Biomedical subjects

B H Westerink

Publications and source records attributed to B H Westerink.

At least 19 recordsLinked to original sources

PubMed2001

Estimation of steady-state culture characteristics during acceleration-stats with yeasts.

Steady-state culture characteristics are usually determined in chemostat cultivations, which are very time-consuming. In contrast, acceleration-stat (A-stat) cultivations in which the dilution rate is continuously changed with a constant acceleration rate are not so time-consuming, especially at high acceleration rates. Therefore, the A-stat could be advantageous to use instead of the chemostat. However, the highest acceleration rate, meaning the fastest A-stat that can be applied for estimating steady-state culture characteristics, is not known yet. Experimental results obtained with Zygosaccharomyces rouxii, an important yeast in soy sauce processes, showed that the culture characteristics during the A-stat with an acceleration rate of 0.001 h(-2) were roughly comparable to those of the chemostat. For higher acceleration rates the deviation between the culture characteristics in the A-stat and those in the chemostat obtained at the same dilution rate generally started to increase. The source of these deviations was examined by simulation for Saccharomyces cerevisiae. The simulations demonstrated that this deviation was not only dependent on the metabolic adaptation rate of the yeast, but also on the rate of change in environmental substrate concentrations during A-stats. From this work, it was concluded that an A-stat with an acceleration rate of 0.001 h(-2) is attractive to be used instead of chemostat whenever a rough estimation of steady-state culture characteristics is acceptable.

Adaptation, Physiological↗

PubMed2001

Glycosphingolipids are required for sorting melanosomal proteins in the Golgi complex.

Although glycosphingolipids are ubiquitously expressed and essential for multicellular organisms, surprisingly little is known about their intracellular functions. To explore the role of glycosphingolipids in membrane transport, we used the glycosphingolipid-deficient GM95 mouse melanoma cell line. We found that GM95 cells do not make melanin pigment because tyrosinase, the first and rate-limiting enzyme in melanin synthesis, was not targeted to melanosomes but accumulated in the Golgi complex. However, tyrosinase-related protein 1 still reached melanosomal structures via the plasma membrane instead of the direct pathway from the Golgi. Delivery of lysosomal enzymes from the Golgi complex to endosomes was normal, suggesting that this pathway is not affected by the absence of glycosphingolipids. Loss of pigmentation was due to tyrosinase mislocalization, since transfection of tyrosinase with an extended transmembrane domain, which bypassed the transport block, restored pigmentation. Transfection of ceramide glucosyltransferase or addition of glucosylsphingosine restored tyrosinase transport and pigmentation. We conclude that protein transport from Golgi to melanosomes via the direct pathway requires glycosphingolipids.

Animals↗

PubMed2001

A method to evaluate the diffusion rate of drugs from a microdialysis probe through brain tissue.

For interpretation of microdialysis experiments in which compounds are applied retrodialysis, it is important to have information about the migration rate of the infused compounds. Here we describe a dual-probe microdialysis method that can be used to evaluate the penetration rate of the infused drug. The basic idea is that not the drug itself is assayed, but that its pharmacological effect is recorded by a second probe positioned at a fixed distance (1 mm) of the infusion probe. Using this approach several compounds, each known to induce specific changes in the extracellular levels of dopamine, were infused into the striatum of the rat. The results indicate that the penetration rate of the pharmacological effect of infused compounds differed widely. No effects were seen at the second probe when high potassium chloride was infused. Apparently dopamine was not able to migrate into brain tissue over a distance of 1 mm. Low penetration rates were observed for the dopamine antagonist sulpiride, the dopamine agonist LY17155, and for amphetamine and nomifensine. A very high penetration rate was observed in case of tetrodotoxin. The fast effects of TTX could also be explained by remote inhibition of neurons passing along the infusion probe. The present study showed that most of the compounds have rather slow infusion rates, indicating that relatively high infusion concentrations are needed (1-10 mM) to reach substantial brain concentrations at a distance of 1 mm from the infusion probe.

Amphetamine↗

PubMed2001

The release of noradrenaline in the locus coeruleus and prefrontal cortex studied with dual-probe microdialysis.

