Differential effects of potassium channel blockers on extracellular concentrations of dopamine and 5-HT in the striatum of conscious rats

Glibenclamide alters serotonin and dopamine levels in the rat striatum and hippocampus, reducing cognitive impairment

RATIONALE

Glibenclamide (GD) is a widely used medical drug; therefore, identifying the mechanisms underlying its pleiotropic effects in the central nervous system is urgent.

OBJECTIVES

The aim of this work was to determine the ability of GD to modulate serotonin (5-hydroxytryptamine, 5-HT) and dopamine (DA) transmission and to assess the dose-dependent effect of GD on cognitive function in rats during natural ageing.

METHODS

In Experiment 1, rats received 10, 25, or 50 μg/kg GD intraperitoneally for 10 days. In Experiment 2, rats received 50 μg/kg GD intraperitoneally for 30 days. Spatial and working memory was assessed in the MWM and Y-maze tests, respectively. In both experiments, the levels of DA and 5-HT, their metabolites, and turnover rate were analysed by HPLC-ED in the rat hippocampus and striatum.

RESULTS

Changes in DA and 5-HT levels occurred only with a dose of 50 μg/kg GD. Therefore, in the second experiment, we administered a dose of 50 μg/kg GD. At this dose, GD prevented the development of impairments in spatial and working memory. The hippocampal concentrations of DA and DOPAC decreased, and the striatal concentrations of DA, DOPAC, 5-HT, and 5-HIAA increased.

CONCLUSION

One of the possible mechanisms of the precognitive effect of GD is its ability to modulate monoamine transmission. Thus, in translating our results to humans, GD can be recommended as a prophylactic agent for natural ageing to reduce the risk of developing cognitive impairments.

SK channel blockade reverses cognitive and motor deficits induced by nigrostriatal dopamine lesions in rats

Parkinson's disease has traditionally been viewed as a motor disorder caused by the loss of dopamine (DA) neurons. However, emotional and cognitive syndromes can precede the onset of the motor deficits and provide an opportunity for therapeutic intervention. Potassium channels have recently emerged as potential new targets in the treatment of Parkinson's disease. The selective blockade of small conductance calcium-activated K+ channels (SK channels) by apamin is known to increase burst firing in midbrain DA neurons and therefore DA release. We thus investigated the effects of systemic administration of apamin on the motor, cognitive deficits and anxiety present after bilateral nigrostriatal 6-hydroxydopamine (6-OHDA) lesions in rats. Apamin administration (0.1 or 0.3 mg/kg i.p.) counteracted the depression, anxiety-like behaviors evaluated on sucrose consumption and in the elevated plus maze, social recognition and spatial memory deficits produced by partial 6-OHDA lesions. Apamin also reduced asymmetric motor deficits on circling behavior and postural adjustments in the unilateral extensive 6-OHDA model. The partial 6-OHDA lesions (56% striatal DA depletion) produced 20% decrease of iodinated apamin binding sites in the substantia nigra pars compacta in correlation with the loss of tyrosine hydroxylase positive cells, without modifying apamin binding in brain regions receiving DAergic innervation. Striatal extracellular levels of DA, not detectable after 6-OHDA lesions, were enhanced by apamin treatment as measured by in vivo microdialysis. These results indicate that blocking SK channels may reinstate minimal DA activity in the striatum to alleviate the non-motor symptoms induced by partial striatal DA lesions.

Altered basal and stimulated accumbens dopamine release in obese OLETF rats as a function of age and diabetic status

The Otsuka Long-Evans Tokushima Fatty (OLETF) rat lacking the CCK-1 receptor is hyperphagic, prefers palatable and high-calorie meals, and gradually develops obesity and type 2 diabetes. To determine dopamine levels in this strain, we used in vivo quantitative (no net flux) microdialysis at three different ages representing nondiabetic (8 wk), prediabetic (18 wk), and diabetic (56 wk) stages in OLETF and age-matched lean Long-Evans Tokushima Otsuka (LETO) controls. Results showed significantly elevated basal dopamine levels in the caudomedial nucleus accumbens of OLETF rats compared with LETO at younger ages (8 wk: 20.10 +/- 5.61 nM vs. 15.85 +/- 5.63 nM; 18 wk: 7.37 +/- 3.71 nM vs. 4.75 +/- 1.25 nM, means +/- SD). In contrast, at 56 wk of age, a profound decline in extracellular dopamine concentrations was seen in both strains with a tendency for a greater effect in OLETF rats (1.78 +/- 0.40 nM vs. 2.39 +/- 0.42 nM). Further, extracellular fraction, an index for reuptake, was higher in 56-wk-old OLETF compared with LETO (0.648 +/- 0.049 vs. 0.526 +/- 0.057). Potassium-stimulated dopamine efflux revealed an increased capacity of vesicular pool in OLETF rats compared with LETO across all age groups with an accentuated strain difference at 56 wk. These findings demonstrate altered striatal dopamine functions (i.e., increased stimulated release and uptake) in obese OLETF rat. This could be due to the lack of functional CCK-1 receptors, or metabolic and hormonal factors associated with the development of obesity and insulin resistance, or both.

