Distribution of monoamines and metabolites in rabbit neostriatum, hippocampus and cortex

The Effects of Acutely Administered 3,4-Methylenedioxymethamphetamine on Spontaneous Brain Function in Healthy Volunteers Measured with Arterial Spin Labeling and Blood Oxygen Level-Dependent Resting State Functional Connectivity

BACKGROUND

The compound 3,4-methylenedioxymethamphetamine (MDMA) is a potent monoamine releaser that produces an acute euphoria in most individuals.

METHODS

In a double-blind, placebo-controlled, balanced-order study, MDMA was orally administered to 25 physically and mentally healthy individuals. Arterial spin labeling and seed-based resting state functional connectivity (RSFC) were used to produce spatial maps displaying changes in cerebral blood flow (CBF) and RSFC after MDMA administration. Participants underwent two arterial spin labeling and two blood oxygen level-dependent scans in a 90-minute scan session; MDMA and placebo study days were separated by 1 week.

RESULTS

Marked increases in positive mood were produced by MDMA. Decreased CBF only was observed after MDMA, and this was localized to the right medial temporal lobe (MTL), thalamus, inferior visual cortex, and the somatosensory cortex. Decreased CBF in the right amygdala and hippocampus correlated with ratings of the intensity of global subjective effects of MDMA. The RSFC results complemented the CBF results, with decreases in RSFC between midline cortical regions, the medial prefrontal cortex, and MTL regions, and increases between the amygdala and hippocampus. There were trend-level correlations between these effects and ratings of intense and positive subjective effects.

CONCLUSIONS

The MTLs appear to be specifically implicated in the mechanism of action of MDMA, but further work is required to elucidate how the drug's characteristic subjective effects arise from its modulation of spontaneous brain activity.

Effects of 3,4-methylenedioxymethamphetamine (MDMA) on serotonin transporter and vesicular monoamine transporter 2 protein and gene expression in rats: implications for MDMA neurotoxicity

3,4-Methylenedioxymethamphetamine (MDMA; 'Ecstasy') is a popular recreational drug used worldwide. This study aimed to determine the effects of this compound on the expression of nerve terminal serotonergic markers in rats. Experiment 1 investigated MDMA-induced changes in levels of the serotonin transporter (SERT) and the vesicular monoamine transporter 2 (VMAT-2) in the hippocampus, a region with sparse dopaminergic innervation, after lesioning noradrenergic input with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4). Adult male Sprague-Dawley rats were administered 100 mg/kg DSP-4 or saline 1 week prior to either an MDMA (10 mg/kg x 4) or saline binge. Two weeks following the binge treatment, the DSP-4/MDMA group unexpectedly showed little change in hippocampal VMAT-2 protein expression compared with DSP-4/Saline controls, despite large reductions in SERT levels in all regions examined in the MDMA-treated animals. Furthermore, animals treated with binge MDMA (Experiment 2) showed a striking decrease in SERT gene expression (and a lesser effect on VMAT-2) measured by quantitative RT-PCR in pooled dorsal and median raphe tissue punches, when compared with saline-treated controls. These results demonstrate that MDMA causes substantial regulatory changes in the expression of serotonergic markers, thus questioning the need to invoke distal axotomy as an explanation of MDMA-related serotonergic deficits.

Drugs, biogenic amine targets and the developing brain

Defects in the development of the brain have a profound impact on mature brain functions and underlying psychopathology. Classical neurotransmitters and neuromodulators, such as dopamine, serotonin, norepinephrine, acetylcholine, glutamate and GABA, have pleiotropic effects during brain development. In other words, these molecules produce multiple diverse effects to serve as regulators of distinct cellular functions at different times in neurodevelopment. These systems are impacted upon by abuse of a variety of illicit drugs, neurotherapeutics and environmental contaminants. In this review, we describe the impact of drugs and chemicals on brain formation and function in animal models and in human populations, highlighting sensitive periods and effects that may not emerge until later in life.

