A middle-latency auditory-evoked potential in the rat

Cholinergic modulation of auditory steady-state response in the auditory cortex of the freely moving rat

As disturbance in auditory steady-state response (ASSR) has been consistently found in many neuropsychiatric disorders, such as autism spectrum disorder and schizophrenia, there is considerable interest in the development of translational rat models to elucidate the underlying neural and neurochemical mechanisms involved in ASSR. This is the first study to investigate the effects of the non-selective muscarinic antagonist scopolamine and the cholinesterase inhibitor donepezil (also in combination with scopolamine) on ASSR. We recorded the local field potentials through the chronic microelectrodes implanted in the auditory cortex of freely moving rat. ASSRs were recorded in response to auditory stimuli delivered over a range of frequencies (10-80 Hz) and averaged over 60 trials. We found that a single dose of scopolamine produced a temporal attenuation in response to auditory stimuli; the most attenuation occurred at 40 Hz. Time-frequency analysis revealed deficits in both power and phase-locking to 40 Hz. Donepezil augmented 40-Hz steady-state power and phase-locking. Scopolamine combined with donepezil had an enhanced effect on the phase-locking, but not power of ASSR. These changes induced by cholinergic drugs suggest an involvement of muscarinic neurotransmission in auditory processing and provide a rodent model investigating the neurochemical mechanism of neurophysiological deficits seen in patients.

Subcortical evoked activity and motor enhancement in Parkinson's disease

Enhancements in motor performance have been demonstrated in response to intense stimuli both in healthy subjects and in the form of 'paradoxical kinesis' in patients with Parkinson's disease. Here we identify a mid-latency evoked potential in local field potential recordings from the region of the subthalamic nucleus, which scales in amplitude with both the intensity of the stimulus delivered and corresponding enhancements in biomechanical measures of maximal handgrips, independent of the dopaminergic state of our subjects with Parkinson's disease. Recordings of a similar evoked potential in the related pedunculopontine nucleus - a key component of the reticular activating system - provide support for this neural signature in the subthalmic nucleus being a novel correlate of ascending arousal, propagated from the reticular activating system to exert an 'energizing' influence on motor circuitry. Future manipulation of this system linking arousal and motor performance may provide a novel approach for the non-dopaminergic enhancement of motor performance in patients with hypokinetic disorders such as Parkinson's disease.

Arousal, motor control, and parkinson's disease

This review highlights the most important discovery in the reticular activating system (RAS) in the last 10 years, the manifestation of gamma (γ) band activity in cells of the RAS, especially in the pedunculopontine nucleus (PPN), which is in charge of the high frequency states of waking and rapid eye movement sleep. This discovery is critical to understanding the modulation of movement by the RAS and how it sets the background over which we generate voluntary and triggered movements. The presence of γ band activity in the RAS is proposed to participate in the process of preconscious awareness, and provide the essential stream of information for the formulation of many of our actions. Early findings using stimulation of this region to induce arousal, and also to elicit stepping, are placed in this context. This finding also helps explain the novel use of PPN deep brain stimulation for the treatment of Parkinson's disease, although considerable work remains to be done.

State-dependent changes in auditory sensory gating in different cortical areas in rats

Sensory gating is a process in which the brain’s response to a repetitive stimulus is attenuated; it is thought to contribute to information processing by enabling organisms to filter extraneous sensory inputs from the environment. To date, sensory gating has typically been used to determine whether brain function is impaired, such as in individuals with schizophrenia or addiction. In healthy subjects, sensory gating is sensitive to a subject’s behavioral state, such as acute stress and attention. The cortical response to sensory stimulation significantly decreases during sleep; however, information processing continues throughout sleep, and an auditory evoked potential (AEP) can be elicited by sound. It is not known whether sensory gating changes during sleep. Sleep is a non-uniform process in the whole brain with regional differences in neural activities. Thus, another question arises concerning whether sensory gating changes are uniform in different brain areas from waking to sleep. To address these questions, we used the sound stimuli of a Conditioning-testing paradigm to examine sensory gating during waking, rapid eye movement (REM) sleep and Non-REM (NREM) sleep in different cortical areas in rats. We demonstrated the following: 1. Auditory sensory gating was affected by vigilant states in the frontal and parietal areas but not in the occipital areas. 2. Auditory sensory gating decreased in NREM sleep but not REM sleep from waking in the frontal and parietal areas. 3. The decreased sensory gating in the frontal and parietal areas during NREM sleep was the result of a significant increase in the test sound amplitude.

Cholinergic gating of hippocampal auditory evoked potentials in freely moving rats

As perturbations in auditory filtering appear to be a candidate trait marker of schizophrenia, there has been considerable interest in the development of translational rat models to elucidate the underlying neural and neurochemical mechanisms involved in sensory gating. This is the first study to investigate the effects of the non-selective muscarinic antagonist scopolamine, the muscarinic M1 antagonist biperiden and the cholinesterase inhibitor donepezil (also in combination with scopolamine and biperiden) on auditory evoked potentials (AEPs) and sensory gating. In the saline condition, only the N50 peak displayed sensory gating. Scopolamine and biperiden both disrupted sensory gating by increasing N50 amplitude for the S2 click. Donepezil was able to fully reverse the effects of biperiden on N50 sensory gating, but had residual effects when combined with scopolamine; i.e., it enhanced sensory gating by increasing N50 amplitude of the S1 stimulus. Donepezil by itself improved sensory gating by enhancing N50 amplitude of S1, and reducing N50 amplitude of the S2 click. In conclusion, due to its relatively more selective effects biperiden is to be preferred over scopolamine as a means for pharmacologically inducing cholinergic impairments in auditory processing in healthy rats. Changes in auditory processing and sensory gating induced by cholinergic drugs may serve as a translational model for aging instead of schizophrenia.

