Reduction In Paired-pulse Inhibition Research Articles

Early exposure to urethane anesthesia: Effects on neuronal activity in the piriform cortex of the developing brain

Exposure to urethane anesthesia reportedly produces selective neuronal cell loss in the piriform cortex of young brains; however, resulting functional deficits have not been investigated. The present study found abnormalities in piriform cortex activity of isolated brains in vitro that were harvested from guinea pigs exposed to urethane anesthesia at 14 days of age. Current source density (CSD) analysis and voltage-sensitive dye (VSD) imaging experiments were conducted 48h after urethane injection. We applied paired-pulse stimulation to the lateral olfactory tract (LOT) and assessed short-interval intra-cortical inhibition in the piriform cortex. CSD analysis revealed that a current sink in layer Ib remained active in response to successive stimuli, with an inter-stimulus interval of 30–60ms, which was typically strongly inhibited. VSD imaging demonstrated stronger and extended neural activity in the urethane-treated piriform cortex, even in response to a second stimulus delivered in short succession. We identified gamma-aminobutyric acid (GABA) ergic neurons in the piriform cortex of sham and urethane-treated animals and found a decrease in GABA-immunoreactive cell density in the urethane group. These results suggest that urethane exposure induces loss of GABAergic interneurons and a subsequent reduction in paired-pulse inhibition in the immature piriform cortex.

P 6. tDCS-induced effects on paired-pulse inhibition in the primary somatosensory cortex

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique used to modulate cortical excitability. In the motor domain, there is consensus that anodal tDCS increases cortical excitability, whereas cathodal tDCS decreases it. On the other hand, inconsistent results of tDCS-induced effects in the sensory domain have been reported. The aim of the present study was to investigate (A) changes in cortical excitability within primary somatosensory cortex (S1) by means of single-pulse somatosensory evoked potentials (SEPs) and (B) intracortical inhibition by means of paired-pulse SEPs when tDCS was applied over left SM1. 10 min of anodal, cathodal or sham tDCS was applied over the left SM1 using saline-soaked sponge electrodes (35 cm 2 electrodes, 1 mA, current density 0.028 mA/cm 2 ). Before, immediately after as well as 10 min after termination of tDCS, single-and paired-pulse SEP recordings were performed. We hypothesized that tDCS will induce polarity specific changes in cortical excitability within left S1. Furthermore, we reasoned that anodal tDCS will reduce paired-pulse inhibition within left S1 while cathodal tDCS will result in an augmentation of inhibition relative to sham stimulation. 10 min of anodal and cathodal tDCS over SM1 did not result in any significant excitability changes within left S1. However, anodal tDCS resulted in a reduction of paired-pulse inhibition within left S1 10 min after termination of stimulation. No change in paired-pulse inhibition could be observed after cathodal tDCS. Here we provide novel evidence that anodal tDCS affects inhibitory processing within S1, a finding that might improve our understanding about the underlying neural mechanisms of tDCS on somatosensory processing.

Alterations in Serotonin Receptors and Transporter Immunoreactivities in the Hippocampus in the Rat Unilateral Hypoxic-induced Epilepsy Model

Unilateral hypoxic-ischemia results in the frequent occurrence of interictal spikes, and occasionally sustained ictal discharges accompanied by a reduction in paired-pulse inhibition within the non-lesioned dentate gyrus. To elucidate the roles of serotonin (5-hydroxytryptamine [5-HT]) in an epileptogenic insult, we investigated the changes in 5-HT receptors and serotonin transporter (5-HTT) immunoreactivities within the lesioned and contralateral hippocampus following unilateral hypoxic-ischemia. During epileptogenic periods following hypoxic-ischemia, both 5-HT(1A) and 5HT(1B) receptor immunoreactivities were decreased within the lesioned and the non-lesioned hippocampus. However, 5-HTT immunoreactivity was transiently increased within the hippocampus bilaterally. These findings indicate that alteration of the 5-HT system results in a "diaschisis" pattern, and may contribute to neuronal death and the development of emotional disorders in epileptic patients accompanied by psychological stress.