Gastric emptying effect by 4-aminopyridine in patients with chronic spinal cord injury

BACKGROUND

4-Aminopyridine (4-AP) given to patients with chronic spinal cord injury (SCI) has shown beneficial effects in some somatic and autonomic functions, although patients often develop dyspeptic symptoms. 4-AP is a potassium-channel blocker capable of altering gastro-pyloric functions as demonstrated experimentally. Our objective was to examine the influence of 4-AP treatment on gastric emptying in patients with chronic SCI.

METHODS

Gastric emptying was measured by the acetaminophen absorption test in 18 patients (9 with cervical and 9 with thoracic injury), and 9 healthy volunteers. Patients received increasing oral doses, 5 mg day(-1) of oral 4-AP (5-30 mg day(-1)), for 12 weeks. Patients were studied before and at the end of the last week of 4-AP treatment, whereas healthy volunteers (without 4-AP treatment) were studied only once. Whole blood samples of 2.5 mL were drawn at 0 (before 1 g of oral acetaminophen) and at 15, 30, 45, 60, 75, 90, 105 and 120 min postdose. Acetaminophen concentration in plasma was determined by high-pressure liquid chromatography.

RESULTS

Treatment with 4-AP significantly delayed stomach emptying in patients with chronic SCI, considering the significant decreasing of acetaminophen absorption (t paired test, p <0.05). This effect did not correlate either to the level or ASIA score of the injury (linear regression correlation analysis, r(2) = 0.003 and 0.015, respectively). No significant differences were observed by comparing data of patients before 4-AP treatment with healthy volunteers.

CONCLUSIONS

4-AP intake in patients with chronic spinal cord injury significantly slowed gastric emptying regardless of level and ASIA score of the injury.

Methylamine-dependent release of nitric oxide and dopamine in the CNS modulates food intake in fasting rats

BACKGROUND AND PURPOSE

Methylamine is an endogenous aliphatic amine exhibiting anorexigenic properties in mice. The aim of this work was to show whether methylamine also modifies feeding behaviour in rats and, if so, to identify the mediator(s) responsible for such effects.

EXPERIMENTAL APPROACH

Microdialysis experiments with the probe inserted in the periventricular hypothalamic nucleus were carried out in 12 h starved, freely moving rats. Collected perfusate samples following methylamine injection (i.c.v.) were analysed for nitric oxide by chemiluminescence and for dopamine and 5-hydroxytryptamine content by HPLC. Kv1.6 potassium channel expression was reduced by antisense strategy and this decrease quantified by semi-quantitative RT-PCR analysis.

KEY RESULTS

Methylamine showed biphasic dose-related effects on rat feeding. At doses of 15-30 microg per rat, it was hyperphagic whereas higher doses (60-80 microg) were hypophagic. Methylamine stimulated central nitric oxide (+115% vs. basal) following hyperphagic and dopamine release (60% over basal values) at hypophagic doses, respectively. Treatment with L-N(G)-nitro-L-arginine-methyl ester (i.c.v. 2 microg 10 microl(-1)) or with alpha-methyl-p-tyrosine (i.p. 100 mg kg(-1)) before methylamine injection, reduced nitric oxide output and hyperphagia, or dopamine release and hypophagia respectively. Moreover, hypophagia and hyperphagia, as well as nitric oxide and dopamine release were significantly reduced by down-regulating brain Kv1.6 potassium channel expression.

CONCLUSIONS AND IMPLICATIONS

The effects of methylamine on feeding depend on the hypothalamic release of nitric oxide and dopamine as a result of interaction at the Kv1.6 channels. The study of methylamine levels in the CNS may provide new perspectives on the physiopathology of alimentary behaviour.

4-methyl benzylamine stimulates food consumption and counteracts the hypophagic effects of amphetamine acting on brain Shaker-like Kv1.1 channels

1.--4-methyl benzylamine (4-MBZ; 28 microg, 231 nmol) elicits a hyperphagic response in starved mice in contrast to the hypophagia induced by the parent compound benzylamine (BZ; 33 microg, 231 nmol) or by amphetamine (AMPH, 2 mug). 2.--In mice starved for only 4 h, and therefore with little stimulation to eat, the maximal increase in food consumption induced by intracerebroventricular (i.c.v.)-injected 4-MBZ was 190% over that of the controls (ED(50) 8.3+/-2.7 microg mouse(-1); 68+/-22 nmol mouse(-1)), whereas after i.p. administration, these values were 160% and approximately 129 mg kg(-1), respectively. 3.--The hyperphagic effect of 4-MBZ was reduced by more than 60% in mice pretreated with antisense oligodeoxyribonucleotide (aODN(1)) previously found to selectively inhibit (over 50%) the expression of Shaker-like Kv1.1 channels. 4.--In mice highly stimulated to eat after 12-h fasting, 4-MBZ (28 microg) significantly reduced (to about 70%) the hypophagic response by AMPH (2 microg) or BZ (33 microg). Conversely, these two compounds reduced (respectively, by 69 and 44%) the hyperphagic response of 4-MBZ in 4-h fasting mice. 5.--4-MBZ (28 microg) also reduced the hypermotility and the stimulation of inspection activity elicited by AMPH in mice and the release of DA stimulated by AMPH (2 microg) from the nucleus accumbens of rats. We hypothesize that 4-MBZ elicits hyperphagic effects probably by opening Shaker-like Kv1.1 subtypes in the brain, whereas AMPH and BZ are hypophagic by blocking these channels.