State-dependent changes in cortical gain control as measured by auditory evoked responses to varying intensity stimuli

STUDY OBJECTIVES

Auditory evoked potential (AEP) components correspond to sequential activation of brain structures within the auditory pathway and reveal neural activity during sensory processing. To investigate state-dependent modulation of stimulus intensity response profiles within different brain structures, we assessed AEP components across both stimulus intensity and state.

DESIGN

We implanted adult female Sprague-Dawley rats (N = 6) with electrodes to measure EEG, EKG, and EMG. Intermittent auditory stimuli (6-12 s) varying from 50 to 75 dBa were delivered over a 24-h period. Data were parsed into 2-s epochs and scored for wake/sleep state.

RESULTS

All AEP components increased in amplitude with increased stimulus intensity during wake. During quiet sleep, however, only the early latency response (ELR) showed this relationship, while the middle latency response (MLR) increased at the highest 75 dBa intensity, and the late latency response (LLR) showed no significant change across the stimulus intensities tested. During rapid eye movement sleep (REM), both ELR and LLR increased, similar to wake, but MLR was severely attenuated.

CONCLUSIONS

Stimulation intensity and the corresponding AEP response profile were dependent on both brain structure and sleep state. Lower brain structures maintained stimulus intensity and neural response relationships during sleep. This relationship was not observed in the cortex, implying state-dependent modification of stimulus intensity coding. Since AEP amplitude is not modulated by stimulus intensity during sleep, differences between paired 75/50 dBa stimuli could be used to determine state better than individual intensities.

The pedunculopontine tegmental nucleus: from basic neuroscience to neurosurgical applications: arousal from slices to humans: implications for DBS

One element of the reticular activating system (RAS) is the pedunculopontine nucleus (PPN), which projects to the thalamus to trigger thalamocortical rhythms and the brainstem to modulate muscle tone and locomotion. The PPN is a posterior midbrain site known to induce locomotion in decerebrate animals when activated at 40-60 Hz, and has become a target for DBS in disorders involving gait deficits. We developed a research program using brainstem slices containing the PPN to study the cellular and molecular organization of this region. We showed that PPN neurons preferentially fire at gamma band frequency (30-60 Hz) when maximally activated, accounting for the effects of electrical stimulation. In addition, we developed the P13 midlatency auditory evoked potential, which is generated by PPN outputs, in freely moving rats. This allows the study of PPN cellular and molecular mechanisms in the whole animal. We also study the P50 midlatency auditory evoked potential, which is the human equivalent of the rodent P13 potential, allowing us to study PPN-related processes detected in vitro, confirmed in the whole animal, and tested in humans. Previous findings on the P50 potential in PD suggest that PPN output in this disorder is overactive. This translational research program led to the discovery of a novel mechanism of sleep-wake control based on electrical coupling, pointing the way to a number of new clinical applications in the development of novel stimulants (e.g., modafinil) and anesthetics. In addition, it provides methods for monitoring therapeutic efficacy of DBS in humans and animal models.

Effects of glutamate receptor agonists on the p13 auditory evoked potential and startle response in the rat

The P13 potential is the rodent equivalent of the P50 potential, which is an evoked response recorded at the vertex (Vx) 50 ms following an auditory stimulus in humans. Both the P13 and P50 potentials are only present during waking and rapid eye movement (REM) sleep, and are considered to be measures of level of arousal. The source of the P13 and P50 potentials appears to be the pedunculopontine nucleus (PPN), a brainstem nucleus with indirect ascending projections to the cortex through the intralaminar thalamus, mediating arousal, and descending inhibitory projections to the caudal pontine reticular formation (CPRF), which mediates the auditory startle response (SR). We tested the hypothesis that intracranial microinjection (ICM) of glutamate (GLU) or GLU receptor agonists will increase the activity of PPN neurons, resulting in an increased P13 potential response, and decreased SR due to inhibitory projections from the PPN to the CPRF, in freely moving animals. Cannulae were inserted into the PPN to inject neuroactive agents, screws were inserted into the Vx in order to record the P13 potential, and electrodes inserted into the dorsal nuchal muscle to record electromyograms and SR amplitude. Our results showed that ICM of GLU into the PPN dose-dependently increased the amplitude of the P13 potential and decreased the amplitude of the SR. Similarly, ICM of N-methyl-d-aspartic acid or kainate into the PPN increased the amplitude of the P13 potential. These findings indicate that glutamatergic input to the PPN plays a role in arousal control in vivo, and changes in glutamatergic input, or excitability of PPN neurons, could be implicated in a number of neuropsychiatric disorders with the common symptoms of hyperarousal and REM sleep dysregulation.

AROUSAL FROM SLICES TO HUMANS: Translational studies on sleep-wake control

Most psychiatric and neurological disorders exhibit sleep disorders, and in some cases presage the disease. Study of the control of sleep and waking has the potential for making a major impact on a number of disorders, making translational neuroscience research on this area critical. One element of the reticular activating system (RAS) is the pedunculopontine nucleus (PPN), which is the cholinergic arm of the RAS, and projects to the thalamus to trigger thalamocortical rhythms and to the brainstem to modulate muscle tone and locomotion. We developed a research program using brainstem slices containing the PPN to tell us about the cellular and molecular organization of this region. In addition, we developed the P13 midlatency auditory evoked potential, which is generated by PPN outputs, preparation in freely moving rats. This allows the study of PPN cellular and molecular mechanisms at the level of the whole animal. We also study the P50 midlatency auditory evoked potential, which is the human equivalent of the rodent P13 potential, allowing us to study processes detected in vitro, confirmed in the whole animal, and tested in humans. This translational research program led to the discovery of a novel mechanism of sleep-wake control, pointing the way to a number of new clinical applications in the development of novel stimulants and anesthetics.

