Thesis (Ph.D. in Neuroscience) -- University of Colorado at Denver and Health Sciences Center, 2006. / Typescript. Includes bibliographical references (leaves 108-118). Free to UCD Anschutz Medical Campus. Online version available via ProQuest Digital Dissertations;

Abstract The dihydropyridine receptor (DHPR) in the skeletal muscle plasma membrane functions as a voltage sensor for excitation-contraction coupling. In the present work the expression and special features of DHPR were studied under various conditions. In order to localize and visualize the DHPRs, a method using fluorophore-conjugated dihydropyridine molecules as a probe was developed. In addition, different laboratory assays and electrophysiological measurements were used to study the expression of the myofibrillar proteins, force production of the muscle and conduction velocity of the plasma membrane. During the postnatal development of mouse skeletal muscle the density of DHPR increased. By the time of DHPR appearance, the expression of sodium channels had started and the typical assembly of basic structural components and membrane compartments was clearly detectable. According to the histochemical analysis, the DHPR was selectively expressed in type IIA muscle fibres of mouse. In addition to the fibre type specificity, the uneven distribution of DHPRs was also seen at the muscle level. The attenuation of the contraction force after addition of DHPR blocker was largest in muscles with a high percentage of type IIA fibres. In fish muscles, the distribution of DHPR was homogenous and the density between different fibre types was quite similar. In gastrocnemius muscle of mouse, the density and mRNA expression of DHPR increased significantly by 21.5 and 66.8%, respectively, after a 15-week aerobic exercise programme. The increase correlated significantly with the raise in % myosin heavy chain IIa isoform. In thigh and heart muscles, no significant changes were observed. In fish, the environmental change following hatchery release and downstream migration of 46.5 km induced an increase in the DHPR density in swimming muscles. Furthermore, a transition of phenotypic profile from fast-to-slow fibres was observed. Taken together, the present data provide evidence for the fact that the expression of DHPR increases during postnatal development. Moreover, the expression correlates with a specific fibre-type metabolism, thus having an impact on the overall contractile properties of the muscle. This is further manifested as an increase in the DHPR density after endurance training in mammalian as well as in fish muscle. In addition, a strong correlation exists between the level of muscle activity and the density of DHPR. / Tiivistelmä Poikkijuovaisen lihaksen solukalvolla esiintyvä dihydropyridiini (DHP) reseptori toimii jännitesensorina lihaksen ärsytys-supistus kytkennässä. Tässä työssä tutkittiin DHP reseptorin erityispiirteitä sekä erilaisten fysiologisten tekijöiden vaikutusta reseptorin ilmenemiseen. DHP reseptorien visualisointiin kehitettiin histologinen värjäysmenetelmä, jossa merkkiaineena toimi fluorofori-konjugoitu DHP molekyyli. Lisäksi työssä tutkittiin lihaksen proteiinien ekspressiota, lihaksen voimantuottoa sekä solukalvon johtonopeutta erilaisten määritysmenetelmien ja elektrofysiologisten mittausmenetelmien avulla. Tulokset osoittivat, että yksilönkehityksen aikana DHP reseptorien tiheys kasvoi hiiren poikkijuovaisessa lihaksessa. DHP reseptorien ekspression alkaessa natriumkanavat olivat jo ilmaantuneet solukalvolle ja lihassolun rakenne oli pitkälle erilaistunut. Histokemiallisten määritysten perusteella DHP reseptorin ekspressio oli selektiivistä. Reseptoreita esiintyi runsaimmin tyypin IIA soluissa. Reseptoriproteiinin solutyyppispesifisyys oli huomattavissa myös lihastasolla. Tulosten mukaan proteiinin salpaaja alensi lihaksen supistumisvoimaa erityisesti niissä hiiren lihaksissa, joiden solutyyppikoostumuksessa tyypillä IIA on suuri prosentuaalinen osuus. Kalan lihaksissa DHP reseptorit olivat homogeenisesti jakautuneet. Lisäksi reseptoritiheys oli samankaltainen eri solutyyppien välillä. 15 viikon aerobinen harjoittelu lisäsi sekä DHP reseptorin proteiini- että mRNA- ekspressiota tilastollisesti merkitsevästi (21,5 ja 66,8 %) hiiren kantalihaksessa. Ekspression kasvu korreloi merkitsevästi samanaikaisesti tapahtuneen myosiini isomuoto IIa määrän kasvun kanssa. Reisi- ja sydänlihaksessa merkittäviä muutoksia ei havaittu. Kalan uintilihaksissa DHP reseptorien tiheys kasvoi vapauttamisen jälkeisen ympäristön muutoksen ja 46,5 km pituisen vaelluksen jälkeen. Lisäksi lihasten solutyyppikoostumuksessa tapahtui muutos kohti hitaasti supistuvia solutyyppejä. Yhteenvetona voidaan todeta, että saatujen tulosten perusteella DHP reseptorien ekspressio kasvaa syntymän jälkeen hiiren poikkijuovaisessa lihaksessa. Solujen erilaistuessa ekspressio korreloi solutyyppimetabolian kanssa vaikuttaen edelleen lihaksen supistumisominaisuuksiin. Tästä johtuen myös kestävyysharjoittelun seurauksena DHP reseptorien määrä kasvaa sekä nisäkkään että kalan lihaksissa. Erityisesti lihaksen aktiivisuudella on merkitystä DHP reseptorin ekspressioon.

