Temporal profile of levels of monoamines and their metabolites in striata of rats implanted with dialysis tubes

Long-lasting effects of feline amygdala kindling on monoamines, seizures and sleep

This report describes the relationship between monoamines, sleep and seizures before and 1-month after amygdala kindling in young cats (<1 year old; n=8; six female and two male). Concentrations (fmoles of norepinephrine or NE, dopamine or DA and serotonin or 5-HT) were quantified in consecutive, 5-min microdialysis samples (2 microl/min infusion rate) from amygdala and locus ceruleus complex (LC) during four, 6-8-h polygraphic recordings before (n=2) and 1 month post-kindling (n=2); 5-min recording epochs were temporally adjusted to correspond to dialysate samples and differentiated according to dominant sleep or waking state (lasting > or =80% of 5-min epoch) and degree of spontaneous seizure activity (number and duration of focal versus generalized spikes and spike trains and behavioral seizure correlates). Post-kindling records in each cat were divided into two groups (n=1 record each) based on higher or lower spontaneous EEG and behavioral seizure activity and compared to pre-kindling records. We found: (1) before and after kindling, NE and 5-HT but not DA concentrations were significantly lower in sleep than waking at both sites; (2) after kindling, each cat showed cyclic patterns, as follows: (a) higher NE, 5-HT and DA concentrations accompanied increased seizure activity with delayed sleep onset latency and increased sleep fragmentation (reduced sleep state percentages, number of epochs and/or epoch duration) in one recording versus (b) lower monoaminergic concentrations accompanied reduced seizure activity, rapid sleep onset and reduced sleep disruption in the other recording. The alternating, post-kindling pattern suggested "rebound" effects which could explain some controversies in the literature about chronic effects of kindling on monoamines and sleep-waking state patterns.

Simultaneous comparison of cerebral dialysis and push-pull perfusion in the brain of rats: a critical review

Over the last 30 years, studies of the in vivo activity of neurotransmitters and other endogenous factors in the brain have comprised a major effort in the neurosciences. Historically, the technology of push-pull perfusion was utilized as a major approach to investigations in this field. In the last 10 years, cerebral dialysis has been used as an alternative method essentially for the same scientific purpose, since the perfusion technique was viewed as difficult and excessively damaging to tissue. This review considers the representative literature in which both systems have been used to study local neurochemical responses to a drug or other chemical factor, a physiological condition or other situation. In addition, new experiments have been undertaken to compare, in the same animal and at the same time, the utility and properties inherent in the techniques of push-pull perfusion and cerebral dialysis in terms of the profile of a neurotransmitter activity and their local histopathological effects. A miniaturized 33/26 ga push-pull needle and a 24 ga dialysis probe were implanted simultaneously in the left and right caudate nuclei, respectively, in the anesthetized rat. An artificial cerebrospinal fluid (CSF) was perfused simultaneously through both devices at a rate of 10 microliters/min in the push-pull cannula and at 1.0 or 2.0 microliters/min in the dialysis probe. Within a series of 8-10 successive perfusions, excess K+ ions in a concentration of either 30 or 60 mM were incorporated in the CSF and delivered simultaneously to both the push-pull cannula and dialysis probe. Samples of perfusate and dialysate were assayed chromatographically by coulometric HPLC detector and quantitated in terms of the pg/min efflux of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA). The results showed that the resting level of DA was almost undetectable in dialysate samples from either structure; in push-pull perfusates the recovery of DA ranged between 7.0 to 10.0 pg/min, which was increased threefold by excess K+ ions. The recovery of DA and the three metabolites in samples of push-pull perfusate was two to four times that in samples of dialysate during the condition of excess K+ ions. Post-mortem histological analysis of the sites of perfusion and dialysis revealed little or no differences in the cytological damage induced by either the perfusion needle or dialysis probe. Finally, the advantages and limitations of each of these two experimental approaches to in vivo analysis of neurotransmitter efflux are reviewed in relation to the selection of an open or closed system for the on-line study of in vivo neurochemical events.