Loudness dependence of auditory evoked potentials as indicator of central serotonergic neurotransmission: simultaneous electrophysiological recordings and in vivo microdialysis in the rat primary auditory cortex

Serotonin released in synapsis is one of the key neurotransmitters in psychiatry and psychopharmacology. The loudness dependence of auditory evoked potentials (LDAEP) has been proposed as a marker for central serotonergic neurotransmission. Several findings in animals and humans support this hypothesis. However, the in vivo measurement of cortical extracellular serotonin levels has never been performed simultaneously with the recording of auditory evoked potentials. The interrelationship between low cortical serotonergic activity and strong LDAEP is yet to be proven. The auditory evoked potentials were recorded in the epidura above the primary auditory cortex of male Wistar rats whereas extracellular serotonin levels in the primary auditory cortex were measured by in vivo microdialysis before and after i.p. application of the selective serotonin reuptake inhibitor citalopram. At baseline, the correlation of coefficients between the LDAEP, especially of the N1 component, and extracellular serotonin levels in the primary auditory cortex was negative. The increase of serotonin levels after citalopram application was significantly related to a decrease of LDAEP of the N1 component (r=-0.86, p=0.003). These data support the view that the LDAEP is closely modulated by cortical serotonergic activity. Thus, the LDAEP might serve as an inversely related marker of synaptically released serotonin in the CNS.

Modafinil increases arousal determined by P13 potential amplitude: an effect blocked by gap junction antagonists

STUDY OBJECTIVES

We recorded the effects of administration of the stimulant modafinil on the amplitude of the sleep state-dependent auditory P13 evoked potential in freely moving rats, a measure of arousal thought to be generated by the cholinergic arm of the reticular activating system, specifically the pedunculopontine nucleus (PPN).

DESIGN

Groups of rats were implanted for recording auditory evoked responses and the effects on P13 potential amplitude of intracranial injections into the PPN of neuroactive agents determined.

MEASUREMENTS AND RESULTS

The effects of intracranial injections into the PPN of modafinil showed that P13 potential amplitude increased in a dose-dependent manner at doses of 100, 200, and 300 microM. The effect was blocked by pretreatment with either of the gap junction antagonists carbenoxolone (300 microM) or mefloquine (25 microM), which by themselves slightly decreased P13 potential amplitude.

CONCLUSIONS

These results suggest that modafinil increases arousal levels as determined by the amplitude of the P13 potential, an effect blocked by gap junction antagonists, suggesting that one mechanism by which modafinil increases arousal may be by increasing electrical coupling.

A mouse model for studying large-scale neuronal networks using EEG mapping techniques

Human functional imaging studies are increasingly focusing on the identification of large-scale neuronal networks, their temporal properties, their development, and their plasticity and recovery after brain lesions. A method targeting large-scale networks in rodents would open the possibility to investigate their neuronal and molecular basis in detail. We here present a method to study such networks in mice with minimal invasiveness, based on the simultaneous recording of epicranial EEG from 32 electrodes regularly distributed over the head surface. Spatiotemporal analysis of the electrical potential maps similar to human EEG imaging studies allows quantifying the dynamics of the global neuronal activation with sub-millisecond resolution. We tested the feasibility, stability and reproducibility of the method by recording the electrical activity evoked by mechanical stimulation of the mystacial vibrissae. We found a series of potential maps with different spatial configurations that suggested the activation of a large-scale network with generators in several somatosensory and motor areas of both hemispheres. The spatiotemporal activation pattern was stable both across mice and in the same mouse across time. We also performed 16-channel intracortical recordings of the local field potential across cortical layers in different brain areas and found tight spatiotemporal concordance with the generators estimated from the epicranial maps. Epicranial EEG mapping thus allows assessing sensory processing by large-scale neuronal networks in living mice with minimal invasiveness, complementing existing approaches to study the neurophysiological mechanisms of interaction within the network in detail and to characterize their developmental, experience-dependent and lesion-induced plasticity in normal and transgenic animals.

Noradrenergic antagonism of the P13 and N40 components of the rat auditory evoked potential

RATIONALE

Two rat auditory evoked potential (AEP) components P13 and N40 are suggested as analogues to the human P50, which has abnormal suppression properties in schizophrenia. However, P50 likely reflects neural activity from several different brain areas. Studies examining each of these components in the rat model have proposed circuitry that involves alpha2 norepinephrine (NE) receptors, and different disruption effects are predicted depending on whether effects are presynaptic or postsynaptic.

OBJECTIVES

The aim of this paper is to test differential effects of NE antagonism on disruption of normal P13 and N40 expression.

MATERIALS AND METHODS

AEPs were recorded simultaneously in alert, freely moving rats using the alpha2 antagonist yohimbine. Amplitudes of P13 and N40 elicited by 500-ms interstimulus interval click pairs were measured after administration of a placebo and three doses of the yohimbine.

RESULTS

A high dose of yohimbine yielded smaller P13 amplitudes to both clicks, consistent with presynaptic action. However, a moderate yohimbine dose yielded increased P13 amplitudes to both clicks. For N40, a moderate dose of yohimbine yielded increased amplitudes to the second stimulus, and a high dose restored normal suppression, which is consistent with previously reported findings.

CONCLUSIONS

This study demonstrated that noradrenergic activity differentially affects P13 and N40 components. As P13 and N40 are each models of human P50, these findings highlight the complex circuitry that likely underlies P50. An appreciation for these complexities is critical for understanding the mechanisms of the P50 suppression deficit in schizophrenia, which may be influenced by both trait and state factors.

