(+/-)-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.

Response of amygdalar norepinephrine to footshock and GABAergic drugs using in vivo microdialysis and HPLC.

These studies used in vivo microdialysis and high-performance liquid chromatography (HPLC) to examine levels of norepinephrine (NE) within the amygdala in response to both footshock and GABAergic compounds. In Experiment 1, microdialysis probes were inserted into a previously implanted guide cannula aimed at the amygdala and the level of NE was examined in response to footshock. A 0.55 mA (2 s) footshock induced a significant increase in NE levels when the microdialysis probe was located within the amygdala; levels of NE did not differ from baseline in rats with microdialysis probes located outside the amygdala. Experiment 2 examined the effects of the GABAergic antagonist, picrotoxin, the GABAergic agonist, muscimol, and saline on NE levels in the amygdala. Rats treated systemically with picrotoxin showed a dramatic increase in levels of NE within the amygdala. In contrast, systemic injection of muscimol resulted in decreased levels of NE. These findings are consistent with the hypothesis that drugs that are capable of modulating memory do so by altering levels of NE within the amygdala.

Excitotoxic basolateral amygdala lesions potentiate the memory impairment effect of muscimol injected into the medial septal area.

In rats, the septo-hippocampal system is important for memory encoding. Previous reports indicate that muscimol, a specific GABAergic agonist induces learning and memory deficits when infused into the medial septal area. The basolateral nucleus of the amygdala (BLA) modulates memory encoding in other brain areas, including the hippocampus. To explore the interactions between the septo-hippocampal system and amygdala in memory, we studied the effects of intra-medial septal infusions of muscimol in rats with BLA lesions. Animals received sham surgery or excitotoxic BLA lesions and were given infusions of either vehicle or muscimol (5 nmol) into the medial septal area 5 min prior to training sessions in inhibitory avoidance and water maze tasks. In the inhibitory avoidance task, muscimol-induced memory impairment was potentiated by BLA amygdala lesions. Additionally, in the water maze task, BLA-lesioned rats given muscimol infusions into the medial septal also showed memory impairment. These findings indicate that the MSA interacts with the BLA in the processing of memory storage.

Stability of long temporal gradients of retrograde amnesia in mice.

Mice were given a single training trial and then received a series of four electroconvulsive shocks (ECS), 1 h apart, at one of several times after training (1-180 days). Retention was then tested at one of three times after ECS: 7, 14, or 28 days. Control animals that received sham treatment exhibited gradual forgetting with increasing training-retention intervals. Mice given ECS exhibited temporally graded retrograde amnesia, which affected memories acquired up to about 14 days before treatment. The retrograde amnesia was relatively stable, maintaining its temporally graded appearance for at least 28 days after ECS. Some recovery may have occurred in the case of memories acquired 7 days or longer before ECS, but memories acquired only 1 or 5 days before ECS did not recover. These findings extend the parallel between experimental amnesia in laboratory animals and human amnesia.

Long gradient of retrograde amnesia in mice: continuity with the findings in humans.

Mice were given a single training trial and then received either sham treatment or electroconvulsive shock (ECS; four treatments at hourly intervals) at 1 of 7 times (1-70 days) after training. Retention was always tested 2 weeks after treatment. Control animals exhibited gradual forgetting with increasing retention intervals. Mice given ECS exhibited severe retrograde amnesia, which diminished as the interval between training and ECS increased from 1 to 21 days. ECS given 21-70 days after training had no effect on memory. The finding of long, temporally graded retrograde amnesia in mice establishes continuity between the results for laboratory animals and those for humans and indicates that the neural changes involved in the consolidation of memory can continue for a significant portion of the lifetime of a memory.

Subsensitivity of catecholaminergic neurons to direct acting agonists after single or repeated electroconvulsive shock.

Spontaneous firing rates and changes in firing rate in response to an intravenously administered dose of apomorphine were measured after various electroconvulsive shock (ECS) treatment regimens from dopaminergic cells of the substantia nigra in urethane-anesthetized rats. Similar measurements were obtained from noradrenergic neurons of the locus coeruleus before and after intravenous injection of clonidine. A significant decrement in the inhibition of spontaneous firing in response to intravenous administration of these agonists was observed following multiple or single ECS treatment in both substantia nigra and locus coeruleus cells. There was a consistent but nonsignificant tendency for cells in both areas of the brain from treated animals to display higher rates of spontaneous firing than their respective sham-shocked controls. Both the effects on base-line rates of spontaneous activity and on the depression of firing rate in response to drug administration were found to be independent of repeated treatment. A significant negative correlation was obtained between base-line firing rate and percentage depression to the autoreceptor agonist, but this correlation alone was insufficient to account for the observed differences in the drug response. These results are discussed with respect to possible mechanisms of action of electroconvulsive therapy in the treatment of depression.

Memory and convulsive stimulation: effects of stimulus waveform.

