It is assumed that plasticity involving up-and down regulation of synaptic strength (i.e., long-term potentiation, LTP; long-term depression, LTD) mediates learning and memory processes. Typically, high-frequency stimulation (HFS) of afferent fibers results in LTP, while low-frequency stimulation (LFS) elicits LTD. In stark contrast to this “HFS- LTP vs. LFS-LTD” dogma, the present thesis characterizes a novel form of LFS-induced LTP in the septohippocampal system. The first set of experiments show that alternating, single pulse stimulation (1 Hz) of the medial septum (MS) and CA3 hippocampal (H) commissural fibres results in a long-lasting potentiation of field excitatory postsynaptic potentials (fEPSPs) in CA1 of urethane-anesthetised rats (MS-H-LTP). MS-H LTP is long lasting (>5 h), requires a specific inter-stimulus interval of 1 s between MS and CA3 stimulation, saturates with repeated stimulation episodes and depends on NMDA receptor activation. In the third chapter (review) I suggest that LFS protocols may more accurately mimic some oscillatory activity patterns (~ 1Hz) present in hippocampal and neocortical circuits during sleep-related memory consolidation. Moreover, I compare the mechanisms underlying classical, HFS-LTP to those mediating MS-H LTP as well as several other types of LFS-LTP in the hippocampus and amygdala in vitro. Subsequently, I investigated cellular mechanisms of MS-H LTP and their similarity to classical HFS-LTP via drug application at the CA1 recording site and showed that MS-H LTP depends on protein kinase A and protein synthesis. This surprising similarity between mechanisms mediating HFS-LTP and MS-H LTP was further supported by occlusion experiments whereby LFS and HFS, delivered to the same animal, competed for the available synaptic potentiation of CA3-CA1 synapses. The final experiments showed that MS-H LTP is compromised in early aged rats, while similar levels of potentiation are expressed in the juvenile and adult hippocampus. Interestingly, MS-H LTP could not be induced (i.e., was occluded) 3 h after training on the hidden platform version of the Morris water maze, while it was unaltered at 8 and 24 h intervals. This thesis characterizes a novel form of hippocampal plasticity at the cellular, synaptic and behavioural level and suggests that LFS-LTP may mediate processes of sleep-related memory consolidation. / Thesis (Ph.D, Neuroscience Studies) -- Queen's University, 2010-10-04 11:35:21.288

Synaptic plasticity reflects the capacity of synapses to undergo changes in synaptic strength and connectivity, and is highly regulated by age and sensory experience. This thesis focuses on the characterization of synaptic plasticity in the primary auditory cortex (A1) of rats throughout development and following sensory deprivation. Initial experiments revealed an age-dependent decline in plasticity, as indicated by reductions in long-term potentiation (LTP). The enhanced plasticity of juvenile rats appeared to be mediated by NR2B subunits of the N-methyl-d-aspartate receptor (NMDAR), as NR2B antagonist application reduced LTP to adult-like levels in juveniles, yet had no effect in adults. The importance of sensory experience in mediating plasticity was revealed in experiments using white noise exposure, which is a sensory deprivation technique known to arrest cortical development in A1. Notably, adult rats reared in continuous white noise maintained more juvenile-like levels of LTP, which normalized upon subsequent exposure to an unaltered acoustic environment. The white noise-induced LTP enhancements also appeared to be mediated by NR2B subunits, as NR2B antagonists reversed these LTP enhancements in white noise-reared rats. Given the strong influence that sensory experience exerts on plasticity, additional experiments examined the effect of shorter episodes of white noise exposure on LTP in adult rats. Exposure to white noise during early postnatal life appeared to “prime” A1 for subsequent exposure in adulthood, resulting in enhanced LTP. The necessity of early-life exposure was evident, as repeated episodes of white noise in adulthood did not enhance plasticity. In older rats that typically no longer express LTP in A1, pharmacological methods to enhance plasticity were explored. Moderate LTP was observed in older rats with cortical zinc application, which may act through its antagonism of NR2A subunits of the NMDAR. Additionally, current source density and cortical silencing analyses were conducted to characterize the distinct peaks of field postsynaptic potentials recorded in A1, with the earlier and later peaks likely representing thalamocortical and intracortical synapses, respectively. Together, this thesis emphasizes the critical role of sensory experience in determining levels of cortical plasticity, and demonstrates strategies to enhance plasticity in the mature auditory cortex. / Thesis (Ph.D, Neuroscience Studies) -- Queen's University, 2011-01-11 14:53:57.677

No description available.

