Hippocampal place cell responses to distal and proximal cue manipulations in dopamine D2 receptor-knockout mice.

The human hippocampus is critical for learning and memory. In rodents, hippocampal pyramidal neurons fire in a location-specific manner and form relational representations of environmental cues. The important roles of dopaminergic D1 receptors in learning and in hippocampal neural synaptic plasticity in novel environments have been previously shown. However, the roles of D2 receptors in hippocampal neural plasticity in response to novel and familiar spatial stimuli remain unclear. In order to clarify this issue, we recorded from hippocampal neurons in dopamine D2 receptor-knockout (D2R-KO) mice and their wild-type (WT) littermates during manipulations of distinct spatial cues in familiar and novel environments. Here, we report that D2R-KO mice showed substantial deficits in place-cell properties (number of place cells, intra-field firing rates, spatial tuning, and spatial coherence). Furthermore, although place cells in D2R-KO mice responded to manipulations of distal and proximal cues in both familiar and novel environments in a manner that was similar to place cells in WT mice, place fields were less stable in the D . The axes represent the differences between the peak and the valley of each waveform of EL2 and EL3.2R-KO mice in the familiar environment, but not in the novel environment. The present results suggested that D2 receptors in the hippocampus are important for place response stability. The place-cell properties of D2R-KO mice were similar to aged animals, suggesting that the alterations of place-cell properties in aged animals might be ascribed partly to alterations in the D2R in the HF of aged animals.

Reward Contingency Modulates Neuronal Activity in Rat Septal Nuclei during Elemental and Configural Association Tasks.

It has been suggested that septal nuclei are important in the control of behavior during various reward and non-reward situations. In the present study, neuronal activity was recorded from rat septal nuclei during discrimination of conditioned sensory stimuli (CSs) of the medial forebrain bundle associated with or without a reward (sucrose solution or intracranial self-stimulation, ICSS). Rats were trained to lick a spout protruding close to the mouth just after a CS to obtain a reward stimulus. The CSs included both elemental and configural stimuli. In the configural condition, the reward contingency of the stimuli presented together was opposite to that of each elemental stimulus presented alone, although the same sensory stimuli were involved. Of the 72 responsive septal neurons, 18 responded selectively to the CSs predicting reward (CS(+)-related), four to the CSs predicting non-reward (CS(0)-related), nine to some CSs predicting reward or non-reward, and 15 non-differentially to all CSs. The remaining 26 neurons responded mainly during the ingestion/ICSS phase. A multivariate analysis of the septal neuronal responses to elemental and configural stimuli indicated that septal neurons encoded the CSs based on reward contingency, regardless of the stimulus physical properties and were categorized into three groups; CSs predicting the sucrose solution, CSs predicting a non-reward, and CSs predicting ICSS. The results suggest that septal nuclei are deeply involved in discriminating the reward contingency of environmental stimuli to manifest appropriate behaviors in response to changing stimuli.

Cognitive and socio-emotional deficits in platelet-derived growth factor receptor-ß gene knockout mice.

Platelet-derived growth factor (PDGF) is a potent mitogen. Extensive in vivo studies of PDGF and its receptor (PDGFR) genes have reported that PDGF plays an important role in embryogenesis and development of the central nervous system (CNS). Furthermore, PDGF and the ß subunit of the PDGF receptor (PDGFR-ß) have been reported to be associated with schizophrenia and autism. However, no study has reported on the effects of PDGF deletion on mice behavior. Here we generated novel mutant mice (PDGFR-ß KO) in which PDGFR-ß was conditionally deleted in CNS neurons using the Cre/loxP system. Mice without the Cre transgene but with floxed PDGFR-ß were used as controls. Both groups of mice reached adulthood without any apparent anatomical defects. These mice were further examined by conducting several behavioral tests for spatial memory, social interaction, conditioning, prepulse inhibition, and forced swimming. The test results indicated that the PDGFR-ß KO mice show deficits in all of these areas. Furthermore, an immunohistochemical study of the PDGFR-ß KO mice brain indicated that the number of parvalbumin (calcium-binding protein)-positive (i.e., putatively γ-aminobutyric acid-ergic) neurons was low in the amygdala, hippocampus, and medial prefrontal cortex. Neurophysiological studies indicated that sensory-evoked gamma oscillation was low in the PDGFR-ß KO mice, consistent with the observed reduction in the number of parvalbumin-positive neurons. These results suggest that PDGFR-ß plays an important role in cognitive and socioemotional functions, and that deficits in this receptor may partly underlie the cognitive and socioemotional deficits observed in schizophrenic and autistic patients.

