Guanosine impairs inhibitory avoidance performance in rats

The nucleoside guanosine, previously found to exert trophic and neuroprotective effects, was found to impair retention of inhibitory avoidance responses, with a complete effect at 7.5 mg/kg i.p. pretraining. Treated animals, when retrained 1 week later, showed normal learning ability. Guanosine injected immediately after training or pretest did not alter retention latency. Combined pretraining and pretest treatments with guanosine failed to reverse its amnestic effect, excluding the contribution of state dependency. Open field parameters and shock sensitivity were mostly unaltered by guanosine. These results suggest an amnestic effect of guanosine on inhibitory avoidance in rats, in a pattern compatible with inhibition of glutamatergic activity. However, the mechanism for the amnestic effect of guanosine is yet to be elucidated.     

Metabotropic glutamate receptor 1 blockade impairs acquisition and retention in a spatial Water maze task

Metabotropic glutamate receptors, including the mGlu1 receptor, have received considerable attention as potential targets for anxiolytic, antidepressant, antipsychotic and antinociceptive drugs. mGlu1 receptors have also been suggested to play a role in the modulation of cognitive processes, but knowledge is still very limited. In the present study the effects of the selective mGlu1 receptor antagonist 3,4-dihydro-2H-pyrano[2,3]beta-quinolin-7-yl)(cis-4-methoxycyclohexyl)methanone (JNJ16259685, 0.63-10 mg/kg s.c.) on more or less spatially demanding learning and spatial memory (retention and re-acquisition) were investigated in mice performing in a water maze. Selective mGlu1 receptor blockade with JNJ16259685 impaired spatial acquisition processes, irrespective of spatial load, as well as spatial re-acquisition, already at the lowest dose tested (0.63 mg/kg). In contrast, effects on spatial retention performance were relatively mild in mice that had learned to locate the position of the escape platform prior to treatment. Thigmotaxic behaviour and locomotor activity appeared to be unaffected by JNJ16259685. These data suggest that blockade of the mGlu1 receptor primarily affects learning of new information, but leaves retention of spatial information relatively unaffected. Blockade of the mGlu5 receptor with MPEP also impaired spatial learning, although only at the highest dose tested (10 mg/kg). An ex vivo receptor occupancy study in rats revealed that MPEP occupied central mGlu5 receptors with an ED(50) of 2.0 mg/kg one hour after subcutaneous administration. This is 50-150 times higher than the ED(50) reported for JNJ16259685 at central mGlu1 receptors and suggests that one reason why the two compounds cause cognitive effects at different doses might be due to differences in central mGlu receptor occupancy, rather than fundamentally different roles of mGlu1 and mGlu5 receptors in the modulation of cognitive function.     

Visual acuity in the water maze: sensitivity to muscarinic receptor blockade in rats and mice

A visual acuity task performed in the water maze in rats [Behav. Brain Res. 119 (2001) 77] was used to reveal the sensitivity of the visual system to muscarinic receptor blockade. Intraperitoneal injection of 0.2 mg/kg scopolamine had no effect, but 2 mg/kg severely compromised visual acuity, but did not affect the swim strategy to solve the task. Spatial learning in a reference memory version of the water maze, however, was impaired by 0.2 mg/kg scopolamine. It was also confirmed that the same visual acuity task is applicable to C57BL6/J mice. The visual deficit induced by 2 mg/kg scopolamine was less severe compared to rats, possibly due to a change in swim strategy in the drug condition. The effect of scopolamine on spatial reference memory in mice was not tested in this study. These data suggest that it may be possible to dissociate drug-induced effects on memory from changes in sensory perception.     

Arginine administration inhibits hippocampal Na(+),K(+)-ATPase activity and impairs retention of an inhibitory avoidance task in rats

In the present study we investigated the effect of acute administration of L-arginine (Arg) on hippocampal Na(+),K(+)-ATPase activity and on retrieval of step-down inhibitory avoidance in adult rats. The action of L-NAME on the effects produced by Arg was also tested. Sixty-day-old rats were treated with a single intraperitoneal injection of saline (group I, control), arginine (0.8 g/kg) (group II), L-NAME (2 mg/kg) (group III) or arginine (0.8 g/kg) plus L-NAME (2 mg/kg) (group IV). Na(+),K(+)-ATPase activity was significantly reduced in arginine-treated rats; this effect was prevented by L-NAME. Retrieval of the avoidance task was also significantly impaired by arginine, whereas the simultaneous injection of L-NAME prevented this effect. Present data strongly indicate that in vivo Arg administration reduces both Na(+),K(+)-ATPase activity and memory modulation in rats probably through NO formation.     

