Microinjection of GABA-A agonist muscimol into the dorsal but not the ventral hippocampus impairs non-mnemonic measures of delayed non-matching-to-position performance in rats

Toxicological and pharmacological profile of Amanita muscaria (L.) Lam. – a new rising opportunity for biomedicine

Amanita muscaria, commonly known as fly agaric, is a basidiomycete. Its main psychoactive constituents are ibotenic acid and muscimol, both involved in ‘pantherina-muscaria’ poisoning syndrome. The rising pharmacological and toxicological interest based on lots of contradictive opinions concerning the use of Amanita muscaria extracts’ neuroprotective role against some neurodegenerative diseases such as Parkinson’s and Alzheimer’s, its potent role in the treatment of cerebral ischaemia and other socially significant health conditions gave the basis for this review. Facts about Amanita muscaria’s morphology, chemical content, toxicological and pharmacological characteristics and usage from ancient times to present-day’s opportunities in modern medicine are presented.

The role of the hippocampus in object discrimination based on visual features

The role of rodent hippocampus has been intensively studied in different cognitive tasks. However, its role in discrimination of objects remains controversial due to conflicting findings. We tested whether the number and type of features available for the identification of objects might affect the strategy (hippocampal-independent vs. hippocampal-dependent) that rats adopt to solve object discrimination tasks. We trained rats to discriminate 2D visual objects presented on a computer screen. The objects were defined either by their shape only or by multiple-features (a combination of filling pattern and brightness in addition to the shape). Our data showed that objects displayed as simple geometric shapes are not discriminated by trained rats after their hippocampi had been bilaterally inactivated by the GABA_A-agonist muscimol. On the other hand, objects containing a specific combination of non-geometric features in addition to the shape are discriminated even without the hippocampus. Our results suggest that the involvement of the hippocampus in visual object discrimination depends on the abundance of object's features.

Abnormal neural activation patterns underlying working memory impairment in chronic phencyclidine-treated mice

Working memory impairment is a hallmark feature of schizophrenia and is thought be caused by dysfunctions in the prefrontal cortex (PFC) and associated brain regions. However, the neural circuit anomalies underlying this impairment are poorly understood. The aim of this study is to assess working memory performance in the chronic phencyclidine (PCP) mouse model of schizophrenia, and to identify the neural substrates of working memory. To address this issue, we conducted the following experiments for mice after withdrawal from chronic administration (14 days) of either saline or PCP (10 mg/kg): (1) a discrete paired-trial variable-delay task in T-maze to assess working memory, and (2) brain-wide c-Fos mapping to identify activated brain regions relevant to this task performance either 90 min or 0 min after the completion of the task, with each time point examined under working memory effort and basal conditions. Correct responses in the test phase of the task were significantly reduced across delays (5, 15, and 30 s) in chronic PCP-treated mice compared with chronic saline-treated controls, suggesting delay-independent impairments in working memory in the PCP group. In layer 2-3 of the prelimbic cortex, the number of working memory effort-elicited c-Fos+ cells was significantly higher in the chronic PCP group than in the chronic saline group. The main effect of working memory effort relative to basal conditions was to induce significantly increased c-Fos+ cells in the other layers of prelimbic cortex and the anterior cingulate and infralimbic cortex regardless of the different chronic regimens. Conversely, this working memory effort had a negative effect (fewer c-Fos+ cells) in the ventral hippocampus. These results shed light on some putative neural networks relevant to working memory impairments in mice chronically treated with PCP, and emphasize the importance of the layer 2-3 of the prelimbic cortex of the PFC.

