Binding of the Novel Serotonin Agonist 8-Hydroxy-2-(Di-n-Propylamino) Tetralin in Normal and Alzheimer Brain

Binding of [3H]8-hydroxy-2-(di-n-propylamino) tetralin, a putative ligand for the 5-hydroxytryptamine (5-HT, serotonin) 1A recognition site, was measured in neocortex from postmortem human brain. The substance was found to bind to a saturable site with a KD value and pharmacological profile similar to that of rat. Binding to membranes from normal human temporal cortex was found to significantly correlate (inversely) with age. A significant reduction in binding, reflecting decreased density of recognition sites, was observed in the frontal cortex of patients with Alzheimer's disease (48% loss). This region in the dement brains showed unaltered presynaptic 5-HT function (5-HT and 5-hydroxyindoleacetic acid content) whereas 5-HT concentration was reduced in the temporal cortex.     

Brain Indoleamines in Alloxan- and Streptozotocin-Induced Diabetic Rats

Previous work by other authors has shown that alloxan-induced diabetes increases whereas streptozotocin-induced diabetes does not alter nonesterified fatty acid (NEFA) plasma levels. The present study replicates these results and demonstrates that fasted, streptozotocin-induced diabetic animals also have increased NEFA levels. In addition, brain levels of 5-hydroxytryptamine (5-HT) and of its immediate precursor and metabolite were measured. Alloxan- and fasted, streptozotocin-induced diabetic rats showed significant increases in brain indoleamine concentrations, whereas fed, streptozotocin-induced diabetic rats had unchanged levels of the same compounds. Levels of brain indoleamines exhibited a strong positive correlation with wet-dog shakes (an index of 5-HT activity) elicited by hippocampal stimulation. Blockade of wet-dog shakes by 5-HT receptor antagonists strengthens the proposal that this behavior is a good index of central 5-HT activity. The increased content of brain indoleamines in alloxan- and fasted, streptozotocin-induced diabetic rats may be related to the increased NEFA plasma levels seen in the same animals. This hypothesis is supported by the positive correlation demonstrated between NEFA and 5-HT levels. In conclusion, it is suggested that alloxan-induced diabetes may represent a useful model for studying the various behavioral changes known to occur in diabetics.     

Effects of Insulin and Streptozotocin-Induced Diabetes on Brain Norepinephrine Metabolism in Rats

Administration of insulin (2 IU/kg, i.p.) produced a significant decrease (18%) in forebrain norepinephrine and a significant increase in the major metabolite of norepinephrine, 3-methoxy-4-hydroxyphenylglycol-sulfate (MOPEG-SO4, +19%) in rats. Streptozotocin-induced diabetes produced the opposite effects, resulting in an increase in forebrain norepinephrine (+17%) and a decrease in MOPEG-SO4 (-26%). In addition, insulin increased (+143%) and diabetes decreased (-41%) the turnover rate of norepinephrine, as measured by the rate of decrease of norepinephrine following inhibition of tyrosine hydroxylase by alpha-methyl-p-tyrosine. All of these effects in diabetic rats were reversed by insulin replacement therapy. These data are discussed within the context of mood disorders characteristic of diabetic patients.     

Clinical chemistry of serotonin and metabolites

Analyses of serotonin and other 5-hydroxyindoles, such as its precursor 5-hydroxytryptophan and major metabolite 5-hydroxyindoleacetic acid (5-HIAA), are indispensable for the elucidation of their (patho)physiological roles. In clinical chemistry attention is mainly focused on the diagnosis and follow-up of carcinoid tumours. For this most laboratories routinely measure urinary 5-HIAA. More recently, measurements of serotonin in platelets and urine have been advocated. Platelet serotonin may be the most sensitive indole marker for the detection of carcinoid tumours that secrete only small amounts of serotonin and/or its precursor 5-hydroxytryptophan. Although several chromatographic techniques have emerged for the analysis of tryptophan-related indoles, HPLC with either electrochemical or fluorometric detection have become the methods of choice for their quantification. HPLC-based methods combine selectivity, sensitivity and high precision, and enable the simultaneous investigation of several metabolically related indoles. This review aims to place the analysis of indoles in biological matrices in a biochemical, physiological and clinical perspective and highlights several important steps in their chromatographic analysis and quantification.     

