Selective Neurotoxic Lesions of Descending Serotonergic and Noradrenergic Pathways in the Rat

The ability of neurotoxic substances to induce selective lesions of the descending monoaminergic pathways in rats was investigated. Saline, 6-hydroxydopamine, 5,6-dihydroxytryptamine, or 5,7-dihydroxytryptamine were administered into the lumbar subarachnoid space through a chronically indwelling catheter. The lesions were evaluated 2-3 weeks later by in vitro uptake of [3H]noradrenaline and [14C]5-hydroxytryptamine into synaptosomal preparations from the frontal cortex, brainstem, cervical spinal cord, and lumbar spinal cord of each animal. There was no difference in uptake between saline-injected and noncatheterized controls and no significant changes in cortical uptake after any of the treatments (dose range of neurotoxins: 0.6-80 micrograms). In the lumbar spinal cord, 6-hydroxydopamine (5-80 micrograms) reduced the [3H]noradrenaline uptake by approximately 90% with no effects on [14C]5-hydroxytryptamine uptake, whereas 5,6-dihydroxytryptamine reduced the uptake of [14C]5-hydroxytryptamine by 90% (20-80 micrograms). [3H]Noradrenaline uptake was unaffected by lower doses of 5,6-dihydroxytryptamine but fell by 45-55% after 40-80 micrograms. 5,7-Dihydroxytryptamine (10-80 micrograms) reduced [3H]noradrenaline uptake by 90-95% and [14C]5-hydroxytryptamine uptake by approximately 80% (5-80 micrograms) in the lumbar cord. It is concluded that intrathecal administration of suitable doses of neurotoxins may produce extensive selective lesions of descending noradrenergic and serotonergic pathways.     

Regulation of Aromatic l-Amino Acid Decarboxylase by Dopamine Receptors in the Rat Brain

Decarboxylation of phenylalanine by aromatic L-amino acid decarboxylase (AADC) is the rate-limiting step in the synthesis of 2-phenylethylamine (PE), a putative modulator of dopamine transmission. Because neuroleptics increase the rate of accumulation of striatal PE, these studies were performed to determine whether this effect may be mediated by a change in AADC activity. Administration of the D1 antagonist SCH 23390 at doses of 0.01-1 mg/kg significantly increased rat striatal AADC activity in an in vitro assay (by 16-33%). Pimozide, a D2-receptor antagonist, when given at doses of 0.01-3 mg/kg, also increased AADC activity in the rat striatum (by 25-41%). In addition, pimozide at doses of 0.3 and 1 mg/kg increased AADC activity in the nucleus accumbens (by 33% and 45%) and at doses of 0.1, 0.3, and 1 mg/kg increased AADC activity in the olfactory tubercles (by 23%, 30%, and 28%, respectively). Analysis of the enzyme kinetics indicated that the Vmax increased with little change in the Km with L-3,4-dihydroxyphenylalanine as substrate. The AADC activity in the striatum showed a time-dependent response after the administration of SCH 23390 and pimozide: the activity was increased within 30 min and the increases lasted 2-4 h. Inhibition of protein synthesis by cycloheximide (10 mg/kg, 0.5 h) had no effect on the striatal AADC activity or on the increases in striatal AADC activity produced by pimozide or SCH 23390. The results indicate that the increases in AADC activity induced by dopamine-receptor blockers are not due to de novo synthesis of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)     

Tryptamine Concentrations in Areas of 5-Hydroxytryptamine Terminal Innervation After Electrolytic Lesions of Midbrain Raphe Nuclei

The possible existence of tryptamine-containing neurons originating in the midbrain raphe is suggested by several reports of tryptamine-mediated responses to electrical stimulation of the raphe nuclei. To assess this hypothesis, we have investigated the effects of electrolytic lesions of the median and dorsal raphe nuclei on striatal, hypothalamic, and hippocampal concentrations of tryptamine, 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid. In addition, the rat striatal tryptophan concentrations were also determined. No changes in the concentrations of tryptamine were observed at 1 or 2 weeks after lesioning the dorsal and median raphe nuclei, at which time the other 5-hydroxyindoles were markedly reduced; furthermore, no reductions were observed in tryptamine concentrations in the striatum, hypothalamus, or hippocampus of rats pretreated with a monoamine oxidase inhibitor. The only change observed in these rats was a limited increase in striatal tryptamine and tryptophan observed at 1 day after lesioning. The results indicate that tryptamine concentration is independent of the integrity of 5-HT-containing neurons of the midbrain raphe nuclei. Furthermore, if tryptamine-containing neurons that have terminal projections to the striatum, hypothalamus, and hippocampus exist, their cell bodies are located in regions outside the dorsal and median raphe nuclei. Another possibility could be that tryptamine is located in glial cells.     

