Functional coupling of GABAA receptors and benzodiazepine recognition site subtypes in the spinal cord of the rat

Pharmacological properties of the GABA(A) receptor complex from brain regions of (hypoemotional) Roman high- and (hyperemotional) low-avoidance rats

The pharmacological properties of benzodiazepine binding sites of the gamma-aminobutyric acid (GABA)A receptor complex from cortical, hippocampal and cerebellar membranes of Roman high-avoidance (RHA/Verh) and Roman low-avoidance (RLH/Verh) rats were investigated. No major differences between the two lines were found in the binding parameters of [3H]flunitrazepam (a non-selective agonist). [3 H]zolpidem (a Type I selective agonist) or [3 H]ethyl 8-azido-6-dihydro-5-methyl-6-oxo-4H-imidazol[1,5-a]-[1,4]benzodiazepine- 3-carboxylate (Ro15-4513) (a partial inverse agonist). Neither the Kd values nor the Bmax for these ligands differed between RHA/Verh and RLA/Verh rats in any of the brain regions studied. As a result, the proportion of Type I binding sites in cortical and hippocampal membranes of RHA/Verh and RLA/Verh rats or the 'diazepam-sensitive' and the 'diazepam-insensitive' binding sites in cerebellar membranes, calculated from the [3H]flunitrazepam and [3H]zolpidem maximal binding sites or from [3H]Ro15-4513 binding (in the absence or in presence of diazepam), respectively, was also similar. Furthermore, there were no differences between the two rat lines in the allosteric interactions between GABA and the benzodiazepine binding sites (labeled with [3H]flunitrazepam) in all three areas tested or the Type I binding sites (labeled with [3H]zolpidem) in the hippocampus. In contrast, RLA/Verh rats showed a significant reduction in the allosteric interactions between GABA and [3H]zolpidem binding sites in the cortex. As a whole, these results indicate the absence of generalized between-line differences in the GABA(A) receptor complex showing, at the same time, the existence of some specific differences in allosterism within the GABA(A) complex. These differences may contribute to the divergent emotional responses which characterize the RHA/Verh and RLA/Verh rat lines.

Intrathecally administered flumazenil and PK 11195 precipitate abstinence syndrome in freely moving diazepam dependent rats

The central and peripheral benzodiazepine (BZ) receptor antagonists, flumazenil (FLU) and PK 11195 (PK), administered intrathecally (IT) to diazepam (DZ)-dependent rats produced a precipitated abstinence syndrome. The scores for abstinence increased with increasing dose of FLU but not with increasing dose of PK. Twitches and jerks increased with increased doses of both. Head and body tremors were produced by FLU, but not by PK. Neither FLU nor PK precipitated abstinence in controls. In DZ-dependent rats IT administered FLU and PK did not significantly change the spectral content and the total power of the EEG. The data indicate that an abstinence syndrome is precipitated at the spinal level in DZ-dependent rats and that both central and peripheral BZ receptors are involved.

A behaviorally selective class of thiophene-containing benzodiazepine receptor ligands

In a continued effort to probe the role of the aromatic rings in classical 1,4-benzodiazepine (BDZ) ligand pharmacology, a series of new thiophene-containing benzodiazepine receptor (BDZR) ligands were synthesized. As a first step in determining the binding profile and selectivity to BDZR functional subtypes, the affinities in two central nervous system (CNS) regions, cerebellum, in which a single 'Type I' BDZR could be labeled; and spinal cord, in which we have previously demonstrated some receptor heterogeneity, were determined. These compounds were also assessed for their compliance with a recently developed three dimensional pharmacophore for recognition and activation of the 'Type I' BDZR, using the techniques of computational chemistry. The computations showed all ligands synthesized fulfilled the minimum requirements for recognition, further validating the current pharmacophore. Using the criteria for activation, the new ligands were all predicted to be agonists at the cerebellar 'Type I' BDZR. Since the compounds showed reasonable affinity, the behavioral profile of one of them at five in vivo endpoints was determined. This compound demonstrated more behavioral selectivity than the typical 1,4-BDZ ligand. While they fulfilled the requirements for agonist activity at the 'Type I' BDZR, these ligands showed significantly greater delocalization in the electron density distribution in the lowest unoccupied molecular orbital (LUMO), so that either aromatic ring could serve as an electron accepting site, not just the one comparable to the more classical BDZR agonist, flunitrazepam. It is possible that the ability of the second ring in the tested compound (5a) to also function as an electron acceptor can affect the recognition and activation of other receptor types leading to the more discriminate behavioral profile of this thiophene analog compared to flunitrazepam.