Effect of brain microdialysis on aminergic transmitter levels in repeated cerebral global transient ischemia in rat

The effect of repeated transient global ischemia and microdialysis on changes in aminergic neurotransmitter release was investigated using the rat four-vessel occlusion model of global ischemia. To examine the possible transient or permanent changes in neurotransmitter release, ischemia was induced at varying time points in 5 groups of rats. The first ischemia occurred either 24 h (groups I, II, IV, V) or 96 h (group III) following vertebral artery electro-coagulation and guide probe implantation(s), and the second ischemia was induced either 48 h (groups I, IV, V) or 72 h (group II) following the first ischemia. To assess the consequence of repeated microdialysis on the results, one group of rats (group IV) was not dialysed during the first ischemia and another group (group V) was bilaterally dialysed during the second ischemia. Finally, amphetamine-induced neurotransmitter release was also studied in rats submitted to ischemia and compared with that in normal rats. In each case, dopamine, serotonin and their main metabolites were measured by HPLC with electrochemical detection. Monoamine release was inhibited during the second episode of transient ischemia; this non-release was linked to the repeated microdialysis and not to the repeated ischemia. Although the results of chronic studies using brain microdialysis have been widely recognized as valid, the findings presented here indicate that combined with ischemia, probe reinsertion modifies the level of neurotransmitter release.

Methamphetamine-stimulated striatal dopamine release declines rapidly over time following microdialysis probe insertion

To investigate changes in striatal dopamine release over a series of brief methamphetamine (METH) exposures, METH was pulsed three times at 2-h intervals, with the first exposure occurring 2 h after microdialysis probe insertion. Whether METH was administered directly into the striatum via the microdialysate (20 microM of METH for 10 min), or via peripheral intraperitoneal (i.p.) injection (1 mg/kg METH, i.p.), the dopamine (DA) peak elicited by the third METH exposure was only 50% as large as that elicited by the first exposure, 4 h earlier. This decline in the magnitude of METH-induced DA release probably continued over at least 24 h, since the magnitude of a single peak 26 h after probe implantation was only one-seventh of that at 2 h. This reduction in the response to METH was a function of time post-probe insertion, and not of prior METH exposure. Thus, peak size was the same at 6 h post-implantation in animals which received two prior METH pulses or no prior METH pulses, and in both cases this 6-h peak was substantially lower than that at 2 h post-implantation. Circadian influences were also excluded as a factor, because size of the initial METH-induced DA peak did not vary as a function of time of probe implantation. It is concluded that METH-stimulated striatal DA release declines rapidly over time post-probe insertion. When METH exposures occur repeatedly at short intervals, this decline can mimic, but is not caused by, desensitization or depletion in response to prior METH exposure.

Differential regional and kinetics effects of piribedil and bromocriptine on dopamine metabolites: a brain microdialysis study in freely moving rats

Brain microdialysis coupled to HPLC was applied to freely moving rats to investigate the regional kinetics of piribedil and bromocriptine on the extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in striatum, nucleus accumbens, and frontal cortex. Both D2 agonists (20 mg/kg i.p.) decreased DOPAC and HVA in the three brain regions. The responsiveness of frontal cortex to both compounds was greater than those previously reported with other dopaminergic drugs. Regional and temporal differences were observed under piribedil: DOPAC and HVA levels decreased more in the nucleus accumbens than in striatum or frontal cortex but increased over basal values from the 5th hour in the frontal cortex suggesting a late stimulatory effect of piribedil on dopamine synthesis in this area. Such regional effects differentiate piribedil from most other D2 agonists and could explain some behavioural and therapeutic actions possibly related to involvement of nucleus accumbens or/and frontal cortex.