In vivo brain microdialysis studies on the striatal dopamine and serotonin release in zitter mutant rats

In the present study, using in vivo brain microdialysis, we investigated the basal extracellular dopamine (DA) and serotonin (5-HT) release in the caudal striatum (cSTR) of young (4-6 months old) and aged (10-12 months old) zitter mutant rats. The basal extracellular levels of DA release in both young and aged zitter rats were significantly lower than that of age-matched Sprague-Dawley (SD) rats, whereas only aged zitter rats showed a significant difference in the basal 5-HT release. Dopaminergic neurons were more vulnerable than serotonergic neurons in the cSTR of zitter mutant rats during aging. Perfusion of 60 mM potassium (K+) enhanced the extracellular levels of cSTR DA in the young zitter rats and the extracellular levels of both DA and 5-HT in the cSTR of the aged zitter rats. The firing rate of K+-stimulated monoamine release in the cSTR was significantly higher in the zitter rats than in the age-matched SD rats. These findings suggest that there are innate quantitative differences in the releasable pool and the availability of monoamines in the cSTR of zitter mutant rats.

Lecozotan (SRA-333): A Selective Serotonin 1A Receptor Antagonist That Enhances the Stimulated Release of Glutamate and Acetylcholine in the Hippocampus and Possesses Cognitive-Enhancing Properties

Recent data has suggested that the 5-hydroxytryptamine (5-HT)(1A) receptor is involved in cognitive processing. A novel 5-HT(1A) receptor antagonist, 4-cyano-N-{2R-[4-(2,3-dihydrobenzo[1,4]-dioxin-5-yl)-piperazin-1-yl]-propyl}-N-pyridin-2-yl-benzamide HCl (lecozotan), which has been characterized in multiple in vitro and in vivo pharmacological assays as a drug to treat cognitive dysfunction, is reported. In vitro binding and intrinsic activity determinations demonstrated that lecozotan is a potent and selective 5-HT(1A) receptor antagonist. Using in vivo microdialysis, lecozotan (0.3 mg/kg s.c.) antagonized the decrease in hippocampal extracellular 5-HT induced by a challenge dose (0.3 mg/kg s.c.) of 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT) and had no effects alone at doses 10-fold higher. Lecozotan significantly potentiated the potassium chloride-stimulated release of glutamate and acetylcholine in the dentate gyrus of the hippocampus. Chronic administration of lecozotan did not induce 5-HT(1A) receptor tolerance or desensitization in a behavioral model indicative of 5-HT(1A) receptor function. In drug discrimination studies, lecozotan (0.01-1 mg/kg i.m.) did not substitute for 8-OH-DPAT and produced a dose-related blockade of the 5-HT(1A) agonist discriminative stimulus cue. In aged rhesus monkeys, lecozotan produced a significant improvement in task performance efficiency at an optimal dose (1 mg/kg p.o.). Learning deficits induced by the glutamatergic antagonist MK-801 [(-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate] (assessed by perceptually complex and visual spatial discrimination) and by specific cholinergic lesions of the hippocampus (assessed by visual spatial discrimination) were reversed by lecozotan (2 mg/kg i.m.) in marmosets. The heterosynaptic nature of the effects of lecozotan imbues this compound with a novel mechanism of action directed at the biochemical pathologies underlying cognitive loss in Alzheimer's disease.

A potassium channel blocker induces a long-lasting enhancement of corticostriatal responses

Disruptions in synaptic plasticity in the dorsal striatum may contribute to the pathophysiology underlying Parkinson's disease. Here we report a novel, chemically-induced form of plasticity induced by application of the potassium channel blocker tetraethylammonium (TEA) in the dorsolateral striatum of the adult rat. Transient application of TEA persistently increased synaptically-evoked extracellularly-recorded corticostriatal responses in an activity-, concentration- and time-dependent manner. Pharmacological experiments suggest that this plasticity is dependent on L-type calcium channel and protein kinase C (PKC) activation. Striatal dopamine depletion induced by nigrostriatal dopamine lesions with 6-hydroxydopamine significantly reduced, but did not abolish, TEA-mediated enhancement of the corticostriatal response. Intracellular recordings demonstrate that this TEA-mediated plasticity is associated with an increase in EPSP size and slope, as well as input resistance. Collectively, these findings demonstrate a novel form of L-type calcium channel-dependent plasticity in the adult dorsal striatum that is induced in the absence of dopaminergic input.