The present study was undertaken to investigate and compare the properties of noradrenaline release in the locus coeruleus (LC) and prefrontal cortex (PFC). For that aim the dual-probe microdialysis technique was applied for simultaneous detection of noradrenaline levels in the LC and PFC in conscious rats. Calcium omission in the LC decreased noradrenaline levels in the LC, but increased its levels in the PFC. Novelty increased noradrenaline levels in both structures. Infusion of the alpha(2)-adrenoceptor agonist clonidine decreased extracellular noradrenaline in the LC as well as in the PFC. Infusion of the alpha(2A)-adrenoceptor antagonist BRL44408, or the alpha(1)-adrenoceptor agonist cirazoline into the LC or PFC caused a similar dose-dependent increase in both structures. When BRL44408 or cirazoline were infused into the LC, few effects were seen in the PFC. Infusion of the 5-HT(1A)-receptor agonist flesinoxan into the LC or the PFC decreased the release of noradrenaline in both structures. When flesinoxan was infused into the LC, no effects were seen in the PFC. When the GABA(A) antagonist bicuculline was applied to the LC, noradrenaline increased in the LC as well as in the PFC. It is concluded that the release of noradrenaline from somatodendritic sites and nerve terminals responded in a similar manner to presynaptic receptor modulation. The possible existence of dendritic noradrenaline release is discussed.

Adrenergic alpha-Agonists↗

PubMed2001

Is the beneficial antidepressant effect of coadministration of pindolol really due to somatodendritic autoreceptor antagonism?

BACKGROUND: We investigated the combination of selective serotonin reuptake inhibitors (SSRIs) with the beta-adrenoceptor/serotonin 1A (5-HT(1A)) antagonist pindolol, based on the concept that 5-HT(1A) receptor blockade would eliminate the need for desensitization of presynaptic 5-HT(1A) receptors and therefore hasten the onset of action and improve the efficacy of SSRIs. However, since pindolol plasma levels after 2.5 mg three times a day are about 60 nmol/L, and the K(i) for the 5-HT(1A) receptor is 30 nmol/L, it is questionable whether pindolol levels in the brain would be sufficient to antagonize 5-HT(1A) receptors. Using microdialysis in the guinea pig, we correlated brain and plasma levels of pindolol with its capability of augmenting paroxetine-induced increases in brain 5-HT levels. In addition, central beta-receptor antagonism of pindolol was studied by investigating blockade of beta-agonist-induced increases in brain cyclic adenosine monophosphate (cAMP) formation. METHODS: Using microdialysis and jugular vein catheterization, we studied the ability of systemically administered pindolol to antagonize central 5-HT(1A) and beta-adrenoceptors, while simultaneously monitoring pindolol plasma and brain concentrations. RESULTS: Augmentation of paroxetine-induced increases in extracellular 5-HT levels in the ventral hippocampus was only observed at steady state plasma levels exceeding 7000 nmol/L (concurrent brain levels 600 nmol/L). In contrast, antagonism of beta-agonist-induced increases of brain cAMP levels was already observed at pindolol plasma levels of 70 nmol/L (concurrent brain levels < 3 nmol/L). CONCLUSIONS: At plasma levels that are observed in patients after 2.5 mg three times a day ( approximately 60 nmol/L), pindolol produces only a partial blockade of presynaptic 5-HT(1A) autoreceptors and does not augment the SSRI-induced 5-HT increase in the guinea pig brain. It is therefore very unlikely that the favorable effects of combining pindolol with SSRIs, as reported in a number of clinical studies, are due to 5-HT(1A) antagonism. Since pindolol completely blocks central beta-adrenoreceptors at clinically relevant plasma levels, it is possible that beta-adrenoceptor antagonism is involved in mediating pindolol's beneficial effects.

Animals↗

PubMed2001

The noradrenaline-dopamine interaction in the rat medial prefrontal cortex studied by multi-probe microdialysis.