Modulation of [35S]TBPS binding by ligands with preferential affinity for benzodiazepine BZ1 sites in the cerebral cortex of newborn and adult rats

The present study was designed to compare the allosteric coupling between the Cl- channel of the GABAA receptor and the different benzodiazepine recognition site subtypes (BZ sites) in the cerebral cortex of newborn (5-day-old) and adult rats (90-day-old). To this aim, we reexamined the heterogeneity of cortical GABAA receptors in self- and cross-competition binding experiments using [3H]flunitrazepam and two ligands with higher affinity for benzodiazepine BZ1 sites relative to benzodiazepine BZ2 sites, the triazolopyridazine 3-methyl-6-[3-(trifluoromethyl)phenyl]-1,2,4-triazolo [4,3-b] pyridazine (CL 218,872) and the imidazopyridine N,N,6-trimethyl-2-(4-methylphenyl)-imidazo[1,2-a]-pyridine-3-acetamide hemitartrate (zolpidem). Benzodiazepine BZ1 sites accounted for 52% of the total number of binding sites in adult rats, but were not detected in newborn rats. On the other hand, two classes of benzodiazepine BZ2 sites with high and low affinity for zolpidem were present in newborn and adult rats. These sites were designated as benzodiazepine BZ2H (high affinity for zolpidem, Kd approximately 150 nM) and benzodiazepine BZ2L (low affinity for zolpidem, Kd approximately 3000 nM). High densities of benzodiazepine BZ2H sites were measured in both newborn and adult rats (75% and 41% of the total number of [3H]flunitrazepam binding sites, respectively), whereas benzodiazepine BZ2L sites accounted for 25% and 7% of the total number of cortical sites in neonates and adults, respectively. Flunitrazepam, CL 218,872 and zolpidem inhibited in a concentration-dependent manner the binding of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) to the convulsant site of cortical GABAA receptors in newborn and adult rats. The IC50 for flunitrazepam was about 3-fold greater in adults than in neonates. This rightward shift in the concentration-response curve may be due to a decrease with age in the intrinsic efficacy of flunitrazepam. In contrast, CL 218,872 and zolpidem were 4-fold more potent at inhibiting [35S]TBPS binding in adult rats relative to neonates. The different affinities of CL 218,872 and zolpidem for benzodiazepine BZ1 and BZ2 receptors may account, at least in part, for the age-related changes in their inhibitory potencies. These results demonstrate that benzodiazepine BZ2 sites mediate the modulation of [35S]TBPS binding by benzodiazepine recognition site ligands in the cerebral cortex of newborn rats. Further, benzodiazepine BZ2 sites may be involved in the inhibition of [35S]TBPS binding by flunitrazepam, CL 218,872 and zolpidem in the cerebral cortex of adult rats.

Molecular determinants of recognition and activation at the cerebellar benzodiazepine receptor site

Semiempirical quantum mechanical and molecular mechanics calculations were carried out to identify and characterize the steric and electronic properties that modulate ligand recognition and activation of the cerebellar GABAA/benzodiazepine (BDZ) receptor. For this hypothesis development, thirteen compounds belonging to structurally diverse chemical families were selected for study. Among the compounds selected were nine that bind and four that do not bind with appreciable affinity to this receptor and some that are known agonists, antagonists and inverse agonists, as measured by their modulation of GABA (gamma-aminobutyric acid) enhanced chloride ion flux in cerebellum. The stereoelectronic requirements for recognition deduced from commonalities among the ligands are the presence of at least two of three hydrogen bonding centers, and a lipophilic aromatic ring, in a specific spatial relationship. The results suggest that the selectivity for the cerebellar or Type I subtype, demonstrated by some of these ligands, could be failure to meet the requirements for binding at other receptors because of the absence of one of the proton accepting centers or the larger surface area and volume of these ligands. The requirement for activation, deduced from comparisons of agonist, antagonist, and inverse agonist properties is the presence of an electron accepting aromatic ring in a specific geometric arrangement with respect to the components of recognition. The validity of the '3D-Pharmacophore' developed was probed by using it for predictions of the behavior of 11 additional compounds not used for its development.

