Thesis (Ph.D.)--University of Western Australia, 2007.

Thesis (M.Sc.) - Carleton University, 2007. / Includes bibliographical references (p. 83-89). Also available in electronic format on the Internet.

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As reações incondicionadas de defesa observadas em mamíferos são organizadas pelo Sistema Encefálico de Aversão (SEA), composto, entre outras estruturas, pela substância cinzenta periaquedutal dorsal (SCPd) e o colículo inferior (CI). Tem sido proposto que o CI seja parte do circuito sensoriomotor para os estímulos auditivos de natureza aversiva e a SCPd como a principal via de saída (output) do SEA para a elaboração de comportamentos defensivos. Ambas as estruturas são reguladas tonicamente pelo neurotransmissor inibitório ácido gama-aminobutírico (GABA). Este trabalho aborda a mediação química GABA/Benzodiazepínica (BZD) do processamento da informação aversiva no CI e das respostas de medo elaboradas pela SCPd. Grupos independentes de animais submetidos ao implante de quimitrodos (eletrodos acoplados a cânulas-guia para injeção de drogas) foram usados para avaliar no CI e SCPd os efeitos de injeções locais de muscimol (agonista de receptores GABA-A), semicarbazida (inibidor da síntese da enzima precursora do GABA descarboxilase do ácido glutâmico) ou midazolam (agonista BZD). Foram registrados potenciais evocados auditivos (PEA) no CI como medida eletrofisiológica da ativação neuronial, além da determinação dos limiares de congelamento e fuga, com o procedimento de estimulação elétrica (EE), tanto do CI quanto da SCPd. A mesma abordagem farmacológica com injeções de drogas intra-CI foi empregada em animais submetidos ao teste do Labirinto em Cruz Elevado (LCE), um modelo animal tradicional de ansiedade. Adicionalmente, investigou-se a participação de ambas as estruturas na expressão do comportamento de desligar uma luz de intensidade aversiva em um novo teste de medo incondicionado (Light Switch Off Test; LSOT) recentemente proposto pelo nosso grupo. Encontramos uma clara segregação funcional entre a porção dorsal e ventral do CI, sendo a última envolvida nos comportamentos defensivos. Mecanismos GABAérgicos em ambas as estruturas influenciam a amplitude do PEA e o congelamento pós-fuga da EE, sugerindo uma relação funcional entre as duas estruturas. Já no LSOT, os resultados indicam o envolvimento de mecanismos GABAérgicos do vCI, mas não da SCPd, na modulação da resposta incondicionada à luz em ratos. Os resultados obtidos permitem ampliar o conhecimento atual sobre a neurobiologia dos estados de medo e ansiedade, em uma abordagem integrada dos mecanismos de processamento das informações sensoriais e da expressão de reações de defesa. / Unconditioned defense reactions observed in mammals are organized by the Brain Aversive System, comprising, among other structures, the dorsal periaqueductal gray matter (dPAG) and the inferior colliculus (IC). It has been proposed that the IC is part of the sensorimotor circuitry that processes aversive auditory information and the dPAG is considered the main neural substrate for the expression of defensive behaviors. Both structures are tonically regulated by the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). This work addresses the chemical mediation of GABA/Benzodiazepine (BZD) on aversive information processing in the IC and the elaboration of fear responses by dPAG. Independent groups of animals implanted with chemitrodes (electrodes attached to a guide cannula for drug injection) have been used to evaluate the IC and dPAG regarding the effects of local injections of GABAergic agents (muscimol, semicarbazide, and midazolam). Auditory evoked potentials (AEP) have been recorded in the IC as a measure of electrophysiological neuronal activation, in addition to determining the thresholds of defensive freezing and flight behaviors, using the electrical stimulation (EE) procedure in both IC and dPAG. The same pharmacological regimen of drug injections intra-dPAG and intra-CI have been applied to animals subjected to the elevated plus maze (EPM), a well-known animal model of anxiety, and also to a novel animal test for innate fear (Light Switch Off Test, LSOT) that has been developed and proposed by our group. We found a clear functional segregation between the dorsal and ventral portions of the IC, the latter being the specific collicular substrate of defensive behaviors. GABAergic mechanisms in both structures influence the amplitude of the AEP and post-stimulation freezing of EE, suggesting a functional link between the two structures. In the LSOT, our data indicate the involvement of GABAergic mechanisms of the ICv, but not the dPAG, in the modulation of the unconditioned response to light in rats. These original findings presented here contribute to broaden the current knowledge on the neurobiology of fear and anxiety, in an integrative approach of the mechanisms underlying sensory processing and the expression of defensive behaviors.

