Differential Response of Central Dopaminergic System in Acute and Chronic Unpredictable Stress Models in Rats

We aimed to evaluate the response of dopaminergic system in acute stress (AS) and chronic unpredictable stress (CUS) by measuring dopamine (DA) levels, its receptor densities in the frontal cortex, striatum, hippocampus, amygdala and orbito-frontal cortex regions of rat brain, and investigated the corresponding behavioral locomotor changes. Involvement of D₁ receptor was also examined during AS and CUS using A 68930, a D₁ selective agonist. Rats were exposed to AS (single immobilization for 150 min) and CUS (two different stressors for 7 days). AS significantly decreased the DA levels in the striatum and hippocampus, and A 68930 pretreatment significantly reverted these changes. However, in the frontal cortex significantly increased DA levels were remain unchanged following A 68930. CUS led to a decrease of DA levels in the frontal cortex, striatum and hippocampus, which were normalized by A 68930. Saturation radioligand binding assays revealed a significant decrease in the number of D₁-like receptors in the frontal cortex during CUS, which were further decreased by A 68930 pretreatment. However, in the striatum and hippocampus, A 68930 pretreatment reduced the CUS induced increase in the number of D₁-like receptors. No significant changes were observed in the amygdala and orbito-frontal cortex during AS and CUS, while D₂-like receptors were unchanged in all the brain regions studied. Locomotor activity was significantly decreased in both the stress models, A 68930 pretreatment significantly increased stereotypic counts and horizontal activity. Thus, present investigation provide insights into the differential regional response of dopaminergic system during AS and CUS. Further, neurochemical and behavioral effects of D₁ agonist pretreatment suggest specific modulatory role of D₁ receptor under such stressful episodes.     

Postnatal Development of Cholecystokinin-Like Immunoreactivity and Its mRNA Level in Rat Brain Regions

Developmental changes of preprocholecystokinin mRNA (CCK mRNA) and cholecystokinin-like immunoreactivity (CCK-LI) were examined in rat brain regions (frontal cortex, colliculi, hippocampus, striatum, and cerebellum) using RNA dot blot assays with cholecystokinin (CCK) cDNA and radioimmunoassay, respectively. The CCK-LI levels in all regions examined were very low at birth. Excluding the cerebellum, the levels in these regions increased postnatally and reached adult values at 28 days of age. In contrast to CCK-LI, CCK mRNA levels changed dramatically during development. A considerable amount of CCK mRNA was detected in the frontal cortex and hippocampus at birth. The changes in the level of CCK mRNA in the frontal cortex and colliculi paralleled those of CCK-LI, including a rapid increase from 7 to 14 days of age. The synthesis of CCK mRNA preceded the appearance of CCK-LI. CCK mRNA levels in the hippocampus and striatum exhibited a transient increase, with a peak at 14 days of age. In the adult brain, the CCK mRNA levels were high in the frontal cortex, moderate in the hippocampus and colliculi, and low in the striatum. The cerebellum contained only a negligible amount of CCK mRNA during development. The relatively high level of CCK-LI compared with the low level of CCK mRNA in the striatum supports the idea that most of the striatal CCK-LI is supplied from extrastriatal regions.(ABSTRACT TRUNCATED AT 250 WORDS)     

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)     

Brain neurotransmitter receptors after long-term haloperidol: dopamine, acetylcholine, serotonin, alpha-noradrenergic and naloxone receptors.

Since long-term neuroleptic therapy is known to alter brain dopaminergic sensitivity, we tested the effects of chronic haloperidol administration (10 mg/kg/day for over 3 weeks) on the amount of the dopamine receptors (using ³H-apomorphine and ³H-haloperidol) in various regions of the rat brain. To test whether the changes in dopamine receptors were selectively produced, we also assayed acetylcholine receptors (with ³H-quinuclidinyl benzilate or ³H-QNB), alpha-noradrenergic receptors (with ³H-WB-4101), ³H-serotonin receptors and ³H-naloxone receptors.The specific binding of ³H-haloperidol increased significantly by 34% in the striatum and by 45% in the mesolimbic region after long-term haloperidol. The specific binding of ³H-apomorphine also increased significantly by 77% in the striatum and 55% in the mesolimbic area. Although there was a small significant increase of 20% in specific ³H-serotonin binding in the striatum, no such increment occurred in the hippocampus or the cerebral cortex. No significantly different binding occurred for the other ³H-ligands in these brain regions except for a 13% increase in alpha-noradrenergic binding in the cerebral cortex. These results indicate that long-term haloperidol treatment produces rather selective increases in dopamine/neuroleptic receptors, without much change in 4 other types of receptors. Such relatively selective increments in these receptors may be the basis of dopaminergic supersensitivity (e.g. tardive dyskinesia) after long-term haloperidol.     

