The visualization of neuronal benzodiazepine receptors in the brain by autoradiography and immunohistochemistry

γ-Aminobutyric acid transporter and GABAA receptor mechanisms in Slc6a1+/A288V and Slc6a1+/S295L mice associated with developmental and epileptic encephalopathies

We have previously characterized the molecular mechanisms for variants in γ-aminobutyric acid transporter 1-encoding solute carrier family 6-member 1 (SLC6A1) in vitro and concluded that a partial or complete loss of γ-aminobutyric acid uptake due to impaired protein trafficking is the primary aetiology. Impairment of γ-aminobutyric acid transporter 1 function could cause compensatory changes in the expression of γ-aminobutyric acid receptors, which, in turn, modify disease pathophysiology and phenotype. Here we used different approaches including radioactive 3H γ-aminobutyric acid uptake in cells and synaptosomes, immunohistochemistry and confocal microscopy as well as brain slice surface protein biotinylation to characterize Slc6a1+/A288V and Slc6a1+/S295L mice, representative of a partial or a complete loss of function of SLC6A1 mutations, respectively. We employed the γ-aminobutyric acid transporter 1-specific inhibitor [3H]tiagabine binding and GABAA receptor subunit-specific radioligand binding to profile the γ-aminobutyric acid transporter 1 and GABAA receptor expression in major brain regions such as cortex, cerebellum, hippocampus and thalamus. We also determined the total and surface expression of γ-aminobutyric acid transporter 1, γ-aminobutyric acid transporter 3 and expression of GABAA receptor in the major brain regions in the knockin mice. We found that γ-aminobutyric acid transporter 1 protein was markedly reduced in cortex, hippocampus, thalamus and cerebellum in both mutant mouse lines. Consistent with the findings of reduced γ-aminobutyric acid uptake for both γ-aminobutyric acid transporter 1(A288V) and γ-aminobutyric acid transporter 1(S295L), both the total and the γ-aminobutyric acid transporter 1-mediated 3H γ-aminobutyric acid reuptake was reduced. We found that γ-aminobutyric acid transporter 3 is only abundantly expressed in the thalamus and there was no compensatory increase of γ-aminobutyric acid transporter 3 in either of the mutant mouse lines. γ-Aminobutyric acid transporter 1 was reduced in both somatic regions and nonsomatic regions in both mouse models, in which a ring-like structure was identified only in the Slc6a1+/A288V mouse, suggesting more γ-aminobutyric acid transporter 1 retention inside endoplasmic reticulum in the Slc6a1+/A288V mouse. The [3H]tiagabine binding was similar in both mouse models despite the difference in γ-aminobutyric acid uptake function and γ-aminobutyric acid transporter 1 protein expression for both mutations. There were no differences in GABAA receptor subtype expression, except for a small increase in the expression of α5 subunits of GABAA receptor in the hippocampus of Slc6a1S295L homozygous mice, suggesting a potential interaction between the expression of this GABAA receptor subtype and the mutant γ-aminobutyric acid transporter 1. The study provides the first comprehensive characterization of the SLC6A1 mutations in vivo in two representative mouse models. Because both γ-aminobutyric acid transporter 1 and GABAA receptors are targets for anti-seizure medications, the findings from this study can help guide tailored treatment options based on the expression and function of γ-aminobutyric acid transporter 1 and GABAA receptor in SLC6A1 mutation-mediated neurodevelopmental and epileptic encephalopathies.

Quantification of [11C]Ro15-4513 GABAAα5 specific binding and regional selectivity in humans

[¹¹C]Ro15-4513 has been introduced as a positron emission tomography radioligand to image the GABA_Aα5 receptor subtype thought to be important in learning, memory and addiction. However, the in vivo selectivity of the ligand remains unknown and a full assessment of different analysis approaches has yet to be performed. Using human heterologous competition data, with [¹¹C]Ro15-4513 and the highly selective GABA_Aα5 selective negative allosteric modulator Basmisanil (RG1662), we quantify the GABA_Aα5 selectivity of [¹¹C]Ro15-4513, assess the validity of reference tissues and evaluate the performance of four different kinetic analysis methods. The results show that [¹¹C]Ro15-4513 has high but not complete selectivity for GABA_Aα5, with α5 representing around 60-70% of the specific binding in α5 rich regions. Competition data indicate that the cerebellum and pons are essentially devoid of α5 signal and might be used as reference regions under certain conditions. Off-target non-selective binding to other GABA_A subtypes means that the choice of analysis method and the interpretation of outcome measures must be considered carefully. We discuss the merits of two tissue compartmental model analyses to derive both V_T and V_S, band-pass spectral analysis for estimation of [Formula: see text] and the simplified reference tissue model for estimation of [Formula: see text].

