Neurobiological substrates of classical conditioning across the life span

Nondeclarative associative learning in Alzheimer's disease: An overview of eyeblink, fear, and other emotion-based conditioning

Alzheimer's disease is a neurodegenerative disorder that is characterized by progressive cognitive decline, particularly in declarative memory, and the presence of β-amyloid plaques, neurofibrillary tangles, and cortical atrophy (especially in the temporal lobe). Unlike the relationship between the temporal cortex and declarative memory, nondeclarative memories (e.g., motor, fear, and other emotion-based memories) involve distinct neural structures. The present review investigates nondeclarative associative learning ability in Alzheimer's disease. We discuss eyeblink conditioning, fear conditioning, and other emotion-based learning and present the functions and brain areas that are involved in each type of learning. Evidence suggests that nondeclarative learning is also affected by Alzheimer's disease, although some forms of learning may be relatively preserved. Details about each nondeclarative associative learning process and the implications of these findings are presented.

Diet-induced Alzheimer's-like syndrome in the rabbit

INTRODUCTION

Although mouse models of Alzheimer's disease (AD) have increased our understanding of the molecular basis of the disease, none of those models represent late-onset Alzheimer's Disease which accounts for >90% of AD cases, and no therapeutics developed in the mouse (with the possible exceptions of aduhelm/aducanumab and gantenerumab) have succeeded in preventing or reversing the disease. This technology has allowed much progress in understanding the molecular basis of AD. To further enhance our understanding, we used wild-type rabbit (with a nearly identical amino acid sequence for amyloid as in humans) to model LOAD by stressing risk factors including age, hypercholesterolemia, and elevated blood glucose levels (BGLs), upon an ε3-like isoform of apolipoprotein. We report a combined behavioral, imaging, and metabolic study using rabbit as a non-transgenic model to examine effects of AD-related risk factors on cognition, intrinsic functional connectivity, and magnetic resonance-based biomarkers of neuropathology.

METHODS

Aging rabbits were fed a diet enriched with either 2% cholesterol or 10% fat/30% fructose. Monthly tests of novel object recognition (NOR) and object location memory (OLM) were administered to track cognitive impairment. Trace eyeblink conditioning (EBC) was administered as a final test of cognitive impairment. Magnetic resonance imaging (MRI) was used to obtain resting state connectivity and quantitative parametric data (R2*).

RESULTS

Experimental diets induced hypercholesterolemia or elevated BGL. Both experimental diets induced statistically significant impairment of OLM (but not NOR) and altered intrinsic functional connectivity. EBC was more impaired by fat/fructose diet than by cholesterol. Whole brain and regional R2* MRI values were elevated in both experimental diet groups relative to rabbits on the control diet.

DISCUSSION

We propose that mechanisms underlying LOAD can be assessed by stressing risk factors for inducing AD and that dietary manipulations can be used to assess etiological differences in the pathologies and effectiveness of potential therapeutics against LOAD. In addition, non-invasive MRI in awake, non-anesthetized rabbits further increases the translational value of this non-transgenic model to study AD.

Contribution of the Cerebellum in Cue-Dependent Force Changes During an Isometric Precision Grip Task

The “raspberry task” represents a precision grip task that requires continuous adjustment of grip forces and pull forces. During this task, subjects use a specialised grip rod and have to increase the pull force linearly while the rod is locked. The positions of the fingers are unrestrained and freely selectable. From the finger positions and the geometry of the grip rod, a physical lever was derived which is a comprehensive measurement of the subject’s grip behaviour. In this study, the involvement of the cerebellum in establishing cued force changes (CFC) was examined. The auditory stimulus was associated with a motor behaviour that has to be readjusted during an ongoing movement that already started. Moreover, cerebellar involvement on grip behaviour was examined. The results show that patients presenting with degenerating cerebellar disease (CBL) were able to elicit CFC and were additionally able to optimise grip behaviour by minimising the lever. Comparison of the results of CBL with a control group of healthy subjects showed, however, that the CFC incidence was significantly lower and the reduction of the lever was less in CBL. Hence, the cerebellum is involved not only in the classical conditioning of reflexes but also in the association of sensory stimuli with complex changes in motor behaviour. Furthermore, the cerebellum is involved in the optimisation of grip behaviour during ongoing movements. Recent studies lead to the assumption that the cerebello-reticulo-spinal pathway might be important for the reduced optimisation of grip behaviour in CBL.

