Limonene inhibits methamphetamine-induced locomotor activity via regulation of 5-HT neuronal function and dopamine release

Methamphetamine is a psychomotor stimulant that produces hyperlocomotion in rodents. Limonene (a cyclic terpene from citrus essential oils) has been reported to induce sedative effects. In this study, we demonstrated that limonene administration significantly inhibited serotonin (5-hydroxytryptamine, 5-HT)-induced head twitch response in mice. In rats, pretreatment with limonene decreased hyperlocomotion induced by methamphetamine injection. In addition, limonene reversed the increase in dopamine levels in the nucleus accumbens of rats given methamphetamine. These results suggest that limonene may inhibit stimulant-induced behavioral changes via regulating dopamine levels and 5-HT receptor function.     

Topographic distribution of dopamine uptake,choline uptake,choline acetyltransferase,and GABA uptake in the striata of weaver mutant mice

The topographic distribution of dopamine (DA) uptake, choline uptake, choline acetyltransferase (ChAT) activity and GABA uptake within the striata of weaver mutant mice and control mice was determined. Uptake of [³H]dopamine, [³H]choline and [¹⁴C]GABA, as well as ChAT activity were determined in samples prepared from the dorsolateral, dorsomedial, ventrolateral and ventromedial portions of the striatum. In 45–60 day old control mice, dopamine uptake was homogeneously distributed throughout the striatum. On the other hand, striata from weaver mice exhibited an uneven distribution with the ventral aspects having greater uptake activity than the dorsal regions. Thus, although the ventral portion of the striatum is less severely affected than the dorsal portion, all areas of the striatum exhibited significantly reduced uptake rates. In 9 and 12 month old mice, choline uptake was higher in lateral than medial zones of the striatum of both genotypes and no differences were observed between genotypes. GABA uptake was higher in the ventral striatum than in the dorsal striatum but again no differences were found between weaver and control mice. The results of this study indicate that the entire weaver striatum is severely deficient in its ability to recapture dopamine and thus is functionally compromised. The results also indicate that the striatal cholinergic and GABAergic interneurons are not directly or indirectly affected by the weaver gene.Special ïssue dedicated to Dr. Morris H. Aprison     

GABA in the entopeduncular nucleus and the subthalamic nucleus participates in mediating dopaminergic striatal output functions

The bilateral intracerebral injection of the specific GABA agonists muscimol (25, 100 ng) and THIP (500 ng) into the pallido-entopeduncular nucleus (EP) and the subthalamic nucleus (STN) of rats induced a behavioural stimulation closely resembling the syndrome evoked by direct stimulation of dopamine receptors in the striatum or by the systemic injection of dopamine agonists. The rats showed strong locomotor and rearing activity followed by characteristic stereotyped behaviour consisting of sniffing and gnawing activity. The stimulation induced by muscimol (25 ng) was found independent of dopamine, since the dopamine antagonist haloperidol (1 mg/kg s.c.) induced no blockade. Injection of the GABA antogonist picrotoxin (100 ng) into the EP or STN induced sedation and catalepsy. The unilateral injection of muscimol and picrotoxin provoked contraversive and ipsiversive postural changes. Related behavioral effects were induced by GABAergic drugs injected in substantia nigra, zona reticulata (SNR). These data provide support for the new hypothesis that GABA in the EP, SNR and STN is important for the expression of behavior related to stimulation of dopamine receptors in the striatum. The effects may be induced by a dopamine activation of the descending striato-EP, striato-SNR GABAergic pathways and possibly also the pallido-STN GABAergic pathway. The findings suggest that in addition to a pathology of the dopamine system there may also be a GABAergic dysfunction in the efferent system of the basal ganglia localized to the EP, SNR and STN in diseases, such as parkinsonism, Huntington's chorea and possibly schizophrenia.     

