Basolateral amygdala lesions do not prevent memory of context-footshock training

The present studies examined the effects of basolateral amygdala (BLA) lesions induced prior to or after context-footshock training on 48-h memory, using several retention measures. In experiment 1, male Sprague-Dawley rats with bilateral BLA lesions (NMDA, 12.5 mg/mL, 0.2 μL) were given footshock training in one compartment of a two-compartment alley. Rats were habituated to the alley and 24 h later were given two footshocks in the shock compartment. Retention was tested 48 h later, using latency to enter the shock compartment and time spent freezing as measures of memory. Two days later, they were tested again and received a footshock on each re-entry of the shock compartment prior to remaining in the safe compartment for 200 consecutive seconds. The BLA lesions did not block retention as assessed by freezing or number of re-entries of the shock compartment. In experiment 2, no prior habituation was given, and only one footshock was used for the training. BLA lesions did not block retention, as indicated by latencies to enter the shock compartment on a 48-h test or by number of entries of the shock compartment. Experiment 3 examined the effects of the GABA_A agonist muscimol infused into the BLA prior to the 48-h retention test. The muscimol infusions decreased retention test entrance latencies but did not block retention as assessed by the number of subsequent entries of the shock compartment. These findings provide additional evidence that an intact BLA is not required for the acquisition or retention of context-footshock training.     

Epinephrine modulates long-term retention of an aversively motivated discrimination

These experiments examined the effects of post-training epinephrine (Epi) on retention of an aversively motivated discrimination task. Male CFW mice were trained to escape from footshock by entering one of two alleys of a Y-maze. On a 24-h retention test (six trials) the correct alley was reversed. The findings of Experiment 1 indicate that errors on the discrimination reversal varied directly with number of trials (criterion of 0, 3, or 6 successive correct choices) on the original training. These findings indicate that errors on discrimination reversal training provide a sensitive index of retention of the original training. In Experiment 2, mice were trained to a criterion of three successive correct choices and were given post-training injections of saline or Epi (0.1, 0.3, or 1.0 mg/kg ip). On a 24-h discrimination reversal test mice given the low doses of Epi made more errors than did saline controls while mice given the high dose made fewer errors. In Experiment 3, mice trained as in Exp 2 received post-training saline or Epi (0.3 or 1.0 mg/kg) and were tested for retention either 1 week or 1 month later. At each retention interval, performance was comparable to that found with a 24-h retention interval. The findings provide additional evidence that post-training Epi produces long-lasting dose-dependent modulating effects on memory storage.     

Glucose effects on memory: behavioral and pharmacological characteristics

Recent findings indicate that post-training glucose injections can modulate memory storage for inhibitory (passive) avoidance training. Experiment I extended these findings to determine whether glucose, like other memory modulating treatments, enhances memory storage when administered after training with low footshock and impairs memory storage after high footshock training. In Experiment I, male Sprague-Dawley rats were trained in a one-trial inhibitory avoidance task using either a brief footshock (0.5 mA, 0.7 s) or slightly more intense footshock kept on until escape (0.7 mA, mean escape latency = 3.4 s). Immediately after training, each rat received a subcutaneous injection of glucose (100 mg/kg). When tested for retention performance 24 h later, the glucose-injected animals exhibited enhanced retention performance for low footshock training and impaired retention for high footshock training. Experiment II determined whether pretreatment with adrenergic antagonists blocked the effects of glucose on memory. Pretreatment with the alpha- or beta-adrenergic receptor antagonists, phenoxybenzamine, or propranolol, respectively, had no effect on acquisition or retention in animals trained with the brief footshock and did not affect glucose facilitation of that memory. In animals trained to escape footshock, phenoxybenzamine did not attenuate the amnesia produced by glucose. Propranolol-pretreated animals had impaired retention whether or not they received post-training amnestic injections of glucose; glucose had no effect on retention in these amnestic animals. These findings add further support to the view that glucose release after training and treatment may represent a physiological response subsequent to epinephrine release in modulating memory storage processing.     

