Spatial memory impairment induced by lesion of the mesohippocampal dopaminergic system in the rat

Modeling cognitive dysfunction in neurofibromatosis-1

Cognitive dysfunction, including significant impairments in learning, behavior, and attention, is found in over 10% of children in the general population. However, in the common inherited cancer predisposition syndrome, neurofibromatosis type 1 (NF1), the prevalence of these cognitive deficits approaches 70%. As a monogenic disorder, NF1 provides a unique genetic tool to identify and dissect mechanistically the molecular and cellular bases underlying cognitive dysfunction. In this review, we discuss Nf1 fly and mouse systems that mimic many of the cognitive abnormalities seen in children with NF1. Further, we describe discoveries from these models that have uncovered defects in the regulation of Ras activity, cAMP generation, and dopamine homeostasis as key mechanisms important for cognitive dysfunction in children with NF1.

Dopaminergic modulation of the hippocampal neuropil proteome identified by bioorthogonal noncanonical amino acid tagging (BONCAT)

Local protein synthesis and its activity-dependent modulation via dopamine receptor stimulation play an important role in synaptic plasticity - allowing synapses to respond dynamically to changes in their activity patterns. We describe here the metabolic labeling, enrichment, and MS-based identification of candidate proteins specifically translated in intact hippocampal neuropil sections upon treatment with the selective D1/D5 receptor agonist SKF81297. Using the noncanonical amino acid azidohomoalanine and click chemistry, we identified over 300 newly synthesized proteins specific to dendrites and axons. Candidates specific for the SKF81297-treated samples were predominantly involved in protein synthesis and synapse-specific functions. Furthermore, we demonstrate a dendrite-specific increase in proteins synthesis upon application of SKF81297. This study provides the first snapshot in the dynamics of the dopaminergic hippocampal neuropil proteome.

Dopamine deficiency underlies learning deficits in neurofibromatosis-1 mice

Children with neurofibromatosis type 1 (NF1) are prone to learning and behavioral abnormalities, including problems with spatial learning and attention. The molecular etiology for these deficits is unclear, as previous studies have implicated defective dopamine, cyclic adenosine monophosphate (cAMP), and Ras homeostasis. Using behavioral, electrophysiological, and primary culture, we now demonstrate that reduced dopamine signaling is responsible for cAMP-dependent defects in neuron function and learning. Collectively, these results establish defective dopaminergic function as a contributing factor underlying impaired spatial learning and memory in children and adults with NF1, and support the use of treatments that restore normal dopamine homeostasis for select individuals.

9-Methyl-β-carboline-induced cognitive enhancement is associated with elevated hippocampal dopamine levels and dendritic and synaptic proliferation

β-Carbolines (BCs) belong to the heterogenous family of carbolines, which have been found exogenously, that is, in various fruits, meats, tobacco smoke, alcohol and coffee, but also endogenously, that is, blood, brain and CSF. These exogenous and endogenous BCs and some of their metabolites can exert neurotoxic effects, however, an unexpected stimulatory effect of 9-methyl-β-carboline (9-me-BC) on dopaminergic neurons in primary mesencephalic cultures was recently discovered. The aim of the present study was to extend our knowledge on the stimulatory effects of 9-me-BC and to test the hypothesis that 9-me-BC may act as a cognitive enhancer. We found that 10 days (but not 5 days) of pharmacological treatment with 9-me-BC (i) improves spatial learning in the radial maze, (ii) elevates dopamine levels in the hippocampal formation, and (iii) results after 10 days of treatment in elongated, more complex dendritic trees and higher spine numbers on granule neurons in the dentate gyrus of 9-me-BC-treated rats. Our results demonstrate that beyond its neuroprotective/neurorestorative and anti-inflammatory effects, 9-me-BC acts as a cognitive enhancer in a hippocampus-dependent task, and that the behavioral effects may be associated with a stimulatory impact on hippocampal dopamine levels and dendritic and synaptic proliferation.

CNS dopamine transmission mediated by noradrenergic innervation

The presynaptic source of dopamine in the CA1 field of dorsal hippocampus is uncertain due to an anatomical mismatch between dopaminergic terminals and receptors. We show, in an in vitro slice preparation from C57BL/6 male mice, that a dopamine (DA) D1 receptor (D1R)-mediated enhancement in glutamate synaptic transmission occurs following release of endogenous DA with amphetamine exposure. It is assumed DA is released from terminals innervating from the ventral tegmental area (VTA) even though DA transporter (DAT)-positive fibers are absent in hippocampus, a region with abundant D1Rs. It has been suggested this results from a lack of DAT expression on VTA terminals rather than a lack of these terminals per se. Neither a knockdown of tyrosine hydroxylase (TH) expression in the VTA by THsiRNA, delivered locally, by adeno-associated viral vector, nor localized pharmacological blockade of DAT to prevent amphetamine uptake into DA terminals, has any effect on the D1R synaptic, enhancement response to amphetamine. However, either a decrease in TH expression in the locus ceruleus (LC) or a blockade of the norepinephrine (NE) transporter prevents the DA-mediated response, indicating LC terminals can release both NE and DA. These findings suggest noradrenergic fibers may be the primary source of DA release in hippocampus and corresponding DA-mediated increase in synaptic transmission. Accordingly, these data imply the LC may have a role in DA transmission in the CNS in response to drugs of abuse, and potentially, under physiological conditions.

Dopamine in the hippocampus is cleared by the norepinephrine transporter

Abnormal dopaminergic neurotransmission in the hippocampus may be involved in certain aspects of cognitive dysfunction. In the hippocampus, there is little, if any, expression of dopamine transporters (DAT), indicating that the mechanism for dopamine clearance differs from that in the striatum. Here, by means of in-vivo microdialysis in freely moving rats, we tested the hypothesis that the norepinephrine transporter (NET) is involved in dopamine clearance in the hippocampus. We found that systemic administration of the selective NET inhibitor reboxetine (3 mg/kg) and the psychostimulants amphetamine (0.5 mg/kg) and cocaine (10 mg/kg) increased hippocampal dopamine efflux. Local administration of reboxetine (300 μM) produced a large increase in hippocampal dopamine levels that could not be further enhanced by the addition of the NET/DAT inhibitor nomifensine (100 μM). Administration of the specific DAT inhibitor GBR12909 at a concentration (1 mM) that robustly increased dopamine in the nucleus accumbens had a comparably smaller effect in the hippocampus. In line with a minor role of DAT in the hippocampus, we detected very little DAT in this area using ligand binding with radiolabelled RTI-55. Moreover, in contrast to raclopride (100 μM), a dopamine D2-autoreceptor antagonist, local administration of the α2-adrenoceptor antagonist idazoxan (100 μM) increased hippocampal dopamine. Taken together, our data demonstrate an interaction between dopamine and norepinephrine systems in the hippocampus. It is proposed that this interaction originates from a shared uptake mechanism at the NET level.

