Excitatory Amino Acid Systems Research Articles

Genome-wide association studies in suicidology — analysis of main results

Studies of suicide genetics can be divided into 3 periods: 1) period of classical methods of behavioral genetics; 2) candidate genes association studies, and 3) genome-wide association studies. The majority of studies using candidate genes strategy has been dedicated to main neurotransmitter systems (serotonine, catecholamines, GABA, excitatory aminoacids system, etc.) and in the last decade — to genes of the stress-reactivity system and neurotrophins. On the other hand among main GWAS findings are genes that are involved in neurodevelopment, neuroplasticity, cell adhesion and migration, proliferation and intracellular signaling, as well as immune system functions. We consider it a confirmation of the relevance of the stressvulnerability models that imply key role of the early development and involvement of the neuroplasticity, as well as models that focus on stress as a systemic reaction of the organism in pathogenesis of suicidal behavior. It should be noted that findings of different GWAS studies most often do not coincide and in general produce rather heterogeneous picture. It may be due to differing bioinformatics approaches, phenotypes description differences and study design peculiarities. We consider that more research is needed in all three directions with involvement of NGS approaches, in particular whole-exome sequencing.

The effect of nutrition on the neural mechanisms potentially involved in melatonin-stimulated LH secretion in female Mediterranean goats

This research examines which neural mechanisms among the endogenous opioid, dopaminergic, serotonergic and excitatory amino acid systems are involved in the stimulation of LH secretion by melatonin implantation and their modulation by nutritional level. Female goats were distributed to two experimental groups that received either 1.1 (group H; n=24) or 0.7 (group L; n=24) times their nutritional maintenance requirements. Half of each group was implanted with melatonin after a long-day period. Plasma LH concentrations were measured twice per week. The effects of i.v. injections of naloxone, pimozide, cyproheptadine and N-methyl-d,l-aspartate (NMDA) on LH secretion were assessed the day before melatonin implantation and again on days 30 and 45. The functioning of all but the dopaminergic systems was clearly modified by the level of nutrition, melatonin implantation and time elapsed since implantation. Thirty days after implantation, naloxone increased LH concentrations irrespective of the level of nutrition (P<0.05), similar to NMDA in the melatonin-implanted H goats (HM; P<0.01). On day 45, naloxone increased LH concentrations in the HM animals (P<0.05), similar to cyproheptadine in both the non-implanted H (HC) and the HM animals (P<0.01). Finally, at 45 days, NMDA increased the LH concentration in all subgroups (P<0.01). These results provide evidence that the effects of different neural systems on LH secretion are modified by nutritional level and melatonin implantation. Endogenous opioids seem to be most strongly involved in the inhibition of LH secretion on days 30 and 45 after melatonin implantation. However, the serotonergic mechanism appears to be most influenced by nutritional level.

Effects of 90-day oral dimethoate exposure on glutamatergic system and neurobehavioral performance in rats

To investigate the spatial learning and exploration along with the CNS excitatory amino acid neurotransmitters profiles in adult rats subchronically exposed to the anticholinesterase organophosphorus insecticide dimethoate. Rats were gavaged daily with dimethoate (0, 5, 10 or 20 mg/kg via oral) in NS. for 90 days. Morris water maze tasks were used to test the spatial learning and memory in the rats after the dimethoate exposure. Simultaneously, rats were decapitated for the determination of brain cholinesterase AChE activities, glutamate concentrations, and the NMDA receptor NMDA-R densities and affinities in hippocampus. Latencies to find a hidden escape platform were significantly longer in dimethoate dosed groups than that of the control group in the place navigation tests. Subsequently, the times of crossing the location of platform which had been removed obviously decreased in the highest dose group compared with that of the control in the spatial probe tests (P < 0.05). AChE activity was significantly reduced 42% approximately 78% by all three doses of dimethoate (P < 0.05). Glutamate concentrations were increased significantly 132.9% approximately 134.5% by the two highest doses of dimethoate (P < 0.05). In addition, the NMDA receptor bindings were reduced 21.2% approximately 23.2% with the statistical significance at the same two highest doses (P < 0.05). Furthermore, the receptor affinities was reduced 33.1% by the highest dose group (P < 0.05). The lesions of spatial memory were statistically corrected with the decrease of the NMDA-R affinities (P < 0.05). The cholinergic lesion as well as the excitatory amino acid system alteration might attribute to the inferior ability in spatial learning and memory in dimethoate subchronically exposed rats.

