Neurophysiology, neuropharmacology and behavioral relationships of visual system evoked after-discharges: A review

Visual Cues Predictive of Behaviorally Neutral Outcomes Evoke Persistent but Not Interval Timing Activity in V1, Whereas Aversive Conditioning Suppresses This Activity

Cue-evoked persistent activity is neural activity that persists beyond stimulation of a sensory cue and has been described in many regions of the brain, including primary sensory areas. Nonetheless, the functional role that persistent activity plays in primary sensory areas is enigmatic. However, one form of persistent activity in a primary sensory area is the representation of time between a visual stimulus and a water reward. This “reward timing activity”—observed within the primary visual cortex—has been implicated in informing the timing of visually cued, reward-seeking actions. Although rewarding outcomes are sufficient to engender interval timing activity within V1, it is unclear to what extent cue-evoked persistent activity exists outside of reward conditioning, and whether temporal relationships to other outcomes (such as behaviorally neutral or aversive outcomes) are able to engender timing activity. Here we describe the existence of cue-evoked persistent activity in mouse V1 following three conditioning strategies: pseudo-conditioning (where unpaired, monocular visual stimuli are repeatedly presented to an animal), neutral conditioning (where monocular visual stimuli are paired with a binocular visual stimulus, at a delay), and aversive conditioning (where monocular visual stimuli are paired with a tail shock, at a delay). We find that these conditioning strategies exhibit persistent activity that takes one of three forms, a sustained increase of activity; a sustained decrease of activity; or a delayed, transient peak of activity, as previously observed following conditioning with delayed reward. However, these conditioning strategies do not result in visually cued interval timing activity, as observed following appetitive conditioning. Moreover, we find that neutral conditioning increases the magnitude of cue-evoked responses whereas aversive conditioning strongly diminished both the response magnitude and the prevalence of cue-evoked persistent activity. These results demonstrate that cue-evoked persistent activity within V1 can exist outside of conditioning visual stimuli with delayed outcomes and that this persistent activity can be uniquely modulated across different conditioning strategies using unconditioned stimuli of varying behavioral relevance. Together, these data extend our understanding of cue-evoked persistent activity within a primary sensory cortical network and its ability to be modulated by salient outcomes.

Methylphenidate and alcohol effects on flash-evoked potentials, body temperature, and behavior in Long-Evans rats

Methylphenidate (MPD) is a psychostimulant used to treat attention deficit hyperactivity disorder (ADHD). Most adult ADHD patients use ethanol in combination with MPD. This research examined the effects of MPD and ethanol on flash-evoked potentials (FEPs; cortical responses frequently used to assess neural activity and sensory processing) recorded from the visual cortex (VC) and superior colliculus (SC; a structure involved in attention and orientation) of chronically implanted male Long-Evans rats, and on body temperature and open field behavior. For one group of rats, either saline or ethanol (2.0 g/kg) was given 5 min prior to either saline or MPD (2.9 mg/kg). FEPs were recorded 10 and 20 min later. In the VC, ethanol decreased amplitudes of several components, but increased P2. MPD increased N3, but decreased P3 and P4. Ethanol increased the latency of several components. In the SC, ethanol decreased all three components, while MPD increased P3. Ethanol increased latency of all components. During FEP testing, ethanol decreased body movement while MPD increased movement. In the open field, line crossings were increased but rearings were decreased by ethanol. Both ethanol and MPD produced hypothermia. A second group of rats was given MPD at 11.6 mg/kg. Ethanol decreased several VC amplitudes, but increased P2. MPD increased N3 amplitude but decreased amplitude for other components. MPD also counteracted the effect of ethanol on the amplitude of P2 and N3. Both ethanol and MPD increased the latency of several components. In the SC, ethanol decreased all component amplitudes, while MPD increased P3 but decreased N4. Ethanol increased all component latencies, while MPD increased latency for two components. During FEP testing, ethanol decreased body movement while MPD increased movement. In the open field, line crossings were increased by ethanol and MPD. Rearings were eliminated by ethanol in the open field but increased by MPD, and MPD counteracted the effect of ethanol on rearings. Both ethanol and MPD produced hypothermia. Some of these results might help explain why users take MPD and ethanol in combination in order to enable consuming larger amounts of alcohol.

Effects of mecamylamine on flash-evoked potentials, body temperature, and behavior in Long-Evans rats

This experiment examined the effects of mecamylamine, a nicotinic acetylcholine receptor antagonist, on flash-evoked potentials (FEPs) recorded from the visual cortex (VC) and superior colliculus (SC) of chronically implanted male Long-Evans rats, and on body temperature and open field behavior. FEPs were recorded at 20 and 35 min following intraperitoneal injections of saline, and of 0.3, 3.0, and 10.0 mg/kg mecamylamine on separate days. The 0.3 mg/kg dose did not produce significant effects. The amplitude of VC components N₃₀, P₄₈, and P₈₇ increased, N₁₅₀ and P₂₃₁ decreased, and P₂₃, N₄₀, N₅₈, and N₆₈ were unchanged following administration of the 10.0 mg/kg dose. In the SC, component P₂₈ was unaffected, P₃₉ was reduced, and N₄₉ was augmented by the 10.0 mg/kg dose. All component peak latencies were increased by the 3.0 and 10.0 mg/kg doses. Significant hypothermia was also produced by the 3.0 and 10.0 mg/kg doses, suggesting that this was the basis for the increased latencies. The 10.0 mg/kg dose produced a significant decrease in movement during the recording sessions. In subsequent open field observations, both line crossings and rearings were reduced by the 3.0 and 10.0 mg/kg doses. The results suggest that endogenous acetylcholine acting on nicotinic acetylcholine receptors plays at most a modest role in producing FEPs recorded from the VC and SC.

Baclofen does not counteract the acute effects of ethanol on flash-evoked potentials in Long-Evans rats

This experiment examined the separate and combined effects of baclofen (5.0 mg/kg, i.p.), a GABA B receptor agonist, and ethanol (2.0 g/kg, i.p.) on flash-evoked potentials (FEPs) recorded from both the visual cortex and superior colliculus (SC) of chronically implanted male Long-Evans rats. In the visual cortex, ethanol significantly decreased the amplitude of positive component P87, but increased P37 and P47. Other component amplitudes were not significantly altered. In contrast, baclofen reduced the amplitude of negative component N31 to such an extent that it became positive. Although P47 was also reduced by baclofen, the amplitude of most other components was increased. Only P24 and P87 were unchanged by baclofen. The combination of baclofen and ethanol resulted in amplitudes very similar to ethanol alone for secondary components P47, N62, and P87, but very similar to baclofen alone for primary component N31 and late components N147 and P230. In the SC, component amplitudes were generally decreased by ethanol, baclofen, and the combination treatment. Latencies of most components in both structures were increased by the drug treatments. Each drug treatment produced significant hypothermia. Locomotor behavior was also altered. These results demonstrate: (1) pharmacological differences between the primary and late components versus the secondary components of the cortical FEP, (2) that baclofen does not counteract significant effects of ethanol on cortical or collicular component amplitudes, and (3) that baclofen enhances N147-P230 amplitude, suggesting reduced cortical arousal.

