Pentobarbital and ketamine alter the pattern of 2-deoxyglucose uptake in the central auditory system of the gerbil

NMDA Receptor Antagonist Effects on Speech-Related Mismatch Negativity and Its Underlying Oscillatory and Source Activity in Healthy Humans

Background: Previous studies in schizophrenia have consistently shown that deficits in the generation of the auditory mismatch negativity (MMN) – a pre-attentive, event-related potential (ERP) typically elicited by changes to simple sound features – are linked to N-methyl-D-aspartate (NMDA) receptor hypofunction. Concomitant with extensive language dysfunction in schizophrenia, patients also exhibit MMN deficits to changes in speech but their relationship to NMDA-mediated neurotransmission is not clear. Accordingly, our study aimed to investigate speech MMNs in healthy humans and their underlying electrophysiological mechanisms in response to NMDA antagonist treatment. We also evaluated the relationship between baseline MMN/electrocortical activity and emergent schizophrenia-like symptoms associated with NMDA receptor blockade.

Methods: In a sample of 18 healthy volunteers, a multi-feature Finnish language paradigm incorporating changes in syllables, vowels and consonant stimuli was used to assess the acute effects of the NMDA receptor antagonist ketamine and placebo on the MMN. Further, measures of underlying neural activity, including evoked theta power, theta phase locking and source-localized current density in cortical regions of interest were assessed. Subjective symptoms were assessed with the Clinician Administered Dissociative States Scale (CADSS).

Results: Participants exhibited significant ketamine-induced increases in psychosis-like symptoms and depending on temporal or frontal recording region, co-occurred with reductions in MMN generation in response to syllable frequency/intensity, vowel duration, across vowel and consonant deviants. MMN attenuation was associated with decreases in evoked theta power, theta phase locking and diminished current density in auditory and inferior frontal (language-related cortical) regions. Baseline (placebo) MMN and underlying electrophysiological features associated with the processing of changes in syllable intensity correlated with the degree of psychotomimetic response to ketamine.

Conclusion: Ketamine-induced impairments in healthy human speech MMNs and their underlying electrocortical mechanisms closely resemble those observed in schizophrenia and support a model of dysfunctional NMDA receptor-mediated neurotransmission of language processing deficits in schizophrenia.

Activation in the auditory pathway of the gerbil studied with 18F-FDG PET: effects of anesthesia

Here, we present results from an ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography (PET) study in the Mongolian gerbil, a preferred animal model in auditory research. One major issue in preclinical nuclear imaging, as well as in most of the neurophysiological methods investigating auditory processing, is the need of anesthesia. We compared the usability of two types of anesthesia which are frequently employed in electrophysiology, ketamine/xylazine (KX), and fentanyl/midazolam/medetomidine (FMM), for valid measurements of auditory activation with ¹⁸F-FDG PET. Gerbils were placed in a sound-shielding box and injected with ¹⁸F-FDG. Two acoustic free-field conditions were used: (1) baseline (no stimulation, 25 dB background noise) and (2) 90 dB frequency-modulated tones (FM). After 40 min of ¹⁸F-FDG uptake, a 30 min acquisition was performed using a small animal PET/CT system. Blood glucose levels were measured after the uptake phase before scanning. Standardized uptake value ratios for relevant regions were determined after implementing image and volume of interest templates. Scans demonstrated a significantly higher uptake in the inferior colliculus with FM stimulation compared to baseline in awake subjects (+ 12%; p = 0.02) and with FMM anesthesia (+ 13%; p = 0.0012), but not with KX anesthesia. In non-auditory brain regions, no significant difference was detected. Blood glucose levels were significantly higher under KX compared to FMM anesthesia (17.29 ± 0.42 mmol/l vs. 14.30 ± 1.91 mmol/l; p = 0.024). These results suggest that valid ¹⁸F-FDG PET measurements of auditory activation comparable to electrophysiology can be obtained from gerbils during opioid-based anesthesia due to its limited effects on interfering blood glucose levels.