Studies on the Formation of 6-Hydroxydopamine in Mouse Brain After Administration of 2,4,5-Trihydroxyphenylalanine (6-HydroxyDOPA)

2,4,5-Trihydroxyphenylalanine (6-OH-DOPA) destroys central and peripheral noradrenergic neurons, while sparing dopaminergic neurons. Previous studies indicate that 6-OH-DOPA toxicity is mediated by the formation of 6-hydroxydopamine. However, levels of 6-hydroxydopamine in brain following peripheral administration of 6-OH-DOPA have not been documented. In the current study, 6-OH-DOPA and 6-hydroxydopamine were measured in brain by HPLC with electrochemical detection after intraperitoneal injection of 6-OH-DOPA. When mice were injected with 100 mg 6-OH-DOPA/kg, 6-hydroxydopamine levels in the striatum were highest (1.9 microgram/g) at 15 min and fell slowly to 24% of the peak value at 4 h. Experiments with reserpine indicated that the relatively stability of 6-hydroxydopamine was largely dependent upon storage in synaptic vesicles. Reserpine (10 mg/kg) lowered striatal 6-hydroxydopamine levels to 21.6% of control (non-reserpine-treated) values at 1 h, and to 8.9% of control values at 4 h. Levels of 6-hydroxydopamine in the striatum at 1 h were increased 113% by pargyline (100 mg/kg), 145% by alpha-methyldopahydrazine (carbidopa; 25 mg/kg), and 261% by pargyline and carbidopa together. Levels of dopamine in the striatum were unchanged at 2.5 h after 200 mg 6-OH-DOPA/kg (with pargyline and 50 mg carbidopa/kg), whereas levels of norepinephrine in the frontal cortex fell by 77%. At the same time, 6-hydroxydopamine levels were 8.8-fold higher in the striatum (5.54 micrograms/g) than in the cortex (0.63 micrograms/g).(ABSTRACT TRUNCATED AT 250 WORDS)     

Selective Depletion of Dopamine, Octopamine, and 5-Hydroxytryptamine in the Nervous Tissue of the Cockroach (Periplaneta americana)

High-performance liquid chromatography with electrochemical detection was used to measure the concentrations of 3,4-dihydroxyphenylethylamine (dopamine), 5-hydroxytryptamine (5-HT), p-hydroxyphenylethanolamine (octopamine), alpha-methyl-p-tyrosine, and tryptophan in the cerebral ganglia of cockroaches (Periplaneta americana) after peripheral administration of alpha-methyl-p-tyrosine and alpha-methyltryptophan. In addition, the levels of dopamine, 5-HT, octopamine, alpha-methyl-p-tyrosine, and tryptophan were determined after injection of alpha-methyl-p-tyrosine, 6-hydroxydopamine, or 5,7-dihydroxytryptamine directly into the cerebral ganglia by means of microinjection needles. Peripheral administration of alpha-methyl-p-tyrosine (400-1,600 micrograms/insect) caused a reduction in dopamine and 5-HT concentrations in cockroach cerebral ganglia, although the reduction in dopamine concentrations was more pronounced. Peripheral injections of alpha-methyl-p-tyrosine also reduced octopamine levels in the cerebral ganglia. Peripheral injection of alpha-methyltryptophan (400-1,600 micrograms/insect) caused a marked reduction in 5-HT and tryptophan concentrations in cockroach cerebral ganglia without altering dopamine or octopamine concentrations. Central injections of alpha-methyl-p-tyrosine (80 micrograms/insect) reduced dopamine concentrations in the cerebral ganglia. However, neither 6-hydroxydopamine (20 micrograms/insect) nor 5,7-dihydroxytryptamine (20 micrograms/insect) caused reductions in amine levels when applied near or directly into the cerebral ganglia. The results suggest that specific lesions of aminergic neurons in insects by either 6-hydroxydopamine or 5,7-dihydroxytryptamine are impractical. The specific, long-lasting depletion of 5-HT by alpha-methyltryptophan suggests that this chemical may be useful in elucidating the functions of 5-HT in insects.     