Improvement of tactile perception and enhancement of cortical excitability through intermittent theta burst rTMS over human primary somatosensory cortex

Adopting the patterns of theta burst stimulation (TBS) used in brain-slice preparations, a novel and rapid method of conditioning the human brain has recently been introduced. Using short bursts of high-frequency (50 Hz) repetitive transcranial magnetic stimulation (rTMS) has been shown to induce lasting changes in brain physiology of the motor cortex. In the present study, we tested whether a few minutes of intermittent theta burst stimulation (iTBS) over left primary somatosensory cortex (SI) evokes excitability changes within the stimulated brain area and whether such changes are accompanied by changes in tactile discrimination behavior. As a measure of altered perception we assessed tactile discrimination thresholds on the right and left index fingers (d2) before and after iTBS. We found an improved discrimination performance on the right d2 that was present for at least 30 min after termination of iTBS. Similar improvements were found for the ring finger, while left d2 remained unaffected in all cases. As a control, iTBS over the tibialis anterior muscle representation within primary motor cortex had no effects on tactile discrimination. Recording somatosensory evoked potentials over left SI after median nerve stimulation revealed a reduction in paired-pulse inhibition after iTBS that was associated but not correlated with improved discrimination performance. No excitability changes could be found for SI contralateral to iTBS. Testing the performance of simple motor tasks revealed no alterations after iTBS was applied over left SI. Our results demonstrate that iTBS protocols resembling those used in slice preparations for the induction of long-term potentiation are also effective in driving lasting improvements of the perception of touch in human subjects together with an enhancement of cortical excitability.

Progesterone withdrawal reduces paired-pulse inhibition in rat hippocampus: dependence on GABA(A) receptor alpha4 subunit upregulation.

Withdrawal from the endogenous steroid progesterone (P) after chronic administration increases anxiety and seizure susceptibility via declining levels of its potent GABA-modulatory metabolite 3alpha-OH-5alpha-pregnan-20-one (3alpha,5alphaTHP). This 3alpha,5alpha-THP withdrawal also results in a decreased decay time constant for GABA-gated current assessed using whole cell patch-clamp techniques on pyramidal cells acutely dissociated from CA1 hippocampus. The purpose of this study was to test the hypothesis that the decreases in total integrated GABA-gated current observed at the level of the isolated pyramidal cell would be manifested as a reduced GABA inhibition at the circuit level following hormone withdrawal. Toward this end, adult, female rats were administered P via subcutaneous capsule for 3 wk using a multiple withdrawal paradigm. We then evaluated paired-pulse inhibition (PPI) of pyramidal neurons in CA1 hippocampus using extracellular recording techniques in hippocampal slices from rats 24 h after removal of the capsule (P withdrawal, P Wd). The population spike (PS) was recorded at the stratum pyramidale following homosynaptic orthodromic stimulation in the nearby stratum radiatum. The threshold for eliciting a response was decreased after P Wd, and the mean PS amplitude was significantly increased compared with control values at this time. Paired pulses with 10-ms inter-pulse intervals were then applied across an intensity range from 2 to 20 times threshold. Evaluation of paired-pulse responses showed a significant 40-50% reduction in PPI for PS recorded in the hippocampal CA1 region after P Wd, suggesting an increase in circuit excitability. At this time, enhancement of PPI by the benzodiazepine lorazepam (LZM; 10 microM) was prevented, while pentobarbital (10 microM) potentiation of PPI was comparable to control levels of response. These data are consistent with upregulation of the alpha4 subunit of the GABA(A) receptor (GABAR) as we have previously shown. Moreover, the reduced PPI caused by P Wd was prevented by suppression of GABAR alpha4-subunit expression following intraventricular administration of specific antisense oligonucleotides (1 microg/h for 72 h). These results demonstrating a reduction in PPI following P Wd suggest that GABAergic-mediated recurrent or feed-forward inhibition occurring at the circuit level were decreased following P Wd in female rats, an effect at least partially attributable to alterations in the GABAR subunit gene expression.

The reduction in paired-pulse inhibition in the rat hippocampus by gabapentin is independent of GABA(B) receptor receptor activation.

Previously we have shown that gabapentin causes a reduction of paired-pulse inhibition in the dentate gyrus of the urethane-anesthetized rat, which looks very much like the effect of baclofen on paired-pulse inhibition. In addition, it has been proposed that gabapentin increases release of GABA from non-vesicular stores and may, therefore, interact with GABA(B) mechanisms. Here we tested the ability of a GABA(B) agonist, baclofen, and a GABA(B) antagonist, CGP35348, to block the effect of gabapentin on paired-pulse inhibition in the dentate gyrus in urethane-anesthetized adult Sprague-Dawley rats. Both baclofen (6 mg/kg) and gabapentin (100 mg/kg) caused a long-lasting reduction of paired-pulse inhibition in the dentate gyrus when given alone or in combination. CGP35348 (45 mg/kg) blocked the effect of baclofen on paired-pulse inhibition, but did not alter the effect of gabapentin. Gabapentin also caused a reduction of inhibition in the CA1 region, indicating that its effect is not specific for the dentate gyrus. These results suggest that gabapentin produces its effect on paired-pulse inhibition independent from the effect of baclofen and not through non-vesicular release of GABA interacting with the GABA(B) receptor system.

