Inhibitory modulation of cat somatosensory cortex: a pharmacological study

Spike-wave discharges in Sprague-Dawley rats reflect precise intra- and interhemispheric synchronization of somatosensory cortex

Spike-wave discharges (SWDs) are among the most prominent electrical signals recordable from the rat cerebrum. Increased by inbreeding, SWDs have served as an animal model of human genetic absence seizures. Yet, SWDs are ubiquitous in inbred and outbred rats, suggesting they reflect normal brain function. We hypothesized that SWDs represent oscillatory neural ensemble activity underlying sensory encoding. To test this hypothesis, we simultaneously mapped SWDs from wide areas (8 × 8 mm) of both hemispheres in anesthetized rats, using 256-electrode epicortical arrays that covered primary and secondary somatosensory, auditory and visual cortex bilaterally. We also recorded the laminar pattern of SWDs with linear microelectrode arrays. We compared the spatial and temporal organization of SWDs to somatosensory-evoked potentials (SEPs), as well as auditory- and visual-evoked potentials (AEPs and VEPs) to examine similarities and/or differences between sensory-evoked and spontaneous oscillations in the same animals. We discovered that SWDs are confined to the facial representation of primary and secondary somatosensory cortex (SI and SII, respectively), areas that are preferentially engaged during environmental exploration in the rat. Furthermore, these oscillations exhibit highly synchronized bilateral traveling waves in SI and SII, simultaneously forming closely matched spread patterns in both hemispheres. We propose that SWDs could reflect a previously unappreciated capacity for rat somatosensory cortex to perform precise spatial and temporal analysis of rapidly changing sensory input at the level of large neural ensembles synchronized both within and between the cerebral hemispheres.NEW & NOTEWORTHY We simultaneously mapped electrocortical SWDs from both cerebral hemispheres of Sprague-Dawley rats and discovered that they reflect systematic activation of the facial representation of somatosensory cortex. SWDs form mirror spatiotemporal patterns in both hemispheres that are precisely aligned in both space and time. Our data suggest that SWDs may reflect a substrate by which large neural ensembles perform precise spatiotemporal processing of rapidly changing somatosensory input.

Study of neurovascular coupling by modulating neuronal activity with GABA

Fundamental to the interpretation of neurovascular coupling is determining the neuronal activity that accounts for functional hyperemia. Recently, synaptic and not spiking activity has been found to be responsible for the hemodynamic response. Using pharmacological manipulation in rats, we want to further determine whether the cortical synaptic activity generated by the thalamic input or the subsequent synaptic activity related to secondary cortical processing is driving the hemodynamic response. In this study, we topically applied γ-aminobutyric acid (GABA) in the somatosensory cortex and used electrical forepaw stimulation to evoke neural and vascular activity. In a group of 8 animals, using laminar electrophysiology, we verified that topical application of GABA for 20min does not affect layer IV synaptic activity but reduces subsequent activity in the supragranular and infragranular layers. In another group of 8 animals, we simultaneously measured the electrical and vascular responses with scalp electroencephalography (EEG) and diffuse optical imaging (DOI), respectively. We decomposed somatosensory evoked potentials (SEP) into three major components: P1, N1, and P2, where P1 represents the thalamic input activity originating in layer IV and N1 and P2 represent the subsequent cortical transmissions. We verified that GABA infusion in the somatosensory cortex does not significantly reduce the P1 SEP component but strongly reduces the N1 and P2 components. We found that GABA also elicits a large reduction in the hemodynamic responses, which correlate with the reduction in N1 and P2 components. These results suggest that the hemodynamic response is predominantly driven by cortico-cortical interactions and not by the initial thalamocortical activity in layer IV.

Dissociation of slow waves and fast oscillations above 200 Hz during GABA application in rat somatosensory cortex

Fast electrical oscillations (FOs; > 200 Hz), superimposed on vibrissa-evoked slow potentials, may support rapid sensory integration in neocortex. Yet, while it is well established that the positive/negative (P1/N1) slow wave of the somatosensory evoked potential primarily reflects sequential activation of supragranular and infragranular pyramidal cells mediated chiefly via excitatory chemical synaptic pathways, little is known about the generation of FOs. In this study, laminar current source-density analysis and principal component analysis indicated that FOs are generated by two dipolar current sources situated in the supra- and infragranular layers, similar in laminar location to the two current dipoles associated with the slow wave. However, exogenous GABA application reversibly abolished the N1 slow wave, leaving the P1 intact, while the FO was unaffected by GABA. Furthermore, reductions in both supra- and infragranular cortical unit discharge during application of GABA suggests that FO generation is not dependent on the same intracortical synaptic circuits that are associated with the N1 slow wave. These data suggest a marked functional dissociation between neural mechanisms underlying the slow and fast components of the vibrissa-evoked response.

