Sex Differences in the Ventral Subiculum's Microcircuitry and Function.

Temporal lobe epilepsy is the most common form of epilepsy in adults. The pilocarpine-treated rat model is used frequently to investigate temporal lobe epilepsy. The validity of the pilocarpine model has been challenged based largely on concerns that seizures might initiate in different brain regions in rats than in patients. The present study used 32 recording electrodes per rat to evaluate spontaneous seizures in various brain regions including the septum, dorsomedial thalamus, amygdala, olfactory cortex, dorsal and ventral hippocampus, substantia nigra, entorhinal cortex, and ventral subiculum. Compared with published results from patients, seizures in rats tended to be shorter, spread faster and more extensively, generate behavioral manifestations more quickly, and produce generalized convulsions more frequently. Similarities to patients included electrographic waveform patterns at seizure onset, variability in sites of earliest seizure activity within individuals, and variability in patterns of seizure spread. Like patients, the earliest seizure activity in rats was recorded most frequently within the hippocampal formation. The ventral hippocampus and ventral subiculum displayed the earliest seizure activity. Amygdala, olfactory cortex, and septum occasionally displayed early seizure latencies, but not above chance levels. Substantia nigra and dorsomedial thalamus demonstrated consistently late seizure onsets, suggesting their unlikely involvement in seizure initiation. The results of the present study reveal similarities in onset sites of spontaneous seizures in patients with temporal lobe epilepsy and pilocarpine-treated rats that support the model's validity.

Role of muscarinic receptors in the activation of the ventral subiculum and the consequences for dopamine release in the nucleus accumbens

We compared neuronal activation, as measured by Fos staining, during different spatial tasks in two experiments. The counts of Fos-stained neurons in the hippocampus increased as the spatial demands of the tasks increased, the tasks having been carefully matched for other factors. In Experiment 1, matched groups of rats either ran a standard eight-arm radial maze task or were trained to run up and down just one arm of the maze; the number of runs and rewards was identical in both conditions. In Experiment 2, rats were trained on the eight-arm maze but in different rooms. On the critical test day, both groups were run in the same room so that one group now performed with novel landmarks. All hippocampal subfields (dentate gyrus, CA3, CA1, dorsal, ventral, and caudal subiculum) showed a relative increases in c-fos activation in the eight-arm (Experiment 1) and novel room (Experiment 2) conditions, the sole exception being the ventral subiculum in Experiment 2. Although increased c-fos activation was found in both dorsal and ventral hippocampus, in Experiment 2 the relative increase was significantly greater in the dorsal hippocampus. Parahippocampal cortices responded heterogeneously: the perirhinal cortex failed to show increased activation in both experiments, in contrast to the entorhinal and postrhinal cortices. Subsequent comparisons confirmed that the perirhinal and postrhinal cortices responded in qualitatively different ways, the perirhinal cortex differing from the rest of the hippocampal formation. These experiments, which provide the first analysis of hippocampal Fos production during tests of allocentric spatial working memory, reveal that all components of the hippocampus are activated, but that under certain conditions the dorsal hippocampus is disproportionately involved.

Ventral subiculum control of avoidance behavior and hypothalamic-pituitary-adrenal axis reactivity via the bed nucleus of the stria terminalis in male and female mice – ISPNE 2024 Dirk Helhammer Award

PSYCHIATRIC SYMPTOMS | Behavioral Hyperactivity and Psychiatric Symptoms Induced by Seizures

