Working and reference memory in rats in the three-panel runway task following dorsal hippocampal lesions

In vivo administration of extracellular cGMP normalizes TNF-α and membrane expression of AMPA receptors in hippocampus and spatial reference memory but not IL-1β, NMDA receptors in membrane and working memory in hyperammonemic rats

Patients with hepatic encephalopathy (HE) show working memory and visuo-spatial orientation deficits. Hyperammonemia is a main contributor to cognitive impairment in HE. Hyperammonemic rats show impaired spatial learning and learning ability in the Y maze. Intracerebral administration of extracellular cGMP restores learning in the Y-maze. The underlying mechanisms remain unknown. It also remains unknown whether extracellular cGMP improves neuroinflammation or restores spatial learning in hyperammonemic rats and if it affects differently reference and working memory. The aims of this work were: Spatial working and reference memory were assessed using the radial and Morris water mazes and neuroinflammation by immunohistochemistry and Western blot. Membrane expression of NMDA and AMPA receptor subunits was analyzed using the BS3 crosslinker. Extracellular cGMP was administered intracerebrally using osmotic minipumps. Chronic hyperammonemia induces neuroinflammation in hippocampus, with astrocytes activation and increased IL-1β, which are associated with increased NMDA receptors membrane expression and impaired working memory. This process is not affected by extracellular cGMP. Hyperammonemia also activates microglia and increases TNF-α, alters membrane expression of AMPA receptor subunits (increased GluA1 and reduced GluA2) and impairs reference memory. All these changes are reversed by extracellular cGMP. These results show that extracellular cGMP modulates spatial reference memory but not working memory. This would be mediated by modulation of TNF-α levels and of membrane expression of GluA1 and GluA2 subunits of AMPA receptors.

M1 muscarinic receptors boost synaptic potentials and calcium influx in dendritic spines by inhibiting postsynaptic SK channels

Acetylcholine release and activation of muscarinic cholinergic receptors (mAChRs) enhance synaptic plasticity in vitro and cognition and memory in vivo. Within the hippocampus, mAChRs promote NMDA-type glutamate receptor-dependent forms of long-term potentiation. Here, we use calcium (Ca) imaging combined with two-photon laser glutamate uncaging at apical spines of CA1 pyramidal neurons to examine postsynaptic mechanisms of muscarinic modulation of glutamatergic transmission. Uncaging-evoked excitatory postsynaptic potentials and Ca transients are increased by muscarinic stimulation; however, this is not due to direct modulation of glutamate receptors. Instead, mAChRs modulate a negative feedback loop in spines that normally suppresses synaptic signals. mAChR activation reduces the Ca sensitivity of small conductance Ca-activated potassium (SK) channels that are found in the spine, resulting in increased synaptic potentials and Ca transients. These effects are mediated by M1-type muscarinic receptors and occur in a casein kinase-2-dependent manner. Thus, muscarinic modulation regulates synaptic transmission by tuning the activity of nonglutamatergic postsynaptic ion channels.

Dorsal hippocampal α7 and α4β2 nicotinic receptors and memory

Nicotinic receptor systems have been shown to be important for working memory. In general, nicotinic agonists have been shown to improve memory, and nicotinic antagonists impair it. All of the neuronal substrates for nicotinic involvement in memory still remain to be discovered. The amygdala and ventral hippocampus have both been found to be important for nicotinic involvement in memory function. Local infusion of the nicotinic antagonist methyllycaconitine (MLA) to block alpha7 nicotinic receptors and dihydro-beta-erythrodine (DHbetaE) to block alpha4beta2 nicotinic receptors into the basolateral amygdala and the ventral hippocampus have been found to impair working memory function, with no additive effects being observed. The current project assessed the roles of alpha7 and alpha4beta2 nicotinic receptors in the dorsal hippocampus for memory function. Adult female Sprague-Dawley rats were trained on the 16-arm radial maze. The rats (n = 10) had bilateral cannulae implanted into the dorsal hippocampus. The rats were given acute infusions of DHbetaE (0, 1.69, 3.38, and 6.75 microg/side) and MLA (6.75 microg/side) alone and in combination in a repeated measures counter-balanced design. DHbetaE and MLA infusion into the dorsal hippocampus significantly increased working memory errors. However, when the two drugs were given in combination, an attenuated effect was seen. No significant effects of MLA or DHbetaE were seen with reference memory errors or response latency. These results confirm the importance of alpha4beta2 and alpha7 nicotinic acetylcholine receptors in the dorsal hippocampus for appetitively-motivated spatial cognitive function.