Baclofen alters flash-evoked potentials in Long–Evans rats

This experiment examined the effects of the GABA-B agonist baclofen on flash-evoked potentials (FEPs) recorded from both the visual cortex (VC) and superior colliculus (SC) of chronically implanted male Long-Evans rats. FEPs were recorded at 5, 25, 45, and 65 min following intraperitoneal injections of saline, and of 1.25, 2.5, 5.0, and 10.0 mg/kg baclofen on separate days. In the VC, the amplitude of components P(23), P(37), N(55), N(150), and P(242) increased, while the amplitude of components N(31) and P(48) decreased following baclofen administration. P(88) was unchanged. In the SC, components P(28), N(49), N(55), and N(59) were reduced in amplitude, while P(39) was unaffected by baclofen. These effects on amplitudes were dose- and time-dependent. Many peak latencies in the VC and SC were altered by baclofen, although there was no obvious pattern of change, with some decreasing, a few increasing, and others unchanged. Body temperature was recorded in a separate group of animals, with both the 5.0 and 10.0 mg/kg doses of baclofen producing significant hypothermia. The 10.0 mg/kg dose of baclofen resulted in a significant decrease in movement during the recording sessions, but not in subsequent open field observations. The results show the involvement of GABA-B receptors in the production/modulation of the various components of FEPs.

Photic-induced sensitization: eye-specific neural plasticity and effect of behavioral state

We reported previously that exposure to repetitive visual stimulation in ordinary adult rats results in acquisition of an enduring increase in magnitude and change in character of visual cortical responses. This sensitization is consistent with experience-dependent neuroplastic changes, but could also reflect alterations in response with behavioral state during testing. The aim of this study was to distinguish the contributions of behavioral state and neural plasticity in this photic-induced sensitization. We used repetitive light-emitting diode flashes delivered monocularly and recorded electrocorticographically in the albino rat in which retino-geniculo-cortical projections are predominantly crossed. This enabled comparison of visual responses of sensitized visual circuitry associated with one eye to responses from effectively unsensitized circuitry associated with the second eye at similar time points in an animal, thus providing an internal control for behavioral state. Following sensitization, monocular stimulation of one eye produced the characteristic high amplitude driven spike-wave response in corresponding contralateral visual cortex, but not ipsilateral cortex. Expression of the sensitized driven response was optimal in the quiet awake state and suppressed during active exploration, drowsiness, or anesthesia. When the animal was in the quiet awake state, producing sensitized responses to stimulation of the first eye, no such response was observed on alternate trials upon stimulation of the second eye. Only after extended exposure of the second eye did the high amplitude driven spike-wave response in contralateral visual cortex develop. The data further suggest some degree of sensitization of ipsilateral pathways may accompany monocular stimulation and that effects of monocular sensitization could include suppression in pathways related to the unstimulated eye. Thus, while behavioral state influences expression of the sensitized driven visual response, the eye-specific nature of the effect provides strong evidence that response enhancement reflects neuroplasticity in visual pathways and not a more general change in behavioral state during testing.

Isoflurane disrupts anterio-posterior phase synchronization of flash-induced field potentials in the rat

Consciousness presumes a set of integrated functions such as sensory processing, attention, and interpretation, and may depend upon both local and long-range phase synchronization of neuronal activity in cerebral cortex. Here we investigated whether volatile anesthetic isoflurane at concentrations that produce loss of consciousness (LOC) disrupts long-range anterio-posterior and local anterior synchronization of neuronal activity in the rat. In six rats, deep electrodes were chronically implanted in the primary visual cortex (V1) and in two areas of the motor cortex (M1 and M2) for recording of intracortical event-related potentials (ERP). Thirty discrete flashes were presented at random interstimulus intervals of 15-45 s, and ERPs were recorded at stepwise increasing isoflurane concentrations of 0-1.1%. Neuronal synchronization was estimated using wavelet coherence computed from the ERP data band-pass filtered at 5-50 Hz. We found that (1) in the waking state, long-range anterio-posterior coherence in 5-25 Hz and 25-50 Hz frequency bands was significantly higher than local anterior coherence; (2) anterio-posterior coherence in both 5-25 Hz and 26-50 Hz bands was significantly reduced by isoflurane in a concentration-dependent manner; (3) local anterior coherence was not affected by isoflurane at any of the concentrations studied. These findings suggest that a disruption of long-range anterio-posterior rather than local anterior synchronization of neuronal activity precedes the anesthetic-induced loss of consciousness.

Photic-induced sensitization: acquisition of an augmenting spike-wave response in the adult rat through repeated strobe exposure

It is well established that patterns of sensory input can affect neuroplastic changes during early development. The scope and consequences of experience-dependent plasticity in the adult are less well understood. We studied the possibility that repeated exposure to trains of stroboscopic stimuli could induce a sensitized and potentially aberrant response in ordinary individuals. Chronic electrocorticographic recording electrodes enabled measurement of responses in awake, freely moving animals. Normal adult rats, primarily Sprague-Dawley, were exposed to 20-40 strobe trains per day after a strobe-free adaptation period. The common response to strobe trains changed in 34/36 rats with development of a high-amplitude spike-wave response that emerged fully by the third day of photic exposure. Onset of this sensitized response was marked by short-term augmentation of response to successive strobe flashes. The waveform generalized across the brain, reflected characteristics of the visual stimulus, as well as an inherent 6- to 8-Hz pacing, and was suppressed with ethosuximide administration. Spike-wave episodes were self-limiting but could persist beyond the strobe period. Sensitization lasted 2-4 wk after last strobe exposure. The results indicate visual stimulation, by itself, can induce in adult rats an enduring sensitization of visual response with epileptiform characteristics. The results raise the question of the effects of such neuroplastic change on sensation and epileptiform events.

