Browsing by Subject "Midbrain"

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Electrophysiological measurement of temporal integration in listeners with normal hearing
(2016-05) Chen, Yu-Fu; Champlin, Craig A.; Liu, Chang; Sussman, Harvey M.; Whittaker, Tiffany A.
Temporal integration refers to the phenomenon whereby the detection threshold of the stimulus decreases (improves) as the signal duration increases. The majority of studies of temporal integration have relied on behavioral methods. As a result temporal integration can be influenced by the subject’s physical and psychological status and these factors may affect signal detection. In the present study, the measurement of the auditory steady-state response (ASSR) was used to investigate temporal integration in listeners with normal hearing. The stimuli were sinusoidally amplitude modulated (SAM) signals varying in the modulation frequency (40 Hz and 80 Hz) and duration (50 ms, 100 ms, 200 ms, 300 ms, 400 ms, and 800 ms). The carrier was 1-kHz tone, 4-kHz tone, or white noise. The ASSRs were analyzed across different stimulus conditions in terms of amplitude, phase, signal-to-noise ratio (SNR), and percentage of detected responses. The results showed that temporal integration was more clearly revealed when the ASSR was recorded with the 40-Hz modulation frequency as compared to 80-Hz modulation frequency. For 40-Hz modulation frequency, the amplitude of the ASSR increased over the first 200 milliseconds after the stimulus onset until reaching a steady-state plateau, and then dropped rapidly after the stimulus offset. Conversely, the phase strength (i.e., variability) decreased over the first 400 milliseconds and remained relatively stable after that. For 80-Hz modulation frequency, the ASSR amplitude did not increase until approximately 200 milliseconds, beyond which the ASSR amplitude increased at the same rate as for 40-Hz modulation frequency. In addition, the ASSR phase was less stable across subjects, which suggests weaker responses overall. An exponential model fit the electrophysiological data best; however, a significant frequency effect on the time constant was not observed. These results suggest that both auditory midbrain and brainstem are able to integrate auditory information over the first 200 milliseconds of stimulus.
Mechanisms in ethanol modulation of GABA release onto dopaminergic neurons of the ventral tegmental area
(2009-12) Theile, Jonathan William; Morrisett, Richard A.; Gonzales, Rueben A.; Morikawa, Hitoshi; Mayfield, R. Dayne (Roy Dayne), 1958-; Mihic, S. J.
Activation of ventral tegmental area (VTA) dopaminergic (DA) neurons by ethanol has been implicated in the rewarding and reinforcing actions of ethanol. GABAergic transmission is thought to play an important role in regulating the activity of DA neurons. While at most central synapses ethanol generally increases inhibitory synaptic transmission, no studies have explored the effect of acute ethanol on GABAergic transmission in the VTA. Here we investigated how ethanol modulates GABAergic transmission in the VTA in relation to the overall action of ethanol on VTA-DA neuron activity. We demonstrated that ethanol dose-dependently enhances action potential-dependent and -independent GABA release onto VTA-DA neurons. Utilizing whole-cell voltage clamp recording techniques, ethanol increased both spontaneous and miniature inhibitory postsynaptic current (s/mIPSC) frequency while having minimal effect on s/mIPSC amplitude. The ethanol enhancement in GABA release was independent of GABAB auto-receptor inhibition of release. Intra-terminal calcium levels regulate neurotransmitter release, thus we investigated how modulation of calcium levels would affect the ethanol-enhancement in GABA release. Ethanol enhanced mIPSC frequency in the presence of the voltage-gated calcium channel blockers, cadmium chloride and nicardipine. However, blockade of intracellular calcium stores with 2-APB and cyclopiazonic acid eliminated the ethanol-enhancement of mIPSC frequency. Intracellular calcium stores are regulated via Gq protein-coupled receptors such as the 5-HT2C receptor. 5-HT2C receptor activation robustly enhanced mIPSC frequency whereas blockade inhibited the ethanol-enhancement in mIPSC frequency. These observations suggest that increased calcium release from intracellular stores via 5-HT2C receptor activation is involved in the ethanol-enhancement of GABA release onto VTA-DA neurons. Utilizing cell-attached current-clamp recordings, we demonstrated that the ethanol-enhancement of VTA-DA neuron activity is modulated by the concurrent enhancement in GABA release. Blockade and activation of GABAA receptors enhanced and reversed, respectively, the stimulatory effect of ethanol on VTA-DA neurons. Mu-opioid receptors (MORs) on GABAergic interneurons have been demonstrated to modulate both basal and ethanol-enhanced VTA-DA activity in vivo, though we failed to demonstrate such an effect in vitro. Overall, the results of this study suggest that the 5-HT2C receptor and intra-terminal calcium-dependent ethanol enhancement in GABA release acts to regulate the overall stimulatory effect of ethanol on VTA-DA activity.
