Effect of monoamine oxidase A and B and of catechol-O-methyltransferase inhibition on L-DOPA-induced circling behavior

The effect of enzyme-inhibiting adjuvants on L-DOPA + benserazide-induced contralateral turning in unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats was studied. Both the number of turns and the duration of turning were examined. Inhibition of MAO-A with 10 mg/kg Ro 41-1049 increased both parameters; inhibition of COMT with 30 mg/kg Ro 40-7592 had a similar effect. In contrast, inhibition of MAO-B with 10 mg/kg Ro 19-6327 did not change turning behavior. A further potentiation of turning behavior was observed after the combined administration of both the MAO-A and COMT inhibitor. MAO-A inhibition in conjunction with MAO-B inhibition prolonged the duration of L-DOPA-induced turning but had no effect on the number of turns. However, in conjunction with COMT inhibition, 10 mg/kg of the MAO-B inhibitor, Ro 19-6327, significantly affected both the number and duration of turning behavior. An even further potentiation of turning behavior was observed after the combined administration of all three enzyme-inhibitors.

Affinity for dopamine D2, D3, and D4 receptors of 2-aminotetralins. Relevance of D2 agonist binding for determination of receptor subtype selectivity

A series of 2-aminotetralins, substituted with a methoxy or a hydroxy group on the 5- or 7-position, and with varying N-alkyl or N-arylalkyl substituents, were prepared and evaluated in binding assays for human dopamine (DA) D2, D3, and D4 receptors. Some members of this series were prepared in former studies, but were never tested in vitro with single receptor subtypes, and these were examined again. None of the tested 2-aminotetralins showed high affinity for the dopamine D4 receptor. However, a number of the 2-aminotetralins showed high affinity for both the D2 and the D3 DA receptors, as exemplified by compounds 11-15 and 21-26, while some had a reasonable selectivity for the DA D3 receptors. The affinities of the 2-aminotetralins for the D21, receptor depended on the type of radioligand (agonist or antagonist) used. The agonist affinity data, obtained by using the agonist ligand [3H]N-0437, are thought to be more relevant for calculating DA receptor subtype selectivity.

Potential anxiolytic properties of R-(+)-8-OSO2CF3-PAT, a 5-HT 1A receptor agonist

The anxiolytic property of R-(+)-8-OSO2CF3-PAT(R-(+)-8- [[(trifluoromethyl)sulfonyl]oxy]-2-(n-propyl-amino)tetralin), a 5-HT1A receptor agonist, was evaluated in Wistar rats by means of animal models of anxiety, the conditioned defensive burying model and the conditioned stress-induced freezing response followed by the elevated plus-maze test, respectively. In addition, the 5-HIAA/5-HT ratio (5-hydroxy-indole acetic acid/5-hydroxytryptamine) of rat brain homogenates was studied. Acute drug administration resulted in abolition of the burying behaviour (3 mg/kg i.p.), a dose-dependent decrease of rearing and induction of hyperphagia. R-(+)-8-OSO2CF3-PAT had no effect on conditioned footshock-induced freezing behaviour but increased open-arm activity in the rats on the plus-maze. The 5-HIAA/5-HT ratio was decreased in the lateral septum (1 and 3 mg/kg), dorsal hippocampus (3 mg/kg) and somatosensory cortex (3 mg/kg), implying that R-(+)-8-OSO2CF3-PAT affects particularly the limbic system in anxiety-inducing situations.

Neurochemical and functional characterization of the preferentially selective dopamine D3 agonist PD 128907

The present study determined the biochemical and pharmacological effects of PD 128907 [R-(+)-trans-3,4,4a,10b-tetrahydro-4-propyl-2H,5H- [1]benzopyrano[4,3-b]-1,4-oxazin-9-ol], a dopamine (DA) receptor agonist that shows a preference for the human D3 receptor. In transfected Chinese hamster ovary cells (CHO K1), PD 128907 displaced [3H]spiperone in a biphasic fashion which fit best to a two-site model, generating Ki values of 20 and 6964 nM for the high- and low-affinity sites for the D2L receptors and 1.43 and 413 nM for the corresponding sites for the D3 receptors. Addition of sodium and the GTP analog Gpp(NH)p to both the D2L and D3 caused a modest reduction in the affinity of the compound suggestive of an agonist type action. In agonist binding ([3H]N-0437), PD 128907 exhibited an 18-fold selectivity for D3 versus D2L, a selectivity similar to that found with antagonist binding to the high-affinity sites. PD 128907 exhibited only weak affinity for D4.2 receptors (Ki = 169 nM). No significant affinity for a variety of other receptors was observed. PD 128907 stimulated cell division (measured by [3H]thymidine uptake) in CHO p-5 cells transfected with either D2L or D3 receptors exhibiting about a 6.3-fold greater potency in activating D3 as compared to D2L receptors. In vivo the compound was active in reducing DA synthesis both in normal and gamma-butyrolactone (GBL) treated rats; in the GBL model, the decrease was greater in the higher D3-expressing mesolimbic region as compared with striatum which has a lower expression of D3 receptors. PD 128907 decreased DA release (as measured by brain microdialysis) both in rat striatum, nucleus accumbens and medial frontal cortex, as well as in monkey putamen. Behaviorally PD 128907 decreased spontaneous locomotor activity (LMA) in rats at low doses, whereas at higher doses stimulatory effects were observed. PD 128907 at high doses reversed the reserpine-induced decrease in LMA and induced stereotypy in combination with the D1 agonist SKF 38393 indicating postsynaptic DA agonist actions. It is unclear which of the subtypes of DA receptors might be mediating the pharmacological effects of PD 128907. However, the present findings indicating that PD 128907 shows a preference for DA D3 over D2L and D4.2 receptors indicates that its action at low doses may be due to interaction with D3 receptors and at higher doses, with both D2 and D3 receptors.

Sexual activity increases dopamine transmission in the nucleus accumbens and striatum of female rats

In vivo microdialysis was used to monitor extracellular concentrations of dopamine (DA), and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens and dorsal striatum of sexually active female rats during tests of locomotor activity, exposure to a novel chamber, exposure to sex odors, the presentation of a sexually active male rat, and copulation. DA increased slightly but significantly in the nucleus accumbens when a sexually active male was placed behind a wire-mesh screen, and further during copulation. DA also increased significantly in the dorsal striatum during copulation; however, the magnitude of this effect was significantly lower than that observed in the nucleus accumbens. The metabolites DOPAC and HVA generally followed DA with a delay, and increased significantly during copulation in both regions. In contrast, forced locomotion on a rotating drum, exposure to a novel testing chamber, and exposure to sex odors did not increase DA significantly in either region, although forced locomotion increased DOPAC significantly in both regions, and HVA significantly in the nucleus accumbens. The magnitude of DA release in the nucleus accumbens was significantly greater during copulation than running, whereas no significant difference was detected for striatal DA release between these two behavioral conditions. These results indicate that novelty or locomotor activity alone do not account for the increase in DA observed in the nucleus accumbens of female rats during copulation, and suggest that DA transmission in the nucleus accumbens is associated with anticipatory and consummatory aspects of sexual activity, as it is in male rats. In the dorsal striatum, however, DA release during copulation may reflect an increase in locomotor activity associated with active pacing of the male.

