Associative fear conditioning of enkephalin mRNA levels in central amygdalar neurons

The central nucleus of the amygdala (CEA) is required for the expression of learned fear responses. This study used in situ hybridization to show that mRNA levels of the neuropeptide enkephalin are increased in CEA neurons after rats are placed in an environment that they associate with an unpleasant experience. In contrast, mRNA levels of another neuropeptide, corticotropin releasing hormone, do not change under the same conditions in the CEA of the same rats. Conditioned neuropeptide levels in amygdalar circuits may act as a reversible "gain control" for long-term modulation of subsequent fear responses.

Potentiation of conditioned freezing following dorsomedial prefrontal cortex lesions does not interfere with fear reduction in mice

In Experiment 1, an auditory conditioned stimulus (CS) was paired with footshock, except when it was preceded by another stimulus (a visual conditioned inhibitor [CI]). After conditioning, all mice displayed less CS-evoked freezing when the CI-CS compound was presented than when the CS was presented alone. However, lesions of the dorsomedial prefrontal cortex (dmPFC) potentiated CS-evoked freezing on each of the 2 sessions (i.e., CI CS and CS alone). In Experiment 2, mice were submitted to fear extinction (CS-alone presentation for 3 days). Lesioned mice exhibited a higher level of freezing behavior than controls on each of the 3 sessions. However, lesioned mice and controls displayed the same rate of reduction of freezing over the 3 days of extinction. These data in mice support previous studies in rats, which suggests that the dmPFC is not critical for either conditioned inhibition or extinction of acquired freezing behavior.

17beta-estradiol suppresses expression of long-term depression in aged rats

It has been recently reported that the female steroid hormone 17beta-estradiol enhances synaptic transmission and the magnitude of long-term potentiation (LTP) in adult rodent hippocampus. Moreover, 17beta-estradiol ameliorates cognitive and memory function in postmenopausal women. Since aging is associated with an alteration of synaptic plasticity (e.g., higher susceptibility to long-term depression [LTD]), we examined whether 17beta-estradiol alters the expression of LTD in aged rats. We now report that the induction of LTD recorded from CA1 hippocampal neurons of aged rats is suppressed by 17beta-estradiol treatment, which produced only a minimal effect in suppressing LTD in adult rats. These results suggest that estrogen may act to improve memory by suppressing forgetfulness via a synaptic mechanism, such as LTD.

Learning- and cerebellum-dependent neuronal activity in the lateral pontine nucleus

The effects of inactivation of cerebellar deep nuclei and the lateral pontine nucleus on classical eyeblink conditioning with tone or lateral reticular nucleus (LRN) stimulation as conditioned stimuli (CSs) were examined. Inactivation of cerebellar deep nuclei abolished eyeblink conditioned responses (CRs) when the CS was either a tone or LRN stimulation. Inactivation of the lateral pontine nucleus prevented only the acquisition and retention of tone-evoked eyeblink CRs. Multiple-unit recording demonstrated that when LRN stimulation was used as the CS, inactivation of the interpositus nucleus abolished learning-related neuronal activity in the lateral pontine nucleus, whereas inactivation of pontine nucleus had little effect on similar activity in the interpositus nucleus. Thus, the learning-induced neuronal activity in the lateral pontine nucleus was most likely driven by the cerebellar interpositus nucleus.

Associative fear conditioning of enkephalin mRNA levels in central amygdalar neurons

The central nucleus of the amygdala (CEA) is required for the expression of learned fear responses. This study used in situ hybridization to show that mRNA levels of the neuropeptide enkephalin are increased in CEA neurons after rats are placed in an environment that they associate with an unpleasant experience. In contrast, mRNA levels of another neuropeptide, corticotropin releasing hormone, do not change under the same conditions in the CEA of the same rats. Conditioned neuropeptide levels in amygdalar circuits may act as a reversible "gain control" for long-term modulation of subsequent fear responses.

Potentiation of conditioned freezing following dorsomedial prefrontal cortex lesions does not interfere with fear reduction in mice

In Experiment 1, an auditory conditioned stimulus (CS) was paired with footshock, except when it was preceded by another stimulus (a visual conditioned inhibitor [CI]). After conditioning, all mice displayed less CS-evoked freezing when the CI-CS compound was presented than when the CS was presented alone. However, lesions of the dorsomedial prefrontal cortex (dmPFC) potentiated CS-evoked freezing on each of the 2 sessions (i.e., CI CS and CS alone). In Experiment 2, mice were submitted to fear extinction (CS-alone presentation for 3 days). Lesioned mice exhibited a higher level of freezing behavior than controls on each of the 3 sessions. However, lesioned mice and controls displayed the same rate of reduction of freezing over the 3 days of extinction. These data in mice support previous studies in rats, which suggests that the dmPFC is not critical for either conditioned inhibition or extinction of acquired freezing behavior.

Intracerebellar conditioning--Brogden and Gantt revisited

In 1942, Brogden and Gantt reported that electrical stimulation of cerebellar white matter elicited specific behavioral responses (limb flexion, eyeblink, etc.) and that these movements so elicited could easily be conditioned to a neural tone CS, using standard Pavlovian procedures. This early evidence for the key role of the cerebellum in learning of discrete movements has in recent years been replicated and much extended. It is now clear that the cerebellum is the essential structure for associative learning of discrete movements elicited by peripheral aversive or intracerebellar stimuli and that the memory traces so formed are stored in the cerebellum.

