Crucial role for CA2 inputs in the sequential organization of CA1 time cells supporting memory.

There is considerable evidence for hippocampal time cells that briefly activate in succession to represent the temporal structure of memories. Previous studies have shown that time cells can be disrupted while leaving place cells intact, indicating that spatial and temporal information can be coded in parallel. However, the circuits in which spatial and temporal information are coded have not been clearly identified. Here we investigated temporal and spatial coding by dorsal hippocampal CA1 (dCA1) neurons in mice trained on a classic spatial working-memory task. On each trial, the mice approached the same choice point on a maze but were trained to alternate between traversing one of two distinct spatial routes (spatial coding phase). In between trials, there was a 10-s mnemonic delay during which the mouse continuously ran in a fixed location (temporal coding phase). Using cell-type-specific optogenetic methods, we found that inhibiting dorsal CA2 (dCA2) inputs into dCA1 degraded time cell coding during the mnemonic delay and impaired the mouse's subsequent memory-guided choice. Conversely, inhibiting dCA2 inputs during the spatial coding phase had a negligible effect on place cell activity in dCA1 and no effect on behavior. Collectively, our work demonstrates that spatial and temporal coding in dCA1 is largely segregated with respect to the dCA2-dCA1 circuit and suggests that CA2 plays a critical role in representing the flow of time in memory within the hippocampal network.

Activating positive memory engrams suppresses depression-like behaviour.

Stress is considered a potent environmental risk factor for many behavioural abnormalities, including anxiety and mood disorders. Animal models can exhibit limited but quantifiable behavioural impairments resulting from chronic stress, including deficits in motivation, abnormal responses to behavioural challenges, and anhedonia. The hippocampus is thought to negatively regulate the stress response and to mediate various cognitive and mnemonic aspects of stress-induced impairments, although the neuronal underpinnings sufficient to support behavioural improvements are largely unknown. Here we acutely rescue stress-induced depression-related behaviours in mice by optogenetically reactivating dentate gyrus cells that were previously active during a positive experience. A brain-wide histological investigation, coupled with pharmacological and projection-specific optogenetic blockade experiments, identified glutamatergic activity in the hippocampus-amygdala-nucleus-accumbens pathway as a candidate circuit supporting the acute rescue. Finally, chronically reactivating hippocampal cells associated with a positive memory resulted in the rescue of stress-induced behavioural impairments and neurogenesis at time points beyond the light stimulation. Together, our data suggest that activating positive memories artificially is sufficient to suppress depression-like behaviours and point to dentate gyrus engram cells as potential therapeutic nodes for intervening with maladaptive behavioural states.

Entorhinal-hippocampal neuronal circuits bridge temporally discontiguous events.

The entorhinal cortex (EC)-hippocampal (HPC) network plays an essential role for episodic memory, which preserves spatial and temporal information about the occurrence of past events. Although there has been significant progress toward understanding the neural circuits underlying the spatial dimension of episodic memory, the relevant circuits subserving the temporal dimension are just beginning to be understood. In this review, we examine the evidence concerning the role of the EC in associating events separated by time--or temporal associative learning--with emphasis on the function of persistent activity in the medial entorhinal cortex layer III (MECIII) and their direct inputs into the CA1 region of HPC. We also discuss the unique role of Island cells in the medial entorhinal cortex layer II (MECII), which is a newly discovered direct feedforward inhibitory circuit to CA1. Finally, we relate the function of these entorhinal cortical circuits to recent findings concerning hippocampal time cells, which may collectively activate in sequence to bridge temporal gaps between discontiguous events in an episode.

A unified mathematical framework for coding time, space, and sequences in the hippocampal region.