Age-dependent impairment of eyeblink conditioning in prion protein-deficient mice

Mice lacking the prion protein (PrP(C)) gene (Prnp), Ngsk Prnp (0/0) mice, show late-onset cerebellar Purkinje cell (PC) degeneration because of ectopic overexpression of PrP(C)-like protein (PrPLP/Dpl). Because PrP(C) is highly expressed in cerebellar neurons (including PCs and granule cells), it may be involved in cerebellar synaptic function and cerebellar cognitive function. However, no studies have been conducted to investigate the possible involvement of PrP(C) and/or PrPLP/Dpl in cerebellum-dependent discrete motor learning. Therefore, the present cross-sectional study was designed to examine cerebellum-dependent delay eyeblink conditioning in Ngsk Prnp (0/0) mice in adulthood (16, 40, and 60 weeks of age). The aims of the present study were two-fold: (1) to examine the role of PrP(C) and/or PrPLP/Dpl in cerebellum-dependent motor learning and (2) to confirm the age-related deterioration of eyeblink conditioning in Ngsk Prnp (0/0) mice as an animal model of progressive cerebellar degeneration. Ngsk Prnp (0/0) mice aged 16 weeks exhibited intact acquisition of conditioned eyeblink responses (CRs), although the CR timing was altered. The same result was observed in another line of PrP(c)-deficient mice, ZrchI PrnP (0/0) mice. However, at 40 weeks of age, CR incidence impairment was observed in Ngsk Prnp (0/0) mice. Furthermore, Ngsk Prnp (0/0) mice aged 60 weeks showed more significantly impaired CR acquisition than Ngsk Prnp (0/0) mice aged 40 weeks, indicating the temporal correlation between cerebellar PC degeneration and motor learning deficits. Our findings indicate the importance of the cerebellar cortex in delay eyeblink conditioning and suggest an important physiological role of prion protein in cerebellar motor learning.

Comparison of the classically conditioned withdrawal reflex in cerebellar patients and healthy control subjects during stance: I. electrophysiological characteristics

The aim of this study was to demonstrate the involvement of the human cerebellum in the classically conditioned lower limb withdrawal reflex in standing subjects. Electromyographic activity was recorded from the main muscle groups of both legs of eight patients with cerebellar disease (CBL) and eight control subjects (CTRL). The unconditioned stimulus (US) consisted of electrical stimulation of the tibial nerve at the medial malleolus. The conditioning stimulus (CS) was an auditory signal given via headphones. Experiments started with 70 paired conditioning stimulus-unconditioned stimulus(CSUS) trials followed by 50 US-alone trials. The general reaction consisted of lifting and flexing the stimulated (stepping) leg with accompanying activation of the contralateral (supporting) leg. In CTRL, the ipsilateral (side of stimulation) flexor and contralateral extensor muscles were activated characteristically. In CBL, the magnitudes of ipsilateral flexor and contralateral extensor muscle activation were reduced comparably. In CTRL, the conditioning process increased the incidence of conditioned responses (CR), following a typical learning curve, while CBL showed a clearly lower CR incidence with a marginal increase, albeit, at a shorter latency. Conditioning processes also modified temporal parameters by shortening unconditioned response (UR) onset latencies and UR times to peak and, more importantly in CBL, also the sequence of activation of muscles, which became similar to that of CTRL. The expression of this reflex in standing subjects showed characteristic differences in the groups tested with the underlying associative processes not being restricted exclusively to the CR but also modifying parameters of the innate UR.