Multi-probe microdialysis was used to investigate the interaction between the release of noradrenaline and dopamine in the medial prefrontal cortex. Retrograde microdialysis was used to stimulate or inhibit the activity of the locus coeruleus for a restricted period of time, and the response of extracellular noradrenaline and dopamine in the ipsilateral and contralateral medial prefrontal cortex was recorded with microdialysis probes. Infusion of clonidine into the locus coeruleus (100 microM for 45 min) suppressed noradrenaline release and slightly inhibited dopamine release in the ipsilateral medial prefrontal cortex. Application of carbachol to the locus coeruleus (100 microM for 45 min) stimulated both the noradrenaline and dopamine release in the ipsilateral medial prefrontal cortex. No changes were seen in the contralateral medial prefrontal cortex. In the ipsilateral nucleus accumbens, extracellular noradrenaline levels increased, but dopamine levels remained unchanged. Application to the locus coeruleus (during 10 min) of the glutamate receptor agonists N-methyl-D-aspartate (NMDA) (300 microM) or kainate (100 microM) strongly increased extracellular noradrenaline and dopamine levels in the ipsilateral medial prefrontal cortex. However, in the contralateral probe the release of dopamine (but not of noradrenaline) was also stimulated. Application of carbachol to the locus coeruleus was used as a model to further investigate the presumed noradrenaline-dopamine interaction. In a series of dual-probe experiments, alpha(1)-, alpha(2)-, and beta-adrenoceptor antagonists (prazosin, idazoxan, propranolol) or a reuptake-inhibitor (nomifensine) was administered during carbachol stimulation of the locus coeruleus. Prazosin and propranolol were administered systemically in a dose of 3 mg/kg, whereas idazoxan (10 microM) and nomifensine (100 microM) were infused into the medial prefrontal cortex. However, none of these pretreatments modified the effects of the control carbachol-infusions. The results did not identify a receptor-interaction or a common reuptake site that explained the presumed interaction between dopamine and noradrenaline in the medial prefrontal cortex. Therefore, the noradrenaline-dopamine interaction hypothesis could not be confirmed or refuted.

Adrenergic beta-Antagonists↗

PubMed2001

Antipsychotic drugs classified by their effects on the release of dopamine and noradrenaline in the prefrontal cortex and striatum.

Dose-effect curves were established for the effects of the antipsychotic drugs haloperidol, clozapine, olanzapine, risperidone and ziprasidone on extracellular levels of dopamine and noradrenaline in the medial prefrontal cortex, and of dopamine in the striatum. Haloperidol was more effective in stimulating the release of dopamine in the striatum, whereas clozapine was much more effective in the medial prefrontal cortex. The efficacy of risperidone, olanzapine and ziprasidone did not differ for the two brain areas. The benzamides sulpiride and raclopride increased dopamine release in the striatum but did not affect the release of dopamine and noradrenaline in the medial prefrontal cortex. In the presence of dopamine/noradrenaline reuptake inhibitors, the benzamides strongly increased the release of dopamine-but not of noradrenaline-in the medial prefrontal cortex. The 5-HT(2) receptor antagonist R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-piperidinemethanol (MDL100,907) (800 nmol/kg) and the dopamine D(2) receptor antagonist raclopride (2 micromol/kg) displayed a clear synergism in increasing the release of dopamine in the medial prefrontal cortex. No such synergism was seen in the case of noradrenaline. Co-administration of the 5-HT(2) receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine HCl (DOI) (850 nmol/kg) with clozapine (10 micromol/kg) or haloperidol (800 nmol/kg) blocked the increase in dopamine as well as noradrenaline in the medial prefrontal cortex. It is concluded that typical and non-benzamide atypical antipsychotics increase extracellular dopamine in the medial prefrontal cortex via a synergistic interaction by blocking 5-HT(2) as well as dopamine D(2) receptors. The increase in extracellular noradrenaline in the medial prefrontal cortex that was observed after administration of antipsychotics is explained by inhibition of 5-HT(2) receptors and not dopamine D(2) receptors. Finally, the significance of the classification of antipsychotic drugs based on their selective action on the release of dopamine and noradrenaline in the medial prefrontal cortex is discussed. In particular, the position of the benzamides is discussed.