Innovative Approaches for the Development of Antidepressant Drugs: Current and Future Strategies

Depression is a highly debilitating disorder that has been estimated to affect up to 21% of the world population. Despite the advances in the treatment of depression with selective serotonin reuptake inhibitors (SSRIs) and serotonin and norepinephrine reuptake inhibitors (SNRIs), there continue to be many unmet clinical needs with respect to both efficacy and side effects. These needs range from efficacy in treatment resistant patients, to improved onset, to reductions in side effects such as emesis or sexual dysfunction. To address these needs, there are numerous combination therapies and novel targets that have been identified that may demonstrate improvements in one or more areas. There is tremendous diversity in the types of targets and approaches being taken. At one end of a spectrum is combination therapies that maintain the benefits associated with SSRIs but attempt to either improve efficacy or reduce side effects by adding additional mechanisms (5-HT1A, 5-HT1B, 5-HT1D, 5-HT2C, alpha-2A). At the other end of the spectrum are more novel targets, such as neurotrophins (BDNF, IGF), based on recent findings that antidepressants induce neurogenesis. In between, there are many approaches that range from directly targeting serotonin receptors (5-HT2C, 5-HT6) to targeting the multiplicity of potential mechanisms associated with excitatory (glutamate, NMDA, mGluR2, mGluR5) or inhibitory amino acid systems (GABA) or peptidergic systems (neurokinin 1, corticotropin-releasing factor 1, melanin-concentrating hormone 1, V1b). The present review addresses the most exciting approaches and reviews the localization, neurochemical and behavioral data that provide the supporting rationale for each of these targets or target combinations.

Leptin Stimulates LH Secretion in Peripubertal Male Rats Through NMDA Receptors

Leptin, a peptide hormone secreted by adipocytes that has been proposed as a metabolic signal in the reproductive system, appears to be linked to the different neuroendocrine processes involved in the onset of puberty. We studied the ontogenic effect of administration of leptin (30 mg/kg i.p) on serum LH levels during different stages of sexual development (7, 30, and 45 days of age) in male rats and on the hypothalamic content of glutamate (GLU) and γ-aminobutyric acid (GABA) in 30-day-old rats.Leptin induced a significant increase (p < 0.01) in LH levels in 30 days old rats. This hormone stimulatory effect was accompanied by a significant enhancement (p < 0.01) of the hypothalamic content of glutamate, the hypothalamic excitatory aminoacid involved in N-methyl-D-aspartate (NMDA) neurotransmission. No changes in the LH plasma levels were observed in 7- and 45-day‐old male rats treated with leptin.MK 801 (0.1 and 0.3 mg/kg i.p.), an antagonist of NMDA receptors of excitatory amino acid system (EAAs), antagonized the stimulatory effect of leptin on LH secretion and on the hypothalamic content of GLU. These results demonstrate that leptin stimulates the reproductive axis in male rats during a determined period of sexual maturation and that NMDA receptors are involved in the facilitatory action of leptin on the gonadal axis of male rats during sexual maturation.