No description available.

The role of T-type calcium channels (TTCCs) in the heart is unclear. TTCCs are transiently expressed throughout the neonatal heart during a period of rapid cardiac development. A few weeks postnatally, TTCCs are no longer found in ventricular myocytes (VMs) and calcium influx via TTCCs (ICa,T) is only detected in the SA node and Purkinje system. However, pathologic cardiac stress is associated with re-expression of TTCCs in VMs. Whether ICa,T in this setting promotes cardiac growth or exacerbates cardiac function is a topic of debate. The focus of this thesis work was to examine the effect of TTCC re-expression in the normal and diseased myocardium. Our experiments were performed in a transgenic mouse model with inducible, cardiac-specific expression of α1G TTCCs. While both the α1G and α1H TTCC subtypes re-appear during cardiac disease, we specifically evaluated the effects of α1G TTCCs since mRNA levels of this TTCC subtype are markedly elevated during cardiac pathology. We found that transgenic mice with α1G overexpression had robust ICa,T with biophysical properties similar to those published in previous studies. α1G mice had a small increase in cardiac function and showed no evidence of cardiac histopathology or increased mortality. These findings were in contrast to the phenotype of transgenic mice with augmented L-type calcium channel (LTCC) activity secondary to overexpression of the β2a regulatory subunit. While the magnitude of calcium influx in α1G and β2a VMs was similar, we found that cardiac contractility of β2a mice was significantly greater than α1G mice. Also, β2a mice had significant cardiac fibrosis, myocyte death, and premature lethality compared to the benign phenotype of α1G mice. We showed that the phenotypic differences are likely related to the differential spatial localization of T- and LTCCs. Whereas α1G TTCCs were principally localized to the surface sarcolemma, LTCCs were primarily found in the transverse tubules in close proximity to the sites of sarcoplasmic reticulum calcium release. We evaluated the effect of TTCC expression during cardiac disease by inducing myocardial infarction (MI) in α1G mice. Acutely (1-week post MI), α1G mice showed similar worsening of cardiac function and mortality rates compared to control post-infarct mice. However, α1G hearts had smaller infarct sizes which correlated with increased Akt and NFAT activation in α1G than control hearts. After chronic heart failure, i.e. 7- weeks post-infarction, α1G hearts had significant hypertrophic response as determined by increased HW/BW ratio, myocyte cross-sectional area, as well as NFAT and Akt activity. Finally, α1G mice had a small survival benefit than control mice, which while statistically non-significant, suggests that TTCC re-expression does not exacerbate cardiac function as hypothesized by some investigators. We conclude that TTCCs play a minimal role in cardiac function and activate pro-survival signaling pathways in the myocardium. / Physiology

Modelos matemáticos de motoneurônios têm sido desenvolvidos para auxiliar na compreensão dos fenômenos que envolvem o sistema neuromuscular. Entretanto, a maioria dos modelos já desenvolvidos baseou-se na premissa de que a árvore dendrítica tem um comportamento passivo, o que ocorre em animais anestesiados, mas pode não ocorrer durante o comportamento motor normal de um animal intacto. Experimentos com animais descerebrados, em que as vias monoaminérgicas encontravam-se ativas, mostraram que os motoneurônios podem apresentar comportamentos mais complexos decorrentes da presença de condutâncias iônicas voltagem-dependentes