Autoradiographic analysis of the distribution of vasoactive intestinal peptide binding sites in the vertebrate central nervous system: a phylogenetic study

The distribution of vasoactive intestinal peptide (VIP) binding sites in the brain of several vertebrate species was examined by in vitro autoradiography on slide-mounted sections. This study included fish, frog, snake, pigeon, rat, mouse, guinea pig, cat and monkey brain. A fully characterized, monoiodinated form of vasoactive intestinal peptide (M-¹²⁵I-VIP), which maintains the biological activity of the native peptide in the central nervous system (CNS), was used throughout the study. Among the lower vertebrate species, no significant specific binding was found in the fish brain, whereas in the frog and snake brain, specific VIP binding sites were observed, mainly in the telencephalon. In the pigeon brain, high densities of VIP binding sites were localized in the hyperstriatum, neostriatum, archistriatum, hippocampal area, dorsolateral cortical area and in the optic tectum. Ectostriatum and paleostriatum augmentatum displayed lower densities of specific binding. In mammals, the highest concentrations of VIP binding sites were observed in the rodent brain. In the rat, mouse and guinea pig brain, high densities were detected in the olfactory bulb, external layers of the cerebral cortex, dentate gyrus, midline thalamic nuclei, geniculate nuclei, some hypothalamic nuclei, superior colliculus coeruleus. Intermediate densities were found in amygdala, caudate-putamen, septum and nucleus accumbens, CA1–CA3 fields of the hippocampus and central gray. The cerebellum of these species presented high densities of VIP binding sites, with species to species differences in their localization. The non-specific binding was, however, increased in the rodent cerebellum. Lower densities of VIP binding sites were observed in the cat and monkey CNS. In these two species, the non-specific binding was considerably higher than in the lower mammals brain. In the cat and monkey brain, as in the lower mammals, the highest densities were revealed in the neocortex, dentate gyrus, thalamic nuclei and some midbrain structures including substantia nigra and locus coeruleus. In all the species studied, the white matter was never labeled with M-¹²⁵I-VIP. This study suggests that VIP binding sites appear relatively early in the evolution of the vertebrate CNS. The most important densities of specific VIP binding sites are observed in the pigeon and rodent brain, whereas the cat and monkey present a marked increase in non-specific binding. It is interesting to note that the distribution of VIP binding sites as revealed by autoradiography is quite conservative in terms of evolution and indicates an association, although non-exclusive, of VIP receptors with brain regions involved in the processing of specific sensory inputs.     

Distribution of VIP mRNA and two distinct VIP binding sites in the developing rat brain: Relation to ontogenic events

The peptide neurotransmitter vasoactive intestinal peptide (VIP) has neurotrophic properties and influences neurobehavioral development. To assess the role of VIP during neural ontogeny, the present work traces the development of VIP mRNA with in situ hybridization and VIP receptors with in vitro autoradiography in rat central nervous system (CNS) from embryonic day 14 (E14) to the adult. VIP mRNA was not evident in the CNS until birth. Postnatally, it was expressed in several distinct brain regions, but its distribution bore little relation to that of VIP receptors. VIP receptors were present and expressed changing patterns of distribution throughout CNS development. The changing patterns were the result of (1) the transient appearance of GTP-insensitive VIP receptors in several regions undergoing mitosis or glial fasciculation and (2) the transient appearance of GTP-sensitive VIP receptors homogeneously distributed throughout the CNS during the first 2 postnatal weeks, the period of the brain growth spurt. At E14-16 VIP binding was dense throughout the brainstem and spinal cord, but limited in the rest of the brain. From E19 to postnatal day 14 (P14), while VIP binding was higher in germinal zones, it tended to be uniformly dense throughout the remainder of the brain. By P21 the adult pattern began to emerge; VIP binding was unevenly distributed and was related to specific cytoarchitectural sites. Since the expression of VIP in the CNS is limited to postnatal development but VIP receptors are abundant prenatally, we suggest that extraembryonic VIP may act upon prenatal VIP receptors to regulate ontogenic events in the brain. © 1994 Wiley-Liss, Inc.     