Antisense knockdown of the Shaker-like Kv1.1 gene abolishes the central stimulatory effects of amphetamines in mice and rats

Amphetamine (AMPH) is an indirect sympathomimetic compound classified as a substrate-type releaser that distinguishes it from other stimulants that act as uptake 1 blockers, such as cocaine (COC). In mammals, AMPH elicits central stimulation, hypermotility, anorexia, analgesia and analeptic activity, mainly through the increase of extracellular brain dopamine (DA). The inversion of vesicular transporters and/or intravesicular alkalinization is assumed to have a role in AMPH-induced exocytosis. However, the action mechanism of this compound has not yet been completely clarified. Recent evidence on the action of AMPHs indicates potassium channel-blocking properties in peripheral tissues. We investigated the possible involvement of a Shaker-like Kv1.1 channel subtype in the central effects of AMPH, using an antisense oligodeoxyribonucleotide (aODN) that specifically and reversibly inhibits the expression of these channels in the brain. The effect of aODN pretreatments was studied by evaluating the modification of behavioral effects induced in mice through the intracerebroventricular administration of AMPH, COC, or other compounds. The aODN in mice almost completely blocked the stimulatory effects of AMPH and other releasers but was ineffective in reducing the central activity of COC. In aODN-pretreated rats a strong reduction of the AMPH, but not of the COC-stimulated DA efflux from nucleus accumbens was observed. Our results suggest that the stimulant effects of AMPH and chemically related compounds, but not COC, require the presence of functionally active Kv1.1 channels in the brain.

Novel truncated isoform of SK3 potassium channel is a potent dominant-negative regulator of SK currents: implications in schizophrenia

The small-conductance calcium-activated K(+) channel SK3 (SKCa3/KCNN3) regulates electrical excitability and neurotransmitter release in monoaminergic neurons, and has been implicated in schizophrenia, ataxia and anorexia nervosa. We have identified a novel SK3 transcript, SK3-1B that utilizes an alternative first exon (exon 1B), but is otherwise identical to SK3. SK3-1B, mRNA is widely distributed in human tissues and is present at 20-60% of SK3 in the brain. The SK3-1B protein lacks the N-terminus and first transmembrane segment, and begins eight residues upstream of the second transmembrane segment. When expressed alone, SK3-1B did not produce functional channels, but selectively suppressed endogenous SK3 currents in the pheochromocytoma cell line, PC12, in a dominant-negative fashion. This dominant inhibitory effect extended to other members of the SK subfamily, but not to voltage-gated K(+) channels, and appears to be due to intracellular trapping of endogenous SK channels. The effect of SK3-1B expression is very similar to that produced by expression of the rare SK3 truncation allele, SK3-Delta, found in a patient with schizophrenia. Regulation of SK3 and SK3-1B levels may provide a potent mechanism to titrate neuronal firing rates and neurotransmitter release in monoaminergic neurons, and alterations in the relative abundance of these proteins could contribute to abnormal neuronal excitability, and to the pathogenesis of schizophrenia.

Pharmacological Studies of Alcohol Susceptibility and Brain Monoamine Function in Stroke-Prone Spontaneously Hypertensive Rats (SHRSP) and Stroke-Resistant Spontaneously Hypertensive Rats (SHRSR)

Differences of alcohol drinking behavior, brain dopamine (DA) and serotonin (5-HT) levels and releases in the striatum were investigated in stroke-prone spontaneously hypertensive rats (SHRSP) and age-matched stroke-resistant spontaneously hypertensive rats (SHRSR). Voluntary alcohol (EtOH) consumption in SHRSP rats increased at 1 and 2 hours in the 4 hour time access. In the DA level, SHRSP showed decreases in the caudate-putamen (C/P) and dorsal raphe nucleus (DRN) compared with in SHRSR. 5-HT levels in the C/P, ventral tegmental area-subtantia nigra (V/S) and DRN of the SHRSP were decreased compared with that in SHRSR. The basal extracellular levels of 5-HT release in the C/P were increased in SHRSP as compared with those in SHRSR. K(+)- or EtOH-induced DA and 5-HT releases in the C/P of the SHRSP were a lower magnitude than those in SHRSR. Increased basal extracellular 5-HT releases showing low levels of 5-HT in the C/P of SHRSP mean an abnormality of serotonergic neuronal functions in a normal physiological condition. Higher voluntary alcohol drinking behavior, so called lower susceptibility to EtOH, in the SHRSP may be associated with the degenerated rewarding system including the DRN. These results suggest that the hypertensive state causes the dysfunction in the striatum of the brain rewarding system and induces the risk for increasing alcohol consumption to compensate for the alteration of serotonergic neurons.

Effect of chronic fluoxetine and WAY-100635 treatment on serotonergic neurotransmission in the frontal cortex

Clinical augmentation strategies have shown that some improvement in antidepressant efficacy can be achieved by combining the beta-adrenergic/serotonin (5-HT)1A/1B receptor antagonist (+/-)pindolol with a selective serotonin reuptake inhibitor (SSRI). This has lead to the hypothesis that a combination of a 5-HT1A receptor antagonist with an SSRI will lead to a faster onset of antidepressant action. Although there is a significant accumulation of acute preclinical data supporting this rationale, until recently, there have been no investigations examining the chronic effects of combining an SSRI with a 5-HT1A receptor antagonist. Here, we determined the chronic effects of fluoxetine (10 mg/kg o.d.), administered in combination with the selective 5-HT1A receptor antagonist WAY-100635 (1 mg/kg b.i.d.), on serotonergic neurotransmission in the frontal cortex using in-vivo microdialysis. Following chronic administration of fluoxetine +/- WAY-100635, functional changes in serotonergic neurotransmission, as well as 5-HT1A autoreceptors, were assessed by administering fluoxetine or (+/-) 8-hydroxy-2-(di-n-propylamino)tetralin [(+/-) 8-OH-DPAT] 24 h after the last chronic dose. Chronic administration of WAY-100635 alone produced no detectable change in the functional status of the 5-HT1A receptor. However, fluoxetine alone produced a time-dependent adaptation in serotonergic transmission such that fluoxetine (acutely administered on day 15) was able to produce a two-fold increase in extracellular 5-HT levels but the decrease in response to 8-OH-DPAT was completely attenuated. These data indicate that the fluoxetine-induced adaptation was mediated by desensitization of the 5-HT1A receptor. WAY-100635 given chronically in combination with fluoxetine blocked the SSRI-induced desensitization of the 5-HT1A receptor. Furthermore, chronic treatment with this combination produced no tolerance in terms of its ability to acutely increase forebrain 5-HT levels. These data suggest that augmentation of an SSRI by combined pharmacotherapy with a 5-HT1A antagonist would be effective upon prolonged exposure.