Estrogen and memory system bias in females across the lifespan

Studies in both rodents and humans have made much progress in shedding light on how fluctuations in ovarian hormones can affect memory in women across the lifespan. Specifically, advances in neuroscience have identified multiple memory systems that are each mediated by different brain areas. Two memory systems used to navigate an environment are ‘place’ and ‘response’ memory. They are defined as either using an allocentric strategy: using a spatial or cognitive map of the surroundings, or an egocentric strategy: using habitual-turns/movements, respectively. Studies in neuroendocrinology have shown that estrogen levels can bias a female to use one memory system over another to solve a task, such that high estrogen levels are associated with using place memory and low levels with using response memory. Furthermore, recent advances in identifying and localizing estrogen receptors in the rodent brain are uncovering which brain regions are affected by estrogen and providing insight into how hormonal fluctuations during the menstrual cycle, pregnancy, and menopause might affect which memory system is facilitated or impaired in women at different life stages. These studies can help point the way to improving cognitive health in women.

Learning-facilitated synaptic plasticity occurs in the intermediate hippocampus in association with spatial learning

The dorsoventral axis of the hippocampus is differentiated into dorsal, intermediate, and ventral parts. Whereas the dorsal part is believed to specialize in processing spatial information, the ventral may be equipped to process non-spatial information. The precise role of the intermediate hippocampus is unclear, although recent data suggests it is functionally distinct, at least from the dorsal hippocampus. Learning-facilitated synaptic plasticity describes the ability of hippocampal synapses to respond with robust synaptic plasticity (>24 h) when a spatial learning event is coupled with afferent stimulation that would normally not lead to a lasting plasticity response: in the dorsal hippocampus novel space facilitates robust expression of long-term potentiation (LTP), whereas novel spatial content facilitates long-term depression (LTD). We explored whether the intermediate hippocampus engages in this kind of synaptic plasticity in response to novel spatial experience. In freely moving rats, high-frequency stimulation at 200 Hz (3 bursts of 15 stimuli) elicited synaptic potentiation that lasted for at least 4 h. Coupling of this stimulation with the exploration of a novel holeboard resulted in LTP that lasted for over 24 h. Low frequency afferent stimulation (1 Hz, 900 pulses) resulted in short-term depression (STD) that was significantly enhanced and prolonged by exposure to a novel large orientational (landmark) cues, however LTD was not enabled. Exposure to a holeboard that included novel objects in the holeboard holes elicited a transient enhancement of STD of the population spike (PS) but not field EPSP, and also failed to facilitate the expression of LTD. Our data suggest that the intermediate dentate gyrus engages in processing of spatial information, but is functionally distinct to the dorsal dentate gyrus. This may in turn reflect their assumed different roles in synaptic information processing and memory formation.

Theta oscillations in the medial prefrontal cortex are modulated by spatial working memory and synchronize with the hippocampus through its ventral subregion

The rodent medial prefrontal cortex (mPFC) is critical for spatial working memory (SWM), but the underlying neural processes are incompletely understood. During SWM tasks, neural activity in the mPFC becomes synchronized with theta oscillations in the hippocampus, and the strength of hippocampal-prefrontal synchrony is correlated with behavioral performance. However, to what extent the mPFC generates theta oscillations and whether they are also modulated by SWM remains unclear. Furthermore, it is not known how theta oscillations in the mPFC are synchronized with theta oscillations in the hippocampus. Although the ventral hippocampus (vHPC) projects directly to the mPFC, previous studies have only examined synchrony between the mPFC and the dorsal hippocampus (dHPC), with which it is not directly connected. To address these issues, we recorded simultaneously from the dHPC, vHPC, and mPFC of mice performing a SWM task in a T-maze. The local field potential recorded in the mPFC displayed robust theta oscillations that were reflected in local measures of neuronal activity and modulated by SWM performance. mPFC theta oscillations were also synchronized with theta oscillations in both the vHPC and dHPC, and the magnitude of theta synchrony was modulated by SWM. Removing the influence of the vHPC either computationally (through partial correlations) or experimentally (through pharmacological inactivation) reduced theta synchrony between the mPFC and dHPC. These results reveal theta oscillations as a prominent feature of neural activity in the mPFC and a candidate neural mechanism underlying SWM. Furthermore, our results suggest that the vHPC plays a major role in synchronizing theta oscillations in the mPFC and the hippocampus.