A antinocicepção é descrita como redução na capacidade de perceber a dor, sendo importante componente para o organismo, quando este está envolvido em situações de emergência. Muitos neurotransmissores e seus receptores estão envolvidos nas diversas formas de analgesia, como por exemplo, monoaminas, acetilcolina e opióides endógenos. A analgesia pós-ictal é uma das muitas formas de antinocicepção em que o recrutamento de cada um desses neurotransmissores é descrito. Algumas evidências mostram o envolvimento de estruturas mesencefálicas em processos antinociceptivos (GEBHART & TOLEIKIS, 1978; COIMBRA e col., 1992; COIMBRA & BRANDÃO, 1997; CASTILHO e col., 1999) O colículo inferior é a estrutura mesencefálica responsável pela origem e expressão de crises audiogênicas (MCCOWN e col, 1987). Estruturas como a substância cinzenta periaquedutal, camadas profundas do colículo superior e núcleo central do colículo inferior, têm sido implicadas em processos convulsivos (DE PAULIS e col., 1990; CARDOSO e col., 1994, MCCOWN e col., 1984). Recentes relatos demonstraram que a estimulação dessas estruturas, em cujo substrato neural há neurônios positivos para beta-endorfina e leu-encefalina (EICHENBERGER e col., 2002; OSAKI e col., 2003) pode gerar processos antinociceptivos (CASTILHO e col., 1999; GEBHART & TOLEIKIS, 1978), seja de natureza opióide (NICHOLS e col., 1989), seja de natureza monoaminérgica (COIMBRA e col., 1992; COIMBRA & BRANDÃO, 1997). Neste trabalho, foi realizado o estudo periférico para investigar o envolvimento, mais especificamente, dos receptores opióides µ1 na analgesia que segue as crises convulsivas evocadas pela administração de um antagonista de canais de Cl- ligados ao GABA, como é o caso do pentilenotetrazol, administrado por via intraperitoneal, após o pré-tratamento com o bloqueador opióide específico, o naloxonazine, administrado em diferentes doses. Assim também, estudou-se a participação do colículo inferior nesse processo de inibição de dor, mensurado pelo teste de retirada de cauda. O pré-tratamento com naloxonazine, administrado por via intraperitoneal por tempo prolongado (24h), mas não agudamente (10 min), antagonizou a antinocicepção evocada por convulsões tônico-clônicas. Microinjeções de naloxonazine realizadas nos núcleos central, cortical externo e cortical dorsal do colículo inferior antagonizaram a analgesia induzida por crises convulsivas tônico-clônicas, efeito que segue uma curva dose-resposta, bem como, causaram a redução do tempo do processo convulsivo induzido pelo bloqueio ionóforo de canais de cloro ligados ao GABA. Em vista disso, podemos sugerir o envolvimento de receptores µ1-opióides e das redes neurais do colículo inferior na elaboração da analgesia pós-ictal, e na modulação de crises convulsivas tônico-clônicas. / The post-ictal analgesia is an impressive kind of antinociception, in wich the involvement of many neural systems has been demonstrated. The inferior colliculus is a brainstem structure responsible for the origin and elaboration of convulsive responses (MCCOWN e col, 1987) in the presence of audiogenic stimulus or during the treatment of supralimiar administration of GABAergic antagonists. Mesencephalic structures such as the periaqueductal gray matter, the deep layers of the superior colliculus and the central nucleus of the inferior colliculus have been implicated in convulsive processes (DE PAULIS e col., 1990; CARDOSO e col., 1994, MCCOWN e col., 1984). The stimulation of these areas, in whose neural substrates there are beta-endorphin- and leu-enkephalin-positive neurons (EICHENBERGER e col., 2002; OSAKI e col., 2003) evokes antinociceptive processes (CASTILHO e col., 1999; GEBHART & TOLEIKIS, 1978), of either opioid (NICHOLS e col., 1989) or monoaminergic COIMBRA e col., 1992; COIMBRA & BRANDÃO, 1997) nature. The aim of the present work is to investigate the involvement of the µ1-opioid receptor-mediated system in the post-ictal analgesia. The antinociceptive responses were recorded by the tail-flick test, after the pre-treatment with the specific opioid antagonist naloxonazine, administrated either by peripheral (intraperitoneally) or central (into the inferior colliculus neural network) way, in different doses. The peripheral lon-lasting (24h) but not acute (10 min) pre-treatment with naloxonazine antagonized the analgesia evoked by tonic-clonic convulsions. Microinjections of naloxonazine in the central, dorsal cortical and external cortical nuclei of inferior colliculus antagonized the analgesia induced by tonic-clonic reactions, whose effect followed a dose-response curve. Also, microinjections of naloxonazine into the inferior colliculus decrease the time of convulsions reactions. These findings suggest the involvement of µ1-opiate receptors and the neural networks of the inferior colliculus in this antinociceptive phenomenon, and, in addition, the involvement of these receptors in the modulation of tonic-clonic convulsive reactions has been suggested. In conclusion, the endogenous opioid peptides-mediated system of the neural networks of the inferior colliculus is clearly implicated in the elaboration of the post-ictal antinociception and in the modulation of tonic-clonic convulsions. In theses processes, µ1-opioid receptors of the central nucleus, as well as of the cortical dorsal and cortical external nuclei of the inferior colliculus are crucially involved.