A Comparison of Biochemical Indices of 5-Hydroxytryptaminergic Neuronal Activity Following Electrical Stimulation of the Dorsal Raphe Nucleus

The activity of 5-hydroxytryptaminergic neurons has been estimated from measurements of: concentrations of 5-hydroxyindoleacetic acid; the ratio of the concentrations of 5-hydroxyindoleacetic acid to 5-hydroxytryptamine; the rate of accumulation of 5-hydroxytryptophan following the administration of an aromatic L-amino acid decarboxylase inhibitor (e.g., NSD 1015); the rate of accumulation of 5-hydroxytryptamine, and the rate of decline of 5-hydroxyindoleacetic acid following the administration of a monoamine oxidase inhibitor (e.g., pargyline). The purpose of the present study was to compare these different methods under conditions of changing neuronal impulse traffic produced by electrical stimulation of 5-hydroxytryptaminergic neurons. Male rats anesthetized with chloral hydrate were killed following 0, 15, or 30 min of electrical stimulation of the dorsal raphe nucleus at a frequency of 0, 5, or 10 Hz. The concentrations of 5-hydroxytryptamine, 5-hydroxyindoleacetic acid, and 5-hydroxytryptophan in nucleus accumbens, amygdala, suprachiasmatic nucleus, and dorsomedial nucleus were measured using HPLC coupled to an electrochemical detector. In each brain region, stimulation elicited an increase in the concentration of 5-hydroxyindoleacetic acid and the 5-hydroxyindoleacetic acid/5-hydroxytryptamine concentration ratio in saline-treated animals and an increase in 5-hydroxytryptophan accumulation in NSD 1015-treated animals, but did not alter the concentration of 5-hydroxytryptamine or 5-hydroxyindoleacetic acid in pargyline-treated rats. The results o f this study indicate that although the first three methods serve as valid indices of 5-hydroxytryptaminergic neuronal activity, the pargyline-dependent techniques are not responsive to changes in the rate of 5-hydroxytryptamine nerve firing.     

Subdivision of Mouse Brain [3H]Imipramine Binding Based on Ion Dependence and Serotonin Sensitivity

The specific binding of [3H]imipramine to mouse brain membranes in an assay containing 120 mM NaCl and 5 mM KCl was similar in regional distribution and pharmacological specificity to that reported previously in rat and human brain. However, the absence of ions decreased the density of the specific binding of [3H]imipramine and did not affect the equilibrium dissociation constant. Sodium was the only cation, and halides were the only anions tested that enhanced the specific binding of [3H]imipramine. Chloride did not increase the density of binding in the absence of sodium. The ion-sensitive binding of [3H]imipramine was regionally dependent and was highly correlated with the uptake of 5-hydroxytryptamine (5-HT, serotonin) into synaptosomes from brain regions. 5-HT did not inhibit the binding of [3H]imipramine in the absence of ions. Antidepressants inhibited binding in the absence and presence of ions, but in the presence of ions inhibition curves were shifted to the left and the apparent complexity of inhibition was increased. Quantitative analysis of the inhibition of [3H]imipramine binding by antidepressants conducted in the presence of ions was consistent with two binding sites. Lesion of the serotonergic input to the cerebral cortex by 5,7-dihydroxytryptamine suggested that both the 5-HT-sensitive and ion-sensitive binding of [3H]imipramine were associated with serotonergic nerve terminals. [3H]Imipramine binding displaced by desipramine, but insensitive to 5-HT and ions, was not affected by the lesion. Thus, the binding of [3H]imipramine that is displaced by desipramine, the most common assay for [3H]imipramine binding, includes a component that is not associated with brain serotonergic nerve terminals and 5-HT uptake, and, in addition, a separable component that is highly correlated with serotonergic function. These data have important implications for studies of serotonergic neurons and for the interpretation of imipramine binding data.     

Postmortem and Regional Changes of Serotonin, 5-Hydroxyindoleacetic Acid, and Tryptophan in Brain

Using a specific and sensitive high pressure liquid chromatographic technique for the measurement of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), and tryptophan (TRP), we found that there were no changes in 5-HT or 5-HIAA in the rat cortex when left in situ for 6 h at room temperature or 24 h at 4 degrees C. Only a minimal 14% increase in 5-HT was observed after 24 h at 4 degrees C in the striatum of the same animals. Concentrations of TRP, however, were increased significantly in both brain regions by these postmortem delay procedures. A second study revealed that there were significant regional 5-HT and 5-HIAA concentration differences within the cerebral cortex. The frontal cortex was shown to have the highest concentrations of 5-HT and 5-HIAA. Further, within the frontal cortex, 5-HIAA levels varied, showing apparent progressive rostral to caudal increases. 5-HT concentrations, however, remained constant within the frontal cortex. These results are discussed in reference to the conflicting reports of the previous human suicide and postmortem studies.     