Emotion-induced amnesia in rats: working memory-specific impairment, corticosterone-memory correlation, and fear versus arousal effects on memory

We have shown previously that psychological stress (predator exposure) impairs spatial memory in rats. We have extended that finding here to show that predator stress selectively impaired recently acquired (hippocampal-dependent) spatial working memory without affecting long-term (hippocampal-independent) spatial reference memory. We also investigated why predator exposure impairs memory. Was spatial memory impaired because of the fear-provoking aspects of predator exposure or only because the cat was a novel and arousing stimulus? If the latter possibility was correct, then any novel and arousing stimulus, independent of its emotional valence (i.e., aversive or appetitive), would impair memory. We found that spatial working memory was not impaired when the male rats were exposed to a sexually receptive female rat, a stimulus that was novel and arousing to them, but not aversive. We also found that there was an equivalent increase in serum corticosterone levels in male rats exposed to either a cat or a female rat, but only the cat-exposed rats exhibited a significant correlation between corticosterone levels and impaired memory. Overall, this series of experiments demonstrates that (1). predator stress selectively impaired working (hippocampal-dependent), but not reference (hippocampal-independent), memory; (2). a fear-provoking stimulus, and not merely novelty and increased arousal, impaired spatial memory; and (3). increased corticosterone levels correlated with impaired spatial working memory only under predator exposure, that is, fear-provoking conditions.

Decursin from Angelica gigas mitigates amnesia induced by scopolamine in mice

We previously reported that a total methanolic extract of the underground part of Angelica gigas Nakai (Umbelliferae) (here-in-after abbreviated AG) significantly inhibited acetylcholinesterase (AChE) activity. We characterized 12 coumarin derivatives including both decursin and decursinol from extracts of AG. In this study, we evaluated the anti-amnestic activity of decursin, a major coumarin constituent isolated from AG, in vivo using ICR mice with amnesia induced by scopolamine (1 mg/kg body weight, s.c.). Decursin, when administered to mice at 1 and 5 mg/kg body weight i.p., significantly ameliorated scopolamine-induced amnesia as measured in both the passive avoidance test and the Morris water maze test. Moreover, decursin significantly inhibited AChE activity by 34% in the hippocampus of treated mice. These results indicate that decursin may exert anti-amnestic activity in vivo through inhibition of AChE activity in the hippocampus.

Involvement of cholinergic and glutamatergic functions in working memory impairment induced by interleukin-1beta in rats

Interleukin-1beta at doses of 32 and 100 ng/side, injected bilaterally into the dorsal hippocampus of rats, significantly increased the working memory errors in a three-panel runway setup, whereas interleukin-1beta at doses affecting working memory errors had no effect on the number of errors in the first trial or the latency. The increase in working memory errors induced by intrahippocampal administration of 100 ng/side interleukin-1beta was significantly decreased by concurrent injection (300 ng/side) of the interleukin-1 receptor antagonist. The cholinesterase inhibitor physostigmine at a dose of 3.2 microg/side and D-cycloserine (1.0 and 10 microg/side), which is a partial agonist acting at the glycine binding site of the NMDA receptor/channel complex, reduced the increase in working memory errors induced by 100 ng/side interleukin-1beta. These results suggest that interleukin-1beta causes disruptions of septohippocampal cholinergic and glutamatergic transmission via its high-affinity receptor, which underlie the impairment of working memory.

Cholinergic enhancement improves performance on working memory by modulating the functional activity in distinct brain regions: a positron emission tomography regional cerebral blood flow study in healthy humans

Previously, we have shown that physostigmine, an acetylcholinesterase inhibitor, improved performance on a working memory for faces task, as reflected by reduced reaction time (RT), and reduced task-specific regional cerebral blood flow (rCBF) in right prefrontal cortex and, further, that these reductions in RT and right frontal rCBF were significantly correlated. Here we investigated the relation between the effects of physostigmine on task performance and task-specific functional brain response throughout the cortex by examining correlations between physostigmine-related changes in rCBF in all brain areas and changes in RT. In subjects who received an infusion of physostigmine, reduced RT correlated (p<0.001) positively with reduced rCBF in right frontal cortex, left temporal cortex, anterior cingulate, and left hippocampus; and correlated with increased rCBF in medial occipital visual cortex. In subjects who received a placebo infusion of saline, no significant correlations between changes in RT and cortical rCBF were observed. The results show that cholinergically induced improvements in working memory performance are related to alterations in neural activity in multiple cortical regions, including increased neural activity in regions associated with early perceptual processing and decreased neural activity in regions associated with attention, memory encoding, and memory maintenance.