Cannabinoid Receptors and Modulation of Cyclic AMP Accumulation in the Rat Brain

The mechanism by which cannabinoid compounds produce their effects in the rat brain was evaluated in this investigation. Cannabinoid receptors, quantitated by [3H]CP-55,940 binding, were found in greatest abundance in the rat cortex, cerebellum, hippocampus, and striatum, with smaller but significant binding also found in the hypothalamus, brainstem, and spinal cord. Using rat brain slice preparations, we evaluated the effect of desacetyllevonantradol on basal and forskolin-stimulated cyclic AMP accumulation in the regions exhibiting the greatest cannabinoid receptor density. Desacetyllevonantradol (10 microM) reduced cyclic AMP levels in the hippocampus, frontal cortex, and striatum. In the cerebellum, however, the response to desacetyllevonantradol was biphasic with cyclic AMP accumulation being decreased at lower and increased at higher concentrations. Desacetyllevonantradol reduced cyclic AMP accumulation in isoproterenol-stimulated slices in the cortex and cerebellum, but not in the hippocampus. Cells that responded to vasoactive intestinal peptide with an increase in cyclic AMP accumulation in the hippocampus and cortex also responded to desacetyllevonantradol. The modulation of cyclic AMP accumulation by desacetyllevonantradol could be attenuated following stereotaxic implantation of pertussis toxin, supporting the involvement of a G protein in the cannabinoid response in the brain. However, other actions of cannabinoid compounds may also affect the cyclic AMP levels in brain slice preparations.     

In vivo binding of 125I-LSD to serotonin 5-HT2 receptors in mouse brain

The binding of 125I-LSD (2-[125I]-lysergic acid diethylamide) was studied in various mouse brain regions following intravenous injection of the radioligand. The high specific activity of 125I-LSD enabled the injection of low mass doses (14 ng/kg), which are well below the threshold for induction of any known physiological effect of the probe. The highest levels of 125I-LSD binding were found in the frontal cortex, olfactory tubercles, extra-frontal cortex and striatum while the lowest level was found in the cerebellum. Binding was saturable in the frontal cortex but increased linearly in the cerebellum with increasing doses of 125I-LSD. Serotonergic compounds potently inhibited 125I-LSD binding in cortical regions, olfactory tubercles, and hypothalamus but had no effect in the cerebellum. Dopaminergic compounds caused partial inhibition of binding in the striatum while adrenergic compounds were inactive. From these studies we conclude that 125I-LSD labels serotonin 5-HT2 receptor sites in cortical regions with no indication that other receptor sites are labeled. In the olfactory tubercles and hypothalamus, 125I-LSD labeling occurs predominantly or entirely at serotonin 5-HT2 sites. In the striatum, 125I-LSD labels approximately equal proportions of serotonergic and dopaminergic sites. This data indicates that 125I-LSD labels serotonin receptors in vivo and suggests that appropriate derivatives of 2I-LSD may prove useful for tomographic imaging of serotonin 5-HT2 receptors in the mammalian cortex.     

The effects of acute ethanol exposure and ageing on rat brain glutathione metabolism

Binge alcohol consumption in adolescents is increasing, and it has been proposed that immature brain deals poorly with oxidative stress. The aim of our work was to study the effect of an acute dose of ethanol on glutathione (GSH) metabolism in frontal cortex, hippocampus and striatum of juvenile and adult rats. We have observed no change in levels of glutathione produced by acute alcohol in the three brain areas studied of juvenile and adult rats. Only in the frontal cortex the ratio of GSH/GSSG was increased in the ethanol-treated adult rats. GSH levels in the hippocampus and striatum were significantly higher in adult animals compared to young ones. Higher glutathione peroxidase (GPx) activity in adult rats was observed in frontal cortex and in striatum. Our data show an increased GSH concentration and GPx activity in different cerebral regions of the adult rat, compared to the young ones, suggesting that age-related variations of total antioxidant defences in brain may predispose young brain structures to ethanol-induced, oxidative stress-mediated tissue damage.     