que se situam nos dendritos e são responsáveis pela gênese de uma corrente de entrada persistente. Nesse sentido, um primeiro objetivo deste trabalho foi o de desenvolver novos modelos matemáticos de motoneurônios de diferentes tipos (i.e. dos tipos S, FR e FF), computacionalmente eficientes e contendo em seus compartimentos dendríticos uma condutância de cálcio do tipo L, de forma que os fenômenos de biestabilidade, potencial platô e amplificação da corrente sináptica efetiva possam ser gerados. Um segundo objetivo foi o de verificar como a presença da condutância iônica ativa no dendrito influencia o comportamento motoneuronal quando o mesmo está sujeito a entradas sinápticas de diferentes tipos. Os novos modelos foram parametrizados baseando-se em dados da literatura experimental para motoneurônios de gatos descerebrados e validados segundo os protocolos experimentais básicos que permitem caracterizar cada tipo de modelo como sendo totalmente ou parcialmente biestável. As entradas sinápticas foram simuladas por processos pontuais de Poisson e os trens de potenciais de ação dos motoneurônios foram analisados. Uma modulação senoidal da intensidade do processo pontual foi usada para estimar as respostas em frequência de cada modelo. Observou-se que, funcionalmente, a presença da condutância iônica dendrítica pode favorecer a ação do motoneurônio durante tarefas posturais, pois, uma vez ativada, a corrente de entrada persistente eleva a excitabilidade motoneuronal tornando os disparos mais regulares, além de prover uma alta sensibilidade dos modelos a entradas sinápticas de baixa frequência, correspondentes às oscilações observadas durante a manutenção da postura ereta quieta. / Mathematical models of motoneurons have been developed as an aid to the understanding of phenomena involving the neuromuscular system, but most of these models have been based on the hypothesis of a passive dendritic tree. This holds for anesthetized animals but not necessarily during normal motor behavior of the intact animal. Experiments with decerebrate animals in which the monoaminergic tracts were maintained intact have shown that more complex behaviors may emerge in motoneurons due to dendritic voltage-gated ionic conductances, which are responsible for a persistent inward current. Therefore, the first aim of this work was to develop computationally-efficient new motoneuron models of different types (i.e. type S, FR and FF) that include a dendritic L-type calcium conductance so that bistability, plateau potential and enhancement of effective synaptic current may be generated. The second aim of this research was to evaluate the effects of the active dendritic ionic conductance on the input-output mapping of presynaptic to postsynaptic spike trains. The new models were parameterized based on data reported in experimental literature on the decerebrate cat preparation, and they were validated using appropriate protocols for either fully or partially bistable dynamics. The synaptic inputs were simulated by Poisson point processes and the output spike trains were analyzed. Sinusoidal modulation of the point process intensity was used for the estimation of each models frequency response. The results suggested that an active dendritic ionic conductance in motoneurons has a functional role during postural tasks, because, when activated, the persistent inward current enhances the motoneuronal excitability, reducing the variability of interspike intervals, and focusing the sensitivity of the models to low frequency inputs that correspond to the low-frequency oscillations that typically occur during quiet standing posture.

