Characterization of exploratory patterns and hippocampal-prefrontal network oscillations during the emergence of free exploration

Mouse strain-specific responses along the gut-brain axis upon fecal microbiota transplantation from children with autism

Several factors are linked to the pathophysiology of autism spectrum disorders (ASD); however, the molecular mechanisms of the condition remain unknown. As intestinal problems and gut microbiota dysbiosis are associated with ASD development and severity, recent studies have focused on elucidating the microbiota-gut-brain axis' involvement. This study aims to explore mechanisms through which gut microbiota might influence ASD. Briefly, we depleted the microbiota of conventional male BALB/cAnNCrl (Balb/c) and C57BL/6J (BL/6) mice prior to human fecal microbiota transplantation (hFMT) with samples from children with ASD or their neurotypical siblings. We found mouse strain-specific responses to ASD hFMT. Notably, Balb/c mice exhibit decreased exploratory and social behavior, and show evidence of intestinal, systemic, and central inflammation accompanied with metabolic shifts. BL/6 mice show less changes after hFMT. Our results reveal that gut microbiota alone induce changes in ASD-like behavior, and highlight the importance of mouse strain selection when investigating multifactorial conditions like ASD.

Age-related differences in long-term memory performance and astrocyte morphology in rat hippocampus

Astrocytes are neuromodulator cells. Their complex and dynamic morphology regulates neuronal signaling, synaptic plasticity, and neurogenesis. The impact of aging on astrocyte morphology is still under ongoing debate. Therefore, this study aimed to characterize astrocyte morphology in the hippocampus of older rats. 2-, 18-, and 20-month-old male Wistar rats were submitted to the object recognition test to assess their short- and long-term memories. CA1, CA2, CA3, and the dentate gyrus were collected for immunohistochemistry analysis and glial fibrillary acid protein (GFAP) immunostaining. Our results indicate that 20-month-old rats did not recognize or discriminate the novel object in the long-term memory test. Also, GFAP staining was greater in the oldest group for all analyzed areas. Morphometric and fractal analysis indicated shorter branch lengths and smaller sizes for astrocytes of 20-month-old rats. Overall, our results suggest that 20-month-old rats have long-term memory impairment, increased GFAP staining, and astrocyte dystrophy. These age-related alterations in astrocyte morphology are a resource for future studies exploring the role of astrocytes in age-related cognitive decline and age-related diseases.

Curcumin attenuates neuroinflammatory damage induced by LPS: Implications for the role of S100B

Inflammation is a common feature of neurological disorders that alters cell function in microglia and astrocytes as well as other neuronal cell types. Astrocytes modulate blood flow, regulate glutamate metabolism, and exert antioxidant protection. When responding to inflammatory damage, astrocytes enhance immune cell infiltration and amplify inflammatory responses via the upregulation of cytokine production. Several molecules have been proposed to attenuate neuroinflammation and control neurological diseases. Curcumin gained attention due to its capacity to cross the blood-brain barrier and its well-described anti-inflammatory and antioxidant activities. Our study aimed to understand if oral curcumin administration could protect against central nervous system inflammatory damage induced by intracerebroventricular injection of LPS while focusing on astrocyte function. Despite its poor bioavailability, we found that curcumin reaches the central nervous system, prevents the locomotory damage caused by LPS, and reduces inflammatory signaling via IL-1β and COX-2. Furthermore, we observed that curcumin was protective against LPS-induced S100B secretion in the cerebrospinal fluid and GSH reduction in the hippocampal tissue. However, curcumin could not protect the animals from anhedonia, assessed by the sucrose preference test, and weight loss induced by LPS. Our results indicate that oral curcumin administration exerts a protective anti-inflammatory action in the central nervous system, attenuating the sickness behavior induced by ICV LPS. This work demonstrates that curcumin has an important modulative effect on astrocytes, thus suggesting that astrocytes are critical to the anti-inflammatory effects of curcumin.

Characterization of neuronal oscillations in the prelimbic cortex, nucleus accumbens and CA1 hippocampus during object retrieval task in rats predisposed to early life stress

BACKGROUND

Early life stress (ELS) during the stress hypo-responsive period (SHRP) alters the curiosity-like behavior later during adolescence. Previous studies have shown maternal separation (MS) stress-induced heightened curiosity and associated risk-taking behavior in the object retrieval task (ORT). However, the neural correlates of curiosity in adolescent rats predisposed to early life stress remain unexplored. Hence, the present study aimed to investigate the neural oscillatory patterns and network characteristics in the regions implicated in curiosity behavior, such as the Prelimbic cortex (PL), Nucleus Accumbens (NAc), and CA1 of the Hippocampus. The local field potentials data were analysed to understand the neural activity patterns in these areas during the risky zone crossing and object retrieval phase of the ORT in MS rats and compared with the normal control (NC) group.

