The effect of neuropeptide FF in the amygdala kindling model

OBJECTIVE

Neuropeptide FF (NPFF) and its receptors (NPFF1 R and NPFF2 R) are differentially distributed throughout the central nervous system. NPFF reduces cortical excitability in rats when administered intracerebroventricularly (i.c.v.), and both NPFF and NPFF1 R antagonists attenuate pilocarpine-induced limbic seizures. In this study, our aim was to determine whether NPFF exerts anticonvulsant or anti-epileptogenic effects in the rat amygdala kindling model for temporal lobe seizures.

METHODS

Male Wistar rats were implanted with a recording/stimulation electrode in the right amygdala and a cannula in the left lateral ventricle. In a first group of animals, the afterdischarge threshold (ADT) was determined after a single i.c.v. infusion of saline (n = 8) or NPFF (1 nmol/h for 2 h; n = 10). Subsequently, daily infusion of saline (n = 8) or NPFF (1 nmol/h for 2 h; i.c.v.; n = 9) was performed, followed by a kindling stimulus (ADT+200 μA). Afterdischarge duration and seizure severity were evaluated after every kindling stimulus. A second group of rats (n = 7) were fully kindled, and the effect of saline or a high dose of NPFF (10 nmol/h for 2 h, i.c.v.) on ADT and the generalized seizure threshold (GST) was subsequently determined.

RESULTS

In naive rats, NPFF significantly increased the ADT compared to control (435 ± 72 μA vs 131 ± 23 μA [P < 0.05]). When rats underwent daily stimulations above the ADT, NPFF did not delay or prevent kindling acquisition. Furthermore, a high dose of NPFF did not alter ADT or GST in fully kindled rats.

CONCLUSIONS

I.c.v. administration of NPFF reduced excitability in the amygdala in naive, but not in fully kindled rats, and had no effect on kindling acquisition.

Using microdialysis to analyse the passage of monovalent nanobodies through the blood-brain barrier

BACKGROUND AND PURPOSE

Nanobodies are promising antigen-binding moieties for molecular imaging and therapeutic purposes because of their favourable pharmacological and pharmacokinetic properties. However, the capability of monovalent nanobodies to reach targets in the CNS remains to be demonstrated.

EXPERIMENTAL APPROACH

We have assessed the blood-brain barrier permeability of Nb_An33, a nanobody against the Trypanosoma brucei brucei variant-specific surface glycoprotein (VSG). This analysis was performed in healthy rats and in rats that were in the encephalitic stage of African trypanosomiasis using intracerebral microdialysis, single photon emission computed tomography (SPECT) or a combination of both methodologies. This enabled the quantification of unlabelled and (99m) Tc-labelled nanobodies using, respectively, a sensitive VSG-based nanobody-detection elisa, radioactivity measurement in collected microdialysates and SPECT image analysis.

KEY RESULTS

The combined read-out methodologies showed that Nb_An33 was detected in the brain of healthy rats following i.v. injection, inflammation-induced damage to the blood-brain barrier, as in the late encephalitic stage of trypanosomiasis, significantly increased the efficiency of passage of the nanobody through this barrier. Complementing SPECT analyses with intracerebral microdialysis improved analysis of brain disposition. There is clear value in assessing penetration of the blood-brain barrier by monovalent nanobodies in models of CNS inflammation. Our data also suggest that rapid clearance from blood might hamper efficient targeting of specific nanobodies to the CNS.

CONCLUSIONS AND IMPLICATIONS

Nanobodies can enter the brain parenchyma from the systemic circulation, especially in pathological conditions where the blood-brain barrier integrity is compromised.

Proconvulsive effects of oxytocin in the generalized pentylenetetrazol mouse model are mediated by vasopressin 1a receptors

The possible involvement of oxytocin (OT) in the generation of seizures has not received a lot of attention in the past, although generalized epileptic convulsions were observed in humans following intravenous OT infusion. We here aimed to investigate the effect of exogenous OT administration on seizure susceptibility in C57Bl/6 mice subjected to the pentylenetetrazol (PTZ) model. In addition, we studied via which receptor possible effects on seizure thresholds could be mediated since OT binds to both the OT receptor (OTR) and the vasopressin 1a receptor (V1aR). We showed that C57Bl/6 mice treated with 0.5 mg/kg OT had decreased PTZ thresholds for ear twitch, myoclonic twitch, tail twitch, forelimb clonus and falling. This pronconvulsive effect was reversed by the OTR antagonist L-368.899, however, it was not mimicked by the OTR agonist carbetocin (CBT). Nevertheless, CBT had antidepressant-like effects in the forced swim test that could be reversed by L-368.899. These experiments shed some doubt on the involvement of OTR in the observed effect of OT on seizure thresholds. Therefore, we investigated the role of the V1aR as a possible mediator of the proconvulsive effects of OT. We found that the proconvulsive effects of both arginine vasopressin and OT were reversed by the V1aR antagonist SR49059. In summary, OT has proconvulsive effects in our mouse model of generalized seizures that could not be mimicked by CBT. Our results suggest that the binding of OT to V1aRs is the most plausible explanation for the proconvulsive effects of OT.

