Effects of K+-stimulation and precursor loading on the in vivo release of dopamine, serotonin and their metabolites in the nucleus accumbens of the rat

Do opioids evoke the release of serotonin in the spinal cord? An in vivo microdialysis study of the regulation of extracellular serotonin in the rat

This study investigated the regulation of serotonin (5-HT) and its major metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the dorsal spinal cord of awake, freely moving rats, using microdialysis coupled to HPLC with electrochemical detection and tested the hypothesis that opioids exert their analgesic effect in part through the increased release of 5-HT in the dorsal horn. A dialysis tube was placed transversely at the L4 segment of the dorsal spinal cord and the basal concentration of 5-HT in the dialysate was characterized by infusion of a variety of substances through the dialysis probe: tetrodotoxin (TTX), KCl, imipramine, fluoxetine and amphetamine (AMPH). To evaluate the contribution of opioids, we also studied the effects of either systemic or intracerebroventricular (i.c.v.) injection of morphine or DAMGO. Extracellular concentrations of 5-HT and 5-HIAA were partially and reversibly reduced by TTX. In the presence of KCl, imipramine, fluoxetine or AMPH, 5-HT levels significantly increased. Under these conditions, extracellular 5-HIAA levels usually decreased. By contrast, the effects of opioids on 5-HT concentrations were highly variable. Low doses of morphine administered systemically increased 5-HT concentrations in only 3 of 6 rats. This was paralleled by a decrease in 5-HIAA. Higher doses of morphine, alone or in the presence of fluoxetine, did not change 5-HT concentrations. Intracerebroventricular injection of morphine or DAMGO increased the extracellular concentrations of 5-HT in only about one third of the animals. After intracerebroventricular opioid injection, extracellular concentrations of 5-HIAA either decreased by about 20% or did not change.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin release varies with brain tryptophan levels

This study examines directly the effects on serotonin release of varying brain tryptophan levels within the physiologic range. It also addresses possible interactions between tryptophan availability and frequency of membrane depolarization in controlling serotonin release. We demonstrate that reducing tryptophan levels in rat hypothalamic slices (by superfusing them with medium supplemented with 100 microM leucine) decreases tissue serotonin levels as well as both spontaneous and electrically-evoked serotonin release. Conversely, elevating tissue tryptophan levels (by superfusing slices with medium supplemented with 2 microM tryptophan) increases both tissue serotonin levels and serotonin release. Serotonin release was found to be affected independently by tryptophan availability and frequency of electrical field-stimulation (1-5 Hz), since increasing both variables produced nearly additive increases in release. These observations demonstrate for the first time that both precursor-dependent elevations and reductions in brain serotonin levels produce proportionate changes in serotonin release, and that the magnitude of the tryptophan effect is unrelated to neuronal firing frequency. The data support the hypothesis that serotonin release is proportionate to intracellular serotonin levels.

Characterization of monoamine release in the lateral hypothalamus of awake, freely moving rats using in vivo microdialysis

Extracellular levels of serotonin (5-HT), dopamine (DA) and their major metabolites 5-hydroxyindoleacetic acid (5-HIAA), 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA), were measured in the lateral hypothalamus of awake, freely moving rats using microdialysis combined with HPLC and electrochemical detection. To characterize the factors which control 5-HT release, the effects of various drugs were assessed. TTX had a reversible inhibitory effect on the basal levels of 5-HT, 5-HIAA, DOPAC and HVA. Infusion of K+ concomitantly increased 5-HT and DA and decreased 5-HIAA and HVA. Imipramine increased extracellular levels of 5-HT and DA and decreased 5-HIAA levels; this effect was TTX-sensitive. Systemic pargyline increased extracellular 5-HT and markedly decreased the metabolic levels. Pargyline pretreatment in the presence of imipramine, infused through the dialysis probe, slowly increased 5-HT levels above that produced by the reuptake blocker alone. Infusion with AMPH produced a dramatic, TTX-insensitive, increase in 5-HT and DA and a decrease in the metabolic levels. These results provide evidence that (1) basal release of 5-HT in the lateral hypothalamus results from neuronal activity, (2) the metabolites in the extracellular fluid derive primarily from intracellular monoamine oxidase (MAO) activity, (3) 5-HT is mainly removed from the extracellular space by a reuptake mechanism, with minimal contribution of an extracellular MAO, and (4) the AMPH-evoked release of 5-HT and DA is a Na+ channel-independent process.