Suppression of the rat’s locomotor activity at low intensities of electric footshock

Over 50% of all just noticeably different footshock intensities lie below ,3 mA for rats. Unfortunately, the suppression of the rat’s locomotor activity by low-intensity footshock precludes the use of these shock intensities in active escape or avoidance training. This activity-suppression effect was demonstrated in an alley and in a tilt cage. One explanation of this effect is that rats remain relatively immobile in order to avoid painful peaks in current density that occur when the animals’ paws make or break contact with “hot” grids. If this explanation is correct, the activity-suppression effect should be more pronounced with higher impedance shock sources. Some evidence is reported that is consistent with this explanation.     

Post-training systemic and intra-amygdala administration of the GABA-B agonist baclofen impairs retention

The effects of the GABA-B receptor agonist baclofen on memory storage were studied in two series of experiments. In the first series, CD-1 mice were trained in two aversively motivated tasks: a one-trial inhibitory avoidance task and a classical conditioning task (conditional emotional response). Immediate post-training ip administration of (+/-)baclofen (10 and 30 mg/kg) impaired retention of animals in both tasks. The effect was time-dependent: Retention was not affected by baclofen administered 120 min after training. In the second series of experiments, which used Sprague-Dawley rats, post-training intra-amygdala administration of baclofen impaired retention of an inhibitory avoidance response. These results support the view that the GABAergic system is involved in the modulation of memory storage and that the amygdaloid complex may be a critical site for effects of drugs affecting the GABAergic system.     

Amnesia by post-training infusion of glutamate receptor antagonists into the amygdala, hippocampus, and entorhinal cortex.

The blockers of glutamate receptors, aminophosphonovaleric acid (AP5) (5.0 micrograms) and cyano-nitroquinoxaline-dione (CNQX) (0.5 microgram), were infused bilaterally into the amygdala, dorsal hippocampus, or entorhinal cortex of rats through indwelling cannulae 0, 90, 180, or 360 min after step-down inhibitory avoidance training. Animals were tested for retention 24 h after training. In the amygdala or hippocampus, AP5 was amnestic when given 0 min after training and CNQX was amnestic when given 0, 90, or 180 min after training. In the entorhinal cortex, AP5 was amnestic when given 90 or 180 min after training and CNQX had no effect. The results suggest that a phenomenon sensitive first to AP5 and then to CNQX in the amygdala and hippocampus, probably long-term potentiation (LTP), is crucial to post-training memory processing. LTP in these two structures could underlie their role in memory consolidation and could explain the late involvement of the entorhinal cortex in post-training memory processing.     

A fully automated two-way locomotor training apparatus

A fully automated two-way locomotor training apparatus is described. The device employs two bilevel chambers that are separated by a length of straight alley. The rat is dropped from the distinctively cued upper level of one chamber onto the grid floor of its lower level. To escape shock, the rat must leave this lower level, traverse the alley segment, and jump into the upper level of the opposite compartment. Although the animal is required to traverse the same alley in two directions, its terminal response always carries it into a distinctive goal region where shock never occurs. A brief experiment is described that was designed to assess the effectiveness of this device in the acquisition and extinction of a locomotor escape response with rats.     

On the requirement of nitric oxide signaling in the amygdala for consolidation of inhibitory avoidance memory

Evidence suggests that the NO/sGC/PKG pathway plays a key role in memory processing but the actual participation of this signaling cascade in the amygdala during memory consolidation remains unknown. Here, we show that when infused in the amygdala immediately after inhibitory avoidance training, but not later, the NO synthase inhibitor L-NNA hindered long-term memory retention without affecting locomotion, exploratory behavior, anxiety state or retrieval of the avoidance response. The amnesic effect of L-NNA was not state-dependent and was mimicked by the soluble guanylyl cyclase inhibitor LY83583 and the PKG inhibitor KT-5823. On the contrary, post-training intra-amygdala infusion of the NOS substrate L-Arg, the NO-releasing compound SNAP or the non-hydrolysable analog of cGMP 8Br-cGMP increased memory retention in a dose-dependent manner. Co-infusion of 8Br-cGMP reversed the amnesic effect of L-NNA and LY83583 but not that of KT-5823. Our data indicate that the NO-induced activation of PKG in the amygdala is a necessary step for consolidation of inhibitory avoidance memory.     