Pathophysiology of migraine

The exact pathogenesis of migraine remains to be determined. In particular there is increasing evidence for the neural basis of migraine. We now have a body of data supporting the concept of central neuronal hyperexcitability as a pivotal physiological disturbance predisposing to migraine. The reasons for increased neuronal excitability may be multifactorial. Most recently, abnormality of calcium channels has been introduced as a potential mechanism of interictal neuronal excitability. Mutant voltage gated P/Q type calcium channel genes likely influence presynaptic neurotransmitter release, possibly of excitatory amino-acid systems or inhibitory. It could therefore be hypothesised that genetic abnormalities result in a lowered threshold of response to trigger factors. There is also evidence from spectroscopic studies that magnesium is low in migraine. We currently conceive of a migraine attack as originating in the brain. Triggers of an attack initiate a depolarising neuroelectric and metabolic event likened to the spreading depression of Leao. This event activates the headache and associated features of the attack by mechanisms that remain to be determined, but appear to involve either peripheral trigeminovascular or brain stem pathways, or both. Excitability of cell membranes, perhaps in part genetically determined, is the brain's route of susceptibility to attacks. Factors that increase or decrease neuronal excitability constitute the threshold for triggering attacks.

Seizure susceptibility to various convulsant stimuli of knockout interleukin-6 mice

In the present study, the susceptibility of knockout interleukin-6 (IL-6 −/−) mice to various convulsant stimuli has been evaluated and compared with other three related mice strains. Animals were treated with chemical convulsants impairing the γ-aminobutyric acid neurotransmission [pentylenetetrazole (PTZ), picrotoxin, bicuculline, methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM), methyl-β-carboline-3-carboxylate (β-CCM)], enhancing glutamatergic neurotransmission [ N-methyl- d-aspartate (NMDA), α-amino-3hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and kainic acid (KA)] or a K +channel blocker [4-aminopyridine (4-AP)]. The behavioural changes of such convulsant stimuli on IL-6 −/− were observed and compared with those observed in C57, IL-6 +/+ and DBA/2 mice. The occurrence of clonic and/or tonic seizures was scored and statistically analysed to observe possible differences on seizure susceptibility. The IL-6 −/− mice exhibited significantly higher seizure susceptibility to PTZ, β-CCM, DMCM, NMDA, AMPA and KA than did the other mice strains, with the exception of DBA/2 mice. This study demonstrates that IL-6 −/− mice possess an increased susceptibility to some convulsant stimuli. In particular, the major convulsant effects produced by NMDA, AMPA and KA suggest that the excitatory amino acid system is more active in the central nervous system (CNS) of IL-6 −/− mice. The present data suggest that IL-6 −/− mice might be a valid novel epileptic model for the study of pathophysiology and pharmacology of epileptic seizures.

Neuropathologic quiz

Epilepsy is a common neurological disease but the mechanism of seizure generation has been only partially unraveled. Furthermore, almost 30% of epileptic patients are resistant to pharmacological treatment. Therefore, elucidation of the basic mechanism of seizures and search for new antiepileptics in order to treat the drug-resistant form of epilepsy and to improve the efficacy of current therapies seem justified. The aim of this overview is a brief presentation of some new concepts and research directions in pathogenesis and pharmacotherapy of epilepsy. Development of ideas on the mechanisms of seizures and antiepileptic drugs reflects the progress in our understanding of the central nervous system physiology, particularly of neurotransmission. Hyperactivity of excitatory amino acid systems, insufficient GABAA receptor-mediated neurotransmission, and disturbances in intrinsic properties of neuronal membranes are still regarded as the most important mechanisms of seizures. New data add to the complexity of GABA-glutamate interaction showing both excitatory and inhibitory role of GABA and glutamatergic neurons in the central nervous system. Moreover, besides synaptic NMDA and GABAA receptors, also extrasynaptic receptors for the amino acid transmitters have been recently implicated in the pathomechanism of epilepsy. Changes in expression, polymorphisms, lost- or gain-function mutations as well as cellular energetic imbalance can contribute to the disturbed function of the ligand- and voltage-dependent sodium, potassium, chloride and calcium channels, resulting in epileptiform activity. Voltage-dependent sodium and calcium channel blockers, and GABA mimetics are the most clinically useful groups of antiepileptic drugs and the newest research in this field is focused on more selective and subtle regulations of their molecular targets. Of interest is an emerging role of extrasynaptic GABAA receptors, various kinds of potassium ion channels, hyperpolarization-activated cyclic nucleotide gated (HCN) channels, acid-sensing ion channels, and gap junctions in the regulation of neuronal excitability and seizures. Iono- and metabotropic glutamate receptors used to be viewed as an attractive target for new anticonvulsants, however, opinions are now less enthusiastic, since their competitive and non-competitive antagonists possess undesired side effects. Positive or negative allosteric modulators of glutamate receptors with fewer side-effects can be more promising. The introduction of new compounds acting through novel pharmacological mechanisms gives hope that the proportion of patients with uncontrolled epilepsy will substantially decrease. However, this may be possible if molecular background of the pharmacoresistance in epilepsy is deciphered.