Ces dernières décennies ont mis à jour une implication pathologique nouvelle del’aldostérone, via le récepteur aux minéralocorticoïdes (RM) dans le coeur. L’ensemble desdonnées issues des études expérimentales et des essais cliniques suggère une association délétèreentre l’aldostérone et la survenue d’arythmies. L’utilisation d’antagonistes du RM prévient cesarythmies. Cependant, les voies de signalisations, comme les mécanismes moléculaires soustendantces effets bénéfiques du blocage des RM demeurent incertains. Nous avons accumulésdes preuves d’une modulation de la signalisation calcique dans le cardiomyocyte, et en particulierde l’influx calcique (Ca2+) au travers du canal Ca2+ de type L (LTCC). Celui-Ci pourrait être unecible primaire de l’aldostérone et du RM dans les cardiomyocytes ventriculaires. Toutefois, lesmécanismes par lesquels l’aldostérone et le RM régulent l’expression du LTCC restent à définir.Au cours de ces travaux menés sur cardiomyocytes de rats nouveau-Nés, nous avonsétudiés les évènements moléculaires par lesquels l’aldostérone exerce ses effets sur le CaV1.2,qui correspond à la sous-Unité principale du LTCC formant le pore du canal ; cette protéine estcodée par le CACNA1C. Par microscopie confocale, nous avons suivi en temps réel le traffickingnucléo-Cytoplasmique du RM couplé à la GFP en réponse à l’aldostérone, démontrant ainsi queles RM cardiaques sont fonctionnels. Le traitement durant 24 heures des cardiomyocytes avec del’aldostérone montre une augmentation dose-Dépendante des protéines et de l’ARN messager duCaV1.2. L’utilisation de la technique du gène rapporteur de la luciférase permet l’analyse del’activité du promoteur du CaCNA1C. Celui-Ci montre une activité transcriptionnelle dose ettemps dépendante en réponse à l’aldostérone. De plus, ces effets sont dépendant des RM carinhibés en présence de RU28318, un antagoniste sélectif du RM, ou par l’utilisation de siRNAdirigés contre le RM. L’analyse in silico de la séquence du promoteur du CaCNA1C nous a permisd’identifier cinq séquences putatives correspondant à des éléments de réponse auxglucocorticoïdes (GRE). La mutation du site le plus lointain du site d’initiation de la transcriptionne révèle aucun changement dans les réponses transcriptionnelles induites par un RM humainconstitutivement actif (hMRΔ5,6) ou dans les réponses doses-Dépendantes de l’aldostérone ou dela déxaméthasone, un glucocorticoïde de synthèse. La mutation des trois sites GRE putatifssuivants provoque une diminution des réponses au hMRΔ5,6 comme à l’aldostérone, alors que lesréponses à la déxaméthasone sont soit inchangées, soit augmentées. En contraste, la mutation dusite le plus proximal du promoteur augmente de façon importante l’activité transcriptionnelle dupromoteur en réponse au hMRΔ5,6, à l’aldostérone comme à la déxaméthasone.Ces résultats démontrent que le LTCC cardiaque constitue une cible directe del’aldostérone et du RM, et apportent de nouvelles perspectives quant aux conséquencesmoléculaires et fonctionnelles engendrées par l’activation délétère du système minéralocorticoïdedans la défaillance cardiaque. / During the past decades, major novel pathogenic roles of the steroid hormone,aldosterone, via the Mineralocorticoid Receptor (MR) have been identified in heart. Collectively,experimental studies and clinical trials, suggest a detrimental association between aldosteroneand life threatening arrhythmias that may be prevented by MR blockade. However, the signalingpathways and underlying mechanisms still remain elusive. We have accumulated evidence thatmodulation of Ca2+ signaling, especially Ca2+ influx via L-Type Ca2+ channel (LTCC), might bethe primary aldosterone/MR target in ventricular cardiomyocytes. Yet, the molecularmechanisms by which MR regulates expression of LTCC remain to be defined. Here, weinvestigated, in primary cultures of neonatal rat ventricular myocytes, the molecular eventscritical for aldosterone-Mediated cardiac effects on CaV1.2, the pore-Forming main subunit ofLTCC, which is encoded by the CaCNA1C gene.We showed that cardiac MR are functional as demonstrated by aldosterone-Induced MRnucleocytoplasmic trafficking observed by time-Lapse imaging of transfected GFP-Labeled MRusing confocal microscopy. Aldosterone exposure for 24 hours, induced a dose-Dependentincrease in CaV1.2 expression at both mRNA and protein levels. Analysis of the CaCNA1Cpromoter activity using luciferase reporter assays, revealed a dose- and time-Dependent activationby aldosterone. These effects were inhibited in the presence of either RU28318, a selective MRantagonist, or MR siRNA. In silico analyze enabled us to identify five putative GlucocorticoidResponse Elements (GRE) within the CaCNA1C promoter sequence. The mutation of the mostdistal GRE from Transcription Start Site (TSS) did not altered responses either elicited by theconstitutively active human MR (hMRΔ5,6) or dose-Dependent effects of aldosterone anddexamethasone (a synthetic glucocorticoïd with minimal MR effect). Mutations of the three nextones decreased responses to hMRΔ5,6 and aldosterone, whereas dexamethasone responses wereeither unchanged or increased. In sharp contrast, the mutation of the most proximal GRE fromTSS, increased responses to hMRΔ5,6, aldosterone and dexamethasone.These results provide new insights into the molecular mechanisms associated with cardiacMR activation, and suggest that LTCC is a primary MR target, with subsequent molecular andfunctional consequences that could lead to MR-Related cardiac dysfunction.