Serotonin microinfusion into the ventral tegmental area increases accumbens dopamine release

The effects of microinfusion of serotonin (5-HT) agents as well as glutamate and muscimol into the ventral tegmental area (VTA) on dopamine (DA) release in the ipsilateral nucleus accumbens (ACC) were investigated in freely moving rats, using a push-pull perfusion procedure. The baseline values for DA, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) were approximately 0.24, 8.4, 2.1 and 2.7 pmol/15 min, respectively, in the push-pull perfusate of the ACC. When microinfused into the VTA, glutamate (0.74 microgram) significantly (p less than 0.05) increased the contents of DOPAC (110%) and HVA (90%) over baseline levels in the perfusate. On the other hand, 0.5 microgram muscimol (a gamma-amino-n-butyric acid, GABA, agonist) significantly, (p less than 0.05) decreased both DA (40%) and DOPAC (20%) levels relative to baseline values. Administration of 2 micrograms 5-HT into the VTA caused a significant (p less than 0.05) elevation in the perfusate levels of DOPAC (80%) and HVA (70%) over baseline values. A similar effect was obtained with a nonselective 5-HT1 agonist but not with a selective 5-HT1A agonist. The results suggest that 5-HT innervations in the VTA may have an excitatory action possibly via 5-HT1B rather than 5-HT1A receptors on the mesolimbic DA system projecting to the ACC and that this DA system may also be regulated by glutamatergic and GABAergic (via GABAA receptors) inputs.

Tryptophan availability modulates serotonin release from rat hypothalamic slices

Application of a novel in vitro experimental system has allowed us to describe the relationship between tryptophan availability and serotonin release from rat hypothalamic slices. Superfusing hypothalamic slices with a physiologic medium containing l-tryptophan (1, 2, 5, or 10 microM) caused dose-dependent elevations in tissue tryptophan levels; the magnitude of the elevations produced by supplementing the medium with less than 5 microM tryptophan was within the physiologic range for rat brain tryptophan levels. Slice serotonin levels rose biphasically as the tryptophan concentration in the medium was increased. Superfusing the slices with medium supplemented with a low tryptophan concentration (1 or 2 microM) caused proportionally greater incremental changes in serotonin levels than the increases caused by further elevating the tryptophan concentration (5 or 10 microM). The spontaneous release of serotonin from the slices exhibited a dose-dependent relationship with the tryptophan concentration of the superfusion medium. Electrically evoked serotonin release, which was calcium-dependent and tetrodotoxin-sensitive, also increased in proportion to the medium tryptophan concentration. These data suggest that the rate at which serotonin is released from hypothalamic nerve terminals is coupled to brain tryptophan levels. Accelerations in hypothalamic serotonin synthesis, caused by elevating brain tryptophan levels, result in proportionate increases in the rates of serotonin release during rest and with membrane depolarization.

Fluoxetine increases the extracellular levels of serotonin in the nucleus accumbens

The effects of an IP injection of the monoamine uptake inhibitor fluoxetine on the extracellular concentration of serotonin (5-HT), dopamine (DA), 5-hydroxyindoleacetic acid (5-HIAA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens of awake and freely moving rats were examined using a push-pull perfusion technique. Baseline values of 5-HT, 5-HIAA, DA, DOPAC and HVA in the perfusates were approximately 0.07, 13, 0.8, 49 and 12 pmol/hr, respectively. The IP administration of 5 and 10 mg/kg fluoxetine dose-dependently elevated the amounts of 5-HT 3- and 13-fold, respectively, in the push-pull perfusate, with the maximum reached within one hour after drug administration. Moreover, 10 mg/kg fluoxetine also significantly decreased the levels of 5-HIAA in the perfusate as much as 50% within 2-3 hours. On the other hand, no significant effect of 5 or 10 mg/kg fluoxetine was observed on the contents of DA, DOPAC and HVA in the push-pull perfusates. The data indicate that fluoxetine, in accord with its role as a 5-HT uptake inhibitor, increases the physiologically active pool of 5-HT in the nucleus accumbens under in vivo conditions.

Amphetamine-induced in vivo release of dopamine in the striatum is influenced by local pH

The effect of local pH on the in vivo efflux of endogenous dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) following administration of d-amphetamine (AMPH) was examined in the striatum of the anesthetized rat using two bilaterally placed push-pull cannulae. At both pH 7.3 and 6.4, the baseline efflux values for DA and DOPAC were approximately 0.2 and 25 pmoles/15 min, respectively. Subcutaneous injection of 2 mg/kg AMPH induced a 3-fold increase of DA release at pH 7.3 and a 21-fold increase of DA release at pH 6.4. In both cases, the maximum was reached at about 30 min after the drug administration. Following the administration of AMPH, the efflux of DOPAC was reduced to the same degree (20% of control values) under both pH conditions. In vitro data showed that the lower pH did not alter the recovery of DA or DOPAC. In addition, release of DA produced by local perfusion with 5 uM AMPH was also greater at the lower pH (50-fold increase over baseline) than at the physiological pH (10-fold increase over baseline). The stimulated DA release produced by local perfusion with 35 mM K+, however, was the same at both pH values. Preliminary experiments also indicated that there was a pH effect for AMPH-induced serotonin (5-HT) release but that the difference in the amount of 5-HT in the two media was not nearly as large as that obtained for DA. The markedly elevated level of extracellular DA at the lower pH might be due to a higher affinity of the DA uptake system for AMPH, thereby producing greater inhibition of DA uptake as well as enhanced DA release. The data also suggest an enhanced affinity of AMPH for 5-HT uptake sites at the lower pH.