Amino acid levels and gamma-aminobutyric acidA receptors in rat neostriatum, cortex, and thalamus after neonatal 6-hydroxydopamine lesion

The amino acid gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in brain, and GABAergic neurons have been proposed to play a major role in basal ganglia physiology. In the neostriatum (caudate putamen), medium-sized aspiny interneurons, as well as neostriatal output neurons that project to several brain regions, use GABA as their neurotransmitter. Dopamine fibers arising from the substantia nigra represent a major input to the neostriatum where, besides their classic neurotransmitter role, they are seemingly involved in the regulation of amino acid neurotransmitter release. To further characterize the nature of some of the amino acid/dopamine interactions, selective dopaminergic deafferentations were produced in neonatal rats (3 days postnatal) by intraventricular administration of the neurotoxin 6-hydroxydopamine (6-OHDA); the noradrenergic neurons were protected by prior administration of desmethylimipramine. After a 3-month survival, levels of catecholamines, indoleamines, and amino acids were determined in cingulate cortex, thalamus, and neostriatum. In addition, GABAA receptors were measured in membrane preparations from these three regions, using the specific agonist [3H]muscimol. In the 6-hydroxydopamine-lesioned rats, levels of dopamine and its metabolites homovanillic acid, 3,4-dihydroxyphenylacetic acid, and 3-methoxytyramine were decreased, as expected, in cortex and neostriatum, but remained unmodified in thalamus. In all three regions, serotonin content was increased; its metabolite, 5-hydroxyindole-3-acetic acid, was also elevated, but only in cortex and neostriatum. The levels of GABA were increased in neostriatum and thalamus, but remained unmodified in cortex. Glycine was increased in all three regions examined. There were also increases of phosphatidylethanolamine and serine in thalamus, and of aspartic acid and alanine in neostriatum. The density of GABAA binding sites was increased in neostriatum, but remained unchanged in cortex and thalamus. The changes in amino acid levels and [3H]muscimol binding sites induced by a neonatal 6-hydroxydopamine treatment differ from those found after similar lesions in adult animals, possibly because of the plastic and synaptic rearrangements that can still occur during early postnatal development. The present results also demonstrate that adaptations occur in response to a dopaminergic deafferentation at an early age and that these exhibit a regional specificity.

Comparative autoradiographic distribution of central omega (benzodiazepine) modulatory site subtypes with high, intermediate and low affinity for zolpidem and alpidem

Pharmacological characterization of [3H]benzodiazepine binding to membrane preparations of adult rat hippocampus and neonatal rat brain have demonstrated, in addition to the omega 1 and omega 2 populations of central omega benzodiazepine binding sites associated with GABAA receptors, the existence of binding sites with microM affinity for the imidazopyridines zolpidem and alpidem. In the present study we have investigated their comparative autoradiographic distribution using [3H]flumazenil as a ligand. In the neonatal rat CNS, the imidazopyridine derivatives zolpidem and alpidem were found to discriminate two [3H]flumazenil binding site populations with an IC50 value ratio of more than 200-fold. In the different regions investigated (spinal cord, striatum, neocortex and inferior colliculus) the low affinity component had IC50 values of 20-40 microM (zolpidem) and 5-15 microM (alpidem) and accounted for ca. 50% of the total binding site population. In the adult rat, these imidazopyridine derivatives displayed a greater displacing potency in the cerebellum (IC50 = 6 and 36 nM, respectively) than in the hippocampus (IC50 = 37 and 403 nM, respectively). In the cerebellum, [3H]flumazenil binding was fully displaced by 1 microM of either compound and Hill coefficients of displacement curves were close to unity. In the hippocampus, 25% of [3H]flumazenil binding were resistant to 3 microM zolpidem or 1 microM alpidem, but were displaced by 100 microM of either compound. CL 218,872 also displayed a greater displacing potency in the cerebellum (IC50 = 83 nM) than in the hippocampus (IC50 = 711 nM) but [3H]flumazenil binding in the hippocampus was fully displaced by 10 microM of this compound. In adult rat hippocampus, zolpidem and alpidem were found to discriminate between three central omega site subtypes which display high (IC50 = 31 and 6.1 nM, for these imidazopyridine derivatives. In contrast, CL 218,872 discriminated between omega 1 and omega 2 sites but not between two omega 2 receptor subpopulations. omega 1 sites were mainly localized in layer IV of the sensorimotor cortex, cerebellum, substantia nigra, olfactory bulb and inferior colliculus. omega 2I sites were present in the cortical mantle (with higher levels in the cingulate and olfactory than in the sensorimotor cortex) and in subcortical (hippocampus, hypothalamus and nucleus accumbens) limbic structures. In the hippocampus, hypothalamus, spinal cord and nucleus accumbens, omega 2L sites accounted for more than 25% of the specific [3H]flumazenil binding; the density of these sites was minor in the cortex and in most pyramidal and extrapyramidal system structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Regional differences in the enhancement by GABA of [3H]zolpidem binding to omega 1 sites in rat brain membranes and sections