Identification of variants and dual promoters of murine serine/threonine kinase KKIAMRE

KKIAMRE is a serine/threonine protein kinase whose transcripts increase in the deep cerebellar nuclei of the rabbit after eyeblink conditioning, a model of associative learning and memory. We here characterized the expression, isoforms, and promoters of murine KKIAMRE gene. The expression of KKIAMRE was detected, by in situ hybridization and immunohistochemistry, in neurons in various brain regions including deep cerebellar nuclei. The gene spans approximately 40 kb and consists of 15 exons. Analysis of cDNA clones revealed multiple variants, having diversity in the putative carboxy-terminal regulatory domain, generated by alternative splicing and intraexonal termination. Furthermore, they had alternative 5' noncoding sequences. Primer extension, RNase protection, and transient expression assays revealed that two alternative promoters linked to distinct noncoding exons direct the expression of KKIAMRE. The gene was mapped on chromosomes 5 and 4 in mouse and human, respectively.

Learning- and cerebellum-dependent neuronal activity in the lateral pontine nucleus

The effects of inactivation of cerebellar deep nuclei and the lateral pontine nucleus on classical eyeblink conditioning with tone or lateral reticular nucleus (LRN) stimulation as conditioned stimuli (CSs) were examined. Inactivation of cerebellar deep nuclei abolished eyeblink conditioned responses (CRs) when the CS was either a tone or LRN stimulation. Inactivation of the lateral pontine nucleus prevented only the acquisition and retention of tone-evoked eyeblink CRs. Multiple-unit recording demonstrated that when LRN stimulation was used as the CS, inactivation of the interpositus nucleus abolished learning-related neuronal activity in the lateral pontine nucleus, whereas inactivation of pontine nucleus had little effect on similar activity in the interpositus nucleus. Thus, the learning-induced neuronal activity in the lateral pontine nucleus was most likely driven by the cerebellar interpositus nucleus.

The tyrosine kinase and mitogen-activated protein kinase pathways mediate multiple effects of estrogen in hippocampus

Estrogen replacement therapy in women is associated with improvement of cognitive deficits and reduced incidence of Alzheimer's disease. The present study indicates that estrogen is neuroprotective against N-methyl-d-aspartate (NMDA)- and kainate-mediated neurotoxicity, an effect mediated by tyrosine kinase/mitogen-activated protein kinase (MAPK) pathways. Estrogen also stimulates tyrosine phosphorylation of NMDA receptors via an src tyrosine kinase/MAPK pathway. Finally, estrogen-mediated enhancement of long-term potentiation in hippocampal slices is mediated by activation of an src tyrosine kinase pathway. Thus, estrogen, by activating an src tyrosine kinase and the extracellular signal-related protein kinase/MAPK signaling pathway, both enhances NMDA receptor function and long-term potentiation and retains neuroprotective properties against excitotoxicity. These findings warrant further evaluation of the usefulness of estrogenic compounds for the treatment of Alzheimer's disease and other neurodegenerative diseases.

The amygdala modulates prefrontal cortex activity relative to conditioned fear

Animals learn that a tone can predict the occurrence of an electric shock through classical conditioning. Mice or rats trained in this manner display fear responses, such as freezing behaviour, when they hear the conditioned tone. Studies using amygdalectomized rats have shown that the amygdala is required for both the acquisition and expression of learned fear responses. Freezing to a conditioned tone is enhanced following damage to the dorsal part of the medial prefrontal cortex, indicating that this area may be involved in fear reduction. Here we show that prefrontal neurons reduce their spontaneous activity in the presence of a conditioned aversive tone as a function of the degree of fear. The depression in prefrontal spontaneous activity is related to amygdala activity but not to the freezing response itself. These data indicate that, in the presence of threatening stimuli, the amygdala controls both fear expression and prefrontal neuronal activity. They suggest that abnormal amygdala-induced modulation of prefrontal neuronal activity may be involved in the pathophysiology of certain forms of anxiety disorder.

Learning induces a CDC2-related protein kinase, KKIAMRE

To elucidate molecular mechanisms in learning and memory, we analyzed expression of mRNAs in brains of rabbits undergoing eyeblink conditioning. Infusion of the transcription inhibitor actinomycin D into the cerebellar interpositus nucleus reversibly blocked learning but not performance of the conditioned response. Differential display PCR analysis of cerebellar interpositus RNAs from trained and pseudotrained rabbits identified a 207 bp band that was induced with learning. The fragment was used to isolate a cDNA from a lambdagt11 rabbit brain library containing a 1698 bp open reading frame. The deduced amino acid sequence contains the KKIAMRE motif, which is conserved among cell division cycle 2 (cdc2)-related kinases. These results suggest that there is a new category of cdc2-related kinases in the brain whose function may be important in learning and memory.

Impaired cerebellar synapse maturation in waggler, a mutant mouse with a disrupted neuronal calcium channel gamma subunit

The waggler, a neurological mutant mouse with a disrupted putative neuronal Ca(2+) channel gamma subunit, exhibits a cerebellar granule cell-specific brain-derived neurotrophic factor deficit, severe ataxia, and impaired eyeblink conditioning. Here, we show that multiple synapses of waggler cerebellar granule cells are arrested at an immature stage during development. Synaptic transmission is reduced at parallel fiber-Purkinje cell synapses. The Golgi cell-granule cell synaptic currents show immature kinetics associated with reduced gamma-aminobutyric acid type A receptor alpha6 subunit expression in granule cells. In addition, the mossy fiber-granule cell synapses exhibit N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSCs), but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated EPSCs. Our results suggest that voltage-dependent Ca(2+) channels are involved in synapse maturation. This deficient synaptic transmission in the waggler cerebellum may account for their behavioral deficits.

