Daily Modifications of 3h-Naloxone Binding Sites in the Rat Brain: A Quantitative Autoradiographic Study

The endogenous opioid peptides, the opiate receptors and several related behaviours, like opioid-mediated analgesia, show daily variations in different animal species including rats. The attempt to correlate the daily rhythm of opiate receptors in the central nervous system (CNS) to opiate related rhythmic phenomena requires an experimental approach with a high anatomical resolution, as the opioid distribution is very heterogeneous. In this paper we present the study of daily variations of 3H-naloxone binding sites in the different regions of the adult male rat brain, performed by means of quantitative autoradiography. Five rats are sacrificed at each investigated time of the day (0200, 0600,1000,1400,1800 and 2200). The ligand is 3H-naloxone(4nM), the quantification is performed by means of densitometric procedures (image analyzer Tesak VDC 501, computer Digital PDP11,3H-microscale). The statistical analysis is performed according to the single Cosinor method and the one-way analysis of variance followed by the multiple range test of Duncan. We analysed 33 different regions of the rat CNS, and the daily variations of opiate receptors are regionally selective. A circadian rhythm is found in the anterior cingulate cortex, hippocampal cortex, periventricular, medial, ventral, reticular and posterior nuclei of the thalamus, rhomboid, gelatinosus and rheuniens nuclei, lateral hypothalamus, locus coeruleus, grey substance of the pons, reticular formation of medulla oblongata, inferior olivary complex, medial part of the nucleus of the solitary tract and nucleus of the spinal tract of the trigeminal nerve. An ultradian rhythm is found in the medial and lateral preoptic areas, in the medial hypothalamus, in the medial and in the lateral nuclei of habenula. No significant variations during 24 hr according to the Cosinor analysis are found in the dorsal and lateral cerebral cortex, striatum, globus pallidus, bed nucleus of the stria terminalis, septal nuclei, lateral nucleus of the thalamus, cochlear nuclei, nucleus of the solitary tract, lateral and caudal parts, dorsal motor nucleus of the vagal nerve, XII and IX nerve nuclei. The amplitude of the daily variations observed ranges from 10 to 40%. Our results demonstrate the high anatomical selectivity of the daily modifications of 3H-naloxone binding sites in the rat CNS. They also indicate that quantitative autoradiography is a suitable and sensitive technique for these studies.     

Chronopharmacokinetics of Imipramine and Desipramine in Rat Forebrain and Plasma After Single and Chronic Treatment with Imipramine

Daily variations in the pharmacokinetics of imipramine (IMI) could contribute to circadian phase-dependent effects of the drug. Therefore, the chronopharmacokinetics of IMI and its metabolite, desipramine (DMI), were studied after single and chronic application. Male rats were synchronized to a 12:12 hour lightdark (L:D) regimen with lights on from 07:00 to 19:00 (dark, 19:00-07:00). In single-dose experiments rats were injected with IMI (10 mg/kg) i.p. or i.v. at 07:30 or 19:30 and groups of rats were killed 0-22 hours thereafter. After chronic application of IMI in drinking water (≈ 15 mg/kg/d) groups of rats were killed during the 14th day of treatment at 02:00, 08:00, 14:00, and 20:00, respectively. Brain and plasma concentrations of IMI and DMI were determined by reversed-phase high-performance liquid chromatography with ultraviolet detection. After single i.p. application of IMI, maximal brain concentrations (C_max) of IMI and DMI were nearly twofold higher in darkness (IMI, 4.8 μg/g; DMI, 1.8 μg/g) than in light (IMI, 2.85 Mg/g; DMI, 0.85 Mg/g). Also, the area under the curve (AUC) (0-22 hours) was about 1.6-fold greater in darkness than in light for IMI and DMI; half-lives were not circadian phase dependent. After i.v. injection of IMI, the AUC in brain was also about 30% greater in darkness than in light. After chronic application of IMI in drinking water, brain concentrations of IMI and DMI varied more than threefold within 24 hours. The data demonstrate that the pharmacokinetics of IMI and DMI are circadian phase dependent. It is assumed that circadian variations in drug distribution are more likely to contribute to the drug's chronopharmacokinetics than variations in the drug's metabolism. The 24-hour variations in the drug's concentrations after chronic IMI application in drinking water can be explained by the drinking behavior of the rats, which by itself is altered by IMI.     

