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T Sharp

Publications and source records attributed to T Sharp.

At least 55 records · Page 3Linked to original sources
PubMed1989

Pharmacological characterization of 8-OH-DPAT-induced inhibition of rat hippocampal 5-HT release in vivo as measured by microdialysis.

1. We have previously found that the putative 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) decreases hippocampal 5-hydroxytryptamine (5-HT) release in the anaesthetized rat, as measured by brain microdialysis. The present study attempted to characterize the receptor involved in this response using a range of monoamine receptor antagonists. 2. The classical 5-HT receptor antagonists, metergoline (5 mg kg-1 s.c.), methysergide (10 mg kg-1 s.c.) and methiothepin (10 mg kg-1 s.c.) each reduced dialysate levels of 5-HT which complicated their use as antagonists in these experiments. Nevertheless, pretreatment with metergoline but not methiothepin and methysergide partially reduced the 5-HT response to a maximally effective dose of 8-OH-DPAT (0.25 mg kg-1 s.c.). 3. The mixed 5-HT 1/beta-adrenoceptor antagonist pindolol (8 mg kg-1 s.c.) was without effect on spontaneous 5-HT output but attenuated the effect of both maximally (0.25 mg kg-1 s.c.) and submaximally (0.05 mg kg-1 s.c.) effective dose of 8-OH-DPAT. In comparison, propranolol (10 mg kg-1 s.c.) did not affect 5-HT output when injected alone and did not alter the response to 8-OH-DPAT (0.25 mg kg-1 s.c.). 4. The 5-HT2 receptor antagonist ritanserin (0.2 mg kg-1 s.c.) and the 5-HT3 receptor antagonist BRL 43694 (0.5 mg kg-1 s.c.) neither altered 5-HT output alone nor significantly changed the response to 8-OH-DPAT (0.25 mg kg-1 s.c.). 5. The 8-OH-DPAT (0.25 mg kg-' s.c.) response was not affected by pretreatment with either the dopamine D2-receptor antagonist sulpiride (10mgkg-1 s.c.) or the alpha/alpha 2-adrenoceptor antagonist phentolamine (10mg kg-1 s.c.). 6. We conclude from these data that the decrease of hippocampal 5-HT output induced by 8-OHDPAT does not involve 5-HT2, 5-HT3, adrenoceptors or dopamine D2-receptors and that activation of a 5-HT1 class of receptor seems probable. Full classification of the 8-OH-DPAT response awaits development of a suitably selective 5-HT1 receptor antagonist with low intrinsic activity at the somatodendritic 5-HT autoreceptor.

8-Hydroxy-2-(di-n-propylamino)tetralin↗
PubMed1988

In vivo measurement of extracellular dopamine and DOPAC in rat striatum after various dopamine-releasing drugs; implications for the origin of extracellular DOPAC.

In order to further examine the likely origin of the dopamine (DA) metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), certain drugs known to release DA from different intraneuronal pools were tested for their effects on extracellular striatal DA and DOPAC levels by means of brain microdialysis in the halothane-anaesthetized rat. Amphetamine (10(-6) and 10(-5) M), nomifensine (10(-5) M), potassium chloride (30 and 60 mM), methylphenidate (10(-5) and 10(-4) M) and tyramine (10(-5) M), when added to the perfusion medium and administered locally into the striatum via the dialysis membrane, increased the level of DA in striatal perfusates during the 20 min of application. In comparison, the level of DOPAC in the perfusates was decreased by both amphetamine (10(-5) M) and potassium chloride (60 mM), but was not significantly changed by nomifensine, methylphenidate or tyramine. The effect of amphetamine (10(-6) M) and nomifensine (10(-5) M) on DA and DOPAC levels was further studied by administering the drugs over a longer period of time (3 X 20 min). Although both of these treatments produced a similar increase of DA, only amphetamine reduced the levels of DOPAC. DA (10(-4) but not (10(-5) M) increased the levels of DOPAC but this effect was also seen in DA-denervated animals. These data indicate that when the DA nerve terminal is exposed to drugs which release newly synthesized DA, DOPAC declines possibly because intraneuronal monoamine oxidase is deprived of its main substrate. We suggest that these findings support the hypothesis that a major portion of the DA metabolite, DOPAC, is derived from an intraneuronal pool of newly synthesized DA.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1988

Rapid postmortem increase in extracellular dopamine in the rat brain as assessed by brain microdialysis.