Vertex-recorded, rather than primary somatosensory cortex-recorded, somatosensory-evoked potentials signal unpleasantness of noxious stimuli in the rat

In the present study, we investigated in the rat whether vertex- or primary somatosensory cortex-recorded somatosensory-evoked potentials (Vx-SEP/SI-SEP, respectively) signal unpleasantness of noxious stimuli. Therefore, initially we characterised fentanyl effects (0, 20, 40 or 50 microg/kg/h) on somatosensory and auditory processing by recording Vx-/SI-SEPs and vertex- and primary auditory cortex-recorded auditory-evoked potentials (Vx-/AI-AEPs, respectively). Subsequently, in a separate experiment, the animals were subjected to a Pavlovian fear-conditioning paradigm. The noxious stimuli applied to evoke Vx-/SI-SEPs (unconditioned stimulus (US)) were paired to a tone (conditioned stimulus (CS)) under 'steady state' conditions of 0, 20, 40 or 50 microg/kg/h fentanyl. Vx-/SI-SEPs were recorded simultaneously during these trials. After CS-US presentation, CS-induced fear-conditioned behaviour was analysed in relation to the SEPs recorded during CS-US presentation and the AEPs recorded in the first experiment. While the SI-SEP and AI-AEP were minimally but significantly affected, fentanyl dose-dependently decreased the Vx-SEP and Vx-AEP. The decrease of the Vx-SEP and Vx-AEP was parallelled by the dose-dependent decrease of the amount of CS-induced fear-conditioned behaviour. These results suggest that the dose-dependent decrease of the Vx-SEP amplitude, rather than of the SI-SEP, indicates that the US was experienced as less unpleasant. Next to an altered US processing, altered CS processing contributed to the decrease of the amount of CS-induced fear-conditioned behaviour as indicated by the dose-dependent decrease of the Vx-AEP.

Differences between somatosensory-evoked potentials recorded from the ventral posterolateral thalamic nucleus, primary somatosensory cortex and vertex in the rat

Somatosensory-evoked potential (SEP) components recorded over the primary somatosensory cortex (SI) and vertex in the rat within the 10-30 ms latency range were characterised with respect to the anatomy and function of the primary somatosensory pathway. To this aim, these components were compared to SEP components in the similar latency range recorded from the ventral posterolateral thalamic (VPL) nucleus, a nucleus known to be part of the subcortical structure of the primary somatosensory pathway and were described with respect to their stimulus-response characteristics and their response to the mu-opioid agonist fentanyl. The VPL positive (P)11-negative (N)18-P22 and SI P13-N18-P22 differed with respect to peak occurrence (P11 versus P13, respectively) and waveform morphology, but did not differ with respect to stimulus-response characteristics and their response to fentanyl. When compared to the vertex P15-N19-P26, the VPL P11-N18-P22 and SI P13-N18-P22 complex follow a relatively fast acquisition in stimulus intensity-response and were affected significantly less to increasing stimulus frequencies and to fentanyl. These results demonstrated that when compared to the VPL-SEP and SI-SEP, the Vx-SEP was modulated differently by the experimental conditions. It is suggested that this may be related to involvement of neural structures within different functional somatosensory pathways.

Nicotine suppresses the P13 auditory evoked potential by acting on the pedunculopontine nucleus in the rat

We identified a potential novel site of action for nicotine (NIC) since (a) systemic injection of NIC led to a dose-dependent decrease in the amplitude of the sleep state-dependent, vertex-recorded, P13 midlatency auditory evoked potential (generated by the reticular activating system, RAS), (b) localized injections of a nicotinic receptor antagonist into the pedunculopontine nucleus (PPN, the cholinergic arm of the RAS) blocked the effects of systemic NIC on the P13 potential (a measure of level of arousal), and (c) localized injection of a nicotinic receptor agonist into the PPN also led to a decrease in the amplitude of the P13 potential, an effect blocked by PPN injection of a nicotinic receptor antagonist. There were minor changes in the manifestation of the startle response (SR) at the concentrations used; however, NIC did decrease the hippocampal N40 potential, although its effects were not affected by antagonist or agonist injections into the PPN. These results suggest a potential mechanism underlying the anxiolytic effects of NIC-suppression of the cholinergic arm of the RAS.

Differences between primary somatosensory cortex- and vertex-derived somatosensory-evoked potentials in the rat

The somatosensory-evoked potential (SEP) elicited by high-intensity stimulation potentially provides a reliable indicator of analgesic efficacy since it reflects the level of activation of the nociceptive system. In the present study, components in the 10-30-ms latency range of SEPs recorded over the primary somatosensory cortex (SI-SEPs) and vertex (Vx-SEP) in the rat were characterized and compared. SEPs were elicited by electrical tail-base stimulation, and SI-SEPs and Vx-SEPs were recorded simultaneously. Responses to increasing stimulus intensity and stimulus frequency while awake and responses to bolus injection of fentanyl, thiopental, and ketamine were investigated. The SI-SEP positive component (P) occurring at 12 ms after stimulation (P12) showed a significantly lower intensity threshold and was significantly less affected by increasing stimulus frequency and by administration of the different drugs when compared to the Vx-SEP P15. The fact that a single stimulus modality results in different signal characteristics dependent on the recording site supports the view that different neural mechanisms involved in primary processing of somatosensory information are responsible for the generation of the SI-SEP P12 and Vx-SEP P15, respectively. This differentiation between SI-SEPs and Vx-SEPs potentially has distinct consequences using the SEP to evaluate nociception and analgesia in the rat model.

Arousal mechanisms related to posture and locomotion: 2. Ascending modulation

An intrinsic function of the reticular activating system (RAS) is its participation in fight vs. flight responses such that alerting stimuli simultaneously activate thalamocortical systems, as well as postural and locomotor systems, in order to enable an appropriate response. The P50 midlatency auditory-evoked potential appears to be an ascending manifestation of the cholinergic arm of the RAS in eliciting changes in arousal state. Abnormalities in the manifestation of the P50 potential are present in disorders which include: (1) dysregulation of sleep-wake cycles; (2) abnormalities in reflex/postural, especially, startle, responses; and (3) malfunctions in flight vs. flight responses. In general, the P50 potential appears to be upregulated (increased amplitude and/or decreased sensory gating) in disorders which are marked by upregulation of RAS outputs (hypervigilance), and downregulated in disorders characterized by decreased RAS outputs (hypovigilance). Many of the disorders discussed have a developmental etiology and a postpubertal age of onset.

