Optogenetic Stimulation of CA1 Pyramidal Neurons at Theta Enhances Recognition Memory in Brain Injured Animals.

Abstract The hippocampus plays a prominent role in learning and memory formation. The functional integrity of this structure is often compromised after traumatic brain injury (TBI), resulting in lasting cognitive dysfunction. The activity of hippocampal neurons, particularly place cells, is coordinated by local theta oscillations. Previous studies aimed at examining hippocampal theta oscillations after experimental TBI have reported disparate findings. Using a diffuse brain injury model, the lateral fluid percussion injury (FPI; 2.0 atm), we report a significant reduction in hippocampal theta power that persists for at least three weeks after injury. We questioned whether the behavioral deficit associated with this reduction of theta power can be overcome by optogenetically stimulating CA1 neurons at theta in brain injured rats. Our results show that memory impairments in brain injured animals could be reversed by optogenetically stimulating CA1 pyramidal neurons expressing channelrhodopsin (ChR2) during learning. In contrast, injured animals receiving a control virus (lacking ChR2) did not benefit from optostimulation. These results suggest that direct stimulation of CA1 pyramidal neurons at theta may be a viable option for enhancing memory after TBI.

Impaired Experience-Dependent Refinement of Place Cells in a Rat Model of Alzheimer's Disease.

BACKGROUND: Hippocampal place cells play an integral role in generating spatial maps. Impaired spatial memory is a characteristic pathology of Alzheimer's disease (AD), yet it remains unclear how AD influences the properties of hippocampal place cells. OBJECTIVE: To record electrophysiological activity in hippocampal CA1 neurons in freely-moving 18-month-old male TgF344-AD and age-matched wild-type (WT) littermates to examine place cell properties. METHODS: We implanted 32-channel electrode arrays into the CA1 subfield of 18-month-old male WT and TgF344-AD (n = 6/group) rats. Ten days after implantation, single unit activity in an open field arena was recorded across days. The spatial information content, in-field firing rate, and stability of each place cell was compared across groups. Pathology was assessed by immunohistochemical staining, and a deep neural network approach was used to count cell profiles. RESULTS: Aged TgF344-AD rats exhibited hippocampal amyloid-ß deposition, and a significant increase in Iba1 immunoreactivity and microglia cell counts. Place cells from WT and TgF344-AD rat showed equivalent spatial information, in-field firing rates, and place field stability when initially exposed to the arena. However, by day 3, the place cells in aged WT rats showed characteristic spatial tuning as evidenced by higher spatial information content, stability, and in-field firing rates, an effect not seen in TgF344-AD rats. CONCLUSION: These findings support the notion that altered electrophysiological properties of place cells may contribute to the learning and memory deficits observed in AD.

Dopaminergic regulation of hippocampal plasticity, learning, and memory.

The hippocampus is responsible for encoding behavioral episodes into short-term and long-term memory. The circuits that mediate these processes are subject to neuromodulation, which involves regulation of synaptic plasticity and local neuronal excitability. In this review, we present evidence to demonstrate the influence of dopaminergic neuromodulation on hippocampus-dependent memory, and we address the controversy surrounding the source of dopamine innervation. First, we summarize historical and recent retrograde and anterograde anatomical tracing studies of direct dopaminergic projections from the ventral tegmental area and discuss dopamine release from the adrenergic locus coeruleus. Then, we present evidence of dopaminergic modulation of synaptic plasticity in the hippocampus. Plasticity mechanisms are examined in brain slices and in recordings from in vivo neuronal populations in freely moving rodents. Finally, we review pharmacological, genetic, and circuitry research that demonstrates the importance of dopamine release for learning and memory tasks while dissociating anatomically distinct populations of direct dopaminergic inputs.

Mild Traumatic Brain Injury Decreases Spatial Information Content and Reduces Place Field Stability of Hippocampal CA1 Neurons.