Modulation of the extinction of two different fear-motivated tasks in three distinct brain areas

The hippocampus, basolateral amygdala and ventromedial prefrontal cortex participate in the extinction of inhibitory avoidance and contextual fear conditioning. We studied the effect of drugs acting on receptors involved in synaptic modulation on extinction of both tasks. The drugs were given bilaterally right after the first of two sessions of extinction in each task through cannulae implanted into the mentioned areas. The doses used are known to influence memory consolidation of the original tasks. Their effects were evaluated on a second extinction session 24h later, and assumed to result from influences on the consolidation of extinction. The glutamate NMDA receptor stimulant d-serine (50 μg/side) and the histamine methyl-transferase inhibitor SKF9188 (12.5 μg/side) enhanced, and the NMDA antagonist amino-phosphonopentanoate (5 μg/side) and the H2 histamine receptor antagonist ranitidine (17.5 μg/side) inhibited, extinction of both tasks regardless of the region into which they were administered. Thus, glutamate NMDA receptors are involved in the consolidation of extinction of both tasks, and histamine H2 receptors modulate that process in all areas studied. Norepinephrine (1 μg/side), the β-adrenoceptor antagonist timolol (1 μg/side), the D1 dopamine receptor agonist SKF38393 (12.5 μg/side) and the D1 antagonist SCH23390 (1.5 μg/side) also affected extinction of both tasks, but their effects varied with the task and with the site of infusion, suggesting that extinction modulation by β- and D1 receptors is more complex. In conclusion, extinction of two different aversive tasks is modulatable by various systems, which bears upon the behavioral and pharmacological treatment of fear-motivated brain disorders.

Functional division of hippocampal area CA1 via modulatory gating of entorhinal cortical inputs

The hippocampus receives two streams of information, spatial and nonspatial, via major afferent inputs from the medial (MEC) and lateral entorhinal cortexes (LEC). The MEC and LEC projections in the temporoammonic pathway are topographically organized along the transverse-axis of area CA1. The potential for functional segregation of area CA1, however, remains relatively unexplored. Here, we demonstrated differential novelty-induced c-Fos expression along the transverse-axis of area CA1 corresponding to topographic projections of MEC and LEC inputs. We found that, while novel place exposure induced a uniform c-Fos expression along the transverse-axis of area CA1, novel object exposure primarily activated the distal half of CA1 neurons. In hippocampal slices, we observed distinct presynaptic properties between LEC and MEC terminals, and application of either DA or NE produced a largely selective influence on one set of inputs (LEC). Finally, we demonstrated that differential c-Fos expression along the transverse axis of area CA1 was largely abolished by an antagonist of neuromodulatory receptors, clozapine. Our results suggest that neuromodulators can control topographic TA projections allowing the hippocampus to differentially encode new information along the transverse axis of area CA1.

D1/D5 dopamine receptors modulate spatial memory formation

We investigated the effect of the intra-CA1 administration of the D1/D5 receptor antagonist SCH23390 and the D1/D5 receptor agonist SKF38393 on spatial memory in the water maze. When given immediately, but not 3h after training, SCH23390 hindered long-term spatial memory formation without affecting non-spatial memory or the normal functionality of the hippocampus. On the contrary, post-training infusion of SKF38393 enhanced retention and facilitated the spontaneous recovery of the original spatial preference after reversal learning. Our findings demonstrate that hippocampal D1/D5 receptors play an essential role in spatial memory processing.

Disrupting effect of drug-induced reward on spatial but not cue-guided learning: implication of the striatal protein kinase A/cAMP response element-binding protein pathway

The multiple memory systems hypothesis posits that different neural circuits function in parallel and may compete for information processing and storage. For example, instrumental conditioning would depend on the striatum, whereas spatial memory may be mediated by a circuit centered on the hippocampus. However, the nature of the task itself is not sufficient to select durably one system over the other. In this study, we investigated the effects of natural and pharmacological rewards on the selection of a particular memory system during learning. We compared the effects of food- or drug-induced activation of the reward system on cue-guided versus spatial learning using a Y-maze discrimination task. Drug-induced reward severely impaired the acquisition of a spatial discrimination task but spared the cued version of the task. Immunohistochemical analysis of the phosphorylated form of the cAMP response element binding (CREB) protein and c-Fos expression induced by behavioral testing revealed that the spatial deficit was associated with a decrease of both markers within the hippocampus and the prefrontal cortex. In contrast, drug reward potentiated the cued learning-induced CREB phosphorylation within the dorsal striatum. Administration of the protein kinase A inhibitor 8-Bromo-adenosine-3',5'-cyclic monophosphorothioate Rp isomer (Rp-cAMPS) into the dorsal striatum before training completely reversed the drug-induced spatial deficit and restored CREB phosphorylation levels within the hippocampus and the prefrontal cortex. Therefore, drug-induced striatal hyperactivity may underlie the declarative memory deficit reported here. This mechanism could represent an important early step toward the development of addictive behaviors by promoting conditioning to the detriment of more flexible forms of memory.

Where is my reward and how do I get it? Interaction between the hippocampus and the basal ganglia during spatial learning

Spatial learning has been recognized over the years to be under the control of the hippocampus and related temporal lobe structures. Hippocampal damage often causes severe impairments in the ability to learn and remember a location in space defined by distal visual cues. Recent experimental evidence in rodents demonstrates, however, that other brain areas might also be involved in the acquisition of spatial information. Amongst these, the cortex--basal ganglia loop is known to be involved in reinforcement learning and has been identified as an important contributor to spatial learning. In particular, it has been shown that altered activity of the basal ganglia striatal complex can impair the ability to perform spatial learning tasks. Until recently, little was known about how the basal ganglia and the hippocampus interact and how their activities evolve during learning. The present review, focusing on rodent studies, provides a glimpse of the findings obtained over the past decade that support a dialog between these two structures during spatial learning. Based on these studies, we propose a new functional spatial decision network with three separate loops encompassing hippocampus and specific basal ganglia regions. Each of the three loops serves a different aspect of spatial decision making and all three are linked by their mutual connections and are under the control of the dopaminergic learning signal.

Effects of DSP4 and methylphenidate on spatial memory performance in rats

In this experiment, we have investigated the spatial memory performance of rats following a central noradrenaline depletion induced by three different doses of the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) and following administration of three different doses of methylphenidate (MPH). The rats were required to find food pellets hidden on a holeboard. The sole administration of DSP4 induced only minor cognitive deficits. However, the treatment with MPH increased the reference memory error, the impulsivity and the motor activity of the DSP4-treated rats. Since the noradrenergic terminals in a DSP4-treated rat are significantly reduced, the administration of MPH has little effect on the noradrenergic system and increases dopaminergic rather than noradrenergic activity, resulting in an imbalance with relatively high dopaminergic and low noradrenergic activities. It is suggested that a reduction of noradrenaline and an increase of dopamine induce ADHD-related deficits and that the depletion of noradrenaline is not sufficient for an appropriate rat model of ADHD.