Increased Extracellular Serotonin in Rat Brain After Systemic or Intraraphe Administration of Morphine

Abstract: The effect of morphine on serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in the CNS of unanesthetized rats was investigated by microdialysis. Morphine was administered either subcutaneously, by local perfusion into the diencephalon, or by intraraphe microinjection. Systemic administration of morphine resulted in a significant increase in both extracellular 5-HT and 5-HIAA in the diencephalon. The effect of morphine on 5-HT was dose dependent during local perfusion of the diencephalon with inhibitors of uptake or monoamine oxidase. Systemic morphine also produced significant increases in extracellular 5-HT in the striatum and hippocampus during uptake inhibition. The site of opioid effects on 5-HT was tested by locally perfusing morphine into the diencephalon. This had no effect on 5-HT or 5-HIAA. In contrast, intraraphe injection of morphine caused a dose-dependent increase in extracellular 5-HT and 5-HIAA in the diencephalon. These results suggest that systemic morphine induces an increase in 5-HT release in widespread areas of the forebrain. This appears to be due to an effect on 5-HT cell bodies and not on 5-HT nerve endings in projection sites.     

Exogenous Tryptophan Increases Synthesis, Storage, and Intraneuronal Metabolism of 5-Hydroxytryptamine in the Rat Hypothalamus

The effects of tryptophan administration on neurochemical estimates of synthesis [5-hydroxytryptophan (5-HTP) accumulation following administration of a decarboxylase inhibitor], storage [5-hydroxytryptamine (5-HT) concentrations], and metabolism [5-hydroxyindoleacetic acid (5-HIAA) concentrations] of 5-HT in selected regions of the hypothalamus were determined using HPLC coupled to an electrochemical detector. Tryptophan methyl ester HCl (30-300 mg/kg i.p.) produced a dose-dependent increase in the rate of 5-HTP accumulation throughout the hypothalamus but had no effect on the rate of accumulation of 3,4-dihydroxyphenylalanine. Peak 5-HTP levels were attained by 30 min following administration of tryptophan (100 mg/kg i.p.) and were maintained for an additional 60 min. Tryptophan also produced concomitant dose-dependent increases in 5-HT and 5-HIAA concentrations in these same regions without changes in the 5-HIAA/5-HT ratio. These results indicate that exogenous tryptophan administration selectively increases the synthesis, storage, and metabolism of 5-HT in the hypothalamus without altering the synthesis of catecholamines. Inhibition of 5-HT uptake with chlorimipramine or fluoxetine produced modest (10-40%) reductions in 5-HIAA concentrations throughout the hypothalamus, revealing that only a minor portion of 5-HIAA is derived from released and recaptured 5-HT, whereas the major portion of this metabolite reflects intraneuronal metabolism of unreleased 5-HT. In both chlorimipramine- and fluoxetine-treated rats, 5-HIAA concentrations were significantly increased by tryptophan administration, indicating that the increase in synthesis of 5-HT following precursor loading is accompanied by an increase in the intraneuronal metabolism of 5-HT.     

Fluorescence derivatizing procedure for 5-hydroxytryptamine and 5-hydroxyindoleacetic acid using 1,2-diphenylethylenediamine reagent and their sensitive liquid chromatographic determination

A pre-column derivatization method using a fluorogenic reagent, 1,2-diphenylethylenediamine (DPE) was studied for the sensitive HPLC determination of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA), which are biosubstances used in the diagnosis of several diseases. For the quantitative determination, the biogenic indole compounds were converted to their corresponding fluorescent derivatives with DPE in the presence of potassium hexacyanoferrate (III) at room temperature, and then the derivatives were separated by reversed-phase liquid chromatography with fluorescence detection. The chromatographic detection limits of the fluorescent peaks at a signal-to-noise ratio of 3 were 0.3 fmol for 5-HT and 0.2 fmol for 5-HIAA. The proposed method permits the simultaneous quantification of 5-HT and 5-HIAA at concentrations higher than 2.4 nM in human urine without a clean-up procedure.     