Electroconvulsive shock increases interstitial concentrations of uric acid in the rat brain

This study examined the effects of electroconvulsive shock (ECS) on striatal interstitial concentrations of the purine metabolite uric acid (UA) using microdialysis in freely moving rats. UA increased to about 200% of baseline following ECS. Intense seizure activity induced by the convulsant agent flurothyl also resulted in a two-fold increase of UA concentrations suggesting that the ECS-induced UA increase is related to the seizure activity per se. Local administration of tetrodotoxin or perfusion with a Ca(2+)-free solution failed to affect the basal or the ECS-induced increase in UA concentrations. These data indicate that both the basal and the stimulated interstitial concentrations of uric acid are not dependent upon neuronal activity and exocytotic release. The UA response to ECS appears to be refractory to a second ECS delivered 2 but not 24 h after the first. Intrastriatal infusion of allopurinol (1 mM), an inhibitor of UA synthesis, decreased basal UA concentrations to 26% but did not influence the ECS-induced UA increase. Systemic injection of allopurinol (20 mg/kg, i.p.) decreased basal UA concentrations to 25% and prevented the ECS-induced UA elevation. ECS also increased serum concentrations of UA to almost 200% of baseline. Allopurinol (20 mg/kg, i.p.) markedly decreased serum UA concentrations to non-detectable levels and completely abolished the ECS-induced increase. The estimated concentration difference between blood and brain interstitial UA strongly suggests that ECS-induced increase in brain interstitial UA concentrations is of peripheral origin possibly due to disruption of the blood brain barrier during seizure activity.

Pharmacological aspects of R-(+)-7-OH-DPAT, a putative dopamine D3 receptor ligand

The R-(+)-isomer of 7-hydroxy-2-(N,N-di-n-propylamino)tetralin (7-OH-DPAT) bound with a more than 200-fold higher affinity to cloned human dopamine D3 receptors (Ki = 0.57 nM) than to dopamine D2 receptors; the corresponding S-(-)-enantiomer had considerably less affinity for both dopamine receptor subtypes, indicating that the known enantiomer selectivity of 7-OH-DPAT for the 'classical' dopamine D2 receptor subtype extends to the recently discovered dopamine D3 receptor subtype. In rats R-(+)-7-OH-DPAT dose dependently (10-1000 nmol/kg) decreased dopamine release and induced yawning, while sniffing behaviour occurred at the highest dose tested (1000 nmol/kg). The possibility that the inhibition of dopamine release and the elicitation of yawning are mediated by dopamine D3 receptors is considered.

Neurochemical effects of electrically and chemically induced seizures: an in vivo microdialysis study in the rat hippocampus

This study examined the effects of electroconvulsive shock (ECS) on interstitial concentrations of serotonin (5-HT), its metabolite 5-hydroxyindoleacetic acid (5-HIAA), acetylcholine and choline, and the dopamine metabolite homovanillic acid (HVA) in the hippocampus of freely moving rats using online brain microdialysis. The effects of ECS on 5-HT, 5-HIAA, and HVA were compared to the effects of seizures induced by the convulsant agent flurothyl. Interstitial concentrations of 5-HT increased several fold in response to ECS and this increase was accompanied by a significant increase in the concentration of HVA. Acetylcholine and choline concentrations were also increased significantly by ECS. The ECS-induced increase in interstitial 5-HT was markedly reduced when the voltage-dependent sodium channel blocker tetrodotoxin (1 mumol/L) was added in the perfusion solution, indicating that the observed increase was of neuronal origin. Interstitial concentrations of 5-HT also increased in response to flurothyl-induced seizures and this increase was accompanied by a significant increase in the concentration of HVA. These results provide direct in vivo evidence that interstitial concentrations of 5-HT increase several fold in response to both ECS- and flurothyl-induced seizures. These observations are discussed in relation to the hypothesized role of 5-HT in ECS-induced memory deficits.

Formation and clearance of interstitial metabolites of dopamine and serotonin in the rat striatum: an in vivo microdialysis study

In vivo microdialysis was employed in order to characterize the steady-state kinetics of the turnover of specific dopamine and serotonin metabolites in the rat striatum 48 h after surgery. Inhibitors of monoamine oxidase (MAO; pargyline) and catechol-O-methyltransferase (COMT; Ro 40-7592) were administered, either separately or in conjunction, at doses sufficient to block these enzymes in the CNS. In some experiments, the acid metabolite carrier was blocked with probenecid. Temporal changes were then observed in the efflux of interstitial dopamine, 3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA). The fractional rate constants for the accumulation or disappearance of the metabolites could be determined after pharmacological blockade of catabolic enzymes or the acid metabolite carrier. Interstitial 5-HIAA was found to be cleared with a half-life of approximately 2 h. After blockade of either MAO or COMT, HVA disappeared with a half-life of 17 min. Experiments employing probenecid suggested that some of the interstitial HVA was cleared by the acid metabolite carrier, the remainder being cleared by a probenecid-insensitive process, possibly conjugation. After MAO inhibition, DOPAC disappeared with an apparent half-life of 11.3 min. The rate of 3-MT accumulation after pargyline indicated that the majority of interstitial HVA (> 95%) is formed from DOPAC rather than 3-MT. The formation of 3-MT from interstitial dopamine, calculated from the accumulation rate of 3-MT after pargyline, appeared to follow first-order kinetics (k = 0.1 min-1).

Effects of chronic bupropion on interstitial concentrations of dopamine in rat nucleus accumbens and striatum

Bupropion is a novel atypical antidepressant that inhibits dopamine (DA) uptake. The present experiments investigated the effects of acute (10 mg/kg, twice daily for 2 days) and chronic (10 mg/kg, twice daily for 21 days) bupropion treatment on interstitial DA concentrations using simultaneous in vivo microdialysis in the nucleus accumbens (NAC) and striatum of awake freely moving rats. Compared to animals that had not previously been exposed to the drug, bupropion (25 mg/kg, IP) induced increases in extracellular DA were significantly enhanced in the NAC of the chronic but not the acute bupropion group. This effect was regionally selective, as it was not observed in the striatum. In accordance with previous reports, concurrent behavioral measurements indicated that the locomotor stimulant effects of bupropion were also enhanced in the chronic group. These results demonstrate that bupropion-induced behavioral sensitization is accompanied by a selective potentiation of the effects of this compound on interstitial DA concentrations in the NAC.