Excitotoxicity and the putative involvement of excitatory amino acids in neurodegenerative diseases

Excitatory amino acids (EAA) represent major brain neurotransmitters. They are present in numerous neuronal systems and thus are involved in almost all aspects of normal and pathological brain activity. Changes in EAA transmission have been associated with the functional impairments characterizing major neurological disorders, including epilepsy and schizophrenia. There is also a suspicion that EAA systems underlie the neuronal death associated not only with acute CNS insults, such as in ischemia or post-traumatic lesions, but also with neurodegenerative diseases such as ALS, Huntington's disease and Parkinson's disease. The neurotoxicity of EAA, referred to as excitotoxicity, is presumably mediated primarily through an excess of EAA synaptic receptor stimulation. Indeed, overstimulation of the ionotropic NMDA or AMPA/kainate receptor subtypes has been shown to produce an intense membrane depolarisation and further a massive increase in intracellular calcium leading to cell damage. The extreme diversity and specific pattern of expression of EAA receptor subunits could account for the differential vulnerability of certain brain areas to such excitotoxic processes. In addition, it is now believed that besides abnormalities in receptor functioning or in release processes, alterations in EAA transmission may result from dysfunction of the EAA uptake system, which represents the mechanism for EAA removal from the synapse. From the five transporter proteins cloned, termed EAAT1-5, the primarily glial transporters EAAT1 and EAAT2 have been shown to mediate the bulk of EAA uptake in the brain and it has then been suggested that they play a major role in the prevention of excitotoxic processes. In this respect, the degeneration of motor neurons in ALS has been associated with altered expression or inactivation of EAAT2. Moreover, recent evidence has been provided that pharmacological alteration of glutamate transport can also induce astrocyte degeneration, as observed in neurodegenerative insults, but through a mechanism independent of stimulation of EAA receptors. Thus, one can obviously consider that these EAATs can represent a key target for further development of new neuroprotective agents.

Involvement of N-methyl-D-aspartate receptors in nociception in the cyclophosphamide-induced vesical pain model in the conscious rat

We previously showed that the intraperitoneal (i.p.) administration of 200mg/kg cyclophosphamide, an antitumoral agent, modified the behaviour of rats with cystitis induced by acrolein, a toxic urinary by-product of cyclophosphamide. This behaviour, (namely decreased breathing rate, closing of the eyes, and specific postures), was scored to indirectly assess the nociception elicited by the cystitis and to use this experimental model as a vesical pain model. Here we investigated the involvement of the N-methyl-D-aspartate (NMDA) receptors and thus of the excitatory amino acid system in this model.We administered dizocilpine (0.01 to 0.1mg/kg intravenously (i.v.) and 1 to 20μg/rat intrathecally (i.t.)) and ketamine (5 and 10mg/kg i.v. and 50 to 1000μg/rat i.t.), two non-competitive NMDA receptor antagonists that bind to the channel site, and AP-5 (0.01 to 1mg/kg i.v. and 20 to 500μg/rat i.t), a competitive antagonist that binds to the glutamate site. Whichever the route of administration (i.v. or i.t.), dizocilpine dose-relatedly reduced the behavioural disorders induced by cyclophosphamide. Systemic ketamine also dose-dependently, though transiently, reduced the effects of cyclophosphamide, but ketamine i.t. and AP-5 i.v. and i.t. did not induce any reduction of these effects.These results (i) demonstrate that in the cyclophosphamide-induced vesical pain model NMDA receptors are involved in the nociception, as shown by the effects of dizocilpine and systemic ketamine, (ii) reveal marked differences in the data obtained with various NMDA receptor antagonists, possibly due to their physicochemical properties, to the animal pain model used, to the noxious stimulus applied or to any combination of these factors. Copyright 2002 European Federation of Chapters of the International Association for the Study of Pain. Published by Elsevier Science Ltd. All rights reserved.

