Involvement of the amygdala in memory storage: interaction with other brain systems

Differential effects of amygdala lesions on early and late plastic components of auditory cortex spike trains during fear conditioning

In auditory fear conditioning, pairing of a neutral acoustic conditioned stimulus (CS) with an aversive unconditioned stimulus (US) results in an enhancement of neural responses to the CS in the amygdala and auditory cortex. It is not clear, however, whether cortical plasticity governs neural changes in the amygdala or vice versa, or whether learning in these two structures is determined by independent processes. We examined this issue by recording single-cell activity in the auditory cortex (areas Te1, Te1v, and Te3) of freely behaving, amygdalectomized rats using a movable bundle of microwires. Amygdala damage did not affect short-latency (0-50 msec) tone responses, nor did it interfere with conditioning-induced increases of these onset responses. In contrast, lesions of the amygdala interfered with the development of late (500-1500 msec) conditioned tone responses that were not present before conditioning. Furthermore, whereas onset conditioned responses in the control group remained elevated after 30 extinction trials (presentation of CS alone), onset responses in lesioned animals returned to their preconditioning firing level after approximately 10 extinction trials. These results suggest that the amygdala enables the development of long-latency (US anticipatory) responses and prevents the extinction of short-latency onset responses to threatening stimuli. The findings further suggest that auditory cortex cells may participate differently in explicit and implicit memory networks.

Glucocorticoid receptor and protein/RNA synthesis-dependent mechanisms underlie the control of synaptic plasticity by stress

Learning and memory are exquisitely sensitive to behavioral stress, but the underlying mechanisms are still poorly understood. Because activity-dependent persistent changes in synaptic strength are believed to mediate memory processes in brain areas such as the hippocampus we have examined the means by which stress affects synaptic plasticity in the CA1 region of the hippocampus of anesthetized rats. Inescapable behavioral stress (placement on an elevated platform for 30 min) switched the direction of plasticity, favoring low frequency stimulation-induced decreases in synaptic transmission (long-term depression, LTD), and opposing the induction of long-term potentiation by high frequency stimulation. We have discovered that glucocorticoid receptor activation mediates these effects of stress on LTD and long-term potentiation in a protein synthesis-dependent manner because they were prevented by the glucocorticoid receptor antagonist RU 38486 and the protein synthesis inhibitor emetine. Consistent with this, the ability of exogenously applied corticosterone in non-stressed rats to mimic the effects of stress on synaptic plasticity was also blocked by these agents. The enablement of low frequency stimulation-induced LTD by both stress and exogenous corticosterone was also blocked by the transcription inhibitor actinomycin D. Thus, naturally occurring synaptic plasticity is liable to be reversed in stressful situations via glucocorticoid receptor activation and mechanisms dependent on the synthesis of new protein and RNA. This indicates that the modulation of hippocampus-mediated learning by acute inescapable stress requires glucocorticoid receptor-dependent initiation of transcription and translation.

Amygdala modulation of multiple memory systems: hippocampus and caudate-putamen

A series of five experiments examined the differential mnemonic roles of the hippocampus and caudate-putamen and the modulatory influence of the amygdala on hippocampal and caudate-putamen memory processes. Findings indicate that (a) posttraining intrahippocampal injections of amphetamine selectively enhance memory in a hidden platform water maze task, (b) posttraining intracaudate injections of amphetamine selectively enhance memory in a visible platform water maze task, (c) posttraining intra-amygdala injections of amphetamine enhance memory in both water maze tasks, (d) preretention intrahippocampal lidocaine injections block expression of the memory enhancing effects of posttraining intrahippocampal amphetamine injections in the hidden platform task, (e) preretention intracaudate lidocaine injections block expression of the memory enhancing effects of posttraining intracaudate amphetamine injections in the visible platform task, (f) preretention intra-amygdala lidocaine injections do not block the memory enhancing effect of posttraining intra-amygdala amphetamine injections on either task, (g) in the hidden platform task, posttraining intrahippocampal, but not intracaudate, lidocaine injections block the memory enhancing effects of posttraining intra-amygdala amphetamine, (h) in the visible platform task, posttraining intracaudate, but not intrahippocampal, lidocaine injections block the memory enhancing effects of posttraining intra-amygdala amphetamine. The findings indicate a double dissociation between the roles of the hippocampus and caudate-putamen in memory and suggest that the amygdala exerts a modulatory influence on both the hippocampal and caudate-putamen memory systems.

Memory suppressor genes: inhibitory constraints on the storage of long-term memory

Synaptic plasticity, the ability of neurons to alter the strength of their synaptic connections with activity and experience, is thought to play a critical role in memory storage. Molecular studies of gene expression during long-lasting synaptic plasticity related to memory storage initially focused on the identification of positive regulators. More recent work has revealed that the establishment of long-lasting synaptic plasticity and long-term memory also requires the removal of inhibitory constraints. By analogy to tumor suppressor genes, which restrain cell proliferation, we propose that these inhibitory constraints of memory storage, which restrain synapse growth, be termed memory suppressor genes.