The auditory temporal processing deficits associated with age-dependent hearing decline have been increasingly attributed to issues beyond peripheral hearing loss. Age-related hearing loss (ARHL), also known as presbycusis, is linked with changes in the expression of both excitatory and inhibitory neurotransmitters in the central auditory system. There are also age-related changes in the expression and function of the ion channels which mediate action potential firing. The slow, Ca2+ activated, K+ channels of the BK-type are essential in controlling both neurotransmitter release and neural communication via alteration of action potential durations, firing frequency, and neural adaptation. There are many subsets of this type of ion channel located throughout the body, and though it is evident that these channels are involved in cellular activation within the peripheral auditory system, little is known about their contribution to auditory processing in the brain. There is a need for further understanding of the functional involvement and mechanisms of neurotransmitter loss and how this relates to the BK channel and auditory disorders such as presbycusis and tinnitus (the perception of a phantom sound). My research focused on investigating how the downregulation of neurotransmitter production and the reductions in BK channel expression affect ARHL. I also evaluated a custom BK-channel modulating peptide as a path towards a possible therapeutic intervention for age-related hearing loss. This custom peptide is especially useful because it reduces the potential for serious side effects, due to mechanisms which best mimic natural occurring peptide systems. The initial investigation described in this dissertation measured auditory system changes in aged mice that occurred following a drug-induced increase in the availability of the inhibitory neurotransmitter GABA. This increase in GABA decreased minimum response thresholds in the auditory midbrain of aged mice, bringing them to levels seen in young adult animals. The other changes that occurred following increased GABA availability were increased acoustically driven neuronal firing rates, frequency-dependent decreases in spontaneous rates, and increases in the symmetry of the receptive fields. The return of clear and fine-tuned acoustically-evoked responses in aged mice was a major finding of this experiment. The second phase of the dissertation built on this demonstration that modulation of the aged auditory system was possible by changing neurotransmitter levels. This second portion of the study focused on how a novel potent neuropeptide (LS3), which increases the probability of the BK channel remaining in the closed conformational state, might invoke alterations in auditory-evoked responses. First, the LS3 neuropeptide was used to modify addictive behavior in the C. Elegans; followed by evaluation of in vitro changes to a human cell line. This study then confirmed that LS3 is a potent BK channel modulator with a greater affinity than those known toxins classified as high-affinity toxins. In vivo testing demonstrated that LS3 could rapidly cross the blood-brain barrier (BBB) following systemic injections, where it altered auditory evoked activity in a manner similar to that of the direct application to the dura over the midbrain. This work demonstrates that the BK channel is highly responsible for the control of auditory-evoked neurological processes, and that a potent BK channel modulator may be useful for the treatment of certain neurological disorders. The third study was designed to confirm that the BK channel plays an important role in sound-evoked activity generated in the auditory midbrain, by testing the effects of a general BK channel pore blocker, PAX. The results established that the BK channel is vital for sound processing in the midbrain of young adult mice, and is responsible for the maintenance of receptive field properties. I also evaluated the role it plays in temporal processing, which is an underlying mechanism for the processing of neurologically-relevant complex acoustic signals such as speech. Here, blocking of the channel increased (worsened) the threshold for the detection of a silent gap-in-noise and the neural recovery functions that occurred following the stimuli. The fourth study significantly expanded the in vivo testing of the custom peptide channel blocker, LS3, and added a behavioral measure of changes to auditory perception in addition to the electrophysiology recordings. The auditory-evoked receptive fields from midbrain neurons were modulated in a dose-dependent manner following the application of LS3. The neural recordings took place in the inferior colliculus, where the dorsal region responds to low-frequency sounds and ventral areas to high frequencies. The LS3-induced suppression or enhancement of evoked responses was different for the various tonotopic regions of the auditory midbrain. The improvements shown in receptive fields and improvement in auditory perception indicates a plausible route for direct translational treatment of auditory disorders through small custom peptide therapeutics. These studies provide supportive information about how auditory evoked responses in the midbrain, including the coding of different sound features, are affected by the down-regulation of a key inhibitory neurotransmitter (GABA), and how GABA-dependent neural evoked responses are altered in older mice through the modulation of BK channel activity.

No description available.

