Grygoriy Tsenov

Intrahippocampal injection of endothelin-1 in immature rats results in neuronal death, development of epilepsy and behavioral abnormalities later in life

The direct injection of endothelin‐1 (ET‐1) into brain parenchyma was recently suggested as a suitable model of stroke. The present study was designed to assess whether intrahippocampal injection of ET‐1 in immature rats causes neurodegeneration and immediate seizures, and results in impairment of motor development, cognitive decline, epilepsy and chronic hippocampal lesion. ET‐1 was injected unilaterally into the dorsal hippocampus in doses of 20 or 40 pmol at the age of 12 (P12) or 25 (P25) days. Video‐electroencephalographic monitoring performed during 100 min after the injection of ET‐1 demonstrated the development of convulsive epileptic seizures in 75–100% of animals of individual age‐and‐dose groups. Long‐term behavioral follow‐up did not reveal impairment of motor development in any dose‐and‐age group. At 2 months after ET‐1 injection, impairment of spatial memory occurred only in rats with 40 pmol of ET‐1 at P12. At 3 months after ET‐1 injection spontaneous electrographic seizures occurred in 62.5–100% animals of both ages with no relation to the dose used. Seizures were always non‐convulsive. The total seizure duration per 24 h was higher in the P12 than the P25 group, suggesting more severe epilepsy. The extent of the hippocampal lesion increased with the dose of ET‐1 and was significantly higher in the P12 than the P25 group. The severity of the ET‐1‐induced lesion correlated positively with total seizure duration per 24 h at both ages. Our results document that early intrahippocampal injection of ET‐1 results in lesion development and both immediate seizures and chronic epilepsy in either age group. Cognitive impairment occurred only in rats with ET‐1 injection at P12.

Intrahippocampal injection of endothelin-1: a new model of ischemia-induced seizures in immature rats.

Summary:  The goal of this study was to develop a new model of ischemia‐induced seizures in immature rats using injection of vasoconstrictor Endothelin‐1 (ET‐1) into the brain. ET‐1 (10, 20, or 40 pmol) was infused into the left dorsal hippocampus of freely moving Wistar rats 12 (P12) and 25 (P25) days old. Animals were then video/EEG‐monitored for 100 min and monitoring was repeated 22 h later. Parameters of electrographic seizures (frequency and mean duration) as well as pattern of their behavioral correlates were evaluated. The pattern of behavioral seizures was used to develop model‐specific scoring system. Cresyl violet and Fluoro Jade‐B‐staining were used to evaluate brain damage. Extension of the lesion was correlated with seizure severity. After ET‐1‐injection, seizures occurred in 83–100% animals of all age‐and‐dose groups and persisted for 24 h except P12 rats with 10 pmol. There were no differences in average seizure duration (18–40 s) or seizure frequency (3–7 seizures/100 min) among individual dose‐groups. Between the 1st and 2nd observation period, total seizure duration decreased in 71% of P12 and 47% of P25 rats. Electrographic seizure activity was most frequently accompanied by clonus, incidence of more severe convulsions (barrel rolling or generalized clonic seizures) increased with dose of ET‐1. Morphologic examination did not reveal any dose‐related difference in damage severity, hippocampal damage was however more extensive in P12 compared to P25 animals. Seizure severity correlated positively with severity of the damage in both age groups. Our study presents focal injection of ET‐1 into the brain as a new and practical model of ischemia‐induced seizures in immature rats.

Neuroprotective effect of the 3α5β-pregnanolone glutamate treatment in the model of focal cerebral ischemia in immature rats