Aversive Contexts Reduce Activity in the Ventral Subiculum- BNST Pathway

Hippocampal hyperactivity driven by GABAergic interneuron deficits and NMDA receptor hypofunction is associated with the hyperdopaminergic state often observed in schizophrenia. Furthermore, previous research in the methylazoxymethanol acetate (MAM) rat model has demonstrated that repeated peripubertal diazepam administration can prevent the emergence of adult hippocampal hyperactivity, dopamine-system hyperactivity, and associated psychosis-relevant behaviors. Here, we sought to characterize hippocampal GABAA and NMDA receptors in MAM-treated rats and to elucidate the receptor mechanisms underlying the promising effects of peripubertal diazepam exposure. Quantitative receptor autoradiography was used to measure receptor density in the dorsal hippocampus CA1, ventral hippocampus CA1, and ventral subiculum. Specifically, [3H]-Ro15-4513 was used to quantify the density of α5GABAA receptors (α5GABAAR), [3H]-flumazenil to quantify α1-3;5GABAAR, and [3H]-MK801 to quantify NMDA receptors. MAM rats exhibited anxiety and schizophrenia-relevant behaviors as measured by elevated plus maze and amphetamine-induced hyperlocomotion (AIH), although diazepam only partially rescued these behaviors. α5GABAAR density was reduced in MAM-treated rats in all hippocampal sub-regions, and negatively correlated with AIH. Ventral hippocampus CA1 α5GABAAR density was positively correlated with anxiety-like behavior. Dorsal hippocampus CA1 NMDA receptor density was increased in MAM-treated rats, and positively correlated with AIH. [3H]-flumazenil revealed no significant effects. Finally, we found no significant effect of diazepam treatment on receptor densities, potentially related to the only partial rescue of schizophrenia-relevant phenotypes. Overall, our findings provide first evidence of α5GABAAR and NMDA receptor abnormalities in the MAM model, suggesting that more selective pharmacological agents may become a novel therapeutic mechanism in schizophrenia.

Anatomical and Functional Characterization of the Ventral Hippocampus in a Rodent Model of Schizophrenia Neuropathology Kelley Elizabeth Remole Schizophrenia is a debilitating, life-long illness with a still-unknown, complex etiology and, currently, no cure. Many studies have implicated the hippocampus and the parahippocampal region as a place of both primary pathology in the disease and as regions correlated to symptom severity. To better understand the pathophysiology of the region and potentially uncover mechanisms of the disease, the appropriate choice of an animal model is essential. The “MAM E17” model of hippocampal pathology shows anatomical, neurophysiological, and behavioral changes relevant to schizophrenia. Because of these wide-ranging disease-relevant changes, we aimed to relate anatomical to neurophysiological phenotypes in this model. We also performed experiments to assess the feasibility and validity of transferring the MAM E17 model to the mouse in order enable future studies of the genetic basis of the vulnerability or resilience to MAM. In adult offspring of rats exposed to to methylazoxymethanol (MAM) at embryonic day 17 (E17), we found changes in regional hippocampal anatomy and subicular pyramidal cell morphology with homology to abnormalities reported in schizophrenia. Specifically, we found a decrease in dendritic spine density in specific regions of the dendrite of ventral subicular neurons. At the neurophysiological level, we observed abnormalities in afferent-evoked synaptic responses in the ventral subiculum. These changes were not however, accompanied by changes in in vivo spontaneous spike activity in subicular neurons . In the mouse, MAM was found to have much less impact on brain development, as observed at the gross morphological level. However, these mice showed an increased sensitivity to some psychostimulants and a weak trend for metabolic abnormalities relevant to schizophrenia. We conclude from the rat studies that prenatal disruption of brain development by MAM at E17 in the rat, a manipulation that leads to a profile of gross anatomical and cognitive deficits relevant to schizophrenia, also leads to „dysconnectivity‟ between the ventral subiculum and its inputs. While further work is needed to understand this, we speculate that this synaptic dysconnectivity may contribute to the cognitive deficits in this model and, further, may model an aspect of hippocampal pathophysiology in schizophrenia. A better understanding of these circuits could point to new strategies for treating this disease.

Experiment 1 examined the effects of electrical stimulation of nucleus basalis magnocellularis (NBM) on a relational odor-association task--the social transmission of food preference (STFP). Rats were stimulated unilaterally in the NBM for 20 min (100 microA, 1 Hz) immediately before the social training. They were tested on their ability to remember preference for the trained food either immediately or following a 24-h delay. Stimulation of NBM improved retention regardless of delay, and additional behavioral measures (social interaction, motor activity, or exploration) did not account for such effects. Experiment 2 investigated brain regions activated after NBM electrical stimulation by examining the induction of c-Fos. This treatment led to bilateral increased c-Fos expression in prefrontal regions, such as orbitofrontal, prelimbic, and infralimbic cortices, and some hippocampal subregions (dorsal CA and ventral dentate gyrus). In contrast, no differences between groups in c-Fos expression were found in basolateral amygdala, dorsal dentate gyrus, ventral CA, or ventral subiculum. Present findings indicate that pretraining NBM electrical stimulation facilitates the acquisition of STFP, supporting a role of NBM in the early stages of memory formation, and suggest that the treatment might cause such effects by inducing neural changes, related to transcription factors such as c-Fos, in the prefrontal cortex or the hippocampal formation.