Glucocorticoid enhancement of memory storage involves noradrenergic activation in the basolateral amygdala

Evidence indicates that the modulatory effects of the adrenergic stress hormone epinephrine as well as several other neuromodulatory systems on memory storage are mediated by activation of beta-adrenergic mechanisms in the amygdala. In view of our recent findings indicating that the amygdala is involved in mediating the effects of glucocorticoids on memory storage, the present study examined whether the glucocorticoid-induced effects on memory storage depend on beta-adrenergic activation within the amygdala. Microinfusions (0.5 microg in 0.2 microl) of either propranolol (a nonspecific beta-adrenergic antagonist), atenolol (a beta1-adrenergic antagonist), or zinterol (a beta2-adrenergic antagonist) administered bilaterally into the basolateral nucleus of the amygdala (BLA) of male Sprague-Dawley rats 10 min before training blocked the enhancing effect of posttraining systemic injections of dexamethasone (0.3 mg/kg) on 48-h memory for inhibitory avoidance training. Infusions of these beta-adrenergic antagonists into the central nucleus of the amygdala did not block the dexamethasone-induced memory enhancement. Furthermore, atenolol (0.5 microg) blocked the memory-enhancing effects of the specific glucocorticoid receptor (GR or type II) agonist RU 28362 infused concurrently into the BLA immediately posttraining. These results strongly suggest that beta-adrenergic activation is an essential step in mediating glucocorticoid effects on memory storage and that the BLA is a locus of interaction for these two systems.

Amygdala beta-noradrenergic influence on hippocampal long-term potentiation in vivo

We have previously shown that hippocampal long-term potentiation (LTP), one form of synaptic plasticity that may underlie learning and memory, is attenuated by blocking neuron activity of the basolateral amygdala (BLA). In the present study we investigated the amygdala noradrenergic or cholinergic contribution to hippocampal LTP formation. When propranolol, a beta-adrenoceptor antagonist, was injected into the BLA 10 min before tetanus, the formation of LTP in the perforant path-dentate granule cell synapses was significantly impaired. Scopolamine, a muscarinic cholinergic receptor antagonist, did not affect the formation of LTP. These results suggest that amygdala beta-noradrenergic activity plays a critical role in modulation of hippocampal LTP.

Intact enhancement of declarative memory for emotional material in amnesia

Emotional arousal has been demonstrated to enhance declarative memory (conscious recollection) in humans in both naturalistic and experimental studies. Here, we examined this effect in amnesia. Amnesic patients and controls viewed a slide presentation while listening to an accompanying emotionally arousing story. In both groups, recognition memory was enhanced for the emotionally arousing story elements. The magnitude of the enhancement was proportional for both amnesic patients and controls. Emotional reactions to the story were also equivalent. The results suggest that the enhancement of declarative memory associated with emotional arousal is intact in amnesia. Together with findings from patients with bilateral amygdala lesions, the results indicate that the amygdala is responsible for the enhancement effect.

Impaired declarative memory for emotional material following bilateral amygdala damage in humans

Everyday experience suggests that highly emotional events are often the most memorable, an observation supported by psychological and pharmacological studies in humans. Although studies in animals have shown that nondeclarative emotional memory (behaviors associated with emotional situations) may be impaired by lesions of the amygdala, little is known about the neural underpinnings of emotional memory in humans, especially in regard to declarative memory (memory for facts that can be assessed verbally). We investigated the declarative memory of two rare patients with selective bilateral amygdala damage. Both subjects showed impairments in long-term declarative memory for emotionally arousing material. The data support the hypothesis that the human amygdala normally enhances acquisition of declarative knowledge regarding emotionally arousing stimuli.

Functional anatomy of long-term memory

Memory in the brain is organized into multiple memory systems that perform different memory functions and have different neurologic substrates. Declarative memory involves conscious memory for facts and events. The medial temporal lobe and structures in the diencephalon are essential in the establishment of new declarative memories, and these memory traces are finally stored in domain-specific regions of the cerebral cortex. The frontal lobe and basal ganglia are important in some forms of declarative memory that require reasoning about the contents of memory. Nondeclarative forms of memory (including skill learning, repetition priming, and classical conditioning) do not involve conscious recollection and are measured through changes in the way in which tasks are performed. These forms of memory rely upon the cerebral cortex, basal ganglia, and cerebellum.

Glucocorticoid receptor agonist and antagonist administration into the basolateral but not central amygdala modulates memory storage

The present study examined the effects, in male Sprague-Dawley rats, or microinfusion of drugs affecting glucocorticoid receptors (GRs or Type II) administered into either the basolateral (BLA) or central nucleus of the amygdala (CEA) on memory for training in an inhibitory avoidance and water-maze escape task. The specific GR agonist RU 28362 (1.0 or 3.0 ng) infused into the BLA immediately after training in a one-trial inhibitory avoidance task dose-dependently enhanced 48-h retention, whereas infusion of the GR agonist into the CEA did not affect retention. Additionally, pretraining microinfusions of the specific GR antagonist RU 38486 (0.3, 1.0, or 3.0 ng) into the BLA, but not infusions into the CEA, impaired memory for escape training to find a submerged platform in a water maze. These findings indicate that glucocorticoids affect memory storage, at least in part, by binding directly to GRs in the BLA and provide further evidence for the view that the BLA is an important brain region in integrating hormonal and neurotransmitter influences on memory storage.

Endogenous opiates: 1996

This paper is the nineteenth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1996 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress, tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.