Molecular Profiling of VGluT1 AND VGluT2 Ventral Subiculum to Nucleus Accumbens Shell Projections

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.

Collateralizing ventral subiculum melanocortin 4 receptor circuits regulate energy balance and food motivation

Hippocampal dysfunction is associated with major depressive disorder, a serious mental illness characterized by not only depressed mood but also appetite disturbance and dysregulated body weight. However, the underlying mechanisms by which hippocampal circuits regulate metabolic homeostasis remain incompletely understood. Here we show that collateralizing melanocortin 4 receptor (MC4R) circuits in the ventral subiculum (vSUB), one of the major output structures of the hippocampal formation, affect food motivation and energy balance. Viral-mediated cell type- and projection-specific input-output circuit mapping revealed that the nucleus accumbens shell (NAcSh)-projecting vSUB^MC4R+ neurons send extensive collateral projections of to various hypothalamic nuclei known to be important for energy balance, including the arcuate, ventromedial and dorsomedial nuclei, and receive monosynaptic inputs mainly from the ventral CA1 and the anterior paraventricular nucleus of thalamus. Chemogenetic activation of NAcSh-projecting vSUB^MC4R+neurons lead to increase in motivation to obtain palatable food without noticeable effect on homeostatic feeding. Viral-mediated restoration of MC4R signaling in the vSUB partially restores obesity in MC4R-null mice without affecting anxiety- and depression-like behaviors. Collectively, these results delineate vSUB^MC4R+ circuits to the unprecedented level of precision and identify the vSUB^MC4R signaling as a novel regulator of food reward and energy balance.

Chronic social defeat stress increases burst firing of nucleus accumbens-projecting ventral subicular neurons in stress-susceptible mice

The ventral subiculum (vSub) is the major output structure of the hippocampus and serves as a main limbic region in mediating the brain's response to stress. Previously, we reported that there are three subtypes of vSub neurons based on their firing patterns: regular-spiking (RS), weak-bursting (WB) and strong-bursting (SB) neurons and chronic social defeat stress (CSDS) increased SB neurons especially in the proximal vSub. Here, we found that neurons in the proximal vSub projected to the nucleus accumbens (NAc). CSDS significantly increased SB neurons but decreased RS neurons among the NAc-projecting vSub neuronal population. Interestingly, these changes were only apparent in mice susceptible to CSDS, but not in CSDS-resilient ones. Given that ventral hippocampal inputs to the NAc regulate susceptibility to CSDS, the bursting activity of NAc-projecting vSub neurons might be functionally relevant to behavioral susceptibility to CSDS.

Chronic social defeat stress-induced enhancement of T-type calcium channels increases burst-firing neurons in the ventral subiculum

The ventral subiculum (vSub), a representative output structure of the hippocampus, serves as a main limbic region in mediating the brain's response to stress. There are three subtypes of subicular pyramidal neurons based on their firing patterns: regular-spiking (RS), weak-bursting (WB) and strong-bursting (SB) neurons, located differently along proximal-distal axis. Here, we found that chronic social defeat stress (CSDS) in mice increased the population of SB neurons but decreased RS neurons in the proximal vSub. Specific blockers of T-type calcium channels inhibited the burst firings with a concomitant reduction of afterdepolarization, suggesting that T-type calcium channels underlie the burst-spiking activity. Consistently, CSDS increased both T-type calcium currents and expression of Cav3.1 proteins, a subtype of T-type calcium channels, in the proximal vSub. Therefore, we conclude that CSDS-induced enhancement of Cav3.1 expression increased bursting neuronal population in the vSub, which may contribute to stress-related behaviors.

Activation of the ventral subiculum reinvigorates behavior after failure to achieve a goal: Implications for dopaminergic modulation of motivational processes

Previous studies confirm that brief electrical stimulation of glutamatergic afferents from the ventral subiculum (vSub) can significantly enhance dopamine release in the ventral striatum for an extended duration (>20 min). However, the functional significance of this effect on motivated behavior remains to be specified. Here we tested the hypothesis that brief electrical stimulation of the ventral subiculum (20 Hz for 10 s) might increase effort expenditure for food rewards. Motivation was assessed by a progressive ratio lever pressing task, which requires continuous escalation of the numbers of lever presses to receive each subsequent sucrose pellet, eventually resulting in the failure to achieve the required ratio for a food reward. vSub stimulation at the start of a session did not affect the rate or total number of lever presses prior to reaching the "break point". In contrast, stimulation of the vSub with identical parameters on a post break point trial resulted in a significant increase in total responses. These findings demonstrate that activation of the vSub with parameters that modulate dopamine efflux in the nucleus accumbens can re-activate goal-directed behavior after failure to achieve a goal. Our data highlight a possible role for the vSub in the pathophysiology and potential treatment of motivational processes linked to psychiatric disease.

Brain-wide map of projections from mice ventral subiculum

The hippocampal formation plays a critical role in episodic memory formation and spatial navigation. Within the hippocampus, the subiculum is considered to be a hub connecting the hippocampal formation to the remainder of the brain. There are functional differences between the dorsal and ventral part of subiculum, while the ventral subiculum (vSub) plays a role in anxiety, stress and emotion. In the present study, we examined the projection of the ventral subiculum to the whole brain in mice by using a modified herpes simplex virus 1 strain H129 with an inserted fluorescent protein gene. In our experiments, the modified H129 transits the primary-order, second-order, and third-order neuronal projections at 36-44, 52-60 and 68-76h after inoculation in mice, respectively. Our data revealed that vSub directly projects to the medial entorhinal cortex, amygdalohippocampal area, anterodorsal thalamic nucleus, medial hypothalamus, supramammillary nucleus, medial septal nucleus and adjacent diagonal band, the connections between median raphe nucleus and interpeduncular nucleus in brain stem, while ventral prefrontal cortex, laterodorsal tegmental nucleus and locus coeruleus receives second-order projections from vSub. Our data would help further understanding the functional connections of vSub with other brain regions.

Long term exposure to combination paradigm of environmental enrichment, physical exercise and diet reverses the spatial memory deficits and restores hippocampal neurogenesis in ventral subicular lesioned rats

Subiculum is an important structure of the hippocampal formation and plays an imperative role in spatial learning and memory functions. We have demonstrated earlier the cognitive impairment following bilateral ventral subicular lesion (VSL) in rats. We found that short term exposure to enriched environment (EE) did not help to reverse the spatial memory deficits in water maze task suggesting the need for an appropriate enriched paradigm towards the recovery of spatial memory. In the present study, the efficacy of long term exposure of VSL rats to combination paradigm of environmental enrichment (EE), physical exercise and 18 C.W. diet (Combination Therapy - CT) in reversing the spatial memory deficits in Morris water maze task has been studied. Ibotenate lesioning of ventral subiculum produced significant impairment of performance in the Morris water maze and reduced the hippocampal neurogenesis in rats. Post lesion exposure to C.T. restored the hippocampal neurogenesis and improved the spatial memory functions in VSL rats. Our study supports the hypothesis that the combination paradigm is critical towards the development of an enhanced behavioral and cognitive experience especially in conditions of CNS insults and the associated cognitive dysfunctions.

Cholinergic immunotoxin 192 IgG-SAPORIN alters subicular theta-gamma activity and impairs spatial learning in rats

Subiculum is an important structure of hippocampal formation and is a part of intra hippocampal network involved in spatial information processing. However, relatively very few studies are available in literature demonstrating the explicit role of subiculum in spatial information processing. The present study investigated the cholinergic modulation of subicular theta-gamma activity on spatial learning and memory functions in rats. The cholinergic projections to ventral subiculum were selectively eliminated using 192 IgG-SAPORIN. Eliminations of cholinergic inputs to ventral subiculum significantly reduced the subicular theta and enhanced the gamma activity during active wake and REM sleep states. In addition, the spatial learning was severely impaired following cholinergic elimination of ventral subiculum. The ChAT immunocytochemical studies showed sparse distribution of cholinergic fibers in the ventral subiculum confirming the cholinergic elimination to ventral subiculum. Cholinotoxic infusions to ventral subiculum did not alter the hippocampal cholinergic innervations and retained the hippocampal theta and gamma activities. The present findings support that cholinergic modulation of subicular theta-gamma oscillations is crucial for spatial information processing.

Footshock-induced responses in ventral subiculum neurons are mediated by locus coeruleus noradrenergic afferents

The ventral subiculum (vSub) of the hippocampus is critically involved in mediating the forebrain's response to stress, particularly with regard to psychogenic stressors. Stress, in turn, is known to aggravate many psychiatric conditions including schizophrenia, depression, anxiety, and drug abuse. Pathological alterations in hippocampal function have been identified in all these disorders; thus, it is of interest to understand how stress affects this brain region. The vSub receives dense projections from the stress-related locus coeruleus (LC); however, it is not known what role this input plays in signaling stressful stimuli. In this study, the direct LC innervation of the vSub was investigated as a potential mediator of stress responses in this region. To examine responses to an acute stressor, the effect of footshock on single vSub neurons was tested in rats. Footshock inhibited 13%, and activated 48% of neurons in this region. Importantly, responses to footshock were correlated with LC stimulation-evoked responses in single neurons, and LC inactivation blocked these responses. Furthermore, prazosin, an alpha-1 antagonist, reversed footshock-evoked inhibition, revealing an underlying activation. Inactivation of the basolateral amygdala (BLA) did not block phasic footshock-evoked activation; however, it reduced tonic activity in the vSub. These results suggest that the LC NE system plays an important role in mediating stress responses in the vSub. Footshock evokes both inhibition and excitation in the vSub, by activating noradrenergic inputs from the LC. These responses may contribute to stress adaptation; while an imbalance of this system may lead to pathological stress responses in mental disorders.