The perinatal hypoxic-ischemic insult frequently leads to mortality, morbidity and plays a key role in the later pathological consequences. The ischemic insult causes a massive release of glutamate and subsequent excitotoxic damage. The neuroactive steroid 3α5β-pregnanolone glutamate (PG) is a NMDA receptor antagonist acting via use-dependent mechanism and can be used as a neuroprotective agent that may alleviate glutamatergic excitotoxicity in the brain. First, a possible neurotoxic effect of the PG, a novel use-dependent NMDA antagonist, was studied in immature rats. In addition, to compare this effect with a well-described non-competitive NMDA antagonist, the MK-801 (positive control) was used. Animals at postnatal day 12 (P12) were injected intraperitoneally with PG in a doses 1 or 10mg/kg or with MK-801 in a dose 1mg/kg. Effect of PG treatment on the immature brain was evaluated on Fluoro Jade B (FJB) stained sections. Second, a neuroprotective effect of the PG was studied in the model of focal cerebral ischemia in P12. Focal cerebral ischemia was induced by the infusion of the endothelin-1 (ET-1) into the right dorsal hippocampus. PG at the doses 1 or 10mg/kg was administrated intraperitoneally 5min after the end of ET-1 infusion. To evaluate the neuroprotective effect after the PG treatment FJB staining was used. Our results demonstrate a lack of the neurotoxicity of the PG in intact P12. In the second part of the study in the model of the focal ischemia we detected significantly lower occurrence of FJB-positive cells in the afflicted hippocampus in PG treated groups, while animals without PG treatment exhibited massive neurodegeneration. The neuroprotective potential of the PG can serve in the development of therapeutic strategies for brain damage induced by the glutamate excitotoxicity.

Changes of Cortical Interhemispheric Responses after Status Epilepticus in Immature Rats

Summary:  Purpose: To study cortical excitability after status epilepticus induced in two age groups of immature rats.

Changes of cortical epileptic afterdischarges after status epilepticus in immature rats

Status epilepticus (SE) in developing rats leads to neuronal degeneration in many brain structures including neocortex but the functional consequences of cortical damage were studied only exceptionally. Lithium-pilocarpine SE was elicited in 12- (P12) and 25-day-old (P25) rats, convulsions were interrupted after 2h by paraldehyde. Cortical electrodes were implanted 3, 6, 9, 13 and/or 26 days after SE. Low-frequency stimulation of sensorimotor cortex was repeated with at least 10-min intervals with a stepwise increasing intensity (0.2-14 mA). Thresholds for movements elicited by stimulation, spike-and-wave afterdischarges (ADs), clonic seizures, mixed ADs (transition into a limbic type of ADs) and recurrent ADs as well as duration of ADs were evaluated. The first three phenomena were not influenced by SE with the exception of lower thresholds for movements during stimulation. Transition into limbic seizures and recurrent seizures were delayed in both age groups and threshold intensities for limbic ADs were at some intervals higher in SE than in control animals. Duration of ADs was changed only at short intervals after SE; it was shortened at 3 and 6 days in P25 and 3 days in P12 rats, respectively. P12 group then exhibited a transient increase in duration of ADs 6 days after SE. Our results did not prove a higher cortical excitability after SE in either age group. On the contrary, there were some signs of a decreased excitability.

Corrigendum: Chronic MK-801 application in adolescence and early adulthood: A spatial working memory deficit in adult Long-Evans rats but no changes in the hippocampal NMDA receptor subunits

[This corrects the article on p. 42 in vol. 9, PMID: 29487522.].

Which component of treatment is important for changes of cortical epileptic afterdischarges after status epilepticus in immature rats

Role of lithium chloride and paraldehyde in acute changes after lithium-pilocarpine status epilepticus (SE) induced at postnatal day 12 was studied in 15-day-old rats. In addition to SE group four other groups were formed: naïve animals without any injection, lithium chloride group, paraldehyde group and lithium-paraldehyde group. Cortical epileptic afterdischarges (CxADs) induced by increasing intensities of stimulation current were used as a measure of excitability. SE animals did not exhibit any change in duration of CxADs with increasing stimulation intensity in contrast to naïve control with a progressive prolongation of CxAD. LiCl group was similar to SE rats whereas paraldehyde and lithium-paraldehyde groups exhibited some progress in duration of ADs. Lithium chloride participates in short-term changes of CxADs after SE. Paraldehyde and combination of lithium and paraldehyde are similar to naïve controls.

Activation of either the ETA or the ETB receptors is involved in the development of electrographic seizures following intrahippocampal infusion of the endothelin-1 in immature rats