Regional brain GABA metabolism and release during hepatic coma produced in rats chronically treated with carbon tetrachloride

Hepatic coma was induced in rats chronically treated with CCl₄, by means of a single injection of ammonium acetate. The activities of glutamate decarboxylase (GAD) and GABA transaminase (GABA-T), as well as the synaptosomal uptake and release of [³H]GABA, were measured in the following brain areas of the comatose rats: cortex, striatum, hypothalamus, hippocampus, midbrain and cerebellum. Hepatic coma was associated with a general decrease of GAD activity, whereas GABA-T activity was diminished only in the hypothalamus, striatum and midbrain. During hepatic coma, the K⁺-stimulated [³H]GABA release was notably diminished in the striatum and cerebellum, whereas a significant increase was observed in the hippocampus. [³H]GABA uptake increased in most regions after CCl₄ treatment, independently of the presence of coma. The results indicate that GABAergic transmission seems to be decreased in most cerebral regions during hepatic coma.     

Chronic nicotine modifies the effects of morphine on extracellular striatal dopamine and ventral tegmental GABA

Previously, we have shown that 7-week oral nicotine treatment enhances morphine-induced behaviors and dopaminergic activity in the mouse brain. In this study, we further characterized the nicotine-morphine interaction in the mesolimbic and nigrostriatal dopaminergic systems, as well as in the GABAergic control of these systems. In nicotine-pretreated mice, morphine-induced dopamine release in the caudate putamen and nucleus accumbens was significantly augmented, as measured by microdialysis. Chronic nicotine treatment did not change basal extracellular concentrations of dopamine and its metabolites in the caudate putamen and nucleus accumbens, nor did it affect the rate of dopamine synthesis, as assessed by 3-hydroxybenzylhydrazine dihydrochloride-induced DOPA accumulation. GABAergic control of dopaminergic activity was studied by measuring extracellular GABA in the presence of nipecotic acid, an inhibitor of GABA uptake. Acute (0.3 mg/kg or 0.5 mg/kg i.p.) and chronic nicotine, as well as morphine (15 mg/kg s.c.) in control mice decreased nipecotic acid-induced increase in extracellular GABA in the ventral tegmental area/substantia nigra (VTA/SN). In contrast, in nicotine-treated mice, morphine increased GABA levels in the presence of nipecotic acid. We did not find any alterations in GABA(B)-receptor function after chronic nicotine treatment. Thus, our data show that chronic nicotine treatment sensitizes dopaminergic systems to morphine and affects GABAergic systems in the VTA/SN.     

Limonene, a natural cyclic terpene, is an agonistic ligand for adenosine A(2A) receptors

Limonene is a major aromatic compound in essential oils extracted from citrus rind. The application of limonene, especially in aromatherapy, has expanded significantly, but its potential effects on cellular metabolism have been elusive. We found that limonene directly binds to the adenosine A_2A receptor, which may induce sedative effects. Results from an in vitro radioligand binding assay showed that limonene exhibits selective affinity to A_2A receptors. In addition, limonene increased cytosolic cAMP concentration and induced activation of protein kinase A and phosphorylation of cAMP-response element-binding protein in Chinese hamster ovary cells transfected with the human adenosine A_2A receptor gene. Limonene also increased cytosolic calcium concentration, which can be achieved by the activation of adenosine A_2A receptors. These findings suggest that limonene can act as a ligand and an agonist for adenosine A_2A receptors.     

A mutation in CLOCK leads to altered dopamine receptor function

Mice with a mutation in the Clock gene (ClockΔ19) have a number of behavioral phenotypes that suggest alterations in dopaminergic transmission. These include hyperactivity, increased exploratory behavior, and increased reward value for drugs of abuse. However, the complex changes in dopaminergic transmission that underlie the behavioral abnormalities in these mice remain unclear. Here we find that a loss of CLOCK function increases dopamine release and turnover in striatum as indicated by increased levels of metabolites HVA and DOPAC, and enhances sensitivity to dopamine receptor antagonists. Interestingly, this enlarged dopaminergic tone results in downstream changes in dopamine receptor (DR) levels with a surprising augmentation of both D1‐ and D2‐type DR protein, but a significant shift in the ratio of D1 : D2 receptors in favor of D2 receptor signaling. These effects have functional consequences for both behavior and intracellular signaling, with alterations in locomotor responses to both D1‐type and D2‐type specific agonists and a blunted response to cAMP activation in the ClockΔ19 mutants. Taken together, these studies further elucidate the abnormalities in dopaminergic transmission that underlie mood, activity, and addictive behaviors.     

