Coexistence of carriers for dopamine and GABA uptake on a same nerve terminal in the rat brain

Neuropharmacological Effects of Terpenoids on Preclinical Animal Models of Psychiatric Disorders: A Review

Terpenoids are widely distributed in nature, especially in the plant kingdom, and exhibit diverse pharmacological activities. In recent years, screening has revealed a wide variety of new terpenoids that are active against different psychiatric disorders. This review synthesized the current published preclinical studies of terpenoid use in psychiatric disorders. This review was extensively investigated to provide empirical evidence regarding the neuropharmacological effects of the vast group of terpenoids in translational models of psychiatric disorders, their relevant mechanisms of action, and treatment regimens with evidence of the safety and psychotropic efficacy. Therefore, we utilized nine (9) electronic databases and performed manual searches of each. The relevant data were retrieved from the articles published until present. We used the search terms "terpenoids" or "terpenes" and "psychiatric disorders" ("psychiatric disorders" OR "psychiatric diseases" OR "neuropsychiatric disorders" OR "psychosis" OR "psychiatric symptoms"). The efficacy of terpenoids or biosynthetic compounds in the terpenoid group was demonstrated in preclinical animal studies. Ginsenosides, bacosides, oleanolic acid, asiatic acid, boswellic acid, mono- and diterpenes, and different forms of saponins and triterpenoids were found to be important bioactive compounds in several preclinical studies of psychosis. Taken together, the findings of the present review indicate that natural terpenoids and their derivatives could achieve remarkable success as an alternative therapeutic option for alleviating the core or associated behavioral features of psychiatric disorders.

Blocking glutamate mGlu5 receptors with the negative allosteric modulator CTEP improves disease course in SOD1G93A mouse model of amyotrophic lateral sclerosis

BACKGROUND AND PURPOSE

The pathogenesis of amyotrophic lateral sclerosis (ALS) is not fully clarified, although excessive glutamate (Glu) transmission and the downstream cytotoxic cascades are major mechanisms for motor neuron death. Two metabotropic glutamate receptors (mGlu₁ and mGlu₅ ) are overexpressed in ALS and regulate cellular disease processes. Expression and function of mGlu₅ receptors are altered at early symptomatic stages in the SOD1^G93A mouse model of ALS and knockdown of mGlu5 receptors in SOD1^G93A mice improved disease progression.

EXPERIMENTAL APPROACH

We treated male and female SOD1^G93A mice with 2-chloro-4-((2,5-dimethyl-1-(4-(trifluoromethoxy)phenyl)-1H-imidazol-4-yl)ethynyl)pyridine (CTEP), an orally available mGlu₅ receptor negative allosteric modulator (NAM), using doses of 2 mg·kg^-1 per 48 h or 4 mg·kg^-1 per 24 h from Day 90, an early symptomatic disease stage. Disease progression was studied by behavioural and histological approaches.

KEY RESULTS

CTEP dose-dependently ameliorated clinical features in SOD1^G93A mice. The lower dose increased survival and improved motor skills in female mice, with barely positive effects in male mice. Higher doses significantly ameliorated disease symptoms and survival in both males and females, females being more responsive. CTEP also reduced motor neuron death, astrocyte and microglia activation, and abnormal glutamate release in the spinal cord, with equal effects in male and female mice. No differences were also observed in CTEP access to the brain.

CONCLUSION AND IMPLICATIONS

Our results suggest that mGlu₅ receptors are promising targets for the treatment of ALS and highlight mGlu5 receptor NAMs as effective pharmacological tools with translational potential.

Elevated striatal γ-aminobutyric acid in youth with major depressive disorder

BACKGROUND

Alterations in γ-aminobutyric acid (GABA) have been hypothesized to play a role in the pathogenesis of psychiatric illness. Our previous work has specifically linked anterior cingulate cortex (ACC) GABA deficits with anhedonia in youth with major depressive disorder (MDD). As anhedonia reflects alterations within the reward circuitry, we sought to extend this investigation and examine GABA levels in another key reward-related region, the striatum, in the same adolescent population.

METHODS

Thirty-six youth [20 with MDD and 16 healthy controls; (HC)], ages 12 to 21 years old, underwent J-edited proton magnetic resonance spectroscopy (¹H MRS) whereby GABA levels were measured in striatal and ACC voxels. GABA levels were compared between groups and between voxel positions and were examined in relation to clinical symptomatology, such as depression severity, anhedonia, anxiety, and suicidality.

RESULTS

Depressed youth had unexpectedly higher GABA levels in the striatum compared to HC. In both depressed and healthy youth, GABA levels were higher in the striatum than in the ACC, while the differences in depressed youth were greater. Moreover, in depressed youth, higher striatal GABA above the mean of HCs was correlated with lower ACC GABA below the mean of HCs. Striatal GABA was not correlated with clinical symptomatology in this small sample.

CONCLUSIONS

Together, these findings suggest that higher striatal GABA levels may serve some compensatory function as a result of lower ACC GABA in depressed adolescents. It is also possible that, like lower ACC GABA, higher striatal GABA might simply be another pathological feature of adolescent depression.