<div> <p>Half of people over 75 in the United States suffer from age related hearing loss and have trouble understanding speech in a noisy background. Even older adults who have normal pure tone audiograms can have trouble understanding speech in a noisy background. Speech is a complex sound and therefore sounds more complex than pure tones are required to understand the differences in processing noisy speech in young and aged individuals. Band passed noise is easily controlled and is more complex than pure tones making it better stimulus for testing. The first place in the ascending auditory pathway that does complex processing is the inferior colliculus. Single unit recordings from the inferior colliculus of young and aged F344 rats were preformed using half octave band passed noise and pure tones. Firing rates, first spike latencies, the number of tuning peaks, normalized peak slope, bandwidth, and Q factors were all evaluated for each unit in response to band passed noise. For 54 of the units their responses to pure tones were also collected. Out of 286 units recorded from young animals, 218 were responsive and 178 of them had a band passed response. Out of 193 aged units, 145 were responsive and 134 had a band passed response. Young units had a significantly higher total firing rate (p = 0.008) and bandwidth (p = 0.004). The normalized peak slopes and Q factors were significantly lower in young units indicating sharper tuning in the aged units. Pure tones elicited a stronger response than band passed noise however, for many units the best frequency was similar for both stimuli. These results show that aged units are less responsive to stimuli containing multiple frequencies which may help explain why older adults have trouble understanding noisy speech. </p> </div> <br>

Hearing loss of various types is increasingly plaguing our modern world (Geneva: World Health Organization 2018). As the life expectancy increased in the industrialized world, age-related hearing loss (ARHL) has become more prevalent. The wars and terrorism of the modern world also created a significant body of blast-induced hearing loss (BIHL) patients. Both types of hearing loss present significant challenges for listeners even at suprathreshold sound levels. However, increasing bodies of clinical and laboratory evidence have suggested that the difficulties in the processing of time-varying auditory features in speech and other natural sounds may not be sufficiently diagnosed by threshold changes and simple auditory electrophysiological measures (Snell and Frisina 2000; Saunders et al. 2015; Bressler et al. 2017; Guest et al. 2018).<br>Studies have emphasized that excitatory/inhibitory neurotransmission imbalance plays important roles in ARHL (Caspary et al. 2008) and may also be key in BIHL, as hinted by the strong presence of GABA regulation in non-blast TBI (O’Dell et al. 2000; Cantu et al. 2015; Guerriero et al. 2015). The current studies focus on age-related and blast-induced hearing deficits by examining changes in the processing of simple, brief stimuli and complex, sustained, temporally modulated sounds.<br>Through post hoc circular analysis of single-unit, in vivo recording of young and aged inferior colliculus (IC) neurons responding to amplitude modulation (AM) stimuli and modulation depth changes, we observed evidence of central compensation in the IC manifesting as increased sensitivity to presynaptic input, which was measured via local field potentials (LFPs). We also found decreased sensitivity to decreasing modulation depth. Age-related central gain in the IC single units, while preserving and even overcompensating for temporal phase coding in the form of vector strength, was unable to make up for the loss of envelope shape coding.<br>Through careful, longitudinal measurements of auditory evoked potential (AEP) responses towards simple sounds, AM and speech-like iterated rippled noise (IRN), we documented the development and recovery of BIHL induced by a single mild blast in a previously established (Song et al. 2015; Walls et al. 2016; Race et al. 2017) rat blast model over the course of two months. We identified crucial acute (day 1-4 post-exposure) and early recovery (day 7-14) time windows in which drastic changes in electrophysiology take place. Challenging conditions and broadband, speech-like stimuli can better elucidate mild bTBI-induced auditory deficits during the sub-acute period. The anatomical significance of the aforementioned time windows was demonstrated with immunohistochemistry methods, showing two distinct waves of GABA inhibitory transmission changes taking place in the auditory brainstem, the IC, and the auditory thalamus. These changes were in addition to axonal and oxidative damage evident in the acute phase. We examined the roles and patterns of excitatory/inhibitory imbalance in BIHL, its distinction compared to that of ARHL, and demonstrated the complexity of its electrophysiological consequences. Blast traumatizes the peripheral auditory system and auditory brainstem, evident through membrane damage and acrolein-mediated oxidative stress. These initial traumas kickstart a unique, interlocking cascade of excitatory/inhibitory imbalances along the auditory neuraxis that is more complex and individually varied than the gradual, non-traumatic degradations in ARHL. Systemic treatment with the FDA-approved acrolein scavenger Hydralazine (HZ) was attempted with limited effects.<br>Taken together, the current study provided insights into the similarities and distinctions between the mechanisms of ARHL and BIHL and called for innovative and individual diagnostic and therapeutic measures.<br>

No description available.