Sexual behavior increases dopamine transmission in the nucleus accumbens and striatum of male rats: comparison with novelty and locomotion

Extracellular concentrations of dopamine (DA) and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were examined concurrently, using in vivo microdialysis, in the nucleus accumbens and dorsal striatum of sexually active male rats during tests of locomotor activity, exposure to a novel chamber, exposure to sex odors, the presentation of a sexually receptive female, and copulation. DA increased significantly in the nucleus accumbens when the males were presented with a sexually receptive female behind a screen and increased further during copulation. Although DA also increased significantly in the dorsal striatum during copulation, the magnitude of the effect was significantly lower than that observed in the nucleus accumbens. In contrast, forced locomotion on a rotating drum, exposure to a novel chamber, and exposure to sex odors did not increase DA significantly in either region, although both DOPAC and HVA increased significantly in both regions during the locomotion test. These results indicate that novelty or locomotor activity alone cannot account for the increased extracellular DA concentrations observed in the nucleus accumbens of male rats during the presentation of a sexually receptive female behind a screen, nor can they account for the increased DA concentrations observed in both the nucleus accumbens and dorsal striatum of male rats during copulation. The preferential increase in DA transmission in the nucleus accumbens, compared with that in the striatum, suggests that anticipatory and consummatory aspects of sexual activity may belong to a class of naturally occurring events with reward values that are mediated by DA release in the nucleus accumbens.

The effect of intrastriatal application of directly and indirectly acting dopamine agonists and antagonists on the in vivo release of acetylcholine measured by brain microdialysis

The effect of intrastriatal application of D-1, D-2 and indirect dopaminergic drugs on the release of striatal acetylcholine as a function of the post-implantation intervals was studied using in vivo microdialysis. The dopamine D-2 agonists LY 171555 and (-)N0437 inhibited the release of striatal acetylcholine to 40% of control values 16-24 h after implantation of the dialysis cannula. When LY 171555 was infused 40-48 h after implantation of the dialysis cannula, the response was attenuated to 20% of control values. Meanwhile, the effectiveness of infusions of the antagonists (-)sulpiride and haloperidol was augmented from a non significant effect at 16-24 h to a 150% increase 40-48 h after implantation of the cannula. Infusions of the dopamine releasing agent amphetamine or the dopamine uptake inhibitor nomifensine resulted in a dose-dependent increase in the overflow of dopamine. Not until a sevenfold increase in the level of dopamine was seen, the release of acetylcholine was significantly affected. This hyporesponsiveness of the striatal cholinergic interneurons to endogenous dopamine could not be attributed to dopamine D-1 receptor activation, since no effects on striatal acetylcholine release were found by intrastriatal infusions of the selective D-1 agonist CY 208-243 or the selective D-1 antagonist SCH 23390. The results indicate that dopamine D-2 receptors are involved in the regulation of striatal acetylcholine release and that these receptors are tonically occupied by endogenous dopamine under the present experimental conditions 40-48 h after probe implantation.(ABSTRACT TRUNCATED AT 250 WORDS)

Sexual behavior increases dopamine transmission in the nucleus accumbens and striatum of male rats: Comparison with novelty and locomotion

Extracellular concentrations of dopamine (DA) and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were examined concurrently, using in vivo microdialysis, in the nucleus accumbens and dorsal striatum of sexually active male rats during tests of locomotor activity, exposure to a novel chamber, exposure to sex odors, the presentation of a sexually receptive female, and copulation. DA increased significantly in the nucleus accumbens when the males were presented with a sexually receptive female behind a screen and increased further during copulation. Although DA also increased significantly in the dorsal striatum during copulation, the magnitude of the effect was significantly lower than that observed in the nucleus accumbens. In contrast, forced locomotion on a rotating drum, exposure to a novel chamber, and exposure to sex odors did not increase DA significantly in either region, although both DOPAC and HVA increased significantly in both regions during the locomotion test. These results indicate that novelty or locomotor activity alone cannot account for the increased extracellular DA concentrations observed in the nucleus accumbens of male rats during the presentation of a sexually receptive female behind a screen, nor can they account for the increased DA concentrations observed in both the nucleus accumbens and dorsal striatum of male rats during copulation. The preferential increase in DA transmission in the nucleus accumbens, compared with that in the striatum, suggests that anticipatory and consummatory aspects of sexual activity may belong to a class of naturally occurring events with reward values that are mediated by DA release in the nucleus accumbens.

Benzodiazepine-induced decreases in extracellular concentrations of dopamine in the nucleus accumbens after acute and repeated administration

In vivo microdialysis was used to assess the effects of acute and repeated injections of the benzodiazepine midazolam on extracellular dopamine (DA) concentrations in the nucleus accumbens. Acute administration of midazolam (5 mg/kg, SC) elicited a 22% decrease in extracellular DA in the nucleus accumbens but failed to affect DA concentrations in the striatum. Similarly, six spaced intravenous infusions of midazolam, at a dose that has previously been found to support self-administration (0.05 mg per infusion), produced a 50% decrease in extracellular DA in the nucleus accumbens. In order to assess the effects of subchronic midazolam injections, two groups of rats were given injections of saline or midazolam (5 mg/kg, SC) for 14 days (two injections per day). A subsequent challenge injection of midazolam (5 mg/kg) decreased extracellular DA in the nucleus accumbens by 25% in both groups, indicating that neither tolerance nor sensitization occurred during the repeated drug administration. These experiments indicate (1) that midazolam differentially affects meso-accumbens and nigrostriatal DA neurons, and (2) that the midazolam-induced decrease in extracellular DA in the nucleus accumbens is not affected by repeated drug administration. The data further suggest that the rewarding effects of midazolam are not associated with increased release of DA in the nucleus accumbens.

Dopaminergic regulation of striatal acetylcholine release: importance of D1 and N-methyl-D-aspartate receptors

The dopaminergic regulation of striatal cholinergic activity was studied using in vivo microdialysis to measure interstitial concentrations of acetylcholine (ACh) and choline in the striata of freely moving rats. The quaternary acetylcholinesterase inhibitor neostigmine (100 nM) was included in the perfusion solution to increase the recovery of ACh. d-Amphetamine (2 mg/kg, s.c.) and nomifensine (5 mg/kg, s.c.) increased the concentration of ACh in the striatal dialysate by 40 to 60%. Interstitial choline concentrations were reduced by both drugs. Administration of the selective D1 receptor antagonist SCH 23390 (0.3 mg/kg, s.c.) decreased the concentration of ACh in the striatal dialysate by 15 to 20%; in contrast, the selective D2 antagonist raclopride (1 mg/kg, s.c.) increased striatal ACh release by 50 to 60%. Raclopride also briefly increased the extracellular concentration of choline. Raclopride blocked the increase in locomotor activity produced by d-amphetamine, but did not further enhance ACh release. In contrast, SCH 23390 completely antagonized the increases in locomotion and striatal ACh release produced by d-amphetamine. These results indicate that d-amphetamine increases ACh release in the striatum via a D1 receptor mechanism. Consistent with this hypothesis, the selective D1 receptor agonist CY 208-243 (1 mg/kg, s.c.) increased striatal ACh release by approximately 60%. In contrast, local application of CY 208-243 (10 microM) and SCH 23390 (10 microM) failed to alter ACh concentrations in the striatal dialysate. Inclusion of the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (10 microM) in the striatal perfusion solution significantly attenuated the increase in striatal ACh release produced by systemic CY 208-243.(ABSTRACT TRUNCATED AT 250 WORDS)

