Sex differences in the sensitivity of CBA mice to convulsions induced by GABA antagonists are age-dependent

Increases in GFAP immunoreactive astrocytes in the cerebellar molecular layer of young adult CBA/J mice

Background

CBA/J mice are standard experimental animals in auditory studies, and age-related changes in auditory pathways are well documented. However, changes in locomotion-related brain regions have not been systematically explored.

Results

We showed an increase in immunoreactivity for glial fibrillary acidic protein (GFAP) in the cerebellar molecular layer associated with Purkinje cells in mice at 24 weeks of age but not in the younger mice. Increased GFAP immunoreactivity appeared in the form of clusters and distributed multifocally consistent with hyperplasia of astrocytes that were occasionally associated with Purkinje cell degeneration. Three out of 12 animals at 16 and 24 weeks of age exhibited pre-convulsive clinical signs. Two of these 3 animals also showed increased GFAP immunoreactivity in the cerebellum. Rotarod behavioral assessments indicated decreased performance at 24 weeks of age.

Conclusions

These results suggest minimal to mild reactive astrocytosis likely associated with Purkinje cell degeneration in the cerebellum at 24 weeks of age in CBA/J mice. These findings should be taken into consideration prior to using this mouse strain for studying neuroinflammation or aging.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42826-021-00100-5.

The role of ligand-gated chloride channels in behavioural alterations at elevated CO2 in a cephalopod

Projected future carbon dioxide (CO2) levels in the ocean can alter marine animal behaviours. Disrupted functioning of γ-aminobutyric acid type A (GABAA) receptors (ligand-gated chloride channels) is suggested to underlie CO2-induced behavioural changes in fish. However, the mechanisms underlying behavioural changes in marine invertebrates are poorly understood. We pharmacologically tested the role of GABA-, glutamate-, acetylcholine- and dopamine-gated chloride channels in CO2-induced behavioural changes in a cephalopod, the two-toned pygmy squid (Idiosepius pygmaeus). We exposed squid to ambient (∼450 µatm) or elevated (∼1000 µatm) CO2 for 7 days. Squid were treated with sham, the GABAA receptor antagonist gabazine or the non-specific GABAA receptor antagonist picrotoxin, before measurement of conspecific-directed behaviours and activity levels upon mirror exposure. Elevated CO2 increased conspecific-directed attraction and aggression, as well as activity levels. For some CO2-affected behaviours, both gabazine and picrotoxin had a different effect at elevated compared with ambient CO2, providing robust support for the GABA hypothesis within cephalopods. In another behavioural trait, picrotoxin but not gabazine had a different effect in elevated compared with ambient CO2, providing the first pharmacological evidence, in fish and marine invertebrates, for altered functioning of ligand-gated chloride channels, other than the GABAAR, underlying CO2-induced behavioural changes. For some other behaviours, both gabazine and picrotoxin had a similar effect in elevated and ambient CO2, suggesting altered function of ligand-gated chloride channels was not responsible for these CO2-induced changes. Multiple mechanisms may be involved, which could explain the variability in the CO2 and drug treatment effects across behaviours.

Behavioral and electrophysiological changes in female mice overexpressing ORAI1 in neurons

Orai proteins form highly selective Ca²⁺ release-activated channels (CRACs). They play a critical role in store-operated Ca²⁺ entry (SOCE; i.e., the influx of external Ca²⁺ that is induced by the depletion of endoplasmic reticulum Ca²⁺ stores). Of the three Orai homologs that are present in mammals (Orai1-3), the physiological function of Orai1 is the best described. CRACs are formed by both homomeric assemblies and heteromultimers of Orais. Orai1 and Orai2 can form heteromeric channels that differ in conductivity during SOCE, depending on their Orai1-to-Orai2 ratio. The present study explored the potential consequences of ORAI1 overexpression in neurons where the dominant isoform is Orai2. We established the Tg(ORAI1)Ibd transgenic mouse line that overexpresses ORAI1 in brain neurons. We observed seizure-like symptoms in aged (≥15-month-old) female mice but not in males of the same age. The application of kainic acid and bicuculline to slices that were isolated from 8-month-old (±1 month) female Tg(ORAI1)Ibd mice revealed a significantly lower frequency of interictal bursts compared with samples that were isolated from wildtype mice. No differences were observed in male mice of a similar age. A battery of behavioral tests showed that context recognition decreased only in female transgenic mice. The phenotype that was observed in female mice suggests that ORAI1 overexpression may affect neuronal activity in a sex-dependent manner. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Effects of acute and chronic administration of neurosteroid dehydroepiandrosterone sulfate on neuronal excitability in mice

Background

Neurosteroid dehydroepiandrosterone sulfate (DHEAS) has been associated with important brain functions, including neuronal survival, memory, and behavior, showing therapeutic potential in various neuropsychiatric and cognitive disorders. However, the antagonistic effects of DHEAS on γ-amino-butyric acid_A receptors and its facilitatory action on glutamatergic neurotransmission might lead to enhanced brain excitability and seizures and thus limit DHEAS therapeutic applications. The aim of this study was to investigate possible age and sex differences in the neuronal excitability of the mice following acute and chronic DHEAS administration.