Differential glutamatergic modulation of monoamine release in the limbic lobe by selective anticonvulsant ionotropic and metabotropic glutamate receptor ligands

Several researchers are currently trying to unravel neurobiological relationships between epilepsy and depression. After all, these disorders often develop in the same vulnerable brain regions and the importance of comorbid depression and epilepsy is still underscored. Facilitation of central serotonin (5-HT), dopamine (DA) and noradrenaline (NAD) release seems to be associated with both anticonvulsant and antidepressant effects. We show that selective ionotropic and metabotropic glutamate receptor ligands with anticonvulsant properties differentially modulate NAD, DA and 5-HT in rat limbic lobe structures.

Postischemic mild hypothermia reduces neurotransmitter release and astroglial cell proliferation during reperfusion after asphyxial cardiac arrest in rats

The present study investigated whether postischemic mild hypothermia attenuates the ischemia-induced striatal glutamate (GLU) and dopamine (DA) release, as well as astroglial cell proliferation in the brain. Anesthetized rats were exposed to 8 min of asphyxiation, including 5 min of cardiac arrest. The cardiac arrest was reversed to restoration of spontaneous circulation (ROSC), by brief external heart massage and ventilation within a period of 2 min. After the insult and during reperfusion, the extracellular glutamate and dopamine overflow increased to, respectively, 3000% and 5000% compared with the baseline values in the normothermic group and resulted in brain damage, ischemic neurons and gliosis. However, when hypothermia was induced for a period of 60 min after the insult and restoration of spontaneous circulation, the glutamate and dopamine overflows were not significantly different from that in the sham group. Histological analysis of the brain showed that postischemic mild hypothermia reduced brain damage, ischemic neurons, as well as astroglial cell proliferation. Thus, postischemic mild hypothermia reduces the excitotoxic process, brain damage, as well as astroglial cell proliferation during reperfusion. Moreover, these results emphasize the trigger effect of dopamine on the excitotoxic pathway.

Formalin-induced spinal glutamate release in freely moving rats: comparison of two spinal microdialysis approaches

Two different spinal microdialysis approaches using either a linear tissue probe (LM-3) or a loop probe were explored on freely-moving rats to investigate the basal and formalin-evoked release of glutamate (Glu) in the spinal dorsal horn or in the cerebrospinal fluid (CSF). Adult male Wistar rats were implanted either with a LM-3 probe transversely through the spinal dorsal horn or with a loop probe in the CSF. After 24 hours recovery, microdialysis was initiated with perfusion of modified Ringer's solution at a flow rate of 5 microliters/min and the basal Glu concentrations were sampled for 1 hour. The effects of altering the microdialysis flow rate and perfusion solution on basal Glu release were next investigated. Following the injection of 50 microliters of formalin 5% into the hind paw, 10-min samples were collected for 90 min. The baseline levels of Glu were 0.82 +/- 0.09 microM with LM-3 probes and 5.96 +/- 0.22 microM with the loop probes. Decreasing the flow rate from 5 to 2 microliters/min increased extracellular Glu concentrations by 222.7 +/- 7.3%, whereas perfusion with artificial CSF reduced baseline Glu by 61.5 +/- 9.5% with LM-3 probes. Injection of formalin induced a short-lasting but significant increase of Glu with a similar profile and time course when using either of the microdialysis approaches. In conclusion, microdialysis in the dorsal horn or in the CSF are both effective techniques to assess the alterations in Glu release following peripheral nociceptive input. The loop probe technique in CSF is more reproducible for routine investigation of drug effects, whereas the microdialysis of the dorsal horn provides a useful tool to precisely locate where the release of the neurotransmitters occurs.

Convulsant and subconvulsant doses of norfloxacin in the presence and absence of biphenylacetic acid alter extracellular hippocampal glutamate but not gamma-aminobutyric acid levels in conscious rats

Fluoroquinolones are antibiotics with central excitatory side effects. These adverse effects presumably result from inhibition of gamma-aminobutyric acid (GABA) binding to GABA(A) receptors. This GABA antagonistic effect is greatly potentiated by the active metabolite of fenbufen, biphenylacetic acid (BPAA). Nevertheless, it remains questionable whether GABA receptor antagonism alone can explain the convulsant activity potentials of these antimicrobial agents. The present study was undertaken to investigate the possible effects of norfloxacin, both in the absence and in the presence of BPAA, on the extracellular hippocampal levels of GABA and glutamate, the main central inhibitory and excitatory amino acid neurotransmitters, respectively. This in vivo microdialysis approach with conscious rats allows monitoring of behavioral alterations and concomitant transmitter modulation in the hippocampus. Peroral administration of 100 mg of BPAA per kg of body weight had no effect on behavior and did not significantly alter extracellular GABA or glutamate concentrations. Intravenous perfusion of 300 mg of norfloxacin per kg did not change the rat's behavior or the concomitant neurotransmitter levels in about half of the experiments, while the remaining animals exhibited severe seizures. These norfloxacin-induced convulsions did not affect extracellular hippocampal GABA levels but were accompanied by enhanced glutamate concentrations. Half of the rats receiving both 100 mg of BPAA per kg and 50 mg of norfloxacin per kg displayed lethal seizures, while the remaining animals showed no seizure-related behavior. In the latter subgroup, again no significant alterations in extracellular GABA levels were observed, but glutamate overflow remained significantly elevated for at least 3 h. In conclusion, norfloxacin exerts convulsant activity in rats, accompanied by elevations of extracellular hippocampal glutamate levels but not GABA levels, even in the presence of BPAA.