Bilateral lesions of the interpositus nucleus completely prevent eyeblink conditioning in Purkinje cell-degeneration mutant mice

The authors have previously demonstrated that Purkinje cell-degeneration (pcd) mutant mice are impaired in eyeblink conditioning (L. Chen et al., 1996a). The present study addresses the following 3 questions: (a) whether pcd mice perceive the conditioned and unconditioned stimuli as well as the wild-type mice, (b) whether pcd mice have a normal sensitization level, and (c) whether the residual learning in pcd mice is cerebellum-dependent. Results indicated that the pcd mice exhibited normal tone-induced responses in the cochlear nucleus and normal sensitivity to heat-induced pain. They showed a similar level of sensitization as the wild-type mice and were completely unable to learn conditioned eyeblinks after bilateral lesions aimed at the anterior interpositus nucleus. Thus, pcd mice are partially impaired in eyeblink conditioning because of a deficiency in learning mechanisms, and the residual learning in the pcd mice is mediated by the cerebellar nuclei.

Acquisition of fear conditioning in rats requires the synthesis of mRNA in the amygdala

In this study, the role of mRNA synthesis in the amygdala was studied during the acquisition of conditional fear. Rats with cannulas placed in the basolateral region of the amygdala were trained with a series of noise-shock pairings in a distinctive observation chamber. One half of the rats were pretreated with the mRNA synthesis inhibitor actinomycin-D (act-D). Responding to the training context and the auditory stimulus in a novel context measured by defensive freezing was assessed. Pretreatment with act-D significantly attenuated fear responses to both stimuli. Animals receiving act-D injections exhibited normal reactions to the conditioned stimulus-unconditioned stimulus pairings in the initial training session and displayed normal learning when retrained 7 days after injections. These results indicate that the transcription of new mRNA and subsequent protein synthesis in the amygdala may be essential for neural plasticity during this form of associative learning.

Transgenic brain-derived neurotrophic factor modulates a developing cerebellar inhibitory synapse

Brain-derived neurotrophic factor (BDNF) has been shown to promote synapse formation and maturation in neurons of many brain regions, including inhibitory synapses. In the cerebellum, the Golgi cell-granule cell GABAergic synaptic responses undergo developmental transition from slow-decaying to fast-decaying kinetics, which parallels a developmental increase of GABA(A) receptor alpha6 subunit expression in the cerebellar granule cells. In culture, BDNF accelerates the expression of GABA(A) receptor alpha6 subunit expression in granule cells. Here we examined synaptic GABA(A) response kinetics in BDNF transgenic mice. The mutant mouse, which carries a BDNF transgene driven by a beta-actin promoter, overexpresses BDNF (two- to fivefold increase compared with wild types) in all brain regions. Recordings of the spontaneous GABA(A) responses indicate that the decay time constant of the GABAergic responses decreases during early postnatal development; this transition is accelerated in the BDNF transgenic mouse. The amplitude of the spontaneous GABA(A) responses was also larger in the transgenic mouse than in the wild-type mouse. However, the frequency of the spontaneous GABA(A) responses were not different between the two groups. Our results suggest that BDNF may modulate GABAergic synapse maturation in the cerebellum.

Transgenic Brain-Derived Neurotrophic Factor Modulates a Developing Cerebellar Inhibitory Synapse

Brain-derived neurotrophic factor (BDNF) has been shown to promote synapse formation and maturation in neurons of many brain regions, including inhibitory synapses. In the cerebellum, the Golgi cell-granule cell GABAergic synaptic responses undergo developmental transition from slow-decaying to fast-decaying kinetics, which parallels a developmental increase of GABAAreceptor α6 subunit expression in the cerebellar granule cells. In culture, BDNF accelerates the expression of GABAA receptor α6 subunit expression in granule cells. Here we examined synaptic GABAA response kinetics in BDNF transgenic mice. The mutant mouse, which carries a BDNF transgene driven by a β-actin promoter, overexpresses BDNF (two- to fivefold increase compared with wild types) in all brain regions. Recordings of the spontaneous GABAA responses indicate that the decay time constant of the GABAergic responses decreases during early postnatal development; this transition is accelerated in the BDNF transgenic mouse. The amplitude of the spontaneous GABAA responses was also larger in the transgenic mouse than in the wild-type mouse. However, the frequency of the spontaneous GABAA responses were not different between the two groups. Our results suggest that BDNF may modulate GABAergic synapse maturation in the cerebellum.

Acquisition of fear conditioning in rats requires the synthesis of mRNA in the amygdala

In this study, the role of mRNA synthesis in the amygdala was studied during the acquisition of conditional fear. Rats with cannulas placed in the basolateral region of the amygdala were trained with a series of noise-shock pairings in a distinctive observation chamber. One half of the rats were pretreated with the mRNA synthesis inhibitor actinomycin-D (act-D). Responding to the training context and the auditory stimulus in a novel context measured by defensive freezing was assessed. Pretreatment with act-D significantly attenuated fear responses to both stimuli. Animals receiving act-D injections exhibited normal reactions to the conditioned stimulus-unconditioned stimulus pairings in the initial training session and displayed normal learning when retrained 7 days after injections. These results indicate that the transcription of new mRNA and subsequent protein synthesis in the amygdala may be essential for neural plasticity during this form of associative learning.