GABAergic interneurons in the basolateral complex (BLC) of the amygdala are a part of the emotional-learning circuitry of the brain and receive excitatory inputs from all sensory modalities via cortex and thalamus. Although the BLC, which is made up of the lateral amygdala (LA), basal amygdala (BA) and accessory basal nucleus, is under the influence of a strong inhibition brought about by local interneurons, little is known about the diversity, characteristics and functioning of these interneurons. In this study, I have characterised the BLC interneuron population using a transgenic mouse model in which enhanced green fluorescent protein has been tagged to the GAD67 promoter. This promoter is specifically expressed in all GABAergic interneurons, enabling us to visualise interneurons under UV light. Whole-cell recordings were made from GAD67 interneurons in the BLA to study their membrane and synaptic properties. On the basis of their firing properties, interneurons in the BLC were classified into six distinct groups. The calcium-binding proteins calbindin, calretinin and parvalbumin were found to be expressed differently in the LA and BA interneurons, with the majority of the interneurons in the LA expressing calretinin, whereas those in the BA mostly expressed parvalbumin. We also found diversity in the expression of postsynaptic glutamate receptors in the BLC. Long-term potentiation induced at the interneurons was specific to the cortical inputs in the LA. LTP was expressed only in interneurons that either lacked NMDA receptors or had NMDA receptors with fast decay kinetics. This form of LTP was mediated by calcium-permeable AMPA receptors and required a postsynaptic calcium rise for its induction This study shows that the interneurons in the BLC are a heterogenous population with respect to the expression of calcium-binding proteins, axonal morphology, synaptic and membrane properties. This heterogeneity in interneuron population may be essential for the specialised roles various types of interneurons play in the functioning of the amygdala and in emotional learning.

The computational ability of principal neurons and interneurons in the brain and their ability to work together in concert are thought to underlie higher order cognitive processes such as learning, memory, and attention. Dendrites play a very important role in neuronal information processing because they receive and integrate incoming input and can undergo experience-dependent changes that will alter the future output of the neuron. Here, I have used whole-cell patch clamp recordings and fluorescent Ca2+-imaging to examine the modulation of dendritic excitability in principal neurons of the rat and human hippocampus and neocortex. First, I determined that dendrites of dentate granule cells of the hippocampus are tuned to high frequencies of both afferent input and backpropagating action potentials. Under these conditions they are also capable of generating regenerative dendritic activity that can propagate to the soma, which is prone to modulation. In particular, Neuropeptide Y (NPY) Y1 receptors can decrease frequency-dependent dendritic Ca2+ influx. Dopamine D1 receptors (D1Rs) have an opposite effect; they potentiate frequency-dependent dendritic excitability. These two neuromodulators also have an opposing effect on plasticity, with dopamine acting to induce, and NPY acting to inhibit long-term potentiation (LTP). Parallel observations of D1-induced LTP and an NPY-mediated decrease in dendritic excitability in rodents were complemented by findings in human dentate granule cells. Second, I examined the role of NPY receptors on dendrites of layer 5 pyramidal neurons. In these neurons I found that NPY acts post-synaptically on distal dendrites via the Y1 receptor to inhibit frequency-dependent Ca2+-currents, similar to the findings in dentate granule cells. NPY also decreased regenerative Ca2+ currents caused by the appropriate pairing of pre- and post-synaptic input. Together, these observations demonstrate that the role of NPY in the hippocampus and neocortex is not solely as an anti-epileptic agent. NPY release, likely to occur during high frequency oscillatory activity, can act locally to limit dendritic excitability, which can have a profound effect on plasticity. In the dentate gyrus, NPY can inhibit a D1R induced increased dendritic excitability and resultant changes in synaptic strength. These findings will further the understanding of dendritic information processing in the hippocampus and neocortex.

No description available.

We have recently established that aberrant alkylacylglycerophosphocholine metabolism results in the increased tissue concentration of platelet activating factors (PAFs) in the temporal cortex of Alzheimer Disease (AD) patients and in TgCRND8 mice over-expressing mutant human amyloid precursor protein. PAF lipids activate a G-protein coupled receptor (PAFR) reported to be expressed by microglia and subsets of neurons in rat. It is not known whether this same expression pattern is recapitulated in mice however, as the expression has only been inferred by use of pharmacological PAFR antagonists, many of which impact on both PAFR-dependent and PAFR-independent signalling pathways. PAFR plays a role in long term potentiation (LTP) induction in rats. PAFR has also been implicated in behavioural indices of spatial learning and memory in rats. Contradictory reports using mice provide ambiguity regarding the role of PAFR in LTP induction in mice. To assess whether PAFR is expressed in murine neurons, I localized PAFR mRNA in wild-type C57BL/6 mice using PAFR KO mice as a negative control. I further showed that the loss of PAFR did not impair learning and memory although this assessment must be considered preliminary as the behavioural test employed was not optimized to detect changes in learning and memory of C57BL/6 mice over time adequately.Finally, I showed that the loss of PAFR in TgCRND8 mouse model of AD had no impact upon Aβ plaque number. My observations suggest that PAFR is restricted to microglial-like cells in mouse hippocampus and as such, it may not play a role in learning and memory.

