Serotonin 2C receptor in a rat model of temporal lobe epilepsy: From brainstem expression to pharmacological blockade in relation to ventilatory function.

Because of its involvement in breathing control and neuronal excitability, dysregulation of the serotonin (5-HT) 2C receptor (5-HT2C) might play a key role in sudden unexpected death in epilepsy. Seizure-induced respiratory arrest is thus prevented by a 5-HT2B/C agonist in different seizure model. However, the specific contribution of 5-HT2C in chronic epilepsy-related respiratory dysfunction remains unknown. In a rat model of temporal lobe epilepsy (EPI rats), in which we previously reported interictal respiratory dysfunctions and a reduction of brainstem 5-HT tone, quantitative reverse transcriptase polymerase chain reaction showed overexpression of TPH2 (5-HT synthesis enzyme), SERT (5-HT reuptake transporter), and 5-HT2C transcript levels in the brainstem of EPI rats, and of RNA-specific adenosine deaminase (ADAR1, ADAR2) involved in the production of 5-HT2C isoforms. Interictal ventilation was assessed with whole-body plethysmography before and 2 h after administration of SB242084 (2 mg/kg), a specific antagonist of 5-HT2C. As expected, SB242084 administration induced a progressive decrease in ventilatory parameters and an alteration of breathing stability in both control and EPI rats. However, the size of the SB242084 effect was lower in EPI rats than in controls. Increased 5-HT2C gene expression in the brainstem of EPI rats could be part of a compensatory mechanism against epilepsy-related low 5-HT tone and expression of 5-HT2C isoforms for which 5-HT affinity might be lower.

Are we there yet? A critical evaluation of sudden and unexpected death in epilepsy models.

Although animal models have helped to elaborate meaningful hypotheses about the pathophysiology of sudden and unexpected death in epilepsy (SUDEP), specific prevention strategies are still lacking, potentially reflecting the limitations of these models and the intrinsic difficulties of investigating SUDEP. The interpretation of preclinical data and their translation to diagnostic and therapeutic developments in patients thus require a high level of confidence in their relevance to model the human situation. Preclinical models of SUDEP are heterogeneous and include rodent and nonrodent species. A critical aspect is whether the animals have isolated seizures exclusively induced by a specific trigger, such as models where seizures are elicited by electrical stimulation, pharmacological intervention, or DBA mouse strains, or whether they suffer from epilepsy with spontaneous seizures, with or without spontaneous SUDEP, either of nongenetic epilepsy etiology or from genetically based developmental and epileptic encephalopathies. All these models have advantages and potential disadvantages, but it is important to be aware of these limitations to interpret data appropriately in a translational perspective. The majority of models with spontaneous seizures are of a genetic basis, whereas SUDEP cases with a genetic basis represent only a small proportion of the total number. In almost all models, cardiorespiratory arrest occurs during the course of the seizure, contrary to that in patients observed at the time of death, potentially raising the issue of whether we are studying models of SUDEP or models of periseizure death. However, some of these limitations are impossible to avoid and can in part be dependent on specific features of SUDEP, which may be difficult to model. Several preclinical tools are available to address certain gaps in SUDEP pathophysiology, which can be used to further validate current preclinical models.

Respiratory dysfunction in two rodent models of chronic epilepsy and acute seizures and its link with the brainstem serotonin system.

Patients with drug-resistant epilepsy can experience respiratory alterations, notably during seizures. The mechanisms underlying long-term alterations in respiratory function remain unclear. As the brainstem 5-HT system is a prominent modulator of respiratory function, this study aimed at determining whether epilepsy is associated with alterations in both the respiratory function and brainstem serotonin (5-HT) system in rats. Epilepsy was triggered by pilocarpine-induced status epilepticus in rats. Our results showed that 30-50% of epileptic (EPI) rats exhibited a sharp decrease in oxygen consumption (SDOC), low metabolic rate of oxygen, and slow regular ventilation (EPI/SDOC + rats). These alterations were detected only in rats with chronic epilepsy, independent of behavioral seizures, were persistent over time, and not associated with death. In these rats, 5-HT fiber density in the nucleus tractus solitarius was lower than that in the control and EPI/SDOC- rats. Both EPI/SDOC + rats and DBA/2 mice that present with audiogenic-induced seizure followed by fatal respiratory arrest-a model of sudden and expected death in epilepsy-had increased transcript levels of tryptophan hydroxylase 2 and 5-HT presynaptic transporter. Thus, our data support that 5-HT alterations are associated with chronic and acute epilepsy-related respiratory dysfunction.

Respiratory regulation by steroids in newborn rats: a sex-specific balance between allopregnanolone and progesterone receptors.

