The medial preoptic nucleus (MPN) of the hypothalamus plays a major role in many functions involved in maintaining bodily homeostasis, such as thermoregulation and osmoregulation, as well as in the control of complex behaviours, e.g. sexual behaviour. A fundamental basis for the control and execution of these functions is the synaptic communication between neurons of the MPN. However, the functional properties of the synapses involved are largely unknown. The present thesis is a study of ligand-gated ion channels involved in the pre- and post-synaptic aspects of neuronal communication in the MPN of rat. The aim was to clarify synaptic properties in the MPN, to identify the major channel types involved and to obtain a better understanding of their functional properties. By fast application of agonists to isolated neurons, it was first demonstrated that all neurons responded to glutamate with currents mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and a majority of neurons also with currents mediated by N-Methyl-D-aspartate (NMDA) receptors. All neurons also responded to γ-aminobutyric acid (GABA) and glycine with currents mediated by GABAA receptors and glycine receptors, respectively. These findings show that fast-acting excitatory and inhibitory amino-acid transmitters are most likely important for communication between hypothalamic neurons. Application of agonists to isolated neurons revealed cross-talk, detected as an apparent cross-desensitization, between the responses to GABA and those to glycine. Parallel analysis of current and conductance, using gramicidin-perforated patches to avoid perturbing intracellular chloride concentration, showed that the cross-talk was not dependent on a direct interaction between the receptors as previously suggested, but was a consequence of the change in the intracellular chloride concentration during receptor activation. Strengthened by a computer model, the analysis also showed that the current decay in the presence of GABA or glycine was mainly due to a change in the chloride driving force and that receptor desensitization played a minor role only. The role of thermo-sensitive transient receptor potential TRPV1 channels in the regulation of glutamate- and GABA-mediated transmission was studied in the slice preparation, where much of the synaptic connections between neurons are preserved. It was shown that application of the TRPV1 agonist capsaicin increased the frequency of excitatory AMPA receptor- mediated as well as inhibitory GABAA receptor-mediated postsynaptic currents. This effect was partly presynaptic and demonstrates that TRP channels play a role in regulating synaptic transmission in the MPN. The results imply that such mechanisms may possibly contribute to the thermoregulation by MPN neurons.

Brain function implies complex information processing in neuronal circuits, critically dependent on the molecular machinery that enables signal transmission across synaptic contacts between neurons. The types of ion channels and receptors in the neuronal membranes vary with neuron types and brain regions and determine whether neuronal responses will be excitatory or inhibitory and often allow for functional synaptic plasticity which is thought to be the basis for much of the adaptability of the nervous system and for our ability to learn and store memories. The present thesis is a study of synaptic transmission in the medial preoptic nucleus (MPN), a regulatory center for several homeostatic functions but with most clearly established roles in reproductive behaviour. The latter behaviour typically shows several distinct phases with dramatically varying neuronal impulse activity and is also subject to experience-dependent modifications. It seems likely that the synapses in the MPN contribute to the behaviour by means of activity-dependent functional plasticity. Synaptic transmission in the MPN, however, has not been extensively studied and is not well understood. The present work was initiated to clarify the synaptic properties in the MPN. The aim was to achieve a better understanding of the functional properties of the MPN, but also to obtain information on the functional roles of ion channel types for neurotransmission and its plastic properties in general. The studies were carried out using a brain slice preparation from rat as well as acutely isolated neurons with adhering nerve terminals. Presynaptic nerve fibres were stimulated electrically or, in a few cases, by raised external K+ concentration, and postsynaptic responses were recorded by tight-seal perforated-patch techniques, often combined with voltage-clamp control of the post-synaptic membrane potential. Glutamate receptors of α-amino-3-hydroxy-5-methyl-4-izoxazole propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) types were identified as mediating the main excitatory synaptic signals and γ-aminobutyric acid (GABA)A receptors as mediating the main inhibitory signals. Both types of signals were suppressed by serotonin. The efficacy of AMPA-receptor-mediated transmission displayed several types of short-term plasticity, including paired-pulse potentiation and paired-pulse depression, depending on the stimulus rate and pattern. The observed plasticity was attributed to mainly presynaptic mechanisms. To clarify some of the presynaptic factors controlling synaptic efficacy, the role of presynaptic L-type Ca2+ channels, usually assumed not to directly control transmitter release, was investigated. The analysis showed that (i) L-type channels are present in GABA-containing presynaptic terminals on MPN neurons, (ii) that these channels provide a means for differential control of spontaneous and impulse-evoked GABA release and (iii) that this differential control is prominent during short-term synaptic plasticity. A model where Ca2+ influx through L-type channels may lead to reduced GABA release via effects on Ca2+-activated K+ channels, membrane potential and other Ca2+-channel types explains the observed findings. In addition, massive Ca2+ influx through L-type channels during high-frequency stimulation may contribute to increased GABA release during post-tetanic potentiation. In conclusion, the findings obtained in the present study indicate that complex neurotransmission mechanisms and different forms of synaptic plasticity contribute to the specific functional properties of the MPN.