Neural systems analysis of decision making during goal-directed navigation

The ability to make adaptive decisions during goal-directed navigation is a fundamental and highly evolved behavior that requires continual coordination of perceptions, learning and memory processes, and the planning of behaviors. Here, a neurobiological account for such coordination is provided by integrating current literatures on spatial context analysis and decision-making. This integration includes discussions of our current understanding of the role of the hippocampal system in experience-dependent navigation, how hippocampal information comes to impact midbrain and striatal decision making systems, and finally the role of the striatum in the implementation of behaviors based on recent decisions. These discussions extend across cellular to neural systems levels of analysis. Not only are key findings described, but also fundamental organizing principles within and across neural systems, as well as between neural systems functions and behavior, are emphasized. It is suggested that studying decision making during goal-directed navigation is a powerful model for studying interactive brain systems and their mediation of complex behaviors.

Functional correlation between GABAergic and dopaminergic systems of dorsal hippocampus and ventral tegmental area in passive avoidance learning in rats

The aim of the present study was to investigate the existence of possible functional correlation between GABA-A and dopamine (DA) receptors of the dorsal hippocampus and the ventral tegmental area (VTA) in passive avoidance learning. Two guide cannulas were stereotaxically implanted in the CA1 region of the dorsal hippocampus and the VTA of male Wistar rats. In order to measure memory retrieval, the animals were trained in a step-through type passive avoidance task and tested 24 h after training. Post-training intra-CA1 administration of a GABA-A receptor agonist, muscimol (0.01-0.02 μg/rat) dose-dependently impaired memory retrieval. Post-training intra-VTA administration of SCH23390 (a dopamine D1 receptor antagonist; 0.1-0.8 μg/rat) or sulpiride (a D2 receptor antagonist; 0.5-1.5 μg/rat) decreased the inhibitory effect of muscimol (0.02 μg/rat, intra-CA1) on memory retrieval. Intra-VTA administration of the same doses of SCH23390, but not sulpiride, decreased the step-through latencies. On the other hand, post-training administration of muscimol (0.02 μg/rat) into the VTA inhibited memory retrieval. The administration of SCH23390 (0.01-0.2 μg/rat) or sulpiride (0.1-1 μg/rat) into the CA1 region, immediately after training, had no effect on memory retrieval. Furthermore, the amnesic effect of intra-VTA administration of muscimol was significantly decreased by intra-CA1 administration of sulpiride (0.5 and 1 μg/rat, intra-CA1), but not SCH23390. The practical conclusion is that the relationship between the hippocampus and the VTA may regulate memory formation in passive avoidance learning. Also, the correlation between the hippocampus and VTA by a dopaminergic system may be involved in mediating muscimol-induced amnesia.

Up-regulation of dopamine D1 receptor in the hippocampus after establishment of conditioned place preference by cocaine

The hippocampus plays an important role in the formation of contextual memory between the environment and the rewarding effect of abused drugs. The dopaminergic neural transmission in the hippocampus seems to be critical for such memory. Using conditioned place preference in rats, we found that the protein level of the dopamine D(1) receptor and its prerequisite mRNA in the hippocampus increased in animals that showed a clear preference for the environment paired with cocaine. The increase was not a simple reflection of the repeated administration of cocaine. Instead, it is attributable to conditioning, because systematic contingency between drug administration and exposure to a particular environment was necessary for the increase. Furthermore, we found that the mRNA of the dopamine D(1) receptors increased in the granule cell layer of the dentate gyrus. These results suggest that the alteration of dopamine D(1) receptor in the hippocampus, especially in the dentate gyrus, is related to the induction of drug-induced contextual memory. The finding implicates the relevance of the dopaminergic signal transduction in the hippocampus to drug dependence.

Hippocampal damage produces retrograde but not anterograde amnesia for a cued location in a spontaneous exploratory task in rats

Performance in several memory tasks is known to be unaffected by hippocampal damage sustained before learning, but is severely disrupted if the same damage occurs after learning. Memories for preferred locations, or home bases, in exploratory tasks can be formed by rats with hippocampal damage, but it is unknown if the memory for a home base survives hippocampal damage. To examine this question, for 30 min each day for five consecutive days, rats explored a circular open field containing one local cue. By Day 5 the rats preferentially went directly to that location, spent the majority of their time at that location, made rapid direct trips to that location when returning from an excursion and so demonstrated that the location was a home base. Memory for the cued location was examined after a 24 h or 14-day interval with the cue removed. In Experiments 1 and 2, control rats and rats with prior N-methyl-D-aspartic acid hippocampal lesions demonstrated memory of the home base location by making direct trips to that location. In Experiment 3, rats that had first explored the open field and cue and then received hippocampal lesions showed no memory for the cued location. The absence of anterograde impairment vs. the presence of retrograde impairment for memory of a spatial home base confirms a role for the hippocampus in the retention of spatial memory acquired during exploration.

Dopaminergic modulation of cortical inputs during maturation of adult-born dentate granule cells

Adult neurogenesis, a particular form of plasticity in the adult brain, is under dynamic control of neuronal activity mediated by various neurotransmitters. Despite accumulating evidence suggesting that the neurotransmitter dopamine (DA) regulates proliferation of neural precursor cells in the neurogenic zones, whether and how it acts on newly generated neurons that integrate into the established network remains unknown. Using patch-clamp recordings from retrovirus-labeled newborn hippocampal dentate granule cells (DGCs) in acute mouse brain slices, we found that DA not only caused a long-lasting attenuation of medial perforant path (MPP) inputs to the young DGCs, but also decreased their capacity to express long-term potentiation (LTP). In contrast, DA suppressed MPP transmission to mature DGCs to a similar extent but did not influence their LTP expression. This difference was linked to activation of distinct subtypes of DA receptors in DGCs at different developmental stages. Our observations suggest that DA is particularly effective in modulating the activities of hyperexcitable young neurons, which may have important implications for the dentate function as a filter for incoming information to the hippocampus.