Effects of dietary sucrose on hippocampal serotonin release: a microdialysis study in the freely-moving rat

The effects of dietary supplementation with either sucrose or starch (50 g/kg regular food for 2 weeks) on central 5-hydroxytryptamine (5HT; serotonin) release were investigated in freely-moving rats. It has been suggested that the amount of transmitter that serotoninergic neurons release might be altered by food intake. We monitored the effects of sucrose and starch on concentrations of extracellular 5HT, its metabolite 5-hydroxyindoleacetic acid (5HIAA), gamma-aminobutyric acid (GABA) and dopamine in the hippocampus, using in vivo microdialysis. The major finding was that baseline levels of extracellular hippocampal 5HT in rats with ad libitum access to food supplemented with sucrose were significantly higher compared with the starch control group. We then verified that sucrose supplementation affected the potency of S(+)fenfluramine to increase hippocampal 5HT levels. In both groups of rats, acute intraperitoneal injection (1 mg/kg) of this anorectic drug induced a response curve of the extracellular hippocampal 5HT levels, with a shape that corresponded with earlier data for different brain areas often using up to 10-fold higher doses of S(+)fenfluramine. Nevertheless, we showed that throughout the experiment the absolute values of the sucrose response curve remained higher than in the starch group. On the other hand, S(+)fenfluramine exerted longer lasting effects in the starch group, as compared with the sucrose group. Significant decreases in levels of extracellular hippocampal 5HIAA levels following S(+)fenfluramine administration were simultaneously observed. A practical implication of the present findings is that dietary sucrose may bias the results of studies investigating brain serotoninergic mechanisms and the effects of (anorectic) drugs interacting with 5HT systems in the hippocampus.

2-chloro-N(6)-cyclopentyladenosine-elicited attenuation of evoked glutamate release is not sufficient to give complete protection against pilocarpine-induced seizures in rats

The effects of 2-chloro-N(6)-cyclopentyladenosine (CCPA) perfused intrahippocampally (1 microM) and injected intraperitoneally (0.5 mg/kg) were investigated in focally-evoked pilocarpine-induced (10 mM) seizures in freely moving rats. While the intrahippocampal perfusion of this highly selective adenosine A(1) receptor agonist gave complete protection against pilocarpine-induced seizures, systemic administration only partially protected the animals, as evaluated by concomitant behavioural and electrocorticographical (ECoG) observations and monitoring of the neurotransmitter alterations. However, pilocarpine-evoked elevation of hippocampal glutamate overflow was significantly attenuated by CCPA irrespective of the mode of administration. Acute pretreatment with systemic 8-cyclopentyl-1,3-dipropylxanthine, a selective A(1) antagonist, reversed both the partial protective effect and the attenuating effect on the extracellular glutamate elicited by systemic CCPA administration. Intrahippocampal CCPA markedly reduced basal hippocampal dopamine efflux but not GABA or glutamate and considerably attenuated the pilocarpine-evoked elevation in dopamine levels. Systemic CCPA appeared to have little influence on the overall pattern of dopamine elevation. The findings give evidence that CCPA-elicited abatement of the evoked glutamate release alone, cannot fully account for its anticonvulsant effect and may suggest that the effects mediated by adenosine on postsynaptic adenosine receptors could be more crucial for its anticonvulsant effect.

The effects of LY393613, nimodipine and verapamil, in focal cerebral ischaemia

This study evaluates the effects of N-(2-[bis (4-fluorophenyl)methoxy]ethyl)-1-butanamine hydrochloride (LY393613), a novel neuronal (N/P/Q-type) Ca(2+) channel blocker, in ischaemia. For comparison, two commonly used L-type Ca(2+) channel blockers; nimodipine and verapamil were also evaluated. Ischaemia was induced in freely moving rats by micro-injection of endothelin-1 near the middle cerebral artery. In vivo microdialysis, laser Doppler flowmetry and histology were used to monitor ischaemia. Administration of LY393613, before and after the insult, attenuated the ischaemia-induced glutamate release, but not the dopamine release. Both nimodipine and verapamil failed to affect transmitter releases significantly, when administered post-occlusion. None of the compounds tested, produced any significant change in striatal blood flow. Histology showed that ischaemic damage was significantly less in LY393613 pre-treated rats. In conclusion, LY393613, a neuronal N/P/Q-Ca(2+) channel blocker, can attenuate ischaemic brain damage. The protective mechanism appears to be mainly the attenuation of the ischaemia-induced glutamate release, rather than its effect on cerebral hemodynamics.