The effects of acute ethanol exposure and ageing on rat brain glutathione metabolism

Abstract

Binge alcohol consumption in adolescents is increasing, and it has been proposed that immature brain deals poorly with oxidative stress. The aim of our work was to study the effect of an acute dose of ethanol on glutathione (GSH) metabolism in frontal cortex, hippocampus and striatum of juvenile and adult rats. We have observed no change in levels of glutathione produced by acute alcohol in the three brain areas studied of juvenile and adult rats. Only in the frontal cortex the ratio of GSH/GSSG was increased in the ethanol-treated adult rats. GSH levels in the hippocampus and striatum were significantly higher in adult animals compared to young ones. Higher glutathione peroxidase (GPx) activity in adult rats was observed in frontal cortex and in striatum. Our data show an increased GSH concentration and GPx activity in different cerebral regions of the adult rat, compared to the young ones, suggesting that age-related variations of total antioxidant defences in brain may predispose young brain structures to ethanol-induced, oxidative stress-mediated tissue damage.     

Characterization of N-[3H]Methylcarbamylcholine Binding Sites and Effect of N-Methylcarbamylcholine on Acetylcholine Release in Rat Brain

The present experiments show that N-[3H]-methylcarbamylcholine ([3H]MCC) binds specifically and with high affinity to rat hippocampus, frontal cortex, and striatum. The highest maximal density of binding sites was apparent in frontal cortex and the lowest in hippocampus. [3H]MCC binding was potently inhibited by nicotinic, but not muscarinic, agonists and by the nicotinic antagonist dihydro-beta-erythroidine in all three brain regions studied. The effect of unlabeled MCC on acetylcholine (ACh) release from slices of rat brain was tested. The drug significantly enhanced spontaneous ACh release from slices of hippocampus and frontal cortex, but not from striatal slices. This effect of MCC to increase ACh release from rat hippocampus and frontal cortex was antagonized by the nicotinic antagonists dihydro-beta-erythroidine and d-tubocurarine, but not by alpha-bungarotoxin or by the muscarinic antagonist atropine. The MCC-induced increase in spontaneous ACh release from hippocampal and frontal cortical slices was not affected by tetrodotoxin. The results suggest that MCC might alter cholinergic transmission in rat brain by a direct activation of presynaptic nicotinic receptors on the cholinergic terminals. That this alteration of ACh release is apparent in hippocampus and frontal cortex, but not in striatum, suggests that there may be a regional specificity in the regulation of ACh by nicotinic receptors in rat brain.     

Agonist efficacy and receptor efficiency in heterozygous CB1 knockout mice: relationship of reduced CB1 receptor density to G-protein activation

Heterozygous CB1 receptor knockout mice were used to examine the effect of reduced CB1 receptor density on G-protein activation in membranes prepared from four brain regions: cerebellum, hippocampus, striatum/globus pallidus (striatum/GP) and cingulate cortex. Results showed that CB1 receptor levels were approximately 50% lower in heterozygous mice in all regions examined. However, maximal stimulation of [(35)S]guanosine-5'-(gamma-O-thio) triphosphate ([(35)S]GTPgammaS) binding by the high efficacy agonist WIN 55,212-2 was reduced by only 20-25% in most brain regions, with the exception of striatum/GP where the decrease in stimulation was as predicted (approximately 50%). Furthermore, although the efficacies of the cannabinoid partial agonists, methanandamide and (9)-tetrahydrocannabinol, were similarly lower in heterozygous mice, their relative efficacies compared with WIN 55,212-2 were generally unchanged. Saturation analysis of net WIN 55,212-2-stimulated [(35)S]GTPgammaS binding showed that decreased stimulation by WIN 55,212-2 in striatum/GP of heterozygous mice was caused by a decrease in the apparent affinity of net-stimulated [(35)S]GTPgammaS binding. The apparent maximal number of binding sites (B(max)) values of net WIN 55,212-2-stimulated [(35)S]GTPgammaS binding were unchanged in cerebellum and striatum/GP of heterozygous mice, but decreased in cingulate cortex, with a similar trend in hippocampus. Moreover, in every region except cingulate cortex, the maximal number of net-stimulated [(35)S]GTPgammaS binding sites per receptor was significantly increased in heterozygous mice. These results indicate region-dependent increases in the apparent efficiency of CB1 receptor-mediated G-protein activation in heterozygous CB1 knockout mice.     

Different kinetic properties of neuroleptic receptor binding in the rat striatum and frontal cortex.