Dopaminergic Regulation of Cone Retinomotor Movement in Isolated Teleost Retinas: II. Modulation by γ-Aminobutyric Acid and Serotonin

In the accompanying paper we reported that 3,4-dihydroxyphenylethylamine (dopamine) induced light-adaptive retinomotor movements in teleost photoreceptors and that this effect was mediated by D2 dopamine receptors located on the photoreceptors themselves. In this study, we investigated the effects on cone retinomotor movement of three agents that have been reported by others to modulate retinal dopamine release: gamma-aminobutyric acid (GABA), 5-hydroxytryptamine (5-HT, serotonin), and melatonin. We report here that the GABA antagonists bicuculline and picrotoxin induced light-adaptive cone contraction in dark-adapted green sunfish retinas cultured in constant darkness; thus they mimic the effect of light or exogenously applied dopamine. Since their effects were blocked by either the D2 dopamine antagonist sulpiride or by Co2+, it seems likely that these agents act by enhancing retinal dopamine release. The GABA agonist muscimol produced effects opposite to those of GABA antagonists. Muscimol inhibited light-induced cone contraction in previously dark-adapted retinas and induced dark-adaptive cone elongation in light-adapted retinas. These results suggest that in green sunfish retinas, as has been reported for other retinas, GABA inhibits dopamine release. 5-HT induced light-adaptive cone contraction in dark-adapted retinas; thus 5-HT also mimics the effect of light or exogenously applied dopamine. The effect of 5-HT was blocked by sulpiride, Co2+, or the 5-HT antagonist mianserin. These results suggest that 5-HT induces cone contraction by stimulating dopamine release. Melatonin neither inhibited dopamine-induced cone contraction in retinas cultured in the dark nor induced cone elongation in retinas cultured in the light. Our results suggest that both GABA and 5-HT (but not melatonin) affect cone retinomotor movements in green sunfish by modulating dopamine release: GABA by inhibiting and 5-HT by stimulating dopamine release. We report in the companion paper that dopamine induced contraction in isolated cone fragments. Together these observations strongly suggest that dopamine serves as the final extracellular messenger directly inducing light-adaptive cone retinomotor movement, and that GABA and 5-HT affect these movements by modulating dopamine release.     

Effect of Toluene and Nutritional Status on Serotonin,Lipid Peroxidation Levels and NA<Superscript>+</Superscript>/K<Superscript>+</Superscript>-ATPase in Adult Rat Brain

The aim was to evaluate the effect of toluene and nutritional status on levels of serotonin (5-HT), 5-hydroxytryptophan (5-HTP), Na⁺/K⁺-ATPase, total ATPase and lipid peroxidation (TBARS) in rat brain. Study was conducted with malnourished (MN), well-nourished (WN) and normal Wistar rats. Three groups were formed for each nutritional status: control group I received 0.9% NaCl; toluene (1 g/kg) was administered to group II, and 1.5 g/kg to group III. Levels of 5-HT decreased (P < 0.05) in WN toluene groups, and 5-HTP decreased (P < 0.05) in the WN 1 g toluene and MN 1.5 g toluene groups. TBARS decreased (P < 0.05) in WN toluene groups. A trend to increase in Na⁺/K⁺-ATPase was found in WN and MN toluene groups, while total ATPase increased (P < 0.05) in the WN 1.5 g toluene group. The results suggest that high concentrations of toluene in single doses induce significant changes in the serotonergic system and alter membrane fluidity more perceptibly in the brain of adult animals with regular diet than in malnourished animals.     

MK-801 Increases Extracellular 5-Hydroxytryptamine in Rat Hippocampus and Striatum In Vivo

The effect of MK-801 (0.25 or 0.5 mg/kg) on the extracellular concentration of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in rat hippocampus and striatum was studied using intracerebral dialysis. The dialysate 5-HT concentration was dose-dependently increased by MK-801 in both regions. In the hippocampus, at the higher drug dose a slow increase in the 5-HIAA level was observed, and this became significant 3 h after treatment. In contrast to this, the extracellular 5-HIAA content in the striatum was significantly decreased 150 min after administration of both doses of MK-801. The data are discussed in the light of the known behavioural effects of MK-801 and possible N-methyl-D-aspartic acid receptor regulation of 5-HT release.     