NMDA and muscarinic blockade in the perirhinal cortex impairs object discrimination in rats.

To determine the possible involvement of NMDA and muscarinic activation of the perirhinal cortex in object discrimination, an NMDA antagonist, D,L-2-amino-5-phosphonopentanoic acid (AP5), and a muscarinic antagonist, scopolamine (SCP) were injected into the perirhinal cortex of rats. Each drug at the higher dose (AP5 60 mM, SCP 80 mM) significantly decreased correct choices on the retention test of object discrimination. SCP, but not AP5, also significantly increased response latency, but this increase was not necessarily related to the time spent for a choice. These results suggest that activation of both NMDA and muscarinic receptors contributes to object discrimination.     

Amygdala norepinephrine levels after training predict inhibitory avoidance retention performance in rats

Previous findings indicate that footshock and several drugs that modulate memory consolidation alter norepinephrine (noradrenaline) release in the amygdala, as assessed by in vivo microdialysis and high-performance liquid chromatography. Such findings suggest that norepinephrine release in the amygdala may be critical for regulating memory consolidation. The present study was the first to examine the relationship between norepinephrine release in the amygdala assessed after inhibitory avoidance training and 24-h retention performance within individual animals. Norepinephrine levels increased to > 300% of pretraining baseline 30 min after training and remained elevated for 2 h. In individual rats, the increase in norepinephrine levels after training correlated highly with 24-h retention performance. These findings indicate that the degree of activation of the noradrenergic system within the amygdala in response to a novel, emotionally arousing experience predicts the extent of long-term memory for that experience.     

Exposing rats to a predator impairs spatial working memory in the radial arm water maze

This series of studies investigated the effects of predator exposure on working memory in rats trained on the radial arm water maze (RAWM). The RAWM is a modified Morris water maze that contains four or six swim paths (arms) radiating out of an open central area, with a hidden platform located at the end of one of the arms. The hidden platform was located in the same arm on each trial within a day and was in a different arm across days. Each day rats learned the location of the hidden platform during acquisition trials, and then the rats were removed from the maze for a 30-min delay period. During the delay period, the rats were placed either in their home cage (nonstress condition) or in close proximity to a cat (stress condition). At the end of the delay period, the rats were run on a retention trial, which tested their ability to remember which arm contained the platform that day. The first experiment confirmed that the RAWM is a hippocampal-dependent task. Rats with hippocampal damage were impaired at learning the location of the hidden platform in the easiest RAWM under control (non-stress) conditions. The next three experiments showed that stress had no effect on memory in the easiest RAWM, but stress did impair memory in more difficult versions of the RAWM. These findings indicate that the capacity for stress to impair memory is influenced not only by the brain memory system involved in solving the task (hippocampal versus nonhippocampal), but also by the difficulty of the task. This work should help to resolve some of the confusion in the literature regarding the heterogeneous effects of stress on hippocampal-dependent learning and memory.     

Estradiol to aged female or male mice improves learning in inhibitory avoidance and water maze tasks

Although 17beta-Estradiol (E2) improves cognitive performance of aged female mice, its mnemonic effects when administered post-training to aged male mice have not been examined. E2 (10 microg, SC) or oil vehicle was administered to intact, 24-month-old female or male congenic (primarily C57BL/6 background) mice immediately after training in the inhibitory avoidance or water maze tasks. Following behavioral testing, effects of 1 or 24 h of E2 exposure on hippocampal levels of E2 and brain-derived neurotrophic factor (BDNF) were examined. Female and male mice administered E2 showed significantly better performance in the inhibitory avoidance task than did vehicle-administered mice. When tested 24 h after training, mice that received E2 had significantly longer latencies to cross-over to the shock-associated side of the chamber than did vehicle-administered mice. Female or male mice administered E2 showed significantly better performance in the reference memory aspect of the spatial water maze task. When tested 30 min after training, mice administered E2 had shorter latencies to, and spent longer swimming in, the quadrant that the hidden platform had previously been located in. E2 administration produced physiological levels of E2 in the hippocampus 1 and 24 h after E2. BDNF levels in the hippocampus were decreased following 1 h of E2 exposure compared to vehicle. These findings suggest that E2 to female and male mice may overcome age-related deficits in reference memory in an emotional or spatial learning task.     