Thyrotropin-Releasing Hormone Receptor Binding Sites: Autoradiographic Distribution in the Rat and Guinea Pig Brain

Thyrotropin-releasing hormone (TRH) binding sites were labeled in vitro in mounted brain tissue sections from rat and guinea pig brains with [3H]methyl TRH and localized autoradiographically using 3H-sensitive film. Regional densities of TRH binding sites were measured by computer-assisted microdensitometry. The distribution of sites in both species was highly heterogeneous. In both guinea pig and rat brains, the highest densities of binding sites were seen in the amygdaloid nuclei and the perirhinal cortex. In contrast, in other brain areas, a clear difference between the distribution of sites in rat and guinea pig was found. The temporal cortex, pontine nuclei, and interpeduncular nucleus, which contained high densities of binding in the guinea pig, were scarcely labeled in the rat. The accessory olfactory bulb and the septohippocampal area presented in the rat higher concentrations of binding sites than in the guinea pig. Other brain areas showing intermediate to low densities in both species were accumbens nucleus, bed nucleus of the stria terminalis, dentate gyrus, facial and hypoglossal nuclei, and gelatinosus subnucleus of the trigeminal nerve, among others. The anterior pituitary also presented low to intermediate concentrations of receptors. The distribution of TRH sites here described does not completely correlate with that of endogenous TRH, but is in good agreement with previous biochemical data. The results are discussed in correlation to the physiological effects that appear to be mediated by TRH.     

Pharmacological and Physicochemical Properties of Pre-Versus Postsynaptic 5-Hydroxytryptamine1A Receptor Binding Sites in the Rat Brain: A Quantitative Autoradiographic Study

Numerous data suggested that the pharmacological and biochemical properties of 5-hydroxytryptamine1A (5-HT1A) receptors exhibit some regional differences in the CNS, notably within the raphe nuclei compared with various forebrain areas (such as the hippocampus). This possibility has been further investigated in the dorsal raphe nucleus and two areas within the hippocampus, the dentate gyrus and the CA1 area, using the quantitative autoradiographic technique. The potencies of 5'-guanylylimidodiphosphate to inhibit the specific binding of 125I-Bolton-Hunter-8-methoxy-2-(N-propyl-N-propylamino)tetralin (125I-BH-8-MeO-N-PAT) to 5-HT1A sites and of N-ethylmaleimide to block these sites irreversibly were identical in the dorsal raphe nucleus and the hippocampal areas in rat brain sections. In contrast, slight but significant differences were noted in the pH dependence and pharmacological properties of 5-HT1A sites labeled by 125I-BH-8-MeO-N-PAT in these three regions. Similarly, heat denaturation experiments and tissue exposure to either phospholipase A2 or the alkylating agent 8-methoxy-2-(N-2'-chloropropyl,N-propyl)aminotetraline revealed regional differences in the properties of 5-HT1A sites. However, in most cases, the observed variations were of greater amplitude between the CA1 area and the dentate gyrus, where 5-HT1A sites are located postsynaptically, than between any one of these areas and the dorsal raphe nucleus where they act as (presynaptic) somatodendritic autoreceptors. These data further support that subtypes of 5-HT1A receptors probably exist in the rat brain, but this heterogeneity seems unrelated to the pre- or post-synaptic location of these receptors.     

High-Affinity Binding of [3H]Desipramine to Rat Brain: A Presynaptic Marker for Noradrenergic Uptake Sites

Abstract: High-affinity binding sites (apparent K _D= 1.5 nM) for [³H]desipramine have been demonstrated and characterized in membranes prepared from rat brain. The binding of [3H]desipramine was found to be saturable, reversible, heat-sensitive, sodium-dependent, and regionally distributed among various regions of the brain. High concentrations of [³H]desipramine binding sites were found in the septum, cerebral cortex, and hypothalamus, whereas lower concentrations were found in the medulla, cerebellum, and corpus striatum. A very good correlation ( r = 0.81, P < 0.001) was observed between the potencies of a series of drugs in inhibiting high-affinity [³H]desipramine binding and their capacity to block norepinephrine uptake into synaptosomes. In 6-hydroxydopamine-lesioned rats there was a marked decrease in [³H]norepinephrine uptake and [3H]desipramine binding with no significant alterations in either [³H]serotonin uptake or [³H]imipramine binding. These results suggest that the high-affinity binding of [³HJdesipramine to rat brain membranes is pharmacologically and biochemically distinct from the high-affinity binding of [3H]imipramine, and that there is a close relationship between the high-affinity binding site for [³H]desipramine and the uptake site for norepinephrine.     