Nicotine-ethanol interactions in flash-evoked potentials and behavior of Long-Evans rats

Although nicotine and ethanol are often used together, little is known about their combined effects on visual system electrophysiology. This experiment examined the separate and combined effects of nicotine and ethanol on flash-evoked potentials (FEPs) recorded from both the visual cortex (VC) and superior colliculus (SC) of chronically implanted male Long-Evans rats. There were four treatment conditions administered on separate days: either saline or ethanol (2.0 g/kg, i.p.) was given 10 min before either saline or nicotine (1.0 mg/kg, s.c.). FEPs were recorded at 5, 20, and 40 min following the second injection. In the VC, ethanol significantly decreased the amplitude of most components, but increased P46. Peaks P22 and N53 were unchanged. Nicotine enhanced most component amplitudes, but decreased N29 and P234, while P22 and N139 were unchanged. In the SC, ethanol depressed the amplitude of all components studied. In contrast, nicotine significantly depressed only P27 and N48. Latencies of most components in both structures were increased by ethanol, nicotine, and the combination treatment, although a nicotine-induced enhancement of the effects of ethanol on latencies was not typically observed. Each drug treatment also produced significant hypothermia, with the combination treatment resulting in the greatest hypothermia. Ethanol, either alone or in combination with nicotine, significantly reduced body movements during the FEP recording sessions. In subsequent open-field observations, ethanol, but not nicotine, significantly increased the number of squares crossed, while the combination treatment produced the greatest increase in movement. Nicotine significantly increased rearing behavior, but both ethanol and the combination treatment eliminated rearings. Overall, data suggesting that nicotine can counteract some of the effects of ethanol was demonstrated in varying degrees in the amplitude of VC components N39, P46, N53, N65, and P88, the latency of VC component N53, the amplitude of SC component N59, and the latency of SC components N48 and N54. In contrast, a nicotine-induced enhancement of the effects of ethanol was found for only the latency of VC components N39, P88, and P234, body temperature, and open-field ambulation.

Nicotine alters flash-evoked potentials in Long–Evans rats

This experiment examined the effects of nicotine on flash-evoked potentials (FEPs) recorded from both the visual cortex (VC) and the superior colliculus (SC) of chronically implanted male Long-Evans rats. FEPs were recorded at 5, 20, 40, and 60 min following subcutaneous injections of saline, and of 0.4, 0.7, and 1.0 mg/kg nicotine on separate days. In the VC, the amplitude of components N(39), N(53), N(67), and P(88) increased, while the amplitude of components N(30) and P(235) decreased following nicotine administration. P(22), P(47), and N(153) were unchanged. In the SC, components P(27), N(48), and N(53) were reduced in amplitude, while P(37) and N(57) were unaffected by nicotine. Many peak latencies in the VC and SC were increased by nicotine, often at all three doses. However, effects of nicotine on FEPs were both dose- and time-dependent. When body temperature was recorded 65 min after drug administration, significant hypothermia was found with both the 0.7- and 1.0-mg/kg nicotine doses. The 1.0-mg/kg dose of nicotine resulted in a significant increase in movement during the recording sessions, but not in subsequent open-field observations. The results demonstrate that nicotinic acetylcholine receptors (nAChRs) play a differential role in the production/modulation of the various components of FEPs.

Effects of ethanol on flash-evoked potentials of rats: lack of antagonism by naltrexone

The present study examined the effects of ethanol and naltrexone hydrochloride (a nonselective opiate receptor antagonist) on flash-evoked potentials recorded from both the visual cortex (VC) and the superior colliculus (SC) of chronically implanted hooded rats. There were four treatment conditions administered on separate days: Either saline or naltrexone (10 mg/kg; volume of 1.0 ml/kg) was given 10 min before either saline or ethanol (2.0 g/kg; 20% ethanol solution in a volume of 1.26 ml/100 g). Evoked potentials were recorded 15 min after the intraperitoneal injections were completed. Animals were tested at 23.1 degrees C room temperature. In the VC, ethanol significantly decreased the amplitude of components N1, P3, and N3, whereas it increased the amplitude of P2. Components P1 and N2 were unaffected by ethanol treatment. The SC components P3 and N4 were reduced in amplitude by ethanol, but component P1 was not altered. Latencies of all components in both structures were increased by ethanol. Naltrexone alone did not significantly affect the potentials, nor did naltrexone pretreatment significantly alter the effects of ethanol on the potentials. Naltrexone produced a modest hypothermia of about 0.25 degrees C, whereas ethanol resulted in hypothermia of about 1.0 degrees C. Ethanol, either alone or in combination with naltrexone, significantly reduced body movement during the evoked-potential recording sessions. The results indicate that endogenous opioid systems do not play a major role in the acute effects of ethanol on flash-evoked potentials recorded from primary areas of the visual system.

Temperature dependence and independence of effects of pentobarbital on visual evoked potentials of rats

Visual cortex flash evoked potentials (FEPs), pattern reversal evoked potentials (PREPs), and body temperature were measured in hooded rats following IP injections on separate days of saline, and of 15 and 30 mg pentobarbital/kg body weight. Two experiments were performed, differentiated by standard (23 degrees C) and warm (31 degrees C) room temperatures. The 30 mg/kg dose produced hypothermia of 2.6 degrees C in the standard environment, but not in the warm environment. Early components of FEPs were generally increased in amplitude by the 15 mg/kg dose, and decreased by the 30 mg/kg dose at 23 degrees C. At 31 degrees C, the 30 mg/kg dose did not decrease early component amplitude, suggesting that hypothermia can potentiate some effects of pentobarbital. Amplitudes of late FEP components were depressed at both ambient temperatures. The main PREP components N1P1 and P1N3 were increased in amplitude by the 15 mg/kg dose, but returned to near baseline levels at 30 mg/kg, at both temperatures. PREP component N2P2 was reduced in amplitude by the 30 mg/kg dose only at 23 degrees C. Treatment with 30 mg/kg pentobarbital increased FEP and PREP latencies at both ambient temperatures, but the magnitudes of the increases at 31 degrees C were typically less than half those observed at 23 degrees C. These results indicate that hypothermia contributes to some pentobarbital-induced changes in both FEPs and PREPs, but that pentobarbital also produces effects independent of hypothermia.

Cortical and thalamic visual evoked potentials during sleep-wake states and spike-wave discharges in the rat

Flash visual evoked potentials (VEP) were simultaneously recorded from the primary visual cortex and the dorsal lateral geniculate nucleus in freely-moving WAG/Rij rats, to investigate whether the thalamic VEP shows the same state-dependent alterations as the cortical VEP. VEPs obtained during active and passive wakefulness (AW and PW), slow-wave sleep (SWS), REM sleep and during the occurrence of spike-wave discharges (SWD), a specific trait of the genetically epileptic WAG/Rij rat, were compared. The general architecture of the thalamic VEP resembles the cortical VEP, although its polarity is reversed. This facilitated the interpretation of components in terms of underlying neuronal events. The primary excitation peak is differently modulated in cortex and thalamus. Whereas the thalamic component (P30) is not affected by brain-state, the cortical component (N1) shows a strong increase in latency during SWS and SWD. In contrast, the modulation of later components is highly similar for cortex and thalamus. VEPs obtained during AW and REM resemble each other. During SWS and SWD there is a considerable, and during PW a moderate, enlargement of primarily inhibitory components. After-discharges are enhanced during SWS, SWD and REM. No evidence is found for a major impairment of sensory transmission during SWD.

