Qing-Shan Yan

Dizocilpine (MK-801) increases not only dopamine but also serotonin and norepinephrine transmissions in the nucleus accumbens as measured by microdialysis in freely moving rats

The extracellular concentrations of dopamine (DA), norepinephrine (NE), and serotonin (5-HT) in the nucleus accumbens (NACC) of freely moving rats were monitored simultaneously via intracerebral microdialysis. Local infusion of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine) (5-250 microM) produced significant increases in extracellular levels of DA, NE and 5-HT in a concentration-dependent fashion. Perfusion with tetrodotoxin (TTX, 1 microM) blocked the ability of focal MK-801 (50 microM) to increase DA, NE and 5-HT in the dialysate. Systemic administration of MK-801 (0.3 mg/kg, i.p.) also produced small, but statistically significant, increases in extracellular concentrations of DA, NE and 5-HT in the NACC. Our microdialysis results are consistent with the hypothesis that, in addition to dopaminergic, serotonergic and noradrenergic neurotransmissions in the NACC are involved in the mechanism by which MK-801 alters behavior in rats. Also, the present study gives further support to the concept that NMDA receptors within the NACC do not regulate DA release through direct excitatory control.

Evidence that a serotonergic mechanism is involved in the anticonvulsant effect of fluoxetine in genetically epilepsy-prone rats

Fluoxetine (15 mg/kg i.p.) decreased the audiogenic seizure intensity in 33% of severe seizure genetically epilepsy-prone rats (GEPR-9s). 5-Hydroxytryptophan (5-HTP, 12.5 mg/kg i.p.) produced no anticonvulsant effect in GEPR-9s. When GEPR-9s were treated with a combination of these two drugs, the combination treatment decreased the audiogenic seizure intensity in 83% of the animals tested. Brain microdialysis studies showed that the same combination of 5-HTP and fluoxetine also produced a marked potentiation of the increase in the extracellular serotonin concentration in the thalamus of freely-moving GEPR-9s when compared with administration of either drug alone. A negative correlation between audiogenic seizure intensity and extracellular serotonin concentration existed after either fluoxetine alone or the combination treatment. No significant changes in extracellular norepinephrine concentrations were observed after the combination treatment. These results coupled with our earlier reports strongly suggest that a serotonergic mechanism is involved in the anticonvulsant effects of fluoxetine in GEPRs.

Activation of 5-HT1B/1D receptors in the mesolimbic dopamine system increases dopamine release from the nucleus accumbens: a microdialysis study

This study was designed to investigate the role of 5-hydroxytryptamine (5-HT)(1B) receptors located in the ventral tegmental area and nucleus accumbens in the modulation of accumbal dopaminergic transmission. The selective 5-HT(1B) receptor agonist CP 93129 [3-(1,2,5,6-tetrahydro-4-pyridyl)pyrrolo[3,2-b]pyrid-5-one] was administered into the ventral tegmental area or nucleus accumbens of freely moving Sprague-Dawley rats via retrograde microdialysis. The effects of intra-accumbal and intra-tegmental CP 93129 on extracellular dopamine levels in the nucleus accumbens were measured using one- and dual-probe microdialysis, respectively. For dual-probe microdialysis, one probe was in the ventral tegmental area for drug administration and the other in the ipsilateral nucleus accumbens for dopamine measurement. The results show that infusion of CP 93129 (2, 5 and 10 microM) into the nucleus accumbens increased local dopamine levels in a concentration-related manner. Infusion of CP 93129 (10 and 20 microM) into the ventral tegmental area also increased dopamine levels in the ipsilateral nucleus accumbens. The increased dopamine release in the nucleus accumbens produced by intra-accumbal or intra-tegmental CP 93129 was antagonized by co-infusion of cyanopindolol (5 microM), a 5-HT(1B/1A) receptor antagonist, but not by WAY-100635 [N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-2-pyridinyl-cyclohexanecarboxamide] (5 microM), a highly selective 5-HT(1A) receptor antagonist. In addition, augmentations of dopamine release in the nucleus accumbens induced by intra-accumbal CP 93129 were sensitive to Na(+) channel blockade with tetrodotoxin. These results are not in opposition to the concept that 5-HT(1B) receptors within the ventral tegmental area and nucleus accumbens are all involved in the modulation of dopamine release in the terminal area of the mesolimbic dopamine system.