Changes in the control of gonadotrophin secretion by neurotransmitters during sexual development in rats.

Gonadotrophin hormone releasing hormone (GnRH) is the primary messenger involved in sexual maturation and the onset of puberty. The activity of these neurons are controlled by several neurotransmitters systems. The onset of puberty implies changes from a prepubertal type of gonadotrophin secretion, characterized by a low activity of GnRH neurons, to an adult pattern of secretion with phasic and synchronous activation of GnRH neurons resulting in an increase in the amplitute and frequency of GnRH pulses. Neurotransmitter systems are involved in these changes of GnRH secretion during the onset of puberty by quantitative and qualitative modifications in the effect on GnRH secretion. Serotonin (5-HT), GABA and catecholamines (CA) have qualitative differences in the effects on GnRH and LH secretion in early prepubertal than in late prepubertal and adult female rats. The administration of 5-hydroxytryptophan a precursor of serotonin (5-HT) which increases 5-HT hypothalamic levels induces GnRH and LH release in early prepubertal female rats, these effects dissapear in late prepubertal stage having an inhibitory action in adult female rats. GABAergic system also stimulates GnRH and LH secretion in early prepubertal female rats and has an inhibitory action on this axis in late prepubertal period and in adult female rats. On the contrary the inhibition of catecholamines synthesis by alpha-methyl-p-tyrosine induced an increase of LH secretion in early prepubertal female rats and inhibitory effect in late prepubertal and adult stage. These effects indicate tha CA has an inhibitory effects on GnRH-LH secretion in early prepubertal female rats changing to an stimulatory action in the late puberty and adult rats. These qualitative modifications were observed only in female rats and are probably connected with the hypothalamic differentiation into a female type of gonadotrophin control. Opiadergic and excitatory amino acid systems have quantitative differences on GnRH-LH secretion during prepubertal and peripubertal and adult stages. Opiates has an high inhibitory tone in early prepubertal rats that is decreasing during sexual maturation to reach puberty. On the contrary EAA increases its stimulatory activity on GnRH-LH secretion during sexual maturation by increasing the hypothalamic release of aspartate and glutamate, the excitatory amino acids involved in GnRH release, and the sensibility of NMDA receptors to these amino acids. In conclusion sexual maturation and the onset of puberty in the female rats involve qualitative and quantitative modifications in the effects of neurotrasmitters system on GnRH secretion.

Involvement of glial glutamate transporters in morphine dependence and naloxone-precipitated withdrawal

A body of evidence supports that excitatory amino acid systems, particularly glutamatergic one, participate in morphine dependence and naloxone-precipitated withdrawal. In this study, we examined the involvement of glial glutamate transporters, GLT-1 and GLAST, in them. Rats were rendered morphine-dependent by subcutaneous implantation of two 75 mg morphine pellets for 5 days. Intracerebroventricular administration of DL-threo-beta-benzyloxyaspartate, a glutamate transporter inhibitor significantly facilitated various naloxone-precipitated withdrawal signs. By northern blot analysis, the expression of GLT-1 mRNA was found to decrease significantly in the striatum and thalamus of morphine-dependent rats, and to increase significantly in the striatum 2 hr after the naloxone-precipitated withdrawal. On the other hand, there were no significant changes in GLAST mRNA levels in any brain regions. In vivo microdialysis experiments revealed that the extracellular glutamate levels was elevated in the striatum and nucleus accumbens, in which the changes of GLT-1 mRNA level were observed, during naloxone-precipitated morphine withdrawal. In cultured astrocytes, the expression of GLT-1 mRNA was regulated by agents activating the cAMP pathway, as well as beta-adrenergic agonist and dopamine, but not morphine. These results suggest that the changes of GLT-1 expression, which alter the glutamate uptake and affect the glutamatergic transmission efficiency, play a role in the development of morphine dependence and the expression of morphine withdrawal.