Neurotoxicologic examination of rats exposed to 1,1,2,2-tetrachloroethylene (perchloroethylene) vapor for 13 weeks

Large evoked potential and EEG changes occurred in a pilot study in Fischer 344 rats during exposure to 800 ppm of 1,1,2,2-tetrachloroethylene [perchloroethylene (Perc)], a cleaning solvent with anesthetic properties. In the main study, rats were evaluated for persistent nervous system effects the week following exposure to 0, 50, 200, or 800 ppm Perc for 6 h/day, 5 days/week, for 13 weeks. The only effect related to treatment was in the flash evoked potential (FEP-V), recorded from the visual cortex. The longer latency potentials (N3) of the FEP-V had a greater amplitude in the 800 ppm Perc group. The FEP-Vs were of normal shape and latency. Although mild neurotoxicity could not be ruled out completely, amplitude changes in N3 can occur for a variety of psychophysiological reasons other than neurotoxicity. Consequently, as a stand-alone finding, the toxicologic significance of the larger FEP in the 800 ppm exposure group was unknown. Other data did not support a diagnosis of neurotoxicity. No treatment-related alterations were noted in expanded clinical observations, in the FEP recorded from the cerebellum (as opposed to visual cortex FEP-V), or in auditory, somatosensory, or caudal nerve evoked potentials. No treatment-related lesions were noted during histopathologic examination of eyes, optic nerves, optic tract, or multiple sections of brain, spinal cord, peripheral nerves, or limb muscles. The no-observed-effect-level (NOEL) was 200 ppm, based on increased amplitude of the longer latency potentials of the FEP at 800 ppm.

Simultaneous quantitative evaluation of visual-evoked responses and background EEG activity in rat: normative data

An integrated quantitative electroencephalography system (Phegra) for pharmacological and toxicological research in rat is described. Peak latencies and amplitudes of visual-evoked potentials, occurrence, duration, and linear excursions of photically evoked afterdischarges, "activity," "mobility," "complexity" of Hjorth, and absolute spectral powers of delta, theta, alpha, and beta frequency bands of background activity of visual cortex and frontal-visual leads were measured in freely moving rats. Counts of small and large movements were also registered. Data of baseline measurements performed in large amount of animals are presented. None of the parameters except the occurrence of photically evoked afterdischarge and the linear excursion of its averaged waveshape changed significantly in five measurements performed within six hours following the intraperitoneal and oral administration of two commonly used drug vehicles.

Possible confounding effects of strobe "clicks" on flash evoked potentials in rats

Flash evoked potentials (FEPs) undergo within- and between-session changes and are modified by auditory white noise (26). We examined whether an auditory potential produced by the "click" associated with the strobe discharge could be recorded, and if alterations in an auditory response could explain the within- and between-session changes in FEPs. We also examined differences between a frontal cortex or a nasal reference electrode location on FEPs and auditory potentials. An auditory potential associated with the strobe discharge could be clearly recorded. This response was eliminated by the presence of 80 dB SPL masking white noise. However, the within- and between-session changes in FEPs could not be explained by modifications of the auditory potential. Animals whose ear drums were ruptured did not exhibit an auditory response, and their FEPs were similar to those of controls tested in the presence of masking white noise. A nasal reference electrode decreased the impact of auditory potentials on FEPs, but allow visual potentials (electroretinogram and optic tract activity) to influence FEPs. The data show that auditory potentials associated with the strobe discharge can be recorded from the visual cortex of rats, and that these auditory responses represent a possible confounding factor in the interpretation of toxicological studies employing FEPs.

Within-session changes in peak N160 amplitude of flash evoked potentials in rats

The negative peak occurring approximately 160 ms after stimulation (peak N160) of flash evoked potentials (FEPs) of rats changes with repeated testing. Habituation, sensitization, and arousal have all been invoked to explain these changes, but few studies have directly tested these explanations. We examined within-session changes in peak N160 amplitude with repeated testing, and the modulatory effects of stimulus intensity and auditory white noise. Peak N160 amplitude increased with daily testing (between-session changes), and was larger at greater stimulus intensities. In contrast, peak N160 amplitude underwent within-session increases on early days and within-session decreases on later days. The within-session changes were not affected by stimulus intensity. In rats previously tested in a quiet environment, exposure to acoustic white noise increased motor activity and transiently decreased peak N160 amplitude, which then increased and subsequently decreased with continued photic and acoustic stimulation. Repeated testing in the presence of noise did not alter the within-session changes in peak N160 amplitude. Heart rate showed both within- and between-session decreases, but was unaffected by noise. The data suggest that the within-session changes in peak N160 amplitude may reflect a habituation-like response to the test environment.

Behavioral and electrophysiological aspects of nitrous oxide dependence

We examined the effect of 70% nitrous oxide (N2O) on locomotion and visual-evoked potentials (VEP) in rats. The animals exposed to N2O showed an initial decrease of locomotion, followed by development of tolerance and unaltered motor activity during N2O withdrawal. Similarly, an initial decrease of VEP amplitudes was followed by tolerance to N2O. In addition, some amplitudes (N2-P3, P3-N3, and N3-P4) exceeded the control values, indicating an increase of neuronal excitability of the visual system during a long lasting exposure to N2O. The increase of VEP amplitudes was further potentiated by cessation of this gas. The VEP latencies after initial increase, returned to normal and remained unaltered during N2O withdrawal suggesting that the speed of neurotransmission is not essentially changed during chronic exposure to N2O. However, a significant increase of neuronal excitability during chronic N2O exposure, which further increased by cessation of N2O, could be of clinical importance. Therefore, monitoring of VEP, particularly the amplitude values, may significantly improve a detection of altered neuronal excitability during anaesthesia and drug withdrawal.

Visual evoked potentials during spontaneously occurring spike-wave discharges in rats

Flash evoked visual potentials (VEPs) were recorded in freely moving WAG/Rij rats. These rats show spontaneously occurring spike-wave discharges in the EEG, interpreted as absence-like seizures. VEPs recorded during the presence of spike-wave discharges were compared with those obtained during normal states of vigilance as quiet wakefulness, slow-wave sleep and REM sleep. Almost similar VEPs were recorded during wakefulness and REM sleep, whereas during slow-wave sleep the second positive peak (P2) was considerably larger. In comparison with normal sleep-wake states, VEPs during spike-wave discharges showed unique changes, such as a decrease in the N1 amplitude, an increase of the P4 amplitude and an enhanced afterdischarge. Other characteristics were similar to those seen during slow-wave sleep, such as an increase of the P2 amplitude and a diminished P2-N3-P3 complex. These findings indicate sensory alterations during a spike-wave discharge. As expressed in the decrease of N1, afferent information cannot enter the thalamus during the rhythmic oscillatory mode. Such alterations may underlie the lowered responsiveness to external stimuli during spike-wave activity.