Animals↗

PubMed2001

Acute and chronic effects of citalopram on postsynaptic 5-hydroxytryptamine(1A) receptor-mediated feedback: a microdialysis study in the amygdala.

Microdialysis was used to assess the involvement of postsynaptic 5-hydroxytryptamine(1A) (5-HT(1A)) receptors in the regulation of extracellular 5-HT in the amygdala. Local infusion of the 5-HT(1A) receptor agonist flesinoxan (0.3, 1, 3 microM) for 30 min into the amygdala maximally decreased 5-HT to 50% of basal level. Systemic administration of citalopram (10 micromol/kg) increased 5-HT to 175% of basal level. Local infusion of 1 microM of the 5-HT(1A) receptor antagonist WAY 100.635 into the amygdala augmented the effect of citalopram to more than 500% of basal 5-HT level. 5-HT(1A) receptor responsiveness after chronic citalopram treatment was determined in two ways. First, by local infusion of 1 microM flesinoxan for 30 min into the amygdala, which showed a significant 63% reduction in response (area under the concentration-time curve; AUC) for the citalopram group compared to the saline group. Second, by systemic administration of citalopram (10 micromol/kg), which increased 5-HT to 350% of basal level. The effect was larger than in untreated animals, but more important, local infusion of 1 microM WAY 100.635 into the amygdala now failed to augment the effect of citalopram. Both the flesinoxan and WAY 100.635 data suggest an involvement of postsynaptic 5-HT(1A) receptor-mediated feedback in the amygdala, which diminishes following chronic citalopram treatment.

Amygdala↗

PubMed2000

Analysis of biogenic amines in microdialysates of the brain.

Microdialysis is a method of sampling a liquid compartment by means of a hollow fibre dialysis membrane. The method was developed in the 1980s as a technique for sampling the extracellular fluid of the brain of conscious animals. When used in combination with sensitive analytical chemical tools, microdialysis can be used to study the regulation of neurotransmission in the living brain. Here we describe the application of microdialysis for sampling and detection of biogenic amines (dopamine, noradrenaline and serotonin) in brain tissue. A short overview of the microdialysis technique and its applications are given. In addition, the analytical chemical methods that are currently used to assay biogenic amines in dialysates are briefly discussed.

Animals↗

PubMed2000

A microdialysis study on the mechanism of action of gabapentin.

To gain insight into the mechanism of action of the anti-epileptic, gabapentin, the effects of gabapentin on the in vivo extracellular gamma-aminobutyric acid (GABA) levels in the rat substantia nigra reticulata were studied using microdialysis. In order to investigate possible interference with different GABA-ergic compartments in the substantia nigra reticulata, we studied the effects of gabapentin under basal, K(+)-, nipecotic acid- and glutamate-stimulated conditions. Intraperitoneally (i.p.) administered gabapentin, at a dose of 100 mg/kg, did not significantly affect extracellular GABA levels under any condition. Thus, our data do not support the involvement of nigral GABA release in the mechanism of action of the anti-epileptic gabapentin.

Acetates↗

PubMed2000

Desensitisation of 5-HT autoreceptors upon pharmacokinetically monitored chronic treatment with citalopram.

Rats were chronically treated with the selective serotonin re-uptake inhibitor citalopram [1-(3-dimethylaminopropyl)-1-(4-fluorophenyl)-5-phtalancarbonitril ], by means of osmotic minipumps. Using an infusion concentration of 50 mg/ml citalopram, steady-state plasma concentrations of approximately 0.3 mcM citalopram were maintained for 15 days. Citalopram plasma levels dropped below pharmacologically active concentrations 48 h after removal of the minipumps. Although chronic treatment with citalopram did induce an attenuated response by extracellular levels of 5-hydroxytryptamine (5-HT) after systemic administration of the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), no effect of chronic citalopram treatment was observed when 5-HT(1B) receptor function was evaluated with a local infusion of 5-HT(1B/D) receptor agonist, sumatriptan (3-[2-dimethylamino]ethyl-N-methyl-1H-indole-5methane sulphonamide). Controversially, no augmentation of the increase of 5-HT levels was observed upon systemic administration of citalopram. It is concluded that, although chronic treatment with citalopram does induce desensitisation of 5-HT(1A) receptors, the absence of augmented effects of citalopram on 5-HT levels indicates that other mechanisms compensate for the loss of autoreceptor control.

