Role of granule-cell transmission in memory trace of cerebellum-dependent optokinetic motor learning

Adaptation of the optokinetic response (OKR) is an eye movement enhanced by repeated motion of a surrounding visual field and represents a prototype of cerebellum-dependent motor learning. Purkinje cells and vestibular nuclei (VN) receive optokinetic and retinal slip signals via the mossy fiber-granule cell pathway and climbing-fiber projections, respectively. To explore the neural circuits and mechanisms responsible for OKR adaptation, we adopted the reversible neurotransmission-blocking (RNB) technique, in which granule-cell transmission to Purkinje cells was selectively and reversibly blocked by doxycycline-dependent expression of transmission-blocking tetanus toxin in granule cells. Blockade of granule-cell inputs abolished both short-term and long-term OKR adaptation induced by repeated OKR training, but normal levels of both responses were immediately evoked in the pretrained RNB mice by OKR retraining once granule-cell transmission had recovered. Importantly, eye movement elicited by electrical stimulation of the cerebellar focculus was elevated by long-term but not by short-term OKR training in adaptive OKR-negative RNB mice. Furthermore, when the flocculus of adaptive OKR-negative RNB mice was electrically excited in-phase with OKR stimulation, these mice exhibited long-term adaptive OKR. These results indicate that convergent information to the VN was critical for acquisition and storage of long-term OKR adaptation with conjunctive action of Purkinje cells for OKR expression. Interestingly, in contrast to conditioned eyeblink memory, the expression of once acquired adaptive long-term OKR was not abrogated by blockade of granule-cell transmission, suggesting that distinct forms of neural plasticity would operate in different forms of cerebellum-dependent motor learning.The cerebellum is the critical site for motor learning, and adaptation of the optokinetic response (OKR) is a prototype of cerebellum-dependent motor learning (1–4). Continuous oscillatory rotation of a screen for a few hours strengthens the gain of the OKR, which goes back to basal levels in the light after training (short-term OKR) (2, 3). Repeated training by screen rotation for a few days progressively increases the OKR and long-lastingly maintains the enhanced OKR (long-term OKR), when animals are kept in the dark after training (2, 3, 5). The basic eye movement (basic OKR) observed in untrained naïve animals is mediated by a closed-loop circuit consisting of nucleus reticularis tegmenti pontis (NRTP), vestibular nuclei (VN), and oculomotor nuclei (OMN) (6, 7) (Fig. S1). In OKR adaptation, optokinetic signals are transmitted to the VN directly through mossy fibers derived from NRTP and indirectly to Purkinje cells via the mossy fiber-granule cell pathway whereas retinal slip signals are transmitted to Purkinje cells via the nuclei of the accessory optic system (AOSn)-inferior olive-climbing fiber pathway (6, 7) ( Fig. S1). A number of studies using lesion analysis (3, 8) and electrophysiological (9–11), pharmacological (3, 12, 13), and gene knock-out techniques (5, 14–16) demonstrated that the circuits of both Purkinje cells and VN are indispensable for memory trace of the adaptive OKR. However, OKR adaptation involves multiple learning processes: at least acquisition, expression, and storage of motor learning. The neural circuits and underlining mechanisms involved in OKR adaptation still largely remain to be elucidated.To address this issue, we adopted a gene-manipulating technique termed reversible neurotransmission-blocking (RNB) (17, 18). In the RNB technique, tetanus toxin is restrictedly expressed in granule cells under the control of a tetracycline-controlled reverse transactivator (rtTA). Tetanus toxin specifically cleaves synaptic vesicle VAMP2 (19), resulting in blockade of transmitter release from synaptic vesicles (19) (Fig. S1). In RNB mice, granule-cell inputs to Purkinje cells can be thus selectively blocked and reversibly recovered by administration and omission, respectively, of doxycycline (DOX). As a consequence, when granule-cell transmission is blocked, the optokinetic signal is not transmitted to Purkinje cells but is still conveyed to the VN via the direct mossy-fiber pathway. This reversible blocking technique thus allows us to delineate distinct roles of Purkinje cells and VN in the memory trace of OKR learning. The present study demonstrates that VN are critical for acquisition and storage of long-term OKR memory in intimate association with the Purkinje-cell circuit for expression of OKR adaptation.     

Modulation of function and gated learning in a network memory.

Memory and learning are studied in a model neural network made from component cells with a variety of realistic intrinsic dynamic behaviors. Modulation of intrinsic cellular characteristics causes a network to switch between two entirely different modes of operation. In one mode the network acts as a selective, long-term associative memory, whereas in the other it is a nonselective, short-term latching memory. Such functional modulation can be used as a mechanism for initiating and terminating learning in a network associative memory.     

Partial learning and recognition memory in the aged.

A recognition-memory paradigm was used to test two hypotheses, storage and retrieval, which account for the adult age decrement seen in recall. Partial storage was minimized by using items in the recognition list which were similar to the to-be-remembered items. Recognition performance was unaffected by adult age differences, thereby supporting the retrieval hypothesis. However, older persons made a greater number of semantic errors in the recognition test list supporting the storage hypothesis. While the error difference did not affect overall recognition performance, the result does indicate some caution is necessary in interpreting age-recognition interactions.     