Brain Dialysis: In Vivo Metabolism of Dopamine and Serotonin by Monoamine Oxidase A but Not B in the Striatum of Unrestrained Rats

A dialysis cannula was implanted into rat striatum while the animals were anesthetized, and the area was perfused with Ringer solution while the animals were unanesthetized after at least 3 days following surgery. Concentrations of the metabolites of 3,4-dihydroxyphenylethylamine (DA) and 5-hydroxytryptamine (5-HT) in the perfusate were determined by HPLC with electrochemical detection. Levels of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the perfusate significantly decreased after pargyline administration (50 mg/kg i.p.), which may inhibit not only monoamine oxidase (MAO)-B but also MAO-A in these high doses. The level of the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) also decreased after pargyline treatment, although change in the relative level of 5-HIAA was less than that of DOPAC or HVA. To clarify the mechanisms for the metabolism of monoamines in rat striatum, highly specific MAO-A and -B inhibitors were used in the following experiments. Treatment with l-deprenyl (10 mg/kg), a specific inhibitor for MAO-B, did not cause any statistically significant change in DOPAC, HVA, and 5-HIAA levels. No significant change was found in rat striatal homogenates at 2 h after the same treatment with l-deprenyl. In contrast, low-dose treatment (1 mg/kg) with clorgyline, a specific inhibitor for MAO-A, caused a significant decrease in levels of these three metabolites in both the perfusates and tissue homogenates. In addition to the above three metabolites, the level of 3-methoxytyramine, which is an indicator of the amount of DA released, greatly increased after treatment with a low dose (1 mg/kg) of clorgyline.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of Melatonin on Cognitive Ability of Rats and Expression of NCAM in the Brain Structures in Streptozotocin-Induced Diabetes

We studied a protective effect of a course injections of melatonin on cognitive deficiency in rats with streptozotocin-induced diabetes (STZD). The mean time necessary for the fulfillment of the Morris' water test in animals with STZD after 7 days of testing was three times greater than the corresponding index in the control group. Rats with STZD, which were injected with 10 mg/kg melatonin daily for 21 days after introduction of STZ, demonstrated a significantly lower level of cognitive deficiency ((in these rats the mean time necessary for the test fulfillment was only 48% greater than that in the control animals). In rats with STZD, substantial changes in the content of NCAM isoforms in the brain structures (significant decreases in the NCAM180 content in the hippocampus, neocortex, and cerebellum, and in that of NCAM140 in the cerebellum) were observed. Course injections of melatonin into the rats with STZD promoted significant normalization of the composition of NCAM isoforms in the structures under study. The data obtained indicate that control of expression of separate NCAM isoforms can be one of the mechanisms through which melatonin prevents the development of cognitive deficiency in diabetic animals.     

Preferential Potentiation of the Effects of Serotonin Uptake Inhibitors by 5-HT1A Receptor Antagonists in the Dorsal Raphe Pathway: Role of Somatodendritic Autoreceptors

Abstract: 5-HT_1A autoreceptor antagonists enhance the effects of antidepressants by preventing a negative feedback of serotonin (5-HT) at somatodendritic level. The maximal elevations of extracellular concentration of 5-HT (5-HT_ext) induced by the 5-HT uptake inhibitor paroxetine in forebrain were potentiated by the 5-HT_1A antagonist WAY-100635 (1 mg/kg s.c.) in a regionally dependent manner (striatum > frontal cortex > dorsal hippocampus). Paroxetine (3 mg/kg s.c.) decreased forebrain 5-HT_ext during local blockade of uptake. This reduction was greater in striatum and frontal cortex than in dorsal hippocampus and was counteracted by the local and systemic administration of WAY-100635. The perfusion of 50 µmol/L citalopram in the dorsal or median raphe nucleus reduced 5-HT_ext in frontal cortex or dorsal hippocampus to 40 and 65% of baseline, respectively. The reduction of cortical 5-HT_ext induced by perfusion of citalopram in midbrain raphe was fully reversed by WAY-100635 (1 mg/kg s.c.). Together, these data suggest that dorsal raphe neurons projecting to striatum and frontal cortex are more sensitive to self-inhibition mediated by 5-HT_1A autoreceptors than median raphe neurons projecting to the hippocampus. Therefore, potentiation by 5-HT_1A antagonists occurs preferentially in forebrain areas innervated by serotonergic neurons of the dorsal raphe nucleus.     