Effects of sub-chronic nandrolone administration on hormonal adaptive response to acute stress in rats

Androgenic-anabolic steroid (AAS) misuse has been associated with depression. It has been proposed that stress has a role in depression and that serotonin is involved in both endocrine responses to stress and depressive physiopathology. Although reports demonstrate that AAS chronic administration modifies components of stress-responsive hypothalamic-pituitary-adrenal axis (HPAA), no study has evaluated AAS effect on the response to stressful stimuli. We studied the effects of the subchronic administration (once a day for 14 days in rats) of a supratherapeutical dose of nandrolone decanoate (ND) on HPAA and cortical serotoninergic system response to acute restraint stress (RS). Acute RS produced the following effects: increase in CORT (in blood) and ACTH (both in blood and in pituitary corticotropes), GR depletion in hippocampus and hypothalamus cytosol and GR translocation in hippocampus nuclear fraction, cortical serotonin re-uptake stimulation and hippocampus cytosolic ERK2 activation. ND by itself, i.e. in non-stressed rats, did not modify these parameters, except for a decrease of plasma CORT and ACTH levels and an increase in hippocampus cytosolic phospho-ERK1/2. On the contrary, in stressed rats ND affected stress-induced plasma ACTH increase and prevented all other above reported stress effects, except the increase in pituitary ACTH positive cell density. Our results show that the prolonged administration of a supratherapeutical dose of ND in rats, albeit did not affect in a notable way HPAA and serotonin transporter activity in the absence of stress, may deregulate the stress-induced hormonal cascade which plays a crucial role in depressive psychopathology.

Effects of swim stress and α-MSH acute pre-treatment on brain 5-HT transporter and corticosterone receptor

The forced swim test (FST) can lead to stress-related diseases such as depression, through activation of hypothalamic-pituitary-adrenal axis (HPAA) and corticosteroid disregulation. Among the proopiomelanocortin (POMC)-derived peptides, alpha-melanocyte-stimulating hormone (alpha-MSH) has been shown to regulate long-lasting behavioral responses. Moreover, serotonergic pathways in various brain areas are activated by stressors, a feature that suggests a role for serotonin in both stress-induced HPAA disregulation and depressive physiopathology. Taking all together these data, we investigated the effects of the FST exposure and the effects of pre-treatment with alpha-MSH on cortical synaptosomal serotonin transporter (SERT) activity, corticosterone (CORT) plasma levels and on glucocorticoid receptor (GR) occupancy and expression in rat hippocampus. Young male rats were divided into three groups treated with saline or with alpha-MSH at doses of 1 or 4 microg/rat, 15 min prior to FST. Our data show that FST increased CORT secretion; GR levels in hippocampus decreased in density after stress without variations in affinity; GR redistributed from the cytosolic to the nuclear tissue fraction; finally, SERT activity strongly increased. All these effects were blocked by pre-treatment with alpha-MSH at the higher dose.

GABAergic dysfunction in mood disorders

The authors review the available literature on the preclinical and clinical studies involving GABAergic neurotransmission in mood disorders. Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter present almost exclusively in the central nervous system (CNS), distributed across almost all brain regions, and expressed in interneurons modulating local circuits. The role of GABAergic dysfunction in mood disorders was first proposed 20 years ago. Preclinical studies have suggested that GABA levels may be decreased in animal models of depression, and clinical studies reported low plasma and CSF GABA levels in mood disorder patients. Also, antidepressants, mood stabilizers, electroconvulsive therapy, and GABA agonists have been shown to reverse the depression-like behavior in animal models and to be effective in unipolar and bipolar patients by increasing brain GABAergic activity. The hypothesis of reduced GABAergic activity in mood disorders may complement the monoaminergic and serotonergic theories, proposing that the balance between multiple neurotransmitter systems may be altered in these disorders. However, low GABAergic cortical function may probably be a feature of a subset of mood disorder patients, representing a genetic susceptibility. In this paper, we discuss the status of GABAergic hypothesis of mood disorders and suggest possible directions for future preclinical and clinical research in this area.

Subcellular distribution of histamine, GABA and galanin in tuberomamillary neurons in vitro

Histamine acts as a neurotransmitter in the brain and regulates e.g. sleep, hibernation, vigilance, and release of several other transmitters. All histaminergic neurons are found in the tuberomamillary nucleus (TM), and send axons to almost all parts of the CNS. Despite the obvious importance of these neurons, their development, transmitter storage, and compartmentalization of cotransmitters are poorly known. Histaminergic neurons from fetal rat hypothalamus were studied in primary explant cultures and analyzed by confocal microscopy. Most histaminergic neurons were oval in shape, but round and triangular ones were also found. The average size of the 212 analyzed neurons was 19.2 microm (length), 12.5 microm (width) and 11.7 microm (thickness). The cells possessed two to five microtubule-associated protein (MAP2) positive processes, putative dendrites, and in general one MAP2-negative thin process, a putative axon. Granular histamine-immunoreactivity was found in the cell bodies, axons, and dendrites. In tuberomamillary neurons, most histamine-containing structures displayed immunoreactivity for vesicular monoamine transporter 2 (VMAT2), indicating that the two markers may coexist in the same structures. Lack of VMAT2 in some histamine-immunoreactive structures indicates that another transporter for histamine may exist. In the same neurons, gamma-aminobutyric acid (GABA)-immunoreactivity was found in structures, distinct from those containing histamine, indicating that the two transmitters may be differentially localized, regulated and released. Galanin-immunoreactivity in the cultured tuberomamillary neurons was partially located in the same structures as VMAT2. The results suggest that histamine and GABA, the two principal transmitters of tuberomamillary neurons, are not costored in the same structures in tuberomamillary neurons.