Ventral tegmental area disruption selectively affects CA1/CA2 but not CA3 place fields during a differential reward working memory task

Hippocampus (HPC) receives dopaminergic (DA) projections from the ventral tegmental area (VTA) and substantia nigra. These inputs appear to provide a modulatory signal that influences HPC dependent behaviors and place fields. We examined how efferent projections from VTA to HPC influence spatial working memory and place fields when the reward context changes. CA1 and CA3 process environmental context changes differently and VTA preferentially innervates CA1. Given these anatomical data and electrophysiological evidence that implicate DA in reward processing, we predicted that CA1 place fields would respond more strongly to both VTA disruption and changes in the reward context than CA3 place fields. Rats (N = 9) were implanted with infusion cannula targeting VTA and recording tetrodes aimed at HPC. Then they were tested on a differential reward, win-shift working memory task. One recording session consisted of 5 baseline and 5 manipulation trials during which place cells in CA1/CA2 (N = 167) and CA3 (N = 94) were recorded. Prior to manipulation trials rats were infused with either baclofen or saline and then subjected to control or reward conditions during which the learned locations of large and small reward quantities were reversed. VTA disruption resulted in an increase in errors, and in CA1/CA2 place field reorganization. There were no changes in any measures of CA3 place field stability during VTA disruption. Reward manipulations did not affect performance or place field stability in CA1/CA2 or CA3; however, changes in the reward locations "rescued" performance and place field stability in CA1/CA2 when VTA activity was compromised, perhaps by trigging compensatory mechanisms. These data support the hypothesis that VTA contributes to spatial working memory performance perhaps by maintaining place field stability selectively in CA1/CA2.

Maternal immune activation alters nonspatial information processing in the hippocampus of the adult offspring

The observation that maternal infection increases the risk for schizophrenia in the offspring suggests that the maternal immune system plays a key role in the etiology of schizophrenia. In a mouse model, maternal immune activation (MIA) by injection of poly(I:C) yields adult offspring that display abnormalities in a variety of behaviors relevant to schizophrenia. As abnormalities in the hippocampus are a consistent observation in schizophrenia patients, we examined synaptic properties in hippocampal slices prepared from the offspring of poly(I:C)- and saline-treated mothers. Compared to controls, CA1 pyramidal neurons from adult offspring of MIA mothers display reduced frequency and increased amplitude of miniature excitatory postsynaptic currents. In addition, the specific component of the temporoammonic pathway that mediates object-related information displays increased sensitivity to dopamine. To assess hippocampal network function in vivo, we used expression of the immediate-early gene, c-Fos, as a surrogate measure of neuronal activity. Compared to controls, the offspring of poly(I:C)-treated mothers display a distinct c-Fos expression pattern in area CA1 following novel object, but not novel location, exposure. Thus, the offspring of MIA mothers may have an abnormality in modality-specific information processing. Indeed, the MIA offspring display enhanced discrimination in a novel object recognition, but not in an object location, task. Thus, analysis of object and spatial information processing at both synaptic and behavioral levels reveals a largely selective abnormality in object information processing in this mouse model. Our results suggest that altered processing of object-related information may be part of the pathogenesis of schizophrenia-like cognitive behaviors.

Dopamine and memory: modulation of the persistence of memory for novel hippocampal NMDA receptor-dependent paired associates

Three experiments investigated the role in memory processing of dopamine (DA) afferents to the hippocampus (HPC) that arise from the ventral tegmental area. One hypothesis is that D(1)/D(5) receptor activation in HPC is necessary for the encoding of novel, episodic-like information; the other is that DA activation ensures the greater temporal persistence of transient hippocampal memory traces. Rats (n = 35) were trained, in separate experiments using an episodic-like memory task, to learn six paired associates (PAs) in an "event arena" involving a repeated association between specific flavors of food and locations in space. After 6 weeks of training, rats had learned a "schema" such that two new paired associates could be acquired in a single trial in one session (episodic-like memory). We show that encoding of novel PAs is sensitive to intrahippocampal microinfusion of the NMDA antagonist d-AP-5. Experiment 1 established that intrahippocampal infusion of the D(1)/D(5) dopaminergic antagonist SCH23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride] before encoding of new PAs caused impaired memory 24 h later but that SCH23390 had no effect on the later memory of previously established PAs. Experiment 2 established that SCH23390 modulated the persistence of new memories over time (30 min vs 24 h) rather than affecting initial encoding. Experiment 3 revealed that the impact of SCH23390 was not mediated by state dependence nor had an effect on memory retrieval. These findings support the second hypothesis and establish that persistent, long-term memory of rapid, hippocampal-mediated acquisition of new paired associates requires activation of D(1)/D(5) receptors in HPC at or around the time of encoding.

Context dependent effects of ventral tegmental area inactivation on spatial working memory

Rats were tested on a hippocampus dependent win-shift working memory task in familiar or novel environments after receiving bilateral ventral tegmental area infusions of baclofen. Baclofen infusion disrupted working memory performance in both familiar and novel environments. In addition, baclofen infusion selectively disrupted short-term working memory in the novel environment. This experiment confirms selective ventral tegmental area support of accurate performance during a context dependent spatial navigation task.

What is remembered? Role of attention on the encoding and retrieval of hippocampal representations

The hippocampus is critically involved in storing explicit memory such as memory for space. A defining feature of explicit memory storage is that it requires attention both for encoding and retrieval. Whereas, a great deal is now known about the mechanisms of storage, the mechanisms whereby attention modulates the encoding and retrieval of space and other hippocampus-dependent memory representations are not known. In this review we discuss recent studies, including our own, which show on the cellular level that attention is critical for the stabilization of spatial and reward-associated odour representations. Our findings support the view that in the hippocampus attention selects the reference frame for task-relevant information. This mechanism is in part mediated by dopamine acting through D1/D5 receptors and involves an increase in neuronal synchronization in the gamma band frequency. We propose that synchronous activity leads to enhancements in synaptic strength that mediate the stabilization of hippocampal representations.

COMT val108/158 met genotype affects neural but not cognitive processing in healthy individuals

The relationship between cognition and a functional polymorphism in the catechol-O-methlytransferase (COMT) gene, val108/158met, is one of debate in the literature. Furthermore, based on the dopaminergic differences associated with the COMT val108/158met genotype, neural differences during cognition may be present, regardless of genotypic differences in cognitive performance. To investigate these issues the current study aimed to 1) examine the effects of COMT genotype using a large sample of healthy individuals (n = 496-1218) and multiple cognitive measures, and using a subset of the sample (n = 22), 2) examine whether COMT genotype effects medial temporal lobe (MTL) and frontal activity during successful relational memory processing, and 3) investigate group differences in functional connectivity associated with successful relational memory processing. Results revealed no significant group difference in cognitive performance between COMT genotypes in any of the 19 cognitive measures. However, in the subset sample, COMT val homozygotes exhibited significantly decreased MTL and increased prefrontal activity during both successful relational encoding and retrieval, and reduced connectivity between these regions compared with met homozygotes. Taken together, the results suggest that although the COMT val108/158met genotype has no effect on cognitive behavioral measures in healthy individuals, it is associated with differences in neural process underlying cognitive output.