Binding sites for pituitary adenylate cyclase activating polypeptide (PACAP): comparison with vasoactive intestinal polypeptide (VIP) binding site localization in rat brain sections.

Pituitary adenylate cyclase activating polypeptide (PACAP) is structurally similar to vasoactive intestinal polypeptide (VIP). We investigated the characteristics and topographical distribution of [125I]PACAP binding sites compared with those of [125I]VIP binding sites in the rat brain. Radiolabeled PACAP and VIP showed highly specific binding to sections at the level of the dorsal hippocampus. The specific binding of [125I]PACAP was 10 times higher than that of [125I]VIP in hippocampal sections. [125I]PACAP binding was scarcely displaced by unlabeled VIP, while [125I]VIP binding was effectively displaced by unlabeled PACAP. Therefore, PACAP binding sites may reflect both PACAP specific binding sites and VIP/PACAP binding sites. However, the amount of VIP/PACAP binding sites was negligibly low. Autoradiography revealed that [125I]PACAP binding sites were dense in the piriform cortex, diagonal band, accumbens nucleus, anterior part of the striatum, hippocampal formation, habenular nucleus, lateral hypothalamic area, superior colliculus and dorsal raphe nucleus. Moderate to high labeling was observed in the medial septal nucleus, olfactory tubercle, caudal part of the striatum, most parts of the thalamus, supraoptic and periventricular hypothalamic nuclei, central gray, substantia nigra pars compacta, locus coeruleus, pontine reticular nucleus and cerebellum. Distribution pattern was remarkably different from that of [125I]VIP binding sites in the hippocampal formation, lateral hypothalamic area, substantia nigra pars compacta, pontine reticular nucleus and cerebellum. The present results suggest that PACAP may have a physiological role in the regulation of the central nervous system.     

Major differences in CNS sulfonylurea receptor distribution between the rat (newborn, adult) and turtle.

Our previous results have shown that KATP channels play an important role in K+ efflux and extracellular K+ accumulation in the rat brain, and this role was quantitatively more important in the adult than in the newborn brain. The purpose of this study was to localize by autoradiographic techniques the binding sites of glibenclamide, a potent sulfonylurea ligand that targets KATP channels, in the adult and newborn rat central nervous system (CNS). Since the adult turtle is resistant to anoxia, we also compared the rat to the turtle brain sulfonylurea receptor distribution. In all three animal groups (newborn rat, adult rat, adult turtle), specific glibenclamide binding was saturable. Scatchard plots were curvilinear in the rat, thus suggesting that glibenclamide binds to two types of sites, i.e., high and low affinity sites. Scatchard analysis on turtle brain tissue showed evidence of one binding site only. We also found that the distribution of glibenclamide binding sites was heterogeneous in the adult rat CNS with a higher density in rostral than in caudal regions. The highest binding densities were seen in the cortex, hippocampus, cerebellum, substantia nigra, and a few thalamic nuclei; intermediate densities were observed in the basal ganglia, septum, thalamus, and the hypoglossal nucleus. There was a low density in most areas of the hypothalamus, midbrain, brainstem, and spinal cord. Compared with the adult rat, the newborn had a very homogeneous distribution of binding sites and densities were very low throughout the CNS; the level of binding density was even lower in some regions undetectable in the adult turtle. Our results indicate that (1) there are high and low affinity sulfonylurea receptors in the rat CNS, (2) there is a striking heterogeneity in the distribution and density of sulfonylurea receptors in the adult rat CNS and this is in sharp contrast to the homogeneous distribution and low density in both newborn rat and adult turtle; (3) sulfonylurea receptors increase in number postnatally in the rat since binding density increases and the Kd in the newborn rat is similar to that in the adult rat. We speculate that KATP channels and sulfonylurea receptors are poorly developed in the turtle and develop mostly after birth in the rat, reaching highest density in adulthood.     