³H-spiperone binding in the striatum is 80% stereospecific and 6 % non-stereospecific. In the frontal cortex stereospecific sites account for 60 % of the total binding and non-stereospecific sites for 25 %. Stereospecific sites are of different nature in both brain regions : dopaminergic in the striatum and serotonergic in the frontal cortex. The non-stereospecific sites are saturable and can only be detected by the use of appropriate blanks.Release experiments demonstrate the occurrence of positive coöperative effects in the dissociation of spiperone from striatal receptors while this is not detectable in the frontal cortex. In both brain regions a rapidly and a slowly dissociating component have been observed. In the frontal cortex the rapid component is ascribed to the dissociation of spiperone from the stereospecific sites and the slow component to the dissociation from non-stereospecific sites. The nature of these sites is yet to be identified.     

High affinity dopamine D2 receptor radioligands. 2. [125I]epidepride, a potent and specific radioligand for the characterization of striatal and extrastriatal dopamine D2 receptors.

Epidepride, (S)-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5-iodo-2,3-dimethoxybenzamide+ ++, the iodine analogue of isoremoxipride (FLB 457), was found to be a very potent dopamine D2 receptor antagonist. Optimal in vitro binding required incubation at 25 degrees C for 4 h at pH 7.4 in a buffer containing 120 mM NaCl, 5 mM KCl, 2 mM CaCl2 and 1 mM MgCl2. Scatchard analysis of in vitro binding to striatal, medial frontal cortical, hippocampal and cerebellar membranes revealed a KD of 24 pM in all regions, with Bmax's of 36.7, 1.04, 0.85, and 0.37 pmol/g tissue, respectively. The Hill coefficients ranged from 0.91-1.00 in all four regions. The IC50's for inhibition of [125I]epidepride binding to striatal, medial frontal cortical, and hippocampal membranes for SCH 23390, SKF 83566, serotonin, ketanserin, mianserin, naloxone, QNB, prasozin, clonidine, alprenolol, and norepinephrine ranged from 1 microM to greater than 10 microM. Partial displacement of [125I]epidepride by nanomolar concentrations of clonidine was noted in the frontal cortex and hippocampus, but not in the striatum. Scatchard analysis of epidepride binding to alpha 2 noradrenergic receptors in the frontal cortex and hippocampus revealed an apparent KD of 9 nM. At an epidepride concentration equal to the KD for the D2 receptor, i.e. 25 pM, no striatal alpha 2 binding was seen and only 7% of the specific epidepride binding in the cortex or hippocampus was due to binding at the alpha 2 site. Correlation of inhibition of [3H]spiperone and [125I]epidepride binding to striatal membranes by a variety of D2 ligands revealed a correlation coefficient of 0.99, indicating that epidepride labels a D2 site. In vitro autoradiography revealed high densities of receptor binding in layers V and VI of prefrontal and cingulate cortices as well as in striatum. In vivo rat brain uptake revealed a hippocampal:cerebellar and frontal cortical:cerebellar ratio of 2.2:1 which fell to 1.1:1 following haloperidol pretreatment. These properties suggest that [125I]epidepride is a superior radioligand for the in vitro and in vivo study of striatal and extrastriatal dopamine D2 receptors.     

Species Differences in the Brain Regional Distribution of Receptor Binding for Thyrotropin-Releasing Hormone

Abstract: A survey of the regional distribution of binding of 1 nM [³H](3-MeHis²)thyrotropin-releasing hormone ([³H]MeTRH) to TRH receptors in the brains of eight mammalian species revealed major species differences in both the absolute and relative values of TRH receptor binding in different brain regions. Several brain regions exhibited binding equal to or exceeding that in the anterior pituitary gland of the same species, including the amygdaia in the guinea pig and rat, the hypothalamus in the guinea pig, the nucleus accumbens in the rabbit, and all these and other regions in the cat and dog, for which pituitary binding was exceptionally low. Species could be divided into two groups according to which brain region appeared highest in binding: rabbits, sheep, and cattle had highest binding in the nucleus accumbens/septal area, whereas guinea pigs, rats, dogs, cats, and pigs had highest binding in the amygdala/temporal cortex area. The nucleus accumbens consistently exceeded the caudate-putamen in receptor binding. For most brain regions, rabbits, rodents, and sheep tended to be higher than carnivores, cattle, or pigs. Further regions that exhibited appreciable binding in most species included the olfactory bulb and tubercle, hippocampus, and various cortical and brain stem areas. In fact, essentially all brain regions appeared to have detectable levels of TRH receptors in at least some species, but no rat peripheral tissues have yet shown detectable receptor binding. The species differences appeared to reflect largely if not entirely differences in receptor density, although this was not tested in every species.     