Differential actions of isoflurane and ketamine-based anaesthetics on cochlear function in the mouse

Isoflurane is a volatile inhaled anaesthetic widely used in animal research, with particular utility for hearing research. Isoflurane has been shown to blunt hearing sensitivity compared with the awake state, but little is known about how isoflurane compares with other anaesthetics with regard to hair cell transduction and auditory neurotransmission. The current study was undertaken in C57Bl/6J and C129/SvEv strains of mice to determine whether isoflurane anaesthesia affects hearing function relative to ketamine-based anaesthesia. Cochlear function and central auditory transmission were assessed using auditory brainstem response (ABR) and distortion product otoacoustic emission (DPOAE), comparing thresholds and input/output functions over time, for isoflurane vs. ketamine/xylazine/acepromazine anaesthesia. ABR thresholds at the most sensitive region of hearing (16 kHz) were initially higher under isoflurane anaesthesia. This reduced hearing sensitivity worsened over the 1 h study period, and also became evident with broadband click stimulus. Ketamine anaesthesia provided stable ABR thresholds. Although the growth functions were unchanged over time for both anaesthetics, the slopes under isoflurane anaesthesia were significantly less. Cubic (2f(1)-f(2)) DPOAE thresholds and growth functions were initially similar for both anaesthetics. After 60 min, DPOAE thresholds increased for both groups, but this effect was significantly greater with ketamine anaesthesia. The isoflurane-mediated increase in ABR thresholds over time is attributable to action on cochlear nerve activation, evident as a right-shift in the P1-N1 input/output function compared to K/X/A. The ketamine-based anaesthetic produced stable ABR thresholds and gain over time, despite a right-shift in the outer hair cell - mediated DPOAE input/output function.

Unilateral electrical stimulation of the inferior colliculus of rats modifies the prepulse modulation of the startle response (PPI): effects of ketamine and diazepam

The magnitude of an acoustic startle response can be reduced by a weak stimulus presented immediately before the startle-eliciting noise. This phenomenon has been termed prepulse inhibition of the startle reaction (PPI). Previous studies indicated that the primary neural pathways mediating PPI belong to the brain stem and that the inferior colliculus (IC) was crucial. Its destruction reduced PPI. Stimulations applied to brain areas may be as deleterious as lesions. Therefore, we looked for the possibility of a brain stimulation applied to the IC during a PPI test to reduce also PPI. Rats were implanted with chronic electrodes, their tips being aimed at the IC. They were located within or close to the inter-colliculus nucleus. A train of stimulations was applied and PPI was tested alternately during and between periods of stimulation. As the most common method used to attenuate PPI consists in administrating drugs, for example ketamine, we also tested the effect of this drug. Another drug was also tested, diazepam, since it alters the functioning of the IC without any known effect on PPI. This allowed a comparative analysis of the neurobiological and the pharmacological effects. It appeared that the stimulation decreased PPI quantitatively as much as ketamine (6 mg/kg) without an effect of the basic startle reaction. These effects did not interfere with each other. Diazepam (1 mg/kg) did not modify PPI, neither under stimulation nor per se. Only for a very high dose (4 mg/kg), a sedative and myo-relaxant one the basic startle and PPI were altered.

Tinnitus retraining therapy for patients with tinnitus and decreased sound tolerance

Our experience has revealed the following: (1) TRT is applicable for all types of tinnitus, as well as for decreased sound tolerance, with significant improvement of tinnitus occurring in over 80% of the cases, and at least equal success rate for decreased sound tolerance. (2) TRT can provide cure for decreased sound tolerance. (3) TRT does not require frequent clinic visits and has no side effects; however, (4) Special training of health providers involved in this treatment is required for this treatment to be effective.

Three distinct auditory areas of cortex (AI, AII, and AAF) defined by optical imaging of intrinsic signals

Using pure-tone sound stimulation, three separate auditory areas are revealed by optical imaging of intrinsic signals in the temporal cortex of the chinchilla (Chinchilla laniger). These areas correlate with primary auditory cortex (AI) and two secondary areas, AII and the anterior auditory field (AAF). We have distinguished AI on the basis of concurrent single-unit electrophysiological recording; neurons within the AI intrinsic signal region have short (<15 ms) onset-response latencies compared with neurons recorded in AII and the AAF. Within AI, AII, and AAF we have been able to define cochleotopic or tonotopic organization from the differences in intrinsic signal areas evoked by pure tones at octave-spaced frequencies from 500 Hz to 16 kHz. The maps in AI and AII are arranged orthogonal to each other.