Gain in sensitivity and loss in temporal contrast of STDP by dopaminergic modulation at hippocampal synapses

Spike-timing-dependent plasticity (STDP) is considered a physiologically relevant form of Hebbian learning. However, behavioral learning often involves action of reinforcement or reward signals such as dopamine. Here, we examined how dopamine influences the quantitative rule of STDP at glutamatergic synapses of hippocampal neurons. The presence of 20 muM dopamine during paired pre- and postsynaptic spiking activity expanded the effective time window for timing-dependent long-term potentiation (t-LTP) to at least -45 ms, and allowed normally ineffective weak stimuli with fewer spike pairs to induce significant t-LTP. Meanwhile, dopamine did not affect the degree of t-LTP induced by normal strong stimuli with spike timing (ST) of +10 ms. Such dopamine-dependent enhancement in the sensitivity of t-LTP was completely blocked by the D1-like dopamine receptor antagonist SCH23390, but not by the D2-like dopamine receptor antagonist sulpiride. Surprisingly, timing-dependent long-term depression (t-LTD) at negative ST was converted into t-LTP by dopamine treatment; this conversion was also blocked by SCH23390. In addition, t-LTP in the presence of dopamine was completely blocked by the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid, indicating that D1-like receptor-mediated modulation appears to act through the classical NMDA receptor-mediated signaling pathway that underlies STDP. These results provide a quantitative and mechanistic basis for a previously undescribed learning rule that depends on pre- and postsynaptic ST, as well as the global reward signal.

Trial outcome and associative learning signals in the monkey hippocampus

In tasks of associative learning, animals establish new links between unrelated items by using information about trial outcome to strengthen correct/rewarded associations and modify incorrect/unrewarded ones. To study how hippocampal neurons convey information about reward and trial outcome during new associative learning, we recorded hippocampal neurons as monkeys learned novel object-place associations. A large population of hippocampal neurons (50%) signaled trial outcome by differentiating between correct and error trials during the period after the behavioral response. About half these cells increased their activity following correct trials (correct up cells) while the remaining half fired more following error trials (error up cells). Moreover, correct up cells, but not error up cells, conveyed information about learning by increasing their stimulus-selective response properties with behavioral learning. These findings suggest that information about successful trial outcome conveyed by correct up cells may influence new associative learning through changes in the cell's stimulus-selective response properties.

Dopamine D1 receptor modulates hippocampal representation plasticity to spatial novelty

The human hippocampus is critical for learning and memory. In rodents, hippocampal pyramidal neurons fire in a location-specific manner, forming relational representations of environmental cues. The importance of glutamatergic systems in learning and in hippocampal neural synaptic plasticity has been shown. However, the role of dopaminergic systems in the response of hippocampal neural plasticity to novel and familiar spatial stimuli remains unclear. To clarify this important issue, we recorded hippocampal neurons from dopamine D(1) receptor knock-out (D1R-KO) mice and their wild-type (WT) littermates under the manipulation of distinct spatial cues in a familiar and a novel environment. Here we report that in WT mice, the majority of place cells quickly responded to the manipulations of distal and proximal cues in both familiar and novel environments. In contrast, the influence of distal cues on spatial firing in D1R-KO mice was abolished. In the D1R-KO mice, the influence of proximal cues was facilitated in a familiar environment, and in a novel environment most of the place cells were less likely to respond to changes of spatial cues. Our results demonstrate that hippocampal neurons in mice can rapidly and flexibly encode information about space from both distal and proximal cues to cipher a novel environment. This ability is necessary for many types of learning, and lacking D1R can radically alter this learning-related neural activity. We propose that D1R is crucially implicated in encoding spatial information in novel environments, and influences the plasticity of hippocampal representations, which is important in spatial learning and memory.

Frequency-dependent signal transmission and modulation by neuromodulators

The brain uses a strategy of labor division, which may allow it to accomplish more elaborate and complicated tasks, but in turn, imposes a requirement for central control to integrate information among different brain areas. Anatomically, the divergence of long-range neuromodulator projections appears well-suited to coordinate communication between brain areas. Oscillatory brain activity is a prominent feature of neural transmission. Thus, the ability of neuromodulators to modulate signal transmission in a frequency-dependent manner adds an additional level of regulation. Here, we review the significance of frequency-dependent signal modulation in brain function and how a neuronal network can possess such properties. We also describe how a neuromodulator, dopamine, changes frequency-dependent signal transmission, controlling information flow from the entorhinal cortex to the hippocampus.

mRNA expression of the Nurr1 and NGFI-B nuclear receptor families following acute and chronic administration of methamphetamine

Nur-related 1 (Nurr1) and nerve growth factor inducible-B (NGFI-B) constitute closely related subgroups of the nuclear receptor superfamily. One to three hours after 4 mg/kg acute methamphetamine (METH) administration, the levels of Nurr1 mRNA were significantly higher in the prelimbic (PrL), primary motor (M1) and primary somatosensory (S1) cortices and ventral tegmental area (VTA), as compared with the basal level. Pretreatment with 0.5 mg/kg of SCH23390 prevented the acute METH-induced increase in Nurr1 mRNA levels in these brain regions. One to three hours after 4-mg/kg acute METH administration, the levels of NGFI-B mRNA increased significantly in the PrL, M1, S1, striatum, and nucleus accumbens core (AcbC). Pretreatment with either 0.5 mg/kg of MK-801 or 0.5 mg/kg of SCH23390 prevented the acute METH-induced increase in NGFI-B mRNA levels in these brain regions. The levels of mRNAs were determined 3 h after a challenge injection of either saline or 4 mg/kg METH at the three-week withdrawal point in rats which had previously been exposed to either saline or METH (4 mg/kg/day) for 2 weeks. After the saline challenge, the group chronically exposed to METH displayed significantly higher levels of Nurr1 mRNA in the PrL, S1 and VTA, and of NGFI-B mRNA in the PrL, M1, S1, striatum and AcbC than did the group chronically treated with saline. The groups chronically exposed to METH failed to increase Nurr1 mRNA in the VTA, and NGFI-B mRNA in the AcbC, when challenged with 4 mg/kg METH. These results suggest that Nurr1 and NGFI-B mRNA play differential roles upon exposure to METH.

Further characterization of the spatial learning deficit in the human immunodeficiency virus-1 transgenic rat

Human immunodeficiency virus (HIV)-associated dementia (HAD) encompasses a spectrum of cognitive and motor deficits resulting from the progression of central nervous system abnormalities caused by the HIV-1 virus. With the advent of highly active antiretroviral therapy (HAART), these deficits have become milder, but more prevalent as the population of HIV-positive individuals ages. Mild impairment in cognition has also been identified in asymptomatic HIV-1 patients. The noninfectious HIV-1 transgenic (Tg) rat recently developed to study the pathogenesis of acquired immunodeficiency syndrome (AIDS) may also be useful for the study of the early and chronic effects of HIV-1 on learning and cognition. In a previous study, we demonstrated that HIV-1Tg rats show a deficit in learning how to swim to a hidden platform in a modified water maze task compared to normal and transgenic controls. In the present study, we replicate this result and demonstrate that HIV-1Tg rats also show a significant deficit in reversal learning and new strategy learning. These results indicate that the HIV-1Tg rat is a promising model in which to study the neuropathogenic mechanisms that can cause cognitive deficits in patients with HAD as well as asymptomatic HIV-positive individuals.