New developments in the pharmacotherapy of alcohol dependence.

Neuroscientific underpinnings and pharmacotherapeutic treatments of substance use disorders are rapidly developing areas of study. In particular, there have been exciting new developments in our understanding of the involvement of excitatory amino acid neurotransmitter systems and the opiate and serotonin systems in the pathophysiology of alcohol withdrawal, alcohol dependence, and in subtypes of individuals with alcoholism. In this article, new developments in the pharmacotherapy of alcohol dependence will be reviewed. In particular, the use of anticonvulsants in alcohol withdrawal and protracted abstinence syndromes will be discussed. New data on opiate antagonists and acamprosate, an agent that exerts actions through excitatory amino acid systems in relapse prevention, will be reviewed. Finally, there will be a review of new data concerning the use of serotonin reuptake inhibitors in subtypes of alcoholism and the use of combination pharmacotherapy.

Changes in the glutamate release and uptake of cerebellar cells in perinatally nicotine-exposed rat pups.

Cerebellar granule and glial cells were cultured from 7 day-old rat pups after pre- and post-natal nicotine treatment. Ten days later, the basal release of glutamate in the granule cells prepared from the pre- and post-natally nicotine-exposed pups was higher and lower than the controls, respectively. The N-methyl-D-aspartate-induced release of glutamate was higher in the granule cells of post-natal nicotine exposed rats. However, the nicotine-induced glutamate release was either unchanged or was lower in the granule cells of all nicotine-treated pups. The basal glutamate uptake was higher in the glial cells from those exposed pre-natally and lower in the continuously nicotine-exposed pups. The sensitivities of L-trans-pyrrolidine-2,4-dicarboxylic acid on glutamate uptake were higher in all nicotine treated groups. There was a higher number of specific [3H]dizocilpine binding sites in the pre- or continuously nicotine-exposed group. These results suggest that the cerebellar cell properties are altered after perinatal nicotine exposure and that the development of an excitatory amino acid system might be affected differently depending on the nicotine exposure time.

Dynorphin A elicits an increase in intracellular calcium in cultured neurons via a non-opioid, non-NMDA mechanism.

The opioid peptide dynorphin A is known to elicit a number of pathological effects that may result from neuronal excitotoxicity. An up-regulation of this peptide has also been causally related to the dysesthesia associated with inflammation and nerve injury. These effects of dynorphin A are not mediated through opioid receptor activation but can be effectively blocked by pretreatment with N-methyl-D-aspartate (NMDA) receptor antagonists, thus implicating the excitatory amino acid system as a mediator of the actions of dynorphin A and/or its fragments. A direct interaction between dynorphin A and the NMDA receptors has been well established; however the physiological relevance of this interaction remains equivocal. This study examined whether dynorphin A elicits a neuronal excitatory effect that may underlie its activation of the NMDA receptors. Calcium imaging of individual cultured cortical neurons showed that the nonopioid peptide dynorphin A(2-17) induced a time- and dose-dependent increase in intracellular calcium. This excitatory effect of dynorphin A(2-17) was insensitive to (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine (MK-801) pretreatment in NMDA-responsive cells. Thus dynorphin A stimulates neuronal cells via a nonopioid, non-NMDA mechanism. This excitatory action of dynorphin A could modulate NMDA receptor activity in vivo by enhancing excitatory neurotransmitter release or by potentiating NMDA receptor function in a calcium-dependent manner. Further characterization of this novel site of action of dynorphin A may provide new insight into the underlying mechanisms of dynorphin excitotoxicity and its pathological role in neuropathy.

Regulation of growth hormone (GH) secretion by different glutamate receptor subtypes in the rat.