Effect of gonadal steroids and gamma-aminobutyric acid on LH release and dopamine expression and activity in the zona incerta in rats

A dopaminergic system in the zona incerta stimulates LH release and may mediate the positive feedback effects of the gonadal steroids on LH release. In this study the mechanisms by which steroids might increase dopamine activity in the zona incerta were investigated. In addition, experiments were conducted to determine whether the inhibitory effects of gamma-aminobutyric acid (GABA) on LH release in the zona incerta are due to suppression of dopamine activity in this area or conversely whether the stimulatory effects of dopamine on LH release are due to suppression of a tonic inhibitory GABAergic system. Ovariectomized rats were treated s.c. with oil, 5 micrograms oestradiol benzoate or 5 micrograms oestradiol benzoate followed 48 h later by 0.5 mg progesterone, and killed 54 h after the oestradiol benzoate injection. At this time the LH concentrations were suppressed in the oestradiol benzoate group and increased in the group treated with oestradiol benzoate and progesterone. The ratio of tyrosine hydroxylase:beta-actin mRNA in the zona incerta was significantly increased by the oestradiol benzoate treatment, but the addition of progesterone resulted in values similar to those in the control group. At the same time, the progesterone treatment increased tyrosine hydroxylase activity in the zona incerta as indicated by an increase in L-dihydroxyphenylalanine (L-DOPA) accumulation after 100 mg 3-hydroxybenzylhydrazine hydrochloric acid (NSD1015) kg-1 and an increase in dopamine release as indicated by a increase in dihydroxyphenylacetic acid (DOPAC) concentrations (one of the major metabolites of dopamine). Ovariectomized rats treated with oestradiol benzoate plus progesterone were also injected i.p. with 75 mg gamma-acetylenic GABA kg-1 (a GABA transaminase inhibitor) to increase GABA concentrations in the brain. This treatment had no effect on the ratio of tyrosine hydroxylase:beta-actin mRNA but decreased L-DOPA accumulation and DOPAC concentrations in the zona incerta, indicating a post-translational inhibition of dopamine synthesis and release. Treatment of ovariectomized rats with oestradiol benzoate followed by 100 mg L-DOPA i.p. to increase dopamine concentrations in the whole brain had no effect on glutamic acid decarboxylase mRNA expression in the zona incerta, although it increased the glutamic acid decarboxylase:beta-actin mRNA ratio in other hypothalamic areas (that is, the medical preoptic area, ventromedial nucleus and arcuate nucleus). In conclusion, the steroids act to increase dopamine activity in different ways: oestrogen increases tyrosine hydroxylase mRNA expression and progesterone acts after translation to increase tyrosine hydroxylase activity and dopamine release (as indicated by increases in DOPAC concentrations). This latter effect may be due to progesterone removing a tonic GABAergic inhibition from the dopaminergic system.     

Effects of Fetal Methylazoxymethanol Acetate Lesion on the Synaptic Neurochemistry of the Adult Rat Striatum