The period around birth is a risky time for stroke in infants, which is associated with two major acute and subacute processes: anatomical damage and seizures. It is unclear as to what extent each of these processes independently contributes to poor outcome. Furthermore, it is unclear whether there is an interaction between the two processes - does seizure activity cause additional brain damage beyond that produced by ischemia and/or does brain damage foster seizures? The model of focal cerebral ischemia induced by the intrahippocampal infusion of endothelin-1 (ET-1) in 12-day-old rat was used to examine the role of the endothelin receptors in the development of focal ischemia, symptomatic acute seizures and neurodegeneration. ET-1 (40pmol/μl) was infused either alone or co-administered with selective antagonists of ETA (BQ123; 70nmol/μl) or ETB receptors (BQ788; 70nmol/1μl). Effects of activation of ETB receptors were studied using selective agonist 4-Ala-ET-1 (40pmol/1μl). Regional cerebral blood flow (rCBF) and tissue oxygenation (pO2) were measured in anesthetized animals with a Doppler-flowmeter and a pO2-sensor, respectively. Seizure development was assessed with video-EEG in freely moving rats. Controls received the corresponding volume of the appropriate vehicle (10mM PBS or 0.01% DMSO-PBS solution; pH7.4). The extent of hippocampal lesion was determined using FluoroJade B staining performed 24h after ET-1 infusion. Infusion of ET-1 or ET-1+ETB receptor antagonist reduced rCBF to ~25% and pO2 to ~10% for about 1.5h, whereas selective ETB agonist, ET-1+ETA antagonist and the PBS vehicle had only negligible effect on the rCBF and pO2 levels. Reduction of rCBF was associated with the development of lesion in the injected hippocampus. In all groups, except sham operated and PBS controls, epileptiform activity was observed after activation of the ETA or the ETB receptors. The data revealed a positive correlation between the severity of morphological damage and all the measured seizure parameters (seizure frequency, average and total seizure duration) in the ET-1 group. In addition, the severity of morphological damage positively correlated with the average seizure duration in animals after infusion of ET-1+ETA receptor antagonist or after infusion of ET-1+ETB receptor antagonist. Our results indicate that the activation of ETA receptors is crucial for ischemia development, however either ETA or ETB receptors mediate the development of seizures following the application of ET-1 in immature rats. The dissociation between the ischemic-producing and seizure-producing processes suggests that damage is not necessary to induce seizures, although it may exacerbate them.

Age-dependent suppression of hippocampal epileptic afterdischarges by metabotropic glutamate receptor 5 antagonist MTEP

Action of an antagonist of metabotropic glutamate receptors subtype 5 MTEP was studied in a model of complex partial seizures. Dorsal hippocampus of rat pups 12, 18 and 25 days old was stimulated six times with 10-min intervals. MTEP (20 or 40 mg/kg) was injected after the first afterdischarge and duration of afterdischarges was measured. MTEP exhibited marked anticonvulsant action in 12-day-old-rats, the similar effect in 18-day-old rats was observed only with the second stimulation. No anticonvulsant action was seen in 25-day-old animals. Our results may qualify antagonists of mGluR5 as potential antiepileptic drugs for some types of childhood epilepsies.

The Free Radical Scavenger N-Tert-Butyl-α-Phenylnitrone (PBN) Administered to Immature Rats During Status Epilepticus Alters Neurogenesis and Has Variable Effects, Both Beneficial and Detrimental, on Long-Term Outcomes

Status epilepticus (SE), especially in immature animals, is known to produce recurrent spontaneous seizures and behavioral comorbidities later in life. The cause of these adverse long-term outcomes is unknown, but it has been hypothesized that free radicals produced by SE may play a role. We tested this hypothesis by treating immature (P25) rats with the free radical scavenger N-tert-butyl-α-phenylnitrone (PBN) at the time of lithium chloride (LiCl)/pilocarpine (PILO)-induced SE. Later, long-term outcomes were assessed. Cognitive impairment (spatial memory) was tested in the Morris water maze (MWM). Emotional disturbances were assessed by the capture test (aggressiveness) and elevated plus maze’s (EPM) test (anxiety). Next, the presence and severity of spontaneous seizures were assessed by continuous video/EEG monitoring for 5 days. Finally, immunochemistry, stereology and morphology were used to assess the effects of PBN on hippocampal neuropathology and neurogenesis. PBN treatment modified the long-term effects of SE in varying ways, some beneficial and some detrimental. Beneficially, PBN protected against severe anatomical damage in the hippocampus and associated spatial memory impairment. Detrimentally, PBN treated animals had more severe seizures later in life. PBN also made animals more aggressive and more anxious. Correlating with these detrimental long-term outcomes, PBN significantly modified post-natal neurogenesis. Treated animals had significantly increased numbers of mature granule cells (GCs) ectopically located in the dentate hilus (DH). These results raise the possibility that abnormal neurogenesis may significantly contribute to the development of post-SE epilepsy and behavioral comorbidities.