Reorganization in the auditory cortex of the rat induced by intracortical microstimulation: a multiple single-unit study

Many manipulations are able to change or perturb various aspects of single neuron properties and interneuronal relationships. Changes of cerebral cortex organization have been observed in different cortical areas and at different time scales in relation to peripheral stimulation, peripheral damage, associative learning, and electrical stimulation. Here we describe studies on separable multineuron recordings in the rat's auditory cortex under two different anesthetics. Acoustic stimuli were used as a normal, physiological input, and weak electrical intracortical microstimulation (ICMS) as a perturbation that forces a rapid cortical reorganization. ICMS induced fast changes in the cortical map and in the receptive field properties of cells at the electrically stimulated and adjacent electrodes. In effect there was an enlargement of the cortical domain tuned to the acoustic frequency that had been represented at the stimulating electrode. ICMS also incremented afterdischarge responses; these consisted of an initial response to the auditory stimulus followed by less intense repetitive activity that was stimulus-time locked and had a period of 8-12 Hz, similar to that of the spontaneous synchronous activity. Cortical activity under ketamine differed from that under pentobarbital sodium, although in both situations we observed that cortical neurons were highly synchronous.

Moderate hypoglycemia impairs the function of the inferior colliculus in the awake rat

To assess the effect of acute hypoglycemia on the brain stem auditory pathway, we implanted electrodes into the inferior colliculus (IC) and subsequently recorded auditory-evoked potentials from the IC (ICEP) and brain stem (BAEP) in normal awake rats during euglycemic and hypoglycemic hyperinsulinemia (2 hours). Latencies of the ICEP and peak V of the BAEP were significantly prolonged by hypoglycemia (approximately 2.7 mmol/L). The change in the BAEP was principally between peak III and peak V. suggesting an effect in or near the IC. ICEP and BAEP latencies did not change during euglycemic hyperinsulinemia. We conclude that the function of the IC is very sensitive to episodes of moderate hypoglycemia. These data provide direct evidence that the scope of adverse central nervous system effects resulting from hypoglycemia extends beyond cognitive centers to include the brain stem.

Changes of single unit activity in the cat's auditory thalamus and cortex associated to different anesthetic conditions

Single unit spike trains were recorded in the auditory cortex (n = 78) and in the auditory thalamus (n = 55) of nitrous oxide anesthetized cats. The electrophysiological activity was studied before and during the application of pentobarbital (P, 7 mg/kg), ketamine (K, 12 mg/kg) and a mixture of these anesthetics (KP). The units were characterized during the spontaneous and acoustically driven activity ('white' noise and pure tone bursts). For the majority of cortical (61%) and thalamic (83%) units both drugs tended to decrease the spontaneous firing rate, but affected differently its time structure: P tended to increase the average size of burst discharges, whereas K and KP tended to decrease it. In the cortex the peak firing rate evoked by 'white' noise tended to be decreased, whereas stronger excitatory responses were observed in the thalamus after injection of K or KP. The overall effect of the anesthetics during stimulation by pure tones was an increase in tonal selectivity due to a decrease in the response bandwidth. The response pattern to tones was also sometimes affected by the drugs. The direct evidence reported here for significant alterations of the discharge properties of auditory neurons in the thalamus and cortex resulting from low dose administration of K and/or P emphasizes difficulties in comparing data derived from experiments conducted in various conditions of anesthesia or in the awake state.

Rapid increase in mitochondrial volume in nucleus magnocellularis neurons following cochlea removal

Second-order auditory neurons in nucleus magnocellularis (NM) of the chick brainstem undergo a series of rapid metabolic changes following unilateral cochlea removal, culminating in the death of 25% of NM neurons. Within hours of cochlea removal, ipsilateral NM neurons show marked increases in histochemical staining for the mitochondrial enzymes succinate dehydrogenase and cytochrome oxidase. We investigated corresponding ultrastructural changes in NM neurons by preparing animals undergoing unilateral cochlea removal for transmission electron microscopy. We quantified changes in NM mitochondrial volume by stereological methods and qualitatively compared mitochondrial morphology between NM neurons destined to survive and those destined to die after cochlea removal. Within hours of cochlea removal, ipsilateral NM neurons show striking increases in mitochondrial volume (84% at 6 hours and 236% at 12 hours after cochlea removal compared to unoperated, control animals). At 2 week survival times, ipsilateral NM neurons contain fewer mitochondria than contralateral neurons. Surprisingly, anesthesia alone causes short-term increases in NM mitochondrial volume. Animals anesthetized with pentobarbital and ketamine and sacrificed 6 or 12 hours later showed a 45% increase in mitochondrial volume compared to previously unanesthetized animals. NM neurons destined to die within days of cochlea removal can be identified within several hours after deafferentation by the appearance of their ribosomes. We observed qualitative differences in mitochondrial morphology in dying neurons. Mitochondria in neurons destined to die consistently showed mitochondrial swelling and vacuolization indicative of metabolic dysfunction. Similar mitochondrial changes have been reported when mitochondria take up excess calcium. Ultrastructural changes in NM after cochlea removal display features of both programmed and pathological cell death, in which increased intracellular calcium is thought to play a role.