Abstract We used the cytotoxic properties of methylazoxymethanol acetate (MAM), which ablates mitotically active neuroblasts, to eliminate neurons in the fetal striatum to define the factors that regulate the development of the synaptic circuitry of this region. Adult rats whose mothers received a single intraperitoneal injection of 20 mg/kg of MAM on gestational days (DG) 14-17 were used in this study. MAM treatment at 14 DG caused a 49% decrease in striatal mass whereas treatment at 17 DG reduced the striatal weight by only 16%; MAM treatment on 15 or 16 DG gave intermediate results. Histologic analysis of Nissl-stained sections did not reveal an obvious disruption of striatal organization, although the region was clearly hypoplastic. The hypoplasia was associated with significant increases in the specific activities of choline acetyltransferase and tyrosine hydroxylase, although total activities of these enzymes per striatum were significantly depressed with the 14 or 15 DG treatments. In contrast, the specific activity of glutamate decarboxylase was unaffected by MAM treatment whereas the total activity of this enzyme was reduced commensurate with the degree of striatal hypoplasia. In rats lesioned at 15 DG, there was a similar 30% increase in the specific activities of all presynaptic dopaminergic markers studied. In contrast, the specific activity of the synaptosomal uptake process for [³H]choline was elevated by 60%, the specific activity of choline acetyltransferase was increased by only 30%, and the concentration of acetylcholine in the striatum was unchanged. Whereas the specific activities of glutamate decarboxylase and of the synaptosomal uptake process for [³H]γ-aminobutyric acid ([³H]GABA) were unaffected by the 15 DG MAM treatment, the concentration of GABA was increased significantly by 20%. The specific binding of [³H]spiroperidol, [³H]quinuclidinyl benzilate ([³H]QNB). and [³H] muscimol to, respectively, dopamine, muscarinic, and GABA receptors was unchanged by the 15 DG MAM lesion. The nigral dopaminergic perikarya appeared unaffected by the 15 DG MAM lesion in that the tyrosine hydroxylase activity remained normal. Consistent with the loss of striatal GABAergic perikarya, the specific activities of glutamate decarboxylase and of the synaptosomal uptake process for [³H]GABA were significantly reduced in the substantia nigra; however, the concentration of endogenous GABA was twofold greater than in control in this terminal region. The results of these studies indicate that the nigro-striatal dopaminergic pathway only partially compensates for the loss of neurons in its terminal field within the hypoplastic striatum. Striatal cholinergic and GABAergic neurons differ considerably in their responses to the MAM lesion, suggesting that they are derived from different neuroblast pools. Finally, the altered synaptic relationships induced by the fetal lesion may affect neurotransmitter turnover as evidenced by disparities in GABA and acetylcholine levels when compared with other presynaptic markers for the GABAergic and cholinergic neurons.     

l-Tetrahydropalmatine inhibits methamphetamine-induced locomotor activity via regulation of 5-HT neuronal activity and dopamine D3 receptor expression

Methamphetamine (METH) is a psychomotor stimulant that produces hyperlocomotion in rodents. l-tetrahydropalmatine (l-THP) is an active ingredient found in Corydalis ternata which has been used as a traditional herbal preparation in Asian countries for centuries, however, the effect of l-THP on METH-induced phenotypes largely unknown. In this study, to evaluate the effect of l-THP on METH-induced psychotropic effects, rats were pretreated with l-THP (10 and 15 mg/kg) before acute METH injection, following which the total distance the rats moved in an hour was measured. To clarify a possible mechanism underlying the effect of l-THP on METH-induced behavioral changes, dopamine receptor mRNA expression levels in the striatum of the rats was measured following the locomotor activity study. In addition, the effect of l-THP (10 and 15 mg/kg) on serotonergic (5-HTergic) neuronal pathway activation was studied by measurement of 5-HT (80 μg/10 μl/mouse)-induced head twitch response (HTR) in mice. l-THP administration significantly inhibited both hyperlocomotion in rats and HTR in mice. l-THP inhibited climbing behavior-induced by dopaminergic (DAergic) neuronal activation in mice. Furthermore, l-THP attenuated the decrease in dopamine D3 receptor mRNA expression levels in the striatum of the rats induced by METH. These results suggest that l-THP can ameliorate behavioral phenotype induced by METH through regulation of 5-HT neuronal activity and dopamine D3 receptor expression.     

Anxiolytic and anticonvulsant properties of doramectin in rats: behavioral and neurochemistric evaluations

Doramecin is an antiparasitic drug that may interfere with gamma-aminobutyric acid (GABA) neurotransmission. Some behavioral manifestations are related with GABAergic neurotransmissions as anxiety and seizures. The objective of the present study was to examine the possible central nervous system (CNS) effects of doramectin (100, 300 and 1000 microg/kg, SC) in rats, using anxiety behavioral models, susceptibility to seizures and central neurotransmitter evaluations. The open-field results showed (i) few alterations in locomotion frequency; (ii) a biphasic effect on rearing frequency that may be the consequence of least habituation in open-field; (iii) the reduction of grooming durations might be attributed to a possible anxiolytic effect of doramectin since GABAergic agonists reduced this parameter in apparatus. Our data in the hole board showed no effects in locomotion and rearing frequencies but increased head dipping frequency of rats administered doramectin similarly to anxiolytic drugs. In plus-maze test, doramectin administration increased the number of entries and time into open arms, indicating also an anxiolytic effect. Doramectin protected animals from convulsant effects of picrotoxin, indicative of an anxiolytic pharmacological profile of a drug with GABAergic properties. The alterations observed in central dopaminergic, noradrenergic and serotoninergic neurotransmissions might be the consequence of reinforcement in central GABAergic neurotransmission induced by doramectin. The present results suggest that doramectin has the pharmacological profile of an anxiolytic/anticonvulsant drug with GABAergic properties.     

GABA Transporter-1 Deficiency Confers Schizophrenia-Like Behavioral Phenotypes

The mechanism underlying the pathogenesis of schizophrenia remains poorly understood. The hyper-dopamine and hypo-NMDA receptor hypotheses have been the most enduring ideas. Recently, emerging evidence implicates alterations of the major inhibitory system, GABAergic neurotransmission in the schizophrenic patients. However, the pathophysiological role of GABAergic system in schizophrenia still remains dubious. In this study, we took advantage of GABA transporter 1 (GAT1) knockout (KO) mouse, a unique animal model with elevated ambient GABA, to study the schizophrenia-related behavioral abnormalities. We found that GAT1 KO mice displayed multiple behavioral abnormalities related to schizophrenic positive, negative and cognitive symptoms. Moreover, GAT1 deficiency did not change the striatal dopamine levels, but significantly enhanced the tonic GABA currents in prefrontal cortex. The GABA_A receptor antagonist picrotoxin could effectively ameliorate several behavioral defects of GAT1 KO mice. These results identified a novel function of GAT1, and indicated that the elevated ambient GABA contributed critically to the pathogenesis of schizophrenia. Furthermore, several commonly used antipsychotic drugs were effective in treating the locomotor hyperactivity in GAT1 KO mice, suggesting the utility of GAT1 KO mice as an alternative animal model for studying schizophrenia pathogenesis and developing new antipsychotic drugs.     

Modulation of a neuronal network by electrical high frequency stimulation in striatal slices of the rat in vitro

The effects of the GABA(A) receptor antagonist bicuculline, the D2-like receptor antagonist sulpiride and the D1-like receptor antagonist SCH-23390 on the electrical high frequency stimulation (HFS)-evoked gamma-aminobutyric acid (GABA) and dopamine (DA) release were measured from slices of the rat striatum by means of HPLC method with electrochemical detection. HFS with 130Hz stimulated veratridine-activated GABAergic neurons resulting in an increased GABA outflow while DA outflow decreased. In the presence of the GABA(A) receptor antagonist bicuculline extracellular GABA and DA outflow were enhanced. When the competitive dopamine D2-like receptor antagonist S-(-)-sulpiride was added to incubation medium, the HFS-evoked stimulatory effect on GABA outflow declined to values found after veratridine (1microM) without HFS. After co-incubation of sulpiride and the competitive D1-like receptor antagonist R-(+)-SCH-23390, the effect of sulpiride on HFS plus veratridine-evoked GABA outflow was completely reversed. Neither sulpiride nor SCH-23390 had any influence on the effect of HFS on veratridine-induced DA outflow. No effect of HFS on glutamate outflow was observed in all experiments. These results led us to suggest that in our model HFS primarily affects GABAergic neurons. These neurons are embedded in a neuronal network with a GABA-dopamine circuit, and thus, HFS interacts with a neuronal network, not only with one neurotransmitter system or one neuron population.     

Determination of Amino Acids and Monoamine Neurotransmitters in Caudate Nucleus of Seizure-Resistant and Seizure-Prone BALB/c Mice

Abstract: Amino acid and monoamine concentrations were examined in tissue extracts of caudate nucleus of genetic substrains of BALB/c mice susceptible or resistant to audiogenic seizures. Amino acids [aspartate, glutamate, glycine, taurine, serine, γ-aminobutyric acid (GABA)], monoamines, and related metabolites were separated by isocratic reverse-phase chromatography and detected by a coulometric electrode array system. In situ activity of tyrosine hydroxylase and tryptophan hydroxylase were determined by measuring the accumulation of L-DOPA and 5-hydroxytryptophan after administration of the decarboxylase inhibitor NSD-1015. Highly significant decreases in concentrations of both excitatory (glutamate and aspartate) and inhibitory amino acids (GABA and taurine) were observed in extracts of caudate nucleus of seizure-prone mice. Substantial decreases in concentrations of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, were also noted. Decreased accumulation of L-DOPA after NSD-1015 administration provided evidence for decreased tyrosine hydroxylase activity and decreased DA synthesis in striatum of seizure-prone mice compared with seizure-resistant mice. Decreased concentrations of the DA metabolite 3-methoxytyramine (after NSD-1015 administration) suggested that DA release was also compromised in seizure-prone mice. No significant difference in 5-hydroxytryptophan accumulation in striatum of seizure-prone and seizure-resistant mice suggested that tryptophan hydroxylase activity and serotonin synthesis were not affected. The data suggest that seizure-prone BALB/c mice have a deficiency in intracellular content of both excitatory and inhibitory amino acids. The data also raise the issue of whether GABAergic interactions with the nigrostriatal DA system are important in the regulation of audiogenic seizure susceptibility.     

Limonene-induced activation of A2A adenosine receptors reduces airway inflammation and reactivity in a mouse model of asthma

Animal models of asthma have shown that limonene, a naturally occurring terpene in citrus fruits, can reduce inflammation and airway reactivity. However, the mechanism of these effects is unknown. We first performed computational and molecular docking analyses that showed limonene could bind to both A_2A and A_2B receptors. The pharmacological studies were carried out with A_2A adenosine receptor knock-out (A_2AKO) and wild-type (WT) mice using ovalbumin (OVA) to generate the asthma phenotype. We investigated the effects of limonene on lung inflammation and airway responsiveness to methacholine (MCh) and NECA (nonselective adenosine analog) by administering limonene as an inhalation prior to OVA aerosol challenges in one group of allergic mice for both WT and KO. In whole-body plethysmography studies, we observed that airway responsiveness to MCh in WT SEN group was significantly lowered upon limonene treatment but no effect was observed in A_2AKO. Limonene also attenuated NECA-induced airway responsiveness in WT allergic mice with no effect being observed in A_2AKO groups. Differential BAL analysis showed that limonene reduced levels of eosinophils in allergic WT mice but not in A_2AKO. However, limonene reduced neutrophils in sensitized A_2AKO mice, suggesting that it may activate A_2B receptors as well. These data indicate that limonene-induced reduction in airway inflammation and airway reactivity occurs mainly via activation of A_2AAR but A_2B receptors may also play a supporting role.

     

Baclofen,a GABAB receptor agonist,enhances ubiquitin-proteasome system functioning and neuronal survival in Huntington’s disease model mice

Huntington’s disease (HD) is an autosomal neurodegenerative disease. Its manifestations is selective degeneration of medium-sized spiny neurons (MSN) in the striatum. The specificity of the vulnerability of these GABAergic MSNs can be explained by abnormal protein accumulation, excitotoxicity, mitochondrial dysfunction, and failure of trophic control, among other dysfunctions. In this study, we used in vitro and in vivo models of HD to study the effects of GABAergic neuron stimulation on the cellular protein degradation machinery. We administered the GABA_B receptor agonist, baclofen, to wild-type or mutant huntingtin-expressing striatal cells (HD19 or HD43). Chymotrypsin-like proteasome activity and cell viability were significantly increased in the mutant huntingtin-expressing striatal cells (HD43) after GABA_B receptor agonist treatment. In addition, we systemically administered baclofen to a HD model containing the entire human huntingtin gene with 128 CAG repeats (YAC128). Chymotrypsin-like proteasome activity was significantly increased in YAC128 transgenic mice after baclofen administration. Baclofen-injected mutant YAC128 mice also showed significantly reduced numbers of ubiquitin-positive neuronal intranuclear inclusions (NIIs) in the striatum. Baclofen markedly improved behavioral abnormalities in mutant YAC128 mice as determined by the rotarod performance test. These data indicate that stimulation of GABAergic neurons with the GABA_B receptor agonist, baclofen, enhances ubiquitin-proteasome system (UPS) function and cell survival in in vitro and in vivo models of HD.     