The medial temporal lobe (MTL) is believed to support episodic memory, vivid recollection of a specific event situated in a particular place at a particular time. There is ample neurophysiological evidence that the MTL computes location in allocentric space and more recent evidence that the MTL also codes for time. Space and time represent a similar computational challenge; both are variables that cannot be simply calculated from the immediately available sensory information. We introduce a simple mathematical framework that computes functions of both spatial location and time as special cases of a more general computation. In this framework, experience unfolding in time is encoded via a set of leaky integrators. These leaky integrators encode the Laplace transform of their input. The information contained in the transform can be recovered using an approximation to the inverse Laplace transform. In the temporal domain, the resulting representation reconstructs the temporal history. By integrating movements, the equations give rise to a representation of the path taken to arrive at the present location. By modulating the transform with information about allocentric velocity, the equations code for position of a landmark. Simulated cells show a close correspondence to neurons observed in various regions for all three cases. In the temporal domain, novel secondary analyses of hippocampal time cells verified several qualitative predictions of the model. An integrated representation of spatiotemporal context can be computed by taking conjunctions of these elemental inputs, leading to a correspondence with conjunctive neural representations observed in dorsal CA1.

Fast maximum likelihood estimation using continuous-time neural point process models.

A recent report estimates that the number of simultaneously recorded neurons is growing exponentially. A commonly employed statistical paradigm using discrete-time point process models of neural activity involves the computation of a maximum-likelihood estimate. The time to computate this estimate, per neuron, is proportional to the number of bins in a finely spaced discretization of time. By using continuous-time models of neural activity and the optimally efficient Gaussian quadrature, memory requirements and computation times are dramatically decreased in the commonly encountered situation where the number of parameters p is much less than the number of time-bins n. In this regime, with q equal to the quadrature order, memory requirements are decreased from O(np) to O(qp), and the number of floating-point operations are decreased from O(np(2)) to O(qp(2)). Accuracy of the proposed estimates is assessed based upon physiological consideration, error bounds, and mathematical results describing the relation between numerical integration error and numerical error affecting both parameter estimates and the observed Fisher information. A check is provided which is used to adapt the order of numerical integration. The procedure is verified in simulation and for hippocampal recordings. It is found that in 95 % of hippocampal recordings a q of 60 yields numerical error negligible with respect to parameter estimate standard error. Statistical inference using the proposed methodology is a fast and convenient alternative to statistical inference performed using a discrete-time point process model of neural activity. It enables the employment of the statistical methodology available with discrete-time inference, but is faster, uses less memory, and avoids any error due to discretization.

Distinct hippocampal time cell sequences represent odor memories in immobilized rats.

Previous studies have revealed the existence of hippocampal "time cells," principal neurons in CA1 that fire at specific moments in temporally organized experiences. However, in all these studies, animals were in motion; and so, temporal modulation might be due, at least in part, to concurrent or planned movement through space or self-generated movement (path integration). Here the activity of hippocampal CA1 neurons was recorded in head-fixed and immobile rats while they remembered odor stimuli across a delay period. Many neurons selectively and reliably activated at brief moments during the delay, as confirmed by several analyses of temporal modulation, during a strong ongoing θ rhythm. Furthermore, each odor memory was represented by a temporally organized ensemble of time cells composed mostly of neurons that were unique to each memory and some that fired at the same or different moments among multiple memories. These results indicate that ongoing or intended movement through space is not necessary for temporal representations in the hippocampus, and highlight the potential role of time cells as a mechanism for representing the flow of time in distinct memories.

Distinct neural ensembles in the rat gustatory cortex encode salt and water tastes.

The gustatory cortex (GC) is important for perceiving the intensity of tastants but it remains unclear as to how single neurons in the region carry out this function. Previous studies have shown that taste-evoked activity from single neurons in GC can be correlated or anticorrelated with tastant concentration, yet whether one or both neural responses signal intensity is poorly characterized because animals from these studies were not trained to report the intensity of the concentration that they tasted. To address this issue, we designed a two-alternative forced choice (2-AFC) task in which freely licking rats distinguished among concentrations of NaCl and recorded from ensembles of neurons in the GC. We identified three neural ensembles that rapidly (<300 ms or ∼2 licks) processed NaCl concentration. For two ensembles, their NaCl evoked activity was anticorrelated with NaCl concentration but could be further distinguished by their response to water; in one ensemble, water evoked the greatest response while in the other ensemble the lowest tested NaCl concentration evoked the greatest response. However, the concentration sensitive activity from each of these ensembles did not show a strong association with the behaviour of the rat in the 2-AFC task, suggesting a lesser role for signalling tastant intensity. Conversely, for a third neural ensemble, its neural activity was well correlated with increases in NaCl concentration, and this relationship best matched the intensity perceived by the rat. These results suggest that this neuronal ensemble in GC whose activity monotonically increases with concentration plays an important role in signalling the intensity of the taste of NaCl.