NMDA receptor-dependent synaptic plasticity in dorsal and intermediate hippocampus exhibits distinct frequency-dependent profiles

The hippocampus may be functionally differentiated along its dorsoventral axis. In contrast to the wealth of data available on synaptic plasticity mechanisms in the dorsal hippocampus, little is known about synaptic plasticity processes in the intermediate hippocampus. Behavioral data suggest that this structure may play a distinct role in learning and memory. Here, we compared amplitudes, frequency-dependency and persistency of long-term potentiation (LTP) and long-term depression (LTD) in the dorsal (DDG) and intermediate dentate gyrus (IDG). In freely moving rats, high-frequency stimulation (HFS) at 200 Hz (10 burst of 15 stimuli) elicited LTP of similar magnitude in both structures that persisted for over 24 h. The intermediate dentate gyrus is more likely to exhibit persistent LTP than its dorsal counterpart, however: HFS at 200 Hz (3 or 1 burst(s)) or 100 Hz elicited short-term potentiation (STP) in DDG, unlike in the IDG, where LTP could be recorded for at least 4 h. Whereas low frequency stimulation (LFS) at 1 Hz elicited long-lasting LTD (>24 h) in the DDG, it had no significant effect on fEPSP profile in the IDG. LFS at 2 Hz elicited short-term depression in DDG and had no effect in IDG. LTP in both IDG and DDG required activation of N-methyl-D-aspartate receptors. Paired-pulse and input-output responses differed in IDG and DDG. Our data suggest that afferent input from the entorhinal cortex generates a different response profile in the dorsal vs. intermediate DG, which may in turn relate to their postulated distinct roles in synaptic information processing and memory formation. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

Reproductive experience modifies the effects of estradiol on learning and memory bias in female rats

Previous studies have shown that estrogen affects whether a hippocampus-mediated place (allocentric) or a striatum-mediated response (egocentric) memory system is employed by female rats when searching for a food reward in a maze. Because it has been suggested that reproductive experience alters some of the responses to E in the brain, two experiments were carried out to investigate whether reproductive experience would also alter the effect of E on place and response learning. In experiment 1, 152 ovariectomized nulliparous (n=77; no reproductive experience) and primiparous (n=74; having had and raised one litter of pups) Wistar rats were trained on an ambiguous t-maze task and tested for memory system bias. In experiment 2, 35 ovariectomized nulliparous (n=16) and primiparous (n=19) Wistar rats were trained on place and response plus-maze tasks. All rats were exposed to no, chronic low or chronic low with pulsatile high 17β-estradiol (E2) replacement. Congruent with previous findings, low E2 nulliparous rats showed predominant use of response memory and faster response learning, whereas high E2 nulliparous rats showed a trend towards predominant place memory use. Interestingly, the facilitatory effect of low E2 on response task learning and memory seen in nulliparous rats was not observed in low E2 primiparous rats in either experiment. In conclusion, E2 levels do dictate the rate at which female rats learn a response task and utilize response memory, but only in those with no reproductive experience.

Involvement of the dorsal hippocampal GABA-A receptors in histamine-induced facilitation of memory in the Morris water maze