A method for recording evoked local field potentials in the primate dentate gyrus in vivo.

Recording evoked local field potentials (LFPs) in the hippocampus in vivo has yielded us useful information about the neural mechanisms of learning and memory. Although this technique has been used in studies of the hippocampus of rodents, lagomorphs, and felines, it has not yet been applied to the primate hippocampus. Here, we report a method for recording evoked LFPs in the hippocampus of monkeys. A stimulation electrode and a recording electrode were implanted in the perforant pathway and dentate gyrus, respectively, under the guidance of electrophysiological recording. With a low stimulus intensity just above the threshold, the potential appeared as a slow positive-wave component, which was regarded as field excitatory postsynaptic potential (putative fEPSP); as stimulation intensity increased, the fEPSP amplitude increased, followed by a sharp negative component which was regarded as putative population spike. When the coordinates of the recording or stimulation electrode were moved stepwise, we observed a systematic change in the waveforms of evoked LFPs; this change corresponded to the structural arrangement through which the electrode passed. In a test for short-term synaptic plasticity by paired-pulse stimulation, potentials evoked by the second pulse were influenced by the first one in a manner dependent on interpulse intervals. In a test for long-term synaptic plasticity by high-frequency stimulation, the slopes of the fEPSPs and the area of population spikes were increased for more than 1 h. These results indicate that the method developed in the present study is useful for testing theories of hippocampal functions in primates.

The inositol phosphatase SHIP2 negatively regulates insulin/IGF-I actions implicated in neuroprotection and memory function in mouse brain.

Impairment of insulin and IGF-I signaling in the brain is one of the causes of dementia associated with diabetes mellitus and Alzheimer's disease. However, the precise pathological processes are largely unknown. In the present study, we found that SH2-containing inositol 5'-phosphatase 2 (SHIP2), a negative regulator of phosphatidylinositol 3,4,5-trisphosphate-mediated signals, is widely expressed in adult mouse brain. When a dominant-negative mutant of SHIP2 was expressed in cultured neurons, insulin signaling was augmented, indicating physiological significance of endogenous SHIP2 in neurons. Interestingly, SHIP2 mRNA and protein expression levels were significantly increased in the brain of type 2 diabetic db/db mice. To investigate the impact of increased expression of SHIP2 in the brain, we further employed transgenic mice overexpressing SHIP2 and found that increased amounts of SHIP2 induced the disruption of insulin/IGF-I signaling through Akt. Neuroprotective effects of insulin and IGF-I were significantly attenuated in cultured cerebellar granule neurons from SHIP2 transgenic mice. Consistently, terminal deoxynucleotide transferase-mediated dUTP nick end labeling assay demonstrated that the number of apoptosis-positive cells was increased in cerebral cortex of the transgenic mice at an elderly age. Furthermore, SHIP2 transgenic mice exhibited impaired memory performance in the Morris water maze, step-through passive avoidance, and novel-object-recognition tests. Importantly, inhibition of SHIP2 ameliorated the impairment of hippocampal synaptic plasticity and memory formation in db/db mice. These results suggest that SHIP2 is a potent negative regulator of insulin/IGF-I actions in the brain, and excess amounts of SHIP2 may be related, at least in part, to brain dysfunction in insulin resistance with type 2 diabetes.

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.

Neural correlates of stimulus-reward association in the rat mediodorsal thalamus.

Neural activity was recorded from the mediodorsal thalamic nucleus of behaving rats during the discrimination of olfactory cues associated with or without a reward. Approximately 10% of mediodorsal thalamic neurons showed significant responses during the presentation of cues. Most of these neurons responded strongly to cues associated with a reward. These neurons also differentiated between cues, even in the same reinforcement (reward/nonreward) group. All of the neurons tested in extinction, relearning, and/or new-learning trials changed their responses flexibly according to the reward contingency. These neurons were located primarily in the central and medial segments of the mediodorsal thalamus. These results suggest that these mediodorsal thalamic neurons are the neural substrates for association learning of olfactory stimuli with rewards.