Neurochemical correlates of antiepileptic drugs in the genetically epilepsy-prone rat (GEPR)

The GEPR model is composed of two independently derived strains of rats each characterized by a broad-based seizure predisposition. Moderate seizure GEPRs (GEPR-3s) exhibit generalized clonus with loss of righting reflex in response to a standardized sound stimulus. The same stimulus in severe seizure GEPRs (GEPR-9s) produces a tonic-clonic convulsion much like that produced by supramaximal electroshock. The numeric descriptors (3 and 9) derive from the ordinal rating scale developed by Jobe and coworkers for evaluation of convulsion intensity. GEPRs experience an anticonvulsant effect in response to all established and many experimental antiepileptic drugs and distinctions between the classes of drugs can be made. Since serotonin plays an anticonvulsant role in nearly all animal seizure models, we examined the effects of antiepileptic drugs on serotonin using microdialysis. Among clinically effective anticonvulsants, carbamazepine, antiepilepsirine (used in China) and loreclezole produced dose-related anticonvulsant effects and increases in extracellular serotonin in GEPRs. Similarly, drugs known to block serotonin reuptake and increase extracellular serotonin (fluoxetine and sertraline) produce dose related anticonvulsant effects in GEPRs and other animal models. Accentuation of serotonin release by treating GEPRs with fluoxetine and 5-hydroxytryptophan enhances the anticonvulsant effect produced by fluoxetine. Depletion of serotonin greatly decreased the anticonvulsant effect produced by carbamazepine, antiepilepsirine and fluoxetine. Phenytoin produced a dose related anticonvulsant effect in GEPRs but did not increase extracellular serotonin. Depletion of serotonin did not diminish the anticonvulsant effect produced by phenytoin. Thus, serotonin appears to play a role in the anticonvulsant effect of several but not all anticonvulsant drugs.

Carbamazepine increases extracellular serotonin concentration: lack of antagonism by tetrodotoxin or zero Ca2+

Carbamazepine administration causes large increases in extracellular serotonin concentration and dose-related anticonvulsant effects in genetically epilepsy-prone rats (GEPRs). In order to determine the generality of the effect on serotonin, we determined the anticonvulsant ED50 for carbamazepine against maximal electroshock seizures in outbred, non-epileptic Sprague-Dawley rats. We then administered anticonvulsant carbamazepine doses to Sprague-Dawley rats and observed extracellular serotonin concentration in hippocampi by way of microdialysis. We found that administration of carbamazepine, either systemically or through the dialysis probe, resulted in significant and dose-related increases in extracellular serotonin concentration. Basal serotonin release was decreased by tetrodotoxin administration through the dialysis probe. Tetrodotoxin administration through the dialysis probe did not decrease the effect of systemically or focally administered carbamazepine on extracellular serotonin concentration. Similarly, elimination of Ca2+ from the dialysate did not alter the release of serotonin caused by carbamazepine. These findings suggest that the serotonin releasing effect of carbamazepine does not take place by exocytosis and does not require action potentials in the brain area in which the release takes place. Further they suggest that the effect is mediated by an action of carbamazepine directly on serotonergic nerve terminals.

Anticonvulsant doses of carbamazepine increase hippocampal extracellular serotonin in genetically epilepsy-prone rats: dose response relationships

The antiepileptic drug carbamazepine produces dose related anticonvulsant effects in genetically epilepsy-prone rats (GEPRs) and most other animal seizure models. Carbamazepine releases serotonin as part of the pharmacodynamic action by which it suppresses convulsions in GEPRs and it releases serotonin in non-epileptic Sprague-Dawley rats. The two strains which make up the GEPR seizure model (moderate seizure GEPR-3s and severe seizure GEPR-9s) experience anticonvulsant effects in response to different doses of carbamazepine (GEPR-3 ED50 = 25 mg/kg; GEPR-9 ED50 = 3 mg/kg). The present study determined that carbamazepine produces a dose related increase in extracellular serotonin in each of the two GEPR strains. The doses of carbamazepine required to increase extracellular serotonin are similar to the doses required for an anticonvulsant effect in each of the strains. This result provides further support for the hypothesis that release of serotonin by carbamazepine is an important part of the pharmacodynamic action by which this drug suppresses seizures.

Involvement of 5-HT1B receptors within the ventral tegmental area in regulation of mesolimbic dopaminergic neuronal activity via GABA mechanisms: a study with dual-probe microdialysis.

This study was designed to assess the involvement of 5-HT1B receptors within the ventral tegmental area (VTA) in the regulation of mesolimbic dopaminergic transmission. Dual-probe microdialysis was performed in freely moving adult Sprague-Dawley rats with one probe within the VTA and the other within the ipsilateral nucleus accumbens (NACC). Drugs were administered into the VTA via retrograde dialysis. Dialysates from both the VTA and the NAC were collected for determination of dopamine (DA) and gamma-aminobutyric acid (GABA) by high-performance liquid chromatography with electrochemical detection. Intra-tegmental infusion of CP 93129 (20, 40, and 80 microM), a 5-HT1B receptor agonist, increased extracellular DA concentrations in a concentration-dependent manner not only in the NACC but also in the VTA, indicating increased mesolimbic DA neuron activity. Administration of CP 93129 at 80 microM into the VTA also significantly decreased extracellular GABA concentrations in this region. Co-infusion of the 5-HT1B receptor antagonist SB 216641 (10 microM), but not the 5-HT1A receptor antagonist WAY 100635 (10 microM) or the 5-HT1D/1A receptor antagonist BRL 15572 (10 microM), antagonized not only the effects of intra-tegmental CP 93129 (80 microM) on VTA DA and NAC DA but also on VTA GABA. The results suggest that activation of VTA 5-HT1B receptors increases mesolimbic DA neuron activities. The increased DA neuron activity may be associated, at least in part, with the 5-HT1B receptor-mediated inhibition of VTA GABA release.