Neurochemical changes and laser Doppler flowmetry in the endothelin-1 rat model for focal cerebral ischemia

Generalized neurotransmitter overflow into the extracellular space, after cerebral ischemia, has been suggested to contribute to subsequent neuronal death. This study aims to investigate the striatal release of the neurotransmitters dopamine (DA), glutamate (Glu) and gamma-aminobutyric acid (GABA) by means of microdialysis, in a rat model for focal transient cerebral ischemia. Ischemia was induced by the application of 120 pmol endothelin-1 (Et-1), adjacent to the middle cerebral artery (MCA) in freely moving rats. Ischemia produced a large increase in extracellular striatal DA concentrations (2400%), Glu (5500%) and GABA (800%) concentrations. Laser Doppler flowmetry in anaesthetized rats, indicated that the blood flow within the striatum decreased by 75+/-11%. The period of sustained drop of blood flow, was dose-dependently related to the concentration Et-1 injected. Histological analysis of brain slices, taken from anaesthetized and conscious animals, indicated a 500 pmol dose of Et-1 was required to produce a similar infarct in anaesthetized rats to a 120 pmol dose of Et-1 in freely moving rats. The immediate drop in striatal blood flow, and the prompt increase of extracellular DA, after the micro-application of Et-1, were quite striking. This suggests that the DA release, rather than the Glu overflow may be the primary event initiating the cascade of processes ultimately leading to cell death and neurological deficits.

In vivo study of the effect of valpromide and valnoctamide in the pilocarpine rat model of focal epilepsy

PURPOSE

We evaluated the effectiveness of the commonly used antiepileptic drug sodium valproate (400 mg/kg) and two of its amide derivatives, valpromide and valnoctamide (both 100 mg/kg), in an in vivo rat model of focal epilepsy. Our main interest was to get insight into possible changes in extracellular amino acid neurotransmitter levels following administration of the drugs, both in control and in epileptic conditions.

METHODS

Seizures were evoked in freely moving rats by intrahippocampal administration of pilocarpine via a microdialysis probe (10 mM for 40 min at 2 microl/min). Microdialysis was also used as in vivo sampling technique and alterations in extracellular hippocampal glutamate and GABA levels were monitored. Electrophysiological evidence for the presence or absence of seizures was simultaneously recorded with electrocorticography.

RESULTS

The focally evoked pilocarpine-induced seizures were completely prevented by acute intraperitoneal pretreatment with each of the three drugs in the respective doses. Effective protection was reflected in the electrocorticographic recordings and in the lack of sustained elevations of the extracellular glutamate levels after pilocarpine perfusion. Little effects were seen on the basal extracellular amino acid levels after systemic administration of each of the compounds, nor after the intrahippocampal administration of sodium valproate.

CONCLUSIONS

Valnoctamide and valpromide (100 mg/kg) proved to be at least as effective as their parent compound sodium valproate (400 mg/kg) against pilocarpine-induced seizures. All three compounds however failed to induce significant initial alterations in extracellular hippocampal GABA release. This questions the enhancement of GABA-mediated inhibition as being one of their mechanisms of action.

Flumazenil prevents diazepam-elicited anticonvulsant action and concomitant attenuation of glutamate overflow

Systemic administration of diazepam (5 mg/kg, i.p.) produced a prompt anticonvulsant effect in pilocarpine-induced seizures in freely moving rats. The anticonvulsant effect was associated with significant attenuation of pilocarpine-evoked increases in extracellular hippocampal glutamate levels to below the baseline levels. The purpose of the present microdialysis study, therefore, was to investigate if the effect of diazepam on glutamate release was mediated at the level of the benzodiazepine gamma-aminobutyric acid(A) (GABA(A)) receptor complex to preclude any non-GABAergic mechanisms. Systemic administration of the specific benzodiazepine-receptor antagonist flumazenil (10 mg/kg, i.p. )-elicited complete reversal of diazepam-evoked anticonvulsant action and concomitant attenuation of extracellular glutamate efflux below the baseline levels. This provides evidence that under the given experimental conditions, diazepam-evoked alterations in glutamate overflow associated with the anticonvulsant action were indeed mediated at the level of benzodiazepine-GABA(A) receptor complex, possibly involving the modulation of both pre- and post-synaptic sites of the receptor complex.