Interstitial 3-methoxytyramine reflects striatal dopamine release: an in vivo microdialysis study

Previous ex vivo studies have provided indirect evidence that the dopamine (DA) metabolite 3-methoxytyramine (3-MT) may be a useful index of DA release in vivo. In the present study, in vivo microdialysis was utilized to assess directly the relationship between extracellular DA and 3-MT in the striatum of rats following a variety of pharmacological manipulations. Apomorphine, a DA receptor agonist, produced a rapid, transient decrease in both DA and 3-MT. Conversely, the DA receptor antagonist haloperidol produced a concomitant increase in extracellular DA and 3-MT. Increases in DA and 3-MT were also noted following the administration of the DA uptake inhibitor, bupropion. Local application of tetrodotoxin resulted in the complete elimination of measurable amounts of DA and 3-MT in the dialysate, gamma-Butyrolactone also greatly decreased DA and 3-MT. Finally, d-amphetamine produced a large increase in DA and 3-MT in animals that had been treated previously with gamma-butyrolactone. The Pearson correlation coefficients for DA and 3-MT following these manipulations ranged from 0.87 to 0.97. These data indicate that interstitial 3-MT is an accurate index of DA release. However, when compared with previous ex vivo findings, the present results also suggest that changes in tissue concentrations of 3-MT may not reliably reflect DA release following certain pharmacological manipulations.

Characterization of dopamine release in the substantia nigra by in vivo microdialysis in freely moving rats

Dopamine (DA) is released not only from the terminals of the nigrostriatal projection, but also from the dendrites of these neurons, which arborize in the substantia nigra pars reticulata (SNR). Although striatal DA release has been extensively studied by in vivo microdialysis, dendritic DA release in the SNR has not been characterized by this technique. Extracellular DA was monitored simultaneously in the ipsilateral striatum and SNR. The nigral probe was implanted at a 50 degree angle, permitting 2.5 mm of SNR to be dialyzed. Delivery of the tracer Fluoro-Gold into the striatal probe retrogradely labeled tyrosine hydroxylase-positive cell bodies and dendrites in the vicinity of the nigral probe. Hence, it could be demonstrated that dopaminergic neurons near the nigral probe projected to the vicinity of the striatal probe. Addition of 50 mM KCl to the SNR perfusion solution produced a 3.5-fold increase in DA and a 50% reduction in dihydroxyphenylacetic acid (DOPAC) in the SNR; in contrast, this manipulation in the SNR caused DA release in the striatum to be decreased by 20%, while striatal DOPAC was increased by 50%. Local administration of nomifensine (10 microM) in the SNR produced a sevenfold increase in SNR DA but had no effect on striatal DA. Systemic injection of d-amphetamine (2 mg/kg, s.c.) elevated DA in the SNR and striatum five- to sevenfold, while DOPAC was decreased in both structures by at least 40%. To determine the effect of tetrodotoxin (TTX), basal concentrations of DA in the SNR were first elevated threefold by including nomifensine (1 microM) in the nigral perfusion solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of extracellular histamine in the striatum and bed nucleus of the stria terminalis of the rat: an in vivo microdialysis study

The intracerebral microdialysis technique, coupled with a sensitive radioenzymatic assay, was employed to study histamine release in the striatum and in the bed nucleus of the stria terminalis (BNST) in conscious, freely moving rats. In these brain regions, extracellular histamine concentrations decreased by 20% when calcium was omitted from the perfusion solution. Extracellular histamine was insensitive to the addition of tetrodotoxin to the perfusion medium. In striatum, extracellular histamine concentrations declined in an apparent biexponential manner after the administration of alpha-fluoromethylhistidine, an inhibitor of histamine synthesis. The half-lives for the disappearance of histamine were 32 min and 7.7 h, indicating the presence of at least two histamine pools. Histidine loading resulted in a nearly twofold increase in histamine outflow in striatum. In the BNST, yohimbine increased the extracellular histamine content by 50%, suggesting that histamine release is subject to alpha 2-adrenergic regulation in vivo. The extent to which histamine detected in cerebral microdialysis samples is of neurogenic origin remains to be established.

Cholinergic activity in the rat hippocampus, cortex and striatum correlates with locomotor activity: an in vivo microdialysis study

The possible relationship between behavioral arousal and acetylcholine release in the striatum, hippocampus and frontal cortex was investigated in rats. In vivo microdialysate concentrations of acetylcholine and choline from these brain structures, and photocell beam interruptions (as a measure of behavioral arousal), were measured simultaneously under three conditions: after injections of 1) vehicle or 2) scopolamine (0.4 mg/kg), and 3) before and after the beginning of the rats' night cycle. Dialysate concentrations of ACh in all 3 brain structures and locomotor activity were increased after scopolamine and the onset of the lights out condition. Vehicle injections transiently increased ACh in the hippocampus and cortex and caused short-lasting increases in locomotor activity. Under all conditions, the release of ACh from each of the 3 brain structures correlated with the level of locomotor activity.

Chronic desipramine enhances amphetamine-induced increases in interstitial concentrations of dopamine in the nucleus accumbens

There is accumulating evidence that some antidepressant treatments can increase the functional output of the meso-accumbens dopaminergic system. For example, chronic administration of tricyclic antidepressant drugs such as imipramine and desipramine (DMI) enhances the locomotor stimulant effects of d-amphetamine. Subsensitivity of inhibitory dopamine (DA) autoreceptors and supersensitivity of postsynaptic DA receptor mechanisms are among the mechanisms that have been suggested to underlie these observations. The present experiments investigated the effects of acute and chronic DMI treatment on interstitial DA concentrations in the nucleus accumbens and striatum using in vivo microdialysis in awake freely moving rats (48 h following implantation of a microdialysis probe). Neither acute (5 mg/kg b.i.d. for 2 days followed by 72 h withdrawal) nor chronic (5 mg/kg b.i.d. for 21 days followed by 72 h withdrawal) DMI influenced the ability of apomorphine (25 micrograms/kg s.c.) to decrease extracellular concentrations of DA or its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens. In contrast, d-amphetamine (1.5 mg/kg s.c.)-induced increases in extracellular DA were significantly enhanced in the nucleus accumbens of the chronic but not the acute DMI group. This effect was at least partially regionally selective, as significant effects were not observed in the striatum. In accordance with previous reports, the locomotor stimulant effects of d-amphetamine were also enhanced in the chronic DMI groups. DMI itself failed to alter the interstitial concentrations of DA and its metabolites in the nucleus accumbens of the control and chronic DMI groups. These results provide in vivo neurochemical confirmation that chronically administered DMI does not produce DA autoreceptor subsensitivity. They also demonstrate that chronic DMI-induced increases in the locomotor stimulant effects of d-amphetamine are accompanied by a selective potentiation of the effects of this stimulant on interstitial DA concentrations in the nucleus accumbens.