Involvement of central cholinergic mechanisms in the effects of oxytocin and an oxytocin receptor antagonist on retention performance in mice

Oxytocin (OT, 0.10 microg/kg, sc) impaired retention of a one-trial step-through inhibitory avoidance task when injected into male Swiss mice 10 min after training, as indicated by retention performance 48 h later. In contrast, the immediate post-training administration of the putative oxytocin receptor antagonist d(CH(2))(5)[Tyr(Me)(2), Thr(4), Thy-NH(9)(2)] OVT (AOT, 0.30 microg/kg, sc) significantly enhanced retention performance. Neither OT nor AOT affected response latencies in mice not given footshock on the training trial, and neither the impairing effects of OT nor the enhancing effects of AOT were seen when the training-treatment interval was 180 min, suggesting that both treatments influenced memory storage. The effects of OT (0.10 microg/kg, sc) on retention were prevented by AOT (0.03 microg/kg, sc) given immediately after training, but 10 min prior to OT treatment. The central acting anticholinesterase physostigmine (35, 70, or 150 microg/kg, i.p.), but not its quaternary analogue neostigmine (150 microg/kg, i.p.), reversed the impairment of retention performance induced by OT, whereas low subeffective doses of the centrally active muscarinic cholinergic antagonist atropine (0.5 mg/kg, i.p.) or the central acting nicotinic cholinergic antagonist mecamylamine (5 mg/kg, i.p.), but not methylatropine (0.5 mg/kg, i.p.) or hexamethonium (5 mg/kg, i.p.) prevented the enhancement of retention performance caused by AOT. We suggest that oxytocin negatively modulates the activity of central cholinergic mechanisms during the posttraining period that follows an aversively motivated learning experience, leading to an impairment of retention performance of the inhibitory avoidance response.     

Orphanin FQ/nociceptin interacts with the basolateral amygdala noradrenergic system in memory consolidation

Extensive evidence indicates that the basolateral complex of the amygdala (BLA) mediates hormonal and neurotransmitter effects on the consolidation of emotionally influenced memory and that such modulatory influences involve noradrenergic activation of the BLA. As the BLA also expresses a high density of receptors for orphanin FQ/nociceptin (OFQ/N), an opioid-like peptide with anxiolytic and amnestic properties, the present experiments investigated whether the BLA is involved in mediating OFQ/N effects on memory consolidation and whether such effects require noradrenergic activity. OFQ/N (0.01-100 pmol in 0.2 microL) administered bilaterally into the BLA of male Sprague-Dawley rats immediately after aversively motivated inhibitory avoidance training induced dose-dependent impairment on a 48-h retention trial. The beta(1)-adrenoceptor antagonist atenolol (2.0 nmol) administered concurrently into the BLA potentiated the dose-response effects of OFQ/N. In contrast, immediate post-training infusions of the peptidergic OFQ/N receptor antagonist [Nphe(1)]nociceptin(1-13)NH(2) (1-100 pmol in 0.2 microL) into the BLA enhanced 48-h retention of inhibitory avoidance training, an effect that was blocked by coadministration of atenolol. Delayed infusions of OFQ/N or [Nphe(1)]nociceptin(1-13)NH(2) into the BLA administered either 6 or 3 h after training, respectively, or immediate post-training infusions of OFQ/N into the adjacent central amygdala did not significantly alter retention performance. These findings indicate that endogenously released OFQ/N interacts with noradrenergic activity within the BLA in modulating memory consolidation.     

Intracerebroventricular effects of angiotensin II on a step-through passive avoidance task in rats

A wealth of evidence indicates that angiotensin II (Ang II) is involved in learning and memory. However, the precise role of this peptide in these cognitive processes is still controversial, with data indicating either an inhibitory or an enhancing action. The present study was designed to further investigate the effects of intracerebroventricular injections of Ang II (0.5, 1 or 3nmol/5microl) on a step-through passive avoidance task in male adult Wistar rats. When administered pretraining, Ang II did not affect the acquisition of passive avoidance, but markedly improved avoidance performance when given before the retrieval test. The latter effect was observed in retest sessions performed up to 72h after training. Administration of the peptide five minutes after training impaired retention of inhibitory avoidance. Therefore, Ang II may exert opposite effects on passive avoidance memory according to its interference with brain mechanisms leading to the storage or retrieval of this aversively motivated task.     

