Somatostatin receptors in the senescent rat brain: a quantitative autoradiographic study.

To examine the effects of aging on the density and distribution of somatostatin receptors (SS-R) in the rat brain, receptor autoradiography for SS-R was carried out in rats aged 3 and 24 months using 125I-labeled Tyr11-SS-14. Autoradiograms were quantitatively assessed by an image analyzer to evaluate changes in the expression of SS-R due to senescence. Statistically significant decreases in SS-R binding were found in specific regions of the brains of senescent rats as compared to young adult rats. The regions affected included the periaqueductal gray matter (73% loss versus young adult rats), the interpeduncular nucleus (73% loss), the pontine nucleus (63% loss), the superior colliculus (46% loss), the ventral tegmental area (46% loss), the temporal cortex (39% loss), the frontal cortex (34% loss), the hippocampus (33% loss), the amygdala (27% loss) and the claustrum (26% loss). There was no significant change in SS-R expression in the spinal cord with aging. Significant reductions in SS-R binding in these brain regions may be involved in the impairment of sensory and cognitive function that can occur with aging.     

Age dependence of somatostatin levels and somatostatin binding in the rat brain

1. Somatostatin-like immunoreactivity (SLI) and 125I-Tyrl-somatostatin binding were measured from the brains of rats aged 1, 8 and 18 months. 2. Somatostatin binding was reduced in the striatum, frontal cortex, hypothalamus and hippocampus of the 8-month-old rats compared to the 1-month-old group. 3. Somatostatin binding was reduced in the striatum, frontal cortex and hippocampus of the 18-month-old rats compared to the 1-month-old group. 4. The reduction (40%) was most striking in the frontal cortex. 5. In no area of the brain did changes in SLI differ significantly between the different age groups.     

Brain amines and neocortical EEG in young and aged rats

1. Frontal and parieto-occipital electroencephalography (EEG) of young (4 months-old) and aged (17 and 22 months-old) Wistar rats were analyzed, both during movement and during waking immobility. 2. The levels of monoamines, serotonin and their metabolites were measured from the frontal cortex, parieto-occipital cortex, hippocampus, brainstem and midbrain. 3. In aged rats, as compared to young rats, the most apparent changes of the quantitative EEG spectrum were the decreased amplitude of alpha (5-10 Hz) and beta (10-20 Hz) frequency bands in the frontal and parieto-occipital cortices during both movement and waking immobility behavior (p less than 0.05). 4. The levels of dopamine (DA), homovanillinic acid (HVA), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) or the ratios of 5-HT/5-HIAA and DA/HVA did not differ between young and aged rats in any brain region studied, with the exceptions of brainstem DA and parieto-occipital 5-HIAA, which were elevated in aged rats (p less than 0.05). 5. In the frontal cortex, hippocampus and midbrain, noradrenaline (NA) levels of aged rats were slightly increased as compared to young rats (p less than 0.05). 6. NA levels of the parieto-occipital cortex and brainstem did not change during aging. 7. Furthermore, there were no clear correlations between the decreased amplitude of the quantitative EEG spectrum and monoamine or serotonin concentrations, or the ratios of 5-HT/5-HIAA and DA/HVA in the cerebral cortex of aging Wistar rat.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reduced muscarinic receptor plasticity in frontal cortex of aged rats after chronic administration of cholinergic drugs

Age-related differences in muscarinic receptor plasticity were observed in young, adult and senescent Fischer 344 rats (3, 9 and 27 months old, respectively) following the chronic, intracerebroventricular (ivt) administration of a cholinergic agonist, oxotremorine, or antagonist, methylatropine. After three weeks treatment of young rats with ivt oxotremorine, the maximum number (Bmax) of 3H-QNB binding sites in frontal cortex, determined by saturation experiments, was reduced by 27%, with no apparent change in the affinity (Kd) of 3H-QNB for the muscarinic receptor. Conversely, chronic ivt methylatropine administered to 3 month old animals caused a 29% increase in Bmax with no significant change in Kd. Adult animals showed a somewhat lesser degree of muscarinic receptor plasticity (16% down-regulation after oxotremorine, 22% up-regulation after methylatropine). However, 3H-QNB binding parameters in frontal cortex from senescent rats were not significantly altered following identical treatments with oxotremorine or methylatropine. Thus, muscarinic receptor adaptation to chronic, cholinergic drug administration was impaired in aged animals. This reduced receptor plasticity with aging could have important implications for the long-term drug treatment of elderly patients and for the therapeutic efficacy of cholinergic drugs in age-related neurological disorders, such as Alzheimer's disease.     