A comparison of the effects of 6 weeks of traditional resistance training, plyometric training, and complex training on measures of strength and anthropometrics.

Complex training (CT; alternating between heavy and lighter load resistance exercises with similar movement patterns within an exercise session) is a form of training that may potentially bring about a state of postactivation potentiation, resulting in increased dynamic power (Pmax) and rate of force development during the lighter load exercise. Such a method may be more effective than either modality, independently for developing strength. The purpose of this research was to compare the effects of resistance training (RT), plyometric training (PT), and CT on lower body strength and anthropometrics. Thirty recreationally trained college-aged men were trained using 1 of 3 methods: resistance, plyometric, or complex twice weekly for 6 weeks. The participants were tested pre, mid, and post to assess back squat strength, Romanian dead lift (RDL) strength, standing calf raise (SCR) strength, quadriceps girth, triceps surae girth, body mass, and body fat percentage. Diet was not controlled during this study. Statistical measures revealed a significant increase for squat strength (p = 0.000), RDL strength (p = 0.000), and SCR strength (p = 0.000) for all groups pre to post, with no differences between groups. There was also a main effect for time for girth measures of the quadriceps muscle group (p = 0.001), the triceps surae muscle group (p = 0.001), and body mass (p = 0.001; post hoc revealed no significant difference). There were main effects for time and group × time interactions for fat-free mass % (RT: p = 0.031; PT: p = 0.000). The results suggest that CT mirrors benefits seen with traditional RT or PT. Moreover, CT revealed no decrement in strength and anthropometric values and appears to be a viable training modality.

Taste-guided decisions differentially engage neuronal ensembles across gustatory cortices.

Much remains to be understood about the differential contributions from primary and secondary sensory cortices to sensory-guided decision making. To address this issue we simultaneously recorded activity from neuronal ensembles in primary [gustatory cortex GC)] and secondary gustatory [orbitofrontal cortex (OFC)] cortices while rats made a taste-guided decision between two response alternatives. We found that before animals commenced a response guided by a tastant cue, GC ensembles contained more information than OFC about the response alternative about to be selected. Thereafter, while the animal's response was underway, the response-selective information in ensembles from both regions increased, albeit to a greater degree in OFC. In GC, this increase depends on a representation of the taste cue guiding the animal's response. The increase in the OFC also depends on the taste cue guiding and other features of the response such as its spatiomotor properties and the behavioral context under which it is executed. Each of these latter features is encoded by different ensembles of OFC neurons that are recruited at specific times throughout the response selection process. These results indicate that during a taste-guided decision task both primary and secondary gustatory cortices dynamically encode different types of information.

Novel dithiolethione-modified nonsteroidal anti-inflammatory drugs in human hepatoma HepG2 and colon LS180 cells.

PURPOSE: Nonsteroidal anti-inflammatory drugs (NSAID) are promising chemopreventive agents against colon and other cancers. However, the molecular basis mediated by NSAIDs for chemoprevention has not been fully elucidated. Environmental carcinogens induce DNA mutation and cellular transformation; therefore, we examined the effect of NSAIDs on carcinogenesis mediated by the aryl hydrocarbon receptor signaling pathway. In this study, we investigated the activities of a new class of NSAIDs containing dithiolethione moieties (S-NSAID) on both arms of carcinogenesis. EXPERIMENTAL DESIGN: We investigated the effects of the S-NSAIDs, S-diclofenac and S-sulindac, on carcinogen activation and detoxification mechanisms in human hepatoma HepG2 and human colonic adenocarcinoma LS180 cells. RESULTS: We found that S-diclofenac and S-sulindac inhibited the activity and expression of the carcinogen activating enzymes, cytochromes P-450 (CYP) CYP1A1, CYP1B1, and CYP1A2. Inhibition was mediated by transcriptional regulation of the aryl hydrocarbon receptor (AhR) pathway. The S-NSAIDs down-regulated carcinogen-induced expression of CYP1A1 heterogeneous nuclear RNA, a measure of transcription rate. Both compounds blocked carcinogen-activated AhR from binding to the xenobiotic responsive element as shown by chromatin immunoprecipitation. S-diclofenac and S-sulindac inhibited carcinogen-induced CYP enzyme activity through direct inhibition as well as through decreased transcriptional activation of the AhR. S-sulindac induced expression of several carcinogen detoxification enzymes of the glutathione cycle including glutathione S-transferase A2, glutamate cysteine ligase catalytic subunit, glutamate cysteine ligase modifier subunit, and glutathione reductase. CONCLUSIONS: These results indicate that S-diclofenac and S-sulindac may serve as effective chemoprevention agents by favorably balancing the equation of carcinogen activation and detoxification mechanisms.