Intra-basolateral amygdala infusions of AP-5 impair or enhance retention of inhibitory avoidance depending on training conditions

Previous evidence has suggested that N-methyl-D-aspartate receptors (NMDARs) in the basolateral amygdala (BLA) are critically involved in the acquisition of aversively based learning tasks. However, the role of NMDARs in the BLA in the consolidation of memory of aversive training has not been well elucidated. In the present study, the NMDAR antagonist AP-5 (1 or 3 microg) was infused into the BLA of male Sprague-Dawley rats immediately before, immediately after, or 6h after training on an inhibitory avoidance task with either a high footshock (HFS; only high dose of AP-5 given) or a low footshock (LFS; both doses of AP-5 given). The 48 h retention of animals given AP-5 (3 microg) immediately before or after HFS training was significantly impaired compared to that of vehicle-controls. In contrast, the retention of rats given AP-5 (3 microg) immediately after LFS training was significantly enhanced compared to that of vehicle-controls. AP-5 (3 microg) infusions administered 6h after training with either an HFS or LFS did not affect retention. These findings suggest that the NMDARs in the BLA are involved in both the acquisition and consolidation of aversive memory. In addition, the AP-5-induced enhancement of memory obtained with LFS training suggests that NMDARs in the BLA are involved in other mechanisms influencing synaptic transmission, in addition to their well-established role in neuroplasticity.     

Glucocorticoid administration into the dorsal striatum [corrected] facilitates memory consolidation of inhibitory avoidance training but not of the context or footshock components

It is well established that glucocorticoid administration into a variety of brain regions facilitates memory consolidation of fear-conditioning tasks, including inhibitory avoidance. The present findings indicate that the natural glucocorticoid corticosterone administered into the dorsal striatum (i.e., caudate nucleus) of male Wistar rats produced dose- and time-dependent enhancement of inhibitory avoidance memory consolidation. However, as assessed with a modified inhibitory avoidance procedure that took place on two sequential days to separate context training from footshock training, corticosterone administration into the dorsal striatum did not enhance memory of either the contextual or aversively motivational aspects of the task.     

A threshold for the protective effect of over-reinforced passive avoidance against scopolamine-induced amnesia.

Acetylcholine-receptor blockers produce amnesia of aversively motivated behaviors. However, when animals are submitted to relatively high intensities of footshock (over-reinforcement), anticholinergic treatment does not induce memory impairments. The aim of this work was to determine whether the antiamnesic effect produced by increasing the magnitude of the negative reinforcer is gradually established or if a threshold should be reached to obtain such an effect. Wistar rats were trained in passive avoidance using 2.5, 2.6, 2.7, 2.8, 2.9 or 3.0 mA; 5 min after training they were given one systemic injection of scopolamine (8 mg/kg). An amnesic state was produced in the groups that were trained with the lower intensities (2.5-2.7 mA); with the three higher intensities near-perfect retention was evident. These results suggest that acetylcholine is critically involved in memory consolidation, and that by increasing the magnitude of the negative reinforcer, a threshold is reached where cholinergic activity of the nervous system is not necessary for the development of the consolidation process.     

Brain catecholamines and memory modulation: effects of footshock, amygdala implantation, and stimulation

The results of previous studies indicate that the extent of a transient decline in brain norepinephrine (NE) levels shortly after training and administration of any of several memory modulating treatments is correlated with later retention performance. The present experiment assessed such changes after one-trial inhibitory (passive) avoidance training and, in addition, measured concentration changes in 3-methoxy-4-hydroxyphenylglycol (MHPG), the major metabolite of brain NE, as well as dopamine (DA) and epinephrine (EPI) levels. The results indicate that the decreases in brain NE after footshock are accompanied by an increase in MHPG, thus providing additional evidence that brain NE is released after training. DA levels were unchanged after training; brainstem EPI levels increased after the training footshock, but forebrain EPI levels were unchanged. A second experiment examined brain catecholamine levels in animals which received post-training electrical stimulation of the amygdala. The findings of this experiment indicate that the amygdala damage which accompanies electrode implantation apparently results in a chronic change in whole brain NE levels and metabolism. After amygdala, NE concentrations in both brainstem and forebrain samples were reduced by 20% and MHPG was increased by 22-34%. Furthermore, NE levels were not responsive to training in implanted animals. Thus, brain NE levels after training were not predictive of retention performance in amygdala-implanted or -stimulated animals. However, the significance of such findings for understanding the possible role of central NE in memory storage is complicated by the severe modification of the dynamics of brain aminergic systems in animals bearing amygdala electrodes.     

