is no change in D2 receptor transcript levels. Together, our results show that hippocampal inhibitory synaptic transmission, CA1 circuit function, and hippocampal dependent behavior are modulated by the antipsychotic haloperidol, and that these effects of haloperidol are lost in PGC-1α-/- mice. These results have implications for the treatment of individuals with conditions involving PGC-1α deficiency.

Neuromodulation of the Feedforward Dentate Gyrus-CA3 Microcircuit

The feedforward dentate gyrus-CA3 microcircuit in the hippocampus is thought to activate ensembles of CA3 pyramidal cells and interneurons to encode and retrieve episodic memories. The creation of these CA3 ensembles depends on neuromodulatory input and synaptic plasticity within this microcircuit. Here we review the mechanisms by which the neuromodulators aceylcholine, noradrenaline, dopamine, and serotonin reconfigure this microcircuit and thereby infer the net effect of these modulators on the processes of episodic memory encoding and retrieval.

Anatomical and molecular characterization of dopamine D1 receptor-expressing neurons of the mouse CA1 dorsal hippocampus

In the hippocampus, a functional role of dopamine D1 receptors (D1R) in synaptic plasticity and memory processes has been suggested by electrophysiological and pharmacological studies. However, comprehension of their function remains elusive due to the lack of knowledge on the precise localization of D1R expression among the diversity of interneuron populations. Using BAC transgenic mice expressing enhanced green fluorescent protein under the control of D1R promoter, we examined the molecular identity of D1R-containing neurons within the CA1 subfield of the dorsal hippocampus. In agreement with previous findings, our analysis revealed that these neurons are essentially GABAergic interneurons, which express several neurochemical markers, including calcium-binding proteins, neuropeptides, and receptors among others. Finally, by using different tools comprising cell type-specific isolation of mRNAs bound to tagged-ribosomes, we provide solid data indicating that D1R is present in a large proportion of interneurons expressing dopamine D2 receptors. Altogether, our study indicates that D1Rs are expressed by different classes of interneurons in all layers examined and not by pyramidal cells, suggesting that CA1 D1R mostly acts via modulation of GABAergic interneurons.

The enigmatic mossy cell of the dentate gyrus

Mossy cells comprise a large fraction of the cells in the hippocampal dentate gyrus, suggesting that their function in this region is important. They are vulnerable to ischaemia, traumatic brain injury and seizures, and their loss could contribute to dentate gyrus dysfunction in such conditions. Mossy cell function has been unclear because these cells innervate both glutamatergic and GABAergic neurons within the dentate gyrus, contributing to a complex circuitry. It has also been difficult to directly and selectively manipulate mossy cells to study their function. In light of the new data generated using methods to preferentially eliminate or activate mossy cells in mice, it is timely to ask whether mossy cells have become any less enigmatic than they were in the past.

Development of early-born γ-Aminobutyric acid hub neurons in mouse hippocampus from embryogenesis to adulthood

Early‐born γ‐aminobutyric acid (GABA) neurons (EBGNs) are major components of the hippocampal circuit because at early postnatal stages they form a subpopulation of “hub cells” transiently supporting CA3 network synchronization (Picardo et al. [2011] Neuron 71:695–709). It is therefore essential to determine when these cells acquire the remarkable morphofunctional attributes supporting their network function and whether they develop into a specific subtype of interneuron into adulthood. Inducible genetic fate mapping conveniently allows for the labeling of EBGNs throughout their life. EBGNs were first analyzed during the perinatal week. We observed that EBGNs acquired mature characteristics at the time when the first synapse‐driven synchronous activities appeared in the form of giant depolarizing potentials. The fate of EBGNs was next analyzed in the adult hippocampus by using anatomical characterization. Adult EBGNs included a significant proportion of cells projecting selectively to the septum; in turn, EBGNs were targeted by septal and entorhinal inputs. In addition, most EBGNs were strongly targeted by cholinergic and monoaminergic terminals, suggesting significant subcortical innervation. Finally, we found that some EBGNs located in the septum or the entorhinal cortex also displayed a long‐range projection that we traced to the hippocampus. Therefore, this study shows that the maturation of the morphophysiological properties of EBGNs mirrors the evolution of early network dynamics, suggesting that both phenomena may be causally linked. We propose that a subpopulation of EBGNs forms into adulthood a scaffold of GABAergic projection neurons linking the hippocampus to distant structures. J. Comp. Neurol. 524:2440–2461, 2016. © 2016 Wiley Periodicals, Inc.

Modulating the map: dopaminergic tuning of hippocampal spatial coding and interactions

Salient events activate the midbrain dopaminergic system and have important impacts on various aspects of mnemonic function, including the stability of hippocampus-dependent memories. Dopamine is also central to modulation of neocortical memory processing, particularly during prefrontal cortex-dependent working memory. Here, we review the current state of the circuitry and physiology underlying dopamine's actions, suggesting that--alongside local effects within hippocampus and prefrontal cortex--dopamine released from the midbrain ventral tegmental area is well positioned to dynamically tune interactions between limbic-cortical circuits through modulation of rhythmic network activity.