Basal ganglia contributions to adaptive navigation

The striatum has long been considered to be selectively important for nondeclarative, procedural types of memory. This stands in contrast with spatial context processing that is typically attributed to hippocampus. Neurophysiological evidence from studies of the neural mechanisms of adaptive navigation reveals that distinct neural systems such as the striatum and hippocampus continuously process task relevant information regardless of the current cognitive strategy. For example, both striatal and hippocampal neural representations reflect spatial location, directional heading, reward, and egocentric movement features of a test situation in an experience-dependent way, and independent of task demands. Thus, continual parallel processing across memory systems may be the norm rather than the exception. It is suggested that neuromodulators, such as dopamine, may serve to differentially regulate learning-induced neural plasticity mechanisms within these memory systems such that the most successful form of neural processing exerts the strongest control over response selection functions. In this way, dopamine may serve to optimize behavioral choices in the face of changing environmental demands during navigation.

Epistasis between dopamine regulating genes identifies a nonlinear response of the human hippocampus during memory tasks

BACKGROUND

Dopamine modulation of neuronal activity in prefrontal cortex maps to an inverted U-curve. Dopamine is also an important factor in regulation of hippocampal mediated memory processing. Here, we investigated the effect of genetic variation of dopamine inactivation via catechol-O-methyltransferase (COMT) and the dopamine transporter (DAT) on hippocampal activity in healthy humans during different memory conditions.

METHODS

Using blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) in 82 subjects matched for a series of demographic and genetic variables, we studied the effect of the COMT valine (Val)(158)methionine (Met) and the DAT 3' variable number tandem repeat (VNTR) polymorphisms on function of the hippocampus during encoding of recognition memory and during working memory.

RESULTS

Our results consistently demonstrated a double dissociation so that DAT 9-repeat carrier alleles modulated activity in the hippocampus in the exact opposite direction of DAT 10/10-repeat alleles based on COMT Val(158)Met genotype during different memory conditions. Similar results were evident in ventrolateral and dorsolateral prefrontal cortex.

CONCLUSIONS

These findings suggest that genetically determined dopamine signaling during memory processing maps to a nonlinear relationship also in the hippocampus. Our data also demonstrate in human brain epistasis of two genes implicated in dopamine signaling on brain activity during different memory conditions.

Positive and negative modulation of word learning by reward anticipation

Recent evidence from neuroscience indicates that the anticipation of external rewards may enhance declarative memory consolidation by increasing dopaminergic-modulated plasticity in the hippocampus. A number of studies in psychology, however, have shown that external rewards may have null, or even negative, effects on learning. To shed light on this issue, we developed a novel task, in which native Japanese speakers were rewarded to learn unknown English words inside a functional MRI scanner. Rewards had no effect on recall performance unless we used a rating of reward-induced anxiety as a covariate. In this case, for highly rewarded words, we found a negative correlation between recall performance and anxiety ratings. For those words, high recall performance and low anxiety ratings were associated with enhanced activity in the midbrain dopaminergic centers, the hippocampus, and the amygdala. On the other hand, low recall performance and high anxiety ratings were associated with enhanced activity in the anterior cingulate and middle frontal gyrus, brain regions that have been shown to be involved with anxiety and divided attention, respectively. A connectivity analysis indicated positive functional connectivity between the midbrain dopaminergic centers and both the hippocampus and the amygdala, as well as negative connectivity between the anterior cingulate and the amygdala. Thus, both our behavioral and imaging results suggest that the anticipation of rewards can, depending on the individual level of reward-induced anxiety, have either a beneficial effect or a negative effect on word learning.

Synergistic effects of selegiline and donepezil on cognitive impairment induced by amyloid beta (25-35)

Selegiline, an irreversible inhibitor of monoamine oxidase B used in the treatment of Parkinson's disease, has been demonstrated to have a potential cognition-improving effect in patients with Alzheimer's disease (AD) undergoing treatment with an acetylcholinesterase inhibitor donepezil. To confirm such clinical events, we investigated whether co-administration of donepezil with selegiline had a synergistic cognition-improving effect in an animal model of AD. Intracerebroventricular injection of amyloid beta protein fragment 25-35 [Abeta(25-35)] induced impairment of learning and memory in a Y-maze, novel object recognition and contextual fear conditioning tests. Either donepezil or selegiline alone improved the cognitive impairments in the Y-maze and conditioned fear learning tasks in Abeta(25-35)-injected mice, whereas donepezil, but not selegiline, failed to improve the impairment in a novel object recognition task. Co-administration of donepezil with selegiline, at doses that do not exert efficacy individually, significantly improved the deficits in all three tests, indicating a synergistic cognition-improving effect. These alleviating effects were antagonized by pretreatment with a muscarinic receptor antagonist scopolamine and a dopamine receptor antagonist haloperidol. These results suggest that selegiline potentiates the effect of donepezil on the cognitive impairment, and that the synergistic effect may be mediated through both the cholinergic and dopaminergic systems.

Performance- and task-dependent effects of the dopamine D1/D5 receptor agonist SKF 38393 on learning and memory in the rat

Dopamine D(1)/D(5) receptor agonists may enhance cognition by mimicking dopamine's neurophysiological actions on the processes underlying learning and memory. The present study examined the task- and performance- dependence of the cognitive effects of a partial agonist at dopamine D(1)/D(5) receptors, SKF 38393 [(+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrobromide], in rats. Spatial working memory was assessed in a T-maze, spatial reference memory in a water maze and habituation learning in a novel environment, a hole board. The muscarinic acetylcholine receptor antagonist scopolamine (1.5 mg/kg, i.p.) was used to cause an impairment of performance of these learning tasks. Administration of SKF 38393 (6 mg/kg, i.p.) alone had no significant effect on spontaneous alternation in the T-maze, latency to escape to a hidden platform in the water maze or the habituation of spontaneous behaviour in the hole board. In contrast, in scopolamine-treated rats, whereas SKF 38393 prevented the scopolamine-induced deficit in the T-maze, it exacerbated the impairment in the water maze and did not significantly alter the disruption of habituation. These results suggest that dopamine D(1)/D(5) receptor activation has performance- and task-dependent effects on cognitive function.