The sensitivity of catecholamine release to botulinum toxin C1 and E suggests selective targeting of vesicles set into the readily releasable pool

The impact of syntaxin and SNAP-25 cleavage on [3H]noradrenaline ([3H]NA) and [3H]dopamine ([3H]DA) exocytotic release evoked by different stimuli was studied in superfused rat synaptosomes. The external Ca2+-dependent K+-induced [3H]catecholamine overflows were almost totally abolished by botulinum toxin C1 (BoNT/C1), which hydrolyses syntaxin and SNAP-25, or by botulinum toxin E (BoNT/E), selective for SNAP-25. BoNT/C1 cleaved 25% of total syntaxin and 40% of SNAP-25; BoNT/E cleaved 40% of SNAP-25 but left syntaxin intact. The GABA uptake-induced releases of [3H]NA and [3H]DA were differentially affected: both toxins blocked the former, dependent on external Ca2+, but not the latter, internal Ca2+-dependent. BoNT/C1 or BoNT/E only slightly reduced the ionomycin-evoked [3H]catecholamine release. More precisely, [3H]NA exocytosis induced by ionomycin was sensitive to toxins in the early phase of release but not later. The Ca2+-independent [3H]NA exocytosis evoked by hypertonic sucrose, thought to release from the readily releasable pool (RRP) of vesicles, was significantly reduced by BoNT/C1. Pre-treating synaptosomes with phorbol-12-myristate-13-acetate, to increase the RRP, enhanced the sensitivity to BoNT/C1 of [3H]NA release elicited by sucrose or ionomycin. Accordingly, cleavage of syntaxin was augmented by the phorbol-ester. To conclude, our results suggest that clostridial toxins selectively target exocytosis involving vesicles set into the RRP.

Coexistence and function of different neurotransmitter transporters in the plasma membrane of CNS neurons

Transporters able to recapture released neurotransmitters into neurons can no longer be considered as cell-specific neuronal markers. In fact, colocalization on one nerve terminal of transporters able to selectively recapture the released endogenously synthesized transmitter (homotransporters) and of transporters that can selectively take up transmitters/modulators originating from neighboring structures (heterotransporters) has been demonstrated to occur on several families of nerve terminals. Activation of heterotransporters often increases the release of the transmitter stored in the terminals on which the heterotransporters are localized. The release caused by heterotransporter activation takes place through multiple mechanisms including exocytosis, either dependent on external Ca(2+) or on Ca(2+) mobilized from intraterminal stores, and homotransporter reversal. Homocarrier-mediated release elicited by heterocarrier activation represents a clear case of transporter-transporter interaction. Although the functional significance of transporter coexpression on one nerve terminal remains to be established, it may in some instances reflect cotransmission. In other cases, heterotransporters may mediate modulation of basal transmitter release in addition to the modulation of the evoked release brought about by presynaptic heteroreceptors. Heterotransporters are also increasingly reported to exist on neuronal soma/dendrites. With the exception of EAAT4, the glutamate transporter/chloride channel situated on GABAergic Purkinje cells in the cerebellum, the functions of somatodendritic heterocarriers is not understood.

The reverse transport of DA, what physiological significance?

It is well established that midbrain dopamine neurons innervating the striatum, release their neurotransmitter through an exocytotic process triggered by the neural firing and involving a transient calcium entry in the terminals. Long ago, it had been proposed, however, that another mechanism of release could co-exist with classical exocytosis, involving the reverse-transport of the cytosolic amine by the carrier, ordinarily responsible for uptake function. This atypical mode of release could be evoked directly at the preterminal level by multiple environmental endogenous factors involving transient alterations of the sodium gradient. It cannot be excluded that this mode of release participates in the firing-induced release. In contrast with the classical exocytosis of a preformed DA pool, the reverse-transport of DA requires simultaneous alterations of intraterminal amine metabolism including synthesis and displacement from storage compartment. The concept of a reverse-transport of dopamine is coming from the observations that releasing substances, such as amphetamine-related molecules, actually induce this type of transport. A large set of arguments advocates that reverse-transport plays a role in the maintenance of basal extracellular DA concentration in striatum. It was also often evoked in physiopathological situations including ischemia, neurodegenerative processes, etc. The most recent studies suggest that this release could occur mainly outside the synapses, and thus could constitute a major feature in the paracrine transmission, sometimes evoked for DA.

125I-CGP 64213 binding to GABA(B) receptors in the brain of monkeys: effect of MPTP and dopaminomimetic treatments

Much evidence indicates that abnormal GABA neurotransmission may be implicated in the pathophysiology of Parkinson's disease (PD) and dopaminomimetic-induced dyskinesias (DID). In this study, autoradiography using (125)I-CGP 64213 was performed to investigate GABA(B) receptor density in the brain of control monkeys as well as monkeys with MPTP-induced nigrostriatal depletion. Three MPTP monkeys received pulsatile administrations of the D1 dopamine (DA) receptor agonist (SKF 82958) whereas a long-acting D2 DA receptor agonist (cabergoline) was given to another three animals. SKF 82958 treatment relieved parkinsonian symptoms but two of three animals developed DID. Cabergoline induced a comparable motor benefit effect without persistent DID. (125)I-CGP 64213 binding to GABA(B) receptors was heterogeneous throughout the brain with the highest levels in the medial habenula of the thalamus. MPTP induced a decrease (-40%) of (125)I-CGP 64213 binding to GABA(B) receptors in the substantia nigra pars compacta (SNpc) and an increase (+29%) in the internal segment of the globus pallidus (GPi). This increase in the GPi was not affected by SKF 82958 but partly reversed by cabergoline. No change was seen in the striatum, the thalamus, the external segment of the globus pallidus, and the substantia nigra pars reticulata following MPTP and dopaminomimetic treatments. The changes of GABA(B) receptors observed in the SNpc and in the GPi suggest that alteration of GABA(B) receptors may play a role in the pathophysiology of PD and DID.