Behavioral and neurochemical interactions between cocaine and buprenorphine: implications for the pharmacotherapy of cocaine abuse

Intravenous self-administration studies in nonhuman primates suggest that the opioid receptor agonist-antagonist buprenorphine may be useful in the pharmacotherapy of cocaine abuse. In the present studies, behavioral and neurochemical interactions between cocaine and buprenorphine were examined using a conditioned place preference (CPP) procedure and in vivo microdialysis. Cocaine-induced CPP was linearly related to the dose administered (0-5.0 mg/kg). Buprenorphine (0-0.9 mg/kg) also elicited CPP in a dose-related manner; an inverted U-shaped function was obtained. Subthreshold doses of cocaine (1.5 mg/kg) and buprenorphine (0.01 mg/kg), themselves incapable of eliciting CPP, produced a significant CPP when given together. Moderate doses of cocaine (5.0 mg/kg) and buprenorphine (0.075 mg/kg), which were individually capable of eliciting CPP, produced a significantly larger CPP when given in combination. In the in vivo microdialysis studies, a low dose of buprenorphine (0.01 mg/kg) produced a progressive increase in extracellular dopamine in the nucleus accumbens, reaching approximately 200% of basal levels after 5 hr. Cocaine (5.0 mg/kg) rapidly increased extracellular dopamine concentrations (180% of basal values within 20 min), which returned to baseline in 2 to 3 hr. This effect of cocaine was significantly potentiated by coadministering buprenorphine (0.01 mg/kg); under this condition the peak increase in extracellular dopamine reached 260% of baseline values. These neurochemical findings are consistent with the CPP results and indicate that buprenorphine can interact with cocaine in a synergistic manner. In contrast to previous speculations, these results suggest that buprenorphine may enhance rather than attenuate the rewarding properties of cocaine.

The effects of anaesthesia and hypothermia on interstitial concentrations of acetylcholine and choline in rat striatum

The effects of the general anaesthetics pentobarbital, chloral hydrate, and halothane on interstitial concentrations of acetylcholine (ACh) in rat striatum were determined using in vivo microdialysis. All 3 anaesthetics decreased ACh. Emergence from anaesthesia coincided with a recovery of ACh to about 80% of basal values. Pentobarbital increased choline in a profile that was the mirror image of ACh. Chloral hydrate had a biphasic effect on choline, consisting of a shortlasting (20 min) initial decrease followed by an increase. When halothane anaesthetized rats were subjected to forced hypothermia by placing them on ice for 30 min, ACh release was further depressed whereas choline was greatly increased. These finding demonstrate that general anaesthetics decrease extracellular concentrations of ACh in the rat striatum and that this effect can be exacerbated by hypothermia.

Electroconvulsive shock produces large increases in interstitial concentrations of dopamine in the rat striatum: an in vivo microdialysis study

This study examined the effects of electroconvulsive shock (ECS) on interstitial concentrations of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA), and the purine metabolite uric acid, in the striatum using on-line microdialysis in freely moving rats. Interstitial striatal DA increased to 1310% of baseline when the ECS was administered 18 to 24 hours after implantation of the dialysis probe. DOPAC (+ 19%), HVA (+ 30%), 5-HIAA (+10%), and uric acid (+111%) were increased to a smaller extent. The ECS-induced increase in DA was derived from a Ca++ sensitive pool since perfusion of a modified solution in which Ca++ had been replaced with Mg++ blocked this effect.

Behavioral pharmacology and biochemistry of central cholinergic neurotransmission

Systemically administered cholinergic (muscarinic) receptor antagonists can impair the acquisition and post-acquisition performance of a variety of learned behaviors. acquisition performance of a variety of learned behaviors. At present, there is no consensus about the psychological mechanisms underlying these deficits. Behavioral inhibition, working (short-term) memory, reference (long-term) memory, attention, movement and strategy selection, and stimulus processing are among the constructs that have been proposed as underlying the effects of muscarinic receptor blockade. On the basis of neuroanatomical and neuropharmacological considerations it is contended that debates about the nature of the mediating events are pointless because they are on an anatomy that does not exist. Specifically, given that cholinergic neurons innervate almost the entire neuraxis and that muscarinic cholinergic receptors are distributed throughout the central nervous system, it is virtually certain that systemically applied antimuscarinic drugs will influence a broad spectrum of brain functions. In addition, the nature of the deficits produced by scopolamine and atropine, which are competitive antagonists, will depend on the regional endogenous rate of acetylcholine release, which may in turn be influenced by the particular environment and/or level of training imposed on the animal. As the literature seems to indicate, therefore, the effects of competitive antagonists will vary as a function of both the behavioral test and the level of training. Accordingly, attempts at unitary formulations of central cholinergic function are ill-conceived and illusory. Another approach to understanding central cholinergic function has been based on the use of local injections of excitotoxins into brain regions such as the basal forebrain that contain cholinergic neurons. Recent published reports indicate, that many of the behavioral deficits observed after ibotenic acid lesions of the basal forebrain are due primarily to the loss of non-cholinergic neurons. The inherent limitations of the excitotoxin lesion approach for unravelling the functions of central cholinergic systems are such that they cannot produce definitive information and might best, therefore, be abandoned. At present, a reliable selective toxin for cholinergic neurons is not available and urgently required. Until such a compound is identified, local intracerebral applications of antimuscarinic agents may be the preferred procedure for studying the behavioral correlates of regional blockade of cholinergic activity. Brain microdialysis in freely moving animals also holds considerable promise with respect to defining the circumstances under which acetylcholine is released in discrete regions of the central nervous system. At present, the function of central cholinergic systems and the possible role of each in learning and memory remain poorly understood.

Effects of chronic electroconvulsive shock on interstitial concentrations of dopamine in the nucleus accumbens

There is accumulating evidence that chronic electroconvulsive shock (ECS) can increase the functional output of central dopaminergic systems. The present experiments investigated the effects of acute and chronic ECS on interstitial concentrations of dopamine (DA) in the nucleus accumbens (NAC) using in vivo microdialysis in awake freely moving rats. ECS (150 V, 0.75 s) increased interstitial concentrations of DA, DOPAC and HVA to approximately 130% of baseline values. The magnitude of the ECS-induced increase in DA was not affected by chronic ECS. In contrast, the response of the DA metabolites was attenuated in the chronic ECS group. Chronic ECS did not influence apomorphine (25 micrograms/kg, SC)-induced decreases in extracellular concentrations of DA or its metabolites in the NAC, thus providing no support for the hypothesis that chronic ECS produces subsensitivity of DA autoreceptors. d-Amphetamine (1.5 mg/kg SC)-induced increases in extracellular DA were significantly prolonged in the NAC of the chronic ECS group. In accordance with previous reports, the locomotor stimulant effects of d-amphetamine were also enhanced in the chronic ECS group. These data provide further evidence that chronic ECS can increase certain behavioral and neurochemical indices of meso-accumbens DA function.