Effect of rivastigmine on scopolamine-induced memory impairment in rats

The effect of rivastigmine on memory impairments induced in rats by scopolamine (0.5 mg/kg) was assessed in the Morris water maze and passive avoidance tests and compared with that of tacrine (2.5-17.7 mg/kg). Rivastigmine, (0.5-2.5 mg/kg) inhibited cholinesterase in the cortex and hippocampus by 21-60% and antagonised the deficits in working and reference memory. Tacrine (12.5 and 17.7 mg/kg) produced significantly less inhibition of cholinesterase in the hippocampus but more in the striatum than rivastigmine (0.75 and 1.5 mg/kg) and only antagonised the deficit in reference memory. Rivastigmine (1.5 and 2.5 mg/kg) or tacrine (12.5 mg/kg), injected immediately after completion of the acquisition trial in the passive avoidance test, antagonised the deficit induced by scopolamine (1 mg/kg) in memory retention. The inability of higher doses of the cholinesterase inhibitors to antagonise memory deficits induced by scopolamine may be related to excessive cholinergic stimulation in the central nervous system.

Hippocampal grafts of acetylcholine-producing cells are sufficient to improve behavioural performance following a unilateral fimbria-fornix lesion

Lesions of the septohippocampal pathway produce cognitive deficits that are partially attenuated by grafts of cholinergic-rich tissue into denervated target regions or by systemic administration of cholinomimetic drugs. In the present study, fibroblasts engineered to produce acetylcholine were used to test the hypothesis that restoration of hippocampal acetylcholine in rats with septohippocampal lesions is sufficient to improve cognitive processing post-damage. Rats received unilateral grafts of acetylcholine-producing or control fibroblasts into the hippocampus immediately prior to an aspirative lesion of the ipsilateral fimbria-fornix. Some rats with fimbria-fornix lesions were implanted with acetylcholine-producing or control fibroblasts into the neocortex, another major target of the basal forebrain cholinergic system, to determine if the site of acetylcholine delivery to the damaged brain is critical for functional recovery. Rats were tested in a hidden platform water maze task, a cued water maze task and activity chambers between one and three weeks post-grafting. Compared to unoperated controls, rats with fimbria fornix lesions only were significantly impaired in hidden platform water maze performance. Hippocampal grafts of acetylcholine-producing cells reduced lesion-induced deficits in the water maze, whereas hippocampal control grafts and cortical grafts of either cell type were without effect. Locomotor activity and cued water maze performance were unaffected by the lesion or the implants. Taken together, these data indicate that water maze deficits produced by fimbria fornix lesions, which disrupt a number of hippocampal neurotransmitter systems, can be attenuated by target specific replacement of acetylcholine in the hippocampus and that this recovery occurs in the absence of circuitry repair.

Effects of nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester on spatial and cued leaning

An investigation was made of the effects of the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) on the acquisition and retention of two operantly conditioned discrimination tasks. Twenty Long-Evans rats were conditioned to approach one of two spatial locations that was either held constant across trials (spatial task) or was associated with a visual cue (illuminated lamp) that was randomly assigned to one of the locations on each trial (cued task). Rats were assigned to one of two treatment groups in which they received intraperitoneal injections of either NG-nitro-L-arginine methyl ester or saline approximately 2 h before sessions on each day of training. Analysis was made of the trial-by-trial performance in order to identify the characteristics of learning under each condition. Assessment of learning acquisition was based on the number of trials required to reach a criterion of 80% correct responses, whereas retention was assessed by the number of trials to criterion on each day after the criterion was initially reached. Analysis indicated that treatment groups did not differ significantly on acquisition or retention of either the spatial or cued task. These results indicate that inhibition of nitric oxide synthase does not interfere with the learning or retention of basic operant tasks that involve simple spatial or visual analysis. Whereas results from biochemical and physiological investigations have suggested an impact of nitric oxide synthase on behavioural function, behavioural investigations indicate a limited impact of nitric oxide synthase inhibition on learning and memory. Although these results do not discount the role of nitric oxide synthase in a hippocampal mechanism, they illustrate that behavioural analysis should be made in the context of multiple interacting neural systems. Viewed with previous behavioural research on the effects of NG-nitro-L-arginine methyl ester, these results indicate that nitric oxide synthase inhibition results in impairment of certain forms of learning whereas other forms are preserved.