Extracellular dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in rat nucleus accumbens were determined before and shortly following death using microdialysis. A maximal 400-fold increase in the output of DA was observed within the first 5 min of death. DA output remained elevated over the following hour at a level of approximately 70-fold above pre-death values. In contrast to that of DA, DOPAC and HVA output gradually declined. Before death the extracellular DOPAC/DA ratio was about 250; after death this ratio dropped to 0.44 at 5 min. These observations may have important implications for experiments measuring the output of (endogenous) DA and its metabolites from brain tissue in vitro: autoregulation of, e.g., transmitter release and synthesis in vitro may be seriously disrupted by the observed depletion of transmitter storage granules.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1987

HPLC-EC analysis of catechols and indoles in rat brain dialysates.

This short article reviews HPLC-EC methodology that we are currently applying to measure DA, 5-HT and their acid metabolites in rat brain dialysates collected in vivo. HPLC-EC systems based on standard-bore HPLC columns are described which are sufficiently sensitive to allow detection of the monoamine transmitters and their metabolites in regional brain dialysates collected at 10-20 min intervals. A large reduction in sample requirement was achieved by "down-scaling" the conventional HPLC-EC assay to incorporate small-bore HPLC columns. The small-bore systems allowed monoamines to be detected in samples collected over 1 to a few minutes.

Animals↗
PubMed1987

A direct comparison of amphetamine-induced behaviours and regional brain dopamine release in the rat using intracerebral dialysis.

An intracerebral microdialysis method was used in awake rats to directly compare the effect of amphetamine on dopamine (DA) release in the striatum and nucleus (n.) accumbens with alterations in behaviour. Amphetamine (0.5-5.0 mg/kg, s.c.) caused a dose-dependent release of DA in both brain regions; however the n. accumbens appeared for the most part more sensitive to amphetamine than the striatum. At each individual dose of the drug, 0.5, 2.0 and 5.0 mg/kg s.c., DA release was closely followed over the time course by the overall behavioural syndrome. Certain components of behaviour showed a regional-specific association with DA release. The intensity of stereotyped head and forepaw movements was closely correlated over the dose range with the amount of DA released in striatum but not n. accumbens. Over the time course, however, the occurrence of this behaviour was delayed compared to increased striatal DA release. In contrast, increased locomotor activity was correlated with the time course change in, and amount of, DA released in n. accumbens by low doses of amphetamine, but not at any dose with DA released in striatum. Repetitive sniffing was better correlated with DA released in n. accumbens than striatum. These in vivo measurements of DA release add further support to the hypothesis that amphetamine-induced stereotypy and locomotion are mediated via DA released in striatum and n. accumbens, respectively. Our data suggest that the occurrence of intense stereotypy rather than locomotor activity at high doses of amphetamine is not due to a selection action in striatum but probably competition between the two behaviours.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Autoregulation of dopamine release and metabolism by intrastriatal nigral grafts as revealed by intracerebral dialysis.