L'épilepsie-absence est un syndrome épileptique dont le principal symptôme est une altération transitoire de la conscience, avec décharges pointes-ondes généralisées, qui ont pour origine un dysfonctionnement dans la boucle cortico-thalamique, et naissant dans une sous-population de neurones pyramidaux localisée dans les couches profondes du cortex somatosensoriel. A l'aide d'enregistrements EEG et intracellulaires in vivo dans un modèle animal: les Genetic Absence Epilepsy Rats from Strasbourg, j'ai examiné comment l'excitation initiale des neurones ictogèniques lors des crises est suivie par une hyperpolarisation synaptique chlore-dépendante, concomitante d'une décharge en bouffées dans les interneurones GABAergiques locaux. Le système GABA exerce un effet strictement inhibiteur et contraint la décharge des neurones ictogéniques dans une fenêtre temporelle étroite. Dans une deuxième étude chez l'homme et chez le GAERS, j'ai exploré comment des informations sensorielles sont traitées au cours des DPO. Chez l'enfant épileptique, des stimulations visuelles résultent en des potentiels évoqués occipitaux, plus amples que chez les sujets non-épileptiques. Des stimulations tactiles chez le GAERS induisent lors des crises des potentiels évoqués dans l'EEG et, dans les neurones pyramidaux sous-jacents, des potentiels synaptiques excitateurs plus amples que dans la condition inter-critique. Les troubles de la conscience lors des absences ne résultent donc pas d'un filtrage des informations sensorielles. L'ensemble des recherches fournit des données nouvelles sur les propriétés fonctionnelles des circuits corticaux exprimant les paroxysmes électriques lors des crises d'absence

Haemonchus contortus is a parasitic nematode that is controlled by several nematocides which target ion channels. We have identified two H. contortus ion channel genes, Hcounc- 49B and C that encode two GABA-gated chloride channel subunits. Electrophysiological analysis shows that the Hco-UNC-49B subunit forms a functional homomeric channel in Xenopus laevis oocytes that produces a robust response to GABA and is highly sensitive to picrotoxin. In contrast, Hco-UNC-49C alone does not respond to GABA but can assemble with Hco-UNC-49B to form a heteromeric channel with an increased sensitivity to GABA and a lower sensitivity to picrotoxin. To investigate the subunit requirements for high agonist sensitivity, we generated cross-assembled channels by co-expressing the H. contortus subunits with UNC-49 subunits from the nematode Caenorhabditis elegans (Cel-UNC-49). Co-expressing the Cel-UNC-49B with Hco- UNC-49C produced a heteromeric channel with a low sensitivity to GABA. In contrast, co-expressing Hco-UNC-49B with Cel-UNC-49C produced a heteromeric channel that was highly sensitive to GABA. These results suggest that the Hco-UNC-49B subunit is the key determinant for the high agonist sensitivity of heteromeric channels.