Effect of the Cerebral Tryptaminergic System on the Turnover of Dopamine in the Striatum of the Rat

The effect of the cerebral 5-hydroxytryptamine system on the turnover of striatal 3,4-dihydroxyphenyl-ethylamine (dopamine) was investigated by measuring the level of dopamine and one of its metabolites in rats depleted of cerebral 5-hydroxytryptamine or treated with a 5-hydroxytryptamine receptor blocker. Treatment with p-chlorophenylalanine induced, in addition to a reduction in striatal 5-hydroxytryptamine and 5-hydroxyindol-3-ylacetic acid, an increase in the striatal concentration of dopamine, a diminution in the concentration of homovanillic acid in the same cerebral area, and a reduction in the rise of this acid after the administration of a butyrophenone derivative or tetrabenazine. Treatment with methysergide also reduced the increase of homovanillic acid induced by the butyrophenone. When probenecid was given to rats treated with p-chlorophenylalanine, homovanillic acid failed to accumulate, whereas the accumulation of 5-hydroxyindol-3-ylacetic acid was unaffected. The decay of dopamine after alpha-methyl-p-tyrosine administration was normal for the first 6 h but was later reduced in rats given p-chlorophenylalanine or methysergide. The results suggest that the lack of activation of 5-hydroxytryptamine receptors leads to a reduction in the turnover of dopamine in the nigrostriatal pathway.     

Effect of Chronic Estradiol and Progesterone Treatments of Ovariectomized Rats on Brain Dopamine Uptake Sites

Abstract: Dopamine released from brain nerve terminals is mainly removed from the synaptic cleft by an uptake mechanism. Despite their functional importance, modulation of the dopamine uptake sites is still not well known. Steroid hormones were shown to modulate brain dopamine transmission. The aim of this study was thus to investigate in ovariectomized rats the effects of 17β-estradiol and progesterone treatments on brain dopamine uptake sites. Treatments consisted of 17β-estradiol (10 μg/0.2 ml), progesterone (0.72 mg/0.2 ml). 17β-estradiol + progesterone, or the vehicle (0.3% gelatin in saline solution) twice daily for 2 weeks. The steroid treatments left the affinity of [³H]GBR 12935 binding to striatal homogenates unchanged (ovariectomized rats, 0.823 ± 0.028 nM), whereas the density was increased by these steroids alone or in combination to a similar extent of 16-23%. Chronic treatment of ovariectomized rats with 17β-estradiol progesterone, or their combination increased to the same extent and uniformly [³H]-GBR 12935 binding in the striatum as measured by autoradiography; the increase was similar in the substantia nigra pars compacta, whereas no steroid effect was observed in the nucleus accumbens and in the substantia nigra pars reticulata. In summary, chronic exposure to 17β-estradiol and/ or progesterone increased dopamine uptake site density in the nigrostriatal dopaminergic system, whereas the nucleus accumbens and the substantia nigra pars reticulata were unaffected.     

Absence of 6-Hydroxydopamine in the Rat Brain After Treatment with Stimulants and Other Dopaminergic Agents: A Mass Fragmentographic Study

Abstract: Formation of 6-hydroxydopamine (6-OHDA) from dopamine has been hypothesized to mediate neuro-degeneration induced by some psychostimulants. Although the emergence of a 6-OHDA-like substance was reported in the striatum of methamphetamine-treated rats, this substance has not been identified by a direct approach. We used mass fragmentography to search for 6-OHDA in the rat frontal cortex and striatum after the administration of a number of drugs including 3,4-dihy-droxyphenyl-L-alanine, methamphetamine, amphetamine, and cocaine, all of which increase synaptic dopamine. No 6-OHDA was detected after the acute systemic administration of these agents. Intraventricular administration of 6-OHDA (10 μg/rat.) produced measurable concentrations of 6-OHDA that were higher in the striatum than in the frontal cortex. Intraventricular administration of 2,4,5-trihydroxy-phenyl-D,L-alanine (6-OHDOPA; 10 μg/rat) produced similar concentrations of 6-OHDA in both regions. Pargyline, but not carbidopa (α-methyldopahydrazine), enhanced the effect of intraperitoneal 6-OHDOPA administration (80 mg/kg). We conclude that (1) 6-OHDOPA can cross the blood-brain barrier and is converted to 6-OHDA in the brain, (2) 6-OHDA is a substrate for monoamine oxidase(s) and therefore a search for its purported deaminated metabolite is warranted, and (3) acute treatment with the above stimulants either does not lead to the formation of 6-OHDA or produces concentrations below the detection limit of the assay (<34 pg/mg of protein).     