Anticonvulsant effect and neurotransmitter modulation of focal and systemic 2-chloroadenosine against the development of pilocarpine-induced seizures

The present microdialysis study was aimed at evaluating the anticonvulsant effect of the adenosine A(1) receptor agonist 2-chloroadenosine (2-CADO) against pilocarpine-induced seizures in rats. The hippocampal neurotransmitter modulation on the action of 2-CADO and its possible activation of hippocampal adenosine A(2a) receptors was also assessed. Intrahippocampal perfusion of 2-CADO (100 microM) produced a sustained attenuation of baseline dopamine levels, while eliciting a delayed augmentation of both glutamate and GABA efflux. When co-perfused with pilocarpine (10 mM) or injected systemically (7.5 mg/kg), 2-CADO prevented the development of seizures as well as pilocarpine-evoked augmentation of the glutamate and dopamine levels. However, the delayed increase in glutamate overflow with intrahippocampal 2-CADO was still observed. Intraperitoneal injection of selective adenosine A(2a) receptor antagonist SCH 58261 reversed the 2-CADO-elicited attenuation of pilocarpine-induced increment in dopamine efflux and completely abolished the delayed augmentation of glutamate levels, irrespective of perfusion with pilocarpine. Intraperitoneal injection of 5 mg/kg 2-CADO mostly prevented the elevation of pilocarpine-induced glutamate efflux but could not confer adequate protection. We conclude that 2-CADO can prevent pilocarpine-induced seizures by both intrahippocampal perfusion and systemic administration. The attenuation of pilocarpine-induced dopamine efflux and the late elevations of glutamate are likely to be mediated by hippocampal A(2a) receptors. Inhibition of presynaptic glutamate release does not appear to be sufficient for the anticonvulsant action. Postsynaptic events could play a more important role.

Kainate receptors are involved in synaptic plasticity

The ability of synapses to modify their synaptic strength in response to activity is a fundamental property of the nervous system and may be an essential component of learning and memory. There are three classes of ionotropic glutamate receptor, namely NMDA (N-methyl-D-aspartate), AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionic acid) and kainate receptors; critical roles in synaptic plasticity have been identified for two of these. Thus, at many synapses in the brain, transient activation of NMDA receptors leads to a persistent modification in the strength of synaptic transmission mediated by AMPA receptors. Here, to determine whether kainate receptors are involved in synaptic plasticity, we have used a new antagonist, LY382884 ((3S, 4aR, 6S, 8aR)-6-((4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-decahydro isoquinoline-3-carboxylic acid), which antagonizes kainate receptors at concentrations that do not affect AMPA or NMDA receptors. We find that LY382884 is a selective antagonist at neuronal kainate receptors containing the GluR5 subunit. It has no effect on long-term potentiation (LTP) that is dependent on NMDA receptors but prevents the induction of mossy fibre LTP, which is independent of NMDA receptors. Thus, kainate receptors can act as the induction trigger for long-term changes in synaptic transmission.

Effects of diazepam on extracellular brain neurotransmitters in pilocarpine-induced seizures in rats

The present study was undertaken to gain insights into the mechanism of action of diazepam in focally-evoked pilocarpine-induced seizures by concomitantly assessing the changes produced in the extracellular levels of glutamate, GABA (gamma-aminobutyric acid) and dopamine. In vivo microdialysis, coupled to continuous monitoring of electrocorticographic (ECoG) recordings, was performed in freely moving rats. Intrahippocampal perfusion with 10 mM pilocarpine (40 min, 2 microl/min) produced limbic seizures. A single dose of intraperitoneal diazepam (5 mg/kg) was administered 2 h after pilocarpine perfusion was started. Dialysates were sampled both from hippocampus and cerebellum and analysed by microbore liquid chromatography. Diazepam produced instant inhibition of behavioural and ECoG seizure activity. Pilocarpine-induced increases in the extracellular levels of glutamate and dopamine in hippocampus were promptly reduced by diazepam. No concurrent alterations in pilocarpine-induced increases in the extracellular levels of GABA in either hippocampus or cerebellum were seen. Pilocarpine enhanced cerebellar glutamate levels only transiently and levels returned to baseline before diazepam administration. No further changes in cerebellar glutamate levels were observed with diazepam. Our findings suggest that the anti-convulsant action of diazepam against pilocarpine-induced seizures is associated with a prompt attenuation of extracellular hippocampal glutamate overflow without concurrent alteration of pilocarpine-induced increases in endogenous GABA levels. Diazepam also significantly decreased pilocarpine-induced increases in extracellular dopamine levels within the hippocampus. No immediate alterations of the basal levels of the neurotransmitters monitored were observed with diazepam.

Dopaminergic regulation of serotonin release in the substantia nigra of the freely moving rat using microdialysis