Prenatal choline supplementation alters the timing, emotion, and memory performance (TEMP) of adult male and female rats as indexed by differential reinforcement of low-rate schedule behavior.

Choline availability in the maternal diet has a lasting effect on brain and behavior of the offspring. To further delineate the impact of early nutritional status, we examined effects of prenatal-choline supplementation on timing, emotion, and memory performance of adult male and female rats. Rats that were given sufficient choline (CON: 1.1 g/kg) or supplemental choline (SUP: 5.0 g/kg) during embryonic days (ED) 12-17 were trained with a differential reinforcement of low-rate (DRL) schedule that was gradually transitioned through 5-, 10-, 18-, 36-, and 72-sec criterion times. We observed that SUP-females emitted more reinforced responses than CON-females, which were more efficient than both groups of males. In addition, SUP-males and SUP-females exhibited a reduction in burst responding (response latencies <2 sec) compared with both groups of CON rats. Furthermore, despite a reduced level of burst responding, the SUP-males made more nonreinforced responses prior to the DRL criterion as a result of maintaining the previous DRL criterion following transition to a new criterion. In summary, long-lasting effects of prenatal-choline supplementation were exhibited by reduced frustrative DRL responding in conjunction with the persistence of temporal memory in SUP-males and enhanced temporal exploration and response efficiency in SUP-females.

Sulindac and its metabolites induce carcinogen metabolizing enzymes in human colon cancer cells.

Sulindac is a nonsteroidal antiinflammatory drug that has been demonstrated to be a potent chemopreventive agent against colorectal cancer in both human and animal models. In vivo, sulindac may be reversibly reduced to the active antiinflammatory compound, sulindac sulfide, or irreversibly oxidized to sulindac sulfone. Sulindac has also been shown to inhibit polycyclic aromatic hydrocarbon (PAH)-induced cancer, but the molecular mechanisms of its antitumor effect remain unclear. In this study, we investigated the effects of sulindac and its metabolites on the expression of enzymes that metabolize and detoxify PAHs in 2 human colon cancer cell lines, LS180 and Caco-2. Sulindac and sulindac sulfide induced a sustained, concentration-dependent increase in CYP enzyme activity as well as an increase in the mRNA levels of CYP1A1, CYP1A2 and CYP1B1. Sulindac and sulindac sulfide induced the transcription of the CYP1A1 gene, as measured by the level of heterogeneous nuclear CYP1A1 RNA and verified by the use of actinomycin D as a transcription inhibitor. Chromatin immunoprecipitation assays demonstrated that sulindac and sulindac sulfide also increased the nuclear level of activated aryl hydrocarbon receptor, the transcription factor which mediates CYP expression. Additionally, sulindac and both metabolites increased the activity and mRNA expression of the carcinogen detoxification enzyme NAD(P)H:quinone oxidoreductase, as well as the expression of UDP-glucuronosyltransferase mRNA. These results show an overall upregulation of carcinogen metabolizing enzymes in colon cancer cells treated with sulindac, sulindac sulfide and sulindac sulfone that may contribute to the established chemoprotective effects of these compounds.

Amygdala inactivation reverses fear's ability to impair divided attention and make time stand still.