Effects of Paired and Unpaired Eye-Blink Conditioning on Purkinje Cell Morphology

This experiment addressed (1) the importance of conjunctive stimulus presentation for morphological plasticity of cerebellar Purkinje cells and inhibitory interneurons and (2) whether plasticity is restricted to the spiny branches of Purkinje cells, which receive parallel fiber input. These issues were investigated in naive rabbits and in rabbits that received paired or unpaired presentations of the conditioned stimulus (CS) and unconditioned stimulus (US). To direct CS input to the cerebellar cortex, pontine stimulation served as the CS. Air puffs to the cornea served as the US. Paired condition rabbits received pontine stimulation for 350 msec paired with a coterminating 100-msec air puff. Unpaired condition rabbits received the same stimuli in a pseudorandom order at 1- to 32-sec intervals. Rabbits were trained for a mean of 12 days. Naive rabbits received no treatment. In Golgi-stained Purkinje neurons in lobule HVI, total dendritic length, main branch length, total spiny branch length, and number of spiny branch arbors were all greater in the naive group than in the paired and unpaired groups, which did not differ. No differences were found between the hemispheres ipsilateral and contralateral to the trained eye. The dendritic length and number of branches for inhibitory interneurons did not differ across groups. The Purkinje cell morphological changes detected with these methods do not appear to be uniquely related to the conjunctive activation of the CS and US in the paired condition.

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Effects of paired and unpaired eye-blink conditioning on Purkinje cell morphology

This experiment addressed (1) the importance of conjunctive stimulus presentation for morphological plasticity of cerebellar Purkinje cells and inhibitory interneurons and (2) whether plasticity is restricted to the spiny branches of Purkinje cells, which receive parallel fiber input. These issues were investigated in naive rabbits and in rabbits that received paired or unpaired presentations of the conditioned stimulus (CS) and unconditioned stimulus (US). To direct CS input to the cerebellar cortex, pontine stimulation served as the CS. Air puffs to the cornea served as the US. Paired condition rabbits received pontine stimulation for 350 msec paired with a coterminating 100-msec air puff. Unpaired condition rabbits received the same stimuli in a pseudorandom order at 1- to 32-sec intervals. Rabbits were trained for a mean of 12 days. Naive rabbits received no treatment. In Golgi-stained Purkinje neurons in lobule HVI, total dendritic length, main branch length, total spiny branch length, and number of spiny branch arbors were all greater in the naive group than in the paired and unpaired groups, which did not differ. No differences were found between the hemispheres ipsilateral and contralateral to the trained eye. The dendritic length and number of branches for inhibitory interneurons did not differ across groups. The Purkinje cell morphological changes detected with these methods do not appear to be uniquely related to the conjunctive activation of the CS and US in the paired condition.

Essential neuronal pathways for reflex and conditioned response initiation in an intracerebellar stimulation paradigm and the impact of unconditioned stimulus preexposure on learning rate

It has been demonstrated previously that pairing of tone CS and intracerebellar stimulation of lobule HVI white matter as the US produces conditioning that is robust and in many ways similar to that obtained with an airpuff US. The first study in this report addressed the effect of interpositus lesions on conditioned performance in rabbits trained with white matter stimulation as the US. It was found that interpositus lesions effectively eliminated the CR irrespective of the behavioral response measured. In addition, it was shown that the interpositus lesions also abolished the UR, providing strong evidence that the effects of the electrical stimulation were confined to the cerebellum and did not require the activation of brainstem structures. The second experiment examined performance on US-alone trials of varying durations. Response initiation within 100 ms of the US onset, regardless of US duration, indicated that reflex generation could not be due to rebound excitation of the interpositus following termination of Purkinje cell inhibition of that structure but instead likely reflects orthodromic activation of interpositus neurons via climbing fiber and/or mossy fiber collaterals. The impact of US preexposure on associative conditioning in this paradigm was also determined. Animals which received only 108 US-alone trials were massively impaired during subsequent training compared to rabbits that received fewer than 12 US-alone trials.

Bilateral lesions of the interpositus nucleus completely prevent eyeblink conditioning in Purkinje cell-degeneration mutant mice

The authors have previously demonstrated that Purkinje cell-degeneration (pcd) mutant mice are impaired in eyeblink conditioning (L. Chen et al., 1996a). The present study addresses the following 3 questions: (a) whether pcd mice perceive the conditioned and unconditioned stimuli as well as the wild-type mice, (b) whether pcd mice have a normal sensitization level, and (c) whether the residual learning in pcd mice is cerebellum-dependent. Results indicated that the pcd mice exhibited normal tone-induced responses in the cochlear nucleus and normal sensitivity to heat-induced pain. They showed a similar level of sensitization as the wild-type mice and were completely unable to learn conditioned eyeblinks after bilateral lesions aimed at the anterior interpositus nucleus. Thus, pcd mice are partially impaired in eyeblink conditioning because of a deficiency in learning mechanisms, and the residual learning in the pcd mice is mediated by the cerebellar nuclei.