Binge-like ethanol exposure during the early postnatal period impairs eyeblink conditioning at short and long CS-US intervals in rats

Binge-like ethanol exposure on postnatal days (PD) 4-9 in rodents causes cerebellar cell loss and impaired acquisition of conditioned responses (CRs) during "short-delay" eyeblink classical conditioning (ECC), using optimal (280-350 ms) interstimulus intervals (ISIs). We extended those earlier findings by comparing acquisition of delay ECC under two different ISIs. From PD 4 to 9, rats were intubated with either 5.25 g/kg of ethanol (2/day), sham intubated, or were not intubated. They were then trained either as periadolescents (about PD 35) or as adults (>PD 90) with either the optimal short-delay (280-ms) ISI, a long-delay (880-ms) ISI, or explicitly unpaired CS and US presentations. Neonatal binge ethanol treatment significantly impaired acquisition of conditioning at both ages regardless of ISI, and deficits in the acquisition and expression of CRs were comparable across ISIs. These deficits are consistent with the previously documented ethanol-induced damage to the cerebellar-brainstem circuit essential for Pavlovian ECC.

Cerebellar agenesis II: motor and language functions

In a former study of a patient with cerebellar agenesis (HK) mild motor deficits, problems in delay eyeblink conditioning and mild to moderate deficits in IQ, planning behavior, visuospatial abilities, visual memory, and attention were found. The present study reports additional findings in the same patient. In the motor domain, impairments in fine motor manipulations, trace eyeblink conditioning and motor imagination in a functional magnetic resonance (fMRI) study were found. Based on fMRI findings; however, cortical areas involved in a tapping task did not significantly differ from a healthy control group. In the cognitive domain, deficits in speech comprehension as well as verbal learning and declarative memory were present. No significant affective symptoms were observed. Although problems in executive, visuospatial and language tasks are in agreement with the so-called cerebellar cognitive affective syndrome-other possibilities remain. Non-motor impairments in HK might also be a consequence of lacking motor abilities in development and motor deficits may interfere with the performance of parts of the cognitive tasks. In addition, lack of promotion and learning opportunities in childhood may contribute and mental retardation based on extracerebellar dysfunction cannot be excluded.

Hippocampal Damage Disrupts Eyeblink Conditioning in Mice Lacking Glutamate Receptor Subunit δ2

Cerebellar long-term depression (LTD) at the parallel fiber-Purkinje cell synapses has been proposed to be a neural substrate for classical eyeblink conditioning. Mutant mice lacking the glutamate receptor subunit δ2 (GluRδ2), in which the cerebellar LTD is disrupted, exhibited a severe impairment in the delay eyeblink conditioning with a temporal overlap of CS and US. However, they learned normally trace and delay conditioning without CS-US overlap, suggesting a learning mechanism which does not require the cerebellar LTD.In the present study, we tested possible involvement of the hippocampus in this cerebellar LTD-independent learning. We examined effects of scopolamine and hippocampal lesion on the delay conditioning without CS-US overlap. TheGluRδ2 mutant mice that received scopolamine or aspiration of the dorsalhippocampus together with its overlying cortex exhibited a severe impairment in learning, while the control mutant mice that received saline or aspiration of the overlying cortex learned normally. In contrast, wild-type mice that received either treatment learned as normally as the control wild-type mice. These results suggest that the hippocampus is essential in the cerebellar LTD-independent learning in the GluRδ2 mutant mice, indicating a newrole of hippocampus in the paradigm with a short trace interval.