Differential Effect of Stress on In Vivo Dopamine Release in Striatum, Nucleus Accumbens, and Medial Frontal Cortex

Microdialysis was used to assess extracellular dopamine in striatum, nucleus accumbens, and medial frontal cortex of unanesthetized rats both under resting conditions and in response to intermittent tail-shock stress. The dopamine metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid also were measured. The resting extracellular concentration of dopamine was estimated to be approximately 10 nM in striatum, 11 nM in nucleus accumbens, and 3 nM in medial frontal cortex. In contrast, the resting extracellular levels of 3,4-dihydroxyphenylacetic acid and homovanillic acid were in the low micromolar range. Intermittent tail-shock stress increased extracellular dopamine relative to baseline by 25% in striatum, 39% in nucleus accumbens, and 95% in medial frontal cortex. 3,4-Dihydroxyphenylacetic acid and homovanillic acid also were generally increased by stress, although there was a great deal of variability in these responses. These data provide direct in vivo evidence for the global activation of dopaminergic systems by stress and support the concept that there exist regional variations in the regulation of dopamine release.     

Regional Effects of Sodium Valproate on Extracellular Concentrations of 5-Hydroxytryptamine, Dopamine, and Their Metabolites in the Rat Brain: An In Vivo Microdialysis Study

The effects of sodium valproate (VPA; 100, 200, and 400 mg/kg, i.p.) on ventral hippocampal and anterior caudate putamen extracellular levels of dopamine (DA) and 5-hydroxytryptamine (5-HT) were examined using in vivo microdialysis. VPA induced dose-related increases in dialysate DA, 3,4-dihydroxyphenylacetic acid, and 5-HT in the ventral hippocampus. Anterior caudate putamen dialysate 5-HT was also dose dependently elevated by the drug, whereas DA levels tended to decrease with increasing VPA dose. In contrast, VPA (200, 400, and 800 mg/kg, i.p.) produced no significant elevation of DA in posterior caudate putamen dialysates, although 5-HT levels were significantly elevated at the 400- and 800-mg/kg doses. In all three regions studied, dialysate concentrations of 5-hydroxyindoleacetic acid and homovanillic acid remained at basal levels following VPA treatments. The results are discussed with regard to the possible anticonvulsant mode of action of VPA.     

Effect of 3,4-Methylenedioxymethamphetamine on 3,4-Dihydroxyphenylalanine Accumulation in the Striatum and Nucleus Accumbens

The effect of the racemic mixture of 3,4-methylenedioxymethamphetamine (MDMA) on the synthesis of dopamine in the terminals of nigrostriatal and mesolimbic neurons was estimated by measuring the accumulation of 3,4-dihydroxyphenylalanine (DOPA) in the striatum and nucleus accumbens 30 min following the administration of the L-aromatic amino acid decarboxylase inhibitor, 3-hydroxybenzylhydrazine. MDMA produced an increase in DOPA accumulation in the striatum which was greater in magnitude and longer in duration than that in the nucleus accumbens. Although the concentrations of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in both the striatum and nucleus accumbens were reduced 3 h following an injection of MDMA (20 mg/kg), 5-HT and 5-HIAA concentrations were significantly reduced only in the striatum 7 days after the administration of MDMA. Pretreatment with a 5-HT2 antagonist, ketanserin, significantly attenuated the reduction in 5-HT concentration in the striatum 3 h following MDMA administration and completely blocked 5-HT depletion at 7 days post administration. Moreover, ketanserin completely blocked MDMA-induced DOPA accumulation in the striatum. The results obtained in these studies suggest that MDMA activates nigrostriatal dopaminergic pathways via 5-HT2 receptors. In addition, these data are supportive of the hypothesis that dopamine plays a role in MDMA-induced 5-HT depletion.     