Effects of bicuculline methiodide on fast (>200 Hz) electrical oscillations in rat somatosensory cortex

Fast oscillatory activity (more than approximately 200 Hz) has been attracting increasing attention regarding its possible role in both normal brain function and epileptogenesis. Yet, its underlying cellular mechanism remains poorly understood. Our prior investigation of the phenomenon in rat somatosensory cortex indicated that fast oscillations result from repetitive synaptic activation of cortical pyramidal cells originating from GABAergic interneurons (). To test this hypothesis, the effects of topical application of the gamma-aminobutyric acid-A (GABA(A)) antagonist bicuculline methiodide (BMI) on fast oscillations were examined. At subconvulsive concentrations (approximately 10 microM), BMI application resulted in a pronounced enhancement of fast activity, in some trials doubling the number of oscillatory cycles evoked by whisker stimulation. The amplitude and frequency of fast activity were not affected by BMI in a statistically significant fashion. At higher concentrations, BMI application resulted in the emergence of recurring spontaneous slow-wave discharges resembling interictal spikes (IIS) and the eventual onset of seizure. High-pass filtering of the IIS revealed that a burst of fast oscillations accompanied the spontaneous discharge. This activity was present in both the pre- and the postictal regimes, in which its morphology and spatial distribution were largely indistinguishable. These data indicate that fast cortical oscillations do not reflect GABAergic postsynaptic currents. An alternate account consistent with results observed to date is that this activity may instead arise from population spiking in pyramidal cells, possibly mediated by electrotonic coupling in a manner analogous to that underlying 200-Hz ripple in the hippocampus. Additionally, fast oscillations occur within spontaneous epileptiform discharges. However, at least under the present experimental conditions, they do not appear to be a reliable predictor of seizure onset nor an indicator of the seizure focus.

Visceral region in the rat primary somatosensory cortex identified by vagal evoked potential

Recent noninvasive human studies have reconfirmed the presence of a viscerally responsive region in the most lateral part of the primary somatosensory cortex (S1). The present electrophysiological study identified the corresponding area in rats as a vagal afferent projection region and examined the cytoarchitecture. Electrical stimulation of the cervical vagus nerve elicited a field potential comparable in waveform, latency, and amplitude to the simultaneously evoked potential in the insular visceral sensory cortex. The potential field adjoined the S1 trigeminal region without overlap, and was rostroventral to the lip representation barrel field, which was identified histochemically, and rostrodorsal to the tongue representation region, which was identified electrophysiologically. The vagal potential underwent a phase reversal in the middle layers; thus, the current sink site was cytoarchitectonically identified as the most rostral part of the parietal granular cortex or the S1, where no somatosensory input has previously been demonstrated. The rat S1 contains a region representing general visceral information, topographically located as if the visceral organs protruded from the mouth.

Evoked potential mapping of auditory and somatosensory cortices in the miniature swine

The miniature swine (Sus scrofa) is a desirable model for certain electrophysiological studies because of its large brain, ready availability and low cost. We report our findings regarding the location of the auditory and somatosensory cortices obtained by cortical surface evoked potential mapping. Data were collected from 11 animals under general endotracheal anesthesia. Consistent positive potentials were obtained for both auditory (P24) and median nerve somatosensory (P26) stimulation. The auditory region was centered around the lateral (sylvian) fissure; the somatosensory region was centered around the central (dorsomedial suprasylvian) fissure. In the miniature swine the locations of the auditory and somatosensory cortices are comparable to those found in other species.

Auditory evoked potentials from the primary auditory cortex of the cat: topographic and pharmacological studies

Wave VI (8.4 msec) of the brain-stem auditory evoked potential (BAEP) was maximal in a discrete region of primary auditory cortex (AI) of the anesthetized cat. Wave VI underwent rapid amplitude decrease over millimeter distances in the AI region and followed high stimulation rates. Wave VI did not show intracortical polarity inversion nor was it abolished by epicortical or intracortical GABA administration. The data are compatible with a wave VI source in the terminal axons of the thalamo-cortical radiations. Middle latency auditory responses (MAEPs) generated 10-40 msec after auditory stimulation were also recorded in a circumscribed area of AI. In contrast to wave VI, these primary auditory cortex potentials (Pa 18.3 msec; Nb 31.9 msec) underwent transcortical polarity inversion, correlated with intracortical multi-unit activity in the AI region and were reversibly altered or abolished by epicortical or intracortical GABA administration to the AI region. The data suggest that the Pa and Nb components of the cat MAEP are intracortically generated by neuronal elements in the AI region.

Phenytoin increases the severity of cortical hemiplegia in rats

The effects of systemic phenytoin administration on the motor deficit resulting from a cortical lesion were studied in rats trained to walk coordinately on a narrow beam. The somatomotor cortex lesion was produced by an indwelling cannula through which saline or GABA were infused chronically via an osmotic minipump. Phenytoin (50 mg/kg i.p.) administered between days 3 and 5 after the intracortical catheter implantation produced a significant increase in the severity of the resulting hemiplegic syndrome. This DPH effect was more noticeable in those animals also receiving intracortical GABA infusions. The anticonvulsant at the dose used had no effect on motor performance when administered preoperatively or when given to the animals 14 days after surgical intervention when their hemiplegic syndrome had cleared. These findings suggest that phenytoin administration to brain-damaged individuals in the initial postlesion stage may be deleterious.

gamma-Aminobutyric acid-induced potentiation of cortical hemiplegia

A novel model of hemiplegia in young and aged rats is described. Osmotic minipumps were used to deliver a chronic (7 days), localized application of gamma-aminobutyric acid (GABA) (100 micrograms/microliter/h), to the somatomotor cortex of unrestrained rats. This resulted in an easily quantifiable, contralateral and reversible motor syndrome in both young and aged animals. In the young group, the motor deficit cleared over 5-day period, while in the aged animals it persisted for at least a 2-week period. Control animals treated with saline-filled minipumps did not develop a long-lasting motor deficit. The GABA-induced facilitation of hemiplegia due to small motor cortex lesions and the age effects on behavioral recovery of function are discussed. Cortical inhibitory mechanisms may play a role in debilitating syndromes such as stroke or post-epileptic paralysis.