Effects of excitotoxic median raphe lesion on working memory deficits produced by the dorsal hippocampal muscarinic receptor blockade in the inhibitory avoidance in rats

The experiments investigated the interactions between median raphe nucleus (MRN) serotonergic and septo-hippocampal muscarinic cholinergic systems in the modulation of forming and storing performances of working memory. Rats with ibotenic acid-induced MRN-lesion bilaterally received scopolamine (2-4 microg/each side) infusion into the dentate gyrus of the dorsal hippocampus and were tested in a single trial step-through inhibitory avoidance. Initial preference to the dark compartment (escape latency) was taken as the measure of non-mnemonic behaviours and response latency to enter the dark compartment immediately after the foot-shock was used to measure working memory. The high-dose scopolamine infusion 10 min before the training decreased escape latencies in the sham-lesioned rats, whereas had no effect in the MRN-lesioned rats. Although MRN lesion per se did not alter response latency, it alleviated pre-training scopolamine-induced decrease, but aggravated post-training scopolamine-induced reduction in this parameter. These results suggest that the antagonistic interactive processes between MRN-serotonergic and hippocampal cholinergic systems modulate non-mnemonic component of working memory formation, whereas the storing performance of working memory is modulated by the synergistic interactions between these systems in the hippocampus, mainly in the dentate gyrus.     

Muscimol infused into the medial septal area impairs long-term memory but not short-term memory in inhibitory avoidance, water maze place learning and rewarded alternation tasks.

These experiments investigated the effects of injections of muscimol (1 or 5 nmol), administered into the medial septal area prior to training, on memory tested at different retention delays after training in 3 tasks: an inhibitory avoidance task, a one-trial place learning task, and a rewarded alternation task. In all 3 tasks, intraseptal injections of muscimol did not impair memory performance at short retention delays, but impaired memory at the longer retention delays. These findings are consistent with the view that GABAergic regulation of the septohippocampal cholinergic system plays a selective role in the establishment of long-term memory.     

Stress impairs performance in spatial water maze learning tasks

The water maze task has been developed to test spatial learning abilities in rats or mice, and is widely used. Though it has been reported before that numerous cognitive abilities are of importance for learning this task, poor performance is usually interpreted as an impairment of spatial memory formation. Previous investigations that tried to correlate long-term potentiation (LTP) of synaptic transmission with spatial learning abilities in rats reported that injection of drugs or specific gene deletions which blocked the expression of LTP correlated with learning impairments of spatial tasks in a water maze. Recent studies, however, have shown that pretraining enables these animals to learn such spatial tasks even though LTP was still found to be blocked. I investigated to what degree altered fear condition and stress perception could account for the impaired spatial learning when no pretraining is given. In a fear habituation task, unhandled rats preferred a dark over a well lit chamber more than handled animals did, but unhandled rats favoured the lit chamber more in an active avoidance task. They also performed poorly in a spatial water maze task compared with handled rats. Rats pretrained in a radial arm maze performed better in a water maze than non-pretrained rats. No difference between groups was found in a non-spatial water maze task. On the other hand, when pretrained in a water maze, rats performed only marginally better in a radial arm maze compared to non-pretrained animals. Since animals have to be handled to learn a radial arm maze, the difference in this task was not due to stress but most probably due to getting accustomed to the room dimensions prior to learning the spatial task. The results suggest that impaired learning of spatial tasks in the water maze can be due to increased stress and decreased fear conditioning without actually affecting spatial learning abilities. These results question the interpretations of the results of some previously published results of spatial water maze tasks.     