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.     

Age-related changes in receptor-mediated phosphoinositide hydrolysis in various regions of rat brain.

The effects of age on cholinergic markers and receptor-stimulated phosphoinositide hydrolysis was examined in the frontal cortex and striatum of male Fischer-344 rats. Choline acetyltransferase activity was decreased 27% in the striatum of aged (24 month) rats compared to young (3 month) controls. Muscarinic receptor density as measured by [3H]-quinuclidinyl benzilate binding showed a similar 26% decrease in the striatum of aged rats. Phosphoinositide hydrolysis was measured by the release of inositol phosphate (IP) from tissue slices prelabeled with [3H]myoinositol in response to carbachol, norepinephrine, and quisqualate. In the cortex, stimulated IP release was significantly greater in slices from aged rats compared to young rats for all three agonists. In contrast, stimulated IP release was significantly decreased in striatal slices from aged rats compared to young for all three agonists. These data indicate a differential effect of age on agonist-stimulated phosphoinositide hydrolysis in the cortex and striatum. The decreased responsiveness in the latter area may result from the age-related loss of postsynaptic receptors.     

Abnormality of α1Adrenergic Receptors in the Frontal Cortex of Epileptic Rats

Abstract: We found that the binding of [³H]prazosin, a selective ligand for α₁-adrenergic recognition sites, is significantly lower in the frontal cortex of the genetically epilepsy-prone rats (GEPRs), as compared with normal Sprague-Dawley rats. Scatchard analysis reveals a decrease in the B _max of [³H]prazosin binding with no change in the apparent K _D, suggesting that there are fewer α₁-adrenergic recognition sites in the frontal cortex of the GEPR. This abnormality is associated with a reduced capacity of norepinephrine (NE) to stimulate [³H]inositol monophosphate ([³H]IP₁) formation in frontal cortex slices prelabeled with [³H]inositol. No significant differences in [³H]prazosin binding as well as NE-stimulated [³H]IP₁ formation have been observed in other brain regions including hippocampus, corpus striatum, and inferior colliculus. These results indicate that a deficit in the α₁-adrenergic receptor system in the frontal cortex may play a role in the seizure process in the GEPR.     

The brain 5-HT4 receptor binding is down-regulated in the Flinders Sensitive Line depression model and in response to paroxetine administration

The 5-hydroxytryptamine (5-HT₄) receptor may be implicated in depression and is a new potential target for antidepressant treatment. We have investigated the brain 5-HT₄ receptor [³H]SB207145 binding in the Flinders Sensitive Line rat depression model by quantitative receptor autoradiography, and related this to 5-HT transporter ( S )-[ N -methyl-³H]citalopram binding. We also determined the regulation of 5-HT₄ receptor binding by 1, 14, and 21 days of paroxetine administration and subchronic 5-HT depletion, and compared this with changes in 5-HT_2A receptor [³H]MDL100907 binding. In the Flinders Sensitive Line, the 5-HT₄ receptor and 5-HT transporter binding were decreased in the dorsal and ventral hippocampus, and the changes in binding were directly correlated within the dorsal hippocampus. Chronic but not acute paroxetine administration caused a 16–47% down-regulation of 5-HT₄ receptor binding in all regions evaluated including the basal ganglia and hippocampus, while 5-HT depletion increased the 5-HT₄ receptor binding in the dorsal hippocampus, hypothalamus, and lateral globus pallidus. In comparison, the 5-HT_2A receptor binding was decreased in the frontal and cingulate cortices after chronic paroxetine administration, and markedly reduced in several regions after 5-HT depletion. Thus, the 5-HT₄ receptor binding was decreased in the Flinders Sensitive Line depression model and in response to chronic paroxetine administration.     