Impairment of eyeblink conditioning in GluRdelta2-mutant mice depends on the temporal overlap between conditioned and unconditioned stimuli

Mice lacking the glutamate receptor subunit delta2 (GluRdelta2) are deficient in cerebellar long-term depression (LTD) at the parallel fibre-Purkinje cell synapses. We conducted delay and trace eyeblink conditioning with these mice, using various temporal intervals between the conditioned stimulus (CS) and unconditioned stimulus (US). During trace conditioning in which a stimulus-free trace interval (TI) of 250, 100 or 50 ms intervened between the 352-ms tone CS and 100-ms US, GluRdelta2-mutant mice learned as successfully as wild-type mice. Even in the paradigm with TI = 0 ms, in which the end of CS and onset of US are simultaneous, there was no difference between the GluRdelta2-mutant and wild-type mice in their acquisition of a conditioned response. However, in the delay paradigm in which the 452-ms CS overlapped temporally with the coterminating 100-ms US, GluRdelta2-mutant mice exhibited severe learning impairment. The present study together with our previous work [Kishimoto, Y., Kawahara, S., Suzuki, M., Mori, H., Mishina, M. & Kirino, Y. (2001) Eur. J. Neurosci., 13, 1249-1254], indicates that cerebellar LTD-independent learning is possible in paradigms without temporal overlap between the CS and US. On the other hand, GluRdelta2 and cerebellar LTD are essential for learning when there is CS-US temporal overlap, suggesting that the cerebellar neural substrates underlying eyeblink conditioning may change, depending on the temporal overlap of the CS and US.

Serial reaction time learning in preschool- and school-age children

Visuomotor sequence learning was assessed in 4- to 10-year-old children using a serial reaction time (SRT) task with both random and sequenced trials. One-half of the children received exposure to the sequence prior to performing the reaction time (RT) task. In Experiment 1, 7- and 10-year-old children demonstrated sequence-specific decreases in RT. As in the adult SRT literature, participants with explicit awareness of the sequence at the end of the session showed larger sequence-specific reaction time decrements than those without explicit awareness. Contrary to expectation, preexposure to the sequence did not reliably predict the level of awareness attained. Results from Experiment 2 indicate that 4-year-olds also demonstrate significant sequence learning on a variant of the SRT task. This article provides preliminary data regarding developmental changes in sequential learning and the development and use of implicit and explicit knowledge. Age-related differences emerged primarily in explicit rather than implicit knowledge.

A demonstration of classical conditioning of the human eyeblink to an olfactory stimulus

While acquisition of the eyeblink conditioned response to a variety of stimuli has been widely studied, it has yet to be established that humans will demonstrate a conditioned response to an olfactory stimulus. In this study we present data to show that humans will demonstrate a classically conditioned eyeblink response to an olfactory stimulus. Ten participants were tested in a delay conditioning procedure with an olfactory stimulus presented in a heated, humidified stream of air via an olfactometer, allowing the precise control over stimulus duration necessary for delay conditioning. Trials on which odor alone was presented were administered to four additional participants. Establishing that humans will demonstrate an eyeblink conditioned response to an olfactory stimulus will allow further exploration of the pathways involved in classical conditioning and associative learning, as well as an analysis of conditioning pathways across sensory modalities.

Frontal, temporal and lateralized brain function in children with attention-deficit hyperactivity disorder: a psychophysiological and neuropsychological viewpoint on development

This article considers deficits in the selective aspects of perception underlying symptoms of impaired attention and impulsivity in children with attention-deficit hyperactivity syndrome (ADHD) in terms of frontal and temporal lobe function and cerebral asymmetry. Tomographic studies suggest a disturbed fronto-striatal function, but have neglected limbic contributions under activating conditions and are equivocal on the nature of apparent lateralized differences. Neuropsychological and psychophysiological studies suggest that early and late stages of information processing are affected in both the frontal and temporal lobes and imply impaired intercortical dialog. Given the evidence for a normal specialization in global processing in the right and the processing of details in the left hemisphere, the lateralized impairment may progress from situational ADHD (impaired selective aspects of perception on the right) to pervasive ADHD (additional impairment in decision-making on the left). Accordingly some ADHD children may experience an early negative neurodevelopmental influence that only appears as the brain region matures while others show a delayed development of CNS function.