The autoregulation of dopamine release and metabolism by intrastriatal grafts of mesencephalic dopamine neurons was examined in vivo using an intracerebral dialysis technique. Dopamine-rich cell suspension grafts were implanted into the head of the caudate putamen in rats with complete unilateral 6-hydroxydopamine lesion of the nigrostriatal dopamine pathway. Six months later behavioural tests indicated that the grafts had reversed the lesion-induced rotational behaviour. Extracellular levels of striatal dopamine and its metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid were monitored bilaterally in the halothane-anaesthetized grafted rat, both under basal conditions, and also following low (0.05 mg/kg) and high (0.5 mg/kg) doses of the dopamine receptor agonist apomorphine. The perfusate from the grafted striatum showed levels of dopamine which were not statistically different from those of the intact contralateral striatum, indicating that the baseline release of dopamine from the graft was close to normal. Similarly, 3-4-dihydroxyphenylacetic acid and homovanillic acid levels were well recovered on the grafted side (67% and 52%, respectively, of control values). Consistent with previous observations, levels of the serotonin metabolite 5-hydroxyindoleacetic acid measured in perfusate collected from the grafted side was elevated significantly above normal. Subsequent histological analysis revealed large grafts, rich in dopamine-containing neurons (mean +/- SEM number equalled 3138 +/- 630), giving rise to an approximately normal density of dopamine-containing fibres in the area of the host caudate putamen surrounding the probe. Treatment with 0.05 mg/kg (subcutaneous) apomorphine did not affect extracellular dopamine recovered from the grafted striatum, while extracellular DA decreased by a maximum of 30% on the intact side. However, a subsequent injection of 0.5 mg/kg apomorphine produced a large decrease of the dopamine recovered from both the grafted (maximum 40% decrease) and intact striata (maximum 80% decrease). Both the low and the high dose of apomorphine reduced extracellular dopamine metabolite levels, a response which was essentially similar for both the intact and grafted sides. Finally, the dopamine reuptake blocker nomifensine (10(-5) M) added to the perfusion medium produced similar large increases in dopamine in perfusates collected from both grafted and intact striata, while 3,4-dihydroxyphenylacetic acid and homovanillic acid did not change.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

p-Chloroamphetamine releases both serotonin and dopamine into rat brain dialysates in vivo.

The effect of p-chloroamphetamine (PCA) on extracellular levels of endogenous 5-hydroxytryptamine (5-HT) and dopamine (DA) in the striatum and frontal cortex of the halothane-anesthetized rat was studied using an intracerebral dialysis method. PCA (5 mg/kg s.c.) induced an immediate, marked release of 5-HT into dialysates collected from the frontal cortex and striatum, an effect which lasted over 2 h. This treatment also caused a marked release of DA in the striatum. The results further emphasize the need to carefully assess the involvement of brain DA as well as 5-HT in PCA-induced behaviors.

Amphetamines↗
PubMed1986

Further evaluation of the mechanism by which amphetamine reduces striatal dopamine metabolism: a brain dialysis study.

An intracerebral dialysis method was used in the halothane-anaesthetized rat to further clarify the site which mediates the amphetamine-induced decrease of the striatal dopamine (DA) metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Following subcutaneous injection of amphetamine (0.1-5.0 mg/kg), DOPAC and HVA in striatal perfusates decreased over the 2 h time course, with 0.5 mg/kg of the drug having maximal effect. In comparison, amphetamine (0.1-5.0 mg/kg) caused a strictly dose-dependent increase of DA in striatal perfusates. Following low (0.1-5.0 mg/kg) but not high (2.0-5.0 mg/kg) doses of amphetamine there was a negative correlation between the increase of DA and decrease of DOPAC in the striatum. Amphetamine (0.5 mg/kg) caused a reduction in DA metabolism in the ibotenic acid-lesioned striatum. Intranigral injection of 10 micrograms, but not of 1 microgram amphetamine, decreased DOPAC (-27%) in the striatal perfusates. However, injection of 1 microgram amphetamine into the striatum caused a strong decrease in striatal DOPAC (-46%) and HVA (-22%). The possible mechanisms of action of amphetamine are discussed in the light of these data.

Amphetamine↗
PubMed1986

Determination of dopamine and its metabolites in small volumes of rat brain dialysates using small-bore liquid chromatography with electrochemical detection.

A miniaturized liquid chromatographic system with electrochemical detection (LC-ED) was developed and applied to the analysis of dopamine and its metabolites in dialysate samples collected from the rat brain in vivo. An existing LC-ED system was down-scaled using a 1 mm I.D. small-bore column operated at a reduced flow-rate and with an injection volume of 1 microliter. With the small-bore system the limit of detection for dopamine of ca. 0.06 pg was almost 50 times less than with the conventional system, which represents a two-fold improvement in concentration sensitivity. Miniaturization was accomplished with negligible loss in resolution by using a conventional commercial amperometric detector with minor modifications. The results indicate that a number of useful advantages could be realized by the combination of this small-bore LC-ED system and the in vivo brain dialysis method.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1986

Effects of subchronic haloperidol and sulpiride on regional brain dopamine metabolism in the rat.