Cholinergic stimulation alters performance and task-specific regional cerebral blood flow during working memory

Modulation of the cholinergic neurotransmitter system results in changes in memory performance, including working memory (WM), in animals and in patients with Alzheimer disease. To identify associated changes in the functional brain response, we studied performance measures and regional cerebral blood flow (rCBF) using positron emission tomography (PET) in healthy subjects during performance of a WM task. Eight control subjects received an infusion of saline throughout the study and 13 experimental subjects received a saline infusion for the first 2 scans followed by a continuous infusion of physostigmine, an acetylcholinesterase inhibitor, for the subsequent 8 scans. rCBF was measured using H215O and PET in a sequence of 10 PET scans that alternated between rest and task scans. During task scans, subjects performed the WM task for faces. Physostigmine both improved WM efficiency, as indicated by faster reaction times, and reduced WM task-related activity in anterior and posterior regions of right midfrontal gyrus, a region shown previously to be associated with WM. Furthermore, the magnitudes of physostigmine-induced change in reaction time and right midfrontal rCBF correlated. These results suggest that enhancement of cholinergic function can improve processing efficiency and thus reduce the effort required to perform a WM task, and that activation of right prefrontal cortex is associated with task effort.

Characteristics of memory dysfunction in olfactory bulbectomized rats and the effects of cholinergic drugs

The memory impairment of olfactory bulbs (OB)-lesioned rats was characterized using 4 different tasks of learning/memory, and the effects of certain cholinergic drugs on such memory impairment were examined. In reference memory and working memory tasks using a 3-panel runway apparatus, OB-lesioned rats showed a marked increase in errors. In the 3-lever operant task using delayed matching to a sample (DMTS) procedure, OB lesions significantly decreased the correct response in choice (test) trials without affecting the in sample (training) trials. An interesting finding is that impairment in the DMTS performance did not appear immediately after the OB lesion, but tended to appear after a delay. Based on this finding, it is unlikely that memory impairment in the OB-lesioned rats is due to the olfactory deficit itself. However, OB lesions significantly reduced the choice accuracy and delayed the choice reaction time during the 3-choice serial time task for assessing attentional function, using a 3-lever operant apparatus. These findings suggest that marked impairment of learning and memory in OB-lesioned rats may be caused by the attention deficit. Furthermore, the memory impairment in OB-lesioned rats was reduced by cholinesterase inhibitors, physostigmine and NIK-247. These finding suggest that dysfunction of the cholinergic system is involved, at least in part, in the memory impairment of OB-lesioned rats.

Blockade of 5-HT1A receptors compensates loss of hippocampal cholinergic neurotransmission involved in working memory of rats

NAN-190, a selective 5-HT1A receptor antagonist, had no effect on the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) in the working memory task with a three-panel runway setup, when injected bilaterlly at 0.32 or 1.0 micrograms/side into the dorsal hippocampus. Intrahippocampal administration of the muscarinic receptor antagonist scopolamine at 3.2 micrograms/side or the competitive NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) at 32 ng/side significantly increased the number of working memory errors. The increase in working memory errors induced by intrahippocampal scopolamine (3.2 micrograms/side) was reduced by concurrent infusion of 0.32 and 1.0 microgram/side NAN-190, an effect that reached significance only for the 1.0 microgram/side dose. In contrast, NAN-190 at 1.0 micrograms/side did not affect the increase in working memory errors when infused intrahippocampally together with 32 ng/side CPP. These results suggest that blockade of hippocampal 5-HT1A receptors does not affect impairment of working memory resulting from block of NMDA receptor-mediated neurotransmission, but that it can compensate deficiency of septohippocampal cholinergic activity involved in working memory performance of rats.

Beta-adrenergic dysfunction exacerbates impairment of working memory induced by hippocampal NMDA receptor blockade in rats

To clarify the interactions between hippocampal glutamatergic and adrenergic systems in the working memory function of rats, the effects of hippocampal NMDA receptor blockade combined with noradrenaline depletion or alpha- and beta-adrenoceptor blockade on this behavior were examined with a three-panel runway task. Intrahippocampal administration of the potent and competitive NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) at a dose of 32 ng/side significantly increased the number of errors (attempts to pass through two incorrect panels of the three panels gates at four choice points) in the working memory task, whereas the 3.2 ng/side dose of CPP did not affect working memory errors. Administration of the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2 -bromobenzylamine (DSP-4) at 50 mg/kg i.p. produced marked reductions in hippocampal and cortical noradrenaline contents, but it had no effect on working memory errors. Intrahippocampal administration of 3.2 ng/side CPP, the behaviorally ineffective dose in intact rats, significantly increased the number of working memory errors in the noradrenaline-depleted rats. The alpha-adrenoceptor antagonist phentolamine (3.2 mg/kg i.p.) did not affect working memory errors whether administered alone or in combination with intrahippocampal CPP (3.2 ng/side). The beta-adrenoceptor antagonist propranolol (10 mg/kg i.p.) also had no effect on working memory errors. However, propranolol (10 mg/kg) produced a significant increase in working memory errors when administered together with intrahippocampal CPP (3.2 ng/side). These results suggest that hippocampal NMDA/beta-adrenergic interactions are involved in neural processes mediating working memory function of rats.