Stress-and Endotoxin-Induced Increases in Brain Tryptophan and Serotonin Metabolism Depend on Sympathetic Nervous System Activity

Stressful treatments and immune challenges have been shown previously to elevate brain concentrations of tryptophan. The role of the autonomic nervous system in this neurochemical change was investigated using pharmacological treatments that inhibit autonomic effects. Pretreatment with the ganglionic blocker chlorisondamine did not alter the normal increases in catecholamine metabolites, but prevented the increase in brain tryptophan normally observed after footshock or restraint, except when the duration of the footshock period was extended to 60 min. The footshock- and restraint-related increases in 5-hydroxyindoleacetic acid (5-HIAA) were also prevented by chlorisondamine. The increases in brain tryptophan caused by intraperitoneal injection of endotoxin or interleukin-1 (IL-1) were also prevented by chlorisondamine pretreatment. The footshock-induced increases in brain tryptophan and 5-HIAA were attenuated by the beta-adrenergic antagonist propranolol but not by the alpha-adrenergic antagonist phenoxybenzamine or the muscarinic cholinergic antagonist atropine. Thus the autonomic nervous system appears to be involved in the stress-related changes in brain tryptophan, and this effect is due to the sympathetic rather than the parasympathetic limb of the system. Moreover, the main effect of the sympathetic nervous system is exerted on beta- as opposed to alpha-adrenergic receptors. We conclude that activation of the sympathetic nervous system is responsible for the stress-related increases in brain tryptophan, probably by enabling increased brain tryptophan uptake. Endotoxin and IL-1 also elevate brain tryptophan, presumably by a similar mechanism. The increase in brain tryptophan appears to be necessary to sustain the increased serotonin catabolism to 5-HIAA that occurs in stressed animals, and which may reflect increased serotonin release.     

Alterations in the Retinal Dopaminergic Neuronal System in Rats with Streptozotocin-Induced Diabetes

Neurochemical alterations, which may be associated with the development of diabetic retinal dysfunction, were investigated using streptozotocin (STZ)-induced hyperglycemia in rats. Young male Wistar rats, weighing 100-150 g, were made diabetic with daily intraperitoneal injections of STZ (30 mg/kg) for 5 days. This treatment caused a continuous hyperglycemia (400-600 mg/dl) and suppressed gain in body weight. Nine weeks after the STZ treatment, a significant increment in retinal valine and a decline in phenylalanine were noted, while the concentrations of other neuroactive amino acids, such as gamma-aminobutyric acid and aspartic acid, in the retina remained unchanged. On the other hand, the concentration of retinal dopamine (DA) was found to decrease significantly from the third week of hyperglycemia, when [3H]spiperone binding showed a tendency to increase in the retinal particulate fraction. However, the activities of tyrosine hydroxylase and aromatic L-amino acid decarboxylase (AADC) and the uptake of [3H]tyrosine showed no alteration in the retina of diabetic rats. The accumulation rate of 3,4-dihydroxyphenylalanine (DOPA) in vivo in the retina of diabetic rats, measured following the administration of the AADC inhibitor m-hydroxybenzyl-hydrazine (100 mg/kg i.p.), was also unchanged. Although [3H]DA uptake by retinal tissue was similar in control and diabetic animals, the spontaneous efflux of [3H]DA from the retina was found to be significantly accelerated in STZ-treated animals. In addition, the release of preloaded [3H]DA, elicited by repeated photic stimulation, was significantly attenuated in retina from diabetic rats. These results suggest that an accelerated efflux of DA, possibly leading to the depletion of DA from the retinal DA system, may account for early retinal dysfunctions known to occur in diabetic subjects.     

Elevated Density of [3H]Imipramine Binding in Aged Human Brain

Aging was associated with an increase in the density of specific binding sites for [3H]imipramine in postmortem specimens of human hypothalamus, frontal cortex, and parietal cortex. In general, [3H]imipramine binding was not affected by factors considered difficult to control in postmortem studies, i.e., time from death to autopsy and cause of death. The in vitro regulation of [3H]imipramine binding by sodium was impaired with age in hypothalamic homogenates. In vitro regulation of [3H]imipramine binding by chloride was intact. Determination of the concentrations of 5-hydroxytryptamine (serotonin) and 5-hydroxyindoleacetic acid in hypothalamus and frontal cortex indicated no apparent age-related changes in indole metabolism. The age-related increase in brain [3H]imipramine binding and impairment in the in vitro regulation of binding by ions are similar to changes observed previously in aged mouse brain. The increase in brain antidepressant binding sites is discussed in relationship to other indices of brain serotonergic function in aging and to the relationship of [3H]imipramine binding and depression.