The DOPAC/dopamine and HVA/dopamine ratios in extracted whole tissue were determined to obtain an index of dopaminergic activity in various rat brain regions 24 h following 10 days' treatment with haloperidol (0.5 and 2 mg/kg) or sulpiride (10 and 100 mg/kg). Both neuroleptics caused a reduction in the metabolite/amine ratio in nucl. accumbens but not frontal cortex or substantia nigra. Haloperidol, but not sulpiride significantly reduced the HVA/dopamine ratio in striatum. A region-specific action of neuroleptics on brain dopamine neurons is discussed.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1986

Effect of sulpiride on amphetamine-induced behaviour in relation to changes in striatal dopamine release in vivo.

Intracerebral dialysis was used to simultaneously monitor extracellular dopamine (DA) in striatum and behaviour in rats following administration of amphetamine and sulpiride. Amphetamine (2 mg/kg s.c.) caused a marked release of DA into striatal perfusates, an effect which was potentiated in rats pretreated with sulpiride (50 mg/kg s.c.). Amphetamine-induced stereotyped head and forepaw movements were potentiated by sulpiride at a time point corresponding with the enhanced release of DA. In comparison, amphetamine-induced locomotor activity was completely inhibited by sulpiride.

Animals↗
PubMed1986

In vivo measurement of spontaneous release and metabolism of dopamine from intrastriatal nigral grafts using intracerebral dialysis.

Spontaneous release and metabolism of dopamine (DA) from intrastriatal grafts of fetal mesencephalic DA neurons was measured by intracerebral dialysis. Mesencephalic DA cell suspensions were implanted into the head of the caudate-putamen in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the mesostriatal DA pathway. Four months later, when tests for amphetamine-induced turning behaviour showed that the grafts had become functional, loops of dialysis tubing were implanted into the striatum on the grafted side and the contralateral non-lesioned side of the grafted rats, and in a similar position in the denervated caudate-putamen of 6-OHDA lesioned control rats. Dialysis perfusates collected from the 6-OHDA lesioned striata showed a reduction of about 95-98% in DA and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). In the grafted animals these levels had recovered to about 40% of control for DA and to 12-16% of control for HVA and DOPAC. In addition, the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) was increased in the grafted striata compared to both the lesioned and non-lesioned controls. Amphetamine had little or no effect on DA release in the 6-OHDA lesioned rats, but caused a marked increase in DA release in the grafted rats, this response being proportional to that seen in intact striata. Since the subsequent histochemical analysis showed that the dialysis probe had been located in the transplant-reinnervated part of the caudate-putamen, the results provide additional evidence that the grafted DA neurons exert their functional effects through a continuous active transmitter release from their newly-established terminals in the reinnervated host target.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1986

Striato-nigral dynorphin and substance P pathways in the rat. II. Functional analysis.

In the present study the functional role of the striato-nigral dynorphin and substance P pathways in rat brain has been studied using the rotational behavioural model and an intracerebral dialysis technique complemented with brain lesions and immunohistochemical analysis. Attempts were made to evaluate whether these striato-nigral neurons have a feed-back modulatory action on the dopaminergic nigro-striatal system, or whether they represent an outflow pathway conveying motor information from the striatum. Unilateral injection of dynorphin A into the substantia nigra reticulata of naive rats induced contralateral rotational behaviour. This effect was dose-dependent and mimicked by the kappa-opioid receptor agonist, U50,488H. Intranigral injection of substance P, as well as substance K, also produced dose-dependent contralateral rotational behaviour. Unilateral injections of ibotenic acid into various sites of the striatum were used to destroy the striato-nigral pathways. The lesions produced a depletion of dynorphin- and substance P-like immunoreactivity in the pars reticulata of the substantia nigra ipsilateral to the lesion and markedly affected the behavioural responses to intranigral peptide injections. Dynorphin A more potently induced contralateral rotation in the lesioned compared to naive non-lesioned rats, suggesting development of supersensitivity for this peptide. Substance P on the other hand, was markedly less potent in inducing rotation in lesioned animals. The rotational responses to both dynorphin A and substance P were potentiated by injection of amphetamine 1 h later, suggesting that both peptides act via nigro-striatal dopamine neurons. However, in rats with unilateral nigro-striatal dopamine denervation, produced with 6-hydroxy-dopamine, dynorphin A retained its potency to induce rotational behaviour; substance P was again much less potent. Thus, both the ibotenic acid and 6-hydroxy-dopamine lesions differently affect the action of dynorphin A and substance P in the zona reticulata of the substantia nigra. The data suggests that substance P requires an intact dopamine pathway to produce the rotational response, while dynorphin A does not. Direct evidence that behavioural activation produced by dynorphin A is not dependent upon dopamine stimulation was obtained by intrastriatal dialysis experiments in which changes in striatal dopamine release were measured following intranigral injection of dynorphin A or substance P. Intranigral dynorphin A in fact reduced, while substance P increased the release of dopamine.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