Absolute Cerebral Blood Flow Infarction Threshold for 3-Hour Ischemia Time Determined with CT Perfusion and 18F-FFMZ-PET Imaging in a Porcine Model of Cerebral Ischemia

CT Perfusion (CTP) derived cerebral blood flow (CBF) thresholds have been proposed as the optimal parameter for distinguishing the infarct core prior to reperfusion. Previous threshold-derivation studies have been limited by uncertainties introduced by infarct expansion between the acute phase of stroke and follow-up imaging, or DWI lesion reversibility. In this study a model is proposed for determining infarction CBF thresholds at 3hr ischemia time by comparing contemporaneously acquired CTP derived CBF maps to ¹⁸F-FFMZ-PET imaging, with the objective of deriving a CBF threshold for infarction after 3 hours of ischemia. Endothelin-1 (ET-1) was injected into the brain of Duroc-Cross pigs (n = 11) through a burr hole in the skull. CTP images were acquired 10 and 30 minutes post ET-1 injection and then every 30 minutes for 150 minutes. 370 MBq of ¹⁸F-FFMZ was injected ~120 minutes post ET-1 injection and PET images were acquired for 25 minutes starting ~155–180 minutes post ET-1 injection. CBF maps from each CTP acquisition were co-registered and converted into a median CBF map. The median CBF map was co-registered to blood volume maps for vessel exclusion, an average CT image for grey/white matter segmentation, and ¹⁸F-FFMZ-PET images for infarct delineation. Logistic regression and ROC analysis were performed on infarcted and non-infarcted pixel CBF values for each animal that developed infarct. Six of the eleven animals developed infarction. The mean CBF value corresponding to the optimal operating point of the ROC curves for the 6 animals was 12.6 ± 2.8 mL·min^-1·100g^-1 for infarction after 3 hours of ischemia. The porcine ET-1 model of cerebral ischemia is easier to implement then other large animal models of stroke, and performs similarly as long as CBF is monitored using CTP to prevent reperfusion.

Reflections on more than 30 years association with Hanns

I first met Hanns in 1977 and soon learnt of his extraordinary ability as a researcher. He became a friend as well as a mentor providing enthusiasm for my own research. I watched closely over the years how his research uncovered details of the association of the benzodiazepines and GABA and delineated the structural composition of the GABAA receptor associated with the action of individual drugs such as antianxiety and antiepileptic agents. His work produced many important contributions to medicine notable of which was the discovery of the first benzodiazepine antagonists, which are now routinely used in clinical practice. But for me his most important contribution was the discovery of the benzodiazepine receptor. During this time, my group uncovered a novel receptor for GABA and my progress in this work was encouraged and enhanced by discussions with Hanns.

Long-term consequences of neonatal exposure to diazepam on cerebral glucose utilization, learning, memory and anxiety

The long-term consequences of neonatal exposure to diazepam (DZP) on behavioral abilities and local cerebral glucose utilization (LCGU) in 12 brain regions involved in the control of memory and anxiety were studied in adult rats. Rat pups received a daily subcutaneous injection of 10 mg/kg DZP or of the dissolution vehicle from postnatal day (P) 2 to 21. Learning and memory were tested in P60-P70 rats over 5 consecutive days in a T maze and an eight-arm maze while anxiety and reaction to novelty were tested in a two-compartment box with a two-step staircase on the enriched side. LCGU was measured in the P60 rat by the quantitative autoradiographic [14C]deoxyglucose method. In the T maze, when performed without delay between the two trials, the rate of alternation was significantly lower in DZP- than in vehicle-exposed rats on the first 2 days of testing and similar in both groups on days 3-5. In the procedure with a 30 s intertrial delay, the rate of alternation was similar in DZP- and vehicle-treated rats on all days of testing. In the eight-arm maze, DZP-treated rats were more active, i.e., entered more arms per minute than control animals. The number of arms entered before the first error was lower on day 1 and higher on day 3 in DZP- compared to vehicle-exposed rats. In the two-compartment box, DZP-treated rats crossed more often and spent more time than controls on the lower step of the staircase while control rats made more rearings and spent more time than DZP-exposed rats in the well protected corner of the box. LCGU were decreased by early DZP exposure in six regions which were mammillary body, septum, visual and prefrontal cortices, dorsomedian caudate nucleus and mediodorsal thalamus. In conclusion, postnatal DZP treatment induced at adulthood an increase in activity, a delay in task acquisition but no learning-memory impairment and reduced the level of anxiety allowing active responding to novelty. These quite subtle behavioral changes were accompanied by discrete metabolic decreases in regions mediating anxiety, reflecting a change in the level of anxiety and emotionality.