Rat flash-evoked potential peak N160 amplitude: modulation by relative flash intensity

The flash-evoked potential (FEP) of rats has a large negative peak (N160) approximately 160 ms following stimulation. This peak has been reported to be modulated by the subject's state of behavioral arousal and influenced by several test parameters. These experiments examined the influences of repeated testing, the number of stimuli/session, interactions of ambient illumination and flash intensity, and the effect of pupillary dilation on the development and amplitude of peak N160. The amplitude of peak N160 increased with daily testing and reached an asymptotic amplitude by about day 10. This amplitude was affected by the intensity of the flash stimulus relative to the ambient illumination (RFI) and appeared to reach a "ceiling" amplitude at greater than 50 dB RFI. The number of stimuli/session and dilation of the subject's pupils did not have a large influence on the growth or asymptotic level of peak N160 amplitude. The data are consistent with the hypothesis that the growth of peak N160 may represent a sensitization-like phenomenon.

Acute interactive pharmacologic effects of inhaled toluene and dichloromethane on rat brain electrophysiology

Toluene (TOL) and dichloromethane (DCM) are widely used industrial solvents and are common components of solvent mixtures that are voluntarily inhaled to produce altered states of consciousness. In previous studies we characterized some of the acute electrophysiologic effects of these solvents. Opposite effects were noted for some measures, suggesting that they might be antagonistic when combined. In this study we examined the solvents again singly (10,700 and 16,000 ppm) and also in combination (16,000 ppm: 33/67 and 67/33% TOL/DCM ratios). The single gases caused effects similar to those observed previously. Combined effects varied, dependent upon the particular variable examined and the major gas in the mixture. In some respects the solvents were concordant, exerting similar effects on a variable, e.g., both solvents prolonged the latencies of components of the brainstem auditory-evoked response. In other respects they were discordant, e.g., whereas toluene caused mean EEG frequency to increase, dichloromethane had the opposite effect. Sometimes the solvents had similar effects alone, but acted independently in combination. Nonindependent interactions were also observed-both additive/subtractive or positively or negatively synergistic. The results further demonstrate and emphasize the unique patterns of acute central nervous system effects that can be effected by solvents that might have a common behavioral endpoint such as anesthesia, and the results characterize a variety of electrophysiologic interactions between these two solvents. Although there were several variables exhibiting synergistic relationships, independent or additive interactions were the most common.

Effect of acute respiratory acidosis on multimodality sensory evoked potentials of Long-Evans rats

Auditory, visual and somatosensory evoked potentials (EPs), recorded epidurally from 31 chronically implanted male Long-Evans rats, were studied to examine the pattern of sensory effects caused by hypercapnia. Recordings were obtained before exposures, 10-20 min after the beginning of exposure to CO2 in synthetic air, and 30 min after the end of exposure. Previous recordings revealed no substantial effects of the extended recording period itself. Blood pH during an average exposure of 18.8% CO2 was about 7.1. During this level of CO2 exposure the somatosensory response was almost completely abolished, but the latencies of early detectable components were not affected. In contrast, the latencies of all brainstem auditory evoked response components and the 1-5 interwave time increased, whereas amplitudes were only slightly affected. Amplitudes and latencies of early and late components of the flash EP were decreased and lengthened, but the after-discharge components appeared to be most sensitive to CO2. Concentration-response relationships were examined by exposure of rats to 8 and 16% CO2. The most sensitive EP parameter was average amplitude of the late somatosensory EP components. These results suggest that EPs might be useful for assessing acute metabolic disturbances as well as more commonly assessed neurologic disorders.

Acute effects of inhaled dichloromethane on the EEG and sensory-evoked potentials of Fischer-344 rats

Acute effects of inhaled dichloromethane on the spontaneous electroencephalogram (EEG) and sensory-evoked potentials (EPs) were characterized and compared to previously observed effects of toluene; both solvents are common components of abused solvent mixtures. Twelve adult male Fischer-344 rats with chronic epidural electrode implants served as subjects. Each rat was exposed for 60 min to 5,000, 10,000, and 15,000 ppm dichloromethane while held in a plastic restrainer that also served as a head-only exposure chamber. The sequence of exposures was counterbalanced across rats, and the exposures were separated by about one week. To characterize the time course of any changes, somatosensory and flash EPs were recorded every 5 min during the first 45 min of the exposures. As was the case with toluene, electrophysiologic waveforms recorded from different sensory systems, and components of these waveforms, reacted in different ways to dichloromethane. With respect to the FEP and SEP the two solvents produced quite different effects. Toluene increased the amplitudes of early FEP components, eliminated late components, induced oscillations in visual cortex, and had no discernible effects on component latencies. In contrast, dichloromethane eliminated the N1 component, at moderate exposure had little or no effects on amplitudes of the later components (N3 through N4), did not induce oscillations, and had significant effects on latencies. Whereas toluene dramatically increased SEP component amplitudes at moderate concentrations with diminishing effect at higher concentrations and exposure times, dichloromethane rather uniformly decreased SEP amplitude in a simple concentration-related way. Toluene and dichloromethane had similar effects on BAER component latencies. They both caused component (P1 through P5) latencies and the P1-P5 interwave time to increase. However, whereas toluene increased early and late (but not middle) component amplitudes, dichloromethane decreased the amplitudes of early and late components and increased the amplitudes of middle components. These results emphasize the acute pharmacologic specificity of different solvents and suggest that differences in chronic neurotoxicity might also be found; they also suggest that predictable interactions might be found with acute and chronic exposure to mixtures that contain such solvents.

Effects of ketamine, naloxone, and physostigmine on flash evoked potentials in rat superior colliculus

Flash evoked potentials were recorded from the superior colliculus of chronically implanted hooded rats at 5 and 20 min following IP injections of saline, ketamine (75 mg/kg), naloxone (10 mg/kg), or physostigmine (0.4 mg/kg) on separate days. Components in an early positive complex were unaffected by ketamine and naloxone, but were reduced in amplitude by physostigmine. A positive spike emerged from the middle of a later negative wave following ketamine administration, but the amplitude of the negative wave was unaltered by naloxone or physostigmine. A succeeding positive component was enhanced by both ketamine and physostigmine. Physostigmine produced the most consistent alterations in latency, with most components increasing in latency. Naloxone pretreatment did not alter ketamine's influence on evoked potential amplitudes. Pretreatment with physostigmine briefly decreased the amplitude of the ketamine-induced positive spike, augmented the amplitude of the succeeding positive component, and also increased most peak latencies. Ketamine, naloxone and physostigmine all produced approximately equivalent hypothermia. Physostigmine, but not naloxone, pretreatment augmented the ketamine-induced hypothermia. The body temperature data suggest that some of the observed latency alterations are secondary to hypothermia. The amplitude data indicate that ketamine and physostigmine produce a combination of similar, distinct, and antagonistic effects on evoked potentials.

Peak N160 of rat flash evoked potential: does it reflect habituation or sensitization?