Effects of different propofol infusion rates on EEG activity and AEP responses in rats

Parameters calculated from the auditory-evoked potential (AEP) recorded over the auditory cortex and from the electroencephalogram (EEG) recorded over the near vertex were compared in rats at three different infusion rates of propofol (62.5, 35 and 25 mg/kg/h). Depth of anaesthesia was assessed clinically using the strength of the pedal withdrawal reflex. Well-defined AEP responses were consistently obtained. As the propofol concentration was reduced, peak latencies decreased and peak to peak amplitudes increased. Amplitude and latency values were closely associated with the strength of the pedal withdrawal responses. Parameters calculated from the EEG showed no significant change as the propofol concentration was reduced. Periods of burst suppression became more frequent as the propofol infusion rate was increased. The study showed some of the difficulties that may be encountered when using EEG as a tool to assess depth of anaesthesia during propofol infusion. The AEP showed dose dependent changes in rats at different infusion rates of propofol. However, large variability between animals limits the use of this technique for monitoring depth of anaesthesia.

Differential characteristics of the middle latency auditory evoked magnetic responses to interstimulus intervals

OBJECTIVE

The human middle latency auditory evoked magnetic fields were recorded with different interstimulus intervals (ISI) to investigate the differential natures of P30m and the P50m, including whether the P50m source was spatially different or not from the P30m source.

METHODS

Twenty right-handed healthy subjects participated in the experiment. Auditory magnetic responses were recorded in the 0.5 s ISI (ISI were between 0.4 and 0.6 s) and the 1.5 s ISI conditions (ISI were between 1 and 2 s). Tone bursts were presented to the right ears 880 times consecutively for each condition. The P30m and the P50m responses were investigated, and the dipole source localization was performed.

RESULTS

The P50m latency was significantly prolonged, while the P30m latency did not vary in the shorter ISI. Both P50m and P30m amplitudes were significantly reduced in the shorter ISI. The P50m was located significantly more anteriorly than P30m.

CONCLUSIONS

These results suggest the existence of differential characteristic and spatially different magnetic responses in the middle latency range.

SIGNIFICANCE

This study has revealed one aspect of the different natures between P30m and P50m, and may provide a key for auditory perceptional processes in humans.

Propofol suppresses the sleep state-dependent P13 midlatency auditory evoked potential in the rat

Propofol (2,6-diisopropylphenol) is a widely used anesthetic agent, but its mechanisms of action are poorly understood. In this report, the effects of three dose levels of propofol (5, 7.5, and 10mg/kg) on the amplitude of the vertex-recorded, sleep state-dependent P13 midlatency evoked potential were investigated. The P13 potential is generated, at least in part, by the ascending cholinergic reticular activating system (RAS). The RAS is known to be affected by anesthetic agents. Intravenous injections of propofol were found to reduce the amplitude of the P13 potential in a dose- and time-dependent manner. At 2min post-injection, the mean P13 amplitude was suppressed to 40% of its pre-injection level by the lowest dose, but was suppressed to 10% of pre-injection levels by the two higher doses of propofol. The duration of the suppression of mean P13 potential amplitude was also dose-dependent such that complete recovery occurred by 5min using 5mg/kg, by 15min using 7.5mg/kg and by 30min using 10mg/kg of propofol. Using a paired stimulus paradigm, transient effects on habituation of the P13 potential were observed but only after the highest dose. Thus, one of the mechanisms of propofol may be to affect portions of the RAS which modulate the level of arousal. It may only transiently affect higher systems known to modulate the degree of habituation of responses by the RAS (i.e. processes which modulate habituation and may participate in sensory gating and distractibility).

Evaluation of methods for eliciting somatosensory-evoked potentials in the awake, freely moving rat

To standardise the method of eliciting somatosensory-evoked potentials (SEPs), SEPs were generated by electrical stimulation of different stimulus sites and recorded bilaterally from the primary somatosensory cortex (S1) and from midline in awake, freely moving rats. Increasing stimulus intensity enhanced amplitudes of all SEPs. At supramaximal stimulation, SEPs following vibrissae and tail stimulation (V-SEP and Ta-SEP, respectively) but not following trunk stimulation (Tr-SEP), fulfilled our criterion of signal-to-noise ratio >or=4. The first V-SEP component coincided with a stimulus artefact, disqualifying these recordings for a standard stimulation protocol. The Ta-SEP generated stable and reproducible recordings and was considered to be the preferred technique. Early components of the contralateral S1 recorded V-SEP and Tr-SEP occurred at latencies different from the other recordings. Increasing stimulus repetition rate (SRR) decreased amplitudes of all SEPs. At the highest obtainable SRR, the amplitude between the V-SEP second positive and second negative components in all recordings was 70-80% of the amplitude at 0.1 Hz, whereas peak amplitudes of subsequent components and those of the Tr-SEP and Ta-SEP were 20-50%. These results indicate that the different SEP components might be generated by different ascending neural pathways.

Habituation and sensitization in rat auditory evoked potentials: a single-trial analysis with wavelet denoising

In this work, systematic changes of single-trial auditory evoked potentials elicited in rats were studied. Single-trial evoked potentials were obtained with the help of wavelet denoising, a very recently proposed method that has already been shown to be useful in the analysis of scalp human evoked potentials. For the evoked components in the 13-24-ms range (i.e. P13, N18, P20 and N24), it was possible to identify slow exponential decreases in the peak amplitudes, most likely related to a slow habituation process, while for N18, an initial increase in amplitude was also found. On the contrary, the slower components (N38 and N52) habituated within a few trials, and we therefore propose that they are related to a different functional process. The outcomes of the present study show that wavelet denoising is a useful technique for analyzing evoked potentials in rats at the single-trial level. In fact, in the present study it was possible to obtain more information than the one described in previous related works. This allows the study of other forms of learning processes in rats with the aid of evoked potentials. Finally, the outcomes of this study may have some relevance for the comparison of human and rat evoked potentials.