Two kinds of mitogen-activated protein kinase phosphatases, MKP-1 and MKP-3, are differentially activated by acute and chronic methamphetamine treatment in the rat brain

Two functionally different MAP kinase phosphatases (MKPs) were investigated to clarify their roles in behavioral sensitization to methamphetamine (METH). MKP-1 mRNA levels increased substantially by about 60-300% in a range of brain regions, including several cortices, the striatum and thalamus 0.5-1 h after acute METH administration. After chronic METH administration its increase was less pronounced, but a more than 50% increase was still seen in the frontal cortex. MKP-1 protein levels also increased 3 h after acute or chronic METH administration. MKP-3 mRNA levels increased by about 30-50% in several cortices, the striatum and hippocampus 1 h after acute METH administration, but only in the hippocampus CA1 after chronic METH administration. Pre-treatment with the D(1) dopamine receptor antagonist, SCH23390, attenuated the METH-induced increase of MKP-1 and MKP-3 mRNA in every brain region, while pre-treatment with the NMDA receptor antagonist, MK-801, attenuated it in some regions. These findings suggest that in METH-induced sensitization, MKP-1 and MKP-3 play important roles in the neural plastic modification in widespread brain regions in the earlier induction process, but in the later maintenance process, they do so only in restricted brain regions such as MKP-1 in the frontal cortices and MKP-3 in the hippocampus.     

Age-related reduction of extrastriatal dopamine D2 receptor measured by PET

Although the aging effect of dopamine D2 receptor in the striatum is well-documented, the effect of age on the extrastriatal dopamine D2 receptor has not been fully examined. Since the density of extrastriatal dopamine D2 receptor is very low, suitable ligands are limited. In this study, we used [11C]FLB 457 to quantify the extrastriatal dopamine D2 receptor in the living human brain. Twenty-seven healthy male subjects aged from 21 to 82 years participated in the positron emission tomography study. Extrastriatal [11C]FLB 457 binding was quantified with a reference tissue model using cerebellum as a reference region. Binding potentials corresponding to Bmax/Kd were used to evaluate age-related change. We found age-related decreases of D2 receptor binding in all measured extrastriatal regions. The decrease of D2 receptor binding was 13.8% per decade in frontal cortex, 12.0% in temporal cortex, 13.4% in parietal cortex, 12.4% in occipital cortex, 12.2% in hippocampus, and 4.8% in thalamus. These findings suggest that the amounts of D2 receptor declines in all brain regions as part of the normal aging process.     

Decreased levels of free d-aspartic acid in the forebrain of serine racemase (Srr) knock-out mice

d-Serine, an endogenous co-agonist of the N-methyl-d-aspartate (NMDA) receptor is synthesized from l-serine by serine racemase (SRR). A previous study of Srr knockout (Srr-KO) mice showed that levels of d-serine in forebrain regions, such as frontal cortex, hippocampus, and striatum, but not cerebellum, of mutant mice are significantly lower than those of wild-type (WT) mice, suggesting that SRR is responsible for d-serine production in the forebrain. In this study, we attempted to determine whether SRR affects the level of other amino acids in brain tissue. We found that tissue levels of d-aspartic acid in the forebrains (frontal cortex, hippocampus and striatum) of Srr-KO mice were significantly lower than in WT mice, whereas levels of d-aspartic acid in the cerebellum were not altered. Levels of d-alanine, l-alanine, l-aspartic acid, taurine, asparagine, arginine, threonine, γ-amino butyric acid (GABA) and methionine, remained the same in frontal cortex, hippocampus, striatum and cerebellum of WT and mutant mice. Furthermore, no differences in d-aspartate oxidase (DDO) activity were detected in the forebrains of WT and Srr-KO mice. These results suggest that SRR and/or d-serine may be involved in the production of d-aspartic acid in mouse forebrains, although further detailed studies will be necessary to confirm this finding.     