RESULTS

In comparison to NC, MS rats showed a reduction in the theta power at 8-12 Hz, beta power at 12-20 Hz, and gamma power at 20-40 Hz range in the PL during risky zone crossing time. MS rats also showed reduced cross-correlation between PL-CA1 and reduced theta coherence between NAc-CA1 during risky zone crossing. During the object retrieval phase, the MS rats showed reduced peak cross-correlation between PL-CA1 and PL-NAc. Behaviourally, MS rats displayed an increased preference for the curiosity platform and retrieved more hidden objects, thus accounting for a higher curiosity index than controls.

CONCLUSION

In summary, a reduced synchronization between the PL, NAc, and CA1 during the object retrieval task indicates how early MS stress during a critical developmental period impacts the limbic circuit connectivity. This corresponded with enhanced curiosity index in adolescent MS rats, predicting an altered intrinsic motivation and hence a higher susceptibility to substance use disorders during adolescence.

Stochastic characterization of navigation strategies in an automated variant of the Barnes maze

Animals can use a repertoire of strategies to navigate in an environment, and it remains an intriguing question how these strategies are selected based on the nature and familiarity of environments. To investigate this question, we developed a fully automated variant of the Barnes maze, characterized by 24 vestibules distributed along the periphery of a circular arena, and monitored the trajectories of mice over 15 days as they learned to navigate towards a goal vestibule from a random start vestibule. We show that the patterns of vestibule visits can be reproduced by the combination of three stochastic processes reminiscent of random, serial, and spatial strategies. The processes randomly selected vestibules based on either uniform (random) or biased (serial and spatial) probability distributions. They closely matched experimental data across a range of statistical distributions characterizing the length, distribution, step size, direction, and stereotypy of vestibule sequences, revealing a shift from random to spatial and serial strategies over time, with a strategy switch occurring approximately every six vestibule visits. Our study provides a novel apparatus and analysis toolset for tracking the repertoire of navigation strategies and demonstrates that a set of stochastic processes can largely account for exploration patterns in the Barnes maze.

Perinatal exposure to atazanavir-based antiretroviral regimens in a mouse model leads to differential long-term motor and cognitive deficits dependent on the NRTI backbone

Background

Combination antiretroviral therapy (ART) use in pregnancy has been pivotal in improving maternal health and reducing perinatal HIV transmission. However, children born HIV-exposed uninfected fall behind their unexposed peers in several areas including neurodevelopment. The contribution of in utero ART exposure to these deficits is not clear. Here we present our findings of neurocognitive outcomes in adult mice exposed in utero to ART.

Methods

Dams were treated with a combination of ritonavir-boosted atazanavir with either abacavir plus lamivudine (ABC/3TC + ATV/r) or tenofovir disoproxil fumarate plus emtricitabine (TDF/FTC + ATV/r), or water as a control, administered daily from day of plug detection to birth. Offspring underwent a battery of behavioral tests that investigated motor performance and cognition starting at 6-weeks of age and ending at 8 months. Changes in brain structure were assessed using magnetic resonance imaging and immunohistochemistry. Expression of genes involved in neural circuitry and synaptic transmission were assessed in the hippocampus, a region strongly associated with memory formation, using qPCR.

Findings

Pups exposed to TDF/FTC + ATV/r showed increased motor activity and exploratory drive, and deficits in hippocampal-dependent working memory and social interaction, while pups exposed to ABC/3TC + ATV/r showed increased grooming, and deficits in working memory and social interaction. Significant volumetric reductions in the brain were seen only in the ABC/3TC + ATV/r group and were associated with reduced neuronal counts in the hippocampus. Altered neurotransmitter receptor mRNA expression as well as changes in expression of the neurotrophic factor BDNF and its receptors were observed in both ART-exposed groups in a sex-dependent manner.

Interpretation

In our model, in utero ART exposure had long-term effects on brain development and cognitive and motor outcomes in adulthood. Our data show that neurological outcomes can be influenced by the type of nucleoside reverse transcriptase inhibitor backbone of the regimen and not just the base drug, and display sex differences.

Hippocampal oscillatory dynamics in freely behaving rats during exploration of social and non-social stimuli

Hippocampal CA2 supports social memory and encodes information about social experiences. Our previous study showed that CA2 place cells responded specifically to social stimuli (Nat Commun, (Alexander et al. 2016)). In addition, a prior study showed that activation of CA2 induces slow gamma rhythms (~ 25-55 Hz) in the hippocampus (Elife, (Alexander 2018)). Together, these results raise the question of whether slow gamma rhythms coordinate CA2 activity during social information processing. We hypothesized that slow gamma would be associated with transmission of social memories from CA2 to CA1, perhaps to integrate information across regions or promote social memory retrieval. We recorded local field potentials from hippocampal subfields CA1, CA2, and CA3 of 4 rats performing a social exploration task. We analyzed the activity of theta, slow gamma, and fast gamma rhythms, as well as sharp wave-ripples (SWRs), within each subfield. We assessed interactions between subfields during social exploration sessions and during presumed social memory retrieval in post-social exploration sessions. We found that CA2 slow gamma rhythms increased during social interactions but not during non-social exploration. CA2-CA1 theta-show gamma coupling was enhanced during social exploration. Furthermore, CA1 slow gamma rhythms and SWRs were associated with presumed social memory retrieval. In conclusion, these results suggest that CA2-CA1 interactions via slow gamma rhythms occur during social memory encoding, and CA1 slow gamma is associated with retrieval of social experience.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11571-022-09829-8.

Performing Enriched Environment Studies to Improve Functional Recovery

Physical therapy and social interactions between the stroke patient and healthcare professionals or relatives facilitate the process of recovery and promote improvement of lost neurological function after stroke. These observations can be mimicked in an experimental setting by multimodal stimulation provided in the concept of enriched environment. The enriched environment is a housing condition combining social interactions and sensorimotor stimulation that improves lost neurological function without affecting the extent of brain damage after experimental stroke. This chapter provides a detailed protocol on how to perform enriched housing experiments including conceptual and technical considerations as a tool to investigate mechanisms of recovery after brain injury.

Dysfunction of the Hippocampal-Lateral Septal Circuit Impairs Risk Assessment in Epileptic Mice

Temporal lobe epilepsy, a chronic disease of the brain characterized by degeneration of the hippocampus, has impaired risk assessment. Risk assessment is vital for survival in complex environments with potential threats. However, the underlying mechanisms remain largely unknown. The intricate balance of gene regulation and expression across different brain regions is related to the structure and function of specific neuron subtypes. In particular, excitation/inhibition imbalance caused by hyperexcitability of glutamatergic neurons and/or dysfunction of GABAergic neurons, have been implicated in epilepsy. First, we estimated the risk assessment (RA) by evaluating the behavior of mice in the center of the elevated plus maze, and found that the kainic acid-induced temporal lobe epilepsy mice were specifically impaired their RA. This experiment evaluated approach-RA, with a forthcoming approach to the open arm, and avoid-RA, with forthcoming avoidance of the open arm. Next, results from free-moving electrophysiological recordings showed that in the hippocampus, ∼7% of putative glutamatergic neurons and ∼15% of putative GABAergic neurons were preferentially responsive to either approach-risk assessment or avoid-risk assessment, respectively. In addition, ∼12% and ∼8% of dorsal lateral septum GABAergic neurons were preferentially responsive to approach-risk assessment and avoid-risk assessment, respectively. Notably, during the impaired approach-risk assessment, the favorably activated dorsal dentate gyrus and CA3 glutamatergic neurons increased (∼9%) and dorsal dentate gyrus and CA3 GABAergic neurons decreased (∼7%) in the temporal lobe epilepsy mice. Then, we used RNA sequencing and immunohistochemical staining to investigate which subtype of GABAergic neuron loss may contribute to excitation/inhibition imbalance. The results show that temporal lobe epilepsy mice exhibit significant neuronal loss and reorganization of neural networks. In particular, the dorsal dentate gyrus and CA3 somatostatin-positive neurons and dorsal lateral septum cholecystokinin-positive neurons are selectively vulnerable to damage after temporal lobe epilepsy. Optogenetic activation of the hippocampal glutamatergic neurons or chemogenetic inhibition of the hippocampal somatostatin neurons directly disrupts RA, suggesting that an excitation/inhibition imbalance in the dHPC dorsal lateral septum circuit results in the impairment of RA behavior. Taken together, this study provides insight into epilepsy and its comorbidity at different levels, including molecular, cell, neural circuit, and behavior, which are expected to decrease injury and premature mortality in patients with epilepsy.

Entorhinal-hippocampal interactions lead to globally coherent representations of space

The firing maps of grid cells in the entorhinal cortex are thought to provide an efficient metric system capable of supporting spatial inference in all environments. However, whether spatial representations of grid cells are determined by local environment cues or are organized into globally coherent patterns remains undetermined. We propose a navigation model containing a path integration system in the entorhinal cortex and a cognitive map system in the hippocampus. In the path integration system, grid cell network and head direction (HD) cell network integrate movement and visual information, and form attractor states to represent the positions and head directions of the animal. In the cognitive map system, a topological map is constructed capturing the attractor states of the path integration system as nodes and the transitions between attractor states as links. On loop closure, when the animal revisits a familiar place, the topological map is calibrated to minimize odometry errors. The change of the topological map is mapped back to the path integration system, to correct the states of the grid cells and the HD cells. The proposed model was tested on iRat, a rat-like miniature robot, in a realistic maze. Experimental results showed that, after familiarization of the environment, both grid cells and HD cells develop globally coherent firing maps by map calibration and activity correction. These results demonstrate that the hippocampus and the entorhinal cortex work together to form globally coherent metric representations of the environment. The underlying mechanisms of the hippocampal-entorhinal circuit in capturing the structure of the environment from sequences of experience are critical for understanding episodic memory.