Cholinergic modulation of the sleep state-dependent P13 midlatency auditory evoked potential in the rat

Injections into the pedunculopontine nucleus (PPN) of the cholinergic receptor agonist, carbachol (CAR), were found to reduce the amplitude of the vertex-recorded, sleep state-dependent P13 midlatency evoked potential in a dose- and time-dependent manner. This effect was blocked or reduced by pretreatment with the muscarinic receptor antagonist, scopolamine, injected into the PPN.

Locus coeruleus involvement in the effects of immobilization stress on the p13 midlatency auditory evoked potential in the rat

1. Adult male rats were prepared for recording midlatency auditory evoked responses from the vertex (Vx, P13 potential) and auditory cortex (ACx, P7 potential). 2. The P13 potential is the rodent equivalent of the human P1 or P50 potential, which exhibits decreased sensory gating in posttraumatic stress disorder. 3. Immobilization (IMB) stress for 60 min led to a significant decrease in P13 potential amplitude and sensory gating of the potential for the first 30-40 min of IMB. 4. The effects of IMB on the P13 potential were reduced by pre-treatment with the alpha-2 adrenergic receptor blocker yohimbine (YOH). 5. Injections of corticotropin releasing factor (CRF) into the locus coeruleus (LC), but not injections dorsal or ventral to the LC, induced a dose-dependent decrease in P13 potential amplitude and sensory gating. 6. The effects of CRF were blocked by cotreatment with the CRF receptor antagonist alpha-helical CRF (alpha-h CRF). 7. The effects of IMB on the P13 potential were mimicked by injections of the alpha-2 adrenergic receptor agonist dexmedetomidine (DEX) into the pedunculopontine nucleus (PPN). 8. The effects of DEX injections into PPN were reduced by pre-treatment with the alpha-2 adrenergic receptor blocker YOH. 9. The effects of IMB on P13 potential amplitude and sensory gating may be mediated in part via CRF activation of LC, which sends inhibitory alpha-2 adrenergic projections to PPN, a major source of the P13 potential.

The rat vertex-middle latency auditory-evoked potential as indicator of anaesthetic depth: a comparison with evoked-reflex testing

We investigated whether components from the rat Vx-MLAEP could be used to assess depth of anaesthesia induced by propofol. Propofol decreased MLAEP amplitudes and increased latencies. We propose that the P(16)-N(22) wave in the rat MLAEP is similar to the human P1, and that recovery of this wave during propofol anaesthesia correlates with behavioural measures of the regaining of consciousness.

Serotonergic modulation of the P13 midlatency auditory evoked potential in the rat

The vertex-recorded, sleep state-dependent P13 midlatency auditory evoked potential in the rat may be generated, in part, by pedunculopontine nucleus (PPN) projections. Injections into the PPN of the 5-HT(1A) serotonin receptor agonist, 8-hydroxy-2-di-n-propylaminotetralin hydrobromide (DPAT), were found to reduce the amplitude of the P13 potential in a dose- and time-dependent manner. The suppressive effect of DPAT was blocked or reduced by pretreatment with the 5-HT(1A) serotonin receptor antagonist, Pindobind. These results show that the P13 potential can be modulated by known inhibitory serotonergic inputs to the PPN.

Neurochemical modulation of the P13 midlatency auditory evoked potential in the rat

Previous studies have shown that the vertex-recorded P13 auditory evoked potential in the rat appears to be the rodent equivalent of the human P1 (or P50) potential. This sleep state-dependent potential appears to be generated, at least in part, by cholinergic pedunculopontine nucleus projections. The present studies used localized microinjections of neuroactive compounds into the region of the pedunculopontine nucleus in order to modulate the vertex-recorded P13 potential. Both the GABAergic agonist, muscimol, and the noradrenergic alpha2 receptor agonist, clonidine, were found to reduce the amplitude of the P13 potential in a dose-dependent manner. The suppressive effect of clonidine on P13 potential amplitude was blocked by pretreatment with the noradrenergic alpha2 receptor antagonist, yohimbine. In addition, habituation of the P13 potential, measured using a paired stimulus paradigm, was increased by micro-injection of a dose of muscimol or clonidine which did not change the amplitude of the P13 potential induced by the first stimulus of a pair. In contrast, microinjection of yohimbine decreased habituation of the P13 potential. These results show that the vertex-recorded P13 potential and its habituation can be modulated by activation of known inhibitory synapses, both GABAergic and noradrenergic, at the level of the pedunculopontine nucleus. This provides further evidence that the P13 potential is generated, at least in part, by pedunculopontine nucleus outputs.

Sensory gating of the P13 midlatency auditory evoked potential and the startle response in the rat

The human P1/P50 midlatency auditory evoked potential and the startle response (SR) have been used as measures of sensory and sensorimotor gating, respectively. In the present study, both prepulse and paired stimulus paradigms were used in order to investigate the relationship between sensory gating mechanisms of the P13 potential, the putative rodent equivalent of the P1 potential, and those of the SR. In addition, these were compared to the properties of the N40 potential, another measure of sensory gating. Simultaneous recordings from the vertex (P13 potential and N40 potential) and neck musculature (SR) showed that (1) in a prepulse paradigm, increasing the intensity of the prepulse or decreasing the interstimulus interval resulted in increased inhibition of the P13 potential, N40 potential (to a lesser degree) and the SR (to a greater degree), (2) when using a low signal-to-noise ratio between the prepulse intensity and the background level, prepulse inhibition of the SR was reduced or absent while that of the P13 potential was present, (3) the amplitude of the 'prepulse evoked' P13 potential was significantly correlated with prepulse inhibition of the P13 potential, the N40 potential and the SR, (4) in a paired identical stimulus paradigm, decreasing the interstimulus interval resulted in increased habituation of the P13 potential, N40 potential (to a lesser degree) and the SR, and (5) increasing the intensity of the paired stimulation resulted in increased habituation of the P13 potential and the N40 potential (to a lesser degree), but not of the SR. These results demonstrate the presence of prepulse inhibition of the P13 potential, the N40 potential and the SR in a parallel manner, but show certain specific differences in their responses to parametric changes.