K(ATP)-dependent neurotransmitter release in the neuronal network of the rat caudate nucleus

K(ATP) channels can couple the bioenergetic metabolism of the cell to membrane excitability. Here, we show gamma-aminobutyric acid (GABA) mediated inhibition of dopamine outflow from slices of the rat caudate nucleus that is regulated by extracellular glucose via high- and low-affinity K(ATP) channels. During glucose reduction, a biphasic dopamine effect could be observed with first a dopamine increase followed by a decline at low glucose concentrations. Both phases were inhibited by glibenclamide. Pinacidil decreased DA outflow without an effect of glucose reduction implying an overall activation of K(ATP) channels. The first phase with dopamine increase was related to reduced GABAergic activity and could be blocked by bicuculline. Our results may be explained by different types of K(ATP) channels with low affinity of ATP and glibenclamide on inhibitory GABAergic and high-affinity on excitatory DAergic neurons. This led us to suggest a biological principle through which neuronal networks are functioning.     

GABA and the behavioral effects of anxiolytic drugs

Much recent research has shown that benzodiazepine binding sites in the central nervous system are associated with GABA receptors. It is therefore possible that the pharmacological and therapeutic effects of benzodiazepines and drugs with similar profiles are mediated through GABAergic mechanisms. In this paper the evidence is considered for a possible involvement of GABA in the behavioral effects of anxiolytic drugs. There are a number of reports that the behavioral actions of anxiolytics can be antagonised by GABA antagonists such as bicuculline or picrotoxin but there are many contradictory findings and these drugs are difficult to use effectively in behavioral studies. In general, GABA agonists do not exert anxiolytic-like behavioral effects after systemic injection but intracerebral administration of muscimol has been shown to produce benzodiazepine-like actions. Although a number of questions remain unanswered, current evidence does not provide strong support for a role for GABA in the behavioral effects of anxiolytic drugs.     

Behavioral and Neurochemical Changes Induced by Repetitive Combined Treatments of Ketamine and Amphetamine in Mice

The combined abuse of recreational drugs such as ketamine (Ket) and amphetamine (Amph) should be seriously considered important social and health issues. Numerous studies have documented the behavioral and neurochemical changes associated with polydrug administration; however, most studies have only examined the acute effects. The consequences following chronic repetitive polydrug use are less studied. In the present study, intraperitoneal injections of saline, Amph (5 mg/kg), low dose Ket (LK, 10 mg/kg), high dose Ket (HK, 50 mg/kg), or Amph plus LK or HK (ALK or AHK) were conducted twice a day for three consecutive days, and one final treatment was administered on day 4. After seven total treatments, animal behaviors, including locomotion, stereotypy and ataxia, were examined in a novel open field. The expression of GAD₆₇ and dopamine (DA) levels were assessed in the striatum and motor-related cortices using immunohistochemistry and high-performance liquid chromatography. Drug-induced hyperactivities and Amph-mediated potentiation of Ket-triggered ataxia manifested after repeated drug treatments. A significant increase in the number of GAD₆₇-positive puncta in the striatum and motor-related cortices was observed, suggesting a neural adaptive change in the GABAergic system. Four hours after the final treatment, while the behavioral hyperactivities had ceased, considerable changes were still evident in the motor-related cortices, suggesting modulation to the DAergic system. Together, our results show the interactive effects of these two drugs in behavioral and neurochemical aspects and neural adaptive changes in the GABAergic and DAergic systems.     

Effect of honokiol on activity of GAD65 and GAD67 in the cortex and hippocampus of mice

Honokiol, an active agent extracted from magnolia bark, has been reported that induces anxiolytic action in a mouse elevated plus-maze test. However, the mechanism of anxiolytic action induced by honokiol remains unclear. This study was to investigate the change in two forms of glutamic acid decarboxylase (GABA synthesized enzymes) GAD₆₅ and GAD₆₇ in the cortex and hippocampus areas while the anxiolytic actions induced by chronic administration of honokiol in mice. Mice treated with 7 daily injection of honokiol (1 mg/kg, p.o.) caused anxiolytic action which was similar to that was induced by 7 daily injection of diazepam (2 mg/kg, p.o.) in the elevated plus-maze test. In addition, the activity of hippocampal GAD₆₅ of honokiol treated mice was significantly increased than that of the vehicle or diazepam treated groups. These data suggest that honokiol causes diazepam-like anxiolytic action, which may be mediated by altering the synthesis of GABA in the brain of mice.