17beta-estradiol enhances NMDA receptor-mediated EPSPs and long-term potentiation

Gonadal steroid hormones influence CNS functioning through a variety of different mechanisms. To test the hypothesis that estrogen modulates synaptic plasticity in the hippocampus, in vitro hippocampal slices from 2-mo-old Sprague-Dawley male rats were used to determine the effect of 17beta-estradiol on both N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic potentials (EPSPs) through intracellular recordings and long-term potentiation (LTP) through extracellular recordings. Intracellular EPSPs and extracellular field EPSPs (fEPSPs) were recorded from CA1 pyramidal cells by stimulating Schaffer collateral fibers. In intracellular experiments, slices were perfused with medium containing bicuculline (5 microM) and low Mg2+ (0.1 mM) to enhance the NMDA receptor-mediated currents and 6, 7-dinitroquinoxaline-2,3-dione (DNQX) (10 microM) to block the alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprianate (AMPA) receptor-mediated component. The effects of 17beta-estradiol on NMDA receptor-mediated activity were excitatory; concentrations >10 nM induced seizure activity, and lower concentrations (1 nM) markedly increased the amplitude of NMDA-mediated EPSPs (both the first and second responses increased during paired pulse stimulation by 180 and 197%, respectively). In extracellular experiments, slices perfused with 17beta-estradiol (100 pM) exhibited a pronounced, persisting, and significant enhancement of LTP of both the fEPSP slope (192%) and fEPSP amplitude (177%) compared with control slices (fEPSP slope = 155%; fEPSP amplitude = 156%) 30 min after high-frequency stimulation. These data demonstrate that estrogen enhances NMDA receptor-mediated currents and promotes an enhancement of LTP magnitude.

Classical eyeblink conditioning in two strains of mice: conditioned responses, sensitization, and spontaneous eyeblinks

Conditioned eyeblink responses (CRs), sensitization, and spontaneous eyeblinks were studied in C57BL/6J and BALB/c mice. Both strains of mice acquired CRs during 10 days of classical delay eyeblink conditioning. The BALB/c mice reached a higher asymptotic CR level than the C57BL/6J mice. The CRs were extinguished and recovered in both strains following conditioned stimulus-alone and paired conditioned stimulus-unconditioned stimulus training. During 10 days of explicitly unpaired training, the control groups showed no signs of sensitization and low incidence of spontaneous eyeblinks. When switched to paired training, the unpaired groups exhibited significant conditioned inhibition. These results suggest that strain differences must be considered in experimental design and data interpretation for these basic aspects of associative learning and memory.

Cerebellar brain-derived neurotrophic factor-TrkB defect associated with impairment of eyeblink conditioning in Stargazer mutant mice

In the spontaneous ataxic mutant mouse stargazer, there is a selective reduction of brain-derived neurotrophic factor (BDNF) mRNA expression in the cerebellum. BDNF protein levels in the cerebellum are reduced by 70%. Despite normal levels of full-length and truncated TrkB receptor, constitutive and neurotrophin-4/5-induced tyrosine phosphorylation was significantly reduced in several signal transduction molecules, including phospholipase-Cgamma1, erk1, and erk2. Morphological examination revealed an increased number of external granule cells at postnatal day 15 and the presence of abnormal neurons resembling immature granule cells in the adult. These abnormalities are associated with a severe impairment in the acquisition of classical eyeblink conditioning, indicating cerebellar malfunction. Our data suggest that normal BDNF expression and TrkB signal transduction in the cerebellum are necessary for learning and plasticity in this model.

Impaired eye-blink conditioning in waggler, a mutant mouse with cerebellar BDNF deficiency

In addition to their trophic functions, neurotrophins are also implicated in synaptic modulation and learning and memory. Although gene knockout techniques have been used widely in studying the roles of neurotrophins at molecular and cellular levels, behavioral studies using neurotrophin knockouts are limited by the early-onset lethality and various sensory deficits associated with the gene knockout mice. In the present study, we found that in a spontaneous mutant mouse, waggler, the expression of brain-derived neurotrophic factor (BDNF) was selectively absent in the cerebellar granule cells. The cytoarchitecture of the waggler cerebellum appeared to be normal at the light microscope level. The mutant mice exhibited no sensory deficits to auditory stimuli or heat-induced pain. However, they were massively impaired in classic eye-blink conditioning. These results suggest that BDNF may have a role in normal cerebellar neuronal function, which, in turn, is essential for classic eye-blink conditioning.

Impaired Eye-Blink Conditioning in waggler, a Mutant Mouse With Cerebellar BDNF Deficiency

In addition to their trophic functions, neurotrophins are also implicated in synaptic modulation and learning and memory. Although gene knockout techniques have been used widely in studying the roles of neurotrophins at molecular and cellular levels, behavioral studies using neurotrophin knockouts are limited by the early-onset lethality and various sensory deficits associated with the gene knockout mice. In the present study, we found that in a spontaneous mutant mouse, waggler, the expression of brain-derived neurotrophic factor (BDNF) was selectively absent in the cerebellar granule cells. The cytoarchitecture of the wagglercerebellum appeared to be normal at the light microscope level. The mutant mice exhibited no sensory deficits to auditory stimuli or heat-induced pain. However, they were massively impaired in classic eye-blink conditioning. These results suggest that BDNF may have a role in normal cerebellar neuronal function, which, in turn, is essential for classic eye-blink conditioning.