Deficits in the Activation and Phosphorylation of Hippocampal Tyrosine Hydroxylase in the Aged Fischer 344 Rat Following Intraventricular Administration of 6-Hydroxydopamine

Abstract: Tyrosine hydroxylase activity was measured under optimal and suboptimal assay conditions in hippocampal extracts from young (2 month), mature (12 month), and old (24 month) Fischer 344 male rats 72 h after the infusion of 200 µg of the neurotoxin 6-hydroxydopamine or vehicle into the lateral ventricle. The lesion resulted in a 45–55% decrease of tyrosine hydroxylase activity measured under optimal conditions (pH 6.1, 3.0 m M 6-methyl-5,6,7,8-tetrahydropterin) and an ∼35% decrease in the relative concentration of immunoreactive tyrosine hydroxylase. When measured under suboptimal conditions (pH 6.6, 0.7 m M 6-methyl-5,6,7,8-tetrahydropterin), tyrosine hydroxylase activity in 2- and 12-month-old lesioned animals was twice that measured in vehicle-treated animals. However, in the old lesioned animals, tyrosine hydroxylase activity measured under suboptimal conditions was not different from that measured in age-matched vehicle-treated animals. Isoforms of tyrosine hydroxylase were identified on immunoblots after two-dimensional gel electrophoresis using enhanced chemiluminescence. The relative proportion of lower pl isoforms of tyrosine hydroxylase in the 2-month-old lesioned animals was greater than that observed in vehicle-treated controls. In contrast, no difference was seen in the relative proportion of tyrosine hydroxylase isoforms in the 24-month-old lesioned versus control animals. These data indicate that the ability of locus ceruleus neurons to rapidly respond to and compensate for insult is attenuated in 24-month-old Fischer 344 rats due to a deficit in stimulus-evoked enzyme phosphorylation.     

Acute Effects of Typical and Atypical Antipsychotic Drugs on the Release of Dopamine from Prefrontal Cortex, Nucleus Accumbens, and Striatum of the Rat: An In Vivo Microdialysis Study

In vivo microdialysis has been used to study the acute effects of antipsychotic drugs on the extracellular level of dopamine from the nucleus accumbens, striatum, and prefrontal cortex of the rat. (-)-Sulpiride (20, 50, and 100 mg/kg i.v.) and haloperidol (0.1 and 0.5 mg/kg i.v.) enhanced the outflow of dopamine in the striatum and nucleus accumbens. In the medial prefrontal cortex, (-)-sulpiride at all doses tested did not significantly affect the extracellular level of dopamine. The effect of haloperidol was also attenuated in the medial prefrontal cortex; 0.1 mg/kg did not increase the outflow of dopamine and the effect of 0.5 mg/kg haloperidol was of shorter duration in the prefrontal cortex than that observed in striatum and nucleus accumbens. The atypical antipsychotic drug clozapine (5 and 10 mg/kg) increased the extracellular concentration of dopamine in all three regions. In contrast to the effects of sulpiride and haloperidol, that of clozapine in the medial prefrontal cortex was profound. These data suggest that different classes of antipsychotic drugs may have distinct effects on the release of dopamine from the nigrostriatal, mesolimbic, and mesocortical terminals.     

High Doses of Vitamin B6 in the Rat Are Associated with Inhibition of Hepatic Tryptophan Metabolism and Increased Uptake of Tryptophan into the Brain

Abstract: Acute administration of vitamin B₆ to rats (10 mg/kg body weight) led to reduced urinary excretion of N ¹-methyl nicotinamide and methyl pyridone carboxamide, indicating inhibition of the oxidative metabolism of tryptophan. There was a considerable reduction in the production of ¹⁴CO₂ from [ ring -2-¹⁴C]tryptophan, and a significant inhibition of hepatic tryptophan oxygenase when measured in liver homogenates, together with an increase in the concentration of tryptophan in plasma. There was an increase in both the concentration of tryptophan in the brain and the uptake into the brain of peripherally administered [³H]tryptophan, accompanied by a small increase in the rate of synthesis of 5-hydroxy-tryptamine in the brain. It is suggested that this increase in the uptake of tryptophan into the brain following a relatively large dose of vitamin B₆ may explain the beneficial action of the vitamin in some cases of depressive illness.     