Flash evoked potentials recorded from awake rats contain a negative peak occurring about 160 msec after the flash (N160). This peak has been associated with a specific level of arousal, and/or habituation by various authors. The current studies attempted to determine whether changes in N160 amplitude which accompany repeated testing reflect processes associated with sensitization or habituation. This paper describes experiments in Long-Evans hooded rats which demonstrate the effects of repeated testing, varying stimulus intensity, varying stimulus frequency, and discharging an alarm bell before and during a test session. Repeated testing produced increases in N160 amplitude which were greater at high than low stimulus intensities. Repeated exposure to the test chamber without flashing did not alter N160 amplitude, nor did altering stimulus rate within the range of 0.5 to 4.0 Hz. Discharging an alarm increased N160 amplitude. Taken together, the data suggest that amplitude of N160 more closely reflects sensitization to the stimulus than habituation to either the stimulus or any feature of the test situation.

gamma-Hydroxybutyrate model of generalized absence seizures: further characterization and comparison with other absence models

gamma-Hydroxybutyrate (GHB) produces a constellation of EEG and behavioral events that respond selectively to antiabsence antiepileptic drugs. The GHB-induced seizure was quantitated in the presence of three other absence seizure models: pentylenetetrazole, systemic penicillin, and the flash evoked afterdischarge (FEAD). Penicillin and pentylenetetrazole produced a significant prolongation of GHB-induced seizure in a dose-dependent fashion. This potentiation of GHB seizure was observed when these compounds were given either before administration of gamma-butyrolactone (GBL), the prodrug of GHB, or at the onset of GHB-induced seizure. Photic stimulation given in a manner to produce FEAD also resulted in a significant prolongation of GHB-induced seizure. All of these maneuvers lowered the threshold to GHB seizure, but none interfered with the brain kinetics of GHB in the animals treated with GBL. Ethosuximide pretreatment significantly shortened the GHB seizure and overcame the potentiating effect of penicillin and pentylenetetrazole in this model. These data confirm the GHB-treated animal as a model of generalized absence seizure. The GHB model meets appropriate criteria for an absence seizure model and compares favorably with other models of absence currently in use.

Hepatic encephalopathy. Application of visual evoked responses to test hypotheses of its pathogenesis in rats

A previous study of the patterns of visual evoked responses (VERs) in rats was interpreted as providing support for the synergistic neurotoxins hypothesis of the pathogenesis of hepatic encephalopathy (HE) due to fulminant hepatic failure (FHF). In contrast, other studies of the patterns of VERs in rabbits with different encephalopathies were interpreted as providing support for the concept that increased GABA-ergic tone may contribute to the neural inhibition of HE due to FHF. To attempt to resolve the discordant findings in these studies, additional studies of VERs have been undertaken in rats. To induce increased tissue levels of ammonia, mercaptans and fatty acids which are found in HE due to FHF, carefully predetermined doses of urease, dimethyldisulphide and octanoic acid were administered. The (pre-seizure) encephalopathy induced by these three agents was associated with abnormalities of the VER waveform that were fundamentally different from the abnormalities of the VER waveform associated with HE due to thioacetamide-induced FHF. However, the VER waveform in this model of HE due to FHF resembled closely that associated with pentobarbital-induced encephalopathy. These findings are in satisfactory agreement with those in the previous analogous studies in rabbits. They do not provide support for the synergistic neurotoxins hypothesis of the pathogenesis of HE, but are entirely consistent with increased GABA-ergic tone contributing to the neural inhibition of HE due to FHF.

The effects of bilateral and unilateral frontal lesions on visual cortical hypersynchronous bursting and behavioral activity

Photically evoked after-discharge (PhAD) bursting was examined in albino rats following either bilateral or unilateral dorsal frontal lesions. The initial effect of either type of frontal lesion was to significantly suppress PhAD occurrence. However, after repeated exposure to the PhAD recording environment no difference could be seen between the lesioned and control animals. In addition, no difference in PhAD could be detected between the two visual cortices in any of the groups. Spontaneous alternation and open-field activity were also tested. Both frontally lesioned groups of animals were significantly more active than control animals in the initial stages of testing. These findings indicate that the frontal cortex can inhibit brain stem mechanisms which are responsible for behavioral arousal and the modulation of hypersynchronous bursting in the cortex. However, because no difference was seen between the two visual cortices in any of the groups, it is suggested that this mechanism is not a strictly unilateral mechanism.

Amphetamine potentiation of anti-conflict action of chlordiazepoxide

A modified Geller-Seifter paradigm was employed to test in male albino rats the effects of subthreshold doses of amphetamine and chlordiazepoxide (CDP), administered separately or in combination, on shock induced suppression of food-reinforced lever-pressing. MK-801, a newly synthesized sympathomimetic with anxiolytic and anticonvulsant properties, was also tested. dl-Amphetamine in doses of 0.1, 0.2, 0.3, and 1.0 mg/kg had no anxiolytic nor anxiogenic effects, but at 1.0 mg/kg it increased non-conflict responding. CDP in doses of 0.1, 0.2, and 0.4 mg/kg had no significant effect on conflict and non-conflict responding. CDP in the dose of 0.8 mg/kg tended to increase conflict responding. Coadministration of amphetamine (0.2 mg/kg) and CDP (0.4 mg/kg) had a significant anti-conflict effect. MK-801 at 50 micrograms/kg and 100 micrograms/kg caused a significant increase in non-conflict responding. MK-801 at 50 micrograms/kg exerted also a significant anti-conflict effect. The disinhibitory effects of amphetamine coadministered with CDP were discussed in terms of a possible enhanced noradrenergic or dopaminergic activity and their interaction with GABA neurotransmission at GABA-benzodiazepine coupled sites.

Superior colliculus lesions and flash evoked potentials from rat cortex

It is generally assumed that the primary response of the rat flash evoked potential (FEP) is activated by a retino-geniculate path, and that the secondary response reflects input to the cortex by way of the superior colliculus (SC) or other brainstem structures. In the present study, male Long-Evans rats were implanted with monopolar screw electrodes placed over the left visual cortex, and a pair of twisted monopolar depth electrodes, which were used to produce electrolytic lesions, were placed in each SC. One half of the animals did not receive the electrolytic treatment (controls). FEP waveforms were obtained from all animals prior to treatment, and 2 and 5 days after treatment. Histological analysis was performed to verify electrode placement and determine lesion size. Electrolytic lesions resulting in massive destruction of the SC produced no decrement in any portion of the rat FEP but did produce an increase in amplitude of the N2P3 component. The data show that the secondary response is not generated by SC in rats, but that SC may modulate amplitude of the response.

The relationship between visual seizures and visual evoked potentials

We have studied a patient (pt) in status epilepticus with visual seizures (szs) arising focally from the right occipital area and have recorded the visual evoked potential (VEP) to three different stimuli under three different conditions (during, between and with no szs). The pt experienced "sparkling" in the contralateral visual field as the sz and the intensity of the "sparkling" was directly related to the frequency of the ictal activity recorded on the EEG. During the szs the VEPs could still be recorded, but the amplitude of the P100 was higher on the contralateral side with pattern reversal (PR) stimuli, and with flashes (FL) the positive peak after the P100 was less evident on the ipsilateral side. Latencies to these latter two positivities were generally shorter than in normals, with much greater standard deviations ipsilaterally with FL and contralaterally with PR, especially during the actual szs. The relationship between VEP generation and visual sz phenomena is discussed.

