Marzia Orlandi

Decrease in the function of the γ-aminobutyric acid-coupled chloride channel produced by the repeated administration of pentylenetetrazol to rats

Abstract: The acute administration of pentylenetetrazol (PTZ; 25–75 mg/kg i.p.) failed to modify the specific binding of t‐[35S]butylbicyclophosphorothionate ([35S]TBPS) to membrane preparations from the cerebral cortex of the rat. In contrast, the repeated administration of PTZ (30 mg/kg i.p., three times a week for 12 weeks) reduced by 26% the density of [33S]TBPS binding sites without modifying the dissociation constant. This effect was observed 3 days after the last PTZ administration. A parallel reduction of 7‐amino‐butyric acid (GABA)‐stimulated 36CI− uptake was measured in the cerebral cortex of PTZ‐treated rats 3 days after the last injection. The repeated administration of PTZ produced sensitization to the drug, or chemical kindling. In fact, no convulsions were observed in the first week of treatment, but all the animals became sensitized to PTZ by the 12th week. The results are consistent with the hypothesis that chronic treatment with PTZ at a subconvulsant dose causes a decrease in GABA‐coupled chloride channel activity that may be related to the chemical kindling produced by this compound.

Pentylenetetrazol-induced kindling in rats: effect of GABA function inhibitors

The repeated administration of subconvulsant doses of pentylenetetrazol (PTZ) produced a progressive sensitization to the effects of this compound (i.e., chemical kindling) in the rat. A very similar time-course for PTZ-induced kindling was observed using two different treatment schedules: 1) one injection every day (30 mg/kg, IP), and 2) one injection (30 mg/kg, IP) every second day. When these treatment schedules were used for eight consecutive weeks, more than 80% of the rats displayed convulsions by the end of treatment. In contrast, only 20% of the rats were sensitized if PTZ was administered twice daily at the dose of 15 mg/kg, IP. The increased sensitivity to the convulsant effect of PTZ was still present one year after completion of the chronic treatment. Moreover, rats kindled with PTZ showed an enhanced susceptibility to convulsions induced by different inhibitors of central GABAergic function, such as the chloride channel blocker picrotoxin, the benzodiazepine receptor ligands FG 7142 and Ro 15-4513, and the inhibitor of GABA synthesis isoniazid. In contrast, the sensitivity to the convulsant action of the glycine receptor antagonist strychnine was unchanged by repeated PTZ administration. It is suggested that kindling produced by PTZ may be associated with a persistent reduction in the inhibitory function of the GABAergic system in the brain.

MK-801 prevents chemical kindling induced by pentylenetetrazol in rats

The repeated administration of pentylenetetrazol (PTZ) at a subconvulsant dose (30 mg/kg i.p., three times a week for nine weeks) produced kindling in 90% of rats under treatment. Pretreatment with the N-methyl-D-aspartate receptor antagonist, MK-801 (1 mg/kg i.p., 40 min before PTZ), prevented the behavioral manifestation (i.e. motor seizures) as well as the development of kindling. In fact, convulsions were not observed in rats pretreated with MK-801 either during the chronic PTZ administration or when challenged with PTZ three and 10 days after completion of the chronic treatment. The results suggest an involvement of excitatory amino acid neurotransmission in PTZ kindling.

GABAergic and dopaminergic transmission in the brain of Roman high-avoidance and Roman low-avoidance rats.

The GABAergic and dopaminergic pathways in the central nervous system (CNS) play a pivotal role in the control of emotions and in the adaptive responses to stressful stimuli. The present study was aimed at characterizing a range of biochemical markers of GABA- and dopamine-mediated neurotransmission in the CNS of Roman high-avoidance (RHA/Verh) and Roman low-avoidance (RLA/Verh) rats, two psychogenetically selected lines that differ in their level of emotionality. The stimulatory effect of GABA on 36Cl- uptake was less pronounced in the cerebral cortex of RLA/Verh rats as compared to RHA/Verh rats, whereas no line-related changes were detected in [3H]GABA and [3H]flunitrazepam binding. On the other hand, the density of D1 dopamine receptors labeled with [3H]SCH 23390 was lower in the nucleus accumbens of RLA/Verh rats as compared to their RHA/Verh counterparts, whilst no line-dependent changes were observed in the binding parameters of D1 dopamine receptors in the striatum, amygdala, and prefrontal cortex. These biochemical differences may contribute to the distinct emotionality and responsiveness to the effects of psychoactive drugs of RHA/Verh and RLA/Verh rats.

MK-801 prevents the decrease in 35S-TBPS binding in the rat cerebral cortex induced by pentylenetetrazol kindling.

Chemical kindling was induced in the rat by chronic treatment with pentylenetetrazol (PTZ, 30 mg/kg, IP, three times a week for eight weeks). PTZ kindling was associated with a reduction in central GABAergic function, as reflected by a significant decrease in the density of 35S-t-butylbicyclophosphorothionate (35S-TBPS) binding sites in the cerebral cortex. The pretreatment with the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (1 mg/kg, IP, 40 min before each PTZ injection) prevented the development of kindling as well as the reduction in 35S-TBPS binding. The results suggest that NMDA receptors may play a role in the alterations of GABAergic function observed in PTZ-kindled rats.