Several types of learning and memory processes are regulated by the hippocampus which is an important subcortical structure in the mammalians' brain. Previous investigations have shown that different receptor systems in the CA1 region of hippocampus are involved in learning and memory functions. Investigating the possible influence of dorsal hippocampal GABA-A receptors on histamine-induced spatial facilitation in adult male Wistar rats was the focus of the current study. Rats were bilaterally implanted with dorsal hippocampal (CA1) cannulae, recovered from surgery and then trained in Morris water maze (MWM) for 4 consecutive days. A block of four trials was given each day. All drugs were injected into CA1 regions, 5min before training. Pre-training intra-CA1 microinjection of muscimol, a GABA-A receptor agonist, at the dose of 0.01 or 0.02μg/rat, increased the traveled distance or the escape latency and traveled distance to the hidden platform, respectively, indicating a water maze spatial acquisition impairment. Intra-CA1 administration of bicuculline, a GABA-A receptor antagonist however, significantly decreased the escape latency and traveled distance to the hidden platform, suggesting a spatial learning facilitation. On the other hand, pre-training intra-CA1 microinjection of the subthreshold dose of muscimol plus different doses of histamine (0.025, 0.05 and 0.1μg/rat) did not alter the histamine response. Meanwhile, the co-administration of the ineffective dose of bicuculline together with histamine potentiated the spatial learning. Moreover, bilateral infusion of histamine (0.025, 0.05 and 0.1μg/rat) by itself, facilitated the spatial learning. Notably, the drug injections had no effect on swimming speed during the MWM training sessions. Our results suggest that the dorsal hippocampal (CA1) GABA-A mechanism(s) may influence the histamine-induced facilitation of spatial acquisition.

Cooperative interaction between the basolateral amygdala and ventral tegmental area modulates the consolidation of inhibitory avoidance memory

The aim of the current study was to examine the existence of a cooperative interaction between the basolateral nucleus of amygdala (BLA) and the ventral tegmental area (VTA) in inhibitory avoidance task. The BLA and the VTA regions of adult male Wistar rats were simultaneously cannulated and memory consolidation was measured in a step-through type inhibitory avoidance apparatus. Post-training microinjection of muscimol, a potent GABA-A receptor agonist (0.01-0.02 μg/rat), into the VTA impaired memory in a dose-dependent manner. Post-training intra-BLA microinjection of NMDA (0.02-0.04 μg/rat), 5 min before the intra-VTA injection of muscimol (0.02 μg/rat), attenuated muscimol-induced memory impairment. Microinjection of a NMDA receptor antagonist, D-AP5 (0.02-0.06 μg/rat) into the BLA inhibited NMDA effect on the memory impairment induced by intra-VTA microinjection of muscimol. On the other hand, post-training intra-BLA microinjection of muscimol (0.02-0.04 μg/rat) dose-dependently decreased step-through latency, indicating an impairing effect on memory. This impairing effect was however significantly attenuated by intra-VTA microinjection of NMDA (0.01-0.03 μg/rat). Intra-VTA microinjection of D-AP5 (0.02-0.08 μg/rat), 5 min prior to NMDA injection, inhibited NMDA response on the impairing effect induced by intra-BLA microinjection of muscimol. It should be considered that post-training microinjection of the same doses of NMDA or D-AP5 into the BLA or the VTA alone had no effect on memory consolidation. The data suggest that the relationship between the BLA and the VTA in mediating memory consolidation in inhibitory avoidance learning may be dependent on a cooperative interaction between the glutamatergic and GABAergic systems via NMDA and GABA-A receptors.

Involvement of basolateral amygdala GABAA receptors in the effect of dexamethasone on memory in rats

In this study we investigated whether GABA(A) receptors of the basolateral amygdala (BLA) interact with the effect of dexamethasone on the retrieval stage of memory. Adult male Wistar rats were bilaterally cannulated in the BLA by stereotaxic surgery. The animals were trained in step-through apparatus by induction of electric shock (1.5 mA, 3 s) and were tested for memory retrieval after 1 d. The time of latency for entering the dark compartment of the instrument and the time spent by rats in this chamber were recorded for evaluation of the animals' retrieval in passive avoidance memory. Administration of dexamethasone (0.3 and 0.9 mg/kg, subcutaneously (s.c.)), immediately after training, enhanced memory retrieval. This effect was reduced by intra-BLA microinjection of muscimol (0.125, 0.250 and 0.500 µg/rat), when administered before 0.9 mg/kg of dexamethasone. Microinjection of bicuculline (0.75 µg/rat, intra-BLA) with an ineffective dose of dexamethasone (0.1 mg/kg, s.c.) increased memory retrieval. However, the same doses of muscimol and bicuculline without dexamethasone did not affect memory processes. Our data support reports that dexamethasone enhances memory retrieval. It seems that GABA(A) receptors of the BLA mediate the effect of dexamethasone on memory retrieval in rats.