Antiepileptic effects of amphetamine may require GABA (benzodiazepine) activity

Secondary components of visual evoked potentials (slow negative wave-SNW, and photically-evoked sensory after discharge-SAD) are known to be precursors of experimentally activated wave-spike discharges, similar to wave-spikes of petit mal epilepsy. Both SNW and SAD may be potently suppressed either by amphetamine or GABAergic compounds such as diazepam and sodium valproate. A hypothesis was tested in the present study, that amphetamine-induced suppression of wave-spike discharges may require GABA-benzodiazepine activity for its expression. Electrocortical activity was recorded and averaged in unrestrained albino rats with chronically implanted epicortical electrodes. SNW and SAD obtained in habituated rats in the predrug state were potently suppressed by amphetamine (1 mg/kg, i.p.). Fifteen minutes after amphetamine injection, a challenging drug (metrazol, picrotoxin, convulsant benzodiazepine, Ro 5-3663, or imidazodiazepine, Ro 15-1788) was administered intraperitoneally. Subconvulsive doses of metrazol (10 mg/kg) reversed amphetamine suppression; imidazodiazepine (20 mg/kg) and picrotoxin (1.5 mg/kg) reliably opposed the SNW suppression; convulsant benzodiazepine, Ro 5-3663 (2 mg/kg), showed modest and nonsignificant effect in the same direction. It is proposed that the antiepileptic potency of amphetamine may be associated with its ability, apparently via modulatory effect of norepinephrine, to facilitate the activation of benzodiazepine-GABA receptors.

Visual evoked potentials in rats selected for high or low self-stimulation

Visual evoked potentials (VEPs) were analyzed in order to distinguish between rats from genetically high (HI) and low (LO) self-stimulation lines (LC2-HI and LC2-LO). Secondary VEP components - slow secondary negative wave (SNW) and sensory afterdischarge (SAD) - which are considered to be most sensitive indices of normal and pharmacologically-induced behavioral changes, were used for the comparison. Small, albeit statistically significant enhancement of SNW and SAD was obtained in LO rats. Unlike LO animals, HI rats gained in SNW amplitude and SAD area during repeated photic stimulation. The difference being highly significant. D,L-Amphetamine (1 mg/kg, i.p.) suppressed SAD and reduced the SNW amplitude in both HI and LO animals, although the predrug difference in their values remained practically unaltered. Apomorphine (0.25, 2.75, 5.25 mg/kg i.p.) had no measurable effect on VEP parameters even though it caused a regular picture of dose-related enhancement of locomotion and stereotypy. The effect of amphetamine can, therefore, be attributed to the activation of the norepinephrinergic system. Correspondingly, VEP variance in the two lines of rats is interpreted as related to the peculiarities of norepinephrine modulation of neocortical activity.

Amygdala kindling in the classical conditioning paradigm

The hypothesis was tested that epileptiform alterations of brain excitability may be elicited by a conditioning stimulus (CS) in a classical conditioning paradigm. Fifteen male albino rats were chronically implanted with electrodes in the amygdala and over the visual cortex. A train of six stroboscopic flashes served as the CS and the unconditioned stimulus was the amygdaloid electrical stimulation, delivered together with the sixth flash in the course of the kindling procedure. Rats were randomly assigned to two groups. Rats in Group 1 were exposed to one or two kindling sessions per day, and those in Group 2 were exposed to 10 such sessions every day. All rats in Group 1 showed fully mature seizures within 15 +/- 5.5 sessions, whereas none of those in Group 2 displayed kindled seizures. There was no evidence of amygdaloid afterdischarges or enhanced spiking as a response to the CS. A gradual selective suppression of the visual evoked potentials (VEP) secondary components, i.e., slow negative wave and sensory afterdischarge, paralleled kindling in Group 1. This effect was weak in Group 2. There was no change in the primary response of VEPs in either group. No recovery of VEPs followed the extinction of conditioning.

Regular and lasting neocortical spiking produced by systemic administration of a steroid derivative in the rat

The hypothesis was tested that sedation and stereotyped behaviour, developing in rats after the administration of the steroid derivative with gamma-aminobutyric acid (GABA) antagonistic properties, R 5135, are of an epileptiform nature. Electroencephalographic (EEG) and visual evoked potentials (VEP) were recorded and behaviour was observed over not less than 5-7 hr after subconvulsive doses of R 5135. Doses of 2-4 mg/kg of the compound produced quasi-rhythmic spikes resembling experimental focal epileptic discharges in all rats. This epileptiform activity was accompanied by behavioural sedation and somnolence, followed by a build-up of stereotyped behaviour and sporadic episodes of epileptiform motor activity, developing 1-2 hr after injection. The secondary components (SNW) of the visual evoked potentials were suppressed by R 5135 and the primary potential (N1) facilitated, virtually reducing the visual evoked potential to the form of an evoked spike. Pretreatment with the anticonvulsant GABAergic drugs gamma-acetylenic GABA (GAG) (100 mg/kg), sodium valproate (VPA) (400 mg/kg) and diazepam (5 mg/kg) suppressed the motor components of seizure activity, producing severe ataxia, but not the electrographic manifestation of seizure activity. Neither gamma-acetylenic GABA nor valproate significantly altered the latency to onset of spiking, although all three drugs did significantly reduce the frequency of discharges. Diazepam was the only anticonvulsant tested which completely suppressed spike activity in 3 of 5 rats. Moreover, R 5135 was found to antagonize diazepam, but not valproate induced suppression of secondary components of the visual evoked potential, suggesting that diazepam and R 5135 may compete for the same receptor.

Acute effects of alcohol on photic evoked potentials of rats: lateral geniculate nucleus and reticular formation

This study examined the effects of ethanol on photic evoked potentials recorded from the dorsal lateral geniculate nucleus (LGN) and midbrain reticular formation (MRF) of chronically implanted albino rats. Animals were given intraperitoneal injections of saline, or of 0.5, 1.0, 1.5 or 2.5 g ethanol/kg body weight on separate days. Evoked potentials were recorded at 5, 20, 40 and 60 min following injection. An early positive component recorded from each structure was depressed in amplitude by only the 2.5 g/kg alcohol dose, while the succeeding negative component was depressed by both the 1.5 and 2.5 g/kg doses. Latencies of both early components in each structure were increased by the 1.5 and 2.5 g/kg alcohol doses. Alcohol doses of 1.0-2.5 g/kg depressed the amplitude of a later positive component in the LGN (latency of 78 msec), but latency was not altered. In contrast, a late positive component in the MRF (latency of 150 msec) was both decreased in amplitude and increased in latency by only the 2.5 g/kg dose. These results on subcortical structures are discussed in relation to alcohol's effects on cortical evoked potentials.