Midlatency auditory-evoked potentials in the rat: effects of interventions that modulate arousal

The vertex-recorded P13 midlatency auditory-evoked potential in the rat shows the same characteristics as the P1 potential in the human, namely, sleep-state dependence, rapid habituation and blockade by the cholinergic antagonist scopolamine. The P13 potential appears to be generated, at least in part, by projections of the pedunculopontine nucleus, the cholinergic arm of the reticular activating system. On the other hand, the auditory cortex-recorded P7 potential appears to be of primary cortical origin. Simultaneous recordings from the vertex and the auditory cortex showed that (1) the P13 potential was suppressed by administration of the anesthetics ketamine, pentobarbital or halothane in a dose-dependent manner, but the P7 potential was not; (2) the P13 potential was suppressed by intragastric injections of ethanol in a dose-dependent manner, but the P7 potential was not; (3) the amplitude of the P13 potential was negatively correlated with blood ethanol levels; (4) both the P13 and P7 potentials were still present following injections of the neuromuscular blocker pancuronium bromide; and (5) both the P13 and P7 potentials were decreased by diffuse brain injury induced by a weight-drop device in a weight-dependent manner. These findings suggest that the P13 potential is more sensitive than the P7 potential to changes in arousal and that the P13 and P7 potentials are not of myogenic but of neural origin.

Auditory steady-state responses to click trains from the rat temporal cortex

In order to investigate the mechanisms underlying the generation of steady-state responses (SSRs), auditory evoked potentials elicited by click trains presented at several stimulation rates (30, 40, 50, 60 Hz) were recorded in 7 awake rats by means of epidural electrodes placed over the temporal cortex. Mean amplitude-rate function calculated on the recorded responses appeared almost flat and showed the maximum value at 50 Hz, while mean phases showed a linear increase when increasing the stimulation rate. In each rat, predictions of the recorded responses at 30, 40, 50 and 60 Hz were synthesized by superimposing middle-latency auditory evoked potentials (MAEPs) at suitable time intervals at each rate. Mean amplitudes calculated on the predicted curves decreased linearly when increasing the stimulation rate and appeared higher in comparison to those obtained from the recorded SSRs. Predicted phases showed a linear increase when increasing the stimulation rate and were leading with respect to corresponding phase values calculated for recorded SSRs. Our findings indicate that the MAEP superimposition mechanism does not adequately predict the generation of temporal recorded SSRs in rats. This was explained by admitting that phenomena related to the recovery cycle and, to a lesser extent, to rate-dependent facilitating effects come into play.

On-line analysis of middle latency auditory evoked potentials (MLAEP) for monitoring depth of anaesthesia in laboratory rats

In laboratory animals as well as in human beings a depth of anaesthesia, where the subject has no pain or recall of events from the surgery, should be provided. Haemodynamic parameters such as heart rate and blood pressure are not a guarantee for an optimal depth of anaesthesia, especially when using neuromuscular blocking agents (NMBA). A number of studies suggest that the Middle Latency Auditory Evoked Potentials (MLAEP) contain information about the state of consciousness in humans. The purpose of this study was to examine whether the AEP could serve as an indicator of depth of anaesthesia in rats. The AEP was elicited with a click stimulus and monitored in an 80 ms window synchronised to the stimulus. The AEP was extracted applying an Auto Regressive Model with Exogenous Input (ARX-model) from which a Depth of Anaesthesia Index (DAI) was calculated. DAI was normalised to 100 while awake and decreasing gradually to a level between 50 and 20 as the rat was anaesthetised. Nine rats were anaesthetised and included in the study. Four doses of Hypnorm vet. and Dormicum were given as a total, each with 5 minutes interval. Clinical signs of the level of anaesthesia were observed simultaneously with the AEP. The results showed that in four rats DAI decreased to a level below 30 while anaesthetised. In the remaining five rats the AEP was only decreased to a level below 45. The results indicated that a simple dosing regimen based on weight was unable to give the same depth of anaesthesia in individual rats. The decrease in the DAI correlated well with the loss of stimulus response. In conclusion, MLAEP could be used as an indicator of depth of anaesthesia in rats during Hypnorm vet. and Dormicum administration. However studies applying other anaesthetic drugs should be carried out, before a conclusion of the general utility of the method can be made.

Disorders of the reticular activating system

The organization of components of the reticular activating system and their role in sleep-wake mechanisms and arousal are described. A functional model is proposed based on known neuroanatomical and neurophysiological findings. The involvement of these elements of the reticular activating system in various neurological and psychiatric disorders is discussed. A series of hypotheses are advanced to account for the role of these nuclei in such diverse disorders as schizophrenia, post-traumatic stress disorder, REM behavior disorder, Parkinson's disease and narcolepsy. This line of reasoning suggests that, when neurological or psychiatric disorders manifest symptoms related to arousal and sleep-wake control, disturbances of elements of the reticular activating system must be considered responsible.

Combat veterans with posttraumatic stress disorder exhibit decreased habituation of the P1 midlatency auditory evoked potential

The current study used a paired stimulus paradigm to investigate the P1 midlatency auditory evoked potential in Vietnam combat veterans with posttraumatic stress disorder (PTSD) and three comparison groups: alcohol dependents, combat-exposed normals, and combat-unexposed normals. Compared to each comparison group, PTSD subjects exhibited significantly diminished habituation of the P1 potential. P1 potential habituation within the PTSD group, correlated significantly with intensity of PTSD reexperiencing symptoms, such as trauma-related nightmares and flashbacks. These findings are discussed as consistent with a sensory gating defect at the brainstem level in PTSD, and are further discussed in the context of other psychophysiological measures in PTSD and of P1 potential findings in psychiatric disorders other than PTSD.