Development and topography of auditory event-related potentials (ERPs): mismatch and processing negativity in individuals 8-22 years of age

How do event-related potentials (ERPs) reflecting auditory processing develop across adolescence? Such development was described for five ERP components in four groups of 11 healthy participants with mean ages of 10, 14, 17, and 21 years. Data from 19 sites during diffuse (passive) and focused (discrimination) attention in a three-tone oddball were analyzed to see how ERP loci varied with age for tone type, attention condition, and for four types of difference waves reflecting nontarget and target comparisons. Age interacted with site for most components. P1 loci sensitive to rare tones moved posteriorly and N1 loci lost their right bias in early puberty. The P2 loci did not move anterior to Cz until adulthood. N2 amplitude, sensitive to attention condition, developed a frontal focus by 17 years. Right-biased P3 loci moved to the midline with focused attention similarly in all age groups. Difference waves developed in three stages: In 10-year-old participants, early deflections (< 150 ms) were diffusely distributed; in midadolescent participants, the main frontal negative component (150-300 ms) became well formed and lost an earlier right bias; and for participants 17 years old and older, the late positive complex developed a right bias in target-derived waves. Latency decreases for early frontal components were marked in participants 10-14 years old and for later posterior components in participants 14-17 years old. Major developments appeared at the onset of adolescence in early stimulus selection processes and during adolescence in the differential use of this information (N2- and P3-like latencies).

The development of conditioned blocking and monoamine metabolism in children with attention-deficit-hyperactivity disorder or complex tics and healthy controls: an exploratory analysis

Conditioned blocking (CB) measures the transient suppression of learning that a new stimulus, added during learning, has the same consequences as the conditioned stimulus already present. Normal CB increases between the age of 8 and 20 years (Oades, R.D., Roepcke, B. and Schepker, R., A. test of conditioned blocking and its development in childhood and adolescence: relationship to personality and monoamine metabolism, Dev. Neuropsychol., 12 (1996) 207-230). In the present study CB development is compared between healthy children (CN), children with attention deficit (ADHD) and those with complex tics or Tourette's syndrome (TS) with mean ages of 10-11 years. All children needed fewer learning trials with increasing age: the ADHD group showed a slight impairment. Only controls improved CB with increasing age. A trend for worse CB in the TS than the other groups was significant for those over 11 years. While ADHD children over 11 years showed less CB than controls, younger ADHD children showed more. A correlational analysis of the status of monoamine metabolism in 24 h urine samples showed a positive relationship for CB with dopamine metabolism in controls and TS children, but a negative relationship in ADHD children. In contrast, increases of serotonin metabolism were negatively related to CB in TS but positively in ADHD patients. In conclusion, when selective information processing abilities reflected by CB start to develop at puberty-onset, there is a relative worsening in ADHD patients. But TS patients show an impairment independent of age. Changes in the balance between dopamine and serotonin systems may contribute to normal and abnormal cognitive development.

Metrifonate treatment enhances acquisition of eyeblink conditioning in aging rabbits

The cholinergic system is known to show deterioration during aging and Alzheimer's disease. In response, a therapeutic approach to Alzheimer's disease has been to attempt to compensate for the decrease in central cholinergic function by potentiating the activity of the remaining intact cholinergic cells with cholinesterase inhibitors. In this study treatment with the long-lasting cholinesterase inhibitor metrifonate enhanced acquisition of eyeblink conditioning in aging rabbits without producing interfering side effects. The effects of metrifonate on central and peripheral cholinesterase activity were evaluated, as was the involvement of plasma atropine esterase activity on the central and peripheral response to metrifonate. Results demonstrate that metrifonate can produce predictable, dose-dependent ChE inhibition. Associative learning in the aging rabbit was improved by metrifonate-induced steady state ChE inhibition within a range of 30-80%. Metrifonate was behaviorally effective in the absence of the severe side effects which typically plague cholinesterase inhibitors, suggesting that metrifonate is a possible treatment for the cognitive deficits resulting from normal aging and Alzheimer's disease.