We have recently established that aberrant alkylacylglycerophosphocholine metabolism results in the increased tissue concentration of platelet activating factors (PAFs) in the temporal cortex of Alzheimer Disease (AD) patients and in TgCRND8 mice over-expressing mutant human amyloid precursor protein. PAF lipids activate a G-protein coupled receptor (PAFR) reported to be expressed by microglia and subsets of neurons in rat. It is not known whether this same expression pattern is recapitulated in mice however, as the expression has only been inferred by use of pharmacological PAFR antagonists, many of which impact on both PAFR-dependent and PAFR-independent signalling pathways. PAFR plays a role in long term potentiation (LTP) induction in rats. PAFR has also been implicated in behavioural indices of spatial learning and memory in rats. Contradictory reports using mice provide ambiguity regarding the role of PAFR in LTP induction in mice. To assess whether PAFR is expressed in murine neurons, I localized PAFR mRNA in wild-type C57BL/6 mice using PAFR KO mice as a negative control. I further showed that the loss of PAFR did not impair learning and memory although this assessment must be considered preliminary as the behavioural test employed was not optimized to detect changes in learning and memory of C57BL/6 mice over time adequately.Finally, I showed that the loss of PAFR in TgCRND8 mouse model of AD had no impact upon Aβ plaque number. My observations suggest that PAFR is restricted to microglial-like cells in mouse hippocampus and as such, it may not play a role in learning and memory.

The mesolimbic dopamine system, including dopaminergic projections from the ventral tegmental area (VTA) to nucleus accumbens (NAc), is critically involved in the development of addiction to many drugs of abuse, including cocaine (CA). Although there is an attractive hypothesis that the modifications of mesolimbic reward circuit following repeated drug exposure are responsible for cocaine-addicted causes behaviors change, however, our understanding in the underlying molecular mechanisms at the neural circuit level is still in its infancy. It has been suggested PKMzeta, a constitutively active atypical isoform of PKC, plays a critical role in spatial memory formation and long-term synaptic potentiation in hippocampus. To define the relationship among PKMzeta, CA-induced synaptic long-term potentiation and CA addiction, we examined the regulation of PKMzeta after CA administration in Sprague-Dawley rat. We found single CA injection elicits an increase in PKMzeta protein expression in the VTA region. The increase was first observed 10 min after CA administration and lasted for 7 days, the longest sampling time point of our experimental design. The PKMzeta protein expression can also be induced in 10 minutes while incubating the acute isolated brain slice with CA, the expression within 1 hr can be eliminated at the present of Chelerythrine (PKC inhibitor) and ZIP (PKMzeta inhibitor) suggests a positive feedback loop. The PKMzeta mRNA expression can be induced within 1 hr, and Actinomycin d (transcription inhibitor) had no effect on the PKMzeta protein expression indicating CA increases PKM£a translation from preexisting PKM£a mRNA. Furthermore,real time PCR-based analysis showed resembling increase profile ofPKM£a mRNA after single CA injection, suggesting a co-upregulation of transcription and translation of PKM£a after CA administration in VTA. Eticlopride (dopamine receptor D2-subtype antagonist) ¡BSCH-23390(dopamine receptor D1-subtype antagonist)¡BH-89 (PKA inhibitor)¡B Wortmannin (PI3K inhibitor)¡BPD98059 (MEK1 inhibitor) decreasedcocaine-induced PKM£a expression within 1 hr in VTA. On the contrary, KN-62 (CaMK II inhibitor) has no obvious effect on PKM£a expression. CA challenge not only induces the PKM£a expression in the VTA region but also in the NAc and hippocampus region. The CA-induced PKM£a expression is more obvious in elder group (>45 days in age) than younger group (18~30 days in age), similar results also showed in the locomotor activity assay. Prenatal CA exposure decreased the postnatal CA-induced PKM£a expression and the locomotor sensitivity in younger group. Overall, results from our current experiments have raised the possibility of PKM£a involvement in CA addiction. How CA regulates PKM£a expression and the context dependence between PKM£a and CA-induced behavior change and synaptic long-term potentiation remains further elucidation.