Immunocytochemical localization of GABAA receptor beta 2/beta 3-subunits in the brain of Atlantic salmon (Salmo salar L)

In order to obtain a basis for future investigations concerning the possible interactions between melatonin, GABA and benzodiazepines in the central nervous system of a teleost fish, the Atlantic salmon, we have studied the expression of immunoreactivity with a monoclonal antibody against the GABAA-receptor beta 2/beta 3-subunits (bd-17) in the salmon brain. Immunoreactivity was found in all parts of the brain, mostly as a diffuse labelling of discrete neuropil areas but in some instances as a granular perikaryal labelling. Strong neuropil labelling is located in the telencephalon, dorsal thalamus/pretectum, optic tectum, torus semicircularis, and ventrolateral tegmentum. Perikaryal labelling was observed in the stratum periventriculare of the optic tectum, torus longitudinalis, torus semicircularis, ventrolateral tegmentum, and in the granular layer of the cerebellum. The general pattern of distribution is similar to that observed in mammals, in which high receptor densities are found in the telencephalon (cerebral cortex), superior and inferior colliculi, and cerebellum. There is a good correlation with the distribution of melatonin binding sites, observed in a previous study, in areas receiving visual input such as the optic tectum, pretectum, and torus semicircularis. Moreover, a correlation was found in the inferior lobes and regions connected with them. Regions containing both bd-17-immunoreactivity and melatonin binding sites may constitute areas of functional interaction between melatonin, GABA and benzodiazepines in the central nervous system.

Immunocytochemical localization of the GABAA/benzodiazepine receptor beta2/beta3 subunits in the optic tectum of the salmon

The optic tectum of the salmon is a primary visual center with direct input from the retina via the optic tract. The structure is homologous with the superior colliculus of the mammalian brain. We have studied the distribution of immunoreactivity against the GABAA/benzodiazepine receptor beta2/beta3 subunits with a monoclonal antibody (BD-17) in the optic tectum of the salmon brain. A weak immunoreactivity is found in the rostral stratum marginale (SM), strong labelling of the neuropil is shown in a thin band in stratum opticum (SO), two bands in stratum fibrosum et griseum superficiale (SFGS) and two bands in stratum griseum centrale (SGC). Immunoreactive perikarya with neurites that extend radially through the stratum album centrale (SAC) are located in the stratum periventriculare. BD-17 immunoreactivity is to a great extent located in tectal layers that receive direct retinal input, i.e. the SO, SFGS and SGC. These layers are known to receive input also from other visual centers, such as the pretectum (SO, SFGS), the nucleus isthmi (SO, SFGS, SGC), as well as non-visual regions as the telencephalon (SGC). High levels of 2-[125I]-iodomelatonin binding sites have previously been demonstrated in all layers of the salmon optic tectum except the SM and SPV. Thus it appears likely that GABA and/or benzodiazepines and melatonin play a role in visual processing in the optic tectum of teleost fish.

Short- and long-term effects of neonatal diazepam exposure on local cerebral glucose utilization in the rat

The short- and long-term consequences of a neonatal exposure to diazepam (DZP) on the postnatal changes in local cerebral metabolic rates for glucose (LCMRglcs) were studied by the quantitative autoradiographic [14C]2-deoxyglucose method in a total number of 66 brain structures of freely moving rats. Rat pups received a daily subcutaneous injection of 10 mg/kg DZP, of the dissolution vehicle or of saline from postnatal day 2 (P2) to 21 (P21). The animals were studied at 4 ages, P10, P14, P21 and P60. DZP induced a decrease in LCMRglcs which was restricted to 13 areas at P10, mainly sensory and limbic regions. At P14, the treatment had significant metabolic effects on 48 structures belonging to all functional systems. By P21, 23 brain areas were still affected by the treatment, mainly sensory, limbic and motor areas. At P60, i.e. at about 40 days after the end of drug exposure, LCMRglcs still decreased in 14 brain regions which were mainly sensory and limbic structures. The structures most sensitive to both short- and long-term consequences of the anticonvulsant treatment are mammillary body, limbic cortices and sensory regions. The dissolution vehicle increased LCMRglcs in a few brain regions at P14 and P60, whereas it decreased metabolic levels in 5 brain regions at P21. The results of the present study show that the brain appears to be particularly vulnerable to the treatment at P14, period of active brain growth, whereas by P21, the drug is actively metabolized and a tolerance to the treatment may occur. The long-term effects of the treatment are in good accordance with the well-known effects of DZP on anxiety, sedation and memory. The structures most sensitive to early neonatal DZP exposure are the mammillary body, limbic cortices and sensory regions that all contain a high density of benzodiazepine binding sites.

Benzodiazepine receptor ligands and sexual behavior in the male rat: the role of GABAergic mechanisms

Diazepam and chlordiazepoxide produced a dose-dependent inhibition of ambulatory activity, motor execution and sexual behavior. The benzodiazepine antagonist flumazenil had no effect on these behaviors, while the inverse agonist FG 7142 inhibited sexual behavior without affecting motor functions. The GABA antagonist bicuculline was ineffective in all behavioral paradigms, while picrotoxin inhibited all behaviours. Picrotoxin blocked the motor effects of low doses of the benzodiazepines, but not those of higher doses. Neither did this drug block the effects of benzodiazepines on sexual behavior. Bicuculline was unable to block the effects of benzodiazepines on all behaviors. FG 7142, in a low dose, inhibited the effects of diazepam and chlordiazepoxide on ambulatory activity, but not their effects on motor execution or sexual behavior. The effects of the benzodiazepines and picrotoxin on sexual behavior could be a consequence of the motor impairment produced by these drugs, since the doses required to affect these two behaviors were similar. However, the fact that picrotoxin could block the motor deficiencies induced by the benzodiazepines without restoring sexual behavior suggests that these behavioral actions of the drugs can be differentiated. While some evidence was obtained suggesting a role of GABA in the motor effects of benzodiazepines, no evidence could be found for a role of GABA in their effects on sexual behavior.

Transient increase in [3H]Ro 15-4513 specific binding in the superficial gray layer of the rat superior colliculus induced by visual deafferentation

The imidazodiazepine compound [3H]Ro 15-4513, a partial inverse agonist of benzodiazepine receptors of the central type, binds with high affinity (order of 10(-8) M) to a single population of benzodiazepine binding sites in the mammalian central nervous system. A quantitative autoradiographic study was carried out to determine the effects of one eye removal on [3H]Ro 15-4513 specific binding to rat brain sections in the superficial gray layer or stratum griseum superficiale (SGS) of the superior colliculus. Retinal afferent degeneration due to right eye removal, performed 3 and 7 days before sacrifice, led to a significant and symmetrical increase in the [3H]Ro 15-4513 specific binding in both right and left SGS by enhancing the binding affinity of the radioligand. This transient phenomenon disappeared when a longer survival period of 45 days was allowed to elapse. Conversely, unilateral lesion of the primary visual areas had no apparent effects on the specific binding of the radioligand. The absence of any loss of binding sites after either type of lesion suggests that the benzodiazepine receptors are probably not situated on the optic nerve axon terminals, nor on the cortical axon terminals originating from primary visual areas. In the SGS, as in other rat brain structures, benzodiazepine receptors of the central type are functionally coupled with GABAA receptors and form 'GABAA receptors/benzodiazepine receptors/chloride channel' complexes. The involvement of the local GABAergic system in the postlesion plasticity of benzodiazepine receptors was studied by testing the effects of exogenously applied GABA on [3H]Ro 15-4513 specific binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative autoradiographic study of the postnatal development of benzodiazepine binding sites and their coupling to GABA receptors in the rat brain

The postnatal development of benzodiazepine binding sites in the rat brain was studied by quantitative receptor autoradiography using [3H]flunitrazepam. The coupling of these sites to GABA receptors was assessed in 43 cerebral structures by examining the effects of in vitro addition of GABA on flunitrazepam specific binding. Benzodiazepine-specific binding was relatively high at birth and exhibited an heterogeneous distribution pattern, anatomically different from the adult one. Data showed a sequential development of benzodiazepine receptors in relation to the time course of maturation of cerebral structures. A proliferation peak which paralleled rapid brain growth was noticed. High levels of benzodiazepine sites were transiently observed in some areas, e.g. thalamus and hypothalamus, and might be related to maturational events. In every brain structure examined, benzodiazepine binding sites were linked to GABA receptors. However, enhancement of flunitrazepam specific binding by exogenous GABA differed according to the structures studied and decreased during development, suggesting some changes in the control of GABA/benzodiazepine regulation during postnatal maturation.

Distribution of benzodiazepine receptors in the rat superior colliculus: a light and electron microscope quantitative autoradiographic study

The distribution of benzodiazepine (Bdz) receptors of the central type was analysed in the superficial grey layer of the rat superior colliculus from light and electron microscope autoradiographs, using the highly specific partial reverse agonist [3H]Ro 15-4513, a radioligand which can be crosslinked to its binding sites by ultraviolet rays. Biochemical characteristics of the binding were first defined by liquid scintillation count on unfixed cryostat mesencephalic brain slices. Saturation curves (1.6-20 nM) and Scatchard plot indicated that the radioligand bound with a high affinity (Kd = 11 nM) to a single population of sites (Bmax = 650 fmol/mg dry tissue). A slight primary chemical fixation of the brain did not significantly modify the binding characteristics. The consolidation of the specific binding by ultraviolet light on prefixed brain slices was found to be optimal after a 45-min illumination period. The distribution of Bdz sites on light and electron microscope autoradiographs was then analysed by applying these binding conditions. Prefixed brain slices (50 micron thick, Vibratome) were incubated in the 15 nM radioligand in the absence (total binding) or in the presence (non-specific binding) of the non-radioactive antagonist Ro 15-1788 (10(-5) M). Quantitative light microscopic study of Epon-embedded semithin sections showed that 95% of the silver grains of the specific label were located on the neuropil to the detriment of the neuronal and glial cell compartments. In the electron microscopic study, the distribution of the specific binding sites was statistically analysed over a total of more than 10 identified single or junctional tissue compartments, using the 50% probability circle method (Williams, 1969). Apart from a slight labeling of varicose profiles, the specific labeling was found to be concentrated on two particular tissue compartments: the percentage of grains associated with contacts between varicosities and dendrites was 32%, and that associated with axodendritic synapses was 16% of the total specific labeling measured over all compartments combined. A low proportion (33%) of the labeled axodendritic interfaces was characterized by a synaptic differentiation. These results suggest that both synaptic and non-synaptic Bdz receptors are present in the rat superior colliculus, and may each modulate neuronal cell activity in a different way.

Quantitative autoradiography of muscarinic and benzodiazepine receptors in the forebrain of the turtle, Pseudemys scripta

The distribution of muscarinic and benzodiazepine receptors was investigated in the turtle forebrain by the technique of in vitro receptor autoradiography. Muscarinic binding sites were labeled with 1 nM 3H-quinuclidinyl benzilate (3H-QNB), and benzodiazepine sites were demonstrated with the aid of 1 nM 3H-flunitrazepam (3H-FLU). Autoradiograms generated on 3H-Ultrofilm apposed to tissue slices revealed regionally specific distributions of muscarinic and benzodiazepine binding sites that are comparable with those for mammalian brain. Dense benzodiazepine binding was found in the anterior olfactory nucleus, the lateral and dorsal cortices, and the dorsal ventricular ridge (DVR), a structure with no clear mammalian homologue. Muscarinic binding sites were most dense in the striatum, accumbens, DVR, lateral geniculate, and the anterior olfactory nucleus. Cortical binding sites were studied in greater detail by quantitative analysis of autoradiograms generated by using emulsion-coated coverslips. Laminar gradients of binding were observed that were specific for each radioligand; 3H-QNB sites were most dense in the inner molecular layer in all cortical regions, whereas 3H-FLU binding was generally most concentrated in the outer molecular layer and was least dense through all layers in the dorsomedial cortex. Because pyramidal cells are arranged in register in turtle cortex, the laminar patterns of receptor binding may reflect different receptor density gradients along pyramidal cell dendrites.

Autoradiographic localization of benzodiazepine receptor binding in dissociated cultures of fetal mouse cerebral cortex

Autoradiography utilizing photoaffinity labelling with [3H]flunitrazepam was used in living cultures of fetal mouse cerebral cortex in situ to localize benzodiazepine receptor binding sites. There was a predominant localization of silver grains over neurons; however, substantial labelling also occurred over nonneuronal background cells. Clonazepam (0.1 microM) and Ro 5-4864 (0.1 microM) displaced substantial numbers of silver grains over neurons and background cells, respectively. In addition, clonazepam displaced 58-68% of specific grains over background cells and Ro 5-4864 displaced 30% of grains over neurons, suggesting that multiple cell types in the CNS may participate in the neuropharmacologic actions of the benzodiazepines.