Classical conditioning of the human flexion reflex

The eyeblink conditioning paradigm is a well established model for studying learning processes in humans and animals. In this study a flexion reflex conditioning paradigm was established using the standard delay paradigm. The flexion reflex was elicited in 10 young, healthy subjects by a train of electrical pulses (100 ms, 100 Hz, 0.65 ms) applied to the medial plantar nerve (unconditioned stimulus, US). A tone (1000 Hz, 550 ms) was presented via headphones as the conditioning stimulus and which coterminated with the US. Responses were recorded from the anterior tibial muscles. Subjects were conditioned within one session of 120 trials of paired stimuli. This was established statistically via the continuous change in characteristic parameters of the responses throughout the experiment. Although the process of limb muscle conditioning takes longer than eyeblink conditioning, this type of flexion-reflex conditioning may possibly serve as a further model for the study of plastic changes within the nervous system. Moreover, the considerable versatility of limb movements offers the advantage of greater possibilities for testing the conditioning result.

The effects of scopolamine, lorazepam, and glycopyrrolate on classical conditioning of the human eyeblink response

Human eyeblink conditioning, a relatively simple form of learning and memory, has previously been shown to be impaired by the central and peripheral anticholinergic scopolamine. The present study compared the behavioral effects of scopolamine with the benzodiazepine lorazepam and a peripherally active anticholinergic, glycopyrrolate. Thirty-six healthy normal volunteers (mean age: 23.7 years) were studied with 12 assigned double-blind to each of three drug conditions (0.5 mg scopolamine IV, 2 mg lorazepam PO, or 0.2 mg glycopyrrolate IV). Subjects underwent classical conditioning of the eyeblink response in which the conditioned stimulus was an 80 dB binaural tone, and the unconditioned stimulus was a 2 psi airpuff to the right eye. Ten trials of unpaired stimulus presentations were followed by 60 paired trials and finally by an extinction period of five tone-alone presentations. An eyeblink response that occurred during the tone but before the airpuff was scored as a conditioned response (CR). Subjects treated with lorazepam (43% mean CRs) and scopolamine (51% mean CRs) exhibited a significantly lower asymptotic level of conditioning than those treated with glycopyrrolate (85% mean CRs; P < 0.01). However, during extinction, lorazepam-treated subjects (35% CRs) showed a lower overall level of responding to the tone than either scopolamine (60% CRs) or glycopyrrolate (62% CRs) treated subjects (P < 0.05). It seems unlikely that these differences could be accounted for by drug-induced alterations in motor responses because there were no significant differences between the three drug conditions in the frequency, latency, or amplitude of unconditioned responses to the airpuff. Overall, our data indicate that scopolamine and lorazepam impair eyeblink conditioning and suggest that some of the effects of benzodiazepines and anticholinergics on learning and memory can be differentiated using this paradigm.

Aging and motor learning: a possible role for norepinephrine in cerebellar plasticity

Norepinephrine is known to act as a neuromodulator in the cerebellar cortex because it can increase the effect of neurotransmitters such as GABA. This neuromodulatory effect of NE is a possible substrate for an effect of NE on cerebellar plasticity. Cerebellar plasticity can be examined by studying the learning of motor skills. A rod walking paradigm is used in our laboratory for such investigations. Learning of this rod walking task is impaired in rats that are depleted of central stores of NE and in rats that have received the beta-adrenergic antagonist propranolol. In addition, in aged rats there is a correlation between the loss of beta-adrenergic receptor mediated neuromodulatory actions of NE in the cerebellum with a decreased ability to learn the rod walking task. Taken together this information supports a role for NE in cerebellar plasticity and suggest that the beta-adrenergic receptor is important for this plasticity.