5-Hydroxytryptamine2 and β-Adrenergic Receptor Regulation in Rat Brain Following Chronic Treatment with Desipramine and Fluoxetine Alone and in Combination

Abstract: A chronic (14-day) study was initiated to investigate the effects of combined fluoxetine (FLU) and desipramine (DMI) treatment on the densities and affinities of β-adrenergic and 5-hydroxytryptamine₂ (5-HT₂) receptors. Male Sprague-Dawley rats were administered the following doses using osmotic minipumps: FLU, 10 mg/kg/day; DMI, 5, 10, or 15 mg/kg/day; FLU, 10 mg/kg/day, plus DMI, 5 mg/kg/day; or vehicle (distilled water). After 14 days the cortex was dissected out and used for [³H]-ketanserin (5-HT₂) binding, [³H]CGP-12177 (β-adrenergic) binding, and drug level analysis. All animals receiving DMI showed significant down-regulation of 5-HT₂ receptors except those receiving FLU in combination. DMI down-regulated β-adrenergic receptors in a dose-dependent manner, with significantly greater down-regulation seen with the combination than with DMI (5 mg/kg/day) alone. This latter effect was apparently the result of greater levels of DMI in cortex with the combination than with DMI (5 mg/kg/day) alone. FLU had no effect on 5-HT₂ or β-adrenergic receptors on its own. Coadministration of FLU and DMI resulted in a doubling of levels of FLU and its demethylated metabolite, norfluoxetine (NFLU), and a tripling of DMI levels compared with values observed when FLU (10 mg/kg/day) or DMI (5 mg/kg/day) was administered alone. These results suggest that with the DMI/FLU combination (a) FLU and/or NFLU block the down-regulation of 5-HT₂ receptors caused by DMI alone, (b) an important factor determining β-adrenergic receptor density may be the elevated DMI levels relative to those with DMI (5 mg/kg/day) alone, (c) FLU and/or NFLU inhibit the metabolism of DMI, and (d) DMI inhibits the metabolism of FLU.     

Autoradiographic Analysis of [3H]Imipramine Binding in the Human Brain Postmortem: Effects of Age and Alcohol

In vitro quantitative autoradiography of high-affinity [3H]imipramine binding sites was performed on 16 human brains postmortem. The densities of binding sites were highest in the hypothalamus. Next, in descending order, were the basal and lateral nuclei of the amygdala; substantia innominata; insular cortex; the central nucleus of the amygdala; the anterior nucleus of the thalamus; the head of the caudate nucleus; portions of the frontal, parietal, and temporal cortex; claustrum; the granular layer of the dentate gyrus; substantia nigra; the pyramidal layer of CA fields; globus pallidus; red nucleus; and white matter. Imipramine binding was found to increase with age in a region-specific manner. The presence of alcohol had a similar effect, which was most pronounced in the hippocampus. Sex and time from death to autopsy did not affect imipramine binding, in our sample.     

Quantitative Autoradiographic Analysis of the Effects of Electroconvulsive Shock on Serotonin-2 Receptors in Male and Female Rats

Chronic electroconvulsive shock (ECS) is known to increase the level of serotonin-2 (S2) receptors in male rat brain. Using quantitative autoradiography, we have studied the distribution pattern of these receptors in female as well as male rats and the effect of repeated ECS on the receptor level in both sexes. We find that although the distribution of S2 receptors is generally similar in males and females, they respond differently to repeated ECS. In males we found the expected increase in S2 binding, which was localized to specific cortical, hippocampal, and septal regions. In females, no increase was found in the cortex or septum and relatively small increases were found in the hippocampus. It appears that the regulation of S2 receptors by ECS is sex-dependent.     

"Specific" Binding of [3H]Imipramine to Protease-Sensitive and Protease-Resistant Sites

A number of 5-hydroxytryptamine (5-HT) uptake inhibitors have been shown to displace the binding of [3H]imipramine to rat cortical membranes in a complex manner with Hill slopes less than unity. Norzimeldine displaced the binding of [3H]imipramine in a biphasic manner with IC50 values for the two components of about 30 nM and 30 microM. This latter site alone was found in tissues that had been treated with a protease. Binding to both of these sites was displaced by 10 microM desipramine. The protease-sensitive [3H]imipramine binding sites were found to be saturable, high-affinity binding sites with a KD of 8 nM. The number of these sites varied between brain regions and was positively correlated with the regional distribution of [14C]5-HT but not [3H]noradrenaline uptake. This was not the case however for the protease-resistant but desipramine-displaceable binding sites. Since most previous [3H]imipramine binding studies have been performed with high concentrations of desipramine (10 microM) to define "specific binding," these data would suggest that either protease-sensitivity or displacability by 1 microM norzimeldine would give more reliable estimates of the specific binding.     