Hippocampal stimulation produces neuronal death in the immature brain

We re-examined the proposed resistance of the immature brain to seizure-induced damage. In awake, freely moving rat pups, intermittent perforant path stimulation produced selective hippocampal cell loss and reduction in paired-pulse inhibition. During 16 h of stimulation, animals showed frequent wet dog shakes and hind-limb scratching movements but no convulsive motor activity. In situ end-labelling performed 2 h after the end of stimulation showed an intense band of positively-labelled eosinophilic cells with condensed profiles bilaterally in the dentate granule cell layer of stimulated animals. Control animals showed no in situ end-labelling positivity in the dentate gyrus. These cells were not observed 24 h later, suggestive of rapidly scavenged apoptotic cells. One day after the end of stimulation, many necrotic interneurons with eosinophilic cytoplasm and pyknotic nuclei were observed in the hilus of the stimulated dentate gyrus in all rats tested. Hippocampal pyramidal cells in CA1, CA3 and subiculum showed bilateral damage greater on the side of stimulation, and prepiriform cortex sustained bilateral symmetrical lesions. One month after perforant path stimulation, Cresyl Violet staining showed the number of large hilar interneurons (>15 μm) was reduced on the stimulated side (54.1±12.2) compared to the non-stimulated side (100.5±10.2 cells, P<0.01). Immunohistochemical analysis showed significant losses in somatostatin (8.5±1.6 stimulated side, 22.8±3.8 unstimulated side, P<0.05) and neuropeptide Y (12.8±3.2 stimulated side, 17.0±4.1 unstimulated side, P<0.05) immunoreactive cells in the stimulated hilus but no loss of parvalbumin-immunoreactive cells. Significant reductions in paired-pulse inhibition were found after stimulation but there was some return of inhibition by one month. These combined data demonstrate that the immature brain can incur damage as a result of prolonged seizure-like hippocampal activity mimicking status epilepticus in immature rats. The hippocampal damage produced by perforant path stimulation is associated with the immediate loss of physiological inhibition suggesting important modification of excitatory control in an extremely epileptogenic region of the brain.

Examination of persistent effects of repeated administration of pentylenetrazol on rat hippocampal CA1: evidence from in vitro study on hippocampal slices

The early and long-lasting effects of pentylenetetrazol-kindling on hippocampal CA1 synaptic transmission were investigated. Experiments were carried out in the hippocampal slices from control and kindled rats at two post-kindling periods, i.e. 48–144 h (early phase) and 30–33 days (long-lasting phase). Field potentials, i.e. population excitatory postsynaptic potential (pEPSP) and population spike (PS) were recorded at the stratum pyramidale following stimulation of the stratum radiatum. Kindling-induced changes in synaptic transmission were assessed by stimulus-response functions and paired-pulse responses. The results showed that 48–144 h after kindling, the PS amplitude in the CA1 of kindled slices enhanced, and a second PS appeared compared to control slices. But at 30–33 days after kindling, the pEPSP slope in the CA1 of kindled slices enhanced without any change in the PS compared with those in the control slices. Evaluation of paired-pulse responses showed a significant reduction in paired-pulse inhibition for PS 48–144 h after kindling and a significant increase in paired-pulse inhibition for pEPSP 30–33 days after kindling. Our results suggest that pentylenetetrazol-kindling is accompanied by enhanced excitability and a reduction of paired-pulse inhibition in hippocampal CA1. The increased paired-pulse inhibition one month after kindling, may be interpreted as an adaptive process to cope with subsequent seizures.