Dorsal versus ventral hippocampal contributions to trace and contextual conditioning: differential effects of regionally selective NMDA receptor antagonism on acquisition and expression

The dorsal and ventral subregions of the hippocampus likely play dissociable roles in some forms of learning. For example, we have previously demonstrated that temporary inactivation of ventral, but not dorsal, hippocampus dramatically impaired the acquisition of trace fear conditioning, while temporary inactivation of dorsal, but not ventral, hippocampus impaired spatially guided reinforced alternation (Czerniawski et al. (2009) Hippocampus 19:20-32). Importantly, emerging data suggest that lesions, temporary inactivation, and NMDA receptor antagonism within these subregions can produce quite different patterns of behavioral effects when administered into the same region. Specifically, while neither lesions nor temporary inactivation of dorsal hippocampus impair the acquisition of trace fear conditioning, learning in this paradigm is severely impaired by pre-training administration of the NMDA receptor antagonist dl-2-phosphonovaleric acid (APV) in dorsal hippocampus; the effect of NMDA receptor antagonism within ventral hippocampus on the acquisition and expression of trace conditioning, or on learning in general, has not yet been systematically explored. The present study extends our previous work examining the differential effect of lesions or inactivation of the dorsal and ventral hippocampal subregions by systematically examining the effect of regionally selective pre-training or pre-testing administration of APV on the acquisition and expression of trace and contextual fear conditioning. The results of these studies demonstrate that while pre-training NMDA receptor antagonism within either the dorsal or ventral subregion of the hippocampus impaired the acquisition of both trace and contextual conditioning, pre-testing NMDA receptor antagonism within ventral, but not dorsal, hippocampus impaired the expression of previously-acquired trace and contextual fear conditioning. These data suggest that selectively manipulating the integrity of individual subregions may result in compensatory mechanisms that can support learning, and that NMDA-dependent plasticity within both dorsal and ventral hippocampus is normally required for the acquisition and maintenance of memory in trace and contextual fear conditioning.

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.

Dissociating space and trace in dorsal and ventral hippocampus

Emerging evidence suggests that the hippocampus can be anatomically and functionally dissociated along its septotemporal axis into dorsal and ventral subregions. With respect to function, we have recently demonstrated that pre-training excitotoxic lesions of ventral, but not dorsal, hippocampus impair the acquisition of trace fear conditioning, whereas post-training lesions of either dorsal or ventral hippocampus impair the subsequent expression of trace fear conditioning (Yoon and Otto (2007) Neurobiol Learn Mem 87:464-475). In addition to trace fear conditioning, dorsal and ventral hippocampus appear to be differentially involved in a number of spatial memory tasks. The present study examined the effects of temporary inactivation of dorsal or ventral hippocampus on the acquisition and expression of trace fear conditioning and on performance of a spatial delayed reinforced alternation task. The findings demonstrate a double dissociation of dorsal and ventral hippocampal function: inactivation of ventral, but not dorsal, hippocampus attenuated the acquisition and expression of trace fear conditioning, whereas inactivation of dorsal, but not ventral, hippocampus dramatically impaired performance in the delayed reinforced alternation task. These data further support the notion that dorsal and ventral hippocampus contribute differentially to performance in a variety of paradigms.