Both clinical and experimental studies have reported that mild traumatic brain injury (mTBI) can result in cognitive impairments in the absence of overt brain damage. Whether these impairments result from neuronal dysfunction/altered plasticity is an area that has received limited attention. In this study, we recorded activity of neurons in the cornu Ammonis (CA)1 subfield of the hippocampus in sham and mild lateral fluid percussion injured (mFPI) rats while these animals were performing an object location task. Electrophysiology results showed that the number of excitatory neurons encoding spatial information (i.e., place cells) was reduced in mFPI rats, and that these cells had broader and less stable place fields. Additionally, the in-field firing rate of place cells in sham operated, but not in mFPI, animals increased when objects within the testing arena were moved. Immunostaining indicated no visible damage or overall neuronal loss in mFPI brain sections. However, a reduction in the number of parvalbumin-positive inhibitory neurons in the CA1 subfield of mFPI animals was observed, suggesting that this reduction could have influenced place cell physiology. Alterations in spatial information content, place cell stability, and activity in mFPI rats coincided with poor performance in the object location task. These results indicate that altered place cell physiology may underlie the hippocampus-dependent cognitive impairments that result from mTBI.

Repeated mild traumatic brain injury produces neuroinflammation, anxiety-like behaviour and impaired spatial memory in mice.

PRIMARY OBJECTIVE: Repeated traumatic brain injuries (rmTBI) are frequently associated with debilitating neuropsychiatric conditions such as cognitive impairment, mood disorders, and post-traumatic stress disorder. We tested the hypothesis that repeated mild traumatic brain injury impairs spatial memory and enhances anxiety-like behaviour. RESEARCH DESIGN: We used a between groups design using single (smTBI) or repeated (rmTBI) controlled cranial closed skull impacts to mice, compared to a control group. METHODS AND PROCEDURES: We assessed the effects of smTBI and rmTBI using measures of motor performance (Rotarod Test [RT]), anxiety-like behaviour (Elevated Plus Maze [EPM] and Open Field [OF] tests), and spatial memory (Morris Water Maze [MWM]) within 12 days of the final injury. In separate groups of mice, astrocytosis and microglial activation were assessed 24 hours after the final injury using GFAP and IBA-1 immunohistochemistry. MAIN OUTCOMES AND RESULTS: RmTBI impaired spatial memory in the MWM and increased anxiety-like behaviour in the EPM and OFT. In addition, rmTBI elevated GFAP and IBA-1 immunohistochemistry throughout the mouse brain. RmTBI produced astrocytosis and microglial activation, and elicited impaired spatial memory and anxiety-like behaviour. CONCLUSIONS: rmTBI produces acute cognitive and anxiety-like disturbances associated with inflammatory changes in brain regions involved in spatial memory and anxiety.

Dopamine receptor activity participates in hippocampal synaptic plasticity associated with novel object recognition.

Physiological and behavioral evidence supports that dopamine (DA) receptor signaling influences hippocampal function. While several recent studies examined how DA influences CA1 plasticity and learning, there are fewer studies investigating the influence of DA signaling to the dentate gyrus. The dentate gyrus receives convergent cortical input through the perforant path fiber tracts and has been conceptualized to detect novelty in spatial memory tasks. To test whether DA-receptor activity influences novelty-detection, we used a novel object recognition (NOR) task where mice remember previously presented objects as an indication of learning. Although DA innervation arises from other sources and the main DA signaling may be from those sources, our molecular approaches verified that midbrain dopaminergic fibers also sparsely innervate the dentate gyrus. During the NOR task, wild-type mice spent significantly more time investigating novel objects rather than previously observed objects. Dentate granule cells in slices cut from those mice showed an increased AMPA/NMDA-receptor current ratio indicative of potentiated synaptic transmission. Post-training injection of a D1-like receptor antagonist not only effectively blocked the preference for the novel objects, but also prevented the increased AMPA/NMDA ratio. Consistent with that finding, neither NOR learning nor the increase in the AMPA/NMDA ratio were observed in DA-receptor KO mice under the same experimental conditions. The results indicate that DA-receptor signaling contributes to the successful completion of the NOR task and to the associated synaptic plasticity of the dentate gyrus that likely contributes to the learning.

Dopamine Regulates Aversive Contextual Learning and Associated In Vivo Synaptic Plasticity in the Hippocampus.