Auditory thalamus integrates visual inputs into behavioral gains.

By binding multisensory signals, we get robust percepts and respond to our surroundings more correctly and quickly. How and where does the brain link cross-modal sensory information to produce such behavioral advantages? The classical role of sensory thalamus is to relay modality-specific information to the cortex. Here we find that, in the rat thalamus, visual cues influence auditory responses, which have two distinct components: an early phasic one followed by a late gradual buildup that peaks before reward. Although both bimodal presentation and reward value had similar effects on behavioral performance, the cross-modal effect on neural activity showed unique temporal dynamics: it affected the amplitude of the early component and starting level of the late component, whereas reward value affected only the slope of the late component. These results demonstrate that cross-modal cueing modulates gain in the sensory thalamus, potentially providing a priming influence on the choice of an optimal behavior.

Effects of prenatal maternal stress by repeated cold environment on behavioral and emotional development in the rat offspring.

It has been reported that many types of stresses, which caused physiological and psychological alterations in dams as prenatal maternal stress, affected behavioral and emotional traits of their offspring. However, effects of environmental temperature changes, which induce various stress responses in both animals and humans, have not been assessed as prenatal maternal stress. Repeated cold stress (RCS) is a type of chronic cold stress in which environmental temperature changes rapidly and frequently several times within a day. In the present study, to investigate effects of chronic maternal stress by the RCS on behavioral and emotional development of the rat offspring (prenatal RCS rats), the RCS stress was loaded to pregnant rats between day 9 and 19 after fertilization. The prenatal RCS rats showed similar locomotor activity in an open field to control rats that were borne by non-stressed pregnant rats. On the other hand, the prenatal RCS rats showed significantly higher startle responses than the control rats in a light enhanced startle paradigm. However, treatment of diazepam decreased the startle responses in the prenatal RCS rats to the same degree as those in the control rats. The results indicated that prenatal RCS affected emotional development of the rat offspring, but not locomotor activity. Comparison of the present results with the previous studies suggests that there might be unknown common mechanisms among different prenatal maternal stresses that induce similar behavioral developmental alteration.

Dopamine D1 receptors involved in locomotor activity and accumbens neural responses to prediction of reward associated with place.

Predicting reward is essential in learning approach behaviors. Dopaminergic activity has been implicated in reward, movement, and cognitive processes, all essential elements in learning. The nucleus accumbens (NAc) receives converging inputs from corticolimbic information-processing areas and from mesolimbic dopamine neurons originating in the ventral tegmental area. Previously, we reported that in mice, a dopamine D2 receptor knockout (D2R-KO) eliminated the prereward inhibitory response, increased place-field size of NAc neurons, and reduced locomotor activity without marked change in intracranial self-stimulation (ICSS) behavior. The present study investigated the specific contribution of dopamine D1 receptor (D1R) in mediating reward, locomotor activity, and spatial associative processes and in regulating NAc neural responses. In contrast to D2R-KO animals, here we find D1R-KO in mice selectively eliminated the prereward excitatory response and decreased place-field size of NAc neurons. Furthermore, D1R-KO impaired ICSS behavior, seriously reduced locomotor activity, and retarded acquisition of a place learning task. Thus, the present results suggest that D1R may be an important determinant in brain stimulation reward (ICSS) and participates in coding for a type of reward prediction of NAc neurons and in spatial learning.

Comparison of brain activity between dopamine D2 receptor-knockout and wild mice in response to dopamine agonist and antagonist assessed by fMRI.