Kinetics, Pharmacology, and Autoradiographic Distribution of l-[3H]Nitroarginine Binding Sites in Rat Cerebellum

Abstract: The kinetics and pharmacology of N ^G-nitro- l -[2,3,4,5-³H]arginine ( l -[³H]NOARG) binding to rat cerebellum were investigated using in vitro radioligand binding. Specific l -[³H]NOARG binding in cerebellum was of nanomolar affinity, reversible, saturable, and best fit to a single-site model. Specific binding was Ca²⁺ dependent and sensitive to pH (with an optimum of 5.5–7.0). Added calmodulin (1.5–40 µg/ml) had no influence on specific l -[³H]NOARG binding. However, the calmodulin antagonists W-5, W-13, and calmidazolium inhibited l -[³H]NOARG binding with IC₅₀ values in the micromolar range, and calmodulin (10 µg/ml) competitively reversed this inhibition. Nitric oxide synthase (NOS) inhibitors ( N ^G-nitro- l -arginine methyl ester and N ^G-monomethyl- l -arginine acetate) and l -arginine displaced l -[³H]NOARG binding with IC₅₀ values in the nanomolar range, whereas d -arginine and basic amino acids ( l -lysine and l -histidine) displaced l -[³H]NOARG binding with IC₅₀ values in the millimolar range. A comparison of the NOS functional assay with l -[³H]NOARG binding in rat cerebellum showed similar profiles of Ca²⁺ dependency and inhibitory kinetics. Quantitative autoradiographic distribution of l -[³H]NOARG binding sites was significantly higher in the molecular layer than in the granular layer of cerebellum. These studies confirm the potential use of l -[³H]NOARG binding to study the regional distribution and functional properties of NOS.     

[3H]Desipramine Binding to Rat Brain Tissue: Binding to Both Noradrenaline Uptake Sites and Sites Not Related to Noradrenaline Neurons

Abstract The pharmacological and biochemical characteristics of [³H]desipramine binding to rat brain tissue were investigated. Competition studies with noradrenaline, nisoxetine, nortriptyline, and desipramine suggested the presence of more than one [³H]desipramine binding site. Most of the noradrenaline-sensitive binding represented a high-affinity site, and this site appeared to be the same as the high-affinity site of nisoxetine-sensitive binding. The [³H]desipramine binding sites were abolished by protease treatment, a result suggesting that the binding sites are protein in nature. When specific binding was defined by 0.1 μM nisoxetine, the binding was saturable and fitted a single-site binding model with a binding affinity of ～1 nM. This binding fraction was abolished by lesioning of the noradrenaline neurons with the noradrenaline neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromo-benzylamine (DSP4). In contrast, when 10 μM nisoxetine was used to define the specific binding, the binding was not saturable over the nanomolar range, but the binding fitted a two-site binding model with K_D values of 0.5 and >100 nM for the high- and low-affinity components, respectively. The high-affinity site was abolished after DSP4 lesioning, whereas the low-affinity site remained. The binding capacity (B_max) for binding defined by 0.1 μM nisoxetine varied between brain regions, with very low density in the striatum (B_max not possible to determine), 60-90 fmol/mg of protein in cortical areas and cerebellum, and 120 fmol/mg of protein in the hypothalamus. The binding capacities of these high-affinity sites correlated significantly with the regional distribution of [³H]noradrenaline uptake but not with 5-[³H]hydroxytryptamine uptake. The low-affinity sites did not correlate with the regional distribution of [³H]noradrenaline uptake. Drug inhibition studies showed that noradrenaline inhibits the binding defined by 0.1 μM nisoxetine in a competitive manner. Together, these findings suggest that only a small fraction of the [³H]desipramine binding can be regarded as “specific” binding, and this binding fraction may represent the substrate recognition site for noradrenaline uptake. Assuming that one molecule of desipramine binds to each carrier molecule, the turnover number for the noradrenaline carrier was calculated to be 20/min, i.e., the duration of one transport cycle was 3 s.     