Corticotropin-releasing factor receptors in the rat central nervous system: characterization and regional distribution

A stable, iodine-125-labeled analog of rat/human corticotropin-releasing factor (CRF) was used to define the characteristics of CRF receptors in a crude mitochondrial/synaptosomal membrane preparation of rat olfactory bulb, and to study the distribution of CRF binding sites in discrete regions of the rat CNS. The binding of 125I-Tyro rat/human CRF (125I-rCRF) was time- and temperature-dependent, was sensitive to the pH, ionic strength, and cationic composition of the incubation buffer, and was linear over a broad range of membrane protein concentrations. 125I-rCRF binding to olfactory bulb membrane was saturable, reversible, and, on Scatchard analysis, revealed a high-affinity component with an apparent equilibrium dissociation constant (Kd) of 0.2 nM and a low-affinity binding site with Kd of approximately 20 nM. Data from pharmacological studies indicated that the ability of a variety of CRF fragments and analogs to inhibit 125I-rCRF to olfactory bulb membranes correlates well with their reported relative potencies in stimulating pituitary adrenocorticotropic hormone secretion in vitro. Consistent with a coupling of CRF receptors to adenylate cyclase, the binding of 125I-rCRF was decreased by guanine nucleotides and increased by magnesium ions. A heterogeneous distribution of 125I-rCRF binding sites was found in the rat CNS, with highest densities present in olfactory bulb, cerebellum, cerebral cortex and striatum, and progressively lower but significant levels of binding were detected in cervical spinal cord, hypothalamus, medulla, midbrain, thalamus, pons, and hippocampus. These data, using a rat CRF ligand homologous to the endogenous peptide, are consistent with those from previous studies demonstrating the presence of specific binding sites for ovine CRF in rat brain, and provide further support for the suggestion that endogenous CRF may function as a neurotransmitter in the CNS.     

Ontogenesis of adenosine receptors in the central nervous system of the rat

The ontogeny of adenosine receptors was studied in rat brain and spinal cord using the specific ligand [3H]cyclohexyladenosine [( 3H]CHA). The [3H]CHA affinity constant (Kd) and the maximum receptor binding capacity (Bmax) were analyzed at all ages and in all CNS regions studied. Throughout development the Kd of [3H]CHA binding remained relatively stable and for cortex, cerebellum, subcortex, midbrain, brainstem and spinal cord ranged from 2.2 +/- 0.2 to 5.5 +/- 0.6 nM (mean +/- S.E.M.). In contrast, the Bmax values from 1- and 90-day animals increased by as little as 2-fold in subcortical regions and by as much as 9- and 16-fold in cortex and cerebellum, respectively. The highest density of binding sites was observed in subcortical structures and the lowest in brainstem and midbrain. In cortex, a steady increase in receptor number began at day 1 and stopped at the adult level by 21 days. In cerebellum, maximum receptor proliferation began at about 14 days and continued to adulthood. Other CNS regions showed intermediate rates of receptor development. These differences may reflect both the pattern of postnatal neurogenesis in the rat CNS and the maturation of those neural elements containing adenosine receptors.     

Localization of Insulin-Like Growth Factor-II Receptors in Rat Brain by in vitro Autoradiography and Immunohistochemistry