Methylamine and benzylamine induced hypophagia in mice: modulation by semicarbazide-sensitive benzylamine oxidase inhibitors and aODN towards Kv1.1 channels

1. In starved mice, the anorectic activity of methylamine (MET) and benzylamine (BZ), both substrates of semicarbazide-sensitive benzylamine oxidases (Bz-SSAO), was compared with that of the potassium channel blocking agents charybdotoxin (ChTX), tetraethylammonium (TEA), gliquidone (GLI), ammonium chloride (NH(4)(+)) and of the anoressants amphetamine (AMPH) and nicotine (NIC). After i.c.v. administration, an approximate ranking order of potency was: ChTX> or =AMPH>NIC=TEA> or =GLI> or =MET>BZ>NH(4)(+). 2. Clorgyline (2.5 mg kg(-1) i.p.) or deprenyl (10 mg kg(-1) i.p.) potentiated the anorectic effect of i.c.v.-administered BZ, NIC and AMPH. The effect of TEA was increased only by deprenyl, while MET, NH(4)(+), ChTX and GLI were not affected by either of the inhibitors. 3. The Bz-SSAO inhibitors alpha-aminoguanidine (50 mg kg(-1) i.p.), B24 (100 mg kg(-1) i.p.) and MDL 72274 (2.5 mg kg(-1) i.p.) potentiated the effect of i.p., but not of i.c.v.-administered MET. 4. Antisense oligodeoxyribonucleotides (aODN) to Kv1.1 potassium channels abolished the effect of BZ and TEA, but was ineffective in reducing the activity of MET and other compounds. 5. These results suggest that MET is endowed with peculiar hypophagic effects at dosage levels that are not able to affect gross behaviour in mice. The effect of MET, differently from BZ, seems unrelated to an increase in the central release of monoaminergic mediators, as well as to a Kv1.1 blocking activity. Through a reduction of the endogenous breakdown of MET, Bz-SSAO inhibitors enhance the central pharmacological activity of this amine.

Nuclear localization and dominant-negative suppression by a mutant SKCa3 N-terminal channel fragment identified in a patient with schizophrenia

The small conductance calcium-activated K+ channel gene SKCa3/KCNN3 maps to 1q21, a region strongly linked to schizophrenia. Recently, a 4-base pair deletion in SKCa3 was reported in a patient with schizophrenia, which truncates the protein at the end of the N-terminal cytoplasmic region (SKCa3Delta). We generated a green fluorescent protein-SKCa3 N-terminal construct (SKCa3-1/285) that is identical to SKCa3Delta except for the last two residues. Using confocal microscopy we demonstrate that SKCa3-1/285 localizes rapidly and exclusively to the nucleus of mammalian cells like several other pathogenic polyglutamine-containing proteins. This nuclear targeting is mediated in part by two polybasic sequences present at the C-terminal end of SKCa3-1/285. In contrast, full-length SKCa3, SKCa2, and IKCa1 polypeptides are all excluded from the nucleus and express as functional channels. When overexpressed in human Jurkat T cells, SKCa3-1/285 can suppress endogenous SKCa2 currents but not voltage-gated K+ currents. This dominant-negative suppression is most likely mediated through the co-assembly of SKCa3-1/285 with native subunits and the formation of non-functional tetramers. The nuclear localization of SKCa3-1/285 may alter neuronal architecture, and its ability to dominantly suppress endogenous small conductance K(Ca) currents may affect patterns of neuronal firing. Together, these two effects may play a part in the pathogenesis of schizophrenia and other neuropsychiatric disorders.

Distribution of the messenger RNA for the small conductance calcium-activated potassium channel SK3 in the adult rat brain and correlation with immunoreactivity

Small conductance calcium-activated potassium channels are voltage independent potassium channels which modulate the firing patterns of neurons by activating the slow component of the afterhyperpolarization. The genes encoding a family of small conductance calcium-activated potassium channels have been cloned and up to now three known members have been described and named small conductance calcium-activated potassium channel type 1, small conductance calcium-activated potassium channel type 2 and small conductance calcium-activated potassium channel type 3; the distribution of their messenger RNA in the rat CNS has already been performed but only in a limited detail. The present study represents the first detailed analysis of small conductance calcium-activated potassium channel type 3 mRNA distribution in the adult rat brain and resulted in a strong to moderate expression of signal in medial habenular nucleus, substantia nigra compact part, suprachiasmatic nucleus, ventral tegmental area, lateral septum, dorsal raphe and locus coeruleus. Immunohistological experiments were also performed and confirmed the presence of small conductance calcium-activated potassium channel type 3 protein in medial habenular nucleus, locus coeruleus and dorsal raphe. Given the importance of dorsal raphe, locus coeruleus and substantia nigra/ventral tegmental area for serotonergic, noradrenergic and dopaminergic transmission respectively, our results pose the morphological basis for further studies on the action of small conductance calcium-activated potassium channel type 3 in serotonergic, noradrenergic and dopaminergic transmission.