Methods

DHEAS was administered intraperitoneally in male and female adult and old mice either acutely or repeatedly once daily for 4 weeks in a 10 mg/kg dose. To investigate the potential proconvulsant properties of DHEAS, we studied the effects of acute and chronic DHEAS treatment on picrotoxin-, pentylentetrazole-, and N-methyl-D-aspartate-induced seizures in mice. The effects of acute and chronic DHEAS administration on the locomotor activity, motor coordination, and body weight of the mice were also studied. We also investigated the effects of DHEAS treatment on [³H]flunitrazepam binding to the mouse brain membranes.

Results

DHEAS did not modify the locomotor activity, motor coordination, body weight, and brain [³H]flunitrazepam binding of male and female mice. The results failed to demonstrate significant effects of single- and long-term DHEAS treatment on the convulsive susceptibility in both adult and aged mice of both sexes. However, small but significant changes regarding sex differences in the susceptibility to seizures were observed following DHEAS administration to mice.

Conclusion

Although our findings suggest that DHEAS treatment might be safe for various potential therapeutic applications in adult as well as in old age, they also support subtle interaction of DHEAS with male and female hormonal status, which may underline observed sex differences in the relationship between DHEAS and various health outcomes.

Sex-specific consequences of early life seizures

Seizures are very common in the early periods of life and are often associated with poor neurologic outcome in humans. Animal studies have provided evidence that early life seizures may disrupt neuronal differentiation and connectivity, signaling pathways, and the function of various neuronal networks. There is growing experimental evidence that many signaling pathways, like GABAA receptor signaling, the cellular physiology and differentiation, or the functional maturation of certain brain regions, including those involved in seizure control, mature differently in males and females. However, most experimental studies of early life seizures have not directly investigated the importance of sex on the consequences of early life seizures. The sexual dimorphism of the developing brain raises the question that early seizures could have distinct effects in immature females and males that are subjected to seizures. We will first discuss the evidence for sex-specific features of the developing brain that could be involved in modifying the susceptibility and consequences of early life seizures. We will then review how sex-related biological factors could modify the age-specific consequences of induced seizures in the immature animals. These include signaling pathways (e.g., GABAA receptors), steroid hormones, growth factors. Overall, there are very few studies that have specifically addressed seizure outcomes in developing animals as a function of sex. The available literature indicates that a variety of outcomes (histopathological, behavioral, molecular, epileptogenesis) may be affected in a sex-, age-, region-specific manner after seizures during development. Obtaining a better understanding for the gender-related mechanisms underlying epileptogenesis and seizure comorbidities will be necessary to develop better gender and age appropriate therapies.

Aging models of acute seizures and epilepsy

Aged animals have been used by researchers to better understand the differences between the young and the aged brain and how these differences may provide insight into the mechanisms of acute seizures and epilepsy in the elderly. To date, there have been relatively few studies dedicated to the modeling of acute seizures and epilepsy in aged, healthy animals. Inherent challenges to this area of research include the costs associated with the purchase and maintenance of older animals and, at times, the unexpected and potentially confounding comorbidities associated with aging. However, recent studies using a variety of in vivo and in vitro models of acute seizures and epilepsy in mice and rats have built upon early investigations in the field, all of which has provided an expanded vision of seizure generation and epileptogenesis in the aged brain. Results of these studies could potentially translate to new and tailored interventional approaches that limit or prevent the development of epilepsy in the elderly.

Animal models of geriatric epilepsy

Geriatric epilepsy is a significant clinical problem that has not been studied adequately in animal models. This chapter will review the available literature with particular attention to models that have demonstrated how acute seizures and epilepsy in aged animals differ from those of younger animals. Studies include several strains of mice [e.g., El, DBA, senescence-accelerated mouse (SAM), Cacnb4 knockout] as well as acute seizure models in common strains of aged mice. Aged rats (including Fischer 344, Wistar, and Sprague-Dawley) have been used in acute seizure, lesion, and epilepsy models. This area of research remains largely unexplored and therefore provides numerous opportunities for new investigations.

