Influence of age and of pre-treatment with D-cycloserine on the behavior of ethanol-treated rats tested in the elevated plus-maze apparatus

Chronic outcomes after mild-moderate traumatic brain injury in adult seizure-prone (FAST) and seizure-resistant (SLOW) rats: A model for understanding genetic contributions to acquired epileptogenesis?

Post-traumatic epilepsy (PTE) is a common, serious, long-term complication of traumatic brain injury (TBI). However, only a minority of individuals will develop epilepsy after a TBI, and the contribution of genetic predisposition to the risk of acquired epilepsy warrants further exploration. In this study, we examined whether innate, genetically determined differences in seizure susceptibility between seizure-prone FAST and seizure-resistant SLOW rat strains would influence chronic behavioral and PTE outcomes after experimental TBI. We hypothesized that FAST rats would show increased vulnerability to PTE and poorer neurobehavioral outcomes. Using the lateral fluid percussion injury model, we first determined the optimal injury parameters to generate a mild-moderate TBI in young adult FAST rats, which had previously shown high mortality to severe TBI. Then, FAST and SLOW rats underwent TBI or sham surgery, and a series of behavioral tests were performed either acutely (within 4 weeks) or chronically (more than 22 weeks) post-injury. Acutely, FAST rats showed an increased physiological response to TBI with a longer apnea duration, delayed pain response, and delayed self-righting, as well as increased acute seizure-like behavior compared to SLOW rats. Conversely, SLOW rats showed greater neuromotor deficits and weight loss sub-acutely compared to FAST rats. Chronically, while strain-specific phenotypes were observed (e.g., FAST rats showing increased anxiety-like behavior, altered nociceptive responses, and polydipsia), no TBI effects were detected. Analysis of continuous video-electroencephalographic recordings over a 1-month period starting at 6 months post-TBI did not reveal any spontaneous seizures. However, periodic epileptiform discharges were only found in FAST rats that had a TBI. Together, these findings reflect fundamental differences in chronic behavior and epileptiform discharges as a result of innate distinctions in epileptogenic susceptibility in FAST versus SLOW rats. However, a lack of spontaneous seizure activity or chronic neurobehavioral deficits in TBI animals confounded our ability to address the initial hypothesis, such that alternative injury models may be more suitable to study genetic contributions to the development of PTE.

Postinjury weight rather than cognitive or behavioral impairment predicts development of posttraumatic epilepsy after lateral fluid-percussion injury in rats

OBJECTIVE

To identify postinjury physiologic, behavioral, and cognitive biomarkers for posttraumatic epilepsy to enrich study populations for long-term antiepileptogenesis studies.

METHODS

The EPITARGET cohort with behavioral follow-up and 1-month 24/7 video-electroencephalography (vEEG) monitoring included 115 adult male Sprague-Dawley rats with lateral fluid-percussion-induced traumatic brain injury (TBI), 23 sham-operated controls, and 13 naive rats. Animals underwent assessment of somatomotor performance (composite neuroscore), anxiety-like behavior (elevated plus maze, open field), spatial memory (Morris water maze), and depression-like behavior (Porsolt forced swim, sucrose preference). Impact force, postimpact apnea time, postimpact seizure-like behavior, and body weight were monitored.

RESULTS

TBI rats were impaired in the composite neuroscore (P < .001) on days (D) 2-14 and in the spatial memory test (P < .001) on D35-39 post-TBI. Differences in the elevated plus-maze (D28 and D126) and in the open field (D29 and D127) between TBI rats and controls were meager. No differences were observed in the Porsolt forced swim and sucrose preference tests as compared with sham-operated controls. Epilepsy developed in 27% of rats by the end of the sixth month. None of the behavioral or cognitive outcome measures discriminated rats with or without epilepsy. The receiver-operating characteristic analysis indicated that a decrease in body weight between D0 and D4 differentiated TBI rats with epilepsy from TBI rats without epilepsy (48% sensitivity, 83% specificity, area under the curve [AUC] 0.679, confidence interval [CI] 95% 0.56-0.80, P < .01). A 16% body weight decrease during D0-D4 could be used as a biomarker to enrich the study population from 27% (observed) to 50%.

SIGNIFICANCE

Single behavioral and cognitive outcome measures showed no power as prognostic/diagnostic biomarkers for posttraumatic epilepsy. A reduction in body weight during the first postinjury week showed some prognostic value for posttraumatic epileptogenesis and could serve as a subacute measure for selectively enriching the study population for long-term preclinical biomarker and therapy discovery studies of posttraumatic epileptogenesis.

Effects of central and systemic injections of peripheral benzodiazepine receptor ligands on the anxiolytic actions of ethanol in rats

The influence of peripheral benzodiazepine receptor ligands Ro5-4864 (0.05 or 1.0 mg/kg, i.p.) or PK11195 (0.05 or 1.0 mg/kg, i.p.) on the anxiolytic effect of ethanol (1.2 g/kg; 14% p/v; i.p.) was investigated in rats tested on the elevated plus-maze. Other animals were injected through intrahippocampal administrations of the ligands (0.5 or 1.0 nmol/0.5 &mgr;l) before ethanol (1.2g/kg; 14% p/v; i.p.) and submitted to the elevated plus-maze test. The results showed that the systemic administration of either ligands 24 hours before the ethanol treatment resulted in a reduced anxiolytic effect of this drug. Only PK11195 reversed the effect of ethanol after intrahippocampal injection. These data suggest that peripheral benzodiazepine receptors play a role in ethanol anxiolysis.

Dexamethasone reverses the ethanol-induced anxiolytic effect in rats

The effects of intraperitoneal and intrahippocampal administration of the glucocorticoid dexamethasone were assessed regarding ethanol-induced anxiolysis in the elevated plus-maze in rats. Animals pretreated with systemic injections of dexamethasone (0.5, 1. 0, or 2.0 mg/kg, IP) 15 min before ethanol (1.2 g/kg, 14% w/v, IP) administration showed a significant dose-dependent attenuation of the increased percentage of frequency and time spent on open arms of the maze. However, IP dexamethasone treatment 4 h before the test had no effect. Unilateral intrahippocampal injection of dexamethasone (2 and 20 nmol in 0.5 microl) also significantly attenuated the increased exploration of the open arms induced by ethanol. The results are interpreted in terms of the modulation of the anxiolytic effects of ethanol by glucocorticoids and the possible involvement of hippocampus in this response. The rapid blockade of ethanol induced anxiolysis by dexamethasone strengthens the suggestion that a nongenomic mechanism may underlie this response.

Experimental anxiety and the reinforcing effects of ethanol in rats

In order to examine the relationship between anxiety and reinforcing effects of alcohol, drug-naive male Wistar rats weighing 250-300 g were classified as "anxious" and "non-anxious" in the elevated plusmaze test. A conditioned place preference test was then used to investigate the reinforcing effects of ethanol (EtOH) on these animals. On 2 alternate days, groups of "anxious", "non-anxious" and "normal" rats received intraperitoneal (i.p.) injections of EtOH (0.5, 1.0 or 1.5 g/kg) immediately before a 15-min confinement to the white compartment. On the 2 intervening days the same rats received i.p. injections of saline before confinement to the opposite compartment. On day 5, a 15-min free-choice test was carried out with no injections. Rats classified as "anxious" showed a significant, though not dose-dependent preference for all doses of ethanol compared to saline-treated animals. These data demonstrate that rats regarded as "anxious" are more sensitive to the reinforcing effects of EtOH than "non-anxious" and "normal" Wistar rats and emphasize the relevance of the basal levels of anxiety of rats when trying to detect the reinforcing effects of EtOH.