Vagal nerve stimulation during muscarinic and beta-adrenergic blockade causes significant coronary artery dilation

Vasoactive intestinal peptide (VIP) is present in post-ganglionic vagal nerve fibers in the coronary arteries and right ventricle but no significant amounts are found in the left ventricle. We determined the effects of VIP, released endogenously from cardiac vagal nerves, on the circumflex mean coronary artery pressure and on right and left ventricular (RV and LV) contractility (dP/dt_max) and relaxation (dP/dt_min). In 20 anesthetized, open chest mongrel dogs, the cervical vagus nerves and cardiac sympathetic ansa subclaviae were isolated and transected. Electrodes were applied to the cardiac segments of the right and left vagus nerves for subsequent stimulation. The muscarinic and β-adrenergic receptors were blocked with atropine and propranolol, respectively. The heart rate was controlled by either producing atrioventricular node block in 10 dogs and pacing the ventricles (series 1) or by right atrial pacing in 10 separate dogs (series 2). Coronary artery blood flow was controlled by perfusing the circumflex coronary artery in each dog with femoral arterial blood at a controlled flow rate. Coronary artery pressure, ventricular and aortic pressures and dP/dt were continuously measured. Experiments were performed prior to and after the administration of [4Cl-D-Phe⁶,Leu¹⁷]VIP, a sensitive and selective VIP antagonist. Vagal nerve stimulation at 20 Hz (0.5 ms, 20 V) for 5 min significantly decreased the circumflex mean coronary artery pressure by 17% from the control value of 95±2 mmHg in series 1 and by 13% from the control value of 109±2 mmHg in series 2 (both p<0.005). Aortic, LV and RV systolic and end-diastolic pressures, LV dP/dt_max and dP/dt_min, and the EKG did not change. In contrast, RV dP/dt_max and dP/dt_min increased by 22% (p<0.04) and 23% (p<0.02), respectively, in series 1 and by 26% (p<0.02) and 33% (p<0.01), respectively, in series 2. The VIP antagonist, [4Cl-D-Phe⁶,Leu¹⁷]VIP, directly injected into the left circumflex coronary artery, had no effect on coronary, aortic or ventricular pressures, ventricular dP/dt or the EKG. However, 20 Hz vagal stimulation in the presence of the VIP antagonist did not decrease circumflex mean coronary artery pressure. In addition, vagal stimulation, in the presence of the VIP antagonist, had no effect on LV pressures or dP/dt but increased RV dP/dt_max and dP/dt_min. RV dP/dt_max increased by 16% (p<0.01) and RV dP/dt_min increased by 22% (p<0.04), respectively, in series 1 and by 27 and 24%, respectively, in series 2 (both p<0.01). Vagal nerve stimulation during muscarinic and β-adrenergic blockade releases VIP or a `VIP-like' substance that significantly decreases circumflex coronary artery vascular resistance and increases RV dP/dt_max and dP/dt_min.     

Attenuation of scopolamine-induced learning deficits by LVV-hemorphin-7 in rats in the passive avoidance and water maze paradigms

Central administration of angiotensin IV (Ang IV) analogues attenuates scopolamine-induced amnesia. Ang IV mediates its effects by binding to a high affinity, binding site, AT(4) receptor, that has recently been identified as insulin regulated aminopeptidase (IRAP). The purpose of this study was to examine the effect of the distinct AT(4) ligand, LVV-hemorphin-7 (LVV-H7), on scopolamine-induced learning deficits, one which involves fear-conditioning and the other spatial learning. Rats were pretreated with an intracerebroventricular (ICV) dose of scopolamine hydrobromide followed by treatment with 1 nmol LVV-H7 or artificial cerebrospinal fluid (aCSF). During the acquisition phase of the water maze task, daily ICV infusions of 1 nmol of LVV-H7 25 min after scopolamine treatment produced marked improvement in both the latency and distance swum in order to locate the submerged platform using visual cues compared to animals treated with scopolamine only. In addition, the same dose of LVV-H7 attenuated the learning deficit observed for scopolamine-treated animals in the passive avoidance task. These studies clearly demonstrate that LVV-H7, like Ang IV, is a pharmacologically active AT(4) ligand that attenuates the deleterious effects of scopolamine on learning performance in two different behavioral paradigms.     

Vagal stimulation during muscarinic and beta-adrenergic blockade increases atrial contractility and heart rate.