Phthalic acid amygdalopetal lesion of the nucleus basalis magnocellularis induces reversible memory deficits in rats

The basolateral amygdala (BLA) is extensively implicated in emotional learning and memory. The current study investigated the contribution of cholinergic afferents to the BLA from the nucleus basalis magnocellularis in influencing aversive learning and memory. Sprague-Dawley rats were given permanent unilateral phthalic acid (300 ng) lesions of the nucleus basalis magnocellularis and were chronically implanted with cannulas aimed at the ipsilateral BLA. Lesioned rats showed a pronounced inhibitory avoidance task retention deficit that was attenuated by acute posttraining infusions of the muscarinic cholinergic agonist oxotremorine (4 ng) or the indirect agonist physostigmine (1 microg) into the BLA. Continuous multiple-trial inhibitory avoidance training and testing revealed that lesioned rats have a mild acquisition deficit, requiring approximately 1 additional shock to reach the criterion, and a pronounced consolidation deficit as indicated by a shorter latency to enter the shock compartment on the retention test. Because lesioned rats did not differ from sham-operated controls in performance on a spatial water maze task or in shock sensitivity, it is not likely that the memory impairments produced by the phthalic acid lesions are due to any general sensory or motor deficits. These findings suggest that the dense cholinergic projection from the nucleus basalis magnocellularis to the BLA is involved in both the acquisition and the consolidation of the aversive inhibitory avoidance task.     

The effect of reserpine, syrosingopine, and guanethidine on the retention of discriminated escape reversal: peripherally administered catecholamines cannot reverse the reserpine amnesia in this situation

In a series of experiments, the effects of reserpine, syrosingopine, and guanethidine on retention of a discriminated escape reversal training were investigated in mice. The peripherally and centrally acting reserpine produced amnesia while the primarily peripherally acting compounds, syrosingopine or guanethidine, did not produce amnesia even when given in high dosages or when training was given with low footshock. Unlike in the passive avoidance situation, peripherally administered norepinephrine or dopamine was not able to attenuate the reserpine-induced amnesia. The results were discussed in terms of the role of biogenic amines in memory formation.     

Intra-amygdala muscimol injections impair freezing and place avoidance in aversive contextual conditioning

Rats were trained by shocking them in a closed compartment. When subsequently tested in the same closed compartment with no shock, normal rats showed an increased tendency to freeze. They also showed an increased tendency to actively avoid the compartment when given access to an adjacent neutral compartment for the first time. Amygdala inactivation with bilateral muscimol injections before training attenuated freezing and eliminated avoidance during the test. Rats trained in a normal state and given intra-amygdala muscimol injections before the test did not freeze or avoid the shock-paired compartment. This pattern of effects suggests that amygdala inactivation during training impaired acquisition of a conditioned response (CR) due either to inactivation of a neural substrate essential for its storage or to elimination of a memory modulation effect that facilitates its storage in some other brain region(s). The elimination of both freezing and active avoidance by amygdala inactivation during testing suggests that neither of these behaviors is the CR. The possibility that the CR is a set of internal responses that produces both freezing and avoidance as well as other behavioral effects is discussed.     

Leupeptin, a thiol proteinase inhibitor, causes a selective impairment of spatial maze performance in rats

The effects of chronic intraventricular infusion of leupeptin, a potent inhibitor of thiol proteinases, were tested on ingestive behaviors, escape and avoidance conditioning, and spatial memory in rats. The drug did not detectably influence feeding, drinking, body temperature, or the latency to escape from a mild footshock or inhibitory avoidance behavior. However, rats treated with leupeptin made numerous errors ( reentries ) in an eight-arm spatial maze. These results are interpreted as supporting the hypothesis that calcium-activated thiol proteinases are involved in the formation of certain types of memory.