Regional effects of amphetamine, cocaine, nomifensine and GBR 12909 on the dynamics of dopamine release and metabolism in the rat brain

1. The effects of single-dose regimens of amphetamine, cocaine, nomifensine and GBR 12909 on the dynamics of dopamine (DA) release and metabolism were evaluated in the frontal cortex, hypothalamus, nucleus accumbens and striatum. The regimens selected are known to produce substantial behavioural effects. 2. 3-Methoxytyramine (3MT) and 3,4-dihydroxyphenylacetic acid (DOPAC) rates of formation were used to assess DA metabolism by catechol-O-methyltransferase and monoamine oxidase respectively. The rate of formation of 3MT was used as an index of synaptic DA. The ratio and sum, respectively, of 3MT and DOPAC rates of formation were used to assess DA reuptake inhibition and turnover. 3. The effects of amphetamine on 3MT production and DOPAC steady-state levels were similar in all regions, suggesting similar pharmacodynamic actions. Amphetamine increased 3MT formation and steady-state levels, and reduced DOPAC steady-state levels. DOPAC formation was significantly reduced only in the nucleus accumbens and striatum. Total DA turnover remained unchanged except in the nucleus accumbens. Apparently, the amphetamine-induced increase in DA release occurred at the expense of intraneuronal DA metabolism and did not require stimulation of synthesis. 4. Nomifensine elevated 3MT formation in all regions. A similar effect was produced by cocaine except in the nucleus accumbens. GBR 12909 elevated 3MT production only in the hypothalamus, the striatum and the nucleus accumbens. 5. Cocaine selectively reduced DOPAC formation in the frontal cortex. Nomifensine increased and reduced, respectively, DOPAC formation in striatum and hypothalamus. GBR 12909 elevated DOPAC formation in all regions except the cortex, where pargyline did not reduce DOPAC levels in GBR 12909-treated rats. 6. Ratios and sum of 3MT and DOPAC rates of formation also exhibited wide regional variations for each drug. In contrast to the other drugs, the ratio was not increased after GBR 12909. Apparently, the DA uptake properties of this drug are poorly related to its in vivo effects on the ratio of 3MTproduction to that of DOPAC, which should increase when DA reuptake is inhibited.7. Total DA turnover was increased by GBR 12909 in the hypothalamus, nucleus accumbens and striatum, while cocaine and nomifensine increased it only in the nucleus accumbens and striatum respectively.8. It is concluded that:(a) 3MT and DOPAC rates of formation provide better indices of DA release and metabolism than do their steady-state concentrations.(b) Some effects of DA uptake blockers on DA transmission, especially those of nomifensine and cocaine, may be attributed to increased DA release.(c) Patterns of regional effects of psychostimulants on the dynamics of DA release and metabolism may be better biochemical correlates of stimulant-induced behaviours than would changes in any single region.

Dopaminergic function in rat brain after oral administration of calcium-channel blockers or haloperidol. A microdialysis study

Microdialysis technique was used to study the effects of both acute and repeated oral administration of calcium-channel blockers (flunarizine, cinnarizine, verapamil, nifedipine and nicardipine) in dopaminergic function in rat brain and to compare them to the effects of haloperidol. Acute flunarizine, nicardipine or haloperidol increased extracellular levels of dopamine (DA) or metabolites. After repeated (18 days) administration, nicardipine, nifedipine, verapamil or haloperidol increased and flunarizine decreased extracellular striatal levels of dopamine or metabolites. Chronic treatment with calcium-channel blockers or haloperidol failed to block K(+)-evoked release of dopamine. This suggests that the calcium-channel blockers used in this study do not influence calcium entry necessary for DA release. An acute challenge with haloperidol caused either no change or a decrease in extracellular levels of DA or metabolites after repeated administration of calcium-channel blockers or haloperidol. This is considered to be due to the lesser response of dopaminergic neurons because of treatment. A neuroleptic-like mechanism of action together with a decrease in firing activity and/or a reduced dopamine re-uptake of dopaminergic neurons are considered.

Dopamine release in the rat cerebellum and hippocampus: a tissue 3-methoxytyramine study

Multiple lines of evidence indicate dopamine is a neurotransmitter or neuromodulator in the cerebellum and hippocampus. In this study, we explored the utility of 3-methoxytyramine as an index of dopamine release in these regions. We found that: (1) cerebellar and hippocampal 3-methoxytyramine levels can be measured by combined gas chromatography-mass fragmentography with negative chemical ionization; (2) basal 3-methoxytyramine accumulation rates following monoamine oxidase inhibition, but not the steady-state tissue levels, are several times lower in these regions than in the frontal cortex; (3) accumulation of 3-methoxytyramine in the hippocampus and cerebellum can be enhanced following electroconvulsive shock, but not acute haloperidol (0.4 mg/kg) treatment. We conclude that 3-methoxytyramine accumulation may be a useful index of dopamine release in the cerebellum and hippocampus, but dopamine release is regulated differently in these regions than in the frontal cortex and striatum.