We determined the effects of continuous cardiac vagal nerve stimulation on atrial contractility and on heart rate in mongrel dogs in which we blocked the muscarinic and beta-adrenergic receptors. Each dog received atropine, 0.5 mg/kg and propranolol, 0.5-1 mg/kg. We stimulated the cardiac vagus nerves in each dog for three separate 5-min periods at frequencies of 0 (control), 20, and 40 Hz (5 ms, 15 V) and measured the changes in atrial contractility and heart rate. Vagal nerve stimulation increased right atrial contractility from the control value by 27% at 20 Hz and by 19% during stimulation at 40 Hz (P < 0.001). Vagal nerve stimulation also increased the heart rate from 114 +/- 5 beats/min during the control period to 146 +/- 10 beats/min (P < 0.01) during stimulation at a frequency of 20 Hz and to 140 +/- 11 beats/min (P < 0.05) during stimulation at 40 Hz. Injection of the vasoactive intestinal peptide (VIP) antagonist, [4Cl-D-Phe6,Leu17]VIP, directly into the dog right coronary artery in concentrations of 0 (control), 2, and 4 micrograms/kg did not influence spontaneous atrial contractility or the heart rate. However, 4 micrograms/kg of the VIP antagonist significantly reduced the augmentation in right atrial contractility and the increase in heart rate during vagal nerve stimulation. Our experiments suggest that cardiac vagal nerve stimulation, during muscarinic and beta-adrenergic receptor blockade, releases VIP or a 'VIP-like substance', that significantly augments atrial contractility and increases heart rate.     

Post-training amygdaloid lesions impair retention of an inhibitory avoidance response

The study examined the effect of pre- and post-training bilateral amygdaloid lesions on retention of a one-trial inhibitory avoidance response. Groups of rats, including unimplanted controls and implanted controls, were trained and tested for retention at 4, 7 or 12 days following training. The lesions were made at one of several intervals before or after training: 2 days before, immediately after, or 2, 5 or 10 days after. At all retention intervals the retention of implanted controls was poorer than that of unimplanted controls and, in comparison with both control groups, the retention of animals lesioned before training was impaired. Retention was also impaired by the post-training lesions. The degree of impairment varied with the interval between the training and the lesion: lesions made within 2 days following training impaired retention, while lesions made 10 days following training had no impairing effect. These findings suggest that posttraining lesions of the amygdala affect retention by impairing time-dependent processes involved in memory storage. With a sufficiently long training-lesion interval (10 days) an intact amygdala is not essential for retention.     

Postnatal hypobaric hypoxia in rats impairs water maze learning and the morphology of neurones and macroglia in cortex and hippocampus

Newborn rats were exposed to intermittent hypobaric hypoxia from birth until the age of 19 days. Spatial memory was tested in a Morris water maze from postnatal day (P) 23 to P32 and from P100 to P109. From P24 to P27 and on days P100 and P101, the escape latencies of hypoxic animals were longer than those of controls. At P24, the number of neuronal bodies increased in cortical layer II of the somatosensory, motor, and auditory areas, and in layer V of the motor area, but the number of neuronal bodies throughout the whole cortical thickness was unchanged. Decreases in the immunostaining density for neurofilaments (anti-NF 160), astrocytes (anti-GFAP), and oligodendrocytes (RIP) were found in the hippocampus, and the typical parallel organisation of neuronal and macroglial processes was lost. Decreases in immunostaining for neurofilaments and oligodendrocytes were also found in the somatosensory cortex and motor cortex. In adult hypoxic rats, at P114-P240, the number of neuronal bodies and the immunostaining density for neurofilaments, astrocytes, and oligodendrocytes in the examined areas were similar to adult controls; however, in the hippocampus we found hypertrophy of fine astrocytic processes and a decreased number of oligodendrocytic processes. We conclude that the neonatal brain damage induced by hypobaric hypoxia impairs spatial memory in infant as well as adult rats. Hypobaric hypoxia delays the maturation of neurones and substantially affects macroglia in the cortex and hippocampus.     