Endogenous GABA potentiates the potassium-induced release of dopamine in striatum of the freely moving rat: a microdialysis study

Using microdialysis, a study was made of the effects of an increase of endogenous GABA on basal and potassium-stimulated release of dopamine in striatum of the awake rat. The dopamine metabolites DOPAC and HVA were also measured. Extracellular concentrations of GABA were increased by inhibiting its uptake with nipecotic acid. TTX (10 microM) reduced basal extracellular concentrations of dopamine, and dopamine metabolites, but not GABA. Nipecotic acid (200, 500, and 1000 microM) produced a dose-related increase in basal extracellular concentrations of GABA, but did not change basal extracellular concentrations of dopamine and dopamine metabolites. However, nipecotic acid significantly enhanced the dopamine release produced by perfusion of potassium (50 mM) and also enhanced the extracellular increase of GABA produced by high potassium. These results suggest that an increase of endogenous GABA is facilitating the stimulated release, but not the basal release, of dopamine in the striatum of the awake rat.

Evidence for calcium-dependent vesicular transmitter release insensitive to tetanus toxin and botulinum toxin type F

Whether exocytosis evoked by a given releasing stimulus from different neuronal families or by different stimuli from one neuronal population occurs through identical mechanisms is unknown. We studied the release of [3H]noradrenaline, [3H]acetylcholine and [3H]dopamine induced by different stimuli from superfused rat brain synaptosomes pretreated with tetanus toxin or botulinum toxin F, known to block exocytosis by cleaving VAMP/synaptobrevin. The external Ca2(+)-dependent [3H]transmitter overflows evoked by KCl were similarly inhibited by tetanus toxin or botulinum toxin F; the toxins cleaved similar amounts of synaptosomal synaptobrevin, as determined by western blot analysis, suggesting prevalent involvement of synaptobrevin-II. GABA uptake-mediated release of the three [3H]transmitters was that differentially sensitive to the toxins: only the release of [3H]noradrenaline, which is dependent on external Ca2+, but not of [3H]acetylcholine and [3H]dopamine was blocked. Neither toxin affected the [3H]transmitter overflows evoked by the Ca2(+) ionophore ionomycin. Cadmium blocked the K(+)-evoked release of all [3H]transmitters and the GABA-evoked release of [3H]noradrenaline; the GABA-evoked releases of [3H]acetylcholine and [3H]dopamine and those elicited by ionomycin were insensitive to cadmium. The results suggest that tetanus toxin and botulinum toxin F selectively affect exocytosis linked to activation of voltage-sensitive Ca2(+) channels; the Ca2(+)-dependent, exocytotic-like release induced by stimuli not leading to activation of voltage-sensitive Ca2+ channels seems insensitive to these clostridial toxins.

Acute, subchronic and discontinuation effects of zopiclone on sleep EEG and nocturnal melatonin secretion

Zopiclone is a new short half-life cyclopyrrolone hypnotic agent acting at the GABA-benzodiazepine receptor complex. In order to characterize its pharmacological profile, the effects of 7.5 mg zopiclone on nocturnal melatonin secretion were investigated under polysomnographic control in 11 healthy subjects following acute and subchronic administration as well as after abrupt discontinuation of the drug. No effect of zopiclone on the melatonin plasma levels could be observed. Regarding both total melatonin production and the temporal pattern of melatonin secretion during the night, there was no difference between placebo baseline condition, acute and subchronic administration, and discontinuation. In contrast, the sleep EEG data demonstrated the hypnotic efficacy of zopiclone under acute administration and indicated a rebound insomnia after abrupt discontinuation. Moreover, alterations of sleep architecture were found under treatment as well as after discontinuation. Whereas, with regard to sleep EEG parameters, zopiclone appears to be comparable with some short-acting benzodiazepines, a discrepancy between the missing effect of zopiclone on pineal function and the suppressing influence of benzodiazepines known from the literature becomes obvious. The fact that zopiclone does not interfere with nocturnal melatonin secretion at pharmacologically active doses as indicated by alterations in sleep EEG parameters might possibly point to a pharmacodynamic difference between the two drug classes.

Decreased GABA release following tonic-clonic seizures is associated with an increase in extracellular glutamate in rat hippocampus in vivo

The effects of maximal electroshock, used as a model of generalized seizures, were studied on extracellular GABA and glutamate levels in the ventral hippocampus of the freely-moving rat, using in vivo microdialysis. Following a maximal electroshock there was a rapid decline in GABA levels (46 +/- 5%) in the 20 min immediately after the seizure and levels remained depressed for a further 60 min. However, although there was a transient small decrease (11 +/- 2%) in glutamate levels in the first 20 min post-ictally, there followed a more prolonged, larger increase in the next 40 min. Maximal electroshock, administered in the absence of extracellular calcium, did not change GABA levels, while glutamate levels were again increased (42 +/- 8%) in the 40-80 min after the shock. Local perfusion with nickel (1 mM) to block T-type calcium channels had no effect on basal GABA or glutamate levels but prevented maximal electroshock-induced changes in both amino acids. Experiments were carried out to test the hypothesis that the post-ictal increased glutamate release was due to the decrease in GABA release. Perfusion with the potent GABA re-uptake inhibitor NNC-711, for 60 min prior to administration of maximal electroshock, increased GABA levels (436 +/- 58%) and abolished the seizure-induced decrease. Basal glutamate levels were not affected by perfusion with NNC-711 but subsequent maximal electroshock also failed to affect levels. Local perfusion with the GABAA receptor antagonist bicuculline (1, 10 and 100 microM) had no effect on basal GABA levels but glutamate levels were increased (46 +/- 5%) after perfusion with 100 microM bicuculline.(ABSTRACT TRUNCATED AT 250 WORDS)

Do nerve terminals and cell bodies of nigrostriatal dopaminergic neurons of the rat contain similar receptors?

The question was investigated whether dopamine release-controlling receptors are evenly distributed over somatodendritic sites and nerve terminals of nigrostriatal dopaminergic neurons of the rat. Prototypical drugs of 5 different (sub)types of receptors (D2, cholinergic, GABAB, NMDA and non-NMDA) were infused via a microdialysis probe into the striatum, and effects on dopamine released from nerve terminals were determined by microdialysis. In separate experiments the same drugs were infused into the substantia nigra and effects on dendritic release of dopamine were recorded. In addition, the effect of calcium depletion and tetrodotoxin infusion (1 mumol/l) was studied in both areas. Infusion of (-)-N0437 (1 mumol/l), (-)-sulpiride (1 mumol/l), NMDA (300 mumol/l), AMPA (100 mumol/l), kainic acid (30 mumol), tetrodotoxin and depletion of calcium induced comparable changes in the release of dopamine when applied into the striatum as well as into the nigra. Carbachol (100 mumol/l) and baclofen (5 mumol/l) inhibited dendritic dopamine release when administered into the nigra; however, the latter drugs were not effective when infused into the striatum. It is concluded that the release-controlling receptors are not evenly distributed over somata and nerve terminals of dopaminergic neurons.

In vivo release of dopamine from rat striatum, substantia nigra and prefrontal cortex: differential modulation by baclofen

1. The effect of baclofen, a GABAB receptor agonist, on the release of dopamine from the striatum (ST), substantia nigra (SN) and prefrontal cortex (PFC) of the rat was examined by intracerebral microdialysis. 2. Perfusion of baclofen 50 microM did not affect the striatal release of dopamine. However, dopamine release was markedly reduced in the SN and PFC. 3. 3,4-Dihydroxyphenylacetic acid and homovanillic acid output increased in the ST and decreased in the SN and PFC when baclofen was perfused through the microdialysis probe. 5-Hydroxyindoleacetic acid levels were not affected in any experimental condition by baclofen perfusion. 4. The results suggest that GABAB receptors modulate the release of dopamine in the SN and PFC, but do not affect the striatal release of dopamine, which indicates that the role of GABA receptor activation is different in the dopaminergic terminals of the ST and PFC.

Glutamic acid and gamma-aminobutyric acid modulate each other's release through heterocarriers sited on the axon terminals of rat brain

The effects of gamma-aminobutyric acid (GABA) on the spontaneous release of endogenous glutamic acid (Glu) or aspartic acid (Asp) and the effects of Glu on the release of endogenous GABA or [3H]GABA were studied in superfused rat cerebral cortex synaptosomes. GABA increased the outflow of Glu (EC50 17.2 microM) and Asp (EC50 18.4 microM). GABA was not antagonized by bicuculline or picrotoxin. Neither muscimol nor (-)-baclofen mimicked GABA. The effects of GABA were prevented by GABA uptake inhibitors and were Na+ dependent. Glu enhanced the release of [3H]GABA (EC50 11.5 microM) from cortical synaptosomes. Glu was not mimicked by the glutamate receptor agonists N-methyl-D-aspartic, kainic, or quisqualic acid. The Glu effect was decreased by the Glu uptake inhibitor D-threo-hydroxyaspartic acid (THA) and it was Na+ sensitive. Similarly to Glu, D-Asp increased [3H]GABA release (EC50 9.9 microM), an effect blocked by THA. Glu also increased the release of endogenous GABA from cortex synaptosomes. In this case the effect was in part blocked by the (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, whereas the 6-cyano-7-nitroquinoxaline-2,3-dione-insensitive portion of the effect was prevented by THA. GABA increased the [3H]D-Asp outflow (EC50 13.7 microM) from hippocampal synaptosomes in a muscimol-, (-)-baclofen-, bicuculline-, and picrotoxin-insensitive manner. The GABA effect was abolished by blocking GABA uptake and was Na+ dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

gamma-Aminobutyric acid and glycine modulate each other's release through heterocarriers sited on the releasing axon terminals of rat CNS

The ability of gamma-aminobutyric acid (GABA) and glycine (Gly) to modulate each other's release was studied in synaptosomes from rat spinal cord, cerebellum, cerebral cortex, or hippocampus, prelabeled with [3H]GABA or [3H]Gly and exposed in superfusion to Gly or to GABA, respectively. GABA increased the spontaneous outflow of [3H]Gly (EC50, 20.8 microM) from spinal cord synaptosomes. Neither muscimol nor (-)-baclofen, up to 300 microM, mimicked the effect of GABA, which was not antagonized by either bicuculline or picrotoxin. However, the effect of GABA was counteracted by the GABA uptake inhibitors nipecotic acid and N-(4,4-diphenyl-3-butenyl)nipecotic acid. Moreover, the GABA-induced [3H]Gly release was Na+ dependent and disappeared when the medium contained 23 mM Na+. The effect of GABA was Ca2+ independent and tetrodotoxin insensitive. Conversely, Gly enhanced the outflow of [3H]GABA from rat spinal cord synaptosomes (EC50, 100.9 microM). This effect was insensitive to both strychnine and 7-chlorokynurenic acid, antagonists at Gly receptors, but it was strongly Na+ dependent. Also, the Gly-evoked [3H]GABA release was Ca2+ independent and tetrodotoxin insensitive. GABA increased the outflow of [3H]Gly (EC50, 11.1 microM) from cerebellar synaptosomes; the effect was not mimicked by either muscimol or (-)-baclofen nor was it prevented by bicuculline or picrotoxin. The GABA effect was, however, blocked by GABA uptake inhibitors and was Na+ dependent. Gly increased [3H]GABA release from cerebellar synaptosomes (EC50, 110.7 microM) in a strychnine- and 7-chlorokynurenic acid-insensitive manner. This effect was Na+ dependent. The effects of GABA on [3H]Gly release seen in spinal cord and cerebellum could be reproduced also with cerebrocortical synaptosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in gamma-aminobutyric acid release induced by topical administration of drugs affecting its metabolism and receptors: studies in freely moving guinea pigs with epidural cups

The effect of local application of drugs affecting gamma-aminobutyric acid metabolism and receptors on cortical aminoacid release has been investigated in freely-moving guinea pigs equipped with epidural cups. Topical treatment with gamma-aminobutyric acid reuptake and/or metabolism inhibitors (alone and in combination) produced a slow and progressive increase in cortical aminoacid release. The inhibition of gamma-aminobutyric acid-transaminase with ethanolamino-O-sulphate seemed to be a suitable procedure for enhancing the gamma-aminobutyric acid efflux without interfering with its autoreceptor-mediated negative feedback, tested with the gamma-aminobutyric acid agonist (+/-)baclofen and antagonist phaclofen. A substantial part of the gamma-aminobutyric acid outflowing from the cortex was of neuronal origin since tetrodotoxin halved the basal efflux in the presence of gamma-aminobutyric acid reuptake and/or metabolism inhibitors. These results, considered together, indicate that the epidural cup technique may be a useful approach to study changes in cortical gamma-aminobutyric acid release induced by drugs acting on gabaergic transmission and directly applied on the surface of the cortex.

Antidepressant and anxiolytic effects of alprazolam versus the conventional. antidepressant desipramine and the anxiolytic diazepam in the forced swim test in rats

The antidepressant and anxiolytic effects of alprazolam were compared to those of desipramine, diazepam and buspirone in the forced swim test. Subchronic alprazolam induced a reduction in immobility similar to that of desipramine in 'non-pretested' and 'pretested' rats. In 'non-pretested' rats, the anti-immobility effect of desipramine was potentiated by diazepam and alprazolam, given before subchronic desipramine, while the anti-immobility effect of subchronic alprazolam was counteracted by diazepam. Diazepam, administered before the pretest session, counteracted, 24 h later, the anti-immobility effect of subchronic desipramine and alprazolam; alprazolam counteracted the anti-immobility effect of alprazolam but not of desipramine, buspirone at the highest doses tested potentiated the anti-immobility effect of subchronic desipramine but not of alprazolam. These data provide further support for the hypothesis that the GABA/benzodiazepine/Cl complex is directly implicated in the action of antidepressants and that systems other than the GABA system are involved in the antidepressant and anxiolytic effects of alprazolam.

Cholinergic modulation of [3H]dopamine release from dendrosomes of rat substantia nigra

Dendrosomes prepared from substantia nigra are able to take up and release [3H]dopamine in a CA(2+)-dependent manner. The Vmax values of [3H]dopamine uptake in substantia nigra dendrosomes was about 5 times lower than that in caudate putamen synaptosomes. The pattern of the K(+)-dependency of the [3H]dopamine release in substantia nigra dendrosomes was significantly different from that found in caudate putamen synaptosomes. The release of [3H]dopamine evoked by 15 mmol/l KCl from superfused dendrosomes was increased in a concentration-dependent manner by acetylcholine. The maximal potentiation produced by acetylcholine was about 40%. The potentiation of [3H]dopamine release by 10 mumol/l acetylcholine was insensitive to mecamylamine but antagonized by atropine and by pirenzepine. The effects of acetylcholine on the release of [3H]acetylcholine from substantia nigra nerve endings was also studied. Exogenous acetylcholine added to the superfusion medium decreased in a concentration-dependent manner the release of acetylcholine. This effect was not antagonized by mecamylamine or pirenzepine but fully antagonized by atropine. The data suggest the existence, in the substantia nigra of the rat, of two distinct muscarinic receptor subtypes regulating respectively dopamine release from dopamine dendrites and acetylcholine release from cholinergic nerve terminals.

Adenosine and glutamate modulate each other's release from rat hippocampal synaptosomes

In rat hippocampal synaptosomes, adenosine decreased the K+ (15 mM) or the kainate (1 mM) evoked release of glutamate and aspartate. An even more pronounced effect was observed in the presence of the stable adenosine analogue, R-phenylisopropyladenosine. All these effects were reversed by the selective adenosine A1 receptor antagonist 8-cyclopentyltheophylline. In the same synaptosomal preparation, K+ (30 mM) strongly stimulated the release of the preloaded [3H]adenosine in a partially Ca(2+)-dependent and tetrodotoxin (TTX)-sensitive manner. Moreover, in the same experimental conditions, both L-glutamate and L-aspartate enhanced the release of [3H]adenosine derivatives ([3H]ADD). The glutamate-evoked release was dose dependent and appeared to be Ca2+ independent and tetrodotoxin insensitive. This effect was not due to metabolism because even the nonmetabolizable isomers D-glutamate and D-aspartate were able to stimulate [3H]ADD release. In contrast, the specific glutamate agonists N-methyl-D-aspartate, kainate, and quisqualate failed to stimulate [3H]ADD release, suggesting that glutamate and aspartate effects were not mediated by known excitatory amino acid receptors. Moreover, NMDA was also ineffective in the absence of Mg2+ and L-glutamate-evoked release was not inhibited by adding the specific antagonists 2-amino-5-phosphonovaleric acid or 6-7-dinitroquinoxaline-2,3-dione. The stimulatory effect did not appear specific for only excitatory amino acids, as gamma-aminobutyric acid stimulated [3H]ADD release in a dose-related manner. These results suggest that, at least in synaptosomal preparations from rat hippocampus, adenosine and glutamate modulate each other's release. The exact mechanism of such interplay, although still unknown, could help in the understanding of excitatory amino acid neurotoxicity.