Age-dependent modulation of in vivo cortical acetylcholine release by benzodiazepine receptor ligands

In vivo microdialysis was utilized to determine the effects of benzodiazepine receptor (BZR) ligands on cortical acetylcholine (ACh) release in awake young and aged rats. There were no significant differences in baseline cortical ACh release as a function of age. While administration of the BZR selective inverse agonist ZK 93 426 increased ACh release in both groups of animals, the aged rats exhibited a greater stimulation. Unexpectedly, under the present testing conditions, the BZR agonist chlordiazepoxide (CDP) had no systematic effect on ACh release in either group. The presence or absence of these drug effects or drug-age interactions was not secondary to the impact of these compounds on behavioral activity. Cortical ACh release could also be stimulated by turning off the lights in the observation room or by the systemic administration of scopolamine. Aged rats were at least as able as their younger counterparts to respond to these manipulations with increased release. These results suggest that basal and stimulated release of cortical ACh is not impaired at the ages studied. Moreover, selective inverse BZR agonists may be a potent way of trans-synaptically stimulating cortical cholinergic transmission.

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).

Quantitative microdialysis of dopamine in the striatum: effect of circadian variation

Two quantitative microdialysis methods were used to determine the concentration of extracellular dopamine in the anterior striatum of the rat. In the first method, the slow perfusion flow rate method, perfusion was at 57 nl/min and dialysate samples were collected every 90 min for 18 h and assayed for dopamine (DA), DOPAC (3,4-dihydroxy-phenylacetic acid), homovanillic acid (HVA) and 5-hydroxy-indoleacetic acid (5-HIAA). There was a significant increase in the concentration of dopamine during the dark cycle compared with the light cycle (14.7 +/- 1 nM vs. 9.3 +/- 0.7 nM; mean +/- SEM; P less than 0.0001), indicating possible circadian variations in the extracellular concentration of DA. There was a steady decrease in the level of DOPAC and HVA, and no change in the level of 5-HIAA. For the point of no-net-flux method, animals were perfused with 4 concentrations of DA or DOPAC, bracketing the extracellular concentrations. The extracellular concentrations of DA and DOPAC using this method were 10.2 +/- 1.7 nM and 17.4 +/- 2.6 microM, respectively. The in vivo recoveries for DA and DOPAC as derived from the slope of the linear regression curves were 72 +/- 3% and 43 +/- 5%. These values were shown to be significantly different (P less than 0.001). Both methods gave similar results for the level of DA in the striatum.

On the use of multiple probe insertions at the same site for repeated intracerebral microdialysis experiments in the nigrostriatal dopamine system of rats

The effects of implantation of a dialysis probe into the striatum of awake rats on indices of dopamine (DA) and serotonin neurotransmission were assessed, first over 24 h following initial insertion of a probe, and then again following reinsertion of a probe at the same site 1 week later. It was found that the basal concentration of DA in dialysate stabilized within 20-40 min after probe implantation, although DA showed a modest decline 24 h later. There was, however, no significant difference in basal DA between two test sessions separated by 1 week. On the other hand, the basal concentrations of the DA metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, progressively increased for 2-3 h after probe implantation and decreased markedly by 24 h later. Furthermore, in contrast to DA, the DA metabolites decreased even further after the second probe insertion. Amphetamine-stimulated DA release was also greatly attenuated following the second probe insertion, relative to the first probe insertion. Two probe insertions had only modest effects on the concentration of 5-hydroxyindoleacetic acid in dialysate, relative to the DA metabolites. It is suggested the effects of two probe insertions on DA metabolism and amphetamine-stimulated DA release described here are indicative of probe-induced damage to the nigrostriatal DA system. If this is the case, multiple probe insertions may not provide a feasible strategy for within-subjects design dialysis experiments over extended periods of time, at least in the DA system of small animals. It is suggested further that a stable basal concentration of DA in dialysate may be an especially poor indicator of the integrity of the dopaminergic input to the striatum.