Identification of D3 and σ Receptors in the Rat Striatum and Nucleus Accumbens Using (±)-7-Hydroxy-N,N-Di-n-[3H]Propyl-2-Aminotetralin and Carbetapentane

Abstract: Cross-reactions between dopamine D₃ and σ receptor ligands were investigated using (±)-7-hydroxy-N,N-di-n-[³H]propyl-2-aminotetralin [(±)-7-OH-[³H]DPAT], a putative D₃-selective radioligand, in conjunction with the unlabeled σ ligands 1,3-di(2-tolyl)guanidine (DTG), carbetapentane, and R(?)-N-(3-phenyl-1-propyl)-1-phenyl-2-aminopropane [R(?)-PPAP]. In transfected CCL1.3 mouse fibroblasts expressing the human D₃ receptor, neither DTG nor carbetapentane (0.1 µM) displaced (±)-7-OH-[³H]DPAT binding. R(?)-PPAP (0.1 µM) displaced 39.6 ± 1.0% of total (±)-7-OH-[³H]DPAT binding. In striatal and nucleus accumbens homogenates, (±)-7-OH-[³H]DPAT labeled a single site (15–20 fmol/mg of protein) with high (1 nM) affinity. Competition analysis with carbetapentane defined both high- and low-affinity sites in striatal (35 and 65%, respectively) and nucleus accumbens (59 and 41%, respectively) tissue, yet R(?)-PPAP identified two sites in equal proportion. Carbetapentane and R(?)-PPAP (0.1 µM) displaced ～20–50% of total (±)-7-OH-[³H]DPAT binding in striatum, nucleus accumbens, and olfactory tubercle in autoradiographic studies, with the nucleus accumbens shell subregion exhibiting the greatest displacement. To determine directly (+)-7-OH-[³H]DPAT binding to σ receptors, saturation analysis was performed in the cerebellum while masking D₃ receptors with 1 µM dopamine. Under these conditions (+)-7-OH-[³H]DPAT labeled σ receptors with an affinity of 24 nM. These results suggest that (a) (±)-7-OH-[³H]DPAT binds D₃ receptors with high affinity in rat brain and (b) a significant proportion of (±)-7-OH-[³H]DPAT binding consists of σ₁ sites and the percentages of these sites differ among the subregions of the striatum and nucleus accumbens.     

Release and Uptake Rates of 5-Hydroxytryptamine in the Dorsal Raphe and Substantia Nigra Reticulata of the Rat Brain

Abstract: Fast scan cyclic voltammetry with carbon fiber electrodes has been used to investigate the dynamics of the neurotransmitter 5-hydroxytryptamine (5-HT) in the extracellular fluid of two brain regions: the dorsal raphe and the substantia nigra reticulata. The method used previously was shown to be optimized to allow the time course of 5-HT concentration changes to be measured rapidly. Measurements were made in slices prepared from the brains of rats with the carbon fiber electrode inserted into the tissue and a bipolar stimulating electrode placed on the slice surface. Identification of 5-HT as the detected substance in both regions was based on voltammetric, anatomical, physiological, and pharmacological evidence. Autoradiography using [³H]paroxetine revealed highest 5-HT transporter binding densities in the regions in which voltammetric measurements were made. Evaluation of the pharmacological actions of tetrodotoxin and tetrabenazine, as well as the effects of calcium removal, suggested that 5-HT storage was vesicular and that the release process was exocytotic. The effects of fluoxetine (0.5 µM) were typical of a competitive uptake inhibitor, changing K_m with little effect on V_max. Release of 5-HT was found to be maximal with wide (2-ms) stimulus pulses in both regions, as expected for release from small unmyelinated processes, and to increase linearly with the number of pulses when high frequencies (100 Hz) were used. At lower frequencies, the concentration observed was a function of both release and uptake. Kinetic simulations of the data revealed that the major difference in 5-HT neurotransmission between the two regions was that release and uptake rates are twice as large in the dorsal raphe ([5-HT] per pulse = 100 ± 20 nM, V_max = 1,300 ± 20 nM/s for dorsal raphe; [5-HT] per pulse = 55 ± 7 nM, V_max = 570 ± 70 nM/s for substantia nigra reticulata). When normalized to tissue content, uptake rates in both regions were identical and similar to rates previously reported for dopamine in dopamine terminal regions. Nonetheless, compared with dopaminergic transmission in terminal regions such as the striatum, the absolute clearance rates in the substantia nigra reticulata and dorsal raphe were lower, resulting in a longer lifetime of 5-HT in the extracellular fluid and allowing long-range interactions.     

Effect of Electroconvulsive Shock on the Uptake and Release of Noradrenaline and 5-Hydroxytryptamine in Rat Brain Slices

Abstract: The uptake and release of [³H]noradrenaline and [³H]-5-hydroxytryptamine (5-HT) were studied in cerebral cortex slices from rats 30 min and 24 h after a single electroconvulsive shock (ECS) and 24 h after a series of five shocks given over 10 days. Both the K _m and V _max for 5-HT uptake were lower than controls 24 h after a single ECS, whereas after 5 ECS spread over 10 days both parameters remained depressed, though only the fall in V_max was significant. Noradrenaline uptake was not altered after a single ECS, but the V_max and K _m were elevated following chronic ECS treatment. Neither ECS treatment schedule had any effect on the potassium-stimulated release of either transmitter. It is possible that the changes in monoamine uptake seen following ECS are an adaptive response to alterations in the synaptic cleft concentration of these transmitters.     

Differential Effects of Forced Locomotion, Tail-Pinch, Immobilization, and Methyl-β-Carboline Carboxylate on Extracellular 3,4-Dihydroxyphenylacetic Acid Levels in the Rat Striatum, Nucleus Accumbens, and Prefrontal Cortex: An In Vivo Voltammetric Study

In vivo voltammetry with carbon fiber electrodes was used to assess extracellular 3,4-dihydroxyphenylacetic acid (DOPAC) levels in striatum, nucleus accumbens, and anteromedial prefrontal cortex of freely moving rats subjected to altered motor activity or anxiogenic stimuli. Forced locomotion on a rotarod for 40 min caused an increase in extracellular DOPAC levels in the striatum and to a lesser extent in the nucleus accumbens but not in the prefrontal cortex. Subcutaneous injection of the anxiogenic agent methyl-beta-carboline carboxylate (10 mg/kg) increased extracellular DOPAC levels to a similar extent in prefrontal cortex and nucleus accumbens. Immobilization for 4 min augmented dopamine (DA) metabolism preferentially in the nucleus accumbens and to a lesser extent in the prefrontal cortex. Tail-pinch caused a selective activation of DA metabolism in the nucleus accumbens. None of these stimuli altered extracellular striatal DOPAC levels. These results confirm the involvement of dopaminergic systems projecting to the striatum and nucleus accumbens in motor function and suggest that mesolimbic and mesocortical dopaminergic systems can be specifically activated by certain kinds of anxiogenic stimuli; the relative activation of either of these latter systems could depend primarily on the nature (sensory modality, intensity) of the acute stressor.     

Effects of Feeding and Drinking on Acetylcholine Release in the Nucleus Accumbens, Striatum, and Hippocampus of Freely Behaving Rats

Extracellular levels of acetylcholine (ACh) were measured in the nucleus accumbens (NAC), striatum (STR), and hippocampus (HIPP) using microdialysis in 30-min intervals before, during, and after free-feeding in 20-h food-deprived rats. The effects on ACh in the NAC and STR were also observed in response to water intake in 20-h water-deprived animals. Neostigmine was used in the perfusate to improve ACh recovery. Basal ACh was sensitive to tetrodotoxin and low calcium, and therefore largely neuronal in origin. Feeding caused a 38% increase in extracellular ACh in the NAC and no change in the STR or HIPP. Dopamine was also increased in the NAC (48%) and to a lesser extent in the STR (21%) following feeding. Drinking caused 18-20% increases in ACh release in both the NAC and STR. In a separate experiment, ACh release in the NAC was monitored in 10-min intervals during free-feeding; ACh increased in the interval immediately following maximal food intake. These results suggest a site-specific increase in ACh release following feeding that cannot be solely attributed to the activation associated with this behavior.