Post-training intra-striatal scopolamine or flupenthixol impairs radial maze learning in rats

Systemic treatments with acetylcholine (ACh) or dopamine (DA) receptor antagonists during hours 0-4 but not during hours 5-8 following training on a radial arm maze (RAM) or lesions of the dorsal striata impair learning. This suggested that intra-striatal infusions of ACh or DA receptor antagonists during hours 0-4 following training may impair learning. Rats were randomly assigned to groups (ns=5-11) receiving dorsal striatal infusions of the ACh receptor antagonist scopolamine (0-18 microg/microL at 0 and 2h or at 4 and 6h after training), the DA receptor antagonist cis-flupenthixol (0-25 microg/microL at 0, 4 or 12h after training) or the inactive isomer trans-flupenthixol (6 microg/microL at 0 h after training). Scopolamine and cis-flupenthixol impaired the habit-learning version of the task. Given after hours 0-4 following training, the effects of scopolamine were diminished but those of cis-flupenthixol were not. Trans-flupenthixol produced less impairment than cis-flupenthixol. Results suggest that ACh and DA receptors in the dorsal striatum during hours 0-4 following training play a role in habit learning.     

(+/-)-Alpha-methyl-4-carboxyphenylglycine, a metabotropic glutamate receptor blocker, impairs retention of an inhibitory avoidance task in rats when infused into the basolateral nucleus of the amygdala

The amygdala is important for memory processes of emotionally motivated learning and the amygdala glutamatergic system may play a key role in this process. In this study we assessed the effect of the infusion of (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG), a metabotropic glutamate receptor (mGluR) antagonist, into the basolateral complex of the amygdala (BLA) on the learning and retention of an emotionally motivated task. Rats received either vehicle or three different doses of MCPG (0.2, or 1.0, or 5.0 microg/0.2 microl/side, respectively) bilaterally into the BLA, 5 min before they were trained in a continuous multiple-trial inhibitory avoidance (CMIA) task. Response latencies during the training were recorded. Retention was assessed 8 days later. MCPG in the doses given did not significantly affect the acquisition of the CMIA task. However, MCPG at a dose of 5.0 microg/0.2 microl/side impaired the long-term retention test performance. Additionally, a nociception test indicated that dose of MCPG infused into the BLA did not affect the footshock sensitivity. Our results indicate that MCPG, when infused into the BLA of rats prior to the training, impaired long-term memory of aversive training without affecting acquisition.     

Atropine sulfate impairs selective parameters of performance in the Morris water maze

Summary Twelve rats were trained to learn the location of a spatially fixed platform hidden in a Morris water maze. Asymptotic performance was achieved over six training days (10 trials/day). Then retention of the spatial task was assessed 30 min after treatment with 5, 25, 50, 75, or 100 mg/kg, ip, atropine sulfate or the equivalent volume of saline. There was a significant drug effect on escape latency, swim distance, swim speed and swim path measures of spatial performance. There was no significant drug effect on heading error; atropinized animals initially headed toward the escape platform over the first 12 cm of their swim path. However, treatment with atropine sulfate significantly disrupted the usual, direct swim path used to reach the hidden escape platform. Atropinized animals frequently swam a spiraled or looping pattern to locate the platform. We suggest that cholinergic blockade may significantly disrupt the processing of visual cues which rats use in place navigation tasks.     

Chronic administration of fluoxetine impairs inhibitory avoidance in male but not female mice

The effects of chronic administration of fluoxetine (20 mg/kg/day i.p.) on a one-trial step-through inhibitory avoidance task were investigated in male and female CD1 mice. In Experiment 1, treatment was administered for 21 days before the training session, whereas in Experiment 2, other subjects were subjected to the same treatment starting 24 h after the training session. The comparison of test versus training latencies showed memory deterioration with pre-training administration of fluoxetine (Experiment 1), which affected males but not females. Sex differences in this task were also observed in Experiment 1, with females showing a better performance. Sex differences were evident in controls as well as in treated animals. The locomotor activity of the animals was also tested in Experiment 1. Due to the absence of sex differences in the drug effects on this measure, the sex differences in the effects of fluoxetine on inhibitory avoidance were hardly attributable to non specific effects on locomotor activity. The lack of effect of post-training administration of fluoxetine (Experiment 2) constitutes additional support of the idea that the observed effect on inhibitory avoidance in Experiment 1 is specifically related to learning and memory.