Biomedical subjects

Paul G Finlayson

Publications and source records attributed to Paul G Finlayson.

4 recordsLinked to original sources

PubMed2006

Differentiation of simple spike waveforms in the hamster dorsal cochlear nucleus.

The dorsal cochlear nucleus (DCN) consists of many cell types with different morphologies and properties. DCN cells belonging to different morphological classes are distinguished by differences in their physiological characteristics such as their spectral and temporal response patterns, their levels of spontaneous activity, and certain biophysical properties. Recent studies suggest that they may also exhibit different action potentials, such as simple and complex spikes. In the present study, we systematically examined the spike waveforms of spontaneously active DCN neurons using extracellular recording methods. Neurons were found to exhibit simple spikes consisting of trains of individual action potentials. Spikes fell into two discrete groups of opposite polarity, those with M-shaped and those with W-shaped waveforms. The shapes of these waveforms recorded from a given unit remained constant, despite large changes in amplitude that occurred as the electrode was moved along its axis of penetration. A quantitative analysis of the fine details of the waveforms demonstrated that, although the durations of W- and M-shaped spikes exhibited considerable variation, the variants within each category fell along a continuous gradient rather than into discrete subgroups. Both M- and W-shaped waveforms were found predominantly in the fusiform cell and deep layers, with smaller numbers found in the dorsal acoustic stria. Consideration of their depths of occurrence, their response properties, and levels of spontaneous activity of the recorded neurons suggests that W-shaped waveforms probably are associated with fusiform cells, whereas M-shaped spikes likely originate from more than one cell type.

Action Potentials↗

PubMed2005

Excitability of auditory brainstem neurons, in vivo, is increased by cyclic-AMP.

Physiological control of auditory neural responses is critical for accurate representation of acoustic information, such as sound source localization and speech perception. Central auditory neural responses are almost certainly regulated by a range of mechanisms, including second messenger systems, such as the cAMP pathway. An increase in spontaneous neural discharge is known to accompany cochlear insults. Here we report that an increase in spontaneous as well as tone-evoked discharge can also be induced by pressure application of forskolin, a pharmacological agent that elevates intracellular cAMP level by activating adenyl cyclase. The forskolin induced increase in superior olivary complex (SOC) brainstem neurons is specific, dose-dependent, and reversible, whereas application of artificial cerebrospinal fluid (aCSF, the vehicle) does not alter activity. Forskolin-application also has a relatively greater effect on spontaneous activity compared to tone evoked responses. Blockade of the hyperpolarization-activated current, Ih, by ZD7288, consistently reversed the effects of forskolin. Based on these findings, we propose that the second messenger, cAMP, can significantly modulate neural excitability and spontaneous discharge in SOC neurons, principally by shifting the activation of Ih channels.

Acoustic Stimulation↗

PubMed2003

Hyperpolarization-activated (I(h)) conductances affect brainstem auditory neuron excitability.

Blockade of the hyperpolarization-activated cyclic-nucleotide-gated mixed-cationic conductance (I(h)) by ZD7288 markedly reduces excitability of neurons in the superior olivary complex (SOC), in vivo. Following pressure ejection application of 100 microM ZD7288, extracellular recorded single unit responses of 47/47 SOC neurons to monaural or binaural pure tone best frequency (BF) stimuli (30 dB above threshold) decreased by 49.7+/-19%, and background activity decreased by 56.3+/-18.1%. Pressure ejection of the vehicle did not affect excitability. The dose- and time-dependence of ZD7288 (10-100 microM) effects are consistent with specific blockade of I(h) currents. SOC neuron responses to pressure-ejected glutamate were also decreased following application of 100 microM ZD7288 by 76.7+/-28.0%, which suggests a predominant direct effect of ZD7288 on auditory cell excitability. The considerable variability in the magnitude of ZD7288 effects between cells was only partially accounted for by greater effects on neurons with BFs greater than 16 kHz. Therefore, I(h) channels significantly contribute to auditory brainstem neuron excitability, affecting their response level to acoustic stimuli. The variability in the ZD7288 reduction in excitability and its variation with the BF of units could be an indication of regulation and plasticity in neuronal encoding of sounds.

Acoustic Stimulation↗

PubMed2002

Paired-tone stimuli reveal reductions and alterations in temporal processing in inferior colliculus neurons of aged animals.

Temporal processing deficits in the central auditory system of aged subjects are apparent from animal and human studies but could be due to peripheral hearing loss. Sequential paired tone stimuli reveal age-related changes in temporal processing properties of neurons in the central nucleus of the inferior colliculus (CIC). A greater proportion of CIC neurons exhibit suppression of excitability following pure tone stimulation in 20 month old ("aged") compared with 3-6 month "young" Long Evans rats. The duration (time constant of exponential curve fit to recovery of excitability) of suppression is also increased in aged compared with young rats, with more neurons exhibiting suppression with time constants over 100 ms. The time course of post-stimulatory suppression is not dependent on the duration or intensity of preceding stimuli and is not correlated with either initial magnitude of suppression or best frequency of IC neurons. Although the increase in unit thresholds is greater for high-frequency units in old animals, the largest post-stimulatory suppression changes occur in neurons with best frequencies of less than 10 kHz. Since the increase in duration of post-stimulatory suppression is not correlated with peripheral hearing loss, the difference is likely attributed to central auditory neuron changes in aging. In addition, the proportion of IC neurons exhibiting other temporal patterns of excitability (post-stimulatory facilitation and delayed-maximum excitability) is reduced in aged animals. Therefore, temporal processing of acoustic information is significantly altered in aged animals. The greater post-stimulatory suppression of excitability, reduced facilitation, and delayed facilitation is expected to reduce and alter the encoded information passing from the brainstem through the IC to higher structures. These changes correlate with reduced speech understanding in noise, elevated thresholds in noisy conditions, and reduced temporal processing capabilities in the elderly.

Acoustic Stimulation↗