Dorsal hippocampal N-methyl-D-aspartate receptors underlie spatial working memory performance during non-matching to place testing on the T-maze

Previous lesion studies have suggested a functional dissociation along the septotemporal axis of the hippocampus. Whereas the dorsal hippocampus has been implicated in spatial memory processes, the ventral hippocampus may play a role in anxiety. However, these lesion studies are potentially confounded by demyelination of fibres passing through the lesion site, and the possibility of secondary, downstream changes in associated brain structures as a consequence of their chronic denervation following the lesion. In the present study, we have used the microinfusion of muscimol to temporarily inactivate either the dorsal or ventral hippocampus in order to re-examine the contribution of the hippocampal sub-regions to spatial memory. Microinfusion studies spare fibres of passage and offer fewer opportunities for compensatory changes because the effects are transient and short-lasting. Rats were infused prior to spatial working memory testing on a non-matching to place T-maze alternation task. Spatial working memory was impaired by dorsal but not ventral hippocampal inactivation. In a second experiment, infusion of the NMDAR antagonist, D-AP5, into dorsal hippocampus also impaired spatial working memory performance, suggesting that NMDAR function within the dorsal hippocampus makes an essential contribution to this aspect of hippocampal information processing.

Hippocampal inactivation disrupts the acquisition and contextual encoding of fear extinction

In recent studies, inactivation of the dorsal hippocampus before the retrieval of extinguished fear memories disrupted the context-dependent expression of these memories. In the present experiments, we examined the role of the dorsal hippocampus in the acquisition of extinction. After pairing an auditory conditional stimulus (CS) with an aversive footshock [unconditional stimulus (US)], rats received an extinction session in which the CS was presented without the US. In experiment 1, infusion of muscimol, a GABAA receptor agonist, into the dorsal hippocampus before the extinction training session decreased the rate of extinction. Moreover, when later tested for fear to the extinguished CS, all rats that had received hippocampal inactivation before extinction training demonstrated renewed fear regardless of the context in which testing took place. This suggests a role for the dorsal hippocampus in both acquiring the extinction memory and encoding the CS-context relationship that yields the context dependence of extinction. In experiment 2, inactivation of the dorsal hippocampus before testing also disrupted the context dependence of fear to the extinguished CS. In experiment 3, quantitative autoradiography revealed the boundaries of muscimol diffusion after infusion into the dorsal hippocampus. Together, these results reveal that the dorsal hippocampus is involved in the acquisition, contextual encoding, and context-dependent retrieval of fear extinction. Learning and remembering when and where aversive events occur is essential for adaptive emotional regulation.

Intrahippocampal muscimol shifts learning strategy in gonadally intact young adult female rats

Learning strategy preferences depend upon circulating estrogen levels, with enhanced hippocampus-sensitive place learning coinciding with elevated estrogen levels. The effects of estrogen on strategy may be mediated by fluctuations in GABAergic function, given that inhibitory tone in the hippocampus is low when estrogen is high. We investigated the effects on learning strategy of intrahippocampal injections of a GABA(A) agonist in gonadally intact female rats. On the day of training, rats received 0.3 microL intrahippocampal infusions of muscimol (0.26 nmol or 2.6 nmol) or saline 20 min prior to training on a T-maze in which place (hippocampus-sensitive) or response (striatum-sensitive) strategies offer effective solutions. Muscimol treatment increased the use of the response strategy in a dose-dependent manner without influencing learning speed, indicating that muscimol modulated strategy and not learning ability. Furthermore, the muscimol-related shift to response strategies varied across the estrous cycle. The results indicate that increasing inhibition in the hippocampus biases rats away from hippocampus-sensitive place learning strategies and toward hippocampus-insensitive response learning strategies without a learning deficit. Furthermore, rats at proestrus demonstrated the most dramatic shift in learning strategy following muscimol treatment compared with control conditions, while rats at estrus demonstrated the most complete bias toward response strategies. The enhanced use of hippocampus-sensitive strategies at proestrus likely results from reduced hippocampal inhibition.