Frequency-dependent gating of synaptic transmission and plasticity by dopamine

The neurotransmitter dopamine (DA) plays an important role in learning by enhancing the saliency of behaviorally relevant stimuli. How this stimulus selection is achieved on the cellular level, however, is not known. Here, in recordings from hippocampal slices, we show that DA acts specifically at the direct cortical input to hippocampal area CA1 (the temporoammonic (TA) pathway) to filter the excitatory drive onto pyramidal neurons based on the input frequency. During low-frequency patterns of stimulation, DA depressed excitatory TA inputs to both CA1 pyramidal neurons and local inhibitory GABAergic interneurons via presynaptic inhibition. In contrast, during high-frequency patterns of stimulation, DA potently facilitated the TA excitatory drive onto CA1 pyramidal neurons, owing to diminished feedforward inhibition. Analysis of DA's effects over a broad range of stimulus frequencies indicates that it acts as a high-pass filter, augmenting the response to high-frequency inputs while diminishing the impact of low-frequency inputs. These modulatory effects of DA exert a profound influence on activity-dependent forms of synaptic plasticity at both TA-CA1 and Schaffer-collateral (SC)-CA1 synapses. Taken together, our data demonstrate that DA acts as a gate on the direct cortical input to the hippocampus, modulating information flow and synaptic plasticity in a frequency-dependent manner.

The effect of antagonization of orexin 1 receptors in CA1 and dentate gyrus regions on memory processing in passive avoidance task

The hippocampal formation plays an essential role in associative learning like passive avoidance (PA) learning. It has been shown; orexin-containing terminals and orexin receptors densely are distributed in the hippocampal formation. We have previously demonstrated that antagonization of orexin 1 receptor (OX1R) in CA1 region of hippocampus and dentate gyrus (DG) impaired spatial memory processing. Although, there are few studies concerning function of orexinergic system on memory processing in PA task, but there is no study about physiological function of OX1R on this process. To address this, the OX1R antagonist, SB-334867-A, was injected into DG or CA1 regions of hippocampus and evaluated the influence of OX1R antagonization on acquisition, consolidation and retrieval in PA task. Our results show that, SB-334867-A administration into CA1 region impaired memory retrieval but not PA acquisition and consolidation. However, SB-334867-A administration into DG region impaired acquisition and consolidation but not PA memory retrieval. Therefore, it seems that endogenous orexins play an important role in learning and memory in the rat through OX1Rs.

The 6-Hydroxydopamine model of parkinson’s disease

The neurotoxin 6-hydroxydopamine (6-OHDA) continues to constitute a valuable topical tool used chiefly in modeling Parkinson's disease in the rat. The classical method of intracerebral infusion of 6-OHDA involving a massive destruction of nigrostriatal dopaminergic neurons, is largely used to investigate motor and biochemical dysfunctions in Parkinson's disease. Subsequently, more subtle models of partial dopaminergic degeneration have been developed with the aim of revealing finer motor deficits. The present review will examine the main features of 6-OHDA models, namely the mechanisms of neurotoxin-induced neurodegeneration as well as several behavioural deficits and motor dysfunctions, including the priming model, modeled by this means. An overview of the most recent morphological and biochemical findings obtained with the 6-OHDA model will also be provided, particular attention being focused on the newly investigated intracellular mechanisms at the striatal level (e.g., A(2A) and NMDA receptors, PKA, CaMKII, ERK kinases, as well as immediate early genes, GAD67 and peptides). Thanks to studies performed in the 6-OHDA model, all these mechanisms have now been hypothesised to represent the site of pathological dysfunction at cellular level in Parkinson's disease.

Mesolimbic novelty processing in older adults

Normal aging is associated with neuronal loss in the dopaminergic midbrain (substantia nigra/ventral tegmental area, SN/VTA), a region that has recently been implicated in processing novel stimuli as part of a mesolimbic network including the hippocampus. Here, we quantified age-related structural degeneration of the mesolimbic system using magnetization transfer ratio (MTR) and correlated it with mesolimbic hemodynamic responses (HRs) to stimulus novelty. Twenty-one healthy older adults between 55 and 77 years performed a visual oddball paradigm allowing to distinguish mesolimbic HRs to novelty from rareness, negative emotional valence, and targetness using functional magnetic resonance imaging. The HRs in the right SN/VTA and the right hippocampus to novelty were positively correlated both with the SN/VTA MTR and hippocampus MTR but not amygdala MTR. However, the HR of the amygdala to negative emotional valence correlated with the amygdala MTR but not with the MTR in SN/VTA or the hippocampus. The results establish a structure-function relationship in support of a hippocampal-SN/VTA loop of mesolimbic novelty processing by showing that the hemodynamic activation in SN/VTA and hippocampus for novelty is selectively affected by age-related degeneration of these structures.

Intrastriatal injection of hypoxanthine reduces striatal serotonin content and impairs spatial memory performance in rats

The aim of this study was to investigate the effects of intrastriatal injection of hypoxanthine, a metabolite accumulated in Lesch-Nyhan disease, on rats' performance in the Morris water maze tasks, along with the monoamine content in striatum of rats. Male adult Wistar rats were divided in two groups: (1) saline-injected and (2) hypoxanthine-injected group. Seven days after solutions infusion, animals were trained in the Morris Water Maze or were sacrificed for evaluation of the striatal monoamine content. Results show that hypoxanthine administration caused impairment on spatial navigation in the acquisition phase in reference memory task in the Morris Water Maze, as well as in the latency to cross over the platform location in probe trial, when compared to the saline group (control). Hypoxanthine also altered rat performance in the working memory. Although striatal dopamine metabolites content did not change, treated animals showed a reduction of tissue levels of serotonin (5-HT) and 5- hydroxyl-indoleacetic acid (5-HIAA). These results show that intra-striatal hypoxanthine administration provoked impairment of spatial learning/memory in rats without affecting striatal dopaminergic system, although serotonergic pathways seem to have been affected.

Insights into the Role of Dopamine Receptor Systems in Learning and Memory

It is well established that learning and memory are complex processes involving and recruiting different brain modulatory neurotransmitter systems. Considerable evidence points to the involvement of dopamine in various aspects of cognition, and interest has been focused on investigating the clinical relevance of dopamine systems to age-related cognitive decline and manifestations of cognitive impairment in schizophrenia, Alzheimer's disease, Parkinson's disease and other neurodegenerative diseases. In the past decade or so, in spite of the molecular cloning of the five dopamine receptor subtypes, their specific roles in brain function remained inconclusive due to the lack of completely selective ligands that could distinguish between the members of the D1-like and D2-like dopamine receptor families. One of the most important advances in the field of dopamine research has been the generation of mutant mouse models permitting evaluation of the dopaminergic system using gene targeting technologies. These mouse models represent an important approach to explore the functional roles of closely related receptor subtypes. In this review, we present and discuss evidence on the role of dopamine receptors in different aspects of learning and memory at the cellular, molecular and behavioral levels. We compare evidence using conventional pharmacological, lesion or electrophysiological studies with results from mice with targeted deletions of different subtypes of dopamine receptor genes. We particularly focus on dopamine D1 and D2 receptors in an effort to delineate their specific roles in various aspects of cognitive function. We provide strong evidence, from our own recent work as well as others, that dopamine is part of the network that plays a very important role in cognitive function, and that although multiple dopamine receptor subtypes contribute to different aspects of learning and memory, the D1 receptor seems to play a more prominent role in mediating plasticity and specific aspects of cognitive function, including spatial learning and memory processes, reversal learning, extinction learning, and incentive learning.