Distribution and morphology of serotonin-immunoreactive axons in the hippocampal region of the New Zealand white rabbit. I. Area dentata and hippocampus

This study provides a detailed light microscopic description of the morphology and distribution of immunohistochemically stained serotonergic axons in the hippocampal region of the New Zealand white rabbit. The serotonergic axons were segregated morphologically into three types: beaded fibers, fine fibers, and stem-axons, respectively. Beaded fibers were thin serotonergic axons with large varicosities, whereas thin axons with small fusiform or granular varicosities were called fine fibers. Finally, thick straight non-varicose axons were called stem-axons. Beaded fibers often formed large conglomerates with numerous boutons (pericellular arrays) in close apposition to the cell-rich layers in the hippocampal region, e.g., the granular and hilar cell layers of the dentate area and the pyramidal cell layer ventrally in CA3. The pericellular arrays in these layers were often encountered in relation to small calbindin-D2BK-positive cells, as shown by immunohistochemical double staining for serotonin and calbindin-D28K. The beaded and fine serotonergic fibers displayed a specific innervation pattern in the hippocampal region and were encountered predominantly within the terminal field of the perforant path, e.g., the stratum moleculare hippocampi and the outer two-thirds of the dentate molecular layer. These fibers were also frequently seen in the deep part of the stratum oriens and the alveus, forming a dense plexus in relation to large multipolar calbindin-D28K-positive cells and their basal extensions. Stem-axons were primarily seen in the fimbria and alveus. This innervation pattern was present throughout the entire hippocampal formation, but there were considerable septotemporal differences in the density of the serotonergic innervation. A high density of innervation prevailed in the ventral/temporal part of the hippocampal formation, whereas the dorsal/septal part received only a moderate to weak serotonergic innervation. These results suggest that the serotonergic system could modulate the internal hippocampal circuitry by way of its innervation in the terminal field of the perforant path, the hilus fasciae dentatae, and ventrally in the zone closely apposed to the mossy fiber layer and the pyramidal cells of CA3. This modulation could be of a dual nature, mediated directly by single serotonergic fibers traversing the hippocampal layers or indirectly by the pericellular arrays and their close relation to the calbindin-D28K-positive cells. The marked septotemporal differences in innervation density point toward a difference between the ventral and dorsal parts of the hippocampal formation with respect to serotonergic function and need for serotonergic modulation.

Prenatal cocaine exposure produces consistent developmental alterations in dopamine-rich regions of the cerebral cortex

Administration of cocaine to pregnant rabbits produces robust and long-lasting anatomical alterations in the dopamine-rich anterior cingulate cortex of offspring. These effects include increased length and decreased bundling of layer III and V pyramidal neuron dendrites, increases in parvalbumin expression in the dendrites of interneurons, and increases in detectable GABAergic neurons. We have now examined multiple cortical regions with varying degrees of catecholaminergic innervation to investigate regional variations in the ability of prenatal cocaine exposure to elicit these permanent changes. All regions containing a high density of tyrosine hydroxylase-immunoreactive fibers, indicative of prominent dopaminergic input, exhibited alterations in GABA and parvalbumin expression by interneurons and microtubule-associated protein-2 labeling of apical dendrites of pyramidal neurons. These regions included the medial prefrontal, entorhinal, and piriform cortices. In contrast, primary somatosensory, auditory and motor cortices exhibited little tyrosine hydroxylase staining and no measurable cocaine-induced changes in cortical structure. From these data we suggest that the presence of dopaminergic afferents contributes to the marked specificity of the altered development of excitatory pyramidal neurons and inhibitory interneurons induced by low dose i.v. administration of cocaine in utero.