In vivo neurochemical effects of electroconvulsive shock studied by microdialysis in the rat striatum

The present study examined the effects of electroconvulsive shock (ECS) on interstitial concentrations of dopamine (DA), its metabolites DOPAC and HVA and the serotonin metabolite 5-HIAA in the striatum of freely moving rats using on-line microdialysis. DA increased sharply following a single ECS. Interstitial concentrations of DOPAC. HVA and 5-HIAA also increased significantly. The ECS-induced increase in DA varied as a function of days following implantation of the microdialysis probe, being 1300%, 305% and 300% of baseline 24, 48 and 72 h after surgery, respectively. In contrast, the response of the metabolites to ECS did not differ across days following surgery, being approximately 130%, 140% and 110% of baseline for DOPAC, HVA and 5-HIAA, respectively. Seizure activity induced by the convulsant agent flurothyl did not influence dialysate DA concentrations, suggesting that the ECS-induced DA release was related to the passage of current and not to the seizure activity. Interstitial concentrations of acetylcholine and choline in the striatum increased by approximately 20% and 140%, respectively, in response to a single ECS. The DA (but not the DOPAC or HVA) response to ECS was refractory to a second ECS delivered 2 h after the first. A second ECS delivered 24 h after the first produced the normal increase in DA. The ECS-induced increase in DA was attenuated following repeated ECS (eight treatments, one every second day). Baseline DOPAC and HVA concentrations were significantly elevated by repeated ECS.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopaminergic-cholinergic interactions in the striatum: the critical significance of calcium concentrations in brain microdialysis

Brain microdialysis experiments were performed to assess the effects of calcium (1.2 mmol/l and 3.4 mmol/l) in the perfusion solution on a variety of pharmacological treatments known to affect the release of dopamine (DA) and/or acetylcholine (ACh). Intrastriatal infusion of the muscarinic receptor agonist oxotremorine (100 microM), the selective dopamine D-2 receptor agonist (-)-N-0437 (1 microM), and the indirect DA agonists (+)amphetamine (10 microM) and nomifensine (1 microM) via the dialysis probe did not affect the overflow of ACh when the perfusion fluid contained 3.4 mmol/l calcium. In contrast, these compounds produced pronounced decreases in the overflow of ACh at 1.2 mmol/l calcium. Intrastriatal infusion of the muscarinic receptor antagonist atropine (1 microM) increased the output of ACh both at 1.2 mmol/l and 3.4 mmol/l calcium. The selective DA D-2 receptor antagonist (-)-sulpiride (1 microM) did not affect the overflow of ACh at either calcium concentration. Infusion of oxotremorine and atropine had no effect on the overflow of DA at either 1.2 mmol/l at 3.4 mmol/l calcium. (-)-N-0437 decreased and (-)-sulpiride increased DA overflow, both effects being independent of the calcium concentration in the perfusion fluid. Nomifensine and (+)amphetamine caused relatively (but not absolutely) larger increases in the overflow of DA at 1.2 mmol/l calcium. These findings emphasize the critical importance of the calcium concentration of the perfusion fluid in determining the nature of pharmacological responses in microdialysis experiments, and demonstrate that locally applied dopaminergic drugs can modulate striatal cholinergic function.

Sexual behavior enhances central dopamine transmission in the male rat

Central dopamine transmission was examined in the nucleus accumbens and striatum of sexually experienced male rats during mating behaviour using in vivo brain microdialysis. Dopamine release increased significantly in the nucleus accumbens when males were placed in a novel mating chamber and when a receptive female was introduced behind a screen partitioning this chamber. Subsequently, during copulation dopamine transmission increased sharply, this being followed by a gradual decrease after the female was removed. In contrast, striatal dopamine transmission increased significantly only during copulation. These data provide a neurochemical basis for the well-known interactions between dopaminergic drugs and male sexual behaviour and demonstrate the feasibility of using brain microdialysis to elucidate the neurochemical correlates of motivated behaviour.

Dopamine D1 receptor stimulation increases striatal acetylcholine release in the rat

The effect of selective D1 receptor agonists on acetylcholine (ACh) release in the striatum was investigated using in vivo microdialysis. Administration of the reactive enantiomer, (+)-SKF 38393 (2, 10 mg/kg s.c.), doses which elevate grooming and sniffing behaviour, increased ACh release by 40 and 75%, respectively. Another D1 receptor agonist CY 204-283 (1 mg/kg s.c.) also produced a 75% increase in ACh output. The racemate (+/-)-SFK 38393 (20 mg/kg s.c.) increased ACh output by 60% and this was completely blocked by the D1 receptor antagonist SCH 23390 (0.3 mg/kg s.c.). In contrast, administration of the D2 receptor antagonist raclopride (1 mg/kg s.c.), 60 min after (+/-)-SKF 38393 (20 mg/kg s.c.), further increased ACh release. These results suggest that activation of D1 receptors increases ACh release in vivo and that D1 and D2 receptors have opposing roles in the regulation of striatal ACh release.

Dopamine-acetylcholine interaction in the striatum studied by microdialysis in the awake rat: some methodological aspects

In order to evaluate the dopamine/acetylcholine balance hypothesis, sulpiride, (-)-N-0437, oxotremorine or physostigmine were administered intraperitoneally to rats, whereas the extracellular levels of acetylcholine and dopamine in the striatum were recorded by microdialysis. The changes in dialysate concentration of the transmitters did not support the supposed interaction between dopaminergic and cholinergic neurons. Next, we infused direct and indirect dopamine agonists such as (-)-N-0437, amphetamine and nomifensine via the dialysis membrane. The dopamine agonists had no effect on the release of acetylcholine when the calcium concentration of the perfusion fluid was 3.4 mmol/l, but the agonists effectively inhibited the release of the transmitter when the calcium concentration was 1.2 mmol/l. The cholinergic drugs physostigmine, oxotremorine, atropine did not affect the release of dopamine.

