Localization and characterization of an essential associative memory trace in the mammalian brain

Acute and Post-Traumatic Stress Disorders: A biased nervous system

Acute stress disorder and post-traumatic stress disorder are generally triggered by an exceptionally intense threat. The consequences of this traumatogenic situation are explored here in chronological order, from exposure to the threat to development of symptoms. Such a situation may disrupt the equilibrium between two fundamental brain circuits, referred to as the "defensive" and "cognitive". The defensive circuit triggers the stress response as well as the formation of implicit memory. The cognitive circuit triggers the voluntary response and the formation of explicit autobiographical memory. During a traumatogenic situation, the defensive circuit could be over-activated while cognitive circuit is under-activated. In the most severe cases, overactivation of the defensive circuit may cause its brutal deactivation, resulting in dissociation. Here, we address the underlying neurobiological mechanisms at every scale: from neurons to behaviors, providing a detailed explanatory model of trauma.

Microbiota and memory: A symbiotic therapy to counter cognitive decline?

The process of aging underlies many degenerative disorders that arise in the living body, including gradual neuronal loss of the hippocampus that often leads to decline in both memory and cognition. Recent evidence has shown a significant connection between gut microbiota and brain function, as butyrate production by microorganisms is believed to activate the secretion of brain-derived neurotrophic factor (BDNF). To investigate whether modification of intestinal microbiota could impact cognitive decline in the aging brain, Romo-Araiza et al. conducted a study to test how probiotic and prebiotic supplementation impacted spatial and associative memory in middle-aged rats. Their results showed that rats supplemented with the symbiotic (both probiotic and prebiotic) treatment performed significantly better than other groups in the spatial memory test, though not in that of associative memory. Their data also reported that this improvement correlated with increased levels of BDNF, decreased levels of pro-inflammatory cytokines, and better electrophysiological outcomes in the hippocampi of the symbiotic group. Thus, the results indicated that the progression of cognitive impairment is indeed affected by changes in microbiota induced by probiotics and prebiotics. Potential future applications of these findings center around combatting neurodegeneration and inflammation associated not only with aging but also with the damaging posttraumatic effects of ischemic stroke.

Probiotics and Prebiotics as a Therapeutic Strategy to Improve Memory in a Model of Middle-Aged Rats

Aging is associated with morphological, physiological and metabolic changes, leading to multiorgan degenerative pathologies, such as cognitive function decline. It has been suggested that memory loss also involves a decrease in neurotrophic factors, including brain-derived neurotrophic factor (BDNF). In recent years, microbiota has been proposed as an essential player in brain development, as it is believed to activate BDNF secretion through butyrate production. Thus, microbiota modulation by supplementation with probiotics and prebiotics may impact cognitive decline. This study aimed to evaluate the effects of probiotics and prebiotics supplementation on the memory of middle-aged rats. Sprague-Dawley male rats were randomized in four groups (n = 13 per group): control (water), probiotic (E. faecium), prebiotic (agave inulin), symbiotic (E. faecium + inulin), which were administered for 5 weeks by oral gavage. Spatial and associative memory was analyzed using the Morris Water Maze (MWM) and Pavlovian autoshaping tests, respectively. Hippocampus was obtained to analyze cytokines [interleukin (IL-1β) and tumor necrosis factor (TNF-α)], BDNF and γ-aminobutyric acid (GABA) by enzyme-linked immunosorbent assay (ELISA). Butyrate concentrations were also evaluated in feces. The symbiotic group showed a significantly better performance in MWM (p < 0.01), but not in Pavlovian autoshaping test. It also showed significantly lower concentrations of pro-inflammatory cytokines (p < 0.01) and the reduction in IL-1β correlated with a better performance of the symbiotic group in MWM (p < 0.05). Symbiotic group also showed the highest BDNF and butyrate levels (p < 0.0001). Finally, we compared the electrophysiological responses of control (n = 8) and symbiotic (n = 8) groups. Passive properties of CA1 pyramidal cells (PCs) exhibited changes in response to the symbiotic treatment. Likewise, this group showed an increase in the N-methyl-D-aspartate receptor (NMDA)/AMPA ratio and exhibited robust long-term potentiation (LTP; p < 0.01). Integrated results suggest that symbiotics could improve age-related impaired memory.