Extensive evidence suggests that glucose has both positive and negative effects on memory and these effects likely involve an influence on the brain. For instance, direct infusions of glucose into the septum (MS) or hippocampus can enhance or impair memory. The present set of experiments attempted to determine the different conditions that dissociate the memory-enhancing and -impairing effects of glucose in rats. Specifically, these experiments examined the effects of glucose in spontaneous alternation, a measure of spatial working memory and shock avoidance, an index of emontional long-term memory. The results showed that the memory-impairing effects of MS infusions of glucose are not concentration-dependent. These data also indicated that the memory-impairing effects of MS glucose elevations are specific to gamma-aminobutyric acid GABA receptor activation but do not depend on increases in MS GABA synthesis or release. Importantly, we showed that the memory-impairing interaction between MS glucose and GABA agonists does not generalize to the hippocampus, suggesting the memory-modulating effects of glucose are brain region-dependent. We showed further that these brain region-dependent effects of glucose are not due to difference in basal extracellular glucose levels. Moreover, these findings showed that the memory-enhancing effects of hippocampus glucose override the memory-impairing interaction between MS glucose and GABA. These findings are important because they are the first to show that the memory-modulating effects of glucose are both neurotransmitter- and brain region-dependent. Furthermore, these findings provide preliminary evidence suggesting that the memory-impairing effects of MS glucose may involve compromised hippocampal function. These data also suggest the memory-impairing effects of MS co-infusions of glucose with GABA agonists likely involve an influence on the GABAergic SH projection. Finally, these findings demonstrate the mnemonic and neurochemical consequences of glucose in the MS and hippocampus, two brain regions affected by normal aging, Alzheimer’s disease, and diabetes.

Brain injury is a serious clinical problem. The success of recovery from brain injury involves functional compensation in the affected brain area. We are interested in general mechanisms that underlie compensatory plasticity after brain damage, particularly when multiple brain areas or multiple modalities are included. In this thesis, I studied the function of auditory cortex after recovery from neonatal midbrain damage as a model system that resembles patients with brain damage or sensory dysfunction. I addressed maladaptive changes of auditory cortex after invasion by ectopic visual inputs. I found that auditory cortex contained auditory, visual, and multisensory neurons after it recovered from neonatal midbrain damage (Mao et al. 2011). The distribution of these different neuronal responses did not show any clustering or segregation. As might be predicted from the fact that auditory neurons and visual neurons were intermingled throughout the entire auditory cortex, I found that residual auditory tuning and tonotopy in the rewired auditory cortex were compromised. Auditory tuning curves were broader and tonotopic maps were disrupted in the experimental animals. Because lateral inhibition is proposed to contribute to refinement of sensory maps and tuning of receptive fields, I tested whether loss of inhibition is responsible for the compromised auditory function in my experimental animals. I found an increase rather than a decrease of inhibition in the rewired auditory cortex, suggesting that broader tuning curves in the experimental animals are not caused by loss of lateral inhibition. These results suggest that compensatory plasticity can be maladaptive and thus impair the recovery of the original sensory cortical function. The reorganization of brain areas after recovery from brain damage may require stronger inhibition in order to process multiple sensory modalities simultaneously. These findings provide insight into compensatory plasticity after sensory dysfunction and brain damage and new information about the role of inhibition in cross-modal plasticity. This study can guide further research on design of therapeutic strategies to encourage adaptive changes and discourage maladaptive changes after brain damage, sensory/motor dysfunction, and deafferentation.

Thèse de doctorat : Chimie organique : Strasbourg 1 : 2008. / Titre provenant de l'écran-titre. Bibliogr. p. 305-321.

Thesis (Ph. D.)--West Virginia University, 2003. / Title from document title page. Document formatted into pages; contains vi, 187 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 156-183).

Thesis (M.A.)--University of North Carolina at Wilmington, 2003. / Includes bibliographical references (leaves : [84]-89).