Effects of transient forebrain ischemia and pargyline on extracellular concentrations of dopamine, serotonin, and their metabolites in the rat striatum as determined by in vivo microdialysis

Striatal microdialysis was performed in rats subjected to 20 min of transient forebrain ischemia produced by occlusion of the carotid arteries during hemorrhagic hypotension. Extracellular changes of dopamine, serotonin, and their metabolites were monitored before, during, and after the ischemic insult at 10-min intervals by on-line HPLC analysis. During ischemia, extracellular dopamine increased dramatically (156 times baseline), as did 3-methoxytyramine (3-MT), whereas 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) decreased (15-25% of baseline). Upon reperfusion, dopamine was cleared from the extracellular fluid within 40 min and reached a stable level (70% of baseline). DOPAC and HVA increased (250-330%) transiently and reached their maximum 1 h following reperfusion, whereas 3-MT decreased to undetectable levels within 20 min. Although baseline levels of serotonin were not detectable, serotonin and 5-hydroxyindoleacetic acid showed a qualitatively similar temporal pattern to dopamine and its acid metabolites. Killing rats by cervical dislocation produced changes in extracellular dopamine, serotonin, and their metabolites that were almost identical to those seen during ischemia. Pargyline pretreatment 2 h before ischemia had marginal effects on the postischemic clearing of dopamine. The pargyline pretreatment, however, did increase the survival rate of rats subjected to ischemia, and this protective effect might be due to the pargyline-induced blockade of the post-ischemic monoamine oxidase-mediated increase in dopamine metabolism and the concurrent production of the potentially neurotoxic molecule, hydrogen peroxide.

In vivo characterization of locally applied dopamine uptake inhibitors by striatal microdialysis

In vivo brain microdialysis was used to characterize the effects of some dopamine uptake inhibitors on the extracellular concentrations of dopamine (DA) and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the striata of awake, freely moving rats. d-Amphetamine, GBR 12909, cocaine, nomifensine, methylphenidate, bupropion, and benztropine were administered directly to the striatum via the perfusion fluid in increasing concentrations (1-1,000 microM). All drugs increased extracellular DA in a dose-dependent manner; however, only d-amphetamine produced dose-dependent decreases in DOPAC and HVA concentrations. The shapes of the dose-response functions differed considerably between the drugs. At 100 and 1000 microM d-amphetamine had biphasic effects (an increase followed by a decrease) on dialysate DA concentrations. GBR 12909, methylphenidate, and benztropine also had biphasic effects when applied at the 1,000 microM concentration. In contrast, cocaine, nomifensine, and bupropion produced relatively monophasic increases in extracellular DA. Tetrodotoxin (TTX), which prevents action potentials by blocking voltage-dependent Na+ channels, did not prevent d-amphetamine induced increases in extracellular DA, but blocked completely the effects of cocaine, nomifensine, bupropion, and methylphenidate. While low doses (10 microM) of GBR 12909 and benztropine were highly sensitive to TTX, the toxin was only partially effective against higher doses of the compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute effects of bupropion on extracellular dopamine concentrations in rat striatum and nucleus accumbens studied by in vivo microdialysis

This study examined the acute effects of the novel antidepressant drug, bupropion, on extracellular concentrations of dopamine (DA), its metabolites, and the serotonin metabolite 5-HIAA in the striatum and nucleus accumbens using on-line microdialysis in freely moving rats. Bupropion HCl (10, 25, and 100 mg/kg intraperitoneally) increased extracellular striatal DA in a dose- and time-dependent manner; 1 mg/kg did not affect extracellular DA. The maximal response occurred within the first 20 minutes (+76%, +164%, and +443% for each dose, respectively) followed by a gradual decrease to a stable but elevated level for the next 2 hours. This neurochemical response was strongly associated with bupropion-induced stereotyped behavior during the first hour but not during the subsequent 2 hours. Bupropion decreased DOPAC concentrations, increased 5-HIAA, and had variable effects on homovanillic acid (HVA) (decreases with 10 mg/kg and increases with 25 and 100 mg/kg). The increase in extracellular DA after bupropion (25 mg/kg) was blocked by tetrodotoxin and was therefore action-potential-dependent. Bupropion produced similar neurochemical responses in the striatum and the nucleus accumbens. These results suggest that increases in DA transmission contribute to the behavioral effects of bupropion and are consistent with a role for DA in the antidepressant effects of this drug. The partial dissociation between DA release and stereotyped behavior suggests that the relationship between neurotransmitter release and behavior may be complex.

Lack of tolerance to nicotine-induced dopamine release in the nucleus accumbens

The extent to which repeated administration produces tolerance to nicotine-induced increases in dopamine transmission in the nucleus accumbens was investigated in rats. In vivo microdialysis was used to sample extracellular dopamine and metabolites after a nicotine challenge (0.35 mg/kg) in (1) naive rats, (2) acutely pretreated rats (1 prior nicotine injection), and (3) chronically pretreated rats (12-15 prior daily nicotine injections, 0.35 mg/kg per injection). Nicotine increased extracellular DA and its metabolites, and these increases were not significantly altered by either acute or chronic prior exposure to the drug. The failure to find evidence of tolerance is compatible with the hypothesis that the mesolimbic dopaminergic system is a substrate for the reinforcing properties of chronically administered nicotine.

Effect of ouabain applied by intrastriatal microdialysis on the in vivo release of dopamine, acetylcholine, and amino acids in the brain of conscious rats

In the present study we have applied a brain microdialysis technique to investigate the effects of ouabain infusion on the release of dopamine, acetylcholine, and amino acids from striatal neurons in freely moving rats. Ouabain caused an increase in the dialysate levels of dopamine; its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC); and the amino acids glutamate, aspartate, taurine, glycine, alanine, serine, asparagine, and threonine. The ouabain-induced increase in dopamine was dose dependent and explosive (100-fold at an infusion concentration of 1 mmol/L) and contrasted strongly with the small effect of the glycoside on the output of DOPAC. We investigated the nature of ouabain-induced transmitter release by determining its sensitivity to coinfusion with tetrodotoxin or the calcium antagonist Mg2+. In the case of dopamine two mechanisms of ouabain-induced release could be established. At lower infusion concentrations ouabain induced an exocytotic type of release whereas at higher concentrations the release was probably carrier mediated. In the case of amino acids we noticed a calcium-independent release which was nerve impulse flow dependent in the case of glutamate and aspartate and impulse flow independent in the case of alanine, serine, glycine, threonine, and asparagine. Ouabain induced a decrease in the release of acetylcholine and glutamine.

The effect of systemically applied cholinergic drugs on the striatal release of dopamine and its metabolites, as determined by automated brain dialysis in conscious rats

The effects of cholinergic drugs on the in vivo release of dopamine (DA) and its metabolites were studied in the striatum of freely moving rats. The endogenous compounds were sampled by microdialysis and analysed by on-line HPLC. High doses of oxotremorine (5 mumol/kg), physostigmine (3.6 mumol/kg), nicotine (3 mumol/kg) and atropine (10 mumol/kg) were injected i.p. Oxotremorine, physostigmine and atropine failed to modify the release of DA, while nicotine induced a slight (30%) but significant increase in the release of the transmitter. In contrast, oxotremorine and physostigmine did produce a significant rise of the dialysate contents of the DA metabolites. Thus, these data demonstrate clearly that changes in DA metabolism do not necessarily reflect changes in the release of DA. The most interesting findings of the present study is the fact that muscarinic receptor stimulation or blockade does not modify the release of DA from the rat striatum, while nicotine receptor stimulation may exert some stimulatory effect on the release of DA. This conclusion does not support the concept that the mode of action of anticholinergic drugs used in the treatment of parkinsonism, can be ascribed to a modulation of striatal dopaminergic activity.