NEW FINDINGS: What is the central question of this study? What are the contributions of allopregnanolone, the neuroactive metabolite of progesterone, and nuclear (nPR) and membrane (mPR) progesterone receptors to the respiratory effect of progesterone in newborn rats? What is the main finding and its importance? Acute progesterone injection increases the apnoea frequency, whereas finasteride (which blocks the conversion of progesterone to allopregnanolone) reduces apnoea frequency. An nPR agonist decreases apnoea frequency in males and an mPR agonist decreases apnoea frequency in males and females. Chronic injection of progesterone decreases the frequency of apnoea more efficiently in males than in females. We tested the hypothesis that the effects of progesterone on apnoea frequency in newborn rats are the result of a balance between its neuroactive metabolite, allopregnanolone (GABAA receptor modulator), and progesterone receptors. We used male and female rats between 10 and 12 days of age and recorded respiratory and metabolic parameters (whole-body plethysmography), and assessed the frequency and duration of apnoeas in normoxia. We tested the effects of a single injection of progesterone (4 mg kg-1 , i.p.), finasteride (10 mg kg-1 , i.p.; a 5α-reductase antagonist, which blocks the conversion of progesterone to allopregnanolone), finasteride plus progesterone, or agonists of the nuclear or membrane progesterone receptors (R5020 or Org-od-02-0, 4 mg kg-1 ). To test the hypothesis that chronic exposure to progesterone reduces the frequency of apnoeas, we used male and female rats treated daily with progesterone between postnatal days 3 and 12. The acute injection of progesterone reduced minute ventilation and metabolic rate and increased the frequency of apnoeas. Finasteride decreased the frequency of apnoeas, and finasteride plus progesterone did not increase apnoea frequency but decreased minute ventilation in female rats. Although R5020 decreased apnoea frequency only in males, Org-od-02-0 decreased apnoea frequency in males and females and decreased respiratory frequency in females. Chronic progesterone treatment reduced apnoea frequency more efficiently in males than in females, but in females (not in males) an acute injection of caffeine (the gold standard for the treatment of apnoea in preterm neonates) further reduced apnoea frequency. Apnoea frequency in newborn rats is, in part, determined by a sex-specific balance between allopregnanolone, GABAA receptors and progesterone receptors.

Sex-specific respiratory effects of acute and chronic caffeine administration in newborn rats.

Caffeine is widely used for the treatment of apnea of prematurity (AoP) but whether this effect varies with sex is unknown. To shed some light on this question, we present a summary of data obtained on the effects of caffeine on the respiratory chemoreflexes and apnea frequency in 1- and 12-days old male and female rats. Caffeine was either administered as a single acute injection (10mg/kg, i.p.) or for 10 consecutive days (7.5mg/kg/day between 3 and 12days of life by gavage, simulating its clinical use). Acute caffeine had little effects on breathing in 1-day old male and female rats. In 12-days old female rats caffeine reduced the response to hypercapnia (not hypoxia) compared to males. During the steady state of hypoxia females had a lower frequency of apneas than males, and acute injection of caffeine decreased the frequency of apnea, suppressing the differences between males and females. In 12-days old rats chronic administration of caffeine stimulated basal breathing and decreased the frequency of apnea similarly in males and females. In response to hypoxia, chronic caffeine administration also masked the difference in respiratory frequency between males and females observed in control rats. Female rats had lower frequency of apnea than males with or without caffeine treatment. These observations indicate that sex influences the respiratory responses to caffeine and this effect seems to depend on the modality of administration (acute vs chronic) and environmental oxygen (normoxia vs hypoxia).

Standardized environmental enrichment supports enhanced brain plasticity in healthy rats and prevents cognitive impairment in epileptic rats.

Environmental enrichment of laboratory animals influences brain plasticity, stimulates neurogenesis, increases neurotrophic factor expression, and protects against the effects of brain insult. However, these positive effects are not constantly observed, probably because standardized procedures of environmental enrichment are lacking. Therefore, we engineered an enriched cage (the Marlau™ cage), which offers: (1) minimally stressful social interactions; (2) increased voluntary exercise; (3) multiple entertaining activities; (4) cognitive stimulation (maze exploration), and (5) novelty (maze configuration changed three times a week). The maze, which separates food pellet and water bottle compartments, guarantees cognitive stimulation for all animals. Compared to rats raised in groups in conventional cages, rats housed in Marlau™ cages exhibited increased cortical thickness, hippocampal neurogenesis and hippocampal levels of transcripts encoding various genes involved in tissue plasticity and remodeling. In addition, rats housed in Marlau™ cages exhibited better performances in learning and memory, decreased anxiety-associated behaviors, and better recovery of basal plasma corticosterone level after acute restraint stress. Marlau™ cages also insure inter-experiment reproducibility in spatial learning and brain gene expression assays. Finally, housing rats in Marlau™ cages after severe status epilepticus at weaning prevents the cognitive impairment observed in rats subjected to the same insult and then housed in conventional cages. By providing a standardized enriched environment for rodents during housing, the Marlau™ cage should facilitate the uniformity of environmental enrichment across laboratories.