Histochemistry of trauma after electrode implantation and stimulation in the hippocampus

Optogenetic stimulus-triggered acquisition of seizure resistance

Unlike an electrical circuit, the hardware of the brain is susceptible to change. Repeated electrical brain stimulation mimics epileptogenesis. After such "kindling" process, a moderate stimulus would become sufficient in triggering a severe seizure. Here, we report that optogenetic neuronal stimulation can also convert the rat brain to a hyperexcitable state. However, continued stimulation once again converted the brain to a state that was strongly resistant to seizure induction. Histochemical examinations showed that moderate astrocyte activation was coincident with resilience acquisition. Administration of an adenosine A1 receptor antagonist instantly reverted the brain back to a hyperexcitable state, suggesting that hyperexcitability was suppressed by adenosine. Furthermore, an increase in basal adenosine was confirmed using in vivo microdialysis. Daily neuron-to-astrocyte signaling likely prompted a homeostatic increase in the endogenous actions of adenosine. Our data suggest that a certain stimulation paradigm could convert the brain circuit resilient to epilepsy without exogenous drug administration.

Improvements in memory after medial septum stimulation are associated with changes in hippocampal cholinergic activity and neurogenesis

Deep brain stimulation (DBS) has been found to have therapeutic effects in patients with dementia, but DBS mechanisms remain elusive. To provide evidence for the effectiveness of DBS as a treatment for dementia, we performed DBS in a rat model of dementia with intracerebroventricular administration of 192 IgG-saporins. We utilized four groups of rats, group 1, unlesioned control; group 2, cholinergic lesion; group 3, cholinergic lesion plus medial septum (MS) electrode implantation (sham stimulation); group 4, cholinergic lesions plus MS electrode implantation and stimulation. During the probe test in the water maze, performance of the lesion group decreased for measures of time spent and the number of swim crossings over the previous platform location. Interestingly, the stimulation group showed an equivalent performance to the normal group on all measures. And these are partially reversed by the electrode implantation. Acetylcholinesterase activity in the hippocampus was decreased in lesion and implantation groups, whereas activity in the stimulation group was not different from the normal group. Hippocampal neurogenesis was increased in the stimulation group. Our results revealed that DBS of MS restores spatial memory after damage to cholinergic neurons. This effect is associated with an increase in hippocampal cholinergic activity and neurogenesis.

Afterdischarge threshold reduction in the kindling model of epilepsy

Female Wistar rats were implanted with electrodes in the right amygdala, and initial--pre-kindling--afterdischarge thresholds (ADT's) were determined 1, 2, 4, and 8 weeks after electrode implantation. Pre-kindling ADT's were highest 1 week after implantation, lower at 2 weeks, and lowest at 4 weeks. This drop in non-stimulated thresholds had largely disappeared by 8 weeks post-implantation. Half of each group was then kindled, while the other half was "sham" kindled. ADTs were re-measured 1 week after the 10th Stage 5 seizure. When kindling was begun 1 or 2 weeks after implantation (when non-stimulated thresholds were dropping), no effect of kindling on ADT's could be seen. When kindling was begun 4 or 8 weeks after implantation (when non-stimulated thresholds were rising), however, kindling clearly lowered thresholds.

Deep brain stimulation for chronic neuropathic pain: Long-term outcome and the incidence of insertional effect

We conducted a retrospective analysis of long-term results of deep brain stimulation (DBS) for the treatment of neuropathic pain. Twenty-one patients had electrodes implanted in the ventrocaudalis thalamic nucleus (Vc) (n=13) or in both Vc and periaqueductal/periventricular gray matter (PAG/PVG) (n=8). After insertion of the electrodes, 9 patients (43%) had a substantial reduction in pain scores in the absence of stimulation (insertional effect). The effects of stimulation were studied right after surgery or upon return of the patients' pain after electrode insertion (stimulation trials). Patients with a greater than 50% reduction in pain scores were implanted with a pulse generator (IPG). Of interest, patients who had an insertional effect had a trend towards a successful stimulation trial (p=0.08). Overall, 13 of the 21 patients operated (62%) had a successful stimulation trial and received an IPG (12 with electrodes in Vc and one in both Vc and PAG/PVG). Seven patients (33%) did not benefit from stimulation and had the electrodes removed. One patient experienced a prolonged insertional effect and has not required stimulation. Of the 13 patients that received an IPG, 8 discontinued stimulation during the first year of treatment. Only 5 patients maintained long-term benefit (4 with stimulation in Vc and one in both Vc and PAG/PVG). The relatively low efficacy of DBS for the treatment of neuropathic pain stresses the need for further investigation and the exploration of new surgical targets.