The effects of zinc on the olfactory neuroepithelium and olfactory bulbs of the Sprague-Dawley rat after oral administration of zinc-gluconate trihydrate

BACKGROUND

The most frequent causes of upper respiratory infections are human rhinoviruses. The nasopharyngeal area, which includes the respiratory epithelium, mucosa, and the olfactory neuroepithelium (ONe), is a first-line of defense against airborne viruses and allergens, some of which manage to penetrate the nasal mucosa and invade the tissues of the nasal respiratory epithelium. Biochemical evidence from several studies suggests that zinc is an effective cold treatment and that over-the-counter (OTC) zinc-gluconate compounds may provide the high pharmacologic doses of zinc needed to act as an effective means of treating and reducing the duration and severity of symptoms of the common cold.

METHODS

A series of male Sprague-Dawley rats were fed an oral preparation of zinc-gluconate trihydrate or received the equivalent through drinking water to investigate the potential cytotoxic and/or neurotoxic insult to the olfactory receptor cells and other tissue in the ONe and afferent neuronal pathways.

RESULTS

Coronal sections of the rat ONe and corresponding olfactory bulbs showed consistent cellular and tissue damage of increasing severity that correlated with the duration of treatment with the zinc compound when compared with the control group animals.

CONCLUSION

The results of this analysis indicate that the repeated oral administration of such zinc-containing compounds have neurotoxic effects on the ONe and to the mitral cells in the olfactory bulbs of treated rats. These findings point toward the need for increased investigation into the potential deleterious effects of zinc-containing compounds to humans as well.

Dopaminergic modulation of the persistence of one-trial hippocampus-dependent memory

The persistence of new memory traces in the hippocampus, encoded following appropriate activation of glutamatergic receptors and the induction of synaptic plasticity, can be influenced by heterosynaptic activation of neuromodulatory brain systems. We therefore investigated the effects of a hippocampus-specific blockade of dopamine D1/D5 receptors on the persistence of spatial memory encoded in one trial using a delayed matching-to-place (DMP) task in a watermaze in which rats learn a new escape location each day. A within-subjects design was used such that both short (20 min) and long (6 h) retention intervals, and both drug (SCH23390, a D1/D5 receptor antagonist) and vehicle (aCSF) infusions were tested on different days in the same animals. Bilateral intrahippocampal infusion of SCH23390 (5 microg in 1 microL per side) prior to trial 1 (encoding) caused a differential impairment as a function of memory delay-with no effect during trial 2 (memory retrieval) after a 20-min interval, but a block of memory at 6 h. Further experiments revealed that infusion of SCH23390 immediately after trial 1 had no effect on retention 6 h later, and the poor memory seen at long retention intervals when the drug was present at encoding was not due to a state-dependent failure of retrieval. These results suggest that activation of D1/D5 receptors during memory encoding is necessary for the formation of a persistent memory trace in the hippocampus. The complementary effects of D1/D5 receptor blockade on the persistence of LTP and the duration of memory are consistent with the idea that changes in synaptic strength underlie memory.

Reward-Motivated Learning: Mesolimbic Activation Precedes Memory Formation

We examined anticipatory mechanisms of reward-motivated memory formation using event-related FMRI. In a monetary incentive encoding task, cues signaled high- or low-value reward for memorizing an upcoming scene. When tested 24 hr postscan, subjects were significantly more likely to remember scenes that followed cues for high-value rather than low-value reward. A monetary incentive delay task independently localized regions responsive to reward anticipation. In the encoding task, high-reward cues preceding remembered but not forgotten scenes activated the ventral tegmental area, nucleus accumbens, and hippocampus. Across subjects, greater activation in these regions predicted superior memory performance. Within subject, increased correlation between the hippocampus and ventral tegmental area was associated with enhanced long-term memory for the subsequent scene. These findings demonstrate that brain activation preceding stimulus encoding can predict declarative memory formation. The findings are consistent with the hypothesis that reward motivation promotes memory formation via dopamine release in the hippocampus prior to learning.

The dopaminergic midbrain participates in human episodic memory formation: evidence from genetic imaging

Recent data from animal studies raise the possibility that dopaminergic neuromodulation promotes the encoding of novel stimuli. We investigated a possible role for the dopaminergic midbrain in human episodic memory by measuring how polymorphisms in dopamine clearance pathways affect encoding-related brain activity (functional magnetic resonance imaging) in an episodic memory task. In 51 young, healthy adults, successful episodic encoding was associated with activation of the substantia nigra. This midbrain activation was modulated by a functional variable number of tandem repeat (VNTR) polymorphism in the dopamine transporter (DAT1) gene. Despite no differences in memory performance between genotype groups, carriers of the (low expressing) 9-repeat allele of the DAT1 VNTR showed relatively higher midbrain activation when compared with subjects homozygous for the 10-repeat allele, who express DAT1 at higher levels. The catechol-O-methyl transferase (COMT) Val108/158Met polymorphism, which is known to modulate enzyme activity, affected encoding-related activity in the right prefrontal cortex (PFC) and in occipital brain regions but not in the midbrain. Moreover, subjects homozygous for the (low activity) Met allele showed stronger functional coupling between the PFC and the hippocampus during encoding. Our finding that genetic variations in the dopamine clearance pathways affect encoding-related activation patterns in midbrain and PFC provides strong support for a role of dopaminergic neuromodulation in human episodic memory formation. It also supports the hypothesis of anatomically and functionally distinct roles for DAT1 and COMT in dopamine metabolism, with DAT1 modulating rapid, phasic midbrain activity and COMT being particularly involved in prefrontal dopamine clearance.

The accessory optic system: basic organization with an update on connectivity, neurochemistry, and function

The accessory optic system (AOS) is formed by a series of terminal nuclei receiving direct visual information from the retina via one or more accessory optic tracts. In addition to the retinal input, derived from ganglion cells that characteristically have large receptive fields, are direction-selective, and have a preference for slow moving stimuli, there are now well-characterized afferent connections with a key pretectal nucleus (nucleus of the optic tract) and the ventral lateral geniculate nucleus. The efferent connections of the AOS are robust, targeting brainstem and other structures in support of visual-oculomotor events such as optokinetic nystagmus and visual-vestibular interaction. This chapter reviews the newer experimental findings while including older data concerning the structural and functional organization of the AOS. We then consider the ontogeny and phylogeny of the AOS and include a discussion of similarities and differences in the anatomical organization of the AOS in nonmammalian and mammalian species. This is followed by sections dealing with retinal and cerebral cortical afferents to the AOS nuclei, interneuronal connections of AOS neurons, and the efferents of the AOS nuclei. We conclude with a section on Functional Considerations dealing with the issues of the response properties of AOS neurons, lesion and metabolic studies, and the AOS and spatial cognition.