Quantitative analysis of capillary and neuron images under normal conditions and in dementia

Stereological investigations (Leitz-Classimat) of the capillary net of young (19-44 yrs), old (85-95 yrs) and age-matched demented patients with Alzheimer's disease show a condensation (40%) of the capillary volume in the cerebral cortex of the Alzheimer group (n-3) compared with the age-matched controls (n-7), without change of the capillary diameter. These results represent gross atrophy of the frontal brain in senile dementia of Alzheimer type (SDAT). No changes of this kind can be observed between young individuals (n-6) and normal aged group. The behaviour of the capillary net in the putamen is different from that of the cortex. Already during normal aging a 80% condensation of the capillary volume is observed (capillary volume and length per unit increase, intercapillary distances decrease). A comparison between the aged group and the Alzheimer patients exhibits neither additional alterations of capillary parameters nor decreased volume of the putamen. In all anatomical layers of the frontal cortex a significant atrophy (27-36%) of neuronal perikarya (size and shape measurements with the TAS of Leitz) occurs in the Alzheimer group, compared with the normal aged ones. In the same way, neuronal surface area decreases by 30% in the putamen. Significant changes of perikaryal shape in both brain regions confirm marked neuronal atrophy in Alzheimer's disease. During normal aging only 85-95 years old group shows significantly smaller (15-35%) neurons in comparison to young individuals. Quantitative image analysis facilitates considerably evaluating new morphometrical data of the aging process in the human brain, which are important for a pharmacological concept of treating cerebral insufficiency symptoms.     

Effect of Chronic Manganese Treatment on Adenosine Tissue Levels and Adenosine A2a Receptor Binding in Diverse Regions of Mouse Brain

In the present study the effects of chronic manganese (Mn) treatment on adenosine A_2a receptor binding in mouse brain have been assessed. Male albino mice were divided in two groups: In the Mn-treated group, the animals were injected intraperitoneally (i.p.) with MnCl₂ (5 mg/kg/day) five days per week during 9 weeks; in the control group, they were injected likewise with a saline solution. A significant decrease of the Kd without alteration of Bmax in the cerebellum and, an increase of the Kd and Bmax in hippocampus of mice treated with Mn were found. Also, an increase of Kd in frontal cortex was observed. The binding parameters in caudate nucleus, olfactory bulb and hypothalamus were not altered by Mn. A significant decrease in the adenosine concentration in caudate nucleus, olfactory bulb and hypothalamus, without significant changes in hippocampus, frontal cortex and cerebellum was also detected. These findings suggest that chronic administration of Mn could affect adenosine receptor function and turnover, depending on the brain region analyzed.     

Serotonin Receptor Changes in Dementia of the Alzheimer Type

Serotonin receptors were assessed in post-mortem brains of control and Alzheimer-type dementia (ATD) patients using ligand binding techniques. Differential losses of serotonin S1 and S2 receptors were present in neocortex, hippocampus, and amygdala of ATD patients, whereas no significant changes were observed in basal forebrain and basal ganglia. Losses of S1 receptors were significantly age-related in the ATD group, suggesting they occurred at a later stage of the disease process. Losses of S2 receptors were considerably greater (with a reduction to 35% of control in temporal cortex) and were not age-related in ATD. Significant correlations were observed within the ATD group between S2 receptor binding and somatostatin immunoreactivity in temporal and frontal cortices. Thus the loss of S2 receptors in ATD may be a relatively early change in the disease process, and may precede the changes in ascending serotonergic neurones.     

Decreased density of [3H]TCP binding following antipsychotic drug withdrawal in rats

Antipsychotic drugs have been reported to increase the expression of subunits of the NMDA receptor at the level of mRNA but it is not clear whether such effects are apparent at the level of the radioligand binding or receptor protein. Therefore, we examined the effect of treatment of, and withdrawal from, haloperidol, chlorpromazine, olanzapine or clozapine on the binding of [3H]N-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP ) to the open ion channel of the NMDA receptor in rat caudate-putamen, hippocampus and frontal cortex. [3H]TCP binding was not significantly different in the caudate-putamen, hippocampus and cortex after three months of treatment with any antipsychotic drug. There were significant decreases in [3H]TCP binding in rat caudate-putamen and cortex, but not hippocampus, one month after ceasing treatment. Decreases in the caudate-putamen were detected in rats previously treated with chlorpromazine (0.1 mg/kg/day) and clozapine (0.1 and 1.0 mg/kg/day). In the cortex, decreases in [3H]TCP binding were also detected in rats previously treated with olanzapine (0.1 mg/kg/day) for three months. These data suggest that changes in the NMDA receptor associated ion channels occur following antipsychotic drug withdrawal.     