Effects of buspirone on brain indoleamines and catecholamines in wild-type mice and Lurcher mutants

The effects of a chronic serotoninergic stimulation on brain monoamine levels and metabolism were studied in wild-type (+/+) mice and Lurcher (Lc/+) mutants. Endogenous serotonin, dopamine, noradrenaline and some of their major metabolites were measured in the frontal cortex, neostriatum, thalamus, brainstem, cerebellum and spinal cord. In +/+ mice, buspirone (1 mg/kg; i.p.) treatment during 40 days increased indoleamines, albeit with moderate changes in the ratios between tissue serotonin metabolites and endogenous serotonin, augmented noradrenaline contents in the spinal cord, and caused elevations of dopamine metabolites in most regions. In Lc/+ mutants, the effects of buspirone were attenuated, but higher L-tryptophan and indoleamine levels, suggest a storage of serotonin in a non-releasable compartment. In the hypoplastic Lc/+ cerebellum, indoleamine content was accrued, but with a decreased [serotonin metabolites]/[serotonin] ratio, indicating that the reorganized nerve terminals in Lc/+ mutants although they can synthesize and accumulate serotonin, may not utilize it efficiently in synaptic transmission.

Monoamine neurotransmitters and their metabolites in the mature rabbit brain following induction of hydrocephalus

Functional and behavioral disturbances associated with hydrocephalus may be due in part to altered neurotransmitter function in the brain. Hydrocephalus was induced in adult rabbits by injection of silicone oil into the cisterna magna. These and controls were killed 3 days, 1 and 4 weeks post-injection. Tissue concentrations of norepinephrine, epinephrine, serotonin, dopamine, and the metabolites 5-hydroxyindoleacetic acid (5-HIAA), homovanillic acid (HVA), and 3,4-dihydroxyphenylacetic acid (DOPAC) levels were determined in fifteen brain regions using HPLC. There were decreases in hypothalamic and medullary dopamine, transient decreases in basal ganglia serotonin, increases in thalamic noradrenaline, and increases in hypothalamic and thalamic epinephrine. Changes in the primary neurotransmitters may be attributable to damage of their axonal projection systems. Metabolite concentrations increased in the cerebrum. Reduced clearance of extracellular fluid which accompanies cerebrospinal fluid stasis may explain the accumulation of metabolites.

Regional distribution of 5-HT transporters in the brain of wild type and 'Purkinje cell degeneration' mutant mice: a quantitative autoradiographic study with [3H]citalopram

The neurological mutant 'Purkinje cell degeneration' (pcd) is characterized by a primary degeneration of Purkinje cells, as well as by retrograde and secondary partial degeneration of cerebellar granule cells and inferior olivary neurons, and can be considered as an animal model of human degenerative ataxias. The serotonin (5-HT) innervation was examined in wild type and pcd mice, by quantifying 5-HT uptake sites, or transporters, using [3H]citalopram binding autoradiography. In both wild type and pcd mutants, the highest densities of 5-HT transporters were in mesencephalic and rostral pontine regions, in limbic structures, in hypothalamus and in discrete thalamic divisions, while the lowest labelling was found in cerebellum and brainstem reticular formation. In pcd mice, although [3H]citalopram labelling was higher in cerebellar cortex and deep cerebellar nuclei, when binding densities were corrected for surface area, the up-regulation of 5-HT transporters was present only in deep cerebellar nuclei. Also, higher labelling was found in nuclei raphe dorsalis and medialis, in ventral divisions of rostral neostriatum, caudal neostriatum, rostral globus pallidus, posteromedial amygdaloid nucleus, septum, olfactory tubercles, vertical limb of Broca's diagonal band, periventricular, latero-ventral and medio-ventral thalamic nuclei, medial geniculate nucleus, anterior hypothalamus and entorhinal cortex. The results indicate a relative integrity of the 5-HT innervation, but with a reorganization of serotoninergic terminals in the cerebellum, in particular in the deep cerebellar nuclei. This suggests that in progressive cerebellar degeneration, as found in the pcd mutant, the modified 5-HT system may still participate in motor functions by exerting an overall modulation of excitatory amino acid neurotransmission, but the availability of 5-HT may be altered in defined brain targets, as is the case for other spontaneous cerebellar mutants, in particular for the 'Lurcher' mutant mouse, a model of human olivopontocerebellar atrophy.