Spontaneous recovery of conditioned eyeblink responses is associated with transiently decreased cerebellar theta activity in guinea pigs

Behavioral studies have demonstrated that extinguished conditioned eyeblink responses (CR) can spontaneously recover after extinction. However, the neural mechanisms underlying this process are still unclear. We have shown that spontaneous cerebellar theta activity was predictive of subsequent CR extinction. Here, we sought to further evaluate the association between spontaneous recovery and cerebellar theta activity in behaving guinea pigs. It was found that trace conditioning training significantly diminished the degree of spontaneous recovery during extinction sessions as compared to delay training. Moreover, by recording local field potential in the cerebellum of guinea pigs undergoing an eyeblink conditioning extinction task, we found that spontaneous recovery of delay-paradigm CRs was associated with transiently decreased CS-evoked theta activity in the cerebellum. These findings suggest that decreased CS-evoked cerebellar theta activity may contribute to the neural process that is important for the spontaneous recovery of extinguished motor memory. Future studies are needed to clarify the neural mechanism underlying changed cerebellar theta activity during altered behavioral contingencies.

Still searching for the engram

For nearly a century, neurobiologists have searched for the engram-the neural representation of a memory. Early studies showed that the engram is widely distributed both within and across brain areas and is supported by interactions among large networks of neurons. Subsequent research has identified engrams that support memory within dedicated functional systems for habit learning and emotional memory, but the engram for declarative memories has been elusive. Nevertheless, recent years have brought progress from molecular biological approaches that identify neurons and networks that are necessary and sufficient to support memory, and from recording approaches and population analyses that characterize the information coded by large neural networks. These new directions offer the promise of revealing the engrams for episodic and semantic memories.

Relational and procedural memory systems in the goldfish brain revealed by trace and delay eyeblink-like conditioning

The presence of multiple memory systems supported by different neural substrata has been demonstrated in animal and human studies. In mammals, two variants of eyeblink classical conditioning, differing only in the temporal relationships between the conditioned stimulus (CS) and the unconditioned stimulus (US), have been widely used to study the neural substrata of these different memory systems. Delay conditioning, in which both stimuli coincide in time, depends on a non-relational memory system supported by the cerebellum and associated brainstem circuits. In contrast, trace conditioning, in which a stimulus-free time gap separates the CS and the US, requires a declarative or relational memory system, thus depending on forebrain structures in addition to the cerebellum. The distinction between the explicit or relational and the implicit or procedural memory systems that support trace and delay classical conditioning has been extensively studied in mammals, but studies in other vertebrate groups are relatively scarce. In the present experiment we analyzed the differential involvement of the cerebellum and the telencephalon in delay and trace eyeblink-like classical conditioning in goldfish. The results show that whereas the cerebellum lesion prevented the eyeblink-like conditioning in both procedures, the telencephalon ablation impaired exclusively the acquisition of the trace conditioning. These data showing that comparable neural systems support delay and trace eyeblink conditioning in teleost fish and mammals suggest that these separate memory systems and their neural bases could be a shared ancestral brain feature of the vertebrate lineage.

Eyeblink Conditioning and Novel Object Recognition in the Rabbit: Behavioral Paradigms for Assaying Psychiatric Diseases

Analysis of data collected from behavioral paradigms has provided important information for understanding the etiology and progression of diseases that involve neural regions mediating abnormal behavior. The trace eyeblink conditioning (EBC) paradigm is particularly suited to examine cerebro-cerebellar interactions since the paradigm requires the cerebellum, forebrain, and awareness of the stimulus contingencies. Impairments in acquiring EBC have been noted in several neuropsychiatric conditions, including schizophrenia, Alzheimer's disease (AD), progressive supranuclear palsy, and post-traumatic stress disorder. Although several species have been used to examine EBC, the rabbit is unique in its tolerance for restraint, which facilitates imaging, its relatively large skull that facilitates chronic neuronal recordings, a genetic sequence for amyloid that is identical to humans which makes it a valuable model to study AD, and in contrast to rodents, it has a striatum that is differentiated into a caudate and a putamen that facilitates analysis of diseases involving the striatum. This review focuses on EBC during schizophrenia and AD since impairments in cerebro-cerebellar connections have been hypothesized to lead to a cognitive dysmetria. We also relate EBC to conditioned avoidance responses that are more often examined for effects of antipsychotic medications, and we propose that an analysis of novel object recognition (NOR) may add to our understanding of how the underlying neural circuitry has changed during disease states. We propose that the EBC and NOR paradigms will help to determine which therapeutics are effective for treating the cognitive aspects of schizophrenia and AD, and that neuroimaging may reveal biomarkers of the diseases and help to evaluate potential therapeutics. The rabbit, thus, provides an important translational system for studying neural mechanisms mediating maladaptive behaviors that underlie some psychiatric diseases, especially cognitive impairments associated with schizophrenia and AD, and object recognition provides a simple test of memory that can corroborate the results of EBC.