It has been firmly established that excitatory amino acids (EAAs), such as glutamate, are pivotal elements in the hypothalamic circuitry involved in the control of pituitary function. The actions of EAAs are mediated by different postsynaptic receptor subtypes, which include N-methyl D-aspartate (NMDA), kainate (KA), 2-amino-3-hydroxy-5 methyl-4-isoxazol propionic acid (AMPA) and metabotropic receptors. In this review, we summarize our experimental work on the role of EAA neurotransmission in the control of GH secretion in the rat. Detailed characterization of the effects of agonists and antagonists of glutamate receptors on GH release revealed that activation of NMDA, KA and AMPA receptors at different age-points resulted in clear-cut stimulation of GH secretion, although age- and sex-dependent differences were detected in the pattern of response to the different agonists. This stimulatory action was proven nitric oxide (NO)-dependent and not exerted at the pituitary level. In addition, evaluation of the role of hypothalamic GH-releasing hormone (GHRH) in the stimulatory action of NMDA by means of immunoneutralization of endogenous GHRH or destruction of GHRH producing neurons suggested the involvement of signals other than GHRH in this response. Further, evidence was obtained on the modulation of the EAA system by gonadal factors, and on the physiological relevance of EAA pathways in the regulation of pulsatile GH release. In conclusion, our data using the rat as animal model provide evidence for a pivotal role of glutamate pathways in the regulation of GH secretion throughout the life-span.

Interrelationships of GABAergic, Serotoninergic and Excitatory Amino Acid Systems in its Regulatory Effect on Prolactin Secretion in Prepubertal Rats

GABAergic, serotoninergic and excitatory amino acid systems (EAAs) regulate the prolactin (PROL) secretion in prepubertal female rats. The aim of the present paper was to determine the interrelationships of these systems on the control of this pituitary hormone. It was carried out through the following scheme: 1. The participation of the EAAs and serotonin in the effect of GABAergic system on PROL release, determined by evaluating the GABA A and GABA B receptor agonists. It was carried out on animals that were previously treated with AAEs receptor antagonist or p-chlorophenylamphetamine (PCA), this one depleting serotonin in the hypothalamus. 2. The participation of GABAergic system in the effect of serotonin and EAAs systems, determined by the evaluation of the effects of EAAs receptor agonists and of 5-HTP, a serotonin precursor. With this purpose the rats were previously treated with GABA A and GABA B receptor antagonists. 3. The interrelationships between the EAAs and the serotoninergic systems in the control of PROL secretion, determined (a) by using EAAs agonists (in rats depleted of serotonin by PCA) and (b) using EAAs antagonists (in rats treated with 5-HTP, a serotonin precursor). The administration of GABAergic agonists significantly increased PROL secretion in prepubertal female rats. Neither EAAs antagonists nor the depletion of serotonin in the brain, modified the stimulatory effects of the GABAergic system on PROL levels. This is a clear indication that the activity of the GABAergic system is independent of the serotoninergic and of the EAAs system effects on the pituitary hormone. The EAAs neurotransmitter system agonists significantly increase PROL levels. This effect was blocked by the GABAergic system antagonists but was not modified by serotonin depletion. Taking into account these facts it may be considered that the GABAergic system is involved in the stimulatory effect of EAAs on PROL secretion, this effect being independent of the serotoninergic system. 5-HTP significantly increased PROL plasma levels, and this effect was modified neither by the GABAergic nor by the EAAs receptor antagonists. These results indicate that the stimulatory effect of serotonin on PROL release is independent of the GABAergic and EAAs systems. In conclusion it may be considered that in prepubertal female rats, the GABAergic and serotoninergic systems stimulate PROL secretion by independent mechanisms that do not include EAAs. On the other hand, the effects of EAAs neurotransmission are exerted via the GABAergic system.