Metabolic activity of the central auditory structures following prolonged deafferentation

The goal of this work was to evaluate, using autoradiographic techniques, the effects of variable periods of deafness on resting and evoked metabolic activity in central auditory structures elicited by direct electrical cochlear nucleus (CN) stimulation. Thirty-five pigmented guinea pigs, divided into five groups, underwent acute implantation of bipolar electrodes in the CN. One group was not deafened and served as hearing controls. The other four groups were deafened using an established protocol of sequential kanamycin/ethacrynic acid treatment and were tested at 4 weeks, 9 weeks, 16 weeks, and 15 months after deafening. Threshold currents for eliciting evoked middle latency responses (EMLRs) with direct CN stimulation were not significantly different between hearing and deafened groups. Autoradiographic data showed progressive reduction of the evoked metabolic response with incremental periods of deafferentation. Nevertheless, central auditory structures remained responsive to direct electrical stimulation of the CN. These data indicate that direct CN stimulation remains capable of activating the auditory tract despite prolonged periods of deafness.

Direct electrical stimulation of the cochlear nucleus: surface vs. penetrating stimulation

Prosthetic stimulation of the cochlear nucleus (CN) has been used for rehabilitation of profoundly deaf patients who are not suitable candidates for cochlear implants. The goal of this article was to assess the relative effectiveness of surface vs. penetrating stimulation of the CN. Electrophysiologic and autoradiographic measures were used to study central auditory system activation elicited by direct stimulation of the CN. Eighteen pigmented guinea pigs, divided into three groups, underwent acute implantation of bipolar electrodes in the CN. One group was not stimulated and acted as a control (n = 7). Electrodes were placed on the surface of the CN in one test group (n = 4) and within the CN in a second test group (n = 7). Thresholds for electrically evoked middle latency responses (EMLR) were determined and input/output (I/O) functions were obtained. The two test groups were then pulsed with [14C]-2-Deoxyglucose (2-DG) intramuscularly and stimulated for 1 hour with biphasic; charge-balanced pulses having a total duration of 400 microseconds, a repetition rate of 100/sec, and an amplitude of 200 microA. After stimulation, animals were killed and brains were harvested and prepared for autoradiography using standard techniques. Threshold current for EMLRs in the surface-stimulated group had a mean of 67.5 +/- 23.9 microA (range, 40 to 100 microA). Thresholds for in-depth stimulated group had a mean of 11.4 +/- 3.5 microA (range, 10 to 20 microA). The saturation level of the I/O function for the surface-stimulated group had a mean of 287.5 +/- 41.5 microA (range, 250 to 350 microA). The saturation level for the in-depth stimulated group had a mean of 192.9 +/- 49.5 mciroA (range, 100 to 250 microA). The dynamic range for the surface electrodes had a mean of 13.1 +/- 2.7 dB (range, 9.9 to 15.9 dB), whereas the dynamic range for the penetrating electrodes had a mean of 24.5 +/- 2.6 dB (range, 20 to 28.0 dB). Autoradiographs generated by CNS tissue from stimulated animals demonstrated no significant difference in metabolic activity of the CN between surface and in-depth stimulated groups. However, there were highly significant differences in 2-DG uptake in the contralateral superior olivary complex, contralateral inferior colliculus, and ipsilateral and contralateral lateral lemniscus, with greater uptake in in-depth stimulated preparations. Electrophysiologic and autoradiographic data suggest that a penetrating CN prosthesis is capable of activating the auditory tract at a lower threshold, with a relatively wider dynamic range than a surface prosthesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of opioid be drugs on auditory evoked potentials suggest a role of lateral olivocochlear dynorphins in auditory function