Ontogeny of epileptogenesis in the rat hippocampus: a study of the influence of GABAergic inhibition

In vivo experiments were carried out to examine whether the period during which γ-aminobutyric acid (GABA)ergic inhibition in the hippocampus matures is associated with a decrease in epileptogenesis. Seizures were elicited with bipolar electrodes stereotactically positioned in the hippocampus of urethane-anesthetized rat pups from postnatal (PN) 7 through 28 days of age. No clinical seizure activity was detected but electrographic seizures (afterdischarges) were induced at all ages. Afterdischarge thresholds (ADT) varied inversely with age. However, the durations of initial afterdischarges and the degree of lengthening of afterdischarges with the rapidly recurring hippocampal seizure (RRHS) protocol were not different for the various age animals studied. Paired pulse inhibition was assessed with a twin pulse paradigm that has been shown to monitor GABAergic inhibition. Measurements were made before and 60 min after a single seizure and again 60 min after the RRHS protocol. At no age was there a significant change in paired pulse inhibition after a single seizure. After RRHS there was a significant reduction of paired pulse inhibition only in the groups that had manifested adult levels of paired pulse inhibition in pre-seizure measurements (≥ PN 21). These studies indicate that heightened epileptogenesis in the young hippocampus cannot simply be explained on the basis of an immaturity of GABA-mediated inhibition.

Chronic benzodiazepine treatment of rats induces reduction of paired-pulse inhibition in CA1 region of in vitro hippocampus

Paired-pulse inhibition was studied extracellularly in in vitro hippocampal slices from rats sacrificed 48 h or 7 days after 1 week flurazepam (FZP) treatment. Population spikes and field excitatory postsynaptic potentials (EPSPs) were recorded with NaCl-containing glass micropipettes in the stratum pyramidale and stratum radiatum, respectively, of the CA1 region. Conditioning pulses were delivered by stimulating Shaffer collaterals (orthodromic) or the alveus (antidromic). Orthodromic test pulses were delivered with interpulse intervals of 10–200 ms. There was a significant reduction in paired-pulse inhibition in slices from treated vs control rats in both the orthodromic-orthodromic and antidromic-orthodromic paradigms. Reduced inhibition was evident 48 h, but not 7 days, after the end of FZP treatment. Furthermore, there was a significant prolongation of the half decay time of the field EPSP, without a significant change in the initial slope or maximum amplitude. The results may suggest an impairment of endogenous γ-aminobutyric acid function in the hippocampus after chronic benzodiazepine (BZ) treatment and may provide a basis for a mechanism of BZ tolerance.

Reduction of paired pulse inhibition in the CA1 region of the hippocampus by pilocarpine in naive and in amygdala-kindled rats

Subconvulsant doses (20 mg/kg) of pilocarpine administered to a kindled rat convert a kindled seizure to status epilepticus. The hippocampus is involved in such status epilepticus. Furthermore, evidence is accumulating that GABA-mediated inhibition in the hippocampus is chronically diminished by kindling. The studies presented here compared the electrophysiologic effects of pilocarpine in vivo in the CA1 region of the hippocampus in naive and amygdala-kindled rats. A paired pulse paradigm previously shown to quantify the potency of GABAergic inhibition was employed. Stimuli were delivered to the CA3 region of urethane-anesthetized rats and population spikes were recorded in the contralateral CA1 region. In naive rats, pilocarpine (6–60 mg/kg) caused a left shift in the input-output curve measuring stimulus intensity vs population spike amplitudes, indicating an increase in neuronal excitability. In addition, paired pulse inhibition was reduced for interpulse intervals < 70 ms. In amygdala-kindled rats, neuronal excitability was also enhanced following pilocarpine administration. The potency of baseline paired pulse inhibition was decreased in kindled rats compared to naive controls. Following pilocarpine, inhibition for interpulse intervals < 70 ms was further reduced, but to a lesser extent than in naive rats. These findings suggest that the ability of subconvulsive doses of pilocarpine to change a discrete kindled seizure triggered by one stimulus to status epilepticus depends on the suppression of GABAergic inhibition below a critical level.

Long-term loss of paired pulse inhibition in the kainic acid-lesioned hippocampus of the rat

A paired pulse stimulation protocol was employed to examine the loss of inhibition in the hippocampus of the kainic acid-treated rat in vivo. Extracellular recordings were obtained from the CA1 pyramidal cell layer 1, 4, 8 and 16 weeks after a unilateral intracerebroventricular injection of kainic acid. Recordings were made both ipsilateral and contralateral to the site of kainic acid injection. The results demonstrated a loss of paired pulse inhibition in hippocampal CA1 area ipsilateral to the site of kainic acid injection which did not alter over 16 weeks. While the contralateral hippocampus showed no change 1 week after kainic acid injection, a reduction of paired pulse inhibition was seen after 4, 8 and 16 weeks. It is therefore apparent that the effects of kainic acid on the state of inhibition of the hippocampus are long-lasting and, furthermore, that the response to an injection of kainic acid is widespread, affecting the inhibitory control of the contralateral hippocampus.