Effect of Unilateral Perinatal Hypoxic-Ischemic Brain Injury in the Rat on Striatal Muscarinic Cholinergic Receptors and High-Affinity Choline Uptake Sites: A Quantitative Autoradiographic Study

The binding characteristics and distribution of M1 and M2 muscarinic cholinergic receptors and high-affinity choline uptake sites were studied in the striatum of the rat at 3-4 and 9-12 weeks of age after exposure to unilateral perinatal hypoxic-ischemic brain injury. High-affinity choline uptake sites were labeled with [3H]hemicholinium-3, M1 receptors with [3H]pirenzepine, and M2 receptors with [3H]AF-DX 116. Saturation experiments revealed a significant decrease in the maximal binding capacity (Bmax) for [3H]pirenzepine-labeled M1 receptors in the lesioned caudate/putamen complex in immature rats with moderate brain injury, in comparison with controls. In contrast, the Bmax value for [3H]hemicholinium-3-labeled high-affinity choline uptake sites was significantly increased. No changes in dissociation constants (KD) were observed. These changes were most pronounced in the dorsolateral region of striatum. Striatal regional distribution of [3H]AF-DX 116 was not affected. In mature rats, binding of [3H]pirenzepine returned to control values, whereas [3H]hemicholinium binding showed a persistent increase (23%). The increase in [3H]hemicholinium-3 binding, as a specific marker of cholinergic nerve terminals, is consistent with our prior morphologic studies demonstrating relative preservation of cholinergic neurons and neuropil, and supports the concept that striatal cholinergic systems are resistant to hypoxic-ischemic injury.     

Quantitative Autoradiography of [3H]Indalpine Binding Sites in the Rat Brain: I. Pharmacological Characterization

The binding of [3H]indalpine (4-[2-(3-indolyl)]ethyl piperidine) to slide-mounted sections of rat brain has been characterized. This 5-hydroxytryptamine (5-HT) uptake blocker binds to sections with high affinity (KD approximately 1 nM). The binding is saturable, and can be displaced by the addition of clomipramine (1 microM). Other drugs inhibiting the uptake of 5-HT also have the capacity to inhibit the binding of [3H]indalpine. A significant correlation (r = 0.86) was found between the capacity of these compounds to inhibit the uptake of 5-HT and their potencies as inhibitors of [3H]indalpine binding. Binding was Na+ - and Cl- -dependent and was inhibited competitively by 5-HT. Furthermore, electrolytic lesions of the dorsal raphe or medial forebrain bundle, which cause a degeneration of 5-HT cell bodies and fibers, respectively, resulted in a 30-40% reduction in the binding of [3H]indalpine. [3H]Indalpine binds to the 5-HT uptake recognition sites in a different manner from imipramine-like compounds.     

Distribution of Specific High-Affinity Binding Sites for [3H]Imipramine in Human Brain

Abstract: [³H]Imipramine binds with high affinity to membranes from different regions of the human brain. The highest density of binding sites was observed in the hypothalamus and substantia nigra and the lowest density in the white matter and cerebellum. As found in rat brain, tricyclic antidepressant drugs are potent inhibitors of [³H]imipramine binding. Atypical antidepressants are, however, much weaker at inhibiting the specific binding. The [³H]imipramine binding site in human cortex is apparently identical to the site already described in the rat brain and in human platelets.     

Effect of Chronic Nicotine Treatment on Nicotinic Autoreceptor Function and N-[3H]Methylcarbamylcholine Binding Sites in the Rat Brain

It has been reported that N-methylcarbamylcholine (MCC), a nicotinic agonist, binds to central nicotinic receptors and causes an increase of acetylcholine (ACh) release from certain central cholinergic nerve terminals. The present experiments determine whether these two phenomena change in response to the chronic administration of nicotine, a procedure known to result in an increase in nicotinic binding sites. Chronic nicotine caused a brain region-specific up-regulation of [3H]MCC sites; binding increased in the frontal cortex, parietal cortex, striatum, and hippocampus, but not in the occipital cortex or cerebellum. The effect of nicotine was selective to nicotinic binding sites, because muscarinic sites, both M1 ([ 3H]pirenzepine) and M2 ([3H]ACh), were unaffected by chronic nicotine treatment. MCC increased the release of ACh from the frontal cortex and hippocampus by a calcium-dependent mechanism; MCC did not alter ACh release from striatum or occipital cortex of control animals. The MCC-induced increase in ACh release was not apparent in those animals which had been treated with nicotine. There was a partial recovery of nicotinic autoreceptor function when animals were allowed to recover (4 days) following chronic nicotine treatment, but the density of binding sites remained increased compared to control. Chronic nicotine did not change the potassium-evoked release of ACh from the frontal cortex or hippocampus, but decreased this measure from striatum. It also decreased the ACh content of the striatum, but not that of the cortex or the hippocampus; the activity of choline acetyltransferase was not altered in any of the regions tested.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of the Tricyclic Antidepressant Desipramine on β-Adrenergic Receptors in Cultured Rat Glioma C6 Cells