Insulin-like growth factor-ll (IGF-II) and its receptor, which is homologous with the mannose-6-phosphate (M6P) receptor, are found in high levels in adult rat and human brain, though their role remains unclear. In order to point to possible regional functions, we have mapped and quantified IGF-II/M6P receptors in sagittal sections of adult rat brain by in vitro autoradiography/computerized densitometry and immunohistochemistry. While in vitro autoradiography allowed mapping and quantitation, immunohistochemistry both confirmed mapping and allowed more detailed determination of cellular distribution of receptors. The two methods were generally in agreement with few areas of mismatching. By in vitro autoradiography, a discrete and characteristic distribution of IGF-II receptor binding was demonstrated, with specific binding representing 85% of total binding. Displacement and specificity competition curves in arcuate nucleus and choroid plexus were typical for authentic IGF-II receptors with half maximal displacement at 1 nM cold IGF-II. IGF-II receptor density, estimated by in vitro autoradiography, was very high in circumventricular organs, especially the median eminence, which had the highest binding in the brain. In the remainder of the brain there was concordance between the distribution of receptors identified by the two techniques, with greatest densities in the olfactory bulb and olfactory pathways, the hippocampus and discrete regions of the cerebral cortex, cerebellum, hypothalamus, thalamus and brainstem. There were however, some notable mismatches. Autoradiographic binding was high to very high in the median eminence, arcuate nucleus, suprachiasmatic nucleus and anterodorsal thalamic nucleus, whereas these areas were only poorly immunostained. Conversely, the septum showed moderate autoradiographic binding, but very prominent immunostaining of neurons in its dorsolat-eral aspect. Using the immunohistochemical technique IGF-II receptors were localized to specific neuronal groups such as the mitral cells of the olfactory bulb, Purkinje cells of the cerebellum and neurons in the red nucleus. Fibre pathways were not labelled by either technique. We conclude that IGF-II/M6P receptors are widespread throughout rat brain, specifically in neurons and blood vessels, with a similar, but distinct distribution to IGF-I and insulin receptors. Many of these regions have in common high rates of metabolic and synthetic activity, which may be mediated by IGF-II/M6P and their receptors.     

Distribution of estrogen receptor alpha and beta in the mouse central nervous system: in vivo autoradiographic and immunocytochemical analyses

Although the distribution of estrogen receptor beta (ERbeta) immunoreactivity in the rat central nervous has been reported, no such data are available in the mouse. The present study used in vivo autoradiography utilizing a (125)I-estrogen that has equal binding affinity for both receptors as well as immunohistochemistry for ERbeta and ERalpha, to investigate and compare the distribution of the two ERs in the mouse CNS. The use specific antisera against ERalpha and ERbeta allowed us to evaluate the contribution of these receptors to the binding detected with autoradiography. In addition, data were collected in ovariectomized wildtype and ERalpha KO (knockout) mice to examine developmental regulation of ERbeta expression by ERalpha. These studies revealed that in the mouse CNS, combining immunoreactivity for ERalpha with that for ERbeta accounted for all regions where binding was seen using autoradiography. Therefore, these data strongly suggest that the major contributors of estrogen binding in the mouse CNS are ERalpha and ERbeta. Together, these data provide an anatomical foundation for future studies and advance our understanding of estrogen action in the CNS. Moreover, since the immunocytochemical images were similar in wildtype and ERalpha KO mice, these studies suggest that the lack of ERalpha does not influence the expression of ERbeta in the central nervous system.     

Identification of endothelin receptors in various regions of rat brain

Endothelins (ET) are a family of 21 amino acid peptides which carry a high degree of hemology. ET-1, originally isolated from porcine endothelium in culture, is an unusually potent vasoconstrictor. Recent studies indicated the presence of ET-1 and ET-3 in the CNS, and autoradiographic evidence supported the presence of specific ET-1 binding sites. The present study aimed to raise evidence in support for the presence of ET-1 and ET-3 receptors in the rat CNS, to establish their spatial distribution, and gain insight on the molecular species which could be associated to ET-1 and ET-3 binding. The data support the presence of specific binding sites for ET-1 and ET-3 and a heterogeneous distribution of these sites in the CNS. The highest densities were found in the cerebellum (greater than 2200 fmol-mg protein) and the lowest in the cerebral cortex (500 fmol/mg protein). Our data also suggest that ET-1 and ET-3 labelled two different proteins of 45 Kd and 37 Kd molecular weight, but no evidence for separate ET-1 and ET-3 receptors could be noted. These data provide further evidence in support for the possible role of ET-1 and ET-3 in neuromodulation/neurotransmission in the CNS.     