Effect of dopamine D-1 and D-2 receptor selective drugs on dopamine release and metabolism in rat striatum in vivo.

The effect of the dopamine (DA) D-1 agonist SKF 38393, the D-2 agonist LY 171555 and the mixed D-1/D-2 agonist apomorphine on striatal DA release and metabolism was tested in vivo using an intracerebral dialysis method in halothane-anaesthetized rats. The specificity of responses to these agonists was tested using the selective DA antagonists SCH 23390 (D-1) and sulpiride (D-2). Both LY 171555, 0.01 mg/kg, and SKF 38393, 10 mg/kg, reduced levels of DA in striatal perfusates. Neither SCH 23390, 0.5 and 5 mg/kg, nor sulpiride, 10 mg/kg, affected levels of DA in striatal perfusates, but 250 mg/kg sulpiride caused a DA increase. The decrease of DA levels induced by LY 171555 (0.01 mg/kg) was prevented by pretreatment with sulpiride (10 mg/kg) but not SCH 23390 (0.5 mg/kg). In comparison, pretreatment with SCH 23390 (0.5 mg/kg) completely inhibited the reduction of DA induced by SKF 38393 (10 mg/kg) while sulpiride (10 mg/kg) was without effect. Apomorphine (0.05 mg/kg) also decreased DA in striatal perfusates and this action was partially inhibited by both SCH 23390 (0.5 mg/kg) and sulpiride (10 mg/kg). Levels of the DA metabolite DOPAC in striatal perfusates also significantly decreased following LY 171555 (0.01 mg/kg) and apomorphine (0.05 mg/kg) but not SKF 38393 (10 mg/kg). The antagonist SCH 23390, in a dose, 0.5 mg/kg, that alone did not increase levels of DOPAC, inhibited the reduction of DOPAC induced by both LY 171555 and apomorphine. Sulpiride, 10 mg/kg, caused a marked increase in striatal DOPAC and this was not affected by a subsequent injection of LY 171555, SKF 38393 or apomorphine.(ABSTRACT TRUNCATED AT 250 WORDS)

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗
PubMed1986

Intracerebral dialysis coupled to a novel activity box--a method to monitor dopamine release during behaviour.

This paper describes a method for monitoring drug-induced changes in brain dopamine (DA) release and metabolism in the awake rat simultaneous to measurements of behavioural activation. Intracerebral dialysis was combined with a novel activity box based on a circular running track and a simple method for perfusate collection. "Locomotor" and "general" activity were automatically monitored using a system of photobeams lining the track, and stereotyped behaviours were scored by direct observation. Striatal perfusates were analysed for endogenous DA and its main metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), using HPLC with electrochemical detection. This methodology was tested by comparing the behavioural effects of systemically administered amphetamine (2 mg/kg SC) and its central DA releasing action. Amphetamine caused a marked increase in striatal DA release (16-fold) which was followed over the time course by increased locomotion and stereotyped sniffing and head and forepaw movements. The limitations and wider applications of the methodology are discussed.

3,4-Dihydroxyphenylacetic Acid↗