Brain localization for arbitrary stimulus categories: a simple account based on Hebbian learning.

A central theme of cognitive neuroscience is that different parts of the brain perform different functions. Recent evidence from neuropsychology suggests that even the processing of arbitrary stimulus categories that are defined solely by cultural conventions (e.g., letters versus digits) can become spatially segregated in the cerebral cortex. How could the processing of stimulus categories that are not innate and that have no inherent structural differences become segregated? We propose that the temporal clustering of stimuli from a given category interacts with Hebbian learning to lead to functional localization. Neural network simulations bear out this hypothesis.     

A mechanism for the Hebb and the anti-Hebb processes underlying learning and memory.

In a previous paper, a model was presented showing how the group of Ca2+/calmodulin-dependent protein kinase II molecules contained within a postsynaptic density could stably store a graded synaptic weight. This paper completes the model by showing how bidirectional control of synaptic weight could be achieved. It is proposed that the quantitative level of the activity-dependent rise in postsynaptic Ca2+ determines whether the synaptic weight will increase or decrease. It is further proposed that reduction of synaptic weight is governed by protein phosphatase 1, an enzyme indirectly controlled by Ca2+ through reactions involving phosphatase inhibitor 1, cAMP-dependent protein kinase, calcineurin, and adenylate cyclase. Modeling of this biochemical system shows that it can function as an analog computer that can store a synaptic weight and modify it in accord with the Hebb and anti-Hebb learning rules.     

Age differences in learning and memory on a digit-symbol substitution task.

Fifteen young (18 to 28 years) and fifteen old (65 to 75 years) persons were given ten 30-second trials on a modified Digit-Symbol task. Both immediate and delayed (24-hour) recall and recognition measures were used to determine how well they remembered the digit-symbol pairs. Both age groups showed similar, significantly improvement over the 10 trials, although the performance level of the young group was higher throughout. The young group showed significantly higher recall and recognition (both immediate and delayed) for the digit-symbol pairs and were more likely to report the use of mnemonic techniques in learning the pairs.     

Monkeys have a limited form of short-term memory in audition

A stimulus trace may be temporarily retained either actively [i.e., in working memory (WM)] or by the weaker mnemonic process we will call passive short-term memory, in which a given stimulus trace is highly susceptible to "overwriting" by a subsequent stimulus. It has been suggested that WM is the more robust process because it exploits long-term memory (i.e., a current stimulus activates a stored representation of that stimulus, which can then be actively maintained). Recent studies have suggested that monkeys may be unable to store acoustic signals in long-term memory, raising the possibility that they may therefore also lack auditory WM. To explore this possibility, we tested rhesus monkeys on a serial delayed match-to-sample (DMS) task using a small set of sounds presented with ~1-s interstimulus delays. Performance was accurate whenever a match or a nonmatch stimulus followed the sample directly, but it fell precipitously if a single nonmatch stimulus intervened between sample and match. The steep drop in accuracy was found to be due not to passive decay of the sample's trace, but to retroactive interference from the intervening nonmatch stimulus. This "overwriting" effect was far greater than that observed previously in serial DMS with visual stimuli. The results, which accord with the notion that WM relies on long-term memory, indicate that monkeys perform serial DMS in audition remarkably poorly and that whatever success they had on this task depended largely, if not entirely, on the retention of stimulus traces in the passive form of short-term memory.     

Ultrastructural localization of a neutral and basic amino acid transporter in rat kidney and intestine.

A sodium-independent neutral and basic amino acid transporter (NBAT) from rat kidney was recently cloned and its amino acid sequence deduced. We used light and electron microscopic immunoperoxidase labeling to determine the cellular localization of NBAT in rat kidney and small intestine. The localization was carried out using site-directed antisera raised against synthetic peptides within NBAT. The most prominent localization of NBAT was in microvilli of epithelial cells lining renal proximal tubules. Microvilli of small intestinal epithelia were less frequently immunoreactive. Unexpectedly, the most intense labeling in the small intestine was seen within enteroendocrine cells and submucosal neurons. The neuronal labeling was highly localized within dense core vesicles in axon terminals apposed to the basal lamina near fenestrated blood vessels. These results support the proposal that NBAT plays a role in reabsorption of amino acids in renal tubules. In addition, they suggest that NBAT (or NBAT-like proteins) may have multiple functions in the small intestine, including luminal uptake of amino acids and vesicular uptake of related substrates into enteroendocrine cells and enteric neurons.     

Traces of learning in the auditory localization pathway

One of the fascinating properties of the central nervous system is its ability to learn: the ability to alter its functional properties adaptively as a consequence of the interactions of an animal with the environment. The auditory localization pathway provides an opportunity to observe such adaptive changes and to study the cellular mechanisms that underlie them. The midbrain localization pathway creates a multimodal map of space that represents the nervous system's associations of auditory cues with locations in visual space. Various manipulations of auditory or visual experience, especially during early life, that change the relationship between auditory cues and locations in space lead to adaptive changes in auditory localization behavior and to corresponding changes in the functional and anatomical properties of this pathway. Traces of this early learning persist into adulthood, enabling adults to reacquire patterns of connectivity that were learned initially during the juvenile period.     