A functional anatomical study of associative learning in humans

The purpose of the study was to map the functional neuroanatomy of simple associative learning in humans. Eyeblink conditioning was studied in eight normal volunteers using positron emission tomography and H215O. Regional cerebral blood flow was assessed during three sequential phases: (i) explicitly unpaired presentations of the unconditioned stimulus (air puff to the right eye) and conditioned stimulus (binaural tone), (ii) paired presentations of the two stimuli (associative learning), and (iii) presentation of the conditioned stimulus alone. During associative learning, relative to the unpaired phase, blood flow was significantly increased in primary auditory and left posterior cingulate cortices and significantly decreased in areas of the right cerebellar, right prefrontal, right parietal, and insular cortices and right neostriatum. The lateralization of the changes may relate to the functional organization of memory and learning processes in the brain. The activation in primary auditory cortex is an example, using a neuroimaging technique, of a learning-related change in primary sensory cortex in humans. The changes in areas such as the cerebellum, prefrontal cortex, and neostriatum provide support for their roles in associative learning as proposed by animal models. Moreover, these findings show that in humans, even simple classical conditioning involves distributed changes in multiple neural systems.

Motor learning deficits in aged rats are correlated with loss of cerebellar noradrenergic function

We have demonstrated that aged rats show impairments in learning patterned motor movements. Similar behavioral impairment is observed in rats with noradrenergic lesions. Norepinephrine is known to act as a neuromodulator in the cerebellar cortex because it can augment the action of GABA and other neurotransmitters. This effect of NE to augment the signal to noise ratio of GABAergic inputs to cerebellar Purkinje neurons is a possible substrate for NE's effect on motor learning. Aged rats demonstrate deficits in the modulatory actions of NE to augment GABAergic inhibitions when both substances are locally applied onto cerebellar Purkinje neurons. In this report, we examined how motor learning and cerebellar noradrenergic function varied in individual young and 20-month-old Fischer 344 rats. There was a significant correlation between the loss of the neuromodulatory actions of norepinephrine (NE) in the cerebellar cortex and the rate of learning a novel motor task in individual rats. This report thus demonstrates for the first time a correlation between age-related impairments in motor plasticity and specific neurophysiological deficits in cerebellar Purkinje neurons in individual animals.

Neurotransmitter receptors involved in post-training memory processing by the amygdala, medial septum, and hippocampus of the rat

Rats were trained and tested in habituation to a novel environment and step-down inhibitory avoidance. Immediately after training in each task the animals received intra-amygdala, intraseptal, or intrahippocampal micro-injections of agonists and antagonists of various neurotransmitter receptors. In the habitation task, intrahippocampal, but not intra-amygdala or intraseptal administration of the NMDA receptor antagonist aminophosphornopentanoic acid (AP5, 5.0 micrograms) or of the muscarinic receptor antagonist, scopolamine (2.0 micrograms) caused amnesia and the indirect antagonist of GABA-A receptors, picrotoxin (0.08 microgram), caused retrograde facilitation. Intrahippocampal administration of the respective agonists, glutamate, oxotremorine, and muscimol, had effects of their own opposite to those of the blockers, and norepinephrine (0.3 microgram) caused memory facilitation. In the avoidance task, results obtained with drug infusions given into the three structures were very similar: in all cases, AP5, scopolamine, and muscimol were amnestic, and glutamate, oxotremorine, norepinephrine, and picrotoxin caused memory facilitation. In addition, also in the three structures, picrotoxin counteracted the amnestic effect of AP5 and/or scopolamine and the beta-adrenoceptor blocker, timolol (0.3 microgram), while ineffective on its own, attenuated all the effects of picrotoxin. The results suggest that similar synaptic mechanisms in the amygdala, medial septum, and hippocampus are involved in memory consolidation: NMDA, muscarinic, and beta-noradrenergic receptors stimulate and GABA-A receptors inhibit this process, and beta-noradrenergic receptors modulate the GABAergic synapses. In the avoidance task these mechanisms operate in the three structures: in habituation only those in the hippocampus are operative. Possibly in each structure these mechanisms regulate, if not actually consolidate, a different aspect, component, or form of memory.