Effect of depth electrode implantation with or without subsequent kindling on GABA turnover in various rat brain regions

Kindling is a chronic model of epilepsy characterized by a progressive increase in response to the same regularly applied electrical stimulus. The biological basis of the kindling phenomenon requires to be determined, but several studies indicate that impairment of GABAergic inhibition may be involved. In the present experiments, GABA turnover was determined in vivo by the GABA aminotransferase (GABA-T) inhibition method in 13 brain regions in three groups of rats: (1) a group which was kindled via electrical stimulation of intra-amygdala electrodes and was sacrificed 36 days after the last fully kindled seizure for neurochemical determinations; (2) a group of implanted but non-stimulated rats (sham control group) in which neurochemical measurements were done at the same time after electrode implantation as in the kindled group; and (3) a group of non-implanted, naive control rats. Regional GABA levels were determined after vehicle injection as well as 30 and 90 min after administration of aminooxyacetic acid (AOAA) at a dose which completely inhibits GABA-T. Compared to naive controls, prolonged electrode implantation in the amygdala induced a significant reduction of AOAA-induced GABA accumulation in amygdala, hippocampus, piriform cortex, olfactory bulb, frontal cortex, striatum, hypothalamus, tectum, and cerebellar cortex. In view of the GABA hypothesis of kindling, reduced GABA turnover in response to electrode implantation would suggest that the implantation per se exerts a pro-kindling effect, which was recently demonstrated in rats with intraamygdala electrodes. However, amygdala kindling itself appeared to antagonize the effect of electrode implantation in most regions. Thus, although, compared to naive controls, the predominant change in kindled rats was a decrease in GABA turnover, this decrease was less marked than in sham controls. In thalamus and brainstem kindling markedly increased GABA turnover above the levels determined in both naive and sham controls, possibly in response to impaired postsynaptic GABAergic function. The data indicate that both electrode implantation and kindling significantly alter regional GABA turnover, which might contribute to the pathophysiology of the kindling phenomenon. Furthermore, the data substantiate that the choice of adequate controls is critical in neurochemical and functional studies on the kindling phenomenon.

Does prolonged implantation of depth electrodes predispose the brain to kindling?

Chronically implanted depth electrodes are widely used for the study of electrical signals generated in deep cerebral locations and for electrical stimulation of such locations. Although the effects of lesions resulting from electrode implantation are generally considered minimal, some reports have shown lasting neurochemical, histological, and behavioral alterations in response to such implantation. Furthermore, there is some evidence that prolonged electrode implantation may decrease the seizure threshold of the implanted region and increases the rate of kindling from this region. This prompted us to undertake a study on different periods of post-surgical delay to onset of electrical stimulation and subsequent characteristics of kindling development. Rats were implanted with a bipolar electrode in the basolateral amygdala, and the threshold for induction of focal paroxysmal activity (afterdischarge threshold, ADT) was determined after post-surgical recovery periods of either 1, 2, 4, or 8 weeks. The animals were then kindled by daily administration of an electrical stimulus until all rats exhibited fully kindled seizures. In fully kindled rats, the ADT was redetermined. Compared to animals with 1 week of electrode implantation, the pre-kindling ADT was significantly lower in rats with 2 and 4 weeks of electrode implantation, but returned towards the 1 week values at 8 weeks. An enhanced kindling rate was seen when kindling stimulations were started after 4 and 8 weeks of electrode implantation. Despite the marked differences in pre-kindling ADT, the post-kindling ADT was similar in the groups with 1, 2, or 4 weeks but significantly lower in the group with 8 weeks post-surgical delay to onset of testing. The data suggest that prolonged implantation of a bipolar electrode into a sensitive region of the limbic system predisposes the brain to kindling. Based on previous observation of iron deposits induced by electrode implantation and the epileptogenic effect of iron in cortical and limbic regions, we propose that the present observations are due to deposition of iron from hemoglobin destruction in local microhemorrhages caused by the implantation.

Endogenous peroxidatic activity in astrocytes after spinal cord injury

After spinal cord injury, endogenous peroxidatic-like activity develops along the axis of the cord. At 2 weeks postinjury, this activity appears in cells whose processes are intimately associated with microvessels. The objectives of this study were to further characterize this response and to identify the cellular localization of endogenous peroxidatic-like activity. After traumatic injury to the rat spinal cord, adjacent sections of spinal cord were processed in medium to visualize antiglial fibrillary acidic protein, endogenous peroxidatic activity, or cytochrome oxidase activity. In addition, certain sections, stained for endogenous peroxidatic-like activity, were prepared for electron microscopy. To identify the nature of the activity, some sections were exposed to an incubation medium that included inhibitors of either catalase or heme protein activity. The distribution of prominent glial fibrillary acidic protein immunoreactivity in the dorsal columns corresponded to that of marked staining for endogenous peroxidatic-like activity and cytochrome oxidase. At the ultrastructural level, endogenous peroxidatic-like activity was identified in the cytoplasmic compartment of the astrocyte. This activity was abolished when potassium cyanide (an inhibitor of heme protein) was added to the incubation medium. Spinal cord injury elicited a pronounced cellular response along the axis of the cord that was characterized by enhanced staining for antiglial fibrillary acidic protein, cytochrome oxidase activity, and endogenous peroxidatic-like activity. It is not clear whether pronounced cytochrome oxidase activity corresponded to astrocytes that also expressed prominent endogenous peroxidatic-like activity. However, according to both light and ultrastructural findings, endogenous peroxidatic-like activity was prominently associated with the astrocytic cytoplasm. The biochemical nature of the peroxidatic activity is unknown, but these results suggest that it is related to a heme-containing protein.