The effects of four days of continuous striatal microdialysis on indices of dopamine and serotonin neurotransmission in rats

The effects of 4 days of continuous microdialysis with a small-diameter concentric-style probe on indices of striatal dopamine (DA) and serotonin neurotransmission were assessed. It was found that over 4 days of dialysis, there was a marked time-dependent decrease in the basal concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in dialysate and in amphetamine-stimulated DA release. In contrast, there was no decrease in basal DA or in the ability of cocaine to elevate the concentration of DA in dialysate over the same period of time. There were only very modest changes in dialysate levels of the serotonin metabolite, 5-hydroxyindoleacetic acid (5-HIAA), relative to the marked changes in DA metabolites. It is suggested that 4 days of continuous dialysis does not result in a non-specific decrease in diffusibility of these compounds into the dialysis probe, but that the changes are more likely due to probe-induced damage to the nigrostriatal DA system. It is also suggested that a "stable" basal concentration of DA in dialysate is an especially poor indicator of the integrity of the dopaminergic input to the striatum. The implications of these findings for within-subjects design microdialysis experiments are discussed.

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.

In vivo characterisation of extracellular dopamine, GABA and acetylcholine from the dorsolateral striatum of awake freely moving rats by chronic microdialysis

Basal extracellular (EC) DA, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), gamma aminobutyric acid (GABA) and acetylcholine (ACh) were measured in dialysates from the dorsolateral striatum (DLS) of awake rats, every 30 min for 4.5 h each day over a 4-day period. The responsiveness of basal EC DA, DOPAC, HVA and GABA to local perfusion with tetrodotoxin (1 micron) was measured 1 and 4 days after implantation. In addition EC ACh was also measured 4 days after probe implantation. The results of this study indicate that EC levels of DA, DOPAC, HVA, GABA and ACh can be reliably monitored for up to 4 days after probe implantation. In addition, we show that striatal EC levels of DA, GABA and ACh may be regarded as a reflection of ongoing neuronal activity for up to 4 days after implantation of a microdialysis probe.

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.

Effect of varying the ionic concentration of a microdialysis perfusate on basal striatal dopamine levels in awake rats

In vivo microdialysis was used to study the effects of Ca2+, Mg2+, and K+ ion concentrations on basal extracellular (EC) levels of striatal DA and metabolites in awake rats on the second day (48 h) after implantation of a microdialysis probe. Basal EC striatal dopamine (DA) levels were markedly (90%) and reversibly reduced by removal and subsequent replacement of Ca2+ ions from the microdialysis perfusate. This implies that the EC DA in this preparation is primarily of synaptic origin. The addition and subsequent removal of 1.7 mM MgCl2 to the Mg2(+)-free perfusate produced a reversible decrease (20%) in basal EC DA levels. This decrease may reflect a competitive interaction between Ca2+ and Mg2+ in the process of vesicular release. Basal EC DA levels were also reduced (27%) by decreasing the K+ concentration of the perfusate from 4 mM to 3 mM. However, after restoring the K+ concentration to 4 mM, EC DA levels were slow to return to pretreatment levels. Basal EC 3,4-dihydroxyphenylacetic acid and homovanillic acid levels exhibited a parallel but diminished response to each manipulation of the ionic concentration of the perfusate. This study demonstrates that small variations in the concentrations of Ca2+, Mg2+, and K+ in the perfusate employed in microdialysis preparations will affect basal EC striatal DA and metabolite levels.

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)

Concurrent on-line sampling of melatonin in pineal microdialysates from conscious rat and its analysis by high-performance liquid chromatography with electrochemical detection

Dynamic changes of melatonin in microdialysates from the pineal gland of a freely moving rat were repeatedly determined by using on-line high-performance liquid chromatography with electrochemical detection. The detection limit for melatonin, ca. 5 pg, was well below that achieved with other systems. We observed a drastic increase of extracellular pineal melatonin during the transitional phase from the light period to the dark period. This application of microdialysis is a useful tool in the study of the physiological role of the mammalian pineal body.