Functional evidence that BDNF is an anterograde neuronal trophic factor in the CNS

In this report, we have tested the hypothesis that brain-derived neurotrophic factor (BDNF) is an anterograde neurotrophic factor in the CNS and have focused on central noradrenergic neurons that synthesize BDNF. Double-label immunocytochemistry for BDNF and dopamine-beta-hydroxylase (DBH), a marker for noradrenergic neurons, demonstrated that BDNF is partially localized to noradrenergic nerve fibers and terminals in the adult rat brain. To test the functional importance of this anterograde BDNF, we analyzed transgenic mice carrying a DBH-BDNF minigene. Increased synthesis of BDNF in noradrenergic neurons of DBH-BDNF mice caused elevated TrkB tyrosine kinase activation throughout postnatal life in the neocortex, a noradrenergic target region. This afferently regulated increase in TrkB receptor activity led to long-lasting alterations in cortical morphology. To determine whether noradrenergic neuron-expressed BDNF also anterogradely regulated neuronal survival, we examined a second noradrenergic target, neonatal facial motoneurons. One week after axotomy, 72% of facial motoneurons were lost in control animals, whereas only 30-35% were lost in DBH-BDNF transgenic mice. Altogether, these results indicate that BDNF is anterogradely transported to fibers and terminals of noradrenergic neurons, that anterogradely secreted BDNF causes activation of TrkB in target regions, and that this secretion has functional consequences for target neuron survival and differentiation. This presynaptic secretion of BDNF may provide a cellular mechanism for modulating neural circuitry, in either the developing or mature nervous system.

Regional distribution of the 5-HT innervation in the brain of normal and lurcher mice as revealed by [3H]citalopram quantitative autoradiography

The neurological cerebellar mutant lurcher is characterized by a primary degeneration of Purkinje cells as well as retrograde secondary partial degeneration of cerebellar granule cells and inferior olivary neurons. Since serotonin (5-HT) has been implicated in the modulation of excitatory amino acid systems of the cerebellum, the 5-HT innervation of the normal and lurcher mice was examined by quantifying uptake sites using [3H]citalopram autoradiography, and by biochemical assays of the indoles 5-HT, 5-hydroxy-L-tryptophan and 5-hydroxyindole-3-acetic acid using high-performance liquid chromatography. Comparable results were found between [3H]citalopram binding and 5-HT tissue concentrations in different brain regions. The highest [3H]citaslopram labelling was observed in defined structures of the mesencephalic and upper pontine regions, in limbic strutures, in hypothalamus and in discrete thalamic divisions, while the lowest labelling of uptake sites was documented in cerebellum and brainstem reticular formation. In lurcher mutants, the histology confirmed cell degeneration and the reduction in width, leading to 65%, 45% and 25% atrophies of total cerebellum, deep nuclei and inferior olivary nucleus, respectively. The [3H]citalopram labelling corrected for surface loss was 45% and 20% higher to cerebellar deep nuclei and red nucleus, respectively, but remained unchanged in the cerebellar cortex and inferior olivary nucleus. Moreover, higher labelling was found in nucleus raphe dorsalis, ventral tegmental area, inferior colliculus, locus coeruleus, pontine central grey and anterior thalamic nuclei, areas known to be part of cerebellar afferent and efferent systems. The present results indicate that in such pathological conditions as described for the lurcher mutant, the 5-HT system may modulate motor function not only at the level of the cerebellum, but also in other forebrain structures functionally related to the motor system.