GABA terminal autoreceptors in the pars compacta and in the pars reticulata of the rat substantia nigra are GABAB

The depolarization-evoked release of gamma-aminobutyric acid (GABA) and its modulation mediated by autoreceptors were studied in superfused synaptosomes prepared from the pars compacta and from the pars reticulata of the rat substantia nigra. The release of [3H]GABA evoked by 9 mM KCl was almost totally calcium-dependent in both nigral subregions. In the presence of SK&F 89976A (N-(4,4-diphenyl-3-butenyl)nipecotic acid), a GABA uptake inhibitor added to minimize carrier-mediated homoexchange, GABA (0.3-10 microM) inhibited, in a concentration-dependent way, the K(+)-evoked overflow of [3H]GABA from both pars compacta and pars reticulata synaptosomes. Similarly to GABA, (-)-baclofen (0.3-10 microM) reduced the [3H]GABA overflow, being roughly equipotent to GABA in both nigral subregions. The (+) enantiomer of baclofen was ineffective. The overflow of [3H]GABA was not consistently affected by muscimol in either the pars compacta or the pars reticulata. The effects of GABA were bicuculline- and picrotoxin-insensitive. However, the inhibition by GABA of the [3H]GABA overflow was antagonized by phaclofen. It is concluded that (a) GABA autoreceptors are sited on GABAergic nerve endings in both the pars compacta and pars reticulata of the rat substantia nigra; (b) these autoreceptors belong to the GABAB type.

A novel type of GABA receptor in rat spinal cord?

The depolarization-evoked release of gamma-aminobutyric acid (GABA) and its possible modulation mediated by autoreceptors were studied in nerve endings isolated from rat spinal cord and prelabeled with the radioactive aminoacid. In the presence of the GABA uptake inhibitor SK&F 89976A [N-(4,4-diphenyl-3-butenyl)-nipecotic acid], used to minimize carrier-mediated homoexchange, exogenous GABA (1-10 mumol/l) decreased in a concentration-dependent way the release of 3H-GABA evoked by 15 mmol/l KCl. The GABAA receptor agonist muscimol (10-100 mumol/l) did not affect the K+ (15 mmol/l)-evoked 3H-GABA release. Similarly ineffective was the GABAB receptor agonist (-)-baclofen (3-100 mumol/l). The effect of GABA was not counteracted by the GABAA receptor antagonists bicuculline,picrotoxin or SR95531 [2-(3'-carbethoxy-2'-propenyl)-3-amino-6-paramethoxy-phenyl-pyr idazinium bromide].(ABSTRACT TRUNCATED AT 250 WORDS)

The dopamine inhibitor GBR 12909: selectivity and molecular mechanism of action

The neurochemical profile of GBR 12909 (1-(2-bis(4-fluorphenyl)-methoxy)-ethyl)-4-(3-phenyl-propyl)pipera zine) was investigated. GBR 12909 was a potent and selective inhibitor of synaptosomal dopamine uptake (KI = 1 nM), with a 20-fold lower affinity for the histamine H1-receptor and a more than 100-fold affinity for the noradrenaline and 5-HT uptake carriers, the dopamine D-1, D-2, 5-HT2, 5-HT1A and alpha 1-receptors and voltage-dependent sodium channels. GBR 12909 (3 microM) was without effect on muscarinic, alpha 2, beta 1 + 2, gamma-aminobutyric acid (GABA) and benzodiazepine receptors, and on choline and GABA uptake carriers. The selective dopamine uptake inhibitory profile of GBR 12909 was confirmed by ex vivo uptake experiments. GBR 12909 inhibited uptake in vitro in a competitive manner as did cocaine and methylphenidate. [3H]GBR 12935 binding was competitively inhibited by GBR 12909 as well as by dopamine, cocaine and methylphenidate. Off-rate analysis of the [3H]GBR 12935 binding excluded the presence of allosteric binding sites on the dopamine carrier complex. Instead, the data favored the notion that GBR 12909 inhibits dopamine uptake by binding to the dopamine binding site on the carrier protein itself, thereby blocking the carrier process. In conclusion, GBR 12909 is a highly selective inhibitor of dopamine uptake, both in vivo and in vitro. At the moment GBR 12909 is the only compound with this neurochemical profile. The selective effect of GBR 12909 on this neuronal system makes it an interesting experimental tool and a potential antidepressant agent.

Presynaptic mechanisms underlying the gamma-aminobutyric acid-evoked receptor-independent release of [3H]norepinephrine in rat hippocampus

The effects of gamma-aminobutyric acid (GABA) on the spontaneous efflux of [3H]norepinephrine ([3H]NE) were studied in synaptosomes prepared from rat hippocampus and prelabelled with [3H]NE. It had been observed previously that, when synaptosomes were exposed in superfusion to GABA, the basal release of the tritiated catecholamine was enhanced, apparently with no involvement of the known GABA receptors. The mechanisms underlying this effect have now been investigated. The potency of GABA as a releaser of [3H]NE was decreased by lowering the Na+ content of the superfusion medium, and its effect disappeared at 23 mM Na+. The GABA-induced [3H]NE release was counteracted by the GABA uptake inhibitor N-(4,4-diphenyl-3-butenyl)nipecotic acid (SKF 89976A), but it was unaffected by the NE uptake blockers desmethylimipramine and nisoxetine. The GABA-induced release of [3H]NE was Ca2+-dependent and tetrodotoxin-sensitive. The data support the hypothesis that GABA provoked [3H]NE release by a novel mechanism which involves penetration into the noradrenergic nerve terminals through a GABA carrier located on the NE terminals themselves. This uptake process might be electrogenic and provoke depolarization of the nerve terminals, causing an exocytotic release of [3H]NE.

GABAB autoreceptors in rat cortex synaptosomes: response under different depolarizing and ionic conditions

Rat cerebral cortex synaptosomes prelabeled with [3H]gamma-aminobutyric acid [( 3H]GABA) were exposed in superfusion to various concentrations of KCl (9-50 mM). The evoked release of [3H]GABA reached a plateau at about 35 mM KCl. The K+-induced release was Ca2+-dependent, particularly at the lowest K+ concentrations. The GABAB agonist (-)-baclofen concentration dependently inhibited the release of [3H]GABA evoked by K+; this effect decreased with increasing K+ concentration and disappeared at 35 mM KCl. The GABAA agonist muscimol (1-100 microM) was totally ineffective to inhibit the release of [3H]GABA. Veratrine (1-30 microM) induced the release of [3H]GABA and the effect was tetrodotoxin-sensitive. (-)-Baclofen, but not muscimol, decreased the veratrine-induced [3H]GABA release; the GABAB agonist was particularly effective in presence of low concentrations of veratrine (1-3 microM) but the effect disappeared when 30 microM of the alkaloid was used. The inhibitory effect of (-)-baclofen on the release of [3H]GABA evoked by 15 mM KCl was dependent on the concentration of Ca2+: the effect increased as the concentration of Ca2+ was raised, reaching a plateau at 0.6 mM Ca2+. Exogenous GABA, in presence of the GABA uptake blocker SK & F 89976A, inhibited the release of [3H]GABA evoked by K+; this effect was antagonized by phaclofen. The data support the idea that terminal GABA autoreceptors in the rat cerebral cortex are of the GABAB type.

GABA-mediated modification of despair behavior in mice

Gamma-aminobutyric acid (GABA) and GABA agonists exhibited biphasic response on forced swimming-induced despair behavior in mice; smaller doses, GABA (100 mg/kg), muscimol (0.05 and 0.1 mg/kg), baclofen (0.5 and 1 mg/kg), sodium valproate (100 mg/kg), piracetam (25 mg/kg), and fengabine (10 and 20 mg/kg) decreasing forced swimming-induced immobility period, while higher doses enhancing the immobility period. GABA, muscimol, and baclofen also reversed reserpine-induced prolongation of the forced swimming-induced immobility. When GABA was administered with antidepressant agents, it potentiated the effect of some classical antidepressants. Bicuculline and picrotoxin, the GABAergic antagonists, by themselves enhanced the forced swimming-induced immobility period, but they had no significant effect on reserpine-induced prolongation of the immobility period. When animals were chronically exposed to forced swimming, there was a gradual increase of the immobility period which was reduced on treatment with GABAergic agents. It is suggested that GABA has a modulatory role in reversing forced swimming-induced despair and also in potentiating the effect of antidepressants.

Release-regulating autoreceptors of the GABAB-type in human cerebral cortex

1. The depolarization-evoked release of gamma-aminobutyric acid (GABA) and its modulation mediated by autoreceptors were investigated in superfused synaptosomes prepared from fresh human cerebral cortex. 2. The release of [3H]-GABA provoked by 15 mM K+ from human cortex nerve endings was almost totally (85%) calcium-dependent. 3. In the presence of the GABA uptake inhibitor SK&F 89976A (N-(4,4-diphenyl-3-butenyl)-nipecotic acid), added to prevent carrier-mediated homoexchange, GABA (1-10 microM) decreased in a concentration-dependent manner the K+-evoked release of [3H]-GABA. The effect of GABA was mimicked by the GABAB receptor agonist (-)-baclofen (1-100 microM) but not by the GABAA receptor agonist muscimol (1-100 microM). Moreover, the GABA-induced inhibition of [3H]-GABA release was not affected by two GABAA receptor antagonists, bicuculline or SR 95531 (2-(3'-carbethoxy-2'-propenyl)-3-amino-6-paramethoxy-phenyl-pyr idazinium bromide). 4. (-)-Baclofen also inhibited the depolarization-evoked release of endogenous GABA from human cortical synaptosomes. 5. It is concluded that GABA autoreceptors regulating the release of both newly taken up and endogenous GABA are present in human brain and appear to belong to the GABAB subtype.

Studies on [3H]GABA and endogenous GABA release in rat cerebral cortex suggest the presence of autoreceptors of the GABAB type

The presence of autoreceptors for gamma-aminobutyric acid (GABA) in the CNS was reinvestigated using rat cortex synaptosomes prelabeled with [3H]GABA and exposed to GABA by superfusion in the presence of a new GABA uptake inhibitor, N-(4,4-diphenyl-3-butenyl)-nipecotic acid (SK&F 89976A). This compound itself did not increase the basal or the depolarization-evoked release of [3H]GABA. GABA reduced in a concentration-dependent way the release of [3H]GABA evoked by 15 mM K+. The effect was not antagonized by bicuculline, picrotoxin or by the new GABAA antagonist SR 95531. The GABAA agonist muscimol did not affect [3H]GABA release. This was reduced by (-)baclofen (but not by the (+) isomer) and the concentration-inhibition curve of (-)baclofen was superimposable on to that of GABA. Also the K+-evoked release of endogenous GABA was stereoselectively and concentration dependently inhibited by the (-) enantiomer of baclofen. It is concluded that the release of GABA from rat cortical nerve endings may be inhibited through the activation of autoreceptors which appear to belong to the GABAB type.