Autoradiographic localization of thyrotropin-releasing hormone receptors in the rat central nervous system

We employed quantitative autoradiography to examine the distribution of thyrotropin-releasing hormone (TRH) receptors in the rat CNS. The binding of [3H]3-methyl-histidine-TRH [( 3H]MeTRH) to TRH receptors in frozen rat brain sections was saturable, of a high affinity (Kd = 5 nM), and specific for TRH analogs. Autoradiograms of [3H]MeTRH binding showed highest concentrations of TRH receptors in the rhinencephalon, including accessory olfactory bulb, nuclei of the amygdala, and the ventral dentate gyrus and subiculum of the hippocampus. Moderate TRH receptor concentrations were found within the thalamus and hypothalamus, in most regions of the rhombencephalon, such as the cranial nerve nuclei, and in the substantia gelatinosa of the spinal cord. Neocortex and basal ganglia contained low densities of TRH receptors. This distribution correlates well with the sensitivity of brain regions to the known effects of TRH, and suggests that TRH receptors may mediate the actions of TRH in the rat CNS.     

Distinct binding sites for calcitonin gene-related peptide and salmon calcitonin in rat central nervous system

Calcitonin gene-related peptide (CGRP) binding sites have been identified in homogenates from the rat brain and spinal cord. Autoradiography with [¹²⁵I]rat CGRP (rCGRP) revealed high grain density over the lateral hypothalamus, vestibular nuclei, colliculi, medial geniculate body, corpus mamillare and the molecular layer of the cerebellum which lacked binding sites for [¹²⁵I]salmon calcitonin (sCT). In contrast, no rCGRP labeling was seen over the anterior and dorsomedial hypothalamus which showed high sCT binding. The different regional distribution of rCGRP and sCT binding sites indicates that the structurally related peptides interact with separate receptors. The overlap between the localization of CGRP binding sites and endogenous CGRP in many regions of the central nervous system suggests that CGRP exerts unique physiological functions in the central nervous system.     

Binding properties of [3H]gacyclidine (cis(pip/me)-1-[1-(2-thienyl)-2-methylcyclohexyl]piperidine) enantiomers in the rat central nervous system

Gacyclidine (cis(pip/me)-1-[1-(2-thienyl)-2-methylcyclohexyl]piperidine) is a TCP derivative, which exhibits potent neuroprotective properties against glutamate-induced neurotoxicity in vitro and in vivo. In order to better understand gacyclidine pharmacological properties, the binding parameters of its enantiomers ((-) and (+)[3H]GK11) were determined in the rat central nervous system (CNS). An autoradiographic study has shown that their binding distributions are correlated with those of N-methyl-D-aspartate (NMDA) receptors throughout the CNS. Globally, the labeling was the highest with (-)[3H]GK11. In the cerebellum, both radioligands similarly labeled the molecular layer. For both radioligands, on telencephalic, cerebellum and spinal cord homogenates, the association and dissociation kinetics were accounted for by multiphasic process. In all regions, (-)[3H]GK11 exhibited the highest affinity in the nanomolar range. The pharmacological study revealed that both enantiomers labeled both high and low affinity sites in all regions. The pharmacological profile of high affinity sites was correlated with those of NMDA receptors. Those of low affinity sites were different in telencephalic and cerebellar homogenates. Overall, this study showed that low affinity sites might constitute a heterogeneous population, which could include sigma receptors in the cerebellum. The autoradiographic study has shown that these sites may be located in the molecular layer. The contribution of low affinity sites to the neuroprotective properties of gacyclidine remains to be investigated.     