Selective inhibition of amine oxidases differently potentiate the hypophagic effect of benzylamine in mice

In mice deprived of food for 12 h, the i.c.v. or i.p. administration of benzylamine, a substrate common to both monoamine oxidase B and semicarbazide-sensitive benzylamine oxidases, dose-dependently inhibited feeding. This effect was significantly potentiated by selective monoamine oxidase A and B inhibition, suggesting that central monoamines, known to be substrates of these enzymes may be released. The i.p. administration of semicarbazide-sensitive benzylamine oxidase inhibitors, B24 (3,5-ethoxy-4-aminomethylpyridine) and MDL 72274 ((E)-2-phenyl-3-chloroallylamine) strongly potentiated the effect of i.p. but not i.c.v.-administered benzylamine. The hypophagic effect of benzylamine was evaluated following i.c.v. administration, in comparison with the effect of the sympathomimetic compound amphetamine or the K(+) channel blocker tetraethylammonium, as reference compounds. Our results make it possible to define benzylamine as a centrally acting hypophagic compound devoid of amphetamine-like motor stimulatory effects and point to a role of B24 and MDL 72274 as specific peripheral enhancers of the pharmacological effects of benzylamine.

Characterization of transient focal ischemia-induced increases in extracellular glutamate and aspartate in spontaneously hypertensive rats

Using middle cerebral artery occlusion (MCAO) and in vivo microdialysis, we have evaluated the changes in extracellular concentrations of the excitatory amino acids (EAA) glutamate and aspartate during varying periods of MCAO (0, 30, 60 min) in the striatum of spontaneously hypertensive rats (SHR). A positive correlation between occlusion time-dependent elevations in EAAs and the resulting ischemic injury was observed. This is the first demonstration of the temporal profile of EAA efflux during transient focal ischemia in SHRs. Possible sources and mechanisms of ischemia-induced EAA efflux were examined during 60 min of MCAO. Removal of Ca(2+) from the microdialysis infusion media significantly attenuated ischemia-induced increases in both glutamate (from ischemic peak of 4892 +/- 1298 to 1144 +/- 666% of preischemic values) and aspartate (from 2703 +/- 682 to 2090 +/- 599% of preischemic values). Similarly, infusion of the voltage dependent Na(+) channel blocker tetrodotoxin (TTX; 10 microM) significantly attenuated MCAO-induced increases in glutamate (to 1313 +/- 648%) and aspartate (to 359 +/- 114%). Infusion of the GLT-1 selective nontransportable inhibitor, dihydrokainate (DHK; 1 mM) also significantly attenuated the ischemia-induced increases in both EAAs (1285 +/- 508 and 1366 +/- 741% of the preischemic levels, respectively). These results indicate that during transient focal ischemia the increase in extracellular EAAs originates from both the neuronal pool, via conventional exocytotic release, and glial sources via the reversal of the GLT-1 transporter.

Effects of chronic fluoxetine treatment in the presence and absence of (+/-)pindolol: a microdialysis study

Using in vivo microdialysis in the frontal cortex of the freely moving rat we evaluated the effects of chronic treatment with the serotonin specific reuptake inhibitor (SSRI) fluoxetine in the presence and absence of the 5-HT(1A)/beta-adrenergic antagonist (+/-)pindolol. Chronic vehicle treated animals produced no significant response to a challenge with fluoxetine (10 mg kg(-1)) on day 8 and 15. Alternatively, a significant (P<0.05) decrease in extracellular 5-HT was observed in control animals upon challenge with the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.03 and 0.1 mg kg(-1)). Conversely, animals treated with fluoxetine (10 mg kg(-1) o.d.) for 7 and 14 days produced a significant (P<0.05) 2 fold increase in extracellular 5-HT when challenged with fluoxetine (10 mg kg(-1)) on day 8 and 15. Moreover, no significant decrease in extracellular 5-HT was observed upon challenge with either dose of 8-OH-DPAT. Animals chronically treated with (+/-)pindolol (10 or 20 mg kg(-1) b.i.d.) produced a significant dose-related increase in extracellular 5-HT upon challenge with fluoxetine on day 15 only. Furthermore, both doses produced a significantly blunted response to the low dose challenge of 8-OH-DPAT (0.03 mg kg(-1)). In addition, 20 mg kg(-1) (+/-)pindolol treated animals also had no response to the higher 0.1 mg kg(-1) dose of 8-OH-DPAT. Animals treated for 14 days with a combination of (+/-)pindolol (10 or 20 mg kg(-1)) and fluoxetine were not significantly different from vehicle treated animals when challenged with fluoxetine or 8-OH-DPAT. Taken together it would therefore appear that although (+/-)pindolol alone has sufficient intrinsic activity to produce a desensitization of the 5-HT(1A) receptor, when given in combination with fluoxetine it is able to prevent the desensitization induced by not only fluoxetine but also itself. This may suggest that the clinical augmentation of antidepressant action by pindolol, when co-administered with a SSRI, is via antagonism of the 5-HT(1A) receptor.