The cognitive and emotional effects of amygdala or frontal cortex lesions were compared in rats trained to time both a 50-s visual signal paired with food and an embedded 10- or 20-s auditory signal that was paired with either appetitive (food) or aversive (footshock) outcomes. When both auditory and visual signals were paired with food, control and amygdalar-lesioned rats were able to divide attention and to time both signals simultaneously, whereas when the embedded auditory signal was paired with footshock, control rats were impaired in their ability to divide attention and were able to time only one signal at a time. In contrast, amygdalar inactivation blocked this fear-related impairment and allowed rats to time both signals simultaneously, whereas rats with frontal cortex lesions demonstrated sequential processing under all conditions. These results support the proposal that the frontal cortex exerts executive control over the allocation of attentional resources, but that under stressful conditions the amygdala is crucial for the emergence of fear-evoked increments in selective attention leading to deficits in the ability to time 2 or more signals simultaneously.

Combined organizational and activational effects of short and long photoperiods on spatial and temporal memory in rats.

The present study examined the effects of photoperiod on spatial and temporal memory in adult Sprague-Dawley rats that were conceived and reared in different day lengths, i.e., short day (SD-8:16 light/dark) and long day (LD-16:8 light/dark). Both male and female LD rats demonstrated increased spatial memory capacity as evidenced by a lower number of choices to criterion in a 12-arm radial maze task relative to the performance of SD rats. SD rats also demonstrated a distortion in the content of temporal memory as evidenced by a proportional rightward shift in the 20 and 60 s temporal criteria trained using the peak-interval procedure that is consistent with reduced cholinergic function. The conclusion is that both spatial and temporal memory are sensitive to photoperiod variation in laboratory rats in a manner similar to that previously observed for reproductive behaviour.

Maximum Strength, Rate of Force Development, Jump Height, and Peak Power Alterations in Weightlifters across Five Months of Training.

The purpose of this monitoring study was to investigate how alterations in training affect changes in force-related characteristics and weightlifting performance. SUBJECTS: Seven competitive weightlifters participated in the study. METHODS: The weightlifters performed a block style periodized plan across 20 weeks. Force plate data from the isometric mid-thigh pull and static jumps with 0 kg, 11 kg, and 20 kg were collected near the end of each training block (weeks 1, 6, 10, 13, 17, and 20). Weightlifting performance was measured at weeks 0, 7, 11, and 20. RESULTS: Very strong correlations were noted between weightlifting performances and isometric rate of force development (RFD), isometric peak force (PF), peak power (PP), and jump height (JH). Men responded in a more predictable manner than the women. During periods of higher training volume, RFD was depressed to a greater extent than PF. JH at 20 kg responded in a manner reflecting the expected fatigue response more so than JH at 0 kg and 11 kg. CONCLUSIONS: PF appears to have been more resistant to volume alterations than RFD and JH at 20 kg. RFD and JH at 20 kg appear to be superior monitoring metrics due to their "sensitivity."

Interaction of raclopride and preparatory interval effects on simple reaction time performance.

In a series of three experiments, simple reaction time (RT) was characterized with respect to a variable preparatory interval (PI) in order to investigate the relationship between interval timing and RT. In Experiment 1, it was shown that RT decreases as a function of PI and that this effect varies with amount of training. In Experiment 2, RT was shown to increase during probe trials that used a novel 6.25s PI, suggesting that the specific durations of the PIs encoded during initial training contribute to the PI effect on RT. In Experiment 3, 100 microg/kg i.p. of raclopride proportionally slowed RT as a function of the PI. These results are discussed within the context of neuropsychological models of interval timing and support an underlying role for cortico-striatal dopaminergic function in temporal processing and simple RT measurements.

Differential effects of cocaine and ketamine on time estimation: implications for neurobiological models of interval timing.