Cognitive dysfunction is a core characteristic of schizophrenia, which can often persist when other symptoms, particularly positive symptoms, may be improved with drug treatment. The non-competitive NMDA receptor antagonist, phencyclidine (PCP), is a psychomotor stimulant drug that has been shown to induce symptoms characteristic of schizophrenia in humans and animals. The aim of these studies was to use the sub-chronic PCP model in rats to investigate cognitive dysfunction in behavioural tests which have been highlighted as relevance by the MATRICS initiative (MATRICS.ucla.edu). The main tests used were attentional set-shifting, operant reversal learning, and novel object recognition tasks. The pharmacology of antipsychotics was studied in the reversal learning task using receptor selective compounds. Following this, experiments were carried out using in vitro electrophysiology and in vivo microdialysis in an attempt to investigate the mechanisms underpinning the PCP-induced cognitive deficits. The attentional set-shifting task is a test of executive function, the extra-dimensional shift (EDS) phase relates to the ability to shift attention to a different stimulus dimension; this is impaired in patients with schizophrenia. The studies presented in chapter 2 showed that sub-chronic PCP administration impaired attentional set-shifting performance selectively in the EDS phase, a deficit which was significantly attenuated by sub-chronic administration of clozapine and risperidone, but not haloperidol. The effect of PCP was also shown to be more robust in female rats compared to males. A deficit in set-shifting ability was also observed in isolation reared rats. However, the deficits produced by PCP were more robust than the deficit produced by isolation rearing. The reversal learning task is another test of executive function. Chapter 3 reported that sub-chronic PCP administration impairs reversal learning ability in an operant task, as demonstrated by reduced percent correct responding in the reversal phase of the reversal learning task. It was found that a D1 agonist (SKF-38398), a 5-HT1A partial agonist (buspirone), a 5-HT2C antagonist (SB-243213A) and an agonist and positive allosteric modulator of the alpha 7 nACh receptor (PNU-282987 and PheTQS respectively) are able to reverse the sub-chronic PCP-induced deficit in reversal learning. Although many antipsychotics have affinity for muscarinic M1 and histamine H1 receptors, selective agents at these receptors were not able to improve the PCP-induced deficit. In chapter 4, the atypical antipsychotics, clozapine and risperidone, when given alone to naïve rats had no effect on reversal learning. Haloperidol when given to naïve rats impaired performance at the highest dose. Sub-chronic PCP was again found to impair reversal learning performance. Investigative experiments revealed that the 2 min time-out could be important as a cue. Following a double reversal, olanzapine-treated rats lost the ability to switch between the rules, whereas clozapine and risperidone-treated rats could perform the double reversal. Experiments with the extended (15 min) reversal phase could allow the investigation of the time-course effects of antipsychotics or selective compounds. The studies presented in chapter 5 found a reduction in gamma oscillations in the CA3 region of the hippocampus, following sub-chronic PCP treatment (2-5 weeks post treatment) that was paralleled by a deficit in parvalbumin immunoreactive (IR) cell density, at a similar time point (2 weeks post treatment). In contrast, a time-dependent increase in gamma oscillations was observed (6-8 weeks post treatment), at which point parvalbumin IR cell density was unchanged (8 weeks post treatment). Gamma oscillations were unchanged in the prefrontal cortex (PFC) following the PCP treatment regime. Locomotor activity tests were also carried out to ensure that the sub-chronic PCP treatment was successful. In-vivo microdialysis revealed that vehicle-treated rats show an increase in dopamine in the PFC which is selective for the retention trial of the novel object recognition task. PCP-treated rats were unable to distinguish between the novel and familiar objects and the increase in dopamine observed in vehicle rats was absent. As a control experiment it was also shown that sub-chronic PCP did not induce anxiety-like symptoms in the elevated plus maze and open field tests. These studies suggest that sub-chronic PCP induces cognitive deficits in behavioural tasks, and these deficits may be due to GABAergic mediated processes in the hippocampus and dopaminergic dysfunction in the PFC. These behavioural and neurochemical results are concurrent to findings observed in schizophrenia.