Changes in extracellular levels of dopamine metabolites in somatosensory cortex after peripheral denervation

This study examined the effects of a nerve transection on monoamine release from primary somatosensory cortex. The technique of microdialysis was employed to sample extracellular levels of norepinephrine (NE), 3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindole-3-acetic acid (5-HIAA) and homovanillic acid (HVA) in the barrel field of freely moving rats following the surgical transection of the contralateral infraorbital nerve. Microdialysates obtained 3, 4, and 5 days after deafferentation were analyzed using high-performance liquid chromatography with electrochemical detection. We found a significant increase in the release of the dopamine metabolites, DOPAC and HVA from the deafferented cortex. Three days after deafferentation the release of DOPAC was three-fold higher in the deafferented than in the control animals, and remained about 100% higher in the next two days in this group of animals. The release of HVA showed a gradual increase following the deafferentation procedure, since a 92% larger value on day 3 increased to a 338% difference on day 5. On the other hand, the release rate of NE and the levels of the serotonin metabolite 5-HIAA were not significantly affected by the deafferentation procedure. These results are discussed in the context of the possible participation of dopamine in the reorganization of the deafferented somatosensory cortex.

Heterogeneity of cortical and hippocampal 5-HT1A receptors: a reappraisal of homogenate binding with 8-[3H]hydroxydipropylaminotetralin

The selective serotonin (5-HT) agonist 8-hydroxydipropylaminotetralin (8-OH-DPAT) has been extensively used to characterize the physiological, biochemical, and behavioral features of the 5-HT1A receptor. A further characterization of this receptor subtype was conducted with membrane preparations from rat cerebral cortex and hippocampus. The saturation binding isotherms of [3H]8-OH-DPAT (free ligand from 200 pM to 160 nM) revealed high-affinity 5-HT1A receptors (KH = 0.7-0.8 nM) and low-affinity (KL = 22-36 nM) binding sites. The kinetics of [3H]8-OH-DPAT binding were examined at two ligand concentrations, i.e., 1 and 10 nM, and in each case revealed two dissociation rate constants supporting the existence of high- and low-affinity binding sites. When the high-affinity sites were labeled with a 1 nM concentration of [3H]8-OH-DPAT, the competition curves of agonist and antagonist drugs were best fit to a two-site model, indicating the presence of two different 5-HT1A binding sites or, alternatively, two affinity states, tentatively designated as 5-HT1AHIGH and 5-HT1ALOW. However, the low correlation between the affinities of various drugs for these sites indicates the existence of different and independent binding sites. To determine whether 5-HT1A sites are modulated by 5'-guanylylimidodiphosphate, inhibition experiments with 5-HT were performed in the presence or in the absence of 100 microM 5'-guanylylimidodiphosphate. The binding of 1 nM [3H]8-OH-DPAT to the 5-HT1AHIGH site was dramatically (80%) reduced by 5'-guanylylimidodiphosphate; in contrast, the low-affinity site, or 5-HT1ALOW, was seemingly insensitive to the guanine nucleotide. The findings suggest that the high-affinity 5-HT1AHIGH site corresponds to the classic 5-HT1A receptor, whereas the novel 5-HT1ALOW binding site, labeled by 1 nM [3H]8-OH-DPAT and having a micromolar affinity for 5-HT, may not belong to the G protein family of receptors. To further investigate the relationship of 5-HT1A sites and the 5-HT innervation, rats were treated with p-chlorophenylalanine or with the neurotoxin p-chloroamphetamine. The inhibition of 5-HT synthesis by p-chlorophenylalanine did not alter either of the two 5-HT1A sites, but deafferentation by p-chloroamphetamine caused a loss of the low-affinity [3H]8-OH-DPAT binding sites, indicating that these novel binding sites may be located presynaptically on 5-HT fibers and/or nerve terminals.