The functional regulation by dopamine (DA) receptors of serotonin (5-HT) release from the rat substantia nigra (SN) was investigated using in vivo microdialysis. A D1- and D2-receptor-mediated inhibition of nigral 5-HT release was demonstrated in this study. Continuous administration of the D1-receptor agonist CY 208243 (10 microM) through the probe did not alter extracellular DA nor 5-HT from the SN, whereas intranigral administration of the D1-receptor antagonist SCH-23390 HCl (10 microM) significantly increased both DA (to 214%) and 5-HT release (to 168%) from the SN. Co-perfusion of the D1-receptor agonist and antagonist did not change nigral DA nor 5-HT release compared to perfusion of the antagonist alone. The continuous intranigral perfusion of the D2-receptor agonist, (-)-quinpirole HCl (1 microM) significantly decreased both DA ad 5-HT release to 71% and 78%, respectively. These decreases were abolished when the D2-receptor antagonist S(-)-sulpiride (10 microM) and the D2-receptor agonist (-)-quinpirole HCl (1 microM) were co-perfused. In contrast, the intranigral perfusion of the DA precursor, L-DOPA (5 microM; 1 h), significantly increased nigral and striatal 5-HT release to 202% and 155%, respectively. This enhanced nigral 5-HT release might not be receptor-mediated. The results of the present study suggest a D1 and D2 regulation of nigral 5-HT release, either directly mediated by DA receptors on nigral 5-HT terminals or indirectly by nigral GABA, Glu or Asp. Alternatively, the observed DA-5HT-interaction in the SN might not reflect a local interaction but might involve an interaction at the level of the serotonin cell body region, the dorsal raphe nuclei (DRN).

Biotransformation of locally applied precursors of dopamine, serotonin and noradrenaline in striatum and hippocampus: a microdialysis study

In vivo microdialysis in freely moving rats was used to study the biotransformation, consisting primarily of decarboxylation by aromatic amino acid decarboxylase (AAAD), of the precursors L-3,4-dihydroxyphenylalanine (L-DOPA), L-5-hydroxytryptophan (L-5HTP), and L-threo-3,4-dihydroxyphenylserine (L-threo-DOPS) on extracellular levels of dopamine (DA), serotonin (5HT) and noradrenaline (NA), respectively. The precursors were administered locally through the microdialysis probe into the striatum and into the hippocampus. The different transmitter systems were compared with respect to the ability of the precursors to elevate extracellular levels of their associated transmitter. The basal extracellular concentrations of NA and DA were found to be tetrodotoxin (TTX, a blocker of fast sodium channels) sensitive in striatum and hippocampus, indicating the neuronal origin of the measured transmitters. The extracellular concentrations of 5HT (in hippocampus) were only 60% TTX-sensitive. L-DOPA and L-5HTP showed to be effective precursors of DA and 5HT, respectively, although their formation profile was quite different. The L-DOPA-induced increase in extracellular DA was large and short-lasting, while the L-5HTP-induced increase in 5HT was slower and less pronounced. The relative increase in extracellular DA or 5HT was more pronounced in the brain region where their baseline values were lower, but the absolute amount of transmitter formed from their precursor was similar in both brain regions. L-threo-DOPS was a poor precursor for NA and also failed to influence extracellular DA in striatum, questioning its use in the treatment of freezing gait in late stages of Parkinson's disease.

Effectiveness of vigabatrin against focally evoked pilocarpine-induced seizures and concomitant changes in extracellular hippocampal and cerebellar glutamate, gamma-aminobutyric acid and dopamine levels, a microdialysis-electrocorticography study in freely moving rats

Limbic seizures were evoked in freely moving rats by intrahippocampal administration of the muscarinic agonist pilocarpine via the microdialysis probe (10 mM for 40 min at 2 microl/min). This study monitored changes in extracellular hippocampal gamma-aminobutyric acid (GABA), glutamate and dopamine levels after systemic (30 mg/kg/day) or local (intrahippocampal or intranigral, 5 mM or 600 microM for 180 min at 2 microl/min) vigabatrin administration, and evaluated the effectiveness of this antiepileptic drug against pilocarpine-induced seizure activity. Extracellular GABA and glutamate overflow in the ipsilateral cerebellum was studied simultaneously. Microdialysis was used as an in vivo sampling technique and as a drug-delivery tool. Electrophysiological evidence for the presence or absence of seizures was recorded with electrocorticography. The observed alterations in extracellular hippocampal amino acid levels support the hypothesis that muscarinic receptor stimulation by the intrahippocampal administration of 10 mM pilocarpine is responsible for the seizure onset, and that the amino acids maintain the sustained seizure activity. The focally evoked pilocarpine-induced seizures were completely prevented by intraperitoneal vigabatrin premedication for 7 days or by a single intraperitoneal injection. Effective protection was reflected in a lack of sustained elevations of hippocampal glutamate levels. Rats receiving vigabatrin intrahippocampally or intranigrally still developed seizures, although there appeared to be a partial protective effect. During the intrahippocampal perfusion with 5 mM vigabatrin, extracellular hippocampal GABA levels increased, whereas the extracellular glutamate and dopamine overflow decreased. The lack of a complete neuroprotection after local vigabatrin treatment is discussed.