Addiction is a chronic disease that characterize as habitual or compulsive involvement in an activity despite it¡¦s bring negative consequences. Some of psystimulants such as cocaine or amphetamine cause a strong reinforcing effects even after prolonged abstinence periods. Such illegal drugs not only hurt on the adult health but also result in fetal physiological damage. For example, that babies born to mothers who abuse with cocaine bring prematurely delivered, low birth weights, smaller head circumferences and increased heart disease in adult offspring. Mesolimbic dopamine system include nucleus accumbens (NAc) and ventral tegmental area (VTA) are critical regions for the neural adaptations that contribute to addiction. VTA that receives inputs from a large number of brain regions. For example, it receives glutamatergic inputs from prefrontal cortex, or GABAergic inputs from NAc. It has been known that VTA play a major role in the acquisition and expression of learned addictive behaviors. Results from many neuropharmacological studies in animal models indicate that exposure to cocaine or some other drugs of abuse seems to induce long-term potentiation (LTP) ¢w like changes of synaptic plasticity among neurons in VTA region. LTP was first described in hippocampus, a region that associated with memory formation, and were found widespread events in many mammalian brain sites. In the present time, theories and investigation indicated that memory and addiction might shared the similar neural circuitry and signal pathways. In general, LTP can be separate into two main phases : induction and maintenance phases. Many of molecules participate in induction phase such as calcium/calmodulin-dependent protein kinase II (CaMKII), cyclic AMP (cAMP), phosphatidylinositol 3-kinases (PI3K) and protein kinase C (PKC). However, until now there was only one molecule has been found associated with LTP maintenance¡Xprotein kinase M£a (PKM£a). PKM£a is a brain specific, constitutively active form of PKC that does not need Ca2+ or diacylglycerol (DAG) for its activation. Molecular evidences showed that PKM£a is translated uniquely by PKM£a mRNA which is generated under the control of an internal promoter in the PKC£a gene. Recently, investigators introduced a PKM£a selective inhibitor¡XZIP, to hippocampus or insular cortex both successful to eliminate long-term spatial memory or conditioned taste aversion (CTA) behavior, respectively, on rat. Therefore, exclude PKM£a by specific inhibitors and then result in abolish long-term synaptic potentiation which had already established seem to be a leading candidate for cure addiction. Here we showed that blocked of PKM£a activity in VTA dopaminergic neuron eliminated mEPSCs or AMPAR/NMDAR ratio increment elicited by cocaine. Otherwise, our results also presented that myristoylatedinhibitory peptide¢wZIP had no effect on spike timing-dependent long-term potentiation in rats previously injected with saline but remarkably restored spike timing-dependent long-term potentiation in VTA dopamine neurons in slices prepared from rats that received single or multiple cocaine exposure. Furthermore, our western blot analyses showed that both single and five consecutive cocaine injections induced a significant increase in PKM£a level in VTA or NAc. Moreover, our ex vivo cocaine incubation results indicated that multiple kinases activation or de novo protein synthesis was required for PKM£a increment. The most important, our data provided the first physiological evidence between PKM£a and drug addiction when intracranial administered specific PKM£a inhibitors to VTA reversed cocaine-induced conditioned-place preference (CPP) behavior. Finally, we investigated the behavioral effect of cocaine-induced locomotor sensitization in an open field apparatus. Our data showed that peri-adolescent (P21) rats exhibited prominently increased in either acute or repeated cocaine-induced locomotor activity than mid-adolescent (P28) and post-adolescent (P41). Interestingly, applied to high dosage cocaine (30 mg/kg) rescued the acute locomotor response in P28 rats but not behavioral sensitization. We further examined the locomotion on rats that were exposed to cocaine in utero after single or multiple cocaine injection. However, cocaine-induced increase in locomotor activity was lower in P21 rats which exposed to cocaine during pregnancy but no significantly difference in P28 rats. Surprisingly, single high dose cocaine treatment caused a marked reduction in locomotor activity on P21 rats prenatally exposed to cocaine. Otherwise, we also provided the first evidences that repeated cocaine injection in pregnant rats induced a significant decreased to KCC2 level in PFC regions prepared from P20 rat. In conclusion, results from our current studies demonstrate for the first time that persistently active PKM£a is necessary in (1) mEPSC facilitation induced by single cocaine exposure; (2) cocaine-induced enhancement in AMPAR/NMDAR ratio; (3) single or repeated cocaine-induced LTP but not in LTP induced by spike-timing stimulation; and (4) cocaine conditioned place preference in the VTA. In addition, our results also present evidence that the expression of PKM£a is increased by either single or repeated cocaine exposure. Furthermore, our behavioral or Western blotting consequence of cocaine treatment in utero was reflected by the diminishion in the sensitivity of locomotor activity in postnatal rats to cocaine and KCC2 level in PFC regions.