Protein kinase A inhibits a consolidated form of memory in Drosophila

Increasing activity of the cAMP/protein kinase A (PKA) pathway has often been proposed as an approach to improve memory in various organisms. However, here we demonstrate that single-point mutations, which decrease PKA activity, dramatically improve aversive olfactory memory in Drosophila. These mutations do not affect formation of early memory phases or of protein synthesis-dependent long-term memory but do cause a significant increase in a specific consolidated form of memory, anesthesia-resistant memory. Significantly, heterozygotes of null mutations in PKA are sufficient to cause this memory increase. Expressing a PKA transgene in the mushroom bodies, brain structures critical for memory formation in Drosophila, reduces memory back to wild-type levels. These results indicate that although PKA is critical for formation of several memory phases, it also functions to inhibit at least one memory phase.     

Aging impairs intermediate-term behavioral memory by disrupting the dorsal paired medial neuron memory trace

How the functional activity of the brain is altered during aging to cause age-related memory impairments is unknown. We used functional cellular imaging to monitor two different calcium-based memory traces that underlie olfactory classical conditioning in young and aged Drosophila. Functional imaging of neural activity in the processes of the dorsal paired medial (DPM) and mushroom body neurons revealed that the capacity to form an intermediate-term memory (ITM) trace in the DPM neurons after learning is lost with age, whereas the capacity to form a short-term memory trace in the α'/β' mushroom body neurons remains unaffected by age. Stimulation of the DPM neurons by activation of a temperature-sensitive cation channel between acquisition and retrieval enhanced ITM in aged but not young flies. These data indicate that the functional state of the DPM neurons is selectively altered with age to cause an age-related impairment of ITM, and demonstrate that altering the excitability of DPM neurons can restore age-related memory impairments.     

Immunohistochemical localization of basic and acidic fibroblast growth factors in the developing rat heart.

BACKGROUND. We used biochemical and immunohistochemical techniques to investigate the expression and distribution of immunoreactive basic and acidic fibroblast growth factors (bFGF and aFGF, respectively) in the hearts of rat embryos (11-20 days of gestation) and of postnatal rats (1-35 days after birth). Our purpose was to assess the relation between the cellular distribution of these growth factors and histogenetic and morphogenetic events in the developing heart. METHODS AND RESULTS. Western-blot analysis of heparin-bound material from neonatal heart extracts identified a single band with a molecular weight of approximately 18 kD for both bFGF and aFGF. Five antibodies for bFGF and three for aFGF showed superimposable distribution of immunoreactive bFGF and aFGF in the heart at each stage examined. At the cellular level, these peptides were localized in the cytoplasm and extracellular matrix. In the myocytes, immunostaining was positive throughout the embryonic and neonatal periods. In the majority of the mesenchymal cells of the cushions and endothelial cells of endocardium and vessels, staining was also positive. In the smooth muscle cells of the aorta, other large arteries, and coronary arteries, immunostaining was intensely positive at early stages of development but became faint or negative with increasing cell differentiation. CONCLUSIONS. The wide distribution of immunoreactive bFGF and aFGF that we identified in the developing rat heart suggests that these growth factors play an important role in heart cytodifferentiation and morphogenesis. Their superimposable distribution may reflect functional interaction. The progressive changes in their distribution suggest a changing role for these peptides during organogenesis.     

Initial localization of regulatory regions of the cardiac sarcolemmal Na(+)-Ca2+ exchanger.

We have analyzed the regulatory properties of the wild-type cardiac Na(+)-Ca2+ exchanger expressed in Xenopus laevis oocytes using the giant excised patch technique. The exchanger is activated by cytoplasmic application of chymotrypsin and exhibits a number of properties that can be changed or abolished by chymotrypsin treatment, including cytoplasmic Na(+)-dependent inactivation, secondary regulation by free cytoplasmic Ca2+, and inhibition by exchanger inhibitory peptide. Thus, the cloned exchanger expressed in oocytes exhibits regulatory properties similar to those of the native sarcolemmal exchanger. The exchanger protein contains a large (520 amino acids) hydrophilic domain modeled to be intracellular. The role of this region in exchanger function and regulation was examined by deletion mutagenesis. Mutants with residues 240-679 and 562-685 deleted exhibited exchange activity, indicating that this extensive region is not essential for transport function. Both mutants were stimulated by chymotrypsin treatment. Neither mutant demonstrated regulation by free cytoplasmic Ca2+ (Ca2+i) or inhibition by exchanger inhibitory peptide (XIP). However, mutant delta 562-685 but not delta 240-679 displayed Na(+)-dependent inactivation. The data suggest that the binding sites for XIP and regulatory Ca2+ may reside in the region encompassed by residues 562-685. A chimera made from renal and cardiac exchangers has normal regulatory characteristics and helps to further define these sites.