NMDA receptor-mediated pilocarpine-induced seizures: characterization in freely moving rats by microdialysis

1. Pilocarpine administration has been used as an animal model for temporal lobe epilepsy since it produces several morphological and synaptic features in common with human complex partial seizures. Little is known about changes in extracellular neurotransmitter concentrations during the seizures provoked by pilocarpine, a non-selective muscarinic agonist. 2. Focally evoked pilocarpine-induced seizures in freely moving rats were provoked by intrahippocampal pilocarpine (10 mM for 40 min at a flow rate of 2 microl min(-1)) administration via a microdialysis probe. Concomitant changes in extracellular hippocampal glutamate, gamma-aminobutyric acid (GABA) and dopamine levels were monitored and simultaneous electrocorticography was performed. The animal model was characterized by intrahippocampal perfusion with the muscarinic receptor antagonist atropine (20 mM), the sodium channel blocker tetrodotoxin (1 microM) and the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine maleate, 100 microM). The effectiveness of locally (600 microM) or systemically (10 mg kg(-1) day(-1)) applied lamotrigine against the pilocarpine-induced convulsions was evaluated. 3. Pilocarpine initially decreased extracellular hippocampal glutamate and GABA levels. During the subsequent pilocarpine-induced limbic convulsions extracellular glutamate, GABA and dopamine concentrations in hippocampus were significantly increased. Atropine blocked all changes in extracellular transmitter levels during and after co-administration of pilocarpine. All pilocarpine-induced increases were completely prevented by simultaneous tetrodotoxin perfusion. Intrahippocampal administration of MK-801 and lamotrigine resulted in an elevation of hippocampal dopamine levels and protected the rats from the pilocarpine-induced seizures. Pilocarpine-induced convulsions developed in the rats which received lamotrigine perorally. 4. Pilocarpine-induced seizures are initiated via muscarinic receptors and further mediated via NMDA receptors. Sustained increases in extracellular glutamate levels after pilocarpine perfusion are related to the limbic seizures. These are arguments in favour of earlier described NMDA receptor-mediated excitotoxicity. Hippocampal dopamine release may be functionally important in epileptogenesis and may participate in the anticonvulsant effects of MK-801 and lamotrigine. The pilocarpine-stimulated hippocampal GABA, glutamate and dopamine levels reflect neuronal vesicular release.

Muscarinic modulation of striatal dopamine, glutamate, and GABA release, as measured with in vivo microdialysis

Intrastriatal microdialysis was used to administer muscarinic drugs in freely moving rats for 40 min at a flow rate of 2 microl/min. Administration of the nonselective agonist pilocarpine at 10 mM increased striatal dopamine release and decreased extracellular GABA and glutamate overflow. Perfusion with the muscarinic M2 antagonist methoctramine at 75 microM increased extracellular dopamine and glutamate concentrations but exerted no changes on extracellular GABA levels. Intrastriatal administration of the M1 antagonist pirenzepine at 0.05 microM decreased extracellular dopamine overflow. Application of pirenzepine (0.05 and 5 microM) exerted no effects on the measured GABA or glutamate levels. There are thus important differences in applied doses of muscarinic drugs needed to obtain modulatory effects. High doses of agonists are probably needed to superimpose on the background of tonic influences of striatal acetylcholine, whereas antagonists can block the receptors in small doses. We further suggest that M1 receptors might tonically facilitate striatal dopamine release, that M2 receptors might tonically inhibit striatal glutamate efflux, and that acetylcholine does not exert tonic effects on striatal GABA release. The link with the pilocarpine animal model for temporal lobe epilepsy will be discussed.

Endurance training effects on neurotransmitter release in rat striatum: an in vivo microdialysis study

In the present study we use the in vivo microdialysis sampling technique to register extracellular levels of neurotransmitters in the striatum of trained and untrained rats. We further evaluate the influence of 1 h of exercise on the striatal release of dopamine (DA), noradrenaline (NA), glutamate (GLU) and gamma-aminobutyric acid (GABA) in trained and untrained rats. Male Wistars were randomly assigned to a training or control group. The exercise training consisted of running on a treadmill for 6 weeks, 5 days week-1, with running time and speed gradually increased from 30 min at 19 m min-1 during the first week to 80 min at 26 m min-1 during the final training week. The animals of the control group were placed on the treadmill twice a week, and received a total of four 'adaptation sessions', in which they exercised 15-45 min at 26 m min-1. Brain dialysates were analysed with microbore liquid chromatography (LC), with electrochemical detection (monoamines and GABA) and fluorescence detection (GLU). Soleus citrate synthase and basal striatal concentrations of DA, NA and GLU were significantly different between the trained and control animals. Sixty minutes of exercise significantly increased extracellular DA, NA and GLU levels in both groups, but there was no statistically significant difference in the exercise-induced increase between trained and control animals. There was no statistical difference in basal or exercise-induced GABA levels between trained and control animals. The results indicate that exercise training appears to result in diminished basal activity of striatal neurotransmitters, while maintaining the necessary sensitivity for responses to acute exercise.