Distinct Changes in Peptide YY Binding to, and mRNA Levels of, Y1 and Y2 Receptors in the Rat Hippocampus Associated with Kindling Epileptogenesis

Abstract: Electrical kindling of the rat dorsal hippocampus induced significant changes in the binding of ¹²⁵I-peptide YY to Y1 and Y2 subtypes of neuropeptide Y receptors and in their mRNA levels in the area dentata as assessed by quantitative receptor autoradiography and in situ hybridization histochemistry. Binding to Y1 receptor sites decreased by 50% ( p < 0.05) in the molecular layer of the stimulated dentate gyrus, 2 days after preconvulsive stage 2 and 1 week or 1 month after generalized stage 5 seizures compared with sham-stimulated rats. Binding to Y2 receptor sites increased bilaterally by 36–87% ( p < 0.05) in the hilus at stage 2 and 1 week or 1 month after stage 5. No significant changes were observed after one afterdischarge or in the other hippocampal subfields or in the cortex. Y1 receptor mRNA signal decreased bilaterally by 50–64% ( p < 0.01) in the granule cell layer, 6 h but not 24 h after stages 2 and 5. The Y2 receptor mRNA signal was enhanced by 283% ( p < 0.01) in the stimulated granule cell layer 24 h after stage 2. At 6 and 24 h after stage 5, mRNA levels were increased both ipsilaterally (283 and 360%, respectively; p < 0.01) and contralaterally (190 and 260%, respectively; p < 0.05). No significant changes in level of either mRNA was found following one afterdischarge. These modifications, and the enhanced neuropeptide Y release previously shown in the hippocampus, suggest that kindling is associated with lasting changes in neuropeptide Y-mediated neurotransmission.     

Characterization of sulpiride-displaceable 3H-YM-09151-2 binding sites in rat frontal cortex and the effects of subchronic treatment with haloperidol on cortical D-2 dopamine receptors

We investigated the pharmacological properties of the sulpiride-displaceable binding sites labeled by 3H-YM-09151-2 in rat frontal cortex, compared to those in striatum. The IC50 value of ketanserin was 486 nM, which was apparently different from its affinity for the 5HT-2 receptor. Various dopamine antagonists showed almost the same inhibitory effects for binding site in frontal cortex and striatum. Sulpiride-displaceable 3H-YM-09151-2 binding sites were considered to be D-2 dopamine receptors. After subchronic treatment with haloperidol, the D-2 receptor density of frontal cortex (0.55 fmol/mg tissue) increased to the same extent (about 25%) as striatum without significant change in apparent affinity.     

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.     

Effects of chronic ethanol treatment and aging on brain phosphoinositide turnover and adenylate cyclase activity

Inositol phosphate accumulation and adenylate cyclase activity were investigated in the cortex of young and aged ethanol-treated rats. Three months of ethanol treatment of young rats decreased maximal stimulation of inositol phosphate accumulation by carbachol by 26%, from 494 ± 76% of basal turnover in control animals to 396 ± 54% in ethanol-treated animals (mean ± SD). In aged rats ethanol-related changes were no longer observed but age-related changes were evident. EC₅₀ was significantly higher than in young animals and maximal stimulation was significantly lower. Basal adenylate cyclase activity in cortical membranes of all groups of animals was not different. Forskolin-stimulated adenylate cyclase activity was not affected by ethanol treatment, but was higher in aged animals. The activity of forskolin-stimulated adenylate cyclase in the presence of carbachol was higher in both young and aged ethanol-treated animals, when compared to young controls. These results suggest that both ethanol and aging impair the efficiency of receptor/effector coupling.     