Several lines of evidence indicate that estrogen affects hippocampal synaptic plasticity through rapid nongenomic mechanisms, possibly by binding to plasma membrane estrogen receptors (ERs). We have previously shown that ERalpha immunoreactivity (ir) is in select interneuron nuclei and in several extranuclear locations, including dendritic spines and axon terminals, within the rat hippocampal formation (Milner et al., [2001] J Comp Neurol 429:355). The present study sought to determine the cellular and subcellular locations of ERbeta-ir. Coronal hippocampal sections from diestrus rats were immunolabeled with antibodies to ERbeta and examined by light and electron microscopy. By light microscopy, ERbeta-ir was primarily in the perikarya and proximal dendrites of pyramidal and granule cells. ERbeta-ir was also in a few nonprincipal cells and scattered nuclei in the ventral subiculum and CA3 region. Ultrastructural analysis revealed ERbeta-ir at several extranuclear sites in all hippocampal subregions. ERbeta-ir was affiliated with cytoplasmic organelles, especially endomembranes and mitochondria, and with plasma membranes primarily of principal cell perikarya and proximal dendrites. ERbeta-ir was in dendritic spines, many arising from pyramidal and granule cell dendrites. In both dendritic shafts and spines, ERbeta-ir was near the perisynaptic zone adjacent to synapses formed by unlabeled terminals. ERbeta-ir was in preterminal axons and axon terminals, associated with clusters of small, synaptic vesicles. ERbeta-labeled terminals formed both asymmetric and symmetric synapses with dendrites. ERbeta-ir also was detected in glial profiles. The cellular and subcellular localization of ERbeta-ir was generally similar to that of ERalpha, except that ERbeta was more extensively found at extranuclear sites. These results suggest that ERbeta may serve primarily as a nongenomic transducer of estrogen actions in the hippocampal formation.

Sex specific recruitment of a medial prefrontal cortex-hippocampal-thalamic system during context-dependent renewal of responding to food cues in rats

The nucleus accumbens shell is a critical node in reward circuitry, encoding environments associated with reward. Long-range inputs from the ventral hippocampus (ventral subiculum) to the nucleus accumbens shell have been identified, yet their precise molecular phenotype remains to be determined. Here we used retrograde tracing to identify the ventral subiculum as the brain region with the densest glutamatergic (VGluT1-Slc17a7) input to the shell. We then used circuit-directed translating ribosome affinity purification to examine the molecular characteristics of distinct glutamatergic (VGluT1, VGluT2-Slc17a6) ventral subiculum to nucleus accumbens shell projections. We immunoprecipitated translating ribosomes from this population of projection neurons and analysed molecular connectomic information using RNA sequencing. We found differential gene enrichment across both glutamatergic projection neuron subtypes. In VGluT1 projections, we found enrichment of Pfkl, a gene involved in glucose metabolism. In VGluT2 projections, we found a depletion of Sparcl1 and Dlg1, genes known to play a role in depression- and addiction-related behaviours. These findings highlight potential glutamatergic neuronal-projection-specific differences in ventral subiculum to nucleus accumbens shell projections. Together these data advance our understanding of the phenotype of a defined brain circuit.