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

PubMed2000

Augmentation with a 5-HT(1A), but not a 5-HT(1B) receptor antagonist critically depends on the dose of citalopram.

Pharmacokinetic and pharmacodynamic parameters of the selective serotonin reuptake inhibitor 1-(3-dimethylaminopropyl)-1-(4-fluorophenyl)-5-phtalancarbonitril (citalopram) were determined in order to find optimal conditions for augmentation of its effect on extracellular serotonin [5-hydroxytryptamine (5-HT)] through blockade of 5-HT(1A) and 5-HT(1B) autoreceptors. Citalopram dose-dependently (0.3-10 micromol/kg s.c.) increased serotonin levels in ventral hippocampus of conscious rats. At plasma levels above approximately 0.15 microM, the effect of citalopram on extracellular 5-HT was augmented by both a 5-HT(1A) [N-[2-[4-(2-mehoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridil) cyclohexa necarboxamide trihydrochloride (Way 100635), 1 micromol/kg s.c.] and a 5-HT(1B) receptor antagonist (2'-methyl-4'-(5-methyl-[1,2, 4]oxadiazol-3-yl)biphenyl-4-carboxylic acid [4-methoxy]-3-(4-methylpiperazin-1-yl)phenyl]amide (GR 127935), 1 micromol/kg s.c.). However, at plasma levels of the selective serotonin reuptake inhibitor below 0.15 microM, the effects of the antagonists diverged viz. the 5-HT(1B) receptor antagonist was still able to potentiate citalopram's effect on extracellular 5-HT, while the 5-HT(1A) receptor antagonist was no longer effective. These results suggest that in contrast to 5-HT(1B) autoreceptors, indirect activation of 5-HT(1A) autoreceptors by citalopram is critically related to the dose of selective serotonin reuptake inhibitor administered. The latter may have consequences for selective serotonin reuptake inhibitor augmentation strategies with 5-HT(1A) receptor antagonists in the therapy of depression and anxiety disorders.

Animals↗

PubMed2000

The role of afferents to the locus coeruleus in the handling stress-induced increase in the release of noradrenaline in the medial prefrontal cortex: a dual-probe microdialysis study in the rat brain.

This study was aimed to identify the neuronal pathways that mediate the handling stress-induced increase in the release of noradrenaline in the medial prefrontal cortex of the rat brain. For that purpose a microdialysis probe was implanted in the vicinity of the locus coeruleus and a second probe was placed in the ipsilateral medial prefrontal cortex. Receptor specific antagonists acting on the alpha(2)-adrenoceptor (50 microM idazoxan), GABA(A) (50 microM bicuculline), GABA(B) (100 microM (3, 4-Dichlorophenyl)methyl]propyl](diethoxymethyl) phosphonic acid; CGP 52432), acetylcholine (10 microM atropine), corticotropin releasing factor (CRF) (100 microM butyl-ethyl-[2,5-dimethyl-7-(2,4, 6-trimethyl-phenyl)-7H-pyrrolo[2,3-d]pyrimidin-4-yl]-amine; CP-154, 526), NMDA glutamate (300 microM (+/-)-3(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid; CPP) and non-NMDA glutamate receptors (500 microM 6,7-dinitroquinoxaline-2, 3-dione; DNQX) were infused into the locus coeruleus by retrograde dialysis, whereas extracellular noradrenaline was recorded in the ipsilateral medial prefrontal cortex. During infusion of the various compounds rats were gently handled for 10 min. Infusion of idazoxan potentiates the handling-induced increase in the release of noradrenaline in the medial prefrontal cortex. The infusions of, atropine, bicuculline, CGP 52432 and DNQX were without effect on the handling response. Infusion of the NMDA receptor antagonist CPP or the non-peptide CRF receptor antagonist CP-154,526 suppressed the stimulation of noradrenaline during stress. It is concluded that alpha(2)-adrenoceptors, NMDA glutamate receptors and CRF receptors modify the handling stress response of locus coeruleus neurones. The data suggest no major role for glutamatergic, GABAergic, or cholinergic afferents to the locus coeruleus in mediating the stress response.