Autoradiographic localization and characterization of atrial natriuretic peptide binding sites in the rat central nervous system and adrenal gland

Atrial natriuretic peptides (ANP) have recently been identified in both heart and CNS. These peptides possess potent natriuretic, diuretic, and vasorelaxant activities, and are all apparently derived from a single prohormone. Specific ANP binding sites have been characterized in the adrenal zona glomerulosa and kidney cortex, and one study reported ANP binding sites in the CNS. However, a detailed examination of the localization of ANP binding sites throughout the brain has not been reported. In this study, quantitative autoradiography was employed to examine the distribution of ANP receptors in the rat CNS. The binding of (3-125I-iodotyrosyl28) rat ANP-28 to binding sites in the rat CNS was saturable, specific for ANP-related peptides, and displayed high affinity (Kd = 600 pM). When the relative concentrations of ANP binding sites were determined throughout the rat brain, the highest levels of ANP binding were localized to the circumventricular organs, including the area postrema and subfornical organ, and the olfactory apparatus. Moderate levels of ANP binding sites were present throughout the midbrain and brain stem, while low levels were found in the forebrain, diencephalon, basal ganglia, cortex, and cerebellum. The presence of ANP binding sites in the subfornical organ and the area postrema, regions considered to be outside the blood-brain barrier, suggests that peripheral ANP levels may regulate some aspects of CNS control of salt and water balance. The possible functions of ANP binding sites in other regions of the rat brain are not known, but, like many other peptides, ANP may act as a neurotransmitter or neuromodulator at these loci.     

Quantitative autoradiographic analysis of the distribution of [3H]muscimol binding to GABA receptors in chick brain

Quantitative receptor autoradiography was used to investigate the distribution of high-affinity GABA receptors (GABAA) in left and right hemispheres of the brains of 3-week-old chicks. The receptors were labelled with the potent GABA agonist [3H]muscimol. High levels of [3H]muscimol labelling were found throughout the fore-, mid-, and hindbrain, though considerable variation was found in different regions. In the telencephalon the highest concentration of specific binding was found in the hyperstriatum ventrale followed by the neostriatum, and then the lobus parolfactorius of the paleostriatal complex, whilst in the diencephalon highest levels of labelling were present in the infundibulum. In the midbrain distinct lamination was observed in the high levels of [3H]muscimol binding in the optic tectum and in the hind brain the highest density of labelling occurred in the granular layers of the cerebellum. Levels of labelling were generally low in the brainstem regions. The distribution of [3H]muscimol binding in the optic tectum and in the hind brain the highest density of labelling occurred in the granular layers of the cerebellum. Levels of labelling were generally low in the brainstem regions. The distribution of [3H]muscimol binding sites is in good agreement with our previous work on the distribution of GABA-immunoreactivity in the chick brain.     

3H]Resiniferatoxin autoradiography in the CNS of wild-type and TRPV1 null mice defines TRPV1 (VR-1) protein distribution

Knowledge of the distribution and function of the vanilloid receptor (VR-1 or TRPV1) in the CNS lacks the detailed appreciation of its role in the peripheral nervous system. The radiolabelled vanilloid agonist [3H]resiniferatoxin (RTX) has been used to indicate the presence of TRPV1 receptor protein in the brain but low specific binding has complicated interpretation of this data. Recently, support for a more widespread CNS distribution of TRPV1 mRNA and protein has been provided by RT-PCR and antibody data. We have exploited the availability of TRPV1 null mice and used [3H]RTX autoradiography in the CNS of TRPV1 wild-type and TRPV1 null mice to identify the component of [3H]RTX binding to TRPV1 receptor protein. In the brains of TRPV1+/+ mice, specific [3H]RTX binding was broadly localised with the greatest binding in the olfactory nuclei, the cerebral cortex, dentate gyrus, thalamus, hypothalamus, periaqueductal grey, superior colliculus, locus coeruleus and cerebellar cortex. Specific binding was also seen in the spinal cord and sensory (dorsal root and trigeminal) ganglia. This binding was much lower but not abolished in most regions in the TRPV1-/- mice. Nonspecific binding was low in all cases. The present study unequivocally demonstrates a widespread and discrete distribution pattern of the TRPV1 receptor protein in the rat central nervous system. The presence of TRPV1 receptors in several brain regions suggests that it may function as a cannabinoid-gated channel in the CNS.     