Histopathologic evaluation of prolonged intracortical electrical stimulation

Chronic stimulating microelectrodes fabricated from platinum-30% iridium (Pt-30%Ir) or activated iridium were implanted in assemblies of three in the left sensorimotor cortex of the cat and pulsed continuously at currents of 10 to 320 microA (100 to 3200 microC/cm2 X ph, 2 to 64 nC/ph) for periods of 24 h or for 23 h/day for 7 days. The microelectrodes had beveled tips with uninsulated geometric surface areas of 20 X 10(-6) cm2. Neuronal activity evoked by the focal stimulation was monitored by recording compound action potentials from the ipsilateral pyramidal tract. By this criterion neuronal activation thresholds were 5 to 15 microA (50 to 150 microC/cm2 X ph, 1 to 3 nC/ph) for both types of electrodes. Histologic evaluations of tissue surrounding the electrode tips were carried out by either light or electron microscopy. No neural damage was induced by 24 or 161 h of pulsing using either type of electrode at currents of 10 to 80 microA. Neural damage attributable to electrical stimulation per se was observed in a few sites pulsed with 320 microA (3200 microC/cm2 X ph, 64 nC/ph, 16 A/cm2) with Pt-30%Ir but not activated iridium electrodes of the same size. Electrode dissolution appears to be best correlated with charge density and current density. Dissolution of the Pt-30%Ir microelectrode tip was observed by scanning electron microscopy at charge densities as low as 200 microC/cm2 X ph (1 A/cm2), whereas erosion of activated iridium microelectrodes occurred only at the highest charge and current densities (3200 microC/cm2 X ph, 16 A/cm2). Thus, the activated iridium electrode is superior to Pt-30%Ir for chronic stimulations, from the standpoint of electrode tip stability, because with the former, in contrast to the alloy, detectable erosion occurred only at an intensity well above that required for activation of nearby neurons.

Enhanced rate of kindling after prolonged electrode implantation into the amygdala of rats

A comparison was made between the rate of amygdaloid kindling in rats which had been implanted with electrodes either 6-8 days or 28-33 days before the start of a series of kindling stimulations. Using both stainless-steel and platinum/iridium electrodes the rate of kindling was significantly reduced after the longer implantation time. Possible mechanisms by which the presence of an electrode in the amygdala may influence the rate of subsequent kindling, and the implications for studies on kindling, are discussed.

Responses to cortical injury: II. Widespread depression of the activity of an enzyme in cortex remote from a focal injury

As a part of a broader study of the reaction of the brain to injury, we report here an interesting loss of the activity of an enzyme in areas quite remote from the site of direct injury. At 36 h following a laceration or contusion injury to the hindpaw area of the motor cortex, a peculiar loss of staining for the enzyme alpha glycerophosphate dehydrogenase (alpha-GPDH) was noted. alpha-GPDH activity was markedly depressed in cortical layers II and III throughout the hemisphere on the side of the injury. The depression of alpha-GPDH activity extended far laterally across the rhinal fissure into the pyriform cortex. The decrease in alpha-GPDH staining was prominent 4 days after the injury: however, the staining pattern had returned to normal at 9 days. Enzyme changes in animals lesioned in the occipital cortex paralleled that seen in animals with a lesion in the motor cortex. Animals which had received an undercut lesion in the motor cortex 56 days earlier were contused in the occipital cortex. The old injury site presented the same sequelae of changes as seen in other lesioned animals. Additionally, a suction ablation injury involving only a small part of motor cortex resulted in the same widespread reduction of staining for alpha-GPDH in layers II and III. The derangement in energy metabolism suggests that cells in layers II and III of the cerebral cortex may be particularly vulnerable to perturbations induced by cortical trauma. These findings may be related to the diffuse and transient functional losses observed after head injury in man.

Cerebral tissue response to electrode implantation

Tissue response to platinum electrodes was assessed after an eight-day implantation period. The regions of study included the cortical areas at the opercular gyri and at the sulcus parieto-occipitalis externus, as well as the sub-cortical white matter in these cases. Perivascular and intraparenchymal hemorrhagic lesions as well as edematous changes characterized both by extensive intra and extracellular swelling were noted. Numerous phagocytic elements and degenerative strucutres were present at the electrode/parenchymal interface. Tissue alterations were asymmetrical as observed both around and at the tip of the implanted electrode. The parenchymal alterations extended from 0.2mm to 3.5mm distance from the electrode path/parenchymal interface. Greater tissue involvement was found in the subcortical white matter as compared to the adjacent cortical gray matter.