Concurrent blockade of hippocampal metabotropic glutamate and N-methyl-D-aspartate receptors disrupts working memory in the rat

In order to clarify the roles of hippocampal metabotropic glutamate and N-methyl-D-aspartate receptors in working and reference memory performance of rats, the effects of intrahippocampal administration of selective antagonists for both receptors on these behaviours were examined with a three-panel runway task. In the working memory task, the potent and competitive N-methyl-D-aspartate receptor antagonist, (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points), when injected bilaterally at 10 and 32 ng/side into the dorsal hippocampus. Intrahippocampal injection of CPP at a dose of 3.2 ng/side had no effect on the number of working memory errors. The metabotropic glutamate receptor antagonist, (+)-alpha-methyl-4-carboxyphenylglycine (+)-(MCPG), injected into the hippocampus at doses up to 3.2 micrograms/side, did not significantly affect the number of working memory errors. Combined administration of (+)-MCPG (3.2 micrograms/side) and CPP (3.2 ng/side) into the hippocampus, neither of which had an individual effect on errors, significantly increased the number of working memory errors. However, intrahippocampal administration of the relatively inactive isomer, (-)-MCPG, at 3.2 micrograms/side did not affect working memory errors, whether given independently or concurrently with the behaviourally ineffective dose of CPP (3.2 ng/side). In the reference memory task, intrahippocampal injection of CPP at doses up to 32 ng/side had no effect on the number of errors. Intrahippocampal (+)-MCPG at doses up to 3.2 micrograms/side did not affect the number of reference memory errors, whether administered alone or together with 3.2 ng/side of CPP. These results indicate that blockade of hippocampal metabotropic glutamate receptors aggravates impairment of working memory resulting from deficiency of N-methyl-D-aspartate receptor-mediated glutamatergic neurotransmission, suggesting that mechanisms regulated by co-activation of hippocampal metabotropic glutamate and N-methyl-D-aspartate receptors are involved in working memory performance of rats.

Concurrent blockade of beta-adrenergic and muscarinic receptors disrupts working memory but not reference memory in rats

In order to clarify the interactions between monoaminergic and cholinergic systems in working and reference memory functions, the effects of administration of the alpha, beta-adrenergic, D1-, D2-dopaminergic or serotonergic receptor antagonist together with the muscarinic receptor antagonist scopolamine on this behavior were examined using a three-panel runway task. Both in working and reference memory tasks, the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) was significantly increased by 0.32 mg/kg scopolamine, but not by the doses up to 0.18 mg/kg. The beta-adrenergic antagonist propranolol at 10 mg/kg had no effect on the number of working memory errors. Combined administration of 10 mg/kg propranolol and scopolamine at 0.1 and 0.18 mg/kg significantly increased the number of working memory errors. However, in a reference memory task, propranolol at 10 mg/kg did not affect the number of errors, whether administered alone or together with 0.1 mg/kg scopolamine. Other monoaminergic receptor antagonists, including the alpha-adrenergic antagonist phentolamine (3.2 and 10 mg/kg), D1-antagonist SCH23390 (0.032 and 0.056 mg/kg). D2-antagonist sulpiride (100 mg/kg) and serotonin antagonist cinanserin (10 and 32 mg/kg) had no significant effect on working or reference memory errors, whether they were administered independently or in combination with scopolamine at 0.1 mg/kg. These results suggest that beta-adrenergic/muscarinic interactions play an important role in mediating processes involved in working memory performance of rats.