Sex differences in bicuculline-induced convulsions: interaction with stress and ligands of benzodiazepine binding sites

The response to i.v. administration of bicuculline and its interaction with the benzodiazepine agonist diazepam and antagonist flumazenil were studied in male and female handling stressed and swim stressed rats. Both handling stressed and swim stressed male rats needed less bicuculline to produce myoclonic twitch and running/bouncing (RB) clonus than females. Besides, a lower dose of bicuculline produced tonic hindlimb extensor convulsion (THE) in male than in female swim stressed rats. Flumazenil failed to affect seizure thresholds for bicuculline either in handling stressed or in swim stressed animals. Sex differences remained present after diazepam pre-treatment as well. While diazepam enhanced doses of bicuculline producing all three convulsive signs similarly in both handling and swim stressed rats (141-162%), swim stress had the lowest anticonvulsive effect for the onset of myoclonic twitch (110% in males and 117% in females) and the highest for THE (148% in males and 188% in females). The anticonvulsive effect of diazepam was not sex-dependent, while the anticonvulsive effect of swim stress was greater in female than in male rats. The results suggest that greater sensitivity of male rats to bicuculline and the anticonvulsive effect of swim stress do not result from the release of endogenous modulators of benzodiazepine binding sites.

Influence of gender and gonadectomy on bicuculline-induced convulsions and on GABAA receptors

The response to IV administration of GABAA receptor antagonist bicuculline was studied in young (30 days) and in adult gonad-intact or gonadectomized male and female rats. The properties of GABAA receptors, obtained from cortex and cerebellum 30 days following gonadectomy, and the affinity of muscimol and bicuculline for cortical and cerebellar GABA binding sites were also studied. While young rats failed to show sex differences, the threshold doses of bicuculline producing the first myoclonic twitch and running/bouncing clonus (RB clonus) were lower in adult male than female rats. Fifteen days after gonadectomy or sham operation male rats needed less bicuculline to the onset of myoclonic twitch and RB clonus than identically treated females, while orchidectomized rats needed more bicuculline to the onset of tonic hindlimb extension than all other groups examined. All sex differences disappeared 30 days following gonadectomy. At the same time, in males gonadectomy decreased the affinity and enhanced the density of cortical 3H-muscimol binding sites. In female rats, gonadectomy only decreased the affinity of cortical GABAA receptors. Only regional but not sex differences were observed in the affinity of muscimol and bicuculline for GABAA receptors. Sex differences in the threshold doses of bicuculline-producing convulsions do not correlate either with the properties of cortical and cerebellar GABAA receptors or with the affinity of bicuculline for the same binding sites.

Gonadal hormones and picrotoxin-induced convulsions in male and female rats

The sensitivity to the GABA-blocking agent picrotoxin was studied in young and adult male and female rats, in rats treated with gonadal hormones and in gonadectomized male and female rats. Picrotoxin was equipotent in producing convulsions in male and female 20-day-old rats. Adult females tended to be more, while adult males were considerably less sensitive to picrotoxin than young rats. Picrotoxin was equipotent in displacing t-[3H]butylbicycloorthobenzoate ([3H]TBOB) binding to crude cortical and cerebellar membranes from male and female rat brain. Chronic treatment of male rats, beginning with 30 days of age, with estradiol benzoate enhanced their sensitivity to picrotoxin, while an analogous treatment of female rats with testosterone propionate was ineffective. Thirty days following castration adult male rats had shorter latencies to the appearance of picrotoxin-induced convulsions and a higher incidence of death. Ovariectomy in females failed to modify the sensitivity to picrotoxin. The results suggest that gonadal hormones have a crucial role in the development of sex related differences in the response of rats to picrotoxin and presumably to other GABA-related drugs. When developed, the male type of reactivity appears to depend more, and the female type less on the presence of circulating hormones in the blood.

Classical genetic analysis of GABA-related seizures

Differences in resistance to 3-mercaptopropionic acid (MP)-induced seizures exist between DBA/2Ibg and C57BL/6Ibg inbred strains of mice; C57BL/6Ibg mice are more resistant to MP-induced seizures. To determine the mode of inheritance for seizure resistance, a classical genetic analysis was conducted using these two parental strains, their F1, F2, and backcross generations. Latencies to seizure onset and tonus after intraperitoneal (IP) injections of MP (25-40 mg/kg) were quantified. For all populations mean latencies to onset and tonus decreased in a dose dependent manner with the hybrid generations exhibiting a seizure resistant phenotype resembling the C57BL/6Ibg strain. In general, female mice were less resistant to MP-induced seizures than males; however, a significant degree of resistance was retained by the C57BL/6Ibg females and their female progeny. A quantitative assessment of the pattern of inheritance for seizure resistance using a weighted least-squares regression approach indicated that an additive-dominance model explained latency to seizure onset data at 25, 35 and 40 mg/kg. However, at 30 mg/kg, the model required the addition of an epistatic parameter to best describe mean scores at this dose. The results of these analyses suggest that resistance to MP-induced seizures is transmitted in a dominant manner.