Animals↗

PubMed2000

Direct interaction between the sympathetic and renin-angiotensin system in myocardial tissue: a microdialysis study in anaesthetised rats.

It has been suggested that local activation of the renin-angiotensin system is involved in early stages of myocardial pathophysiology. To date, there is increasing evidence for interactions between the renin-angiotensin system and the sympathetic nervous system; consequently, local sympathetic activation may also be involved in this. Microdialysis has great potential in the direct investigation of neurohormonal interactions. Therefore, the present study employs microdialysis to study the local effects of exogenous angiotensin II on the interstitial norepinephrine concentration of the normally innervated left ventricle of the anaesthetised rat. The present study investigates the effect of increasing dosages of exogenous angiotensin II on local interstitial norepinephrine. Furthermore, a single dose of losartan was infused on top of the highest dose of angiotensin II, in order to study possible involvement of angiotensin II type 1 (AT1) receptors. Both infusion and sampling were carried out locally, via the microdialysis probes. Concomitantly, circulating norepinephrine levels, heart rate and respiratory rate were monitored to evaluate physiologic stability of the preparation throughout the experiment. Time controls consisted of rats that were perfused with only a Ringer's solution. Angiotensin II induced a dose dependent increase in norepinephrine that was significantly reduced by losartan. Norepinephrine levels in both plasma (infusion experiment and time controls) and the left ventricular wall (time controls) remained stable throughout the experiment, just as heart rate and respiratory rate did. This study for the first time employs microdialysis to demonstrate direct interaction between the sympathetic nervous system and the renin-angiotensin system in the rat left ventricle. The data strongly suggest that AT1 receptors are involved in this interaction, since selective AT1 receptor blockade with losartan significantly reduced the angiotensin II induced norepinephrine concentration.

Angiotensin II↗

PubMed2000

Combined 5-HT2/D2 receptor blockade inhibits the firing rate of SNR neurons in the rat brain.

1. The aim of the present study was to evaluate the contribution of serotonin (5-HT) and dopamine (DA) receptor antagonism to the distinct inhibitory effects of the atypical antipsychotics clozapine and risperidone on SNR neurons, we have shown previously. 2. Utilizing extracellular recordings in the SNR in chloral hydrate anaesthetized rats, raclopride, a selective DA D2/D3 receptor antagonist and LY 53857, a 5-HT2A:2c receptor antagonist were studied separately and in combination for their effects on the firing rate of the SNR neurons. 3. Both raclopride and LY 53857 induced a slight but significant increase in the firing rate of the SNR neurons in a limited dose range. 4. Upon pretreatment with a single dose of raclopride, LY 53857 induced a dose-dependent inhibitory effect on the firing rate of the SNR neurons. 5. Concurrent 5-HT2 and moderate DA D2 receptor antagonism can mimic the in vivo effects of the atypical antipsychotics clozapine and risperidone on the firing rate of SNR neurons.

Animals↗

PubMed1999

Novel non-indolic melatonin receptor agonists differentially entrain endogenous melatonin rhythm and increase its amplitude.