Intrahippocampal administration of (+)-SKF 10,047, a sigma ligand, reverses MK-801-induced impairment of working memory in rats

In order to clarify the roles of the hippocampal sigma site and phencyclidine (PCP) binding site on the NMDA receptor/channel complex in the regulation of working memory in rats, the effects of intrahippocampal administration of ligands for both binding sites on this behavior were examined with a three-panel runway task. MK-801, a potent noncompetitive NMDA antagonist with high affinity for the PCP site, significantly increased the number of working memory errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points), when injected bilaterally at 0.1 and 0.18 microgram/side into the dorsal hippocampus. However, intrahippocampal injection of (+)-SKF 10,047, a sigma ligand, at doses up to 1.0 microgram/side had no significant effect on the number of working memory errors. The working memory impairment induced by intrahippocampal MK-801 (0.18 microgram/side) was attenuated by concurrent injection of 1.0 microgram/side (+)-SKF 10,047, but not by that of 1.0 microgram/side (-)-SKF 10,047. These results suggest that activation of hippocampal sigma and PCP binding sites exerts antagonistic effects on working memory function, possibly through modulation of NMDA receptor-mediated glutamatergic neurotransmission.

Pharmacology of memory: cholinergic-glutamatergic interactions

Both acetylcholine and glutamate are now thought to play important roles in memory. Recent evidence suggests that the interaction of these two neurotransmitters may be important for some forms of memory, and that acetylcholine, in particular, may function to facilitate glutamate activity by coordinating states of acquisition and recall in the cortex and hippocampus.

The selective CCK-B agonist, BC 264, impairs socially reinforced memory in the three-panel runway test in rats

The role of CCK-B receptor activation in memory processes has been reassessed using the three-panel runway task, under conditions which avoid the effects of CCK on satiety and reduce emotional responses. For this purpose the food reinforcement usually used was replaced by a social reinforcement. The results show that learning and memory can be assessed using this procedure. Moreover, under these experimental conditions, drugs such as scopolamine, amphetamine or kinurenic acid injected into the nucleus accumbens produced behavioral deficits. BC 264, a highly selective CCK-B agonist, peripherally administered or infused into the anterolateral part of the nucleus accumbens also impaired memory. These effects were suppressed by L-365,260 supporting the involvement of CCK-B receptors and of the nucleus accumbens in memory processes.

Attenuated hippocampal long-term potentiation in basolateral amygdala-lesioned rats

Possible involvement of the amygdaloid input in long-term potentiation (LTP) in the medial perforant path-dentate gyrus granule cell synapses in vivo was investigated by evaluating the effects of lesions of the amygdaloid nucleus. The dentate gyrus synaptic potential evoked by low-frequency test stimulation did not change following lesions of the basolateral and central amygdala. However, when tetanic stimulation (30 pulses at 60 Hz) was applied 60 min after lesioning of the ipsilateral basolateral amygdala, the magnitude of LTP was significantly attenuated. Since lesions of the ipsilateral central amygdala and the contralateral basolateral amygdala did not affect the dentate gyrus LTP, the attenuation of the dentate gyrus LTP is a specific effect of acute lesions of the ipsilateral basolateral amygdala. The basolateral amygdaloid lesions significantly attenuated both LTP induced by weak (20 pulses at 60 Hz) and strong (100 pulses at 100 Hz) tetanus, indicating that the effect of the lesions does not depend on the strength of tetanus applied to induce LTP. When the ipsilateral basolateral amygdala was destroyed after application of tetanus, it did not affect the established LTP. The attenuation of LTP was also observed after the basolateral amygdala-lesioned rats were allowed a recovery period of 2 weeks. This is the first report providing evidence that the ipsilateral basolateral amygdala modulates hippocampal synaptic plasticity in vivo.

Blockade of hippocampal M1 muscarinic receptors impairs working memory performance of rats

In order to clarify the roles of hippocampal M1 and M2 muscarinic receptors in working and reference memory performance of rats, the effects of intrahippocampal injections of selective antagonists at both receptors on this behavior were examined with a three-panel runway task. In the working memory task, the M1 muscarinic receptor antagonist pirenzepine, injected bilaterally at 0.32 and 1.0 microgram/side into the dorsal hippocampus, significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points). This effect of intrahippocampal pirenzepine (1.0 microgram/side) on working memory was attenuated by concurrent injection of 10 micrograms/side AF102B, the selective M1 muscarinic receptor agonist. Intrahippocampal injection of the M2 muscarinic receptor antagonist methoctramine at doses up to 1.0 microgram/side had no significant effect on the number of working memory errors. Intrahippocampal methoctramine injection at 3.2 micrograms/side produced a significant increase in working memory errors, an effect that was reversed by concurrent injection of 10 micrograms/side AF102B. Concurrent injection of 0.32 microgram/side methoctramine significantly reduced the increase in working memory errors induced by intrahippocampal pirenzepine (1.0 microgram/side). In the reference memory task, neither pirenzepine nor methoctramine affected the number of errors when injected into the hippocampus at doses up to 1.0 and 3.2 micrograms/side, respectively. These results suggest that processes mediated by M1 muscarinic receptors in the hippocampus are involved in working memory, but not in reference memory, and that blockade of hippocampal M2 muscarinic receptors ameliorates working memory deficits produced by M1 muscarinic blockade, possibly by increasing acetylcholine release.