The role of the GABA(A) agonist muscimol on memory performance: reward contingencies determine the nature of the deficit

A matching-to-position (MTP) paradigm was altered to influence the type of associations a rat would use to solve the task. Our main behavioral manipulation was the application of the differential outcomes procedure (DOP). The DOP involves correlating each to-be-remembered event with a distinct reward condition. This procedure results in the development of unique reward expectancies that enhance and guide choice behavior. Such distinct reward expectancies are not formed when either a common or random assignment of reward is used (a non-differential outcomes procedure [NOP]). Intracerebroventricular infusions of the amnestic agent muscimol (GABA(A) agonist) or aCSF were delivered to male rats trained on a delayed MTP task that implemented either the DOP or the NOP. Muscimol impaired performance in a dose dependent fashion in both groups--but the nature of the deficit differed as a function of reinforcement contingencies. Rats trained with the DOP displayed a non-mnemonic delay-independent impairment: performance at all delay intervals was disrupted. In contrast, NOP-trained rats displayed a delay-dependent impairment demonstrating that muscimol can also have memory-disrupting effects. The difference in pattern of impairment appears to be a function of the associations formed during training and the type of cognitive strategies involved in maintaining behavior on a conditional delayed discrimination task when reinforcement contingencies are varied. Thus, these results demonstrate that increasing GABA(A) receptor activation impairs a range of associative and memory functions.

Deficits induced by quinolinic acid lesion to the striatum in a position discrimination and reversal task are ameliorated by permanent and temporary lesion to the globus pallidus: a potential novel treatment in a rat model of Huntington's disease

Symptoms in the early stages of Huntington's disease (HD) are assumed to reflect basal ganglia circuit dysfunction secondary to degeneration of striatal projections to the external segment of the globus pallidus (GPe). The hypothesis that GPe lesion would ameliorate HD symptoms by "normalizing" the circuit's functioning was tested in a rat model of this disease. The performance of rats sustaining quinolinic acid lesion to the striatum (a rat model of HD) in a position discrimination and reversal task was compared with the performance of rats sustaining in addition a bilateral excitotoxic lesion to the globus pallidus (GP) carried out simultaneously with the striatal lesion (Experiment 1) or 1 month after the striatal lesion (Experiment 2), as well as a unilateral temporary lesion of the GP (Experiment 3). The striatal lesion-induced deficit in the task was effectively reversed by a bilateral excitotoxic GP lesion carried out simultaneously or 1 month after the striatal lesion, as well as by a temporary unilateral GP inactivation. Given that a similar dysfunction of basal ganglia circuitry is thought to subserve the behavioral alterations seen in quinolinic acid lesioned rats and some of the symptoms in HD, these results raise the possibility that lesion or inactivation of the GPe may alleviate some of HD symptoms.

Differential roles of dorsal hippocampal subregions in spatial working memory with short versus intermediate delay

In order to determine the role of subregions of the hippocampus in spatial working memory, this study combined selective neurotoxic lesions of the hippocampal subregions with a simple delayed nonmatching-to-place task on a radial maze in rats. Lesions of the dentate gyrus or the CA3, but not the CA1, subregion of the hippocampus induced a deficit in the acquisition of the task with short-term delays (i.e., 10 sec) and impaired performance of the task in a novel environment. All subregional lesions produced sustained impairment in performing the task with intermediate-term delays (i.e., 5 min) when rats were tested in a familiar environment. The results suggest a dynamic interaction among the dorsal hippocampal subregions in processing spatial working memory, with the time window (i.e., delay) of a task recognized as an essential controlling factor.

Differences in paired-pulse facilitation and long-term potentiation between dorsal and ventral CA1 regions in anesthetized rats

To clarify hippocampal regional differences in synaptic plasticity, paired-pulse facilitation (PPF, a form of short-term plasticity), long-term potentiation (LTP, a form of long-term plasticity), and their interactions were studied in the dorsal and ventral hippocampal CA1 regions of anesthetized rats. Baseline PPF and post-LTP PPF experiments were conducted at interstimulus intervals (ISIs) of 20-320 ms. A general protocol (100 Hz, 1 s) and a stronger protocol (250-Hz pulse series) were applied for LTP induction. PPF were observed in both regions; however, the degree was lower and the range of ISIs was narrower in the ventral region compared with the dorsal region. The degree of ventral LTP was lower than that of the dorsal LTP. The interaction between PPF and LTP was observed in both regions (PPF change correlated inversely with degree of baseline PPF). However, this was also different in each region. Dorsal PPF increased or decreased; in contrast, ventral PPF of short ISIs after LTP only decreased. These regional differences in short-term and long-term synaptic plasticity may explain a consequence of different afferent inputs and information processing.