Selective changes in AMPA receptors in rabbit cerebellum following classical conditioning of the eyelid-nictitating membrane response

alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors are critically involved in several forms of synaptic plasticity proposed to be neural substrates for learning and memory, e.g., long-term potentiation and long-term depression (LTD). The present study was designed to determine changes in cerebellar AMPA receptors following classical conditioning of the eyeblink-nictitating membrane response (NMR) in the rabbit. Quantitative autoradiography was used to assess changes in ligand binding properties of cerebellar AMPA receptors following NMR conditioning elicited by pairing electrical stimulation of the pontine nuclei with an airpuff to the eye. [3H]AMPA and [3H]-6-cyano-7-nitroquinoxaline-2,3-dion (CNQX) binding were determined following preincubation of frozen-thawed brain tissue sections at 0 or 35 degreesC. With 0 degreesC preincubation, no significant differences in [3H]AMPA binding to cerebellar AMPA receptors were seen between any of the experimental groups tested. In contrast, preincubation at 35 degreesC revealed significant decreases in [3H]AMPA binding to the trained side of the cerebellar cortex resulting from paired presentations of the conditioned and the unconditioned stimuli, while unpaired presentations of the stimuli resulted in no significant effect. With 35 degreesC preincubation, there were no significant differences in [3H]CNQX binding between any of the experimental groups and no significant differences in [3H]AMPA binding in the untrained side of the cerebellum. These results indicate that NMR conditioning is associated with a selective modification of AMPA-receptor properties in brain structures involved in the storage of the associative memory. Furthermore, they support the hypothesis that cerebellar LTD, resulting from decreased synaptic efficacy at parallel fiber-Purkinje cell synapses mediated by a change in AMPA-receptor properties, is a form of synaptic plasticity that supports this type of learning.

Evidence of plasticity in the pontocerebellar conditioned stimulus pathway during classical conditioning of the eyeblink response in the rabbit

Electrical stimulation thresholds required to elicit eyeblinks with either pontine or cerebellar interpositus stimulation were measured before and after classical eyeblink conditioning with paired pontine stimulation (conditioned stimulus, CS) and corneal airpuff (unconditioned stimulus, US). Pontine stimulation thresholds dropped dramatically after training and returned to baseline levels following extinction, whereas interpositus thresholds and input-output functions remained stable across training sessions. Learning rate, magnitude of threshold change, and electrode placements were correlated. Pontine projection patterns to the cerebellum were confirmed with retrograde labeling techniques. These results add to the body of literature suggesting that the pons relays CS information to the cerebellum and provide further evidence of synaptic plasticity in the cerebellar network.

AMPA receptor properties in adult rat hippocampus following environmental enrichment

In adult rats, environmental enrichment has been shown to selectively increase -AMPA binding in the hippocampus but the molecular mechanisms underlying this effect remain unknown. We used in situ hybridization with antisense oligonucleotides to determine possible changes in the hippocampal expression of messenger RNAs for different subunits of AMPA receptors in adult rats following exposure to an enriched environment. Quantitative analysis revealed that mRNA levels for three subtypes of AMPA glutamate receptors (GluR1-3; Flip and Flop variants) were not modified in any hippocampal region after environmental enrichment. In addition, no differences were detected in the levels of GluR1 and GluR2/3 proteins in Western blots of hippocampal membranes from enriched rats. Nevertheless, quantitative ligand binding autoradiography indicated that environmental enrichment evoked a significant and uniform decrease in the capacity of calcium or phosphatidylserine (PS) to up-regulate -AMPA binding in various hippocampal regions but not in the cerebral cortex. These findings support previous observations suggesting that post-translational changes in AMPA receptor properties, as a result of the activation of calcium-dependent processes, may represent an important mechanism underlying long-term modifications of synaptic efficacy in the rat hippocampus.

The nature of reinforcement in cerebellar learning

In a now classic study, W. J. Brogden and W. H. Gantt (1942, American Journal of Physiology, 119, 277-278) demonstrated that movements (limbs, head, eyelid) elicited by direct electrical stimulation of certain regions of the cerebellum (particularly the ansiform lobe) could be trained to respond to neutral auditory or visual conditioned stimuli with appropriate pairing. In recent work we have replicated these results in detail and presented considerable evidence that the reinforcing or "teaching" pathway so activated for the learning of discrete movements is the inferior olive-climbing fiber projection system to the cerebellum. Very strong evidence now indicates that the memory traces for this "skilled response" learning are formed and stored in the cerebellum. The climbing fiber system and inhibitory pathway from the interpositus nucleus to the inferior olive appear to form a neural instantiation of the Resorla-Wagner formulation of classical conditioning and accounts for the "cognitive" phenomenon of blocking. It is concluded that reinforcement in this form of learning is not due simply to contiguity/contingency or to unconditioned stimulus aversiveness, per se, but rather to activation of a particular brain circuit, here the climbing fiber system, a circumstance that may apply to other forms of learning, with other reinforcement circuits, as well.

Classical eyeblink conditioning in two strains of mice: conditioned responses, sensitization, and spontaneous eyeblinks

Conditioned eyeblink responses (CRs), sensitization, and spontaneous eyeblinks were studied in C57BL/6J and BALB/c mice. Both strains of mice acquired CRs during 10 days of classical delay eyeblink conditioning. The BALB/c mice reached a higher asymptotic CR level than the C57BL/6J mice. The CRs were extinguished and recovered in both strains following conditioned stimulus-alone and paired conditioned stimulus-unconditioned stimulus training. During 10 days of explicitly unpaired training, the control groups showed no signs of sensitization and low incidence of spontaneous eyeblinks. When switched to paired training, the unpaired groups exhibited significant conditioned inhibition. These results suggest that strain differences must be considered in experimental design and data interpretation for these basic aspects of associative learning and memory.

Feedback and self-efficacy, arousal, and performance of introverts and extraverts

Research has suggested that introverts and extraverts differ in their responses to performance feedback. The present study examined the effect on introverts and extraverts of a short-term memory task. Subjects (ns = 8) were randomly assigned to one of three feedback conditions: positive, negative, or control. On posttest, introverts performed better than extraverts regardless of feedback condition. These findings suggest that individual differences in introversion-extraversion mediate differences in performance through subjective arousal, namely, state-anxiety.

Evidence of plasticity in the pontocerebellar conditioned stimulus pathway during classical conditioning of the eyeblink response in the rabbit

Electrical stimulation thresholds required to elicit eyeblinks with either pontine or cerebellar interpositus stimulation were measured before and after classical eyeblink conditioning with paired pontine stimulation (conditioned stimulus, CS) and corneal airpuff (unconditioned stimulus, US). Pontine stimulation thresholds dropped dramatically after training and returned to baseline levels following extinction, whereas interpositus thresholds and input-output functions remained stable across training sessions. Learning rate, magnitude of threshold change, and electrode placements were correlated. Pontine projection patterns to the cerebellum were confirmed with retrograde labeling techniques. These results add to the body of literature suggesting that the pons relays CS information to the cerebellum and provide further evidence of synaptic plasticity in the cerebellar network.

Importance of the intracellular domain of NR2 subunits for NMDA receptor function in vivo

NMDA receptors, a class of glutamate-gated cation channels with high Ca2+ conductance, mediate fast transmission and plasticity of central excitatory synapses. We show here that gene-targeted mice expressing NMDA receptors without the large intracellular C-terminal domain of any one of three NR2 subunits phenotypically resemble mice made deficient in that particular subunit. Mice expressing the NR2B subunit in a C-terminally truncated form (NR2B(deltaC/deltaC) mice) die perinatally. NR2A(deltaC/deltaC) mice are viable but exhibit impaired synaptic plasticity and contextual memory. These and NR2C(deltaC/deltaC) mice display deficits in motor coordination. C-terminal truncation of NR2 subunits does not interfere with the formation of gateable receptor channels that can be synaptically activated. Thus, the phenotypes of our mutants appear to reflect defective intracellular signaling.

Inhibitory cerebello-olivary projections and blocking effect in classical conditioning

The behavioral phenomenon of blocking indicates that the informational relationship between the conditioned stimulus and the unconditioned stimulus is essential in classical conditioning. The eyeblink conditioning paradigm is used to describe a neural mechanism that mediates blocking. Disrupting inhibition of the inferior olive, a structure that conveys unconditioned stimulus information (airpuff) to the cerebellum prevented blocking in rabbits. Recordings of cerebellar neuronal activity show that the inferior olive input to the cerebellum becomes suppressed as learning occurs. These results suggest that the inferior olive becomes functionally inhibited by the cerebellum during conditioning, and that this negative feedback process might be the neural mechanism mediating blocking.

Exposure to a conditioned aversive environment interferes with long-term potentiation induction in the fimbria-CA3 pathway

The effect of re-exposure of rats to an aversive environment on the induction of long-term potentiation was investigated in the CA3 region 3 and 12 h after contextual conditioning. Electro-physiological recordings showed that re-exposure of rats to the conditioning chamber produced a significant and long-lasting decrease in population spike amplitude at both post-conditioning delays. High-frequency stimulation of the fimbria induced a large and persistent increase in CA3 population spike amplitude (about 400% of baseline) in animals of control groups and shocked animals that were not re-exposed to the conditioning environment. However, high-frequency stimulation applied during re-exposure of shocked subjects 3 h after the initial exposure resulted in a small and transient increase in population spike amplitude (about 140% of baseline); when applied 12 h after the initial exposure, it produced a persistent depression of the response (-30% of baseline). Behavioural testing indicated that re-exposure of shocked animals to the conditioning environment elicited a qualitatively and quantitatively similar freezing behaviour at both post-conditioning delays (3 or 12 h). In contrast to the long-lasting decrease in CA3 population spike amplitude produced by re-exposure to the aversive environment, the level of freezing behaviour diminished rapidly within 10 min of exposure. These results suggest that, during exposure to a conditioned aversive environment, alterations in fimbria-CA3 neural processing may be dissociated from contextual fear-induced freezing behaviour. In addition, processes underlying long-term potentiation induction in fimbria-CA3 pathway may be opposite to those taking place during hippocampal processing of conditioned aversive contexts.

Impaired motor coordination and persistent multiple climbing fiber innervation of cerebellar Purkinje cells in mice lacking Galphaq

Mice lacking the alpha-subunit of the heterotrimeric guanine nucleotide binding protein Gq (Galphaq) are viable but suffer from ataxia with typical signs of motor discoordination. The anatomy of the cerebellum is not overtly disturbed, and excitatory synaptic transmission from parallel fibers to cerebellar Purkinje cells (PCs) and from climbing fibers (CFs) to PCs is functional. However, about 40% of adult Galphaq mutant PCs remain multiply innervated by CFs because of a defect in regression of supernumerary CFs in the third postnatal week. Evidence is provided suggesting that Galphaq is part of a signaling pathway that is involved in the elimination of multiple CF innervation during this period.