Rat glioma C6 cells, cultured in the presence of the tricyclic antidepressant desipramine, lost a significant number of beta-adrenergic receptors in a time- and dose-dependent manner. A similar loss was observed whether binding was determined on intact cells with the hydrophilic beta-adrenergic antagonist (+/-)-[3H]4-(3-tert-butylamino-2-hydroxypropoxyl)benzimidazole-2-o n HCl ([3H]CGP-12177) or on cell lysates with the more hydrophobic antagonists [125I]iodocyanopindolol or [3H]dihydroalprenolol. When stimulated with the agonist isoproterenol, desipramine-treated cells accumulated less cyclic AMP than control cells. The affinity of the beta-adrenergic receptors for either antagonist or agonist was unchanged after desipramine treatment. Desipramine interacted only weakly with the receptors and competed for [125I]iodocyanopindolol binding with a Ki of 30 microM. The presence in the culture medium of alprenolol or propranolol, potent beta-adrenergic antagonists, however, did not prevent the reduction in receptors by desipramine. Desipramine also caused a loss of beta-adrenergic receptors from cells maintained in serum-free medium and the cells themselves did not contain or secrete endogenous catecholamines. Although desipramine is a potent inhibitor of catecholamine uptake, it appears unlikely that the observed loss of beta-adrenergic receptors in rat glioma C6 cells exposed to the drug is due to an increase in extracellular catecholamine levels or to a direct interaction with the receptors.     

No Orcadian Rhythms of Serotoninergic,Alpha-, Beta-Adrenergic and Imipramine Binding Sites in Rat Brain Regions

B_max values of the specific binding of [³H]-WB 4101, [³H]-dihydroalprenolol, [³H]-spiperone and [³H]-imipramine to various rat brain regions were determined at 4 hr intervals over 24 hr under circadian conditions. No significant circadian rhythm of binding sites number was found for any receptor investigated in cerebral cortex, hypothalamus or brain stem. Some methodological issues are discussed.     

No Orcadian Rhythms of Serotoninergic, Alpha-, Beta-Adrenergic and Imipramine Binding Sites in Rat Brain Regions

B_max values of the specific binding of [³H]-WB 4101, [³H]-dihydroalprenolol, [³H]-spiperone and [³H]-imipramine to various rat brain regions were determined at 4 hr intervals over 24 hr under circadian conditions. No significant circadian rhythm of binding sites number was found for any receptor investigated in cerebral cortex, hypothalamus or brain stem. Some methodological issues are discussed.     

Quantitative Autoradiography of [3H]Indalpine Binding Sites in the Rat Brain: II. Regional Distribution

The localization of binding sites for [3H]indalpine to sections of rat brain was studied by a quantitative autoradiographic technique. Binding sites for this specific neuronal 5-hydroxytryptamine (5-HT) uptake inhibitor are concentrated in areas rich in 5-HT neuronal cell bodies, fibers, and synaptic terminals. One of the most interesting features of this regional distribution is the very high density of these sites found in the dorsal raphe, substantia nigra, ventral tegmental area, and locus ceruleus. Components of the visual system also show pronounced labelling with [3H]indalpine. The finding that limbic structures are strongly labelled by this drug may be related to the antidepressant activity of indalpine. The anatomical distribution of binding sites demonstrated is consistent with the specific labelling of 5-HT neurons by [3H]indalpine and confirms previous studies carried out with another 5-HT uptake inhibitor, [3H]imipramine.     

Aging and Brain Cholinergic Muscarinic Receptors: An Autoradiographic Study in the Rat

Cholinergic muscarinic receptors in aged and young rat brains were studied by quantitative autoradiography of tritiated quinuclidinyl benzilate. A selective pattern of decreased binding density was observed in the aged rat. A large number of regions showed no effect of aging; these include subdivisions of the hippocampal formation and most thalamic and hypothalamic nuclei. Small but significant decreases were found in cortical regions and in the striatum. The largest effects were seen in ventral forebrain cholinergic nuclei, where 40-60% depletions were found in the diagonal band, nucleus basalis magnocellularis, ventral pallidum, and substantia innominata.