Intrahippocampal administration of a glycine site antagonist impairs working memory performance of rats

The effects of 7-chlorokynurenic acid, an antagonist at the glycine site associated with the NMDA receptor/channel complex, on memory functions were investigated using a three-panel runway task. In a working memory task, 7-chlorokynurenic acid at a dose of 3.2 micrograms/side, injected bilaterally into the dorsal hippocampus, significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points). The working memory deficit induced by intrahippocampal 7-chlorokynurenic acid (3.2 micrograms/side) was reversed by concurrent injection (32 micrograms/side) of D-serine, the glycine site agonist. In a reference memory task, 7-chlorokynurenic acid had no effect on the number of errors when injected into the hippocampus at doses up to 3.2 micrograms/side. These results suggest that activation of the hippocampal glycine site coupled to the NMDA receptor is required for working memory function in rats.

Intrahippocampal administration of the NO synthase inhibitor L-NAME prevents working memory deficits in rats exposed to transient cerebral ischemia

A 5-min period of cerebral ischemia increased the number of errors in a working memory task with three-panel runway paradigm, while it had no effect on reference memory errors. The nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), infused into the bilateral dorsal hippocampus at 100 micrograms/side immediately after blood flow reperfusion, significantly reduced the increase in working memory errors expected to occur 24 h after 5 min of ischemia. Intrahippocampal administration of the inactive isomer D-NAME at 100 micrograms/side immediately after reperfusion had no effect on the increase in working memory errors in the ischemic rats. These findings suggest that the mechanism mediated by hippocampal NO synthesis during the early reperfusion phase contributes to the postischemic impairment of working memory.

Deficits in working memory following inhibition of hippocampal nitric oxide synthesis in the rat

In order to elucidate the roles of hippocampal nitric oxide (NO) synthesis in working and reference memory performance of rats, the effects of intrahippocampal injections of the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), on this behavior were examined with a three-panel runway task. In the working memory task, L-NAME, injected bilaterally at 10 and 32 micrograms/side into the dorsal hippocampus, significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points). This effect of intrahippocampal L-NAME (32 micrograms/side) on working memory was attenuated by concurrent injection of 100 micrograms/side L-arginine, the precursor of NO. Intrahippocampal injection of the inactive isomer D-NAME at doses up to 32 micrograms/side had no effect on the number of working memory errors. In the reference memory task, neither L-NAME nor D-NAME affected the number of errors when injected into the hippocampus at doses up to 32 micrograms/side. These results suggest that processes mediated by NO synthesis in the hippocampus are involved in working memory, but not in reference memory.

Amygdaloid NMDA and muscarinic receptors involved in working memory performance of rats

In a three-panel runway task, the competitive N-methyl-D-aspartate (NMDA) receptor antagonist, 3-[(+-)-2-carboxypiperazin-4-yl]propyl-1-phosphonic acid (CPP) (56 ng/side), injected bilaterally into the basolateral subdivision of the amygdala, significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel gates at four choice points) in a test of working memory. Intra-amygdaloid injection of the muscarinic receptor antagonist scopolamine (5.6 micrograms/side) also significantly increased working memory errors. In a test of reference memory, neither CPP nor scopolamine affected the number of errors when injected into the basolateral amygdala at doses up to 56 ng/side or 5.6 micrograms/side, respectively. These results indicate that processes mediated by NMDA and muscarinic receptors in the basolateral amygdala are involved in the performance of working memory, but not in reference memory.

Blockade of hippocampal nicotinic receptors impairs working memory but not reference memory in rats

In a three-panel runway task, intrahippocampal injection of the nicotinic receptor antagonist, mecamylamine (10 and 18 micrograms/side), significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) in a test of working memory. This increase in errors also occurred after rats were given IP mecamylamine (10 mg/kg). Mecamylamine did not affect the number of errors in a test of reference memory whether it was given at doses up to 18 micrograms/side intrahippocampally or up to 10 mg/kg IP. These results suggest that mechanisms mediated by hippocampal nicotinic receptors play a role in working memory but not in reference memory.