The role of 5-HT(1A) and 5-HT(1B/1D) receptors on the modulation of acute fluoxetine-induced changes in extracellular 5-HT: the mechanism of action of (+/-)pindolol

Some clinical evidence has suggested that (+/-)pindolol can be effective at producing a shortened time to onset of antidepressant activity when co-administered with a serotonin specific reuptake inhibitor (SSRI). This effect has been attributed to the antagonist effects of pindolol at the 5-HT(1A) receptor. In the present study, we compared the pharmacology of (+/-)pindolol, WAY-100635 (a 5-HT(1A) antagonist), GR127935 (a 5-HT(1B/1D) antagonist), and isamoltane (a 5-HT(1B) antagonist), when given acutely in combination with fluoxetine, using in vivo microdialysis in the frontal cortex of the freely moving rat. We have determined that the acute fluoxetine-induced increases in extracellular 5-HT can be augmented by (+/-)pindolol, WAY100635, GR127935 and isamoltane with maximum increases of 216+/-32%, 235+/-49%, 240+/-18% and 171+/-47% of preinjection control levels, respectively. Combination of both 5-HT(1A) and 5-HT(1B/1D) autoreceptor antagonists with fluoxetine produced additive increases in extracellular 5-HT (i.e. WAY100635+GR127935+fluoxetine and WAY100635+isamoltane+fluoxetine produced a four- and five-fold potentiation, respectively), suggesting that this strategy may be useful in further augmenting the action of a SSRI in the treatment of depression. In addition, by comparing the combined administration of (+/-)pindolol with either WAY100635, GR127935 or isamoltane, we have determined that (+/-)pindolol produces much of its acute potentiation of fluoxetine-induced increases in extracellular 5-HT via its action at the 5-HT(1B/D) receptor in addition to any activity it has at the presynaptic 5-HT(1A) receptor.

Excitotoxicity in neurological disorders--the glutamate paradox

Beneficial effects of glutamate-receptor antagonists in models of neurological disorders are often used to support the notion that endogenous excitotoxicity (i.e. resulting from extracellular accumulation of endogenous glutamate) is a major contributor to neuronal death associated with these conditions. However, this interpretation conflicts with a number of robust and important experimental evidence. Here, emphasis is placed on two key elements: (i) very high extracellular levels of glutamate must be reached to initiate neuronal death, far above those measured in models of neurological disorders; and (ii) changes in extracellular glutamate as measured by microdialysis are not related to changes in the synaptic cleft, i.e. the compartment where neurotransmitter glutamate interacts with its receptors. It has become clear that the diversity and complexity of glutamate-mediated processes allow for a wide range of potential abnormalities (e.g. loss of selectivity of glutamate-operated ion channels, abnormal modulation of glutamate receptors). In addition, as neuronal death subsequent to ischemia and other insults is likely to result from multifactorial processes that may be inter-related, inhibition of glutamate-mediated synaptic transmission may be neuroprotective by increasing the resistance of neurons to other deleterious mechanisms (e.g. inadequate energy supply) that are not directly related to glutamatergic transmission.

High extracellular glutamate and neuronal death in neurological disorders. Cause, contribution or consequence?

In models of neurological disorders, increased extracellular glutamate and beneficial effects produced by glutamate-receptor antagonists are consistently taken as supporting evidence of excitotoxicity. This systematic interpretation is over-simplified and potentially misleading. High extracellular glutamate is not a reliable indicator of endogenous excitotoxicity, i.e., the intrinsic, potential neurotoxicity of endogenous glutamate whenever it accumulates extracellularly. Firstly, because the extracellular levels of glutamate necessary to produce depolarization and death in vivo, are far above those measured in models of neurological disorders. Secondly, because changes in the concentration of glutamate in the synaptic cleft (i.e., the relevant compartment for endogenous excitotoxicity) are not reflected extracellularly. Protection by glutamate-receptor antagonists does not necessarily imply inhibition of excitotoxic abnormalities. Indeed, neuronal death initiated by insults such as ischemia results from multifactorial processes that may be interrelated. Therefore, beneficial effects resulting from an interaction with glutamate-mediated transmission may actually render the cell more resistant to other deleterious mechanisms (e.g., mitochondrial injury, oxidative stress).