Platelet-induced vasomotion of isolated canine coronary artery in the presence of halothane or isoflurane

To determine the effect of 1.5 MAC of two volatile anesthetics (halothane and isoflurane) on platelet-induced contraction of canine coronary artery, isolated, denuded coronary rings were suspended between two stirrups, placed into organ chambers filled with an oxygenated Krebs-Ringer solution, and connected to an isometric force transducer. Human platelets were obtained from healthy donors and introduced into the organ chambers in increasing amounts to reach 20,50 and, 70 x 10(9) platelets/L. The tension generated in both the control and anesthetic-treated rings was recorded; only halothane reduced the tension induced by platelet activation in the organ chambers. In some experiments, aliquots of Krebs-Ringer solution were taken to determine the amount of 5-HT and TB2 released by 70 x 10(9) human platelets in the presence and absence of the anesthetics. Only halothane reduced TA2 production by the activated platelets. Finally, the contractile response of isolated denuded canine coronary artery rings to U46619, a thromboxane analog, was measured in the presence and absence of the anesthetics. Neither halothane nor isoflurane attenuated the response. In another series of experiments, in vitro platelet aggregation was induced by epinephrine, collagen, ADP, or arachidonic acid in the presence or absence of 1.5 MAC isoflurane or halothane. Both anesthetics significantly reduced the aggregation.

Distribution of indoleamines and [3H]paroxetine binding in rat brain regions following acute or perinatal delta 9-tetrahydrocannabinol treatments

The effects of delta 9-tetrahydrocannabinol (delta 9-THC) administration on the central serotoninergic system were evaluated by biochemical assays of tissue levels of indoleamines; a measure of the serotonin (5-HT) innervation was obtained by using [3H]paroxetine as a marker of 5-HT uptake sites. Two different delta 9-THC treatments were chosen, i.e.: acute and chronic perinatal maternal exposure. Following acute treatment (5 mg/kg), the 5-HT content increased in dorsal hippocampus (+35%), Substantia nigra (+61%) and neostriatum (+62%) but remained unchanged in cingulate cortex, Raphe nuclei, Locus coeruleus and anterior hypothalamus. Endogenous 5-hydroxyindole-3-acetic acid (5-HIAA) decreased in anterior hypothalamus (-23%) and Raphe nuclei (-21%). Following maternal exposure to delta 9-THC (5 mg/kg per day; from gestational day 13 to postnatal day 7), levels of 5-HT were increased in the neostriatum (+22%) but decreased in anterior hypothalamus (-25%), Raphe nuclei (-29%) and Locus coeruleus (-20%) of the litters. Tissue 5-HIAA was increased in anterior hypothalamus (+23%) and Substantia nigra (+48%). There were no changes in 5-HT uptake site density, determined by [3H]paroxetine binding, except for an increase (+50%) in the cingulate cortex of perinatal-treated rats when compared to acutely-treated animals. The present results show that acute and maternal exposure to delta 9-THC produced different effects on the central 5-HT system of the offspring, with a clear regional specificity, but with no changes in the densities of 5-HT uptake sites.

Changes of amino acid and monoamine levels after neonatal 6-hydroxydopamine denervation in rat basal ganglia, substantia nigra, and Raphe nuclei