Anticonvulsant effect of felbamate in the pentylenetetrazole kindling model of epilepsy in the rat

The present study was designed to examine the ability of felbamate, a novel antiepileptic agent, to antagonize the increase in seizure severity (i.e., chemical kindling) produced by chronic treatment with initially subconvulsant doses of pentylenetetrazol (PTZ). Rats were treated with PTZ (30 mg/kg, i.p., three times a week) for 8 consecutive weeks. Two other groups of rats received felbamate (300 or 400 mg/kg, i.p.), 90 min before each dose of PTZ. Pretreatment with felbamate at either dose prevented the progression of rank of seizures during chronic treatment with PTZ. Thus, the mean seizure score by the end of the chronic treatment (0–5 scale) was 0 in vehicle treated controls, 3.3 in rats treated with PTZ alone, 1.5 in rats treated with PTZ plus felbamate (300 mg/kg, i.p.) and 0.9 in the group treated with PTZ plus felbamate (400 mg/kg, i.p.). Felbamate also antagonized the long-term increase in the sensitivity to the convulsant effects of GABA function inhibitors observed in PTZ-kindled rats. Thus, the administration of a challenge dose of isoniazid (120 mg/kg, s.c.), picrotoxin (1.5 mg/kg, i.p.) or PTZ itself (15 mg/kg, i.p.), 15 to 45 days after the end of the chronic treatment regimen, induced convulsions in > 80% of PTZ-kindled rats and in < 20% of rats treated with PTZ + felbamate (400 mg/kg). The results are discussed in terms of the multiple mechanisms that can contribute to the anticonvulsant action of felbamate in the PTZ kindling model of epilepsy.

Allosteric modulation of [35S]TBPS-binding in the cerebral cortex of the rat during postnatal development

The ontogenesis of the GABA-gated Cl- channel was investigated in the cerebral cortex of the rat by monitoring the binding parameters of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) at intervals after birth (1-90 days). To investigate the influence of the developmental changes in the content of GABA on [35S]TBPS-binding, the assays were carried out in unwashed membranes, in which the concentration of GABA was dependent on its content in vivo, and in repeatedly washed membranes in the presence of defined concentrations of exogenous GABA. At birth, the density (Bmax) of [35S]TBPS-binding sites in unwashed membranes was similar to that found in well-washed membranes. However, in unwashed membranes, the number of [35S]TBPS-binding sites increased by two-fold within 10 days after birth whereas in washed membranes it increased by four-fold during the same period. The higher density of [35S]TBPS-binding sites in washed membranes as compared with the unwashed counterparts persisted throughout development. In unwashed membranes, the apparent Kd for [35S]TBPS-binding increased with age whereas in washed membranes the affinity of [35S]TBPS for its binding sites remained constant throughout development. The binding of [35S]TBPS to the GABA-gated Cl- channel is allosterically modulated by drugs acting on different sites of the GABAA receptor complex. Thus, GABA and diazepam decrease [35S]TBPS-binding whereas the GABAA receptor antagonist, bicuculline, and the inverse agonist for benzodiazepine receptors, 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylic acid methyl ester, increase it.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related decrease in the density of benzodiazepine recognition sites in the brain of the fresh water eel (Anguilla anguilla)

The binding parameters of benzodiazepine receptors labeled with [3H]flunitrazepam were investigated in the brain of male adult eels at the trophic phase (4 years old) and of male silver eels (6 year old ocean migrants). The density of [3H]flunitrazepam binding sites was significantly decreased (-26%) in the brain of silver eels compared with younger counterparts. In contrast, the apparent dissociation constant (Kd) was not significantly different in the two experimental groups. These results are discussed in terms of the possible involvement of stress, behavioral and environmental factors and age-related degenerative processes.

Kinetics of tert-[35S]butylbicyclophosphorothionate binding in the cerebral cortex of newborn and adult rats : Effects of GABA and receptor desensitization

Abstract: The effects of GABA on the kinetics of tert‐[35S]butylbicyclophosphorothionate ([35S]TBPS) binding to the convulsant site of GABAA receptors were studied in membrane suspensions from the cerebral cortex of newborn (1‐day‐old) and adult (90‐day‐old) rats. TBPS dissociation was biphasic in neonates and adults, indicating that more than one interconvertible state of [35S]TBPS binding sites may be present in the cerebral cortex. In the absence of GABA, the fast (t1/2, 11 min) and slow (t1/2, 77 min) components of TBPS dissociation in newborn rats were approximately fourfold slower than in adults. The acceleration of the dissociation rates caused by 2 µM GABA, however, was more robust in neonates than in adults (six‐ to ninefold vs. twofold increase, respectively). Moreover, the dissociation rates of TBPS in membranes preincubated with 2 µM GABA (dissociation started by adding 40 µM picrotoxin) were two‐ to fourfold slower than in membranes preincubated without GABA (dissociation started by adding 40 µM picrotoxin plus 2 µM GABA). Taken together, these results suggest that (1) the closed state of GABAA receptors is associated with a more effective steric barrier for the binding of TBPS in neonates compared with adults, (2) GABA produces a larger acceleration of the binding kinetics of TBPS in neonates than in adults, and (3) long incubations with GABA may cause receptor desensitization, which in turn slows down the dissociation rates of TBPS.