Effects of rat ventral and dorsal hippocampus temporal inactivation on delayed alternation task

To determine the involvement of the hippocampal regions in a operant-type delayed alternation task of short delay or long delay, microinjections of muscimol into the hippocampus were used for temporal inactivation during the behavioral test in each rat. Dorsal hippocampal inactivation impaired the correct ratio of long delay. Ventral hippocampal inactivation showed no changes in the correct ratio, however, it increased the tendency of perseveration in long delay. These findings suggest hippocampus has regional differentiation in delayed alternation task.

Muscimol inactivation of the dorsal hippocampus impairs contextual retrieval of fear memory

Some models of hippocampal function have suggested a role of the hippocampus in contextual memory retrieval. We have examined this hypothesis by assessing the impact of reversible inactivation of the dorsal hippocampus (DH) on the context-specific expression of latent inhibition, a decrement in conditional responding produced by preexposure to a to-be-conditional stimulus. In Experiment 1, rats received tone preexposure either in the context that would later be used for extinction testing (context A) or in a different context (context C); a third group of rats did not receive tone preexposure. All rats then received fear conditioning, which consisted of tone-footshock pairings, in a third distinct context (context B). The following day conditional fear to the tone was assessed in one of the preexposure contexts (context A) by measuring freezing during a tone extinction test. Rats preexposed and tested in the same context exhibited less freezing to the tone than either rats preexposed and tested in different contexts or non-preexposed rats. These results indicate that the expression of latent inhibition is context specific. In Experiment 2, DH inactivation eliminated the context-specific expression of latent inhibition. Compared with saline-infused rats, rats infused with muscimol into the DH exhibited low levels of tone freezing independent of whether they had received tone preexposure in the test context or in a different context. Experiment 3 revealed normal contextual discrimination in rats after DH inactivation. These results suggest the DH is required for contextual memory retrieval in a latent inhibition paradigm.

Contribution of cholinergic and gabaergic functions to memory processes in BALB/cANnCrlBR mice

Several lines of evidence indicate that glucose influences on memory depend on interactions between glucose, glucoregulation and hippocampal cholinergic function. We previously demonstrated that glucose and scopolamine differentially affected memory consolidation for an operant bar pressing task in two closely-related BALB/c mouse strains. Whereas glucose normally improves memory in several animal strains, memory consolidation was not effected by systemic glucose injections in BALB/cANnCrlBR mice. Moreover, these mice were relatively insensitive to the normally observed amnestic effects of scopolamine. We therefore sought to determine whether cholinergic mechanisms in the dorsal hippocampus were involved in such atypical drug effects on memory processing in that strain of mice. In Experiment 1, we examined whether post-training oxotremorine would also atypically influence memory consolidation for an appetitively reinforced operant bar pressing task following microinjection in the dorsal hippocampus. In Experiment 2, we examined the effects of intrahippocampal GABAA drugs on memory consolidation. The non-selective muscarinic agonist, oxotremorine, dose-dependently impaired memory and the GABAA antagonist, bicuculline, improved retention in BALB/cANnCrlBR mice. It was concluded that GABA-mediated influences on hippocampal pyramidal output in BALB/cANnCrlBR mice and other strains are similar; but the amnestic effects of oxotremorine from the dorsal hippocampus were opposite to facilitating effects normally observed in other animal strains. Results are discussed relative to possible altered septo-hippocampal cholinergic neurotransmission in BALB/cANnCrlBR mice.