Journal of neurophysiology
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The medial-olivocochlear (MOC) acoustic reflex is thought to provide frequency-specific feedback that adjusts the gain of cochlear amplification, but little is known about how frequency specific the reflex actually is. We measured human MOC tuning through changes in stimulus frequency otoacoustic emissions (SFOAEs) from 40-dB-SPL tones at probe frequencies (f(p)s) near 0.5, 1.0, and 4.0 kHz. MOC activity was elicited by 60-dB-SPL ipsilateral, contralateral, or bilateral tones or half-octave noise bands, with elicitor frequency (f(e)) varied in half-octave steps. ⋯ Ipsilateral and contralateral MOC reflexes often showed dramatic differences in plots of MOC effect vs. elicitor frequency, indicating that the contralateral reflex does not give an accurate picture of ipsilateral-reflex properties. These differences in MOC effects appear to imply that ipsilateral and contralateral reflexes have different actions in the cochlea. The implication of these results for MOC function, cochlear mechanics, and the production of SFOAEs are discussed.
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Comparative Study
Kinetics of GABAB autoreceptor-mediated suppression of GABA release in rat insular cortex.
Release of GABA is controlled by presynaptic GABA receptor type B (GABA(B)) autoreceptors at GABAergic terminals. However, there is no direct evidence that GABA(B) autoreceptors are activated by GABA release from their own terminals, and precise profiles of GABA(B) autoreceptor-mediated suppression of GABA release remain unknown. To explore these issues, we performed multiple whole-cell, patch-clamp recordings from layer V rat insular cortex. ⋯ Paired-pulse stimulation (interstimulus interval = 150 ms) of presynaptic FS cells revealed that the second uIPSC was also facilitated by CGP 52432, which had little effect on the amplitude and interevent interval of miniature IPSCs. In contrast, uEPSCs, responding to all five stimulations of a presynaptic pyramidal cell, were less affected by CGP 52432. These results suggest that a single presynaptic action potential is sufficient to activate GABA(B) autoreceptors and to suppress GABA release in the cerebral cortex.
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The cellular proteins that underlie mechanosensation remain largely enigmatic in mammalian systems. Mechanically sensitive ion channels are thought to distinguish pressure, stretch, and other types of tactile signals in skin. Transient receptor potential canonical 1 (TRPC1) is a candidate mechanically sensitive channel that is expressed in primary afferent sensory neurons. ⋯ Additionally, we used repeated light, presumably innocuous punctate stimuli with a low threshold von Frey filament (0.68 mN). In agreement with our electrophysiological data in light-touch afferents, TRPC1-deficient mice exhibited nearly a 50% decrease in behavioral responses to both the light-stroke and light punctate mechanical assays when compared with wild-type controls. In contrast, TRPC1-deficient mice exhibited normal paw withdrawal response to more intense mechanical stimuli that are typically considered measures of nociceptive behavior.
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Paired-pulse transcranial magnetic stimulation (ppTMS) is a safe and noninvasive tool for measuring cortical inhibition in humans, particularly in patients with disorders of cortical inhibition such as epilepsy. However, ppTMS protocols in rodent disease models, where mechanistic insight into the ppTMS physiology and into disease processes may be obtained, have been limited due to the requirement for anesthesia and needle electromyography. To eliminate the confounding factor of anesthesia and to approximate human ppTMS protocols in awake rats, we adapted the mechanomyogram (MMG) method to investigate the ppTMS inhibitory phenomenon in awake rats and then applied differential pharmacology to test the hypothesis that long-interval cortical inhibition is mediated by the GABA(A) receptor. ⋯ With pharmacological testing, time course observations showed that ppTMS-MMG inhibition was acutely reduced following PTZ (P < 0.05), acutely enhanced after PB (P < 0.01) injection, and then recovered to pretreatment baseline after 1 h. Our data support the application of the ppTMS-MMG technique for measuring the cortical excitability in awake rats and provide the evidence that GABA(A) receptor contributes to long-interval paired-pulse cortical inhibition. Thus ppTMS-MMG appears a well-tolerated biomarker for measuring GABA(A)-mediated cortical inhibition in rats.
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The amygdala plays a central role in evaluating the significance of acoustic signals and coordinating the appropriate behavioral responses. To understand how amygdalar responses modulate auditory processing and drive emotional expression, we assessed how neurons respond to and encode information that is carried within complex acoustic stimuli. We characterized responses of single neurons in the lateral nucleus of the amygdala to social vocalizations and synthetic acoustic stimuli in awake big brown bats. ⋯ In most neurons, variation in response duration depended, in part, on persistent excitatory discharge that extended beyond stimulus duration. Information in persistent firing duration was significantly greater than in spike rate, and the majority of neurons displayed more information in persistent firing, which was more likely to be observed in response to aggressive vocalizations (64%) than appeasement vocalizations (25%), suggesting that persistent firing may relate to the behavioral context of vocalizations. These findings suggest that the amygdala uses a novel coding strategy for discriminating among vocalizations and underscore the importance of persistent firing in the general functioning of the amygdala.