Age-related reductions in the activities of antioxidant enzymes in the rat inferior colliculus

The inferior colliculus (IC) is a major relay and processing center of auditory signals in the midbrain and receives inputs from most other auditory nuclei. A number of studies have indicated age-related declines in the GABAergic and excitatory amino acid systems in the IC, including losses in both GABA immunoreactive (+) and GABA immunonegative (−) synapses. The goal of this project was to identify potential biochemical and morphological changes in the IC that may contribute to deficits in the functions of these neurotransmitters, using three age groups of Fischer-344 rats. Homogenates obtained from the IC showed age-dependent reductions in activities of the antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT), with a concomitant increase in lipid peroxidation. Dephosphorylation of IC homogenates with alkaline phosphatase reduced the activities of SOD and CAT in all age groups, which could be restored by protein kinase C (PKC)-dependent phosphorylation. Restoration of enzyme activity was specific to the PKC-α isozyme, but not to the β1, β2, δ or γ forms. No age-dependent change in the levels of PKC isoforms (α, β1, β2 and γ) was detectable in IC homogenates. Morphological analyses indicate decreases in mitochondrial density in the somata of both GABA+ and GABA− IC neurons in 19- and 28-month-old rats when compared to 3-month-olds, along with significantly higher matricial abnormalities. These data indicate age-related increases in oxidative stress in the IC, which could be partially restored by PKC. The progressive increase in oxidative stress with age may underlie changes in neuronal morphology and function of the IC.

The "orphan" Na+/Cl(-)-dependent transporter, Rxt1, is primarily localized within nerve endings of cortical origin in the rat striatum.

Previous studies have shown that the striatum expresses very low levels of Na+/Cl(-)-dependent "orphan" transporter Rxt1 transcripts but contains high levels of protein. This study investigated the origin of Rxt1 expression in rat striatum. Striatal Rxt1 contents assessed by immunocytochemistry or western blotting were found to be significantly reduced after corticostriatal denervation but not after striatal or thalamic lesion with kainic acid or selective 6-hydroxydopamine-induced nigrostriatal deafferentation. Corticostriatal neurons retrogradely labeled by intrastriatal fluorogold injections were shown to express Rxt1 mRNA. Combination of anterograde biotin-dextran amine labeling of the corticostriatal pathway with Rxt1 immunogold detection at the ultrastructural level demonstrated the presence of Rxt1 in about one-third of the corticostriatal synaptic terminals and in numerous unidentified synaptic terminals. All the Rxt1-positive terminals formed asymmetrical contacts on spines. These data provide evidence that striatal Rxt1 immunoreactivity is mainly of extrinsic origin and more specifically associated with the corticostriatal pathway. Rxt1 appears as a selective presynaptic marker of synapses formed by presumably excitatory amino acid afferents, but it segregates a subclass of these synapses, thereby revealing a functional heterogeneity among excitatory amino acid systems.

Enhanced susceptibility of pentylenetetrazole kindled mice to quinolone effects

The present study was designed to examine the ability of different quinolones to affect the seizure severity and the latency of development of chemical kindling produced by repeated treatment using a subconvulsant dose of pentylenetetrazole (PTZ). A group of mice (kindled control) were treated subcutaneously (s.c.) with vehicle+PTZ (30 mg/kg, three times a week) for 6 consecutive weeks and the changes in excitability associated with the kindling state were observed over the following 2 h. A second group of mice were injected intraperitoneally (i.p.) with the following quinolone derivatives, ciprofloxacin (ciprox), pefloxacin (peflox), ofloxacin (oflox), cinoxacin (cinox), nalidixic acid (nalidixic), 1-cyclopropyl-6-amino-7-tetrahydroisoquinoline-8-methyl-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (M5) and 1-cyclopropyl-7-tetrahydro-isoquinoline-8-methyl-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (MH5) at a dose of 20 mg/kg 15 min before receiving a subconvulsant dose of PTZ (30 mg/kg, s.c.). The results showed that pretreatment with some of the quinolones tested facilitated the development of kindling to PTZ-induced seizures. In particular, ciprox, peflox, oflox, M5 and MH5 derivatives variously increased the development of kindling to PTZ induced seizures, whilst cinox and nalidix did not significantly affect it. Additionally we determined whether the enhanced susceptibility of kindled mice only occurred after relatively short intervals following the last seizure or whether it was a more permanent phenomenon. For the study of the persistence of kindling, the animals were rechