Relationship between photically evoked after-discharge occurrence and hippocampal EEG rhythms in restrained and unrestrained albino rats

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.

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.

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.

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.

Food-deprivation induced arousal: evidence against "behavioral inhibition" in electrocortical hypersynchrony

Hypersynchronous activity was recorded from the visual cortex of the rat in the form of photically evoked after-discharges following 0, 24, 48, and 72 hr. of food deprivation. After-discharge activity was affected at only the 48-hr. level of deprivation and only in terms of a decrease in the frequency of occurrence. All other measures—after-discharge burst duration, after-discharge spindle amplitudes, and spindle waves per after-discharge burst—were unaffected by conditions of food deprivation. These results are discussed in terms of evidence against the uniform role of behavioral inhibition in hypersynchronous brain activity.

The photically evoked afterdischarge: a model for the study of drugs useful in the treatment of petit mal epilepsy

Lightly restrained albino rats were administered dipropylacetic acid, trimethadione, diphenylhydantoin, saline, and a pentylenetetrazol challenge. The results were attributed to the locus of action of the anticonvulsants and strongly support the usefulness of the photically evoked after discharge as a model for the evaluation of thalamically active drugs, with particular reference to those useful in the control of petit mal epilepsy.

Metrazol potentiated after-discharges: dose-response relationships and effects of selective lesions

In Experiment 1 the dose-response effects of pentylentetrazol (Metrazol) on photically evoked after-discharge (PhAD) parameters were examined. Metrazol potentiated PhAD activity by affecting all measured parameters - PhAD frequency, amplitude, burst duration and spindle composition - in particular PhAD burt duration and spindle composition. A mimimum effective dose of 10 mg/Kg of Metrazol was required for some statistically reliable potentiation. Metrazol dosage levels of 20 and 25 mg/Kg induced lengthy bouts of EEG spindling and spiking as well as near maximized PhAD component augmentation. In Experiment 2 stereotaxically oriented knife-cuts isolated the thalamus from cortical and/or midbrain and brainstem input. Such lesions did not block the capacity of Metrazol to potentiate PhADs, although the lesions altered evoked activity. These findings are discussed in terms of the current thought of Metrazol action and thalamic mechanisms in the control of after-discharge activity.

The effect of ACTH-analogues on motor behavior and visual evoked responses in rats

Averaged visual evoked responses (VER) in cortical area 17 were recorded one hour after the administration of 7-l-phe ACTH4-10 or 7-d-phe ACTH4-10 to artificially ventilated rats, paralysed with gallamine. In addition, the effects of these peptides on spontaneous motor behavior were analysed. The results show that the latencies of all VER components remain unchanged and the amplitudes of the primary VER were unaffected. Measured at a wide variety of light intensities, however, the amplitudes of the VER afterdischarge were significantly and very similarly diminished by both peptides, the effect of 7-l-phe ACTH4-10 being somewhat stronger than that of 7-d-phe ACTH4-10. These results support the notion, advanced by others, that these peptides have an effect on a CNS vigilance regulating system, yet do not explain the reported opposite effects on active avoidance behavior of the two related peptides. The effects appear specific since spontaneous motor behavior, as index of changes in generalised arousal, is unaffected by these two peptides.

Pharmacological suppression of photically evoked after-discharges in rats: incremental dose, hippocampal EEG and behavioral activity correlates

In subjects lightly restrained (Experiment I) pharmacological arousal via pilocarpine, physostigmine, or amphetamine administration, as compared to saline and methyl atropine treated controls, suppressed photically evoked after-discharge (PhAD) activity in visual cortex, while concomitantly inducing rhythmical slow-wave activity (RSA) in dorsal hippocampus. Incremental doses of amphetamine and pilocarpine correspondingly suppressed PhAD parameters in a dose-response fashion. While physostigmine treatment resulted in significant PhAD suppression, this effect could not be quantified in a dose-response manner. Incrementally increased cholinergic blockade via atropine administration also suppressed PhAD bursting yet simultaneously induced large amplitude irregular slow-wave activity (LIA) in dorsal hippocampus. When tested under identical conditions but in an unrestrained environment (Experiment II) PhADs were similarly suppressed with the concomitant induction of hippocampal patterns as specified in Experiment I; however, general ambulatory activity (grid-crossing) was differentially affected by the drugs. Amphetamine and atropine markedly enhanced while pilocarpine and physostigmine suppressed such activity. In both experiments, previously established PhAD-movement, PhAD-RSA-LIA, and RSA-LIA-movement relationships, occurring as hypothesized in amphetamine, methyl atropine, and saline treated animals, did not remain fully intact during cholinergic alteration.

Neuroheptapeptide influence on attention and memory in man

Twenty normal, male volunteers were administered a subcutaneous injection of either ACTH/MSH 4-10 or diluent and two weeks later received the alternate injection in a complete crossover, double-blind design with order balanced. Subjects were given a battery of psychological tests, including a continuous performance task (CPT), following each injection. Visual evoked responses were recorded during the CPT. Resting, eyes-closed EEGs were also obtained. ACTH/MSH 4-10 improved attention and in so doing improved visual-motor learning and visual, but not verbal, memory, EEG data were consistant with activation of the diffuse thalamic projection system.

Lateral geniculate multiple-unit activity related to metrazol potentiated after-discharges

Following parenteral administration of subconvulsive levels of pentylenetetrazol (Metrazol) photic stimulation induced an augmented rhythmic sequence of late neuron population burst-inhibition periods in the dorsal lateral geniculate nucleus (LGN) of the rat. This late bursting-inhibition activity was in turn observed to be associated with the augmentation of cortically recorded photically evoked after-discharges (PhADs). Multiple-unit activity (MUA) was also recorded from superior colliculus (SC), reticular formation (RF), posterior thalamic area (PTN), and dorsal hippocampus (HIPP). Only SC and RF exhibited an initial discharge to photic stimulation with late bursting infrequently observed and only in the SC. PTN showed some tonic increases in MUA following photic stimulation. HIPP MUA was essentially unaffected by photic pulse stimulation. The results document the neuronal role of the LGN in PhAD activity and were discussed in terms of a recurrent LGN inhibitory system governing cortical PhAD production and elaboration.