Decreased Insulin-Like Growth Factor-I and Its Receptor Expression in the Hippocampus and Somatosensory Cortex of the Aged Mouse

Insulin-like growth factor-I (IGF-I) is a multifunctional polypeptide and has diverse effects on brain functions. In the present study, we compared IGF-I and IGF-I receptor (IGF-IR) immunoreactivity and their protein levels between the adult (postnatal month 6) and aged (postnatal month 24) mouse hippocampus and somatosensory cortex. In the adult hippocampus, IGF-I immunoreactivity was easily observed in the pyramidal cells of the stratum pyramidale in the hippocampus proper and in the granule cells of the granule cell layer of the dentate gyrus. In the adult somatosensory cortex, IGF-I immunoreactivity was easily found in the pyramidal cells of layer V. In the aged groups, IGF-I expression was dramatically decreased in the cells. Like the change of IGF-I immunoreactivity, IGF-IR immunoreactivity in the pyramidal and granule cells of the hippocampus and in the pyramidal cells of the somatosensory cortex was also markedly decreased in the aged group. In addition, both IGF-I and IGF-IR protein levels were significantly decreased in the aged hippocampus and somatosensory cortex. These results indicate that the apparent decrease of IGF-I and IGF-IR expression in the aged mouse hippocampus and somatosensory cortex may be related to age-related changes in the aged brain.     

Effect of p-chloroamphetamine on 5-HT1A and 5-HT7 serotonin receptor expression in rat brain

The aim of this study was to investigate if p-chloroamphetamine (PCA), which is neurotoxic to serotonin (5-HT) nerve terminals, was able to induce, like 3,4-methylenedioxymethamphetamine, a region-specific regulation of 5-HT1A receptor mRNA expression. The effect of PCA on the expression of 5-HT7 receptors, which share some pharmacological properties with 5-HT1A receptors, was comparatively studied. PCA (2 x 5 mg/kg) produced a lasting depletion of 5-HT content in the rat frontal cortex and hippocampus. In the hippocampus, the maximal 5-HT depletion was found on day 21 (-70%), whereas in the cortex, the highest 5-HT depletion was found on day 14 (-73%), with a partial but significant recovery on day 21. At the latter time point, 5-HT1A receptor mRNA expression was increased by 80% in the cortex and decreased by 50% in the hippocampus. The 5-HT1A receptor mRNA expression was also enhanced after exposure to PCA of rat cortical but not of hippocampal primary cultures. In regard to 5-HT7 receptor mRNA expression, the most remarkable change after PCA was the great increase (+200%) in the brain-stem. Binding studies to 5-HT1A receptors matched the changes in receptor mRNA expression. Gel shift assays revealed enhanced nuclear protein binding to the KB sequence with use of cortical but not hippocampal extracts of PCA-treated rats. Overall, the data show region-specific changes in 5-HT receptor-type expression that may not be entirely dependent on the neurotoxic effect of PCA on 5-HT terminals.     

Effect of age and monosodium-L-glutamate (MSG) treatment on neurotransmitter content in brain regions from male fischer-344 rats

Peripheral administration of monosodium-L-glutamate (MSG) has been found to be neurotoxic in neonatal rats. When administered in an acute, subconvulsive dose (500 mg/kg i.p.), MSG altered neurotrnnsmitter content in discrete brain regions of adult (6 month old) and aged (24 month old) male Fischer-344 rats. Norepinephrine (NE) content was reduced in both the hypothalamus (16%) and cerebellum (11%) of adult rats, but was increased in both the hypothalamus (7%) and cerebellum (14%) of aged rats after MSG treatment. MSG also altered the dopamine content in adult rats in both the posterior cortex and the striatum, causing a reduction (23%) and an increase (12%), respectively. Glycine content in the midbrain of aged rats increased (21%) after MSG injection. Of particular interest is the widespread monoamine and amino acid deficits found in the aged rats in many of the brain regions examined. NE content was decreased (11%) in the cerebellum of aged rats. Dopamine content was reduced in both the posterior cortex (35%) and striatum (10%) of aged rats compared to adult animals. Cortical serotonergic deficits were present in aged rats with reductions in both the frontal (13%) and posterior cortex (21%). Aged rats also displayed deficits in amino acids, particularly the excitatory amino acids. There were glutamate deficits (9–18% reductions) in the cortical regions (posterior and frontal) as well as midbrain and brain stem. Aspartate, the other excitatory amino acid transmitter, was reduced 10% in the brainstem of aged rats. These data indicate that an acute, subconvulsive, dose of MSG may elicit neurochemical changes in both adult and aged male Fisher-344 rats, and that there are inherent age-related deficits in particular neurotransmitters in aged male Fisher-344 rats as indicated by the reductions in both monoamines and amino acids.