Electrical stimulation with brief pulses can produce a seizure requiring less energy than conventional sine-wave stimulation, and it has been suggested that brief-pulse stimulation might reduce the memory loss associated with electroconvulsive therapy (ECT). The authors evaluated the effects of electroconvulsive shock (ECS) on memory in mice by using various waveforms, current intensities, training-ECS intervals, pulse widths, and stimulus durations. When equated for ability to produce seizures, low-energy, brief-pulse stimulation caused as much amnesia as sine-wave stimulation and sometimes more. In the absence of comparisons of the amnesic effects of brief-pulse and sine-wave stimulation in humans, the use of brief pulses for administering ECT is unwarranted.

Impairment of retention of avoidance responses in rats by posttraining diethyldithiocarbamate.

Diethyldithiocarbamate (680 mg/kg), administered immediately after training, impaired rats' retention, 6 days later, of a one-way active avoidance task and a discriminated active avoidance task. In the discrimination task a lower dose (340 mg/kg) also impaired retention. Delayed posttraining injections did not affect retention in either task. The findings indicate that DDC can have similar effects on retention of tasks requiring quite different behavioral responses.

Impaired retention of visual discriminated escape training produced by subseizure amygdala stimulation.

These experiments examined the effects on memory of posttrial, subseizure, electrical stimulation of the amygdala. Rats were trained in a visual discriminated avoidance Y-maze. Each animal received 6 tirals on the training day. Retention, tested the following day, was measured both by the number of correct choices on the first 6 retraining trials and by the number of trials to a criterion of 5 of 6 correct choices. If administered 2 min, 1 h, or 4 h, but not 10 h, after training, bilateral amygdala stimulation significantly impaired retention as measured 24 h after training. In a second experiment, rats received unilateral amygdala stimulation in order to examine better the anatomical localization of effective stimulation sites. The unilateral stimulation was administered either 2 min, 10 min, 1 h, or 4 h after training. The behavioral procedures were the same as those used in the first experiment. For animals stimulated 2 min after training, the optimal stimulation region was one which extended rostrally from the ventrolateral portion of the basomedial nucleus to the dorsomedial region of the amygdala near the stria terminalis and nucleus centralis. For animals stimulated after a 10 min training-treatment interval, this amygdala region was not an effective stimulation site. However, in these animals, stimulation of the basolateral nucleus impaired later retention. Unilateral, posttraining amygdala stimulation administered 1 or 4 h after training did not appear to produce retention deficits. The findings of these experiments thus indicate that posttrial unilateral or bilateral amygdala stimulation impairs retention of discriminated avoidance training. Furthermore, the specific amygdala site at which posttrial stimulation impairs later retention varies with the training-treatment interval.

Neurohumoral correlates of sleep: further biochemical and physiological characterization of sleep perfusates.

Twenty cats were prepared surgically with electrodes for recording the EEG, Eye Movements and EMG and a push-pull cannula system in the midbrain reticular formation (MRF) allowing the extraction of perfusates during wakefulness or REM sleep. Proteins in the perfusates were analyzed by Isoelectric Focusing (IEF) polyacrylamide gels and Sodium Dodesyl Sulphate (SDS) slab gels. In addition analysis of glycoproteins was done by gas chromatography. In some cats the contribution of cerebrospinal fluid (CSF) proteins to perfusate proteins from brain tissue was studied by intraventricular injections of labelled leucine. The effect of sleep alterations on the protein cycle during sleep and wakefulness was also studied. The results of these experiments showed that most of the proteins in the perfusates are acidic, and that REM sleep perfusates contain 2 proteins M.W. 73.000 and 45.000 not present in awake perfusates, CSF or serum. It was also shown that CSF proteins do not appear to contribute to the proteins in perfusates, and that altering the sleep wake cycle, induces changes in the rhythm of protein release in perfusates. It is suggested that some relatively large polypeptides may participate in the regulation of REM sleep.

Inhibition of cerebral protein synthesis: performance at different times after passive avoidance training.

Inhibition of cerebral protein synthesis impairs long-term memory in a variety of species and tasks. Recently it was reported that subcutaneous injection of the protein synthesis inhibitor cycloheximide impaired short-term retention, measured 10 min after training in a passive avoidance task. To examine the possibility that inhibition of cerebral protein synthesis may sometimes disrupt short-term memory, mice were injected subcutaneously with cycloheximide (120 mg/kg) or anisomycin (150 mg/kg), or bitemporally with cycloheximide or anisomycin (100 mug/side) and given one training trial in a passive avoidance box. Subcutaneously injected cycloheximide reduced step-through latencies 10 min after training as reported previously, but anisomycin or bitemporally injected cycloheximide did not. All 4 drug groups exhibited impaired long-term memory. Since the results obtained at short intervals after training varied depending on the drug and route of injection, the impairment produced by subcutaneous cycloheximide at 10 min after training cannot be attributed to inhibition of cerebral protein synthesis. It is suggested that performance at short intervals after training reflects drug side effects on step-through behavior. By contrast, the impairment obtained at long intervals after training is consistent with the hypothesis that cerebral protein synthesis is required for formation of long-term memory.