Motor cortex lesions do not affect learning or performance of the eyeblink response in rabbits

The possible modulatory role of motor cortex in classical conditioning of the eyeblink response was examined by ablating anterior neocortex in rabbits and training them with an auditory conditioned stimulus (CS) and an airpuff unconditioned stimulus (US) in either a delay (Experiment 1) or a trace (Experiment 2) conditioning paradigm. Topographic measures such as amplitude and onset latency were assessed during conditioning sessions for conditioned responses (CRs) and on separate test days for unconditioned responses (URs) by using a range of US intensities. No lesion effects were observed for learning or performance measures in acquisition or retention of either delay or trace conditioning. During trace conditioning, lesioned rabbits did, however, exhibit a trend toward impairment and demonstrated significantly longer CR latencies. Damage to motor and frontal cortex does not significantly affect eyeblink response performance or learning in either a delay or a trace conditioning paradigm.

Associative learning

This chapter reviews evidence demonstrating the essential role of the cerebellum and its associated circuitry in the learning and memory of classical conditioning of discrete behavioral responses (e.g., eyeblink, limb flexion, head turn). It now seems conclusive that the memory traces for this basic category of associative learning are formed and stored in the cerebellum. Lesion, neuronal recording, electrical microstimulation, and anatomical procedures have been used to identify the essential conditioned stimulus (CS) circuit, including the pontine mossy fiber projections to the cerebellum; the essential unconditioned stimulus (US) reinforcing or teaching circuit, including neurons in the inferior olive (dorsal accessory olive) projecting to the cerebellum as climbing fibers; and the essential conditioned response (CR) circuit, including the interpositus nucleus, its projection via the superior cerebellar peduncle to the magnocellular red nucleus, and rubral projections to premotor and motor nuclei. Each major component of the eyeblink CR circuit was reversibly inactivated both in trained animals and over the course of training. In all cases in trained animals, inactivation abolished the CR (and the UR as well when motor nuclei were inactivated). When animals were trained during inactivation (and not exhibiting CRs) and then tested without inactivation, animals with inactivation of the motor nuclei, red nucleus, and superior peduncle had fully learned, whereas animals with inactivation of a very localized region of the cerebellum (anterior interpositus and overlying cortex) had not learned at all. Consequently, the memory traces are formed and stored in the cerebellum. Several alternative possibilities are considered and ruled out. Both the cerebellar cortex and the interpositus nucleus are involved in the memory storage process, suggesting that a phenomenon-like long-term depression (LTD) is involved in the cerebellar cortex and long-term potentiation (LTP) is involved in the interpositus. The experimental findings reviewed in this chapter provide perhaps the first conclusive evidence for the localization of a basic form of memory storage to a particular brain region, namely the cerebellum, and indicate that the cerebellum is indeed a cognitive machine.

Time-dependent blockade of STP and LTP in hippocampal slices following acute stress in mice

The characteristics of short-term potentiation (STP) and long-term potentiation (LTP) in the CA1 region of hippocampal slices were determined at various times following exposure to acute stress produced by restraint and tail-shock in mice. In slices prepared from control animals, theta-burst stimulation resulted in a large increase in evoked field excitatory postsynaptic potentials (EPSPs) amplitude and slope that remained stable at least up to 30 min after stimulation. Slices prepared 1 h after stress exhibited a marked decrease in the extent of both STP and LTP. STP and LTP magnitude were still significantly decreased 24 h after stress exposure and were completely restored to control levels by 48 h. These results provide evidence for a reversible impairment of STP and LTP in CA1 following an acute episode of stress, and suggest that stress activates processes different from those activated by LTP-inducing stimuli.

The conditioned eyeblink response: a role for the GABA-B receptor?

In well-trained animals, infusion of the GABA-B agonist baclofen into the cerebellar interpositus nucleus and overlying cortex abolished the conditioned response (CR) with no effect on the unconditioned response (UR) with doses at or above 5.0 mM. Infusion of the GABA-B antagonist CGP 5584-5A alone had no effect on the CR or UR. However, administration of 5 mM baclofen soon after infusion of CGP 5584-5A (15 min) resulted in no reduction of percent CR and only partial reduction of CR amplitude. Naive animals given interpositus infusions of baclofen during training showed no learning, yet learned normally in postinfusion training. The distribution of (radiolabelled) baclofen was localized and remained within the cerebellum. The results presented here are consistent with a growing body of literature supporting the hypothesis that the memory trace for eyeblink conditioning is formed and stored in the cerebellum and may involve GABAergic mechanisms.

Motor cortex lesions do not affect learning or performance of the eyeblink response in rabbits

The possible modulatory role of motor cortex in classical conditioning of the eyeblink response was examined by ablating anterior neocortex in rabbits and training them with an auditory conditioned stimulus (CS) and an airpuff unconditioned stimulus (US) in either a delay (Experiment 1) or a trace (Experiment 2) conditioning paradigm. Topographic measures such as amplitude and onset latency were assessed during conditioning sessions for conditioned responses (CRs) and on separate test days for unconditioned responses (URs) by using a range of US intensities. No lesion effects were observed for learning or performance measures in acquisition or retention of either delay or trace conditioning. During trace conditioning, lesioned rabbits did, however, exhibit a trend toward impairment and demonstrated significantly longer CR latencies. Damage to motor and frontal cortex does not significantly affect eyeblink response performance or learning in either a delay or a trace conditioning paradigm.