The role of bone morphogenetic proteins in the development of the vertebrate midbrain
(2010-12) Eom, Dae Seok; Agarwala, Seema; Wallingford, John; Gross, Jeffrey; Thompson, Wesley; De Lozanne, Arturo
The purpose of the thesis is to explore the role of BMP signaling in developing vertebrate midbrain. BMP signaling plays important roles in various tissues and stages of neural development to regulate cell fate, proliferation, differentiation, morphogenesis and more. We observed that several major BMPs are expressed not only at the roof plate but also the floor plate of the midbrain. This has led us to ask the role of BMP signaling in dorsal and ventral midbrain patterning. Despite ventral experiments, we found that BMP signaling does not regulate ventral cell fate specification in the midbrain. Instead BMPs profoundly influence the shape and early morphogenesis of the midbrain neural plate as it closes into a neural tube. During neural tube closure, one of the early events occurring at the ventral midline is median hinge point (MHP) formation. Failure to form MHP leads to neural tube closure defects, the 2nd most common birth defects in humans. However, the molecular mechanisms underlying MHP formation are not well known. We found that the lowest BMP signaling occurs at the MHP during early neurulation and BMP blockade is necessary and sufficient for MHP formation. Interestingly, we also demonstrated that BMP blockade directs MHP formation by regulating the apicobasal polarity pathway and this regulation may be mediated by biochemical interactions between pSMAD5 and the apical protein, PAR3. Additionally, our time-lapse data suggest that BMP blockade slows cell cycle progression by increasing duration of G1 to S transition and S phase which leads cell nuclei stay at the basal location longer. This mimics basal nuclear migration seen at the MHP where low BMP signaling occurs. Thus, we conclude that BMP signaling regulates neural tube closure via the apicobasal polarity pathway and in a cell cycle dependent manner at the ventral midline. We observed that BMP signaling is necessary and sufficient for the dorsal cell fate specification in a context-dependent manner and ventral BMP signaling affects dorsal cell fates. Taken together, we propose the idea that BMP signaling has distinct roles in different contexts. BMPs regulate tissue morphogenesis in the ventral midbrain and dorsally cell fate specification.
Understanding the mechanisms of floor plate specification in the vertebrate midbrain and its functions during development
(2009-08) Bayly, Roy Downer, 1981-; Agarwala, Seema
We have previously shown that the arcuate organization of cell fates within the ventral midbrain critically depends upon the morphogen, Sonic Hedgehog (SHH), which is secreted from a signaling center located along the ventral midline, called the floor plate (FP). Thus, it is ultimately the specification of the FP that is responsible for the patterning and specification of ventral midbrain cell fates. Interestingly, we have found that the chick midbrain FP can be divided into medial (MFP) and lateral (LFP) regions on the basis of gene expression, mode of induction and function. Overexpression of SHH alone is sufficient to recapitulate the entire pattern of ventral cell fates, although remarkably it cannot induce MFP, consistent with the observation that the MFP is refractory to any perturbations of HH signaling. In contrast, overexpression of the winged-helix transcription factor FOXA2/HNF3[beta]robustly induced the MFP fate throughout ventral midbrain while blocking its activity resulted in the absence of the MFP. Thus, by analyzing the differences between SHH and FOXA2 blockade and overexpression, we were able to attribute functions to each the LFP and the MFP. Notably, we observed that FOXA2 overexpression caused a bending of the midbrain neurepithelium that resembled the endogenous median hinge-point observed during neurulation. Additionally, FOXA2 misexpression led to a robust induction of DA progenitors and neurons that was never observed after SHH expression alone. In contrast, we found that all other ventral cell types required HH signaling directly, at a distance and early on in the development of the midbrain when its tissue size is relatively small. Additionally, HH blockade resulted in increased cell-scatter of the arcuate territories and in the disruption of the regional boundaries between the ventral midbrain and adjacent tissue. Thus, we bring new insight into the mechanism by which midbrain FP is specified and ascribe functional roles to its subregions. We propose that while the MFP regulates the production of dopaminergic progenitors and the changes in cellshape required for bending and shaping the neural tube, the LFP appears to be largely responsible for cell survival and the formation of a spatially coherent pattern of midbrain cell fates.