In this study we have examined the ability of melatonin and four synthetic melatonin receptor agonists to entrain endogenous melatonin secretion in rats, free running in constant darkness. The circadian melatonin profile was measured by trans-pineal microdialysis, which not only reveals the time of onset and end of production (phase), but also the amplitude of the rhythm. Exogenous melatonin given at the onset of subjective darkness (clock time 12 h) was effective to entrain endogenous melatonin production. Only one agonist, 2-chloroacetamido-8-methoxytetralin (AH-017), mimicked this action. Two other agonists, 4-methoxy-2-(methylene propylamide)indan (GG-012) and N-[2-[2,3,7,8-tetrahydro-1H-furo(2, 3-g)indol-1-yl]ethyl]acetamide (GR196429), induced a phase-delay under free running conditions, possibly by increasing tau (tau) period. One agonist, 2-acetamido-8-methoxytetralin (AH-001) did not show any phase effect on the free running rhythm. Unexpectedly, all melatonin receptor agonists increased the amplitude of melatonin secretion. The amount of the increase varied from just below the level of significance (AH-001) to an approximately 2-fold increase (GG-012 and GR196429). This is in clear contrast to entrainment with melatonin, which significantly decreased the amplitude. It is hypothesized that entrainment and effects on amplitude of melatonin secretion are mediated by different mechanisms which can be differentially modulated using specific ligands.

Animals↗

PubMed1999

Effects of handling on extracellular levels of glutamate and other amino acids in various areas of the brain measured by microdialysis.

Upon a physiological and pharmacological challenge, the responsiveness of extracellular glutamate levels in the prefrontal cortex, ventral tegmental area and locus coeruleus were studied using microdialysis. A 10-min handling period was used as a mild stressful stimulus. In all three brain areas, handling induced an immediate and short-lasting increase in glutamate levels, but the responses were highly variable. Only in the ventral tegmental area and the locus coeruleus, but not in the prefrontal cortex, the increases were significantly different from basal values. In rats with relatively low basal glutamate levels, both in the ventral tegmental area and locus coeruleus, handling had a more pronounced effect on glutamate levels than in rats with high basal levels, although in some rats with relatively low basal levels of glutamate, handling had hardly any effect. Potassium stimulation also induced variable responses in all three brain areas. Again, relatively low basal glutamate levels were more responsive to the stimulation than higher basal values, although there appeared to be a lower limit. These data suggest that relatively high basal levels contain sources of glutamate that mask the neuronal pool of glutamate and are therefore less responsive to physiological or pharmacological stimulation. However, this interpretation was questioned by the findings that basal levels and handling-induced increases in glutamate levels were found to be (partly) TTX-independent. As carrier-mediated release as a possible non-exocytotic release mechanism has only been described in vivo under pathological conditions, it seems plausible to ascribe TTX-independent glutamate increases to aspecific, non-neuronal processes. This interpretation was further supported by the observation that in all three brain areas, other amino acids, i.e., aspartate, taurine, glutamine, serine, alanine and glycine also increased upon handling in a very similar way as glutamate did. Thus, these results question a direct correlation between stimulated extracellular glutamate levels induced by handling and measured by microdialysis and glutamatergic neurotransmission.

Amino Acids↗

PubMed1999

Tonic regulation of the activity of noradrenergic neurons in the locus coeruleus of the conscious rat studied by dual-probe microdialysis.

In the present study, receptor specific compounds were infused via a microdialysis probe in the vicinity of the right locus coeruleus (LC). During the infusion of these compounds, the extracellular content of noradrenaline was recorded in the ipsilateral medial prefrontal cortex (mPFC) with a second microdialysis probe. Agonists and antagonists of various subtypes receptors that have been described to be localized on LC cells, were infused near the LC. The receptors investigated were: alpha2-adrenergic, muscarinic, nicotinic, GABAergic (GABAA and GABAB), glutamatergic (NMDA and non-NMDA). The compounds infused were: clonidine (100 microM), idazoxan (50 microM), bicuculline (50 microM), muscimol (50 microM), baclofen (50 microM), CGP52432 (100 microM), NMDA (300 microM), CPP (300 microM), kainate (100 microM), DNQX (500 microM), oxotremorine (100 microM), atropine (10 microM), nicotine (100 microM) and mecamylamine (100 microM). Evidence was provided that GABAA, NMDA, non-NMDA glutamate, and muscarinic cholinergic receptors in the LC played roles in controlling the activity of noradrenaline neurons. The LC noradrenergic neurons were not tonically excitated by glutamatergic or cholinergic afferent neurons, and were not tonically inhibited by alpha2 autoreceptors. Tonic inhibition was evident for GABAergic neurons, acting via GABAA receptors.

Animals↗