Modelos matemáticos de motoneurônios têm sido desenvolvidos para auxiliar na compreensão dos fenômenos que envolvem o sistema neuromuscular. Entretanto, a maioria dos modelos já desenvolvidos baseou-se na premissa de que a árvore dendrítica tem um comportamento passivo, o que ocorre em animais anestesiados, mas pode não ocorrer durante o comportamento motor normal de um animal intacto. Experimentos com animais descerebrados, em que as vias monoaminérgicas encontravam-se ativas, mostraram que os motoneurônios podem apresentar comportamentos mais complexos decorrentes da presença de condutâncias iônicas voltagem-dependentes que se situam nos dendritos e são responsáveis pela gênese de uma corrente de entrada persistente. Nesse sentido, um primeiro objetivo deste trabalho foi o de desenvolver novos modelos matemáticos de motoneurônios de diferentes tipos (i.e. dos tipos S, FR e FF), computacionalmente eficientes e contendo em seus compartimentos dendríticos uma condutância de cálcio do tipo L, de forma que os fenômenos de biestabilidade, potencial platô e amplificação da corrente sináptica efetiva possam ser gerados. Um segundo objetivo foi o de verificar como a presença da condutância iônica ativa no dendrito influencia o comportamento motoneuronal quando o mesmo está sujeito a entradas sinápticas de diferentes tipos. Os novos modelos foram parametrizados baseando-se em dados da literatura experimental para motoneurônios de gatos descerebrados e validados segundo os protocolos experimentais básicos que permitem caracterizar cada tipo de modelo como sendo totalmente ou parcialmente biestável. As entradas sinápticas foram simuladas por processos pontuais de Poisson e os trens de potenciais de ação dos motoneurônios foram analisados. Uma modulação senoidal da intensidade do processo pontual foi usada para estimar as respostas em frequência de cada modelo. Observou-se que, funcionalmente, a presença da condutância iônica dendrítica pode favorecer a ação do motoneurônio durante tarefas posturais, pois, uma vez ativada, a corrente de entrada persistente eleva a excitabilidade motoneuronal tornando os disparos mais regulares, além de prover uma alta sensibilidade dos modelos a entradas sinápticas de baixa frequência, correspondentes às oscilações observadas durante a manutenção da postura ereta quieta. / Mathematical models of motoneurons have been developed as an aid to the understanding of phenomena involving the neuromuscular system, but most of these models have been based on the hypothesis of a passive dendritic tree. This holds for anesthetized animals but not necessarily during normal motor behavior of the intact animal. Experiments with decerebrate animals in which the monoaminergic tracts were maintained intact have shown that more complex behaviors may emerge in motoneurons due to dendritic voltage-gated ionic conductances, which are responsible for a persistent inward current. Therefore, the first aim of this work was to develop computationally-efficient new motoneuron models of different types (i.e. type S, FR and FF) that include a dendritic L-type calcium conductance so that bistability, plateau potential and enhancement of effective synaptic current may be generated. The second aim of this research was to evaluate the effects of the active dendritic ionic conductance on the input-output mapping of presynaptic to postsynaptic spike trains. The new models were parameterized based on data reported in experimental literature on the decerebrate cat preparation, and they were validated using appropriate protocols for either fully or partially bistable dynamics. The synaptic inputs were simulated by Poisson point processes and the output spike trains were analyzed. Sinusoidal modulation of the point process intensity was used for the estimation of each models frequency response. The results suggested that an active dendritic ionic conductance in motoneurons has a functional role during postural tasks, because, when activated, the persistent inward current enhances the motoneuronal excitability, reducing the variability of interspike intervals, and focusing the sensitivity of the models to low frequency inputs that correspond to the low-frequency oscillations that typically occur during quiet standing posture.

Calmodulin (CaM) is a ubiquitous calcium-binding protein responsible for the binding and activation of a vast number of enzymes and signaling pathways. It contains two lobes that bind two calcium ions each, separated by a flexible central linker. This structural flexibility allows CaM to bind and regulate a large number of diverse protein targets within the cell in response to Ca2+ gradients. Voltage gated calcium channels (CaVs), as main sources of extracellular Ca2+, are crucial for a number of physiological processes, from muscle contraction to neurotransmission and endocrine function. These large transmembrane proteins open in response to membrane depolarization and allow gated entry of Ca2+ ions into the cytoplasm. Their regulation is currently the subject of intense investigation due to its pharmacological and scientific importance. CaM has been previously shown to pre-associate and act as a potent inhibitor of one class of high-voltage activated (HVA) channels called L-type channels via its interaction with their C-terminal cytoplasmic region. This interaction is primarily mediated by a conserved CaM-binding motif called the ‘IQ’ motif (for conserved isoleucine and glutamine residues), although the exact molecular details of its involvement in inactivation are currently unclear. Elucidation of these details was the primary objective of this dissertation. Recently, a novel sequence motif within this channel called ‘NSCaTE’ (N-terminal spatial calcium transforming element) has been described as an important contributor to calcium-dependent inactivation (CDI) of L-type channels. It was presumed to be unique to vertebrates, but we also show its conservation in a distantly related L-type channel homolog of Lymnaea stagnalis (pond snail). The interaction of CaM with a number of peptides representing the different regulatory motifs (IQ and NSCaTE) for both mammalian and snail isoforms was characterized in an attempt to better understand their role in CDI. Biophysical work with peptides as well as electrophysiology recordings with an N-terminal truncation mutant of Lymnaea CaV1 homolog were performed to expand our understanding of how the interplay between these channel elements might occur. In brief, the most striking feature of the interaction concerns the strong evidence for a CaM-mediated bridge between the N- and C-terminal elements of L-type channels. Further investigation of the CaM interaction with both IQ and NSCaTE peptides using Ca2+-deficient CaM mutants reveals a preference of both peptides for the Ca2+-C-lobe of CaM, and a much higher affinity of CaM for the IQ peptide, suggesting that the N-lobe of CaM is the main interaction responsible for the physiological effects of NSCaTE. These results are consistent with our electrophysiology findings that reveal a distinct buffer-sensitive CDI in wild type LCaV1 that can be abolished by the N-terminal truncation spanning the NSCaTE region. In addition to L-type channels, CaM has also been shown to have an indirect role in the regulation of low-voltage activated (LVA) or T-type channels (CaV3.x), via their phosphorylation by CaM-dependent protein kinase II (CaMKII). Using a primary sequence scanning algorithm, a CaM-binding site was predicted in a cytoplasmic region of these channels that was also previously shown to be important in channel gating. Biophysical experiments with synthetic peptides spanning this gating brake region from the three human and the single Lymnaea isoform strongly suggest that there is a novel, bona fide CaM interaction in this channel region, and also hint that this interaction may be a Ca2+-dependent switch of some sort. The results confirm a possible new role for CaM in the direct regulation of these channels, although the exact mechanism remains to be elucidated.

Thesis (Ph.D. in Neuroscience) -- University of Colorado Denver, 2008. / Typescript. Includes bibliographical references (leaves 110-125). Online version available via ProQuest Digital Dissertations.

Synaptic plasticity is thought to be integral to learning and memory. The two most common forms of plasticity are long-term potentiation (LTP) and long-term depression (LTD), both of which can be supported either by presynaptic changes in transmitter release probability (Pr), or by postsynaptic changes in AMPA receptor number. It is generally thought that the induction of LTP and LTD at Schaffer-collateral synapses in the hippocampus depends on the activation of NMDA receptors (GluN). Recent studies, however, have demonstrated that both increases and decreases in Pr can be induced under blockade of postsynaptic GluN receptors, suggesting that the activation of postsynaptic GluN receptors by glutamate is only a strict requirement for postsynaptic plasticity. In this thesis, I therefore re-examined the role of glutamate in presynaptic plasticity. I used single synapse imaging along with electrophysiological and pharmacological techniques to independently manipulate and monitor the levels of glutamatergic signalling during synaptic activity. I discovered that glutamate is inhibitory and unnecessary for the induction of LTP at the presynaptic locus. My findings support a novel model of presynaptic plasticity in which the net activity-dependent changes in Pr at an active presynaptic terminal is jointly determined by two opposing processes that can be simultaneously active: 1) postsynaptic depolarization, which, via the activation of L-type voltage-gated Ca²⁺ channels, increases Pr by driving the synthesis and release of nitric oxide from neuronal dendrites and 2) glutamate release, which through the activation of presynaptic GluN receptors, decreases Pr. Computationally, this model suggests that plasticity functions to reduce prediction-errors that arise during synaptic activity, and, thereby offers a biologically plausible mechanism by which neuronal networks may optimize learning at the level of single synapses.