The effects of a neonatal dopaminergic deafferentation with the neurotoxin 6-hydroxydopamine (6-OHDA) on endogenous tissue levels of catecholamines, indoleamines, and amino acids were investigated in discrete rat brain regions. After producing the lesion at postnatal day 3 by intraventricular injections of 6-OHDA, with a desipramine pretreatment to protect noradrenaline neurons, the animals were kept for 3 months. Their brains were dissected to obtain samples of neostriatum, Globus pallidus, Substantia nigra, and Raphe nuclei, which were then analyzed by high-performance liquid chromatography, coupled either to electrochemical detection for aromatic monoamines, or to post-column ninhydrin derivatization with spectrophotometry for amino acids. The neonatal 6-OHDA treatment depleted dopamine (DA) levels in neostriatum, Globus pallidus, and Substantia nigra, but in Raphe nuclei DA was increased. The main metabolites of DA were also decreased in neostriatum, Globus pallidus, and Substantia nigra but remained unchanged in Raphe nuclei. Serotonin (5-HT) and its metabolite 5-hydroxy-indole-3-acetic acid increased in neostriatum and Raphe nuclei; in Substantia nigra there was a slight increase in 5-HT only. The 6-OHDA lesion caused heterogeneous alterations in amino acid contents, which varied according to the region. In the neostriatum there were increases of gamma-aminobutyric acid (GABA), aspartic acid, and glycine. In the Globus pallidus taurine, GABA, glutamic acid, glutamine, aspartic acid, serine, and alanine were elevated. In the Substantia nigra only increases in taurine, GABA, glutamic acid, and glutamine could be documented. This study shows important changes in amino acid levels and in some of their ratios, occurring in different anatomical subdivisions of the basal ganglia and related brainstem nuclei following a neonatal treatment with 6-OHDA. The results thus demonstrate major biochemical modifications in amino acids in the aftermath of a DA denervation and/or a 5-HT hyperinnervation during an early developmental period.

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 biochemical pharmacology of central nervous system dopamine D1 and D2 receptors

The biochemical properties of central nervous system (CNS) dopamine (DA) D1 and D2 receptors were examined using the specific antagonists [3H]SCH23390 and [3H]raclopride, respectively. There is a different participation of sulfhydryl (-SH) and disulfide (-SS-) groups in the binding site and/or coupling to second messenger systems of D1 and D2 receptors. The ionic studies with [3H]SCH23390 showed slight agonist and antagonist affinity shifts for the D1 receptor. On the other hand, the D2 receptor is very sensitive to cations; even if lithium and sodium influence specific [3H]raclopride binding in a similar manner, there appear to be quantitative differences between these two ions that cannot be explained by surface charge mechanisms. The distribution of D1 and D2 receptors was heterogenous in both species, with the greatest densities in the neostriatum, where the highest concentrations of DA and metabolites were measured. Regions with low endogenous DA content (cerebral cortex and hippocampus) had lower densities of DA receptors. Furthermore, these binding sites were differentially localized within the various regions, and there were substantially more D1 than D2 receptors. The functional significance and heterogeneities in the distribution of D1 and D2 receptors can be related to dopaminergic innervation and turnover.

[3H]paroxetine binding and serotonin content of rat cortical areas, hippocampus, neostriatum, ventral mesencephalic tegmentum, and midbrain raphe nuclei region following p-chlorophenylalanine and p-chloroamphetamine treatment

The agents p-chlorophenylalanine (PCPA) and p-chloroamphetamine (PCA) deplete brain serotonin (5-HT) levels by two different mechanisms; PCPA inhibits the enzyme tryptophan hydroxylase, whereas PCA has a neurotoxic action on certain 5-HT neurons. The parameters of [3H]paroxetine binding to homogenates prepared from the cerebral cortex of rats treated with PCPA, PCA, or saline; vehicle were investigated. The tissue concentrations of 5-HT and 5-hydroxyindole-3-acetic acid (5-HIAA) were also determined by HPLC in the same brain samples. After PCPA treatment, neither the maximum binding capacity (Bmax) nor the dissociation constant (KD) of [3H]paroxetine for the 5-HT uptake recognition site differed from controls despite a substantial reduction in the concentration of 5-HT and 5-HIAA. In contrast, significant changes in both the Bmax and KD values were observed in the cerebral cortex of rats treated with PCA. Furthermore, [3H]paroxetine binding and tissue concentrations of 5-HT and 5-HIAA were measured in the following different regions of the rat brain: cingulate, parietal, and visual cortical areas; dorsal and ventral hippocampus; rostral and caudal halves of neostriatum; ventral mesencephalic tegmentum; and midbrain raphe nuclei region after administration of PCPA, PCA, or saline vehicle. There was an excellent correlation between regional 5-HT levels and specific [3H]paroxetine binding in control and PCA-treated rats although this correlation was lost after PCPA treatment. Under these conditions, the 5-HT innervation remains unchanged whereas the concentration of 5-HT and 5-HIAA is greatly reduced. Thus, [3H]paroxetine binding appears to provide a reliable marker of 5-HT innervation density within the mammalian CNS.