Liquid chromatography with electrochemical detection for the determination of choline and acetylcholine in plasma and red blood cells. Failure to detect acetylcholine in blood of humans and mice

An assay is described for the measurement of choline in plasma and red blood cells using liquid chromatography, an enzyme reactor and electrochemical detection after a simple sample pretreatment. The intra-assay coefficient of variation for choline was 6.2 and 3.8% in plasma and in red blood cells, respectively. Using this method we have re-investigated the presence of acetylcholine in blood constituents. We were not able to demonstrate acetylcholine with a limit of detection of 10 pmol per ml of plasma or per ml of red blood cells.

The use of in vivo brain dialysis of dopamine, acetylcholine, amino acids and lactic acid in studies on the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

The use of intracerebral brain dialysis in freely moving rats in neurochemical and neurotoxicological research is discussed and exemplified by studies on the neurotoxin MPTP. Intrastriatal administration of its toxic metabolite MPP+, via the dialysis tube, induced massive changes in the release of neurotransmitters and metabolites. Release enhancing effects could not be repeated by a second MPP+ perfusion and decreases in neurotransmitter or metabolite output were persistent. This indicates that MPP+ has irreversible, toxic effects on various neuortransmitter systems. The MPP+-induced release of DA has been characterized by studying the effect of pretreatment with various drugs, as well by comparison of the time courses of MPP+-induced DA release with those of amphetamine-induced DA release and of MPP+-induced lactate overflow.

The significance of extracellular calcium for the release of dopamine, acetylcholine and amino acids in conscious rats, evaluated by brain microdialysis

The influence of the calcium concentration of the perfusion fluid on the release of striatal dopamine recorded by brain dialysis was investigated. The release of dopamine appeared very sensitive to the calcium concentration of the Ringer. Next we studied whether three different methods known to antagonize the effects of calcium entry, were able to affect the release of dopamine. The conditions investigated were: the use of calcium-free Ringer, infusion of magnesium, and infusion of the calcium-antagonist verapamil. Calcium-antagonism was studied on the day of implantation of the cannula as well as on several days thereafter. It appeared that magnesium infusion was the most effective condition to antagonize the effects of calcium on the release of dopamine. Magnesium infusion was also most effective in preventing drug-evoked voltage-dependent dopamine release (induced by coadministration of haloperidol and GBR 12909). In addition magnesium infusion appeared a potent antagonist of acetylcholine release. In contrast, the dialysate content of aminoacids was not influenced by magnesium infusion.

Differential effects of 4-aminopyridine and 2,4-diaminopyridine on the in vivo release of acetylcholine and dopamine in freely moving rats measured by intrastriatal dialysis

The central effects of 4-aminopyridine (4-AP), a blocking agent of voltage-dependent potassium channels, and its more polar analogue, 2,4-diaminopyridine (2,4-DAP), were studied after i.p. injection and direct intrastriatal administration in rats. The effects of the drugs on the release of acetylcholine (ACh) and dopamine (DA) were quantified by means of an in vivo microdialysis sampling technique. Both neurotransmitters were determined by on-line HPLC analysis. Both aminopyridines increased the release of ACh dose dependently when administered intrastriatally. After peripheral administration, however, 4-AP but not 2,4-DAP induced an increase in the release of ACh. These results are interpreted as being due to the greater lipid solubility of 4-AP compared to 2,4-DAP and hence its better penetration through the blood-brain barrier. Intrastriatal administration of 4-AP induced a much lower increase in the release of DA compared to ACh, whereas there was no change in the release of DA after peripheral administration. These results indicate that the sensitivity of excitable membranes to the releasing effects of 4-AP is not the same for DA- and ACh-containing neurotransmitter systems.

Basal acetylcholine release in freely moving rats detected by on-line trans-striatal dialysis: pharmacological aspects

The basal release of acetylcholine (without the use of an esterase inhibitor) from brain tissue was quantified by means of transversal striatal dialysis in freely moving rats, coupled on-line to an HPLC analysis system. Basal release of acetylcholine was shown to be fully calcium dependent and tetrodotoxin sensitive. A comparison between a U-shaped and a transversally localized dialysis probe was made and some important differences were noticed. The use of a transversal probe resulted in a 20 times higher recovery when compared with the U-shaped cannula. The effect of the cholinomimetic oxotremorine and the anticholinergic atropine on the basal acetylcholine output was determined. Atropine increased the output of acetylcholine, whereas oxotremorine induced a decrease in the output of the transmitter. Application of various degrees of esterase inhibition (by addition of neostigmine to the perfusion fluid) did not qualitatively interfere with the effects of oxotremorine or atropine on the release of acetylcholine.

The use of tetrodotoxin for the characterization of drug-enhanced dopamine release in conscious rats studied by brain dialysis

The effect of TTX (infused during brain dialysis of the striatum and nucleus accumbens) on the in vivo release of dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid, was investigated. In addition it was studied whether the increase in the release of dopamine, induced by various pharmacological treatments, was sensitive to TTX infusion. The following drugs were studied: haloperidol, amphetamine, haloperidol co-administered with GBR 12909, morphine and MPP+. Dialysis was carried out in the striatum, except for morphine, which was studied in the nucleus accumbens. The infusion of TTX revealed three different types of pharmacologically enhanced dopamine release in conscious rats. First, action potential dependent dopamine release (exocytosis), which was observed in untreated animals as well as in animals treated with haloperidol, haloperidol + GBR 12909, and morphine. Second, action potential independent release (carrier-mediated) was established in the case of amphetamine. Third, action potential independent DA release, probably caused by neurotoxic reactions was observed during MPP+ infusion.

Scope and limitations of in vivo brain dialysis: a comparison of its application to various neurotransmitter systems

Brain dialysis is rapidly becoming a routine research method with a wide range of applications. Since 1982 this sampling technique is frequently used as a method to study the in vivo release of endogenous neurotransmitters such as dopamine, noradrenaline, serotonin, acetylcholine and certain amino acids. In this review most of the studies that have appeared in this field, are evaluated. Special attention was given to the question whether the neurotransmitter content in the dialysate is related to neurotransmission. Criteria such as the presence of a high tissue/dialysate concentration ratio, the sensitivity of the transmitters to membrane active compounds and the occurrence of receptor-mediated effects, are discussed. It is concluded that dopamine, noradrenaline and acetylcholine found in the dialysate are directly derived from neurotransmission, whereas the overflow of excitatory amino acid neurotransmitters is related to neurogenic as well as to metabolic events.