Role of serotonin in the anticonvulsant effect of fluoxetine in genetically epilepsy-prone rats

This study was designed to demonstrate a role of serotonin in the anticonvulsant effect of fluoxetine, a serotonin reuptake inhibitor, in genetically epilepsy-prone rats. When varied doses of 5-hydroxytryptophan (12.5, 25, 50 mg/kg) were administered i.p. along with a fixed dose of fluoxetine (15 mg/kg) to severe seizure genetically epilepsy-prone rats, the severity of audiogenic seizures was decreased dose-dependently, and the combination treatment also produced a marked potentiation of the anticonvulsant effect when compared with administration of either drug alone. Pretreatment of severe seizure genetically epilepsy-prone rats with p-chlorophenylalanine depleted brain serotonin and reduced the anticonvulsant effectiveness of fluoxetine. By using intracerebral microdialysis, the depletion of serotonin after p-chlorophenylalanine treatment was confirmed by measuring thalamic extracellular serotonin and 5-hydroxyindoleacetic acid concentrations during basal release and in response to a challenge dose of fluoxetine. We concluded that serotonergic transmission may be involved in the anticonvulsant effect of fluoxetine in severe seizure genetically epilepsy-prone rats.

Anticonvulsant effect of enhancement of noradrenergic transmission in the superior colliculus in genetically epilepsy-prone rats (GEPRs) : a microinjection study

An expanding body of data has indicated that the seizure prone state in genetically epilepsy-prone rats (GEPRs) is partially caused by deficits in central nervous system noradrenergic transmission. Several lines of evidence suggest that the noradrenergic terminals in the superior colliculus (SC) may act as determinants of seizure predisposition in the GEPR. In order to assess the role of the noradrenergic transmission in the SC in the regulation of seizure severity, several drugs with different mechanisms of enhancing noradrenergic transmission were bilaterally microinfused into the SC of GEPR-9s (severe seizure GEPRs). The rats were tested for audiogenic seizure intensity at 0.25, 1, 2, 3, and 4 h after treatments. Bilateral infusion of vehicle produced no reduction in the severity of the audiogenic seizure. Desipramine (2, 4, 8 micrograms/side), nisoxetine (2, 4, 8 micrograms/side), and idazoxan (0.25, 1, 4 micrograms/side) all decreased the seizure severity in a dose-dependent fashion. Significant decreases in the seizure severity were also observed after administration of methoxamine (0.15 microgram/side) or phenylephrine (0.15 microgram/side). Pretreatment with prazosin (1 microgram/side) significantly diminished the anticonvulsant effectiveness of methoxamine and nisoxetine while prazosin, by itself, had no effects on the seizure intensity. These results suggest that noradrenergic transmission in the SC may be involved in the seizure regulation in GEPR-9s, and that this regulation may be mediated, at least in part, through alpha 1 receptors.

Effects of prenatal alcohol exposure on brain-derived neurotrophic factor and its receptor tyrosine kinase B in offspring.

Prenatal alcohol exposure produces many developmental defects in the central nervous system. The underlying molecular mechanism, however, has not been fully understood. The present study was undertaken to examine the effects of prenatal alcohol exposure on brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase B (TrkB) in offspring. The pregnant Sprague-Dawley rats received 1 or 3 g/kg of alcohol or an isocaloric solution by intragastric intubation once a day from gestational day (GD) 5 to GD 20. On postnatal day 7-8, pups were killed and the hippocampus, striatum, cortex, and cerebellum dissected out. Levels of BDNF mRNA and proteins, total TrkB proteins and receptor phosphorylation were measured. The results showed that prenatal alcohol exposure at the dose of 1 g/kg/day did not significantly affect BDNF protein levels in any region examined. However, administration of alcohol at the dose of 3 g/kg/day markedly reduced levels of BDNF protein and mRNA in the cortex and hippocampus of offspring. Western blotting showed that prenatal alcohol exposure at the dose of 3 g/kg/day also inhibited TrkB phosphorylation in the hippocampus although no changes in total TrkB protein levels were observed in any region examined. Our data suggest that prenatal alcohol exposure alters both presynaptic and postsynaptic BDNF function in certain brain areas of offspring. These alterations in BDNF function may contribute to the development of alcohol-related birth defects.