[3H]paroxetine binding and serotonin content of rat and rabbit cortical areas, hippocampus, neostriatum, ventral mesencephalic tegmentum, and midbrain raphe nuclei region

The high-affinity binding of [3H]paroxetine to membranes was measured in different regions of the rat and rabbit brain: cingulate, frontal, parietal, piriform, entorhinal, and visual cortical areas; dorsal and ventral hippocampus; rostral and caudal halves of neostriatum (rat) or caudate nucleus and putamen (rabbit); ventral mesencephalic tegmentum; and midbrain raphe nuclei region. The tissue concentrations of serotonin (5-HT), 5-hydroxyindole-3-acetic acid (5-HIAA) and 5-hydroxy-l-tryptophan (5-HTP) were also determined by high-performance liquid chromatography (HPLC) in the same brain samples. The regional density of [3H]paroxetine binding varied in both species; the highest values (Bmax) were found in the midbrain raphe region and ventral mesencephalic tegmentum. The cortical values ranged from moderate to low, with a significantly higher density in the cingulate cortex of the rat compared with rabbit. In the rat, there was also a higher density in the ventral than dorsal hippocampus, and the caudal than rostral neostriatum. In the rabbit, the hippocampal and neostriatal values were generally lower and more uniform. In both species, there was an excellent correlation between regional 5-HT levels and specific [3H]paroxetine binding (r = 0.87 in the rat and 0.96 in the rabbit). Considering the available quantitative data on the number of 5-HT nerve cell bodies and axon terminals in different regions of the rat brain, it appears likely that the high amount of [3H]paroxetine binding in the midbrain raphe region and ventral mesencephalic tegmentum reflects the presence of 5-HT uptake sites on 5-HT nerve cell bodies and dendrites as well as axon terminals. In other brain regions, the heterogeneous distribution of [3H]paroxetine binding parallels that of the number of 5-HT axon terminals, emphasizing the potential usefulness of this radioligand as a marker of 5-HT innervation density.

[3H]ketanserin binds to non-5-HT2 sites in rabbit cerebral cortex and neostriatum

A characterization of [3H]ketanserin ([3H]KTS) binding in the frontal cortex (fCTX) and neostriatum (caudate-putamen, CPU) of rabbit was carried out to determine whether this ligand labels a non-serotoninergic receptor. The association and dissociation kinetics in fCTX were rapid, and could be fitted to two-site models, suggesting [3H]KTS is labeling two cortical sites. Using the serotonin-2 (5-HT2) antagonist mianserin to determine nonspecific binding, the saturation curves revealed a single high-affinity binding site. In contrast, when unlabeled ketanserin was used for nonspecific counts, the Scatchard plots were best fitted to a two-site model but the binding parameters of the high-affinity site were similar to that obtained in the presence of mianserin. The 5-HT2 antagonists mianserin, methysergide and ritanserin inhibited [3H]KTS binding in fCTX at nanomolar concentrations, however, the curves were best fitted to two-site models. In contrast, [3H]KTS binding to membrane preparations from the CPU could only be inhibited by high (micromolar) concentrations of these antagonists. Low micromolar concentrations of the monoamine uptake blockers GBR12909, desipramine, nomifensine, cocaine and fluoxetine competed with [3H]KTS in both fCTX and CPU. This study demonstrates that [3H]KTS labels a non-serotoninergic recognition site in the rabbit fCTX and CPU similar to that found in the rat neostriatum, i.e.: probably a monoamine transport site.