Ventral subiculum (vSUB) is the major output region of ventral hippocampus (vHIPP) and sends major projections to nucleus accumbens medial shell (NAcMS). Hyperactivity of the vSUB-NAcMS circuit is associated with substance use disorders and the modulation of vSUB activity alters drug seeking and drug reinstatement behavior in rodents. However, to the best of our knowledge, the cell type-specific connectivity and synaptic transmission properties of the vSUB-NAcMS circuit have never been directly examined. Instead, previous functional studies have focused on total ventral hippocampal (vHIPP) output to NAcMS without distinguishing vSUB from other subregions of vHIPP, including ventral CA1 (vCA1). Using ex vivo electrophysiology, we systematically characterized the vSUB-NAcMS circuit with cell type- and synapse-specific resolution in male and female mice and found that vSUB output to dopamine receptor type-1 (D1R) and type-2 (D2R) expressing medium spiny neurons (MSNs) displays a functional connectivity bias for D2R MSNs. Furthermore, we found that vSUB-D1R and vSUB-D2R MSN synapses contain calcium-permeable AMPA receptors in drug-naive mice. Finally, we find that, distinct from other glutamatergic inputs, cocaine exposure selectively induces plasticity at vSUB-D2R synapses. Importantly, we directly compared vSUB and vCA1 output to NAcMS and found that vSUB synapses are functionally distinct and that vCA1 output recapitulated the synaptic properties previously ascribed to vHIPP. Our work highlights the need to consider the contributions of individual subregions of vHIPP to substance use disorders and represents an important first step toward understanding how the vSUB-NAcMS circuit contributes to the etiologies that underlie substance use disorders.SIGNIFICANCE STATEMENT Inputs to nucleus accumbens (NAc) dopamine receptor type 1 (D1R) and D2R medium spiny neurons (MSNs) are critically involved in reward seeking behavior. Ventral subiculum (vSUB) provides robust synaptic input to nucleus accumbens medial shell (NAcMS) and activity of this circuit is linked to substance use disorders. Despite the importance of the vSUB to nucleus accumbens circuit, the functional connectivity and synaptic transmission properties have not been tested. Here, we systematically interrogated these properties and found that basal connectivity and drug-induced plasticity are biased for D2R medium spiny neurons. Overall, we demonstrate that this circuit is distinct from synaptic inputs from other brain regions, which helps to explain how vSUB dysfunction contributes to the etiologies that underlie substance use disorders.

Anatomical studies indicate that the ventral subiculum is in a prime position to mediate hippocampal inhibition of the hypothalamo-pituitary-adrenocortical (HPA) axis. The present study evaluated this hypothesis by assessing HPA function following ibotenic acid lesion of the ventral subiculum region. Rats with lesions of the ventral subiculum (vSUB) or ventral hippocampus (vHIPPO) did not show changes in basal corticosterone (CORT) secretion at either circadian peak or nadir time points when compared to sham-lesion rats (SHAM) or unoperated controls. However, rats with vSUB lesions exhibited a prolonged glucocorticoid stress response relative to all other groups. Baseline CRH mRNA levels were significantly increased in the medial parvocellular paraventricular nucleus (PVN) of the vSUB group relative to controls. CRH mRNA differences were particularly pronounced at caudal levels of the nucleus, suggesting topographic organization of vSUB interactions with PVN neurons. Notably, the vHIPPO group, which received large lesions of ventral CA1, CA3 and dentate gyrus without significant subicular damage, showed no change in stress-induced CORT secretion, suggesting that the ventral subiculum proper is principally responsible for ventral hippocampal actions on the HPA stress response. No differences in medial parvocellular PVN AVP mRNA expression were seen in either the vSUB or vHIPPO groups. The results indicate a specific inhibitory action of the ventral subiculum on HPA activation. The increase in CRH biosynthesis and stress-induced CORT secretion in the absence of changes in baseline CORT secretion or AVP mRNA expression suggests that the inhibitory actions of ventral subicular neurons affect the response capacity of the HPA axis.

Background:Loss of parvalbumin interneurons in the hippocampus is a robust finding in schizophrenia brains. Rats exposed during embryonic day 17 to methylazoxymethanol acetate exhibit characteristics consistent with an animal model of schizophrenia, including decreased parvalbumin interneurons in the ventral hippocampus. We reported previously that peripubertal administration of diazepam prevented the emergence of pathophysiology in adult methylazoxymethanol acetate rats.Methods:We used an unbiased stereological method to examine the impact of peripubertal diazepam treatment on parvalbumin interneuron number in the ventral subiculum, dentate gyrus of the hippocampus and the basolateral amygdala.Results:Methylazoxymethanol acetate rats with peripubertal diazepam showed significantly more parvalbumin interneurons (3355±173 in the ventral subiculum, 1211±76 in the dentate gyrus) than methylazoxymethanol acetate without diazepam (2375±109 and 824±54, respectively). No change was found in the basolateral amygdala.Conclusions:Peripubertal diazepam attenuated the decrease of parvalbumin in the ventral hippocampus of methylazoxymethanol acetate rats.