The present experiment examined the effects of cocaine (0.0 and 15 mg/kg, i.p.) and ketamine (0.0, 10.0 and 15 mg/kg, i.p.) on timing behavior using a 12-s differential reinforcement of low rates (DRL) procedure and a 2- vs. 8-s bisection procedure in rats. DRL (time production) and bisection (time perception) procedures are sensitive to effects of dopaminergic drugs and provide an assessment of the accuracy and precision of interval timing as well as the subject's level of impulsivity. When administered to rats trained on either the DRL or the bisection procedure, cocaine shifted the psychophysical functions leftward relative to control conditions. In contrast, ketamine produced no change in the temporal control of behavior on either procedure. These differential effects of cocaine and ketamine are consistent with previous reports suggesting that dopamine levels in the dorsal striatum, but not in prefrontal cortex, ventral striatum or hippocampal regions, are crucial for the regulation of the speed of an internal clock.

The drug salicylamide is an antagonist of the aryl hydrocarbon receptor that inhibits signal transduction induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a widespread environmental contaminant, that has been linked with a variety of deleterious effects on human health, including increased cancer rates and reproductive anomalies. The detrimental effects of TCDD are mediated via the aryl hydrocarbon receptor (AhR), a transcription factor that regulates the expression of the carcinogen-activating enzymes cytochromes P-450 (CYP) 1A1, 1A2, and 1B1. In the present study, we examined the ability of synthetic derivatives of salicylic acid to affect TCDD-stimulated AhR-mediated signal transduction in human hepatoma HepG2 cells. Salicylamide (SAL), an analgesic drug, caused a potent and long-lasting inhibition of TCDD-induced CYP enzyme activity. Acetylsalicylic acid (aspirin) and the naturally occurring phytochemical salicylic acid had no effect on CYP activity. SAL inhibited the increase in CYP1A1, -1A2, and -1B1 mRNA levels that occurs on exposure to TCDD. TCDD-induced transcription of these genes was also inhibited by SAL, but not by aspirin or salicylic acid, as demonstrated by luciferase reporter assays. The transcription of the CYP1 family of genes is regulated by the interaction of TCDD-activated AhR with the xenobiotic-responsive element present in the promoter regions of these genes. As shown by electrophoretic mobility shift assay, SAL completely blocked the binding of TCDD-activated AhR to the xenobiotic responsive element. Also, SAL substantially blocked the binding of TCDD to the cytosolic AhR. These results demonstrate that SAL, a commonly used analgesic, is a potent inhibitor of AhR-mediated signal transduction, and may be an effective agent in the prevention of TCDD-associated disease.

Inhibition of carcinogen-activating enzymes by 16alpha-fluoro-5-androsten-17-one.

In the present study, we examined the effect of a synthetic analogue of the chemopreventive hormone dehydroepiandrosterone, 16alpha-fluoro-5-androsten-17-one, also known as fluasterone, on the activity and expression of carcinogen-activating enzymes in MCF-7 cells. The increase in cytochrome P450 (CYP) 1A1 and 1B1 activity, as measured by ethoxyresorufin-O-deethylase activity, in cells treated with the carcinogens dimethylbenzanthracene (DMBA) or 2,3,5,7-tetrachlorodibenzo-p-dioxin (TCDD), was inhibited by cotreatment with fluasterone. However, treatment of the cells with fluasterone after induction with DMBA or TCDD failed to decrease enzyme activity, indicating that inhibition was not the result of direct enzyme inhibition. Therefore, we examined the effect of fluasterone on gene expression at the mRNA level. Both DMBA and TCDD caused a dramatic increase in the amount of CYP1A1 and CYP1B1 mRNA, the two major isoforms involved in carcinogen activation in these cells. In cells cotreated with fluasterone, however, there was a dose-dependent decrease in CYP1A1 and CYP1B1 mRNA. Fluasterone also inhibited the basal level of CYP1A1 mRNA but not CYP1B1. Fluasterone inhibited the rate of CYP1A1 promoter-controlled transcription, indicating that it affects the transcriptional regulation of the gene. Actinomycin D chase experiments showed that fluasterone also caused an increase in the degradation of CYP1A1 mRNA, while leaving CYP1B1 mRNA unaffected. These results indicate that fluasterone inhibits the increase in the expression of CYP1A1 normally caused by exposure to carcinogens by both transcriptional and post-transcriptional mechanisms and that CYP1B1 expression is not susceptible to the same post-transcriptional mechanism.