Pharmacological evaluation of a Br-76 analog of epibatidine: a potent ligand for studying brain nicotinic acetylcholine receptors

[(76)Br]-Norchlorobromoepibatidine ([(76)Br]BrPH) is a specific and high affinity radioligand for the nicotinic acetylcholine receptors (nAChRs). In vitro, on rat thalamus membranes [(76)Br]BrPH bound to two sites with apparent affinities of 8 pM and 3 nM. The density of binding sites were 1.9 and 70 fmol/mg protein, respectively. In vivo, in biodistribution and autoradiographic studies in rats the regional distribution of [(76)Br]BrPH paralleled the neuroanatomical localization of nAChRs. Two hours postinjection, the highest concentration in the brain was found in thalamus and colliculi (4% ID/g). Competition experiments with specific nicotinic, muscarinic, dopaminergic, and serotoninergic drugs confirmed that the in vivo binding of [(76)Br]BrPH was consistent with neuronal nicotinic receptors. PET imaging of [(76)Br]BrPH in baboon demonstrated a rapid and high uptake in the brain. Peak uptake occurred at 30-40 min for the thalamus. Due to the constant washout in the cerebellum, the thalamus to cerebellum ratio was 5 at 2 h postinjection. Subcutaneous injection of cytisine (1 mg/kg), 3 h postinjection of [(76)Br]BrPH reduced the radioactivity concentration in thalamus and cortex by 58 and 50%, respectively, as observed 1 h later. Cytisine pretreatment (5 mg/kg s.c.) inhibited completely the radioligand accumulation in the thalamus. Chronic MPTP pretreatment resulted in reduction of [(76)Br]BrPH uptake in all brain regions except in cerebellum. These preliminary results suggest that [(76)Br]BrPH has the potential to be a useful radioligand for studying the pharmacology of nicotinic acetylcholine receptors in preclinical experiments.     

Autoradiographic localization of alpha5 subunit-containing GABAA receptors in rat brain

Multiple subtypes of GABAA receptors are expressed in the rat central nervous system (CNS). To determine the distribution and proportion of alpha5 subunit containing receptors, quantitative autoradiographic analyses were performed with both [3H]L-655,708 and [3H]Ro15-1788, an alpha5 selective and a non selective benzodiazepine binding site ligand, respectively. High densities of [3H]L-655,708 binding sites were observed in hippocampus and olfactory bulb, where alpha5 receptors accounted for 20-35% of total [3H]Ro15-1788 binding sites. Low levels of [3H]L-655,708 sites were associated with the cortex as well as amygdala, thalamic, hypothalamic and midbrain nuclei. These observations indicate that although [3H]L-655,708 binding sites have an overall low expression in rat CNS, they may contribute significantly to GABAergic inhibition in specific brain regions.     

The Functional Neuroanatomy of Brain Cannabinoid Receptors

The effects of the primary psychoactive constituent of marijuana, Δ⁹-tetrahydrocannabinol, are mediated by cannabinoid receptors, CB1 and CB2. The CB1 receptors display a unique central nervous system (CNS) distribution and are present in mammalian brain at higher levels than most other known G-protein-coupled receptors. The highest levels occur in several areas involved in motor control and hippocampus. Cannabinoid effects on CNS activities, including movement, memory, nociception, endocrine regulation, thermoregulation, sensory perception, cognitive functions, and mood, correlate with the regional distribution of cannabinoid receptors and their activation of specific G-protein-mediated signal transduction systems in various brain regions.