Hippocampal and cerebellar extracellular amino acids during pilocarpine-induced seizures in freely moving rats

Limbic seizures were provoked in freely moving rats by intrahippocampal administration of the muscarinic receptor agonist pilocarpine via a microdialysis probe (10 mM for 40 min at 2 microliters/min). Changes in extracellular hippocampal and cerebellar glutamate, aspartate and gamma-aminobutyric acid (GABA) levels were monitored during and after pilocarpine administration. Effects of systemic or local administration of anticonvulsants on the seizures and concomitant changes in amino-acid concentrations, were investigated. Pilocarpine-induced seizures were completely abolished after intraperitoneal premedication for 7 days with phenobarbital (15 mg/kg per day) and after intrahippocampal administration of 10 mM phenobarbital and 1 mM carbamazepine (180 min at 2 microliters/min). Rats premedicated with carbamazepine (5 mg/kg per day) still developed seizures. The changes in extracellular hippocampal amino-acid levels suggest that glutamate, aspartate and GABA are not involved in seizure onset, but may play a role in seizure maintenance and/or spread in the pilocarpine animal model of epilepsy. The increases in extracellular amino acids in ipsi- and contralateral cerebellum following limbic seizures provoked in the hippocampus, probably play a role in the 'reversed' diaschisis phenomenon.

Extracellular striatal dopamine and glutamate after decortication and kainate receptor stimulation, as measured by microdialysis

Disruption of corticostriatal glutamate input in the striatum decreased significantly extracellular striatal glutamate and dopamine levels. Local administration of 300 microM concentration of excitatory receptor agonist kainic acid increased significantly extracellular striatal dopamine in intact freely moving rats. These findings support the hypothesis that glutamate exerts a tonic facilitatory effect on striatal dopamine release. The effect of kainic acid on extracellular striatal glutamate concentration in intact rats was a biphasic increase. The first glutamate increase can be explained by stimulation of presynaptic kainate receptors present on corticostriatal glutamatergic nerve terminals; the second increase is probably the result of a continuous interaction of the different striatal neurotransmitters after disturbance of their balance. Release of dopamine and glutamate was modulated differently in the intact striatum and in the striatum deprived of corticostriatal input. Dopamine release in the denervated striatum after kainate receptor stimulation was significantly lower than in intact striatum, confirming the so-called cooperativity between glutamate and kainic acid. Loss of presynaptic kainate receptors on the glutamatergic nerve terminals after decortication resulted in a loss of effect of kainic acid on glutamate release in denervated striatum. Aspartate showed no significant changes in this study.

Tonic GABA-ergic modulation of striatal dopamine release studied by in vivo microdialysis in the freely moving rat

GABAA and GABAB receptor agonists and antagonists were administered locally in the striatum of intact and kainic acid lesioned rats. (+/-)-Baclofen, a GABAB receptor agonist, significantly decreased the level of extracellular dopamine in the striatum of intact rats. (+/-)-Phaclofen, a GABAB receptor antagonist, increased the level of extracellular dopamine in the striatum of intact rats and to a lesser extent in the striatum after kainic acid lesion. Pregnanolone (5 beta-pregnan-3 alpha-ol-20-one), a positive allosteric modulator of the GABAA receptor, significantly decreased the level of extracellular dopamine in intact rats. (-)-Bicuculline, a GABAB receptor antagonist, increased the level of extracellular dopamine in the striatum of intact rats, but failed to increase the level of extracellular dopamine after kainic acid lesion. The release of extracellular dopamine, due to infusion of phaclofen or bicuculline, was totally suppressed by tetrodotoxin. These results support a direct influence of GABA on the dopaminergic terminals via presynaptic GABAB receptors, while the effects via the GABAA receptor seem to be postsynaptic and mediated by striatal interneurons or the striatonigral feedback loop.

The analysis of excitatory, inhibitory and other amino acids in rat brain microdialysates using microbore liquid chromatography

Three microbore liquid chromatography (LC) assays for determination of amino acids in rat brain dialysates are described: one for separation of amino acids by gradient elution and electrochemical detection, one for analysis of GABA by isocratic elution and electrochemical detection, and one for fast measurement of glutamate and aspartate by gradient elution and fluorescence detection. The assays are reliable, reproducible and sensitive. In comparison with conventional LC, a 5-fold increase in sensitivity was obtained for GABA. Optimization of the derivatization chemistry and the microbore LC system are discussed, as well as important practical aspects.

The use of microdialysis for the determination of plasma protein binding of drugs

Microdialysis sampling was used for the determination of the protein binding and the free therapeutic drug concentration of drugs in plasma in vitro. Several drugs with varying extent of protein binding and for which the plasma monitoring is important were studied. To mimic the in vivo situation, an artificial blood vessel was constructed and filled with spiked plasma circulating at the flow rate of human blood at 37 degrees C. The microdialysis probe (16 mm membrane length, 20000 MW cut off) was placed in the vessel and perfused with 0.9% NaCl at 5 microliters min-1. Dialysates were collected every 10 min and were analysed by reversed-phase LC with UV detection. The free concentration of the drug was calculated by correcting the concentration in the dialysate for the recovery of the probe, which was also determined in the artificial blood vessel after the experiment. The data confirm that microdialysis is a valid alternative technique for the determination of protein binding or free therapeutic plasma concentration of drugs on a comparative basis. Reference to literature values indicates that the results of the proposed method correspond reasonably well with accepted values.