Functional structure of the intermediate and ventral hippocampo-prefrontal pathway in the prefrontal convergent system

Prelimbic proBDNF Facilitates Retrieval-Dependent Fear Memory Destabilization by Regulation of Synaptic and Neural Functions in Juvenile Rats

Fear regulation changes as a function of the early life is a key developmental period for the continued maturation of fear neural circuitry. The mechanisms of fear retrieval-induced reconsolidation have been investigated but remain poorly understood. The involvement of prelimbic proBDNF in fear memory extinction and its mediated signaling have been reported previously. Specifically, blocking the proBDNF/p75^NTR pathway during the postnatal stage disrupts synaptic development and neuronal activity in adulthood. Given the inherent high expression of proBDNF during the juvenile period, we tested whether the prelimbic proBDNF regulated synaptic and neuronal functions allowing to influencing retrieval-dependent memory processing. By examining the freezing behavior of auditory fear-conditioned rats, we found the high level of the prelimbic proBDNF in juvenile rats enhanced the destabilization of the retrieval-dependent weak but not strong fear memory through activating p75^NTR-GluN2B signaling. This modification of fear memory traces was attributed to the increment in the proportion of thin-type spine and promotion in synaptic function, as evidenced by the facilitation of NMDA-mediated EPSCs and GluN2B-dependent synaptic depression at the prelimbic projection. Furthermore, the strong prelimbic theta- and gamma-oscillation coupling predicted the suppressive effect of juvenile proBDNF on the recall of postretrieval memory. Our results critically emphasize the importance of developmental proBDNF for modification of retrieval-dependent memory and provide a potential critical targeting to inhibit threaten memories associated with neurodevelopment disorders.

Neuromodulation of Hippocampal-Prefrontal Cortical Synaptic Plasticity and Functional Connectivity: Implications for Neuropsychiatric Disorders

The hippocampus-prefrontal cortex (HPC-PFC) pathway plays a fundamental role in executive and emotional functions. Neurophysiological studies have begun to unveil the dynamics of HPC-PFC interaction in both immediate demands and long-term adaptations. Disruptions in HPC-PFC functional connectivity can contribute to neuropsychiatric symptoms observed in mental illnesses and neurological conditions, such as schizophrenia, depression, anxiety disorders, and Alzheimer's disease. Given the role in functional and dysfunctional physiology, it is crucial to understand the mechanisms that modulate the dynamics of HPC-PFC communication. Two of the main mechanisms that regulate HPC-PFC interactions are synaptic plasticity and modulatory neurotransmission. Synaptic plasticity can be investigated inducing long-term potentiation or long-term depression, while spontaneous functional connectivity can be inferred by statistical dependencies between the local field potentials of both regions. In turn, several neurotransmitters, such as acetylcholine, dopamine, serotonin, noradrenaline, and endocannabinoids, can regulate the fine-tuning of HPC-PFC connectivity. Despite experimental evidence, the effects of neuromodulation on HPC-PFC neuronal dynamics from cellular to behavioral levels are not fully understood. The current literature lacks a review that focuses on the main neurotransmitter interactions with HPC-PFC activity. Here we reviewed studies showing the effects of the main neurotransmitter systems in long- and short-term HPC-PFC synaptic plasticity. We also looked for the neuromodulatory effects on HPC-PFC oscillatory coordination. Finally, we review the implications of HPC-PFC disruption in synaptic plasticity and functional connectivity on cognition and neuropsychiatric disorders. The comprehensive overview of these impairments could help better understand the role of neuromodulation in HPC-PFC communication and generate insights into the etiology and physiopathology of clinical conditions.

Neurocircuit differences between memory traces of persistent hypoactivity and freezing following fear conditioning among the amygdala, hippocampus, and prefrontal cortex

We aimed to investigate the persistent trace of one traumatic event on neurocircuit controls in rats. Conditioning was reflected by reductions in rates of 'freezing' and 'other-than-freezing' motor activities, between which rats could alternate on delivery of pulsed footshocks of intensity 0.5 mA but not 1.0 mA. At the latter intensity, freezing began to suppress motor activity. The conditional responses evident during both the context and tone sessions persisted when the tests were repeated on post-conditioning days 7 and 8. Thus, difficulties with fear extinction/reduction remained. However, persistence was not evident on post-conditioning days 1 and 2. One day after the 1.0 mA pulsed footshock, ibotenate lesions and corresponding sham surgeries were performed in unilateral and bilateral hemispheres of the amygdala, hippocampus, and prefrontal cortex, as well as three different disconnections (one unilateral and another contralateral lesions out of three regions, a total of nine groups), and were tested on days 7-8. The drastic restoration of freezing following bilateral amygdala lesions was also evident in animals with three types of disconnection; however, this was not the case for hypoactivity. These results imply that a serious experience can drive different neurocircuits that all involve the amygdala, forming persistent concurrent memories of explicit (e.g., 'freezing') or implicit (e.g., 'other-than-freezing' motor activity) emotions, which may exhibit mutual interference.

Early life esophageal acid exposure reduces expression of NMDAR1 in the adult rat dorsal hippocampus and medial prefrontal cortex: Potential relationship with hyperlocomotion

OBJECTIVE

Early life esophageal acid exposure causes long-term molecular alterations in the rostral cingulate cortex; however, whether it induces behavioral changes remains unverified. Little is known about the molecular changes resulting from this event in the developing hippocampus and medial prefrontal cortex (mPFC). This study aimed to investigate the influence of early life esophageal acid exposure on spontaneous locomotor behavior and N-methyl-D-aspartate receptor (NMDAR), expression in these brain regions of adult rats.

METHODS

Male Sprague-Dawley rats were administered with an esophageal acid or saline infusion once per day (postnatal days 7-14). Some of these rats were given acute esophageal acid rechallenge in adulthood (postnatal day 60). The spontaneous locomotor behavior and expressions of esophageal epithelial caludin-1 and NMDAR subunits in the dorsal hippocampus (DH), ventral hippocampus (VH) and mPFC of the adult rats were recorded.

RESULTS

Neonatal esophageal acid stimulation caused long-term impairment of the tight junctions in the adult esophagus. Simultaneously, hyperlocomotion and reduced expression of NMDAR1 subunits in both the DH and mPFC were observed, but not in the VH regions. Adult acute acid rechallenge reversed the decreased NMDAR1 expression in the DH and mPFC. The glycine ligand to NMDAR1 subunits was also changed.

CONCLUSIONS

Peripheral visceral stimulation such as esophageal acid exposure during cerebral development induces increased locomotor activity, which may be related to the alteration of central sensitivity via NMDAR1 subunit reduction in the DH and mPFC. The impairment of tight junctions in the esophageal epithelium may contribute to the formation of central neuroplasticity.

The dynamics of disordered dialogue: Prefrontal, hippocampal and thalamic miscommunication underlying working memory deficits in schizophrenia

The prefrontal cortex is central to the orchestrated brain network communication that gives rise to working memory and other cognitive functions. Accordingly, working memory deficits in schizophrenia are increasingly thought to derive from prefrontal cortex dysfunction coupled with broader network disconnectivity. How the prefrontal cortex dynamically communicates with its distal network partners to support working memory and how this communication is disrupted in individuals with schizophrenia remain unclear. Here we review recent evidence that prefrontal cortex communication with the hippocampus and thalamus is essential for normal spatial working memory, and that miscommunication between these structures underlies spatial working memory deficits in schizophrenia. We focus on studies using normal rodents and rodent models designed to probe schizophrenia-related pathology to assess the dynamics of neural interaction between these brain regions. We also highlight recent preclinical work parsing roles for long-range prefrontal cortex connections with the hippocampus and thalamus in normal and disordered spatial working memory. Finally, we discuss how emerging rodent endophenotypes of hippocampal- and thalamo-prefrontal cortex dynamics in spatial working memory could translate into richer understanding of the neural bases of cognitive function and dysfunction in humans.

The effect of hippocampal NMDA receptor blockade by MK-801 on cued fear extinction

Extinction of conditioned fear has been suggested to be a new form of learning instead of erasure of what was originally learned, and the process is NMDA (N-methyl d-aspartate) receptor (NMDAR) dependent. Most of studies have so far revealed the important roles of NMDARs in the amygdala and medial prefrontal cortex (mPFC) in cued fear extinction. Although the ventral hippocampus has intimately reciprocal connections with the amygdala and mPFC, the role of its NMDARs in cued fear extinction remains unclear. The present experiment explored the issue by bilateral pre-extinction microinjection of the noncompetitive NMDAR antagonist MK-801 into the ventral hippocampus. Four groups of rats were given habituation, tone cued fear conditioning, fear extinction training and extinction test. Prior to extinction training, rats received bilateral infusions of either MK-801 (1.5, 3, or 6μg/0.5μl) or saline. Our results showed that MK-801 reduced freezing on the first trial of extinction training with no impact on within-session acquisition of extinction, and that the lower doses of MK-801 resulted in increased freezing on the extinction retrieval test. These findings suggest that ventral hippocampal NMDARs are necessary for the consolidation of tone cued fear extinction.

Acetazolamide potentiates the afferent drive to prefrontal cortex in vivo

The knowledge on real-time neurophysiological effects of acetazolamide is still far behind the wide clinical use of this drug. Acetazolamide - a carbonic anhydrase inhibitor - has been shown to affect the neuromuscular transmission, implying a pH-mediated influence on the central synaptic transmission. To start filling such a gap, we chose a central substrate: hippocampal-prefrontal cortical projections; and a synaptic phenomenon: paired-pulse facilitation (a form of synaptic plasticity) to probe this drug's effects on interareal brain communication in chronically implanted rats. We observed that systemic acetazolamide potentiates the hippocampal-prefrontal paired-pulse facilitation. In addition to this field electrophysiology data, we found that acetazolamide exerts a net inhibitory effect on prefrontal cortical single-unit firing. We propose that systemic acetazolamide reduces the basal neuronal activity of the prefrontal cortex, whereas increasing the afferent drive it receives from the hippocampus. In addition to being relevant to the clinical and side effects of acetazolamide, these results suggest that exogenous pH regulation can have diverse impacts on afferent signaling across the neocortex.

Loss of promoter IV-driven BDNF expression impacts oscillatory activity during sleep, sensory information processing and fear regulation

Posttraumatic stress disorder is characterized by hyperarousal, sensory processing impairments, sleep disturbances and altered fear regulation; phenotypes associated with changes in brain oscillatory activity. Molecules associated with activity-dependent plasticity, including brain-derived neurotrophic factor (BDNF), may regulate neural oscillations by controlling synaptic activity. BDNF synthesis includes production of multiple Bdnf transcripts, which contain distinct 5' noncoding exons. We assessed arousal, sensory processing, fear regulation and sleep in animals where BDNF expression from activity-dependent promoter IV is disrupted (Bdnf-e4 mice). Bdnf-e4 mice display sensory hyper-reactivity and impaired electrophysiological correlates of sensory information processing as measured by event-related potentials (ERP). Utilizing electroencephalogram, we identified a decrease in slow-wave activity during non-rapid eye movement sleep, suggesting impaired sleep homeostasis. Fear extinction is controlled by hippocampal-prefrontal cortical BDNF signaling, and neurophysiological communication patterns between the hippocampus (HPC) and medial prefrontal cortex (mPFC) correlate with behavioral performance during extinction. Impaired fear extinction in Bdnf-e4 mice is accompanied by increased HPC activation and decreased HPC-mPFC theta phase synchrony during early extinction, as well as increased mPFC activation during extinction recall. These results suggest that activity-dependent BDNF signaling is critical for regulating oscillatory activity, which may contribute to altered behavior.

The persistence of long-term potentiation in the projection from ventral hippocampus to medial prefrontal cortex in awake rats

A potentially vital pathway in the processing of spatial memory is the pathway from ventral hippocampus to medial prefrontal cortex (vHPC-mPFC). To assess long-term potentiation (LTP) induction and maintenance across days in this pathway, the effects of several induction paradigms were compared in awake, freely moving rats. Two different high-frequency stimulation (HFS) protocols generated LTP lasting no longer than 1 week. However, after delivering HFS on three consecutive days, LTP lasted an average of 20 days, due mainly to the greater initial induction. Thus the pathway does not require extensive multi-day stimulation to induce LTP, as for other intra-neocortical pathways, but also it does not exhibit the extremely long-lasting and stable LTP previously observed in area CA1 and the dentate gyrus. By using bilaterally placed stimulating and recording electrodes, we found that HFS in one vHPC generated responses and LTP in the contralateral mPFC, even when the ipsilateral mPFC was inactivated by CNQX. We attribute this crossed response to a polysynaptic pathway from the vHPC to the contralateral mPFC. Finally, we found that repeated overnight exposure to an enriched environment also potentiated the vHPC-mPFC response, but this too was a transient effect lasting < 9 days, declining to baseline even before the enriched environment treatment was completed. Overall, these findings are consistent with the view that potentiation of vHPC-mPFC pathway may play a key role in promoting the hippocampus-mPFC interplay that, over days, leads to long-term storage in the frontal cortex of memories that are independent of the hippocampus.

Early weaning influences short-term synaptic plasticity in the medial prefrontal-anterior basolateral amygdala pathway

Early weaning in rodents reportedly influences behavioral and emotional traits and triggers precocious myelin formation in the anterior basolateral amygdala (aBLA; Ono et al., 2008), where prefrontal efferents terminate. We studied the correlation between behavior and the synaptic properties of the prefrontal-aBLA pathway. Open-field behaviors of adult male rats weaned at either 16 days or 30 days were measured on two consecutive days. On the first day, the rats received a slight footshock that was reportedly insufficient for fear conditioning. Electrophysiological recordings in the prefrontal-aBLA were then performed under urethane anesthesia. Without group differences in the stimulus intensity or the first evoked response, the overall paired-pulse facilitation was significantly lower in the early-weaned group from 25 to 100 ms. At the 25-ms interval, regression values between paired-pulse facilitation and locomotion on the second day were positive/insignificant and negative/significant in early- and control-weaned groups, respectively, and were statistically different between the groups.

The hippocampo-prefrontal pathway: a possible therapeutic target for negative and cognitive symptoms of schizophrenia

The hippocampo-prefrontal (H-PFC) pathway has been linked to cognitive and emotional disturbances in several psychiatric disorders including schizophrenia. Preclinical evidence from the NMDA receptor antagonism rodent model of schizophrenia shows severe pathology selective to the H-PFC pathway. It is speculated that there is an increased excitatory drive from the hippocampus to the prefrontal cortex due to dysfunctions in the H-PFC plasticity, which may serve as the basis for the behavioral consequences observed in this rodent model. Thus, the H-PFC pathway is currently emerging as a promising therapeutic target for the negative and cognitive symptom clusters of schizophrenia. Here, we have reviewed the physiological, pharmacological and functional characteristics of the H-PFC pathway and we propose that allosteric activation of glutamatergic and cholinergic neurotransmission can serve as a plausible therapeutic approach.

A ventral view on antidepressant action: roles for adult hippocampal neurogenesis along the dorsoventral axis

Adult hippocampal neurogenesis is implicated in antidepressant action, stress responses, and cognitive functioning. The hippocampus is functionally segregated along its longitudinal axis into dorsal (dHi) and ventral (vHi) regions in rodents, and analogous posterior and anterior regions in primates, whereby the vHi preferentially regulates stress and anxiety, while the dHi preferentially regulates spatial learning and memory. Given the role of neurogenesis in functions preferentially regulated by the dHi or vHi, it is plausible that neurogenesis is preferentially regulated in either the dHi or vHi depending upon the stimulus. We appraise here the literature on the effects of stress and antidepressants on neurogenesis along the hippocampal longitudinal axis and explore whether preferential regulation of neurogenesis in the vHi/anterior hippocampus contributes to stress resilience and antidepressant action.

Hippocampal-cortical interaction in decision making

When making a decision it is often necessary to consider the available alternatives in order to choose the most appropriate option. This deliberative process, where the pros and cons of each option are considered, relies on memories of past actions and outcomes. The hippocampus and prefrontal cortex are required for memory encoding, memory retrieval and decision making, but it is unclear how these areas support deliberation. Here we examine the potential neural substrates of these processes in the rat. The rat is a powerful model to investigate the network mechanisms underlying deliberation in the mammalian brain given the anatomical and functional conservation of its hippocampus and prefrontal cortex to other mammalian systems. Importantly, it is amenable to large scale neural recording while performing laboratory tasks that exploit its natural decision-making behavior. Focusing on findings in the rat, we discuss how hippocampal-cortical interactions could provide a neural substrate for deliberative decision making.