Multiple gene products of opioid peptide families (e.g., enkephalins, dynorphins) with differing opioid receptor specificities are present within olivocochlear efferent terminals. Enkephalins activate mu- and delta-opioid receptors, and are generally inhibitory in the nervous system, and dynorphins are kappa-receptor agonists, which may be excitatory to postsynaptic neurons. We have examined the effects of intravenously administered opioid agonists and antagonists on click-evoked N1 and N2 amplitudes and latencies of the compound action potential in the chinchilla recorded at the round window. Parenteral administration of the opioid receptor antagonist naloxone or the potent mu-receptor agonist fentanyl did not alter N1 and N2 amplitudes or latencies. The kappa-receptor agonist, mu-receptor antagonist pentazocine caused marked increases in N1 and N2 amplitudes over baseline values at threshold intensities. These effects were not abolished by naloxone. No effects were seen on the cochlear microphonic, supporting a site of action of these effects at the lateral olivocochlear efferent terminals on auditory nerve dendrites under inner hair cells. Similar results were obtained when far field auditory evoked responses were recorded. Results were obtained under ketamine/pentobarbital anesthesia, which provided stable recording baselines in contrast to tiletamine/zolezepam/pentobarbital, with which an upward drift in auditory potentials was observed. This stimulatory action of kappa-agonists on auditory-evoked potential amplitudes appears to represent a physiological role of the lateral olivocochlear efferent innervation. The different neurotransmitters of the olivocochlear efferents (e.g. enkephalins, dynorphins, acetylcholine) may have antagonistic actions on auditory potentials, as may the lateral and medial systems themselves.

Spatial distribution of neural activity evoked by electrical stimulation of the cochlea

Activity in the central auditory system was mapped with 2-deoxyglucose (2-DG) autoradiography, using either pure tones or electrical stimulation of the normal cochlea. Electrical stimulation with both monopolar (distant reference electrode) and bipolar prostheses near threshold increased 2-DG uptake in auditory nuclei in a manner similar to that seen with a pure tone: increased 2-DG uptake was restricted to a small frequency region of brainstem and mid-brain auditory nuclei. The position of this area was related to the cochlear location of the prosthesis. At higher current amplitudes only the bipolar prosthesis retained spatial restriction of evoked neural activity, while stimulation through a monopolar prosthesis produced evoked activity in all frequency regions of auditory nuclei, and in non-auditory nuclei. Activation of non-auditory structures was consistent with spread of current through the brainstem, rather than activation of peripheral nerves. At all current amplitudes, a monopolar prosthesis evoked higher levels of 2-DG uptake than a bipolar prosthesis. The results suggest that while a bipolar prosthesis provides greater spatial restriction of evoked neural activity and a greater dynamic range, a monopolar prosthesis produces higher levels of evoked activity.

Non-N-methyl-D-aspartate receptors may mediate ipsilateral excitation at lateral superior olivary synapses

Principal cells of the lateral superior olivary nucleus (LSO) are thought to receive a direct excitatory input from spherical bushy cells located in the ipsilateral ventral cochlear nucleus (VCN) and an indirect input from the contralateral VCN globular bushy cells via a secure synapse in the medial nucleus of the trapezoid body (MNTB). MNTB bushy cells project to the somata and proximal dendrites of LSO principal cells. LSO neurons display phasic 'chopper' temporal response patterns to ipsilateral tone-burst stimuli at characteristic frequency (CF), while binaural stimuli suppress this ipsilaterally evoked activity. This suppression is sensitive to interaural differences in intensity, phase and time, suggesting a role for these neurons in the localization of sound in space. In the present study, the nature of the neurotransmitter mediating fast ipsilateral excitation of LSO neurons was examined using iontophoretic application of excitant amino acid (EAA) agonists and antagonists. N-methyl-D-aspartate (NMDA) and quisqualate (QUIS) were used as agonists, while the selective NMDA receptor antagonist D. L-2-amino-5-phosphonovaleric acid (APV), and the non-selective receptor EAA antagonist cis-2,3-piperidine-dicarboxylic acid (PDA) were used to study ipsilaterally evoked neuronal responses. In 3 additional experiments the selective non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) replaced PDA. Ipsilateral, tone-evoked and spontaneous activities were generally enhanced by EAA agonists while partial blockade of tone-evoked, ipsilateral excitation was observed with EAA antagonists. Both PDA and DNQX more effectively blocked ipsilateral tone-evoked excitations and spontaneous activity than did the NMDA-receptor antagonist, APV.(ABSTRACT TRUNCATED AT 250 WORDS)