It has been reported that antipsychotic dopamine-D2-receptor (D2R) antagonists affected other neurotransmitter systems. In the present study, the effects of a D2R agonist, bromocriptine, and a D2R antagonist, spiperone, on brain activity were investigated using wild-type mice (WT) with intact D2Rs, and D2R-knockout mice (D2R-KO) lacking D2Rs by functional magnetic resonance imaging. In the WT, flow-weighted signal intensity significantly increased after administration of bromocriptine in the hippocampal formation. In contrast, signal intensity significantly decreased after administration of spiperone in the somatosensory-motor cortices, thalamus, anterior cingulate cortex, caudate-putamen, nucleus accumbens, hippocampal formation, and amygdala. In the D2R-KO, however, no significant changes were observed after administration of either bromocriptine or spiperone. The present results indicated that the D2R-KO lacked sensitivity to D2R agonist and antagonist in agreement with its genetic defects, which confirmed that the changes in brain activity in the WT after administration of either drug were mediated through D2Rs. These results suggest that antipsychotic D2R antagonists affect activity of the same brain regions of human patients through D2Rs, as observed in the present study. These changes in brain activity might be related to therapeutic efficacy as well as side effects of antipsychotic drugs on schizophrenic patients.

Effects of nonsaponin fraction of red ginseng on learning deficits in aged rats.

Previously we reported that oral application of red ginseng significantly ameliorated learning deficits in aged rats and young rats with hippocampal lesions. In the present study, we investigated the effects of the nonsaponin fraction of red ginseng on learning deficits in aged rats in behavioral studies and those on long-term potentiation (LTP) in the hippocampal CA3 subfield in young rats in electrophysiological studies. In the behavioral studies, three groups of rats [aged rats with and without oral administration of the nonsaponin fraction of red ginseng and young rats] were tested with the three types of spatial-learning task [distance movement task (DMT), random-reward place search task (RRPST), and place-learning task (PLT)] in a circular open field. The results in the DMT and RRPST indicated that motivational and motor activity was not significantly different among the three groups of rats. However, performance of the aged rats without nonsaponin was significantly impaired in the PLT when compared with the young rats. Treatment with nonsaponin significantly ameliorated deficits in place-navigation learning in the aged rats in the PLT. In the electrophysiological studies, effects of nonsaponin on the LTP in the CA3 subfield of the hippocampal slices were investigated in vitro. Pretreatment with nonsaponin significantly augmented the increase in population spike amplitudes in the CA3 subfield after LTP induction. These results suggest that the nonsaponin fraction of red ginseng contains important substances to improve learning and memory in aged rats and that this amelioration by nonsaponin might be attributed partly to augmentation of LTP in the CA3 subfield.

Proof of the mysterious efficacy of ginseng: basic and clinical trials: effects of red ginseng on learning and memory deficits in an animal model of amnesia.

Ameliorating effects of red ginseng on learning and memory deficits due to hippocampal lesions and aging were reviewed; the performance of young rats with selective hippocampal lesions with or without red ginseng (p.o.), and aged rats with or without red ginseng (p.o.) in the spatial learning tasks was compared with that of sham-operated or intact young rats. Each rat was tested with 3 types of spatial learning tasks (distance movement task, DMT; random reward place search task, RRPST; and place learning task, PLT) in a circular open field using intracranial self-stimulation (ICSS) as reward. The results in the DMT and RRPST indicated that motivational and motor activity of young rats with hippocampal lesions with and without ginseng and aged rats with and without ginseng were not significantly different from that of control young rats. However, young rats with hippocampal lesions without ginseng and aged rats without ginseng displayed significant deficits in the PLT. Treatment with red ginseng significantly ameliorated place-navigation deficits in young rats with hippocampal lesions in the PLT. Similarly, red ginseng improved performance of aged rats in the PLT. The results, along with previous studies showing significant effects of red ginseng on the central nervous system, suggest that red ginseng ameliorates learning and memory deficits through effects on the central nervous system, partly through effects on the hippocampal formation. However, its mechanisms are still unclear, and further studies are required.

Regulated transgene delivery by ganciclovir in the brain without physiological alterations by a live attenuated herpes simplex virus vector.