Cholecystokinin receptor binding levels in the genetically obese rat brain

Cholecystokinin (CCK) receptor binding levels were compared between groups of genetically obese (fa/fa) and non-obese (Fa/-) Zucker rats of both sexes. The radioligand used was the iodinated octapeptide (CCK-8). Binding was measured in eight brain regions. The relative distribution among different brain regions of specifically bound CCK per mg protein was similar in all groups of animals. High binding levels were present in the olfactory bulb, cortex and caudate nucleus. Moderate levels were seen in hippocampus and hypothalamus, and low levels were observed in hindbrain, midbrain and thalamus. Obese animals of both sexes had significantly higher CCK receptor binding levels in the hippocampus and in the midbrain in comparison to lean controls. The male obese animals also had significantly elevated binding levels in the thalamic sample. These results demonstrate a correlation between genetic obesity and elevated CCK receptor binding levels in specific brain regions.     

n-3 polyunsaturated fatty acid supplementation reverses stress-induced modifications on brain monoamine levels in mice

The aim of this study was to examine the effects of supplementation with n-3 polyunsaturated fatty acids (PUFAs) on stress responses in mice subjected to an unpredictable chronic mild stress (UCMS) procedure. Stress-induced modifications in coat and aggressiveness were evaluated, and phospholipid PUFA profiles and monoamine levels were analyzed in the frontal cortex, hippocampus, and striatum. The results showed that repeated exposure to mild stressors induced degradation in the physical state of the coat, lowered body weight gain, and increased aggressiveness, without any effect of n-3 PUFA supplementation. The UCMS induced a significant decrease in the levels of norepinephrine in the frontal cortex and striatum, and a nonsignificant decrease in the hippocampus. The tissue levels of serotonin (5-HT) were 40% to 65% decreased in the three brain regions studied. Interestingly, the n-3 PUFA supplementation reversed this stress-induced reduction in 5-HT levels. These findings showed that supplementation in n-3 long-chain PUFAs might reverse certain effects of UCMS in cerebral structures involved in stress-related behaviors.     

Selective changes of receptor binding in brain regions of aged rats

Binding to several receptors was compared in brain regions of 3 and 21-23 month-old rats. In crude membrane preparations of aged rats the number of dopamine antagonist receptors in striatum was much reduced (-53%). beta-Noradrenergic receptors (cortex) and benzodiazepine receptors (hippocampus and cerebellum) were less but significantly reduced and serotonergic receptors, alpha 1 noradrenergic receptors (both in cortex) and dopamine agonist receptors (striatum) were unchanged. For each receptor binding the KD values were the same in young and old animals. GABA receptor binding (hippocampus and cerebellum) evaluated at only one 3H-GABA concentration (8 nM) was similar in both groups when expressed per protein content but significantly reduced in aged rats when expressed per tissue wet weight because of the partial purification of the synaptic membranes used for 3H-GABA binding. In our experimental conditions age-related changes of specific binding sites in the central nervous system were selective for some receptors studied and did not seem to be due to general non-specific modification of brain tissue composition.     

Autoradiography of CCK receptors in the rat brain using [H]Boc[Nle]CCK27–33 and [I]bolton-hunter CCK8. Functional significance of subregional distributions

[³H]Boc[Nle^28,31]CCK₂₇–₃₃ ([³H]BDNL-CCK₇) is a new ligand for cholecystokinin (CCK) receptors, endowed with a high specific activity (100 Ci/mmol). Binding sites for this ligand were visualized in the rat brain by autoradiography [³H]BDNL-CCK₇ binds specifically to an apparent single class of CCK receptors on rat striatum sections with a K_d of 1.76 nM and a B_max of 57 fmol/mg protein. Unsulfated CCK₈ was two times less potent than sulfated CCK₈ to displace binding of [³H]BDNL-CCK₇. Binding sites for [³H]BDNL-CCK₇ were present in many brain regions, the highest concentrations occurring in cortex, olfactory bulbs, nucleus accumbens, and medium to high concentrations in striatum, hippocampus, and several nuclei of thalamus, hypothalamus and amygdala. In the same experimental conditions, the binding sites for [¹²⁵I]BH-CCK₈ showed similar specificity and localization. We thus used both ligands to investigate the subregional distributions of CCK receptors in nucleus accumbens and hippocampus, where a highly organized topography of action of CCK has been reported. In nucleus accumbens, the CCK binding sites were concentrated in the anterior portion of the nucleus, whereas very low densities were observed within medial posterior nucleus accumbens, where injection of CCK has been shown to potentiate dopamine-induced hyperlocomotion. p]In hippocampus, CCK receptors were concentrated in the polymorphic zone of the hilus of the dentate gyrus and in stratum lacunosum moleculare of Ammon's horn. Very few receptors were observed in other regions of hippocampus, including stratum pyramidale and stratum moleculare. This is in contrast with the presence of numerous CCK terminals and the potent effect of CCK in these areas. The distributions of CCK receptors reported here in both nucleus accumbens and hippocampus were discussed in correlation with the distribution of CCK neurons and terminals, the related anatomical pathways, and the pharmacological profiles of the effects of CCK in these regions.