Benzodiazepine antagonists abolish electrophysiological effects of sodium valproate in the rat

A hypothesis was considered that anti-epileptic potency of sodium valproate (VPA) may be associated with its action via the benzodiazepine system. The ability of anti-petit mal drugs to suppress the slow secondary negative wave (SNW) of the visually evoked potential was used as a sensitive electrophysiological "tag" for comparison of VPA (200 mg/kg, i.p.) and Diazepam (5 mg/kg, i.p.) effects. Both drugs induced a profound inhibition of the SNW. Benzodiazepine antagonists Ro 5-3663 (2 mg/kg, i.p.) and Ro 15-1788 (5 mg/kg, i.p.) caused recovery of the SNW amplitude within several minutes of injection. Both antagonists abolished immobility and sedation produced by VPA and Diazepam. The possibility should be considered that therapeutic effects of VPA are mediated through the benzodiazepine receptor coupled to GABA.

alpha 2-adrenergic antagonists suppress epileptiform EEG activity in a petit mal seizure model

Experiments were performed to determine the effects of yohimbine, piperoxan, and mianserin on the flash-evoked afterdischarge (FEAD) in rat. All three drugs are alpha-adrenergic antagonists, which block the inhibition of norepinephrine-release mediated by presynaptic (alpha 2) receptors. It was predicted that these drugs would exert dose-dependent biphasic effects on FEAD: at low doses, which act presynaptically, the FEAD would be suppressed; while at higher doses, which also block postsynaptic (alpha 1) receptors, the FEAD would be disinhibited. The dose response curves for yohimbine and piperoxan were, in fact, biphasic with low doses decreasing the amount of FEAD, while higher doses returned the amount of FEAD to baseline. No biphasic effect was seen with mianserin. These observations are consistent with the fact that yohimbine and piperoxan are more potent antagonists at alpha 2- than at alpha 1-receptors, whereas the affinities of mianserin for the two receptor types is comparable. The results of these experiments support the hypothesis that norepinephrine modulates FEAD.

Suppression by sodium valproate of gamma-vinyl GABA-induced facilitation of visual evoked potentials in rats

A single dose (500 mg/kg, i.p.) of GABA-transaminase inhibitor, gamma-vinyl GABA (GVG), administered to Wistar rats caused time-related 2--3-fold enhancement of the slow negative wave (SNW) and sensory after-discharge (SAD) of the VEP. This effect was detectable at 1 h, reached plateau at 3--4 h and remained at this level 7 h after GVG. Sodium valproate (VPA, di-n-propylacetic acid), which is also believed to block GABA-transaminase and succinic semialdehyde dehydrogenase, when administered in a dose of 200 mg/kg (i.p.) 3 h after GVG reversibly antagonized the SNW-SAD hypersynchronization. The time-course of this effect was similar to that obtained after VPA administration alone. These findings are interpreted as suggesting that VPA acts via a mechanism using a transmitter intimately connected to but not identical with GABA.

Pharmacologic analysis of sodium valproate-induced suppression of secondary components of visual evoked potentials in albino rats

Sodium Valproate (VPA) administered to rats in a dose of 10 or 200 mg/kg IP suppressed the slow negative wave (SNW) and photically-induced afterdischarge (SAD) of VEP (when they were present) within 15-30 min. The recovery of VEP amplitude began at 3 hr. This effect was antagonized by subconvulsive doses of convulsant benzodiazepine RO 5-3663 (2 mg/kg) and metrazol (15 mg/kg) but not by picrotoxin (2 mg/kg) and naloxone (10 mg/kg). The SNW suppression may be attributed to a disinhibitory action of a system located presynaptically on recurrent collaterals of the output neurons, or nerve terminals of inhibitory interneurons or both. Alternative conjecture suggests that VPA depolarizes the dendritic tree masking thereby somatic inhibition produced by recurrent circuits.

Thalamocortical coupling and component properties of visually evoked afterdischarge

Dorsal lateral geniculate nucleus (dLGN) multiple-unit activity (MUA) and single visual evoked responses (VER) followed by afterdischarge (AD) were examined in lightly restrained albino rats. It was found that VER AD spike components are initially quite prominent, exceeding at times the amplitude of the broad surface negative wave components. As VER AD progresses toward termination spike amplitude systematically declines. Wave amplitude, however, remains relatively stable until spike components disappear. Averaged dLGN MUA and VER AD responses to photic stimuli suggest the existence of a precise time-locked relationship between peak dLGN MUA and VER AD spike components. It is proposed that thalamic and cortical mechanisms sustaining VER AD bursting might be operative during sensory information ;processing, serving to sustain neural activity functionally related to stimulus input.

Flash-evoked afterdischarge in rat as a model of the absence seizure: dose-response studies with therapeutic drugs

The flash-evoked afterdischarge (FEAD) is a self-sustained burst of wave-and-spike complexes recorded from occipital cortex in the rat and other animals in response to a single light flash. On the basis of behavioral experiments and studies employing single doses of antiepileptic drugs, FEAD has been proposed as a model of the absence seizure. In order to test the validity of FEAD as an absence seizure model, the present experiments determined dose-response relationships for the suppression of FEAD by six antiepileptic drugs with established clinical profiles. It was found that phenobarbital, ethosuximide, and trimethadione suppressed FEAD in a dose-related manner, and that ethosuximide was approximately three times as potent as trimethadione. Mephenytoin produced a maximal reduction of FEAD of only 30 to 40%, which was not dose-related. Neither phenytoin nor acetazolamide suppressed FEAD. The results obtained with ethosuximide, trimethadione, and phenytoin are qualitatively similar to their therapeutic effects in absence epilepsy. The FEAD model failed, however, to unequivocally predict the therapeutic efficacy of mephenytoin or acetazolamide. In this respect, it is similar to the metrazol seizure model. It is concluded that FEAD is a valid absence seizure model with a pharmacological predictive value that is at least as good as the metrazol model.

Photically evoked potentials and afterpotentials recorded from the visual cortex of the unanesthetized hedgehog

Evoked potentials to visual stimuli (VEP) were recorded from the visual cortex of the unanesthetized hedgehog (Erinaceus europaeus), a primitive placental mammal with relatively little differentiation of cortex and thalamus. The VEPs consisted of several distinct positive and negative voltage deflections. Reproducibility of the response was high, as indicated from the small intrasession and intersession response variability. Rhythmic afterpotentials (AP), previously reported for higher mammals, were readily elicited. They had a lower frequency (3/sec) than APs observed in other mammals. The use of the hedgehog in electrophysiological and psychophysiological research is suggested because data obtained from this primitive placental mammal may shed light on CNS functions of higher mammals, as well as mammalian forms 'lower' on the evolutionary scale.