The distribution of a live attenuated herpes simplex virus (betaH1)-mediated gene delivery into the central nervous system (CNS) was regulated by growth inhibition with ganciclovir (GCV) and the effect of this transgene expression system on the physiologic response was characterized by the acoustic startle response and its prepulse inhibition. We inoculated betaH1 expressing beta-galactosidase (beta-gal) driven by the latency associated transcripts promoter into the right caudate putamen of rats. Histochemical analysis demonstrated that the inoculation of betaH1 in the right caudate putamen resulted in a high level of beta-gal expression in the neurons of the area projecting to the inoculation site. On 14 days after inoculation without GCV-treatment, beta-gal activity localized in the anterior olfactory nucleus, frontal, insular, orbital, parietal, perirhinal, piriform cortices and the temporal region including the amygdala. In contrast, the distribution of beta-gal activity was regulated by the interval between virus inoculation and GCV-treatment and maintained after its cessation without significant alteration. The whole process of transgene expression did not influence the emotional behavior, indicating that this vector system is a suitable model for analyzing the transgene function or applying the gene therapy for the CNS diseases.

Altered accumbens neural response to prediction of reward associated with place in dopamine D2 receptor knockout mice.

Midbrain dopaminergic activity seems to be important in forming the prediction of future events such as rewards. The nucleus accumbens (NAc) plays an important role in the integration of reward with motor function, and it receives dense dopamine innervation and extensive limbic and cortical afferents. Here, we examined the specific role of the dopamine D2 receptor (D2R) in mediating associative learning, locomotor activity, and regulating NAc neural responses by using D2R-knockout (KO) mice and their wild-type littermates. D2R-KO mice displayed reduced locomotor activity and slower acquisition of a place-learning task. D2R-KO eliminated the prereward inhibitory response of neurons in the NAc. In contrast, an increased number of neurons in D2R-KO mice displayed place-related activity. These results provide evidence that D2R in the NAc participates in coding for a specific type of neural response to incentive contingencies and partly in spatial learning.

Neuronal responses of the rat amygdala during extinction and reassociation learning in elementary and configural associative tasks.

To investigate functional heterogeneity within the amygdala in appetitive conditioned instrumental behaviours, neuronal activity was recorded from the amygdala of behaving rats during learning and discrimination of conditioned sensory stimuli associated with or without reinforcement [sucrose solution, intracranial self-stimulation (ICSS)]. Sensory stimuli included auditory (tone), visual (light) and configural (simultaneous presentation of tone and light) stimuli. The rat was trained to lick a spout protruded close to its mouth just after a conditioned sensory stimulus to obtain a reward. Of the 609 neurons recorded from the amygdala and amygdalostriatal transition area, 154 responded to one or more sensory stimuli. The 62 amygdalar neurons responded strongly to certain conditioned sensory stimuli associated with rewards. Of these 62 neurons, 45 were tested with the extinction trials. Responses of 31 neurons to conditioned stimuli were finally extinguished, and those of the remaining 14 were not extinguished. Furthermore, responses of 26 of these 31 neurons resumed in the relearning trials (plastic neurons), suggesting that these sensory responses were associative rather than just responses to physical properties of the stimuli. These plastic neurons were located mainly in the basolateral nucleus of the amygdala, and responses of the plastic neurons were correlated with behavioural responses. These results suggest that the basolateral nucleus is crucial in associative learning between sensory information and affective significance for behavioural outputs in appetitive conditioned instrumental behaviours.

Effects of a novel arginine-vasopressin derivative, NC-1900, on the spatial memory impairment of rats with transient forebrain ischemia.

NC-1900, an arginine-vasopressin derivative, has been reported to enhance memory for avoidance behavior. Specifically, NC-1900 ameliorated cycloheximide-induced learning impairments in a passive avoidance test in rats. In the present study, we investigated that effects of NC-1900 on place learning in rats with selective lesions in the CA1 subfield of the hippocampal formation produced by transient forebrain ischemia. NC-1900 was administered daily (1 microg/kg, p.o.) 1 h before the place learning task. A rat was required to alternate between 2 small circular areas located diametrically opposite each other on the circumference of an open field in order to obtain intracranial electrical stimulation reward (the spatial navigation task). Rats with hippocampal lesions showed severe place learning impairments both in task performance (indicated by number of rewards obtained per a session) and in navigation performance (forming efficient trails) over the 30-day test period. Treatment with NC-1900 ameliorated deficits in the place learning exhibited by rats with the same hippocampal lesions, such that their performance reached normal levels. There were no significant differences in the ischemic hippocampal lesions, spontaneous locomotor activity, and stimulation current intensity between the treated and untreated rats. The results demonstrated that NC-1900 reduced place learning impairments produced by hippocampal lesions.