Midlatency auditory evoked potentials and the startle response in the rat

The P13 midlatency auditory evoked potential in the rat is (i) sleep state dependent, (ii) undergoes rapid habituation and (iii) is blocked by the cholinergic antagonist scopolamine. As such, the P13 potential in the rat shows the same characteristics as the P1 (or P50) potential in the human. These potentials are thought to be mediated, at least in part, by the cholinergic arm of the reticular activating system. Previous studies have linked the reticular activating system with the startle response. The present study was undertaken to explore this relationship by simultaneously recording the P13 potential and the electromyographically recorded startle response using stimuli designed to elicit each response. Simultaneous recordings from the vertex and neck musculature following auditory click stimuli showed that: (i) the mean threshold of the P13 potential was 69.3 +/- 1.9 dB, while that for the startle response was 87.9 +/- 6.4 dB; (ii) the P13 potential was present during waking and paradoxical sleep, but absent during slow-wave sleep, while the startle response was present reliably only during waking; (iii) both responses habituated in response to paired stimuli, but the startle response was more habituated than the P13 potential; and (iv) both responses were facilitated by trains of stimuli in a similar manner. Recordings carried out from the auditory cortex verified that the primary cortical response had properties different from the P13 potential; i.e. it was present during all sleep-wake states, had a lower threshold and did not habituate rapidly. Finally, different patterns of startle responses were detected in the neck muscles. In every case, the P13 potential occurred during the middle, inhibitory phase of the startle response. These results suggest that the P13 potential and the startle response share response features, but the P13 potential appears to be more sensitive to auditory stimulation and to sleep-wake states. The startle response may be modulating descending systems by priming the spinal cord to respond in a "fight vs flight" fashion. On the other hand, the P13 response may be modulating ascending systems by triggering thalamocortical activity and resetting descending systems to allow novel motor strategies.

Dependence of rat vertex auditory evoked potentials on central muscarinic receptor activation

Auditory evoked potentials (AEPs) were recorded epidurally at vertex in the freely moving rat, yielding the prominent components P18 and N40 from the average waveform. Both components were severely attenuated at tone presentation rates faster than 1 Hz. Dependence of the vertex AEP on central cholinergic activation was assessed by comparing the effects of subcutaneous injections of the muscarinic antagonist scopolamine (0.1-1.0 mg/kg) to methyl-scopolamine (0.5, 1.0 mg/kg) to test for peripheral effects. Scopolamine produced a significant decrease in amplitude for both the P18 and N40 components in overall tests. N40 amplitude also showed a significant decrease at each dose of scopolamine, and the magnitude of the amplitude decrement was dose dependent. At 23 h after scopolamine, only N40 amplitude remained significantly depressed. There were no effects of injection of saline, or of methyl-scopolamine. These results confirm that middle-latency vertex AEPs in the rat are attenuated by central muscarinic receptor blockade, consistent with previous findings in cats.

The pedunculopontine nucleus--auditory input, arousal and pathophysiology

This review describes the role of the pedunculopontine nucleus (PPN) in various functions, including sleep-wake mechanisms, arousal, locomotion and in several pathological conditions. Special emphasis is placed on the auditory input to the PPN and the possible role of this nucleus in the manifestation of the P1 middle latency auditory evoked response. The importance of these considerations is evident because the PPN is part of the cholinergic arm of the reticular activating system. As such, the auditory input to this region may modulate the level of arousal of the CNS and, consequently, abnormalities in the processing of this input can be expected to have serious consequences on the level of excitability of the CNS. The involvement of the PPN in such disorders as schizophrenia, anxiety disorder and narcolepsy is discussed.

Auditory input to the pedunculopontine nucleus: II. Unit responses

The pedunculopontine nucleus (PPN) has been implicated in sleep-wake control, arousal responses, and motor functions. The PPN also has been implicated in the generation of the P1 middle-latency auditory-evoked potential. The present study was undertaken to determine the nature of the responsiveness of single neurons in and around the PPN following auditory stimulation. Somatosensory responsiveness also was tested in some cells. These results demonstrate a) the presence of a significant proportion of PPN neurons that respond to auditory click stimuli; b) two populations of neurons showing either low threshold/short latency/low habituation or high threshold/longer latency/high habituation; c) the responses of longer latency neurons precede the onset and peak of depth- and vertex-recorded middle-latency auditory-evoked potentials; d) thresholds of longer latency neurons similar to the threshold for wave A in the intact cat, the P13 potential in the intact rat, or the startle reflex; and e) convergent somatosensory and auditory responses at a similar latency in a number of PPN neurons. These findings suggest that neurons in and around the PPN may participate in auditory and somatosensory information processing related to arousal, and may contribute to the manifestation of the P1 auditory middle-latency evoked potential.

Auditory input to the pedunculopontine nucleus: I. Evoked potentials

The pedunculopontine nucleus (PPN) has been implicated in sleep-wake control, arousal responses, and motor functions. The PPN also has been implicated in the generation of the P1 middle-latency auditory-evoked potential. The present study was undertaken to determine the topographical distribution, threshold, and response properties of depth-recorded potentials following auditory click stimulation. Experiments were conducted in both decerebrate cat and rat, with a view towards determining the presence of P1-like middle-latency auditory-evoked potentials in the midbrain of both species. These results demonstrate a) the presence in and around the PPN of a P1-like potential in the decerebrate rat similar to that described in the accompanying article as the P13 in the intact rat; b) the presence in and around the PPN of a P1-like potential in the decerebrate cat similar to that previously described by others as wave A in the intact cat; c) although thresholds for these potentials were similar to those of intact preparations, following frequencies were higher in the decerebrate preparations, i.e., responsiveness to repetitive stimulation was higher; and d) depth-recorded somatosensory-evoked potentials also were studied in the cat and found to show an evoked potential at a similar latency as middle-latency auditory depth-recorded potentials. These findings suggest that click stimulus-evoked, depth-recorded potentials are present in and around the PPN in the decerebrate rat and cat, i.e., in the absence of cortex, at a similar latency as in intact preparations.