Working memory deficits induced by intrahippocampal administration of 8-OH-DPAT, a 5-HT1A receptor agonist, in the rat

In a test of working memory using a three-panel runway task, the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), at 1.0 mg/kg i.p. significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel gates at four choice points), but it had no effect on errors in a test of reference memory. Intrahippocampal injection of 8-OH-DPAT at 10 micrograms/side also significantly increased the number of working memory errors, without affecting that of reference memory errors. The effect of intrahippocampal 8-OH-DPAT (10 micrograms/side) on working memory was blocked by the 5-HT1A receptor antagonist, (-)-propranolol, at 5 mg/kg i.p. These results suggest that activation of hippocampal 5-HT1A receptors impairs working memory, but does not affect retention of reference memory.

Ameliorative effects of the centrally active cholinesterase inhibitor, NIK-247, on impairment of working memory in rats

Using a three-panel runway task, the effects of NIK-247 on impairment of working memory produced by scopolamine, hippocampal lesions, and cerebral ischemia were investigated in rats; these effects were compared with those of the well-known cholinesterase inhibitors, tetrahydroaminoacridine (THA) and physostigmine. Intraperitoneal injection of scopolamine (0.56 mg/kg) significantly increased the number of errors (pushes made on the two incorrect panels of the three-panel gates located at four choice points). NIK-247 (3.2-18 mg/kg PO), THA (1-10 mg/kg PO), and physostigmine (0.1 and 0.32 mg/kg IP) dose-dependently reduced the increase in errors induced by scopolamine. NIK-247 (32 mg/kg) was also effective in reducing the increase in errors produced by lesions of the dorsal hippocampus. A 5-min period of cerebral ischemia markedly increased the number of errors. NIK-247 (3.2 and 10 mg/kg), given immediately after blood flow recirculation and again 20 min before the runway test carried out 24 h after ischemia, significantly reduced the increase in errors expected to occur after ischemia. Tetrahydroaminoacridine (3.2 mg/kg) and physostigmine (0.1 mg/kg) similarly reversed the increased errors in ischemic rats. These results suggest that NIK-247 alleviates the impairment of working memory produced by scopolamine, hippocampal lesions, and cerebral ischemia, possibly through activation of the central cholinergic system.

Involvement of cholinergic mechanisms in impairment of working memory in rats following basolateral amygdaloid lesions

In order to clarify the role of the amygdala in the working and reference memory of rats in the three-panel runway task, the effects of lesions of subnuclei of the amygdaloid complex on this behavior were studied. Rats that had been trained preoperatively, until they achieved the criterion of learning, were subjected to lesions of the amygdala. In the working memory task, lesions of the basolateral subdivision of the amygdala caused a significant increase in the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points), while lesions of the corticomedial amygdala had no effect on working memory errors. The increase in working errors, observed in basolateral amygdaloid-lesioned rats, declined gradually as retraining sessions were given once each day, reverting to control levels on and after the sixth session. In the reference memory task, the number of errors was not affected by lesions of the basolateral or corticomedial amygdala. The increase in working memory errors, induced by lesions of the basolateral amygdala was significantly reduced by intraperitoneal administration of the inhibitors of cholinesterase, tetrahydroaminoacridine (0.32-1.0 mg/kg) and physostigmine (0.032-0.1 mg/kg), and the muscarinic receptor agonist, oxotremorine (0.1 mg/kg), before the runway test. These findings suggest that the basolateral amygdala is selectively involved in working memory but not in reference memory and that the lowering of central cholinergic function may account for the impairment of working memory, induced by lesions of the basolateral amygdala.

Intrahippocampal injections of benzodiazepine and muscimol impair working memory but not reference memory of rats in the three-panel runway task

In a three-panel runway task, the benzodiazepine chlordiazepoxide at 3.2 and 10 mg/kg i.p. significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) in a test of working memory, but it had no effect on errors in a test of reference memory. This effect of 10 mg/kg chlordiazepoxide on working memory was blocked by the benzodiazepine receptor antagonist flumazenil at 10 mg/kg. Intrahippocampal injection of chlordiazepoxide at 10 and 32 micrograms/side significantly increased the number of working memory errors. This effect of intrahippocampal chlordiazepoxide (32 micrograms/side) was attenuated not only by flumazenil at 10 mg/kg but also by the gamma-aminobutyric acid (GABA)A receptor antagonist bicuculline at 3.2 mg/kg. Intrahippocampal injection of the GABAA receptor agonist muscimol at 100 and 320 ng/side also significantly increased working memory errors. Neither chlordiazepoxide nor muscimol affected the number of reference memory errors when injected into the hippocampus at doses up to 32 micrograms/side or 320 ng/side, respectively. These results suggest that activation of the GABAA/benzodiazepine receptor complex in the hippocampus impairs working memory, but does not affect reference memory.