Effects of venlafaxine on extracellular concentrations of 5-HT and noradrenaline in the rat frontal cortex: augmentation via 5-HT1A receptor antagonism

Venlafaxine is a novel serotonin/noradrenaline reuptake inhibitor (SNRI) which has been shown clinically to be an effective antidepressant (AD) with a faster onset of action than serotonin specific reuptake inhibitors (SSRI). Preclinically, venlafaxine has been shown to potently inhibit dorsal raphe neuronal (DRN) firing through a 5-HT1A mediated mechanism, in a similar manner to SSRIs. Here we demonstrate the acute neurochemical effects of venlafaxine on extracellular concentrations of 5-HT and noradrenaline (NA) from the rat frontal cortex using in vivo microdialysis. Administration of venlafaxine (3-50 mg/kg s.c.) resulted in a significant dose-dependent increase in extracellular NA, but produced no significant increase in 5-HT concentrations. Combination treatment with the selective 5-HT1A antagonist WAY100635 produced a dose-dependent augmentation of venlafaxine-induced (3-30 mg/kg s.c) extracellular 5-HT concentrations, but had no further effect on NA above that produced by venlafaxine alone. WAY100635, at doses as low as 0.03 mg/kg s.c., maintained this potentiation effect. The beta-adrenergic/5-HT1A receptor antagonist (+/-)pindolol and the selective 5-HT1B/D antagonist GR127935 produced no significant augmentation of venlafaxine-induced changes in either 5-HT or NA. Using the alpha1 and alpha2-adrenoceptor antagonists, prazosin and idazoxane, we also demonstrate the role of the alpha-adrenoceptors in the augmentation of venlafaxine-induced changes. The possible mechanisms underlying venlafaxines improved clinical AD action and the potential for further enhancement of this SNRIs clinical effects are discussed.

Relationships among seizures, extracellular amino acid changes, and neurodegeneration induced by 4-aminopyridine in rat hippocampus: a microdialysis and electroencephalographic study

4-Aminopyridine is a powerful convulsant that induces the release of neurotransmitters, including glutamate. We report the effect of intrahippocampal administration of 4-aminopyridine at six different concentrations through microdialysis probes on EEG activity and on concentrations of extracellular amino acids and correlate this effect with histological changes in the hippocampus. 4-Aminopyridine induced in a concentration-dependent manner intense and frequent epileptic discharges in both the hippocampus and the cerebral cortex. The three highest concentrations used induced also a dose-dependent enhancement of extracellular glutamate, aspartate, and GABA levels and profound hippocampal damage. Neurodegenerative changes occurred in CA1, CA3, and CA4 subfields, whereas CA2 was spared. In contrast, microdialysis administration of a depolarizing K+ concentration and of tetraethylammonium resulted in increased amino acid levels but no epileptic activity and no or moderate neuronal damage. These results suggest that seizure activity induced by 4-aminopyridine is due to a combined action of excitatory amino acid release and direct stimulation of neuronal firing, whereas neuronal death is related to the increased glutamate release but is independent of seizure activity. In addition, it is concluded that the glutamate release-inducing effect of 4-aminopyridine results in excitotoxicity because it occurs at the level of nerve endings, thus permitting the interaction of glutamate with its postsynaptic receptors, which is probably not the case after K+ depolarization.

Effects of 5-HT1A receptor antagonists on fluoxetine-induced changes in extracellular serotonin concentrations in rat frontal cortex

Clinical studies in which serotonin specific reuptake inhibitors have been co-administered with pindolol have demonstrated a shortened time to onset of antidepressant activity. This effect has been attributed to the antagonist effects of pindolol at the presynaptic 5-HT1A receptor which augments the action of the serotonin specific reuptake inhibitors. In the present study, we demonstrate that acute fluoxetine-induced increases in extracellular serotonin concentrations, as measured by microdialysis in the frontal cortex, can be potentiated by 5-HT1A receptor blockade using N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(pyridinyl)cyclohexa necarboxamide (WAY100635), the silent and selective 5-HT1A receptor antagonist. WAY100635 at doses as low as 0.03 mg/kg s.c. maintained this potentiation effect across a range of fluoxetine doses. In addition, using antagonists with different intrinsic agonist activities for the 5-HT1A receptor, we have determined that only compounds with very low intrinsic agonist activity can produce a potentiation of the acute fluoxetine-induced increases in extracellular serotonin.

Effect of quinine on autoreceptor-regulated serotonin release in the rat hippocampus

The involvement of K+ channels in the autoregulation of terminal serotonin (5-hydroxytryptamine, 5-HT) release was investigated by microdialysis in the hippocampus of conscious rats. Extracellular 5-HT was increased concentration-dependently by the K+ channel blocker quinine (10, 100 and 1000 microM in perfusate), and tetrodotoxin (10 microM) but not fluoxetine (5 microM) exerted a partially attenuating influence. The 5-HT1/2/6 receptor antagonist methiothepin (50 microM) increased dialysate 5-HT, most likely through 5-HT1B autoreceptors tonically activated in the hippocampus of awake rats as opposed to the previously reported lack of effect 5-HT1B autoreceptor blockade in anesthetized rats. The effect of methiothepin was greatly reduced by preperfusion with quinine (100 microM), consonant with a role for quinine-sensitive K+ channels in the autoregulation of 5-HT release in the hippocampus by 5-HT receptor antagonism. In contrast, the reduction in dialysate 5-HT induced by the 5-HT1 receptor agonist RU 24969 (1 microM), in the presence of fluoxetine (5 microM), persisted in the co-presence of quinine, consonant with the involvement of (extrasynaptic?) 5-HT autoreceptors not coupled with quinine-sensitive K+ channels.