The potential heterogeneity in the allosteric coupling between GABA and omega 1 binding sites within the native GABAA receptor complex has been evaluated in the rat by measuring the enhancement by GABA of [3H]zolpidem binding to omega 1 site in membranes from three rat brain structures (neocortex, cerebellum and hippocampus) and brain sections. The maximal stimulatory effect of GABA was significantly higher (265 +/- 47%) in cortical membranes than in cerebellar (165 +/- 48%) or in hippocampal (118 +/- 17%) membranes. These differences are not due either to the presence of different amounts of residual GABA in the membrane preparations or to the labeling, in presence of GABA, of binding sites other than omega 1 since: (1) the pharmacological properties of the [3H]zolpidem binding sites were similar in the three regions; (2) the degree of allosteric enhancement was unrelated to the relative proportion of omega 1 sites in each structure; and (3) GABA did not increase the Bmax for [3H]zolpidem. Regional differences in the effect of 100 microM GABA on [3H]zolpidem binding were also observed by quantitative autoradiography. Regions where the strongest (3-4-fold) effects of GABA in [3H]zolpidem binding were observed included the substantia nigra, lateral geniculate body, olfactory tubercule and red nucleus. A large increase in [3H]zolpidem binding was also demonstrated in the cingulate and frontoparietal cortices with higher effects in deep (4.2-fold) rather than in superficial layers (3.3-fold). Heterogeneous subregional increases in [3H]zolpidem binding in the presence of GABA were quantified within the cerebellum, hippocampus and superior colliculus.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of substantia nigra pars reticulata neurons based on response to iontophoretically applied GABA and flurazepam

Previous work had suggested that neurons in the pars reticulata of the substantia nigra (SNpr) might be differentiated based on responsiveness to GABA and benzodiazepines (BZs). To evaluate this possibility, multi-barreled glass capillary assemblies were used to examine the effects of GABA and a BZ, flurazepam (FZP), on the spontaneous activity of single SNpr neurons in chloral hydrate anesthetized rats. Both FZP and GABA, as a function of increasing ejection current, decreased the rate of neuronal discharge. SNpr neurons differed according to the maximum effect of each agent. For over half of the cells, the spontaneous discharge could be inhibited at least 90% by GABA, while the maximum FZP effect in the same cells ranged from 20 to 100% inhibition. Except for 3 neurons, the maximum inhibition produced by GABA was about the same or greater than that produced by FZP. No clear anatomical segregation according to BZ effect was found.

GABAA sodium independent receptor sites in a strain of rats presenting generalized non-convulsive seizures

The role of gamma-aminobutyric acid (GABA), a major inhibitor neurotransmitter in the central nervous system (CNS), is well established in the genesis and the control of epilepsies. The purpose of this work was to study the binding parameters of the Na(+)-independent GABA receptors in the brain of a strain of rats presenting spontaneous generalized non-convulsive seizures. The high- and low-affinity binding sites were evaluated in cerebral cortex, cerebellum, and hippocampus using [3H]muscimol. No significant modification was observed for the Bmax and the Kd of high-affinity binding sites, although a slight decrease of Bmax was noted in the three brain areas in rats with seizures. Concerning the low-affinity binding sites, significant decreases were observed in the values of Bmax in the cortex, cerebellum, and hippocampus of animals with spontaneous seizures, without modification of Kd values. Such changes could be considered to be involved in some of the physiological and behaviour activities observed in this strain of rats.

Heterogeneity in the allosteric interaction between the gamma-aminobutyric acid (GABA) binding site and three different benzodiazepine binding sites of the GABAA/benzodiazepine receptor complex in the rat nervous system

In the present communication we have investigated the allosteric coupling between the gamma-aminobutyric acidA (GABAA) receptor and the pharmacologically different benzodiazepine (BZD) receptor subtypes in membranes from various rat nervous system regions. Two types of BZD receptors (type I and type II) have been classically defined using CL 218.872. However, using zolpidem, three different BZD receptors have been identified by binding displacement experiments in membranes. These BZD receptor subtypes displayed high, low, and very low affinity for zolpidem. The distribution of the high- and low-affinity binding sites for zolpidem was similar to that of type I and type II subtypes in cerebellum, prefrontal cortex, and adult cerebral cortex. On the other hand, the very-low-affinity binding site was localized in relative high proportion in spinal cord, hippocampus, and newborn cerebral cortex and, to a minor extent, in superior colliculus. The allosteric coupling between the GABAA receptor and the BZD receptor subtypes was different. The high- and low-affinity binding sites for zolpidem seemed to have a similar high degree of coupling, except in spinal cord. On the other hand, the very-low-affinity binding site for zolpidem displayed a low degree of coupling with the GABAA receptor. These results seem to indicate that the different efficacy of GABA in enhancing the [3H]flunitrazepam binding could be due to the different BZD receptor subtypes present in the GABAA/BZD receptor complex and, moreover, led us to speculate that the low GABA efficacy found in membranes from spinal cord, hippocampus, and newborn cerebral cortex might be due to the presence in relatively high proportion of the very-low-affinity binding site for zolpidem.

Ovarian steroid modulation of [3H]muscimol binding in the spinal cord of the rat

[3H]Muscimol binding was measured in the lumbar spinal cord of female rats by in vitro quantitative autoradiography. Ovariectomized rats were treated subcutaneously with either oil, estradiol benzoate (EB) or EB plus progesterone (P) in a regime known to reliably induce sexual receptivity. The level of [3H]muscimol binding was highest in laminae I-III and in the region around the central canal. Binding was lower in laminae IV-VI and was frequently undetectable in the ventral horn. There was a significant increase in the level of binding in laminae I-III after EB treatment. There was also a significant increase after treatment with EB+P in comparison to both the ovariectomized and EB-treated groups in this same region of the spinal cord.

The effects of pentobarbital and benzodiazepines on GABA-responses in the periphery and spinal cord in vitro

Pentobarbital and benzodiazepines were compared in their interaction with the gamma-aminobutyric acid (GABA) antagonists picrotoxin and bicuculline on GABAA receptor-mediated events. On excised vagal nerves and dorsal roots pentobarbital, in contrast to the benzodiazepines diazepam, lorazepam and flurazepam, was able to enhance GABA-induced depolarizations recorded in the presence of picrotoxin or bicuculline. On hemicord preparations picrotoxin simultaneously depressed the electrically evoked dorsal root-dorsal root potential and enhanced the dorsal root-ventral root potential. Pentobarbital overcame the effects of picrotoxin, whereas diazepam and midazolam were without effect. These results may be explained by the suggestion that the GABA receptors in these test systems are not tightly associated with the benzodiazepine receptor activated by diazepam, lorazepam, midazolam and flurazepam, and correspond to the recently described GABAA2 subdivision of GABA receptors.

Effect of acute and chronic administration of buspirone on serotonin and benzodiazepine receptor subtypes in the rat brain: an autoradiographic study

The affinity of buspirone and its main metabolite 1-(2-pyrimidinyl)piperazine (PmP) for serotonin1 (5-HT1) and benzodiazepine receptors was first evaluated by computerized receptor autoradiography. The results confirmed that buspirone is a selective 5-HT1A ligand, since it inhibited the binding of [3H]5-HT with lower IC50 values (about 100 nM) in regions of the brain of the rat where this receptor subtype is predominant (such as hippocampal areas). Larger IC50 values than 3 microM were found in areas of the brain richer in 5-HT1 receptors, other than the 5-HT1A subtype (e.g. striatum, substantia nigra and the ventricles). The PmP was not selective, inhibiting the binding of [3H]5-HT with similar affinity (about 4-10 microM) in all the regions of the brain examined. Neither buspirone nor PmP, up to 100 microM, were active on benzodiazepine receptors. The autoradiographic technique was therefore used to evaluate the effects of acute (10 mg/kg, p.o., 1 hr before killing) and chronic (10 mg/kg, i.p., twice a day for 21 days, 24 hr washout) treatment with buspirone in male rats. Acute treatment reduced the binding of [3H]5-HT in all the regions of the brain studied, including those with low levels of 5-HT1A receptors, indicating the occupancy of 5-HT1 receptors by either buspirone or its metabolite. The binding of [3H]flunitrazepam was decreased (16%) only in the substantia nigra.(ABSTRACT TRUNCATED AT 250 WORDS)

In situ hybridization histochemistry reveals a diversity of GABAA receptor subunit mRNAs in neurons of the rat spinal cord and dorsal root ganglia

The distribution and relative abundance of gene transcripts for diverse GABAA receptor subunits (alpha 1-3,5, beta 1-3, gamma 2) in neurons of the rat cervical spinal cord and dorsal root ganglia were determined by in situ hybridization histochemistry using 35S-labeled 60mer oligonucleotide probes. The receptor proteins (mapped by benzodiazepine receptor radioautography and immunohistochemistry with [3H]flumazenil and a monoclonal antibody for the beta 2 + beta 3 subunits, respectively) were most abundant in the dorsal horn (layers II and III) and in layer X around the central canal. Although diverse receptor subunit mRNAs were detected (to varying degrees) in neurons throughout layers II-X of the spinal cord, motoneurons in layer IX were particularly strongly labeled. The gamma 2 mRNA was the most ubiquitous and abundant of the subunit variants investigated. The labeling of motoneurons in layer IX was particularly strong for alpha 2, moderate for beta 3 and gamma 2 and extremely weak for alpha 1 and alpha 3. In layers VII, VIII and X the beta 3 and gamma 2 transcripts were moderately expressed whereas the alpha 1 and beta 2 transcript levels differed markedly among the cells of these layers. Although the mRNAs of all subunit variants could be detected in layers IV-VI, only alpha 3, alpha 5, beta 3 and gamma 2 hybridization signals were observed in layers II and III. In the dorsal root ganglia, whereas alpha 2 transcripts were abundant in virtually all large sensory neurons and to a much lower degree in the small diameter cells, gamma 2 transcripts were confined to a subpopulation of large and small neurons. Furthermore, beta 2 and alpha 1 transcripts were even more restricted in their distribution. The findings provided a basis for the mediation of synaptic inhibition in the spinal cord by diverse GABAA receptors and further strong evidence for the long-established view that presynaptic inhibition of inter- and motoneurons, via axoaxonic synapses between GABAergic interneurons and primary afferent terminals, is mediated by GABAA receptors. The physiological roles and pharmacological implications of this receptor diversity have yet to be determined.

Anticonvulsant actions and interaction of GABA agonists and a benzodiazepine in pars reticulata of substantia nigra

In a previous study, infusion of flurazepam and midazolam, but not GABA or muscimol into the pars reticulata of substantia nigra (SNpr), blocked the tonus induced by a high dose of pentylenetetrazol (PTZ). It was hypothesized that the divergence of GABAA agonist and benzodiazepine (BZ) actions might be due to a complex action of the GABAA agonists in SNpr that is not shared by the BZs, and which is specific for certain experimental seizures. This was tested in the present study in which GABA, muscimol, midazolam, and combinations of midazolam with GABA and with muscimol were tested against seizures induced by PTZ (40 or 100 mg/kg, i.p.), bicuculline (0.5 mg/kg, i.v.) and maximal electroshock (MES). Intranigral midazolam was effective against PTZ, bicuculline and MES seizures. Intranigral GABA or muscimol, infused together with midazolam, prevented midazolam from blocking tonus induced by the high dose of PTZ. For bicuculline seizures, midazolam alone had a better overall anticonvulsant effect than did the combination of midazolam plus muscimol. Intranigral muscimol, midazolam and a combination of two were equally effective against the low dose PTZ (40 mg/kg, i.p.) and MES seizures. Since the effect of intranigral drugs was model-specific, it was suggested that different populations of nigral output neurons were involved in regulating the generalization of these seizures.

Regional distribution of low-affinity GABA receptors coupled to benzodiazepine receptor subtypes in rat brain: an autoradiographic evaluation

Quantitative autoradiography of rat brain coronal sections show that maximum enhancement (more than 80%) of [3H]flunitrazepam binding by GABA occurs in brain regions particularly rich in type I benzodiazepine receptors (inferior colliculus, medium raphe, central gray and substantia nigra); conversely, brain areas where type II predominates show the lowest enhancement by GABA (about 50%). These results, suggesting that the coupling of GABA receptors with type I sites is more efficient than that with type II sites, are at variance with those reported on GABA-benzodiazepine receptors expressed in transfected cells, where the greater GABA potentiation of benzodiazepine binding is due to a subtype of the type II site containing the alpha 3 subunit of the GABAA receptor. One possible explanation of these discrepancies is that the type II receptors found in type I-enriched tissues (inferior colliculus, median raphe, central gray and substantia nigra) are associated with the alpha 3-subunit, while the type II sites present in limbic and cortical regions represent a subpopulation carrying the alpha 2-subunit of the GABAA receptor, with lower GABA potentiation.