Dopamine release during reward-driven behaviors influences synaptic plasticity. However, dopamine innervation and release in the hippocampus and its role during aversive behaviors are controversial. Here, we show that in vivo hippocampal synaptic plasticity in the CA3-CA1 circuit underlies contextual learning during inhibitory avoidance (IA) training. Immunohistochemistry and molecular techniques verified sparse dopaminergic innervation of the hippocampus from the midbrain. The long-term synaptic potentiation (LTP) underlying the learning of IA was assessed with a D1-like dopamine receptor agonist or antagonist in ex vivo hippocampal slices and in vivo in freely moving mice. Inhibition of D1-like dopamine receptors impaired memory of the IA task and prevented the training-induced enhancement of both ex vivo and in vivo LTP induction. The results indicate that dopamine-receptor signaling during an aversive contextual task regulates aversive memory retention and regulates associated synaptic mechanisms in the hippocampus that likely underlie learning.

Dopamine and norepinephrine receptors participate in methylphenidate enhancement of in vivo hippocampal synaptic plasticity.

Attention-deficit hyperactive disorder (ADHD) is the most commonly studied and diagnosed psychiatric disorder in children. Methylphenidate (MPH, e.g., Ritalin) has been used to treat ADHD for over 50 years. It is the most commonly prescribed treatment for ADHD, and in the past decade it was the drug most commonly prescribed to teenagers. In addition, MPH has become one of the most widely abused drugs on college campuses. In this study, we examined the effects of MPH on hippocampal synaptic plasticity, which serves as a measurable quantification of memory mechanisms. Field potentials were recorded with permanently implanted electrodes in freely-moving mice to quantify MPH modulation of perforant path synaptic transmission onto granule cells of the dentate gyrus. Our hypothesis was that MPH affects hippocampal synaptic plasticity underlying learning because MPH boosts catecholamine signaling by blocking the dopamine and norepinephrine transporters (DAT and NET respectively). In vitro hippocampal slice experiments indicated MPH enhances perforant path plasticity, and this MPH enhancement arose from action via D1-type dopamine receptors and ß-type adrenergic receptors. Similarly, MPH boosted in vivo initiation of long-term potentiation (LTP). While there was an effect via both dopamine and adrenergic receptors in vivo, LTP induction was more dependent on the MPH-induced action via D1-type dopamine receptors. Under biologically reasonable experimental conditions, MPH enhances hippocampal synaptic plasticity via catecholamine receptors.

Posterior parietal cortex dynamically ranks topographic signals via cholinergic influence.

The hypothesis to be discussed in this review is that posterior parietal cortex (PPC) is directly involved in selecting relevant stimuli and filtering irrelevant distractors. The PPC receives input from several sensory modalities and integrates them in part to direct the allocation of resources to optimize gains. In conjunction with prefrontal cortex, nucleus accumbens, and basal forebrain cholinergic nuclei, it comprises a network mediating sustained attentional performance. Numerous anatomical, neurophysiological, and lesion studies have substantiated the notion that the basic functions of the PPC are conserved from rodents to humans. One such function is the detection and selection of relevant stimuli necessary for making optimal choices or responses. The issues to be addressed here are how behaviorally relevant targets recruit oscillatory potentials and spiking activity of posterior parietal neurons compared to similar yet irrelevant stimuli. Further, the influence of cortical cholinergic input to PPC in learning and decision-making is also discussed. I propose that these neurophysiological correlates of attention are transmitted to frontal cortical areas contributing to the top-down selection of stimuli in a timely manner.

Neurophysiology of Nicotine Addiction.

Tobacco use is a major health problem, and nicotine is the main addictive component. Nicotine binds to nicotinic acetylcholine receptors (nAChR) to produce its initial effects. The nAChRs subtypes are composed of five subunits that can form in numerous combinations with varied functional and pharmacological characteristics. Diverse psychopharmacological effects contribute to the overall process of nicotine addiction, but two general neural systems are emerging as critical for the initiation and maintenance of tobacco use. Mesocorticolimbic circuitry that includes the dopaminergic pathway originating in the ventral tegmental area and projecting to the nucleus accumbens is recognized as vital for reinforcing behaviors during the initiation of nicotine addiction. In this neural system ß2, α4, and α6 are the most important nAChR subunits underlying the rewarding aspects of nicotine and nicotine self-administration. On the other hand, the epithalamic habenular complex and the interpeduncular nucleus, which are connected via the fasciculus retroflexus, are critical contributors regulating nicotine dosing and withdrawal symptoms. In this case, the α5 and ß4 nAChR subunits have critical roles in combination with other subunits. In both of these neural systems, particular nAChR subtypes have roles that contribute to the overall nicotine addiction process.

Low frequency oscillations in rat posterior parietal cortex are differentially activated by cues and distractors.

The posterior parietal cortex (PPC) is hypothesized to detect visual cues among competing distractors. Anatomical and neurophysiologic evidence indicates that the rat PPC is part of a network of brain areas involved in directed attention, specifically when new task parameters or conditions are introduced. Here, we test the hypothesis that changes in the local field potential (LFP) of the PPC of rats performing a sustained attention task reflect aspects of detection. Two event-related potentials were observed during detection: the P300 response and the contingent negative variation (CNV). Spectrogram analysis also indicated a detection-specific increase in alpha power in the retention interval of this task. This is consistent with observations from human studies, which indicate that tasks requiring a subject to withhold a response produced a pronounced synchronization of alpha rhythms during the delay, and desynchronization during retrieval. We also found cycles of alpha synchrony and desynchrony in response to a periodic distractor. These cycles were most pronounced in the initial trial block of the distractor when the false alarm rate was highest, and as task performance improved these cycles significantly diminished. This result suggests that alpha cycling in the PPC represent neural activity critical for learning to inhibit distractors. The occurrence of alpha synchronization and desynchronization to attention-demanding stimuli, in addition to the P300 and CNV responses observed during detection, is evidence that rat PPC is involved in sustained attention, particularly in the presence of distractors.

Cholinergic optimization of cue-evoked parietal activity during challenged attentional performance.

The detection of salient or instrumental stimuli and the selection of cue-evoked responses are mediated by a fronto-parietal network that is modulated by cholinergic inputs originating from the basal forebrain. Visual cues that guide behavior are more strongly represented in the posterior parietal cortex (PPC) than are similar cues that are missed or task-irrelevant. Although the crucial role of cholinergic inputs in cue detection has been demonstrated by lesion studies, the role of PPC neurons in the cholinergic modulation of cue detection is unclear. We recorded extracellular spikes from PPC neurons of rats performing a sustained attention task, before and after selective removal of cholinergic inputs to the recording site. Visual cues that were subsequently detected evoked significant increases in the PPC firing rate. In the absence of cholinergic input, the activation of PPC neurons by detected cues was greatly diminished. When a visual distractor was introduced during task performance, a population of PPC neurons selectively responded to the distractor. As a result of cholinergic deafferentation, distractor-related neuronal activity was enhanced, and the detection-related activity was further suppressed. Thus, in deafferented subjects, the distractor lowered the signal-to-noise ratio of cue-evoked responses. This impairment in cue-evoked neuronal activity may have mediated the increased response latencies observed for detected cues in the presence of the distractor. Additional experiments demonstrated that the effects of cholinergic deafferentation were not confounded by extended practice or electrode depth. Collectively, these findings indicate that cholinergic inputs to PPC neurons amplify cue detection, and may also act to suppress irrelevant distractors.

Behavioral responses to stress following central and peripheral injection of the 5-HT(2) agonist DOI.

Evidence suggests that serotonin (5-HT) systems are involved in the regulation of an organism's response to stress. Experiments were conducted to evaluate the possibility that central (20, 100, or 200 microg icv), peripheral (0.1, 0.5, or 1.0 mg/kg sc), or combined central (200 microg) plus peripheral (0.1 mg/kg) injections of the selective 5-HT(2) agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI) would alter behavioral responses to stress in rats. Animals were evaluated during tail pinch stress, in an open field, and on a rotarod task. Across the three modes of administration (icv, sc, icv+sc), DOI resulted in a dose-related decrease in five of seven classes of behaviors observed during tail pinch. This reduction was most pronounced following subcutaneous injections, but occurred following intracerebroventricular and combined subcutaneous and intracerebroventricular injections as well. An additive effect of combined intracerebroventricular and subcutaneous administration was suggested by the fact that doses which were ineffective when given singly by these two routes resulted in a reduction in stress-evoked behavior when given together. Reduced responding seemed not to be attributable to general motoric impairment as DOI did not affect locomotion, grooming, or rotarod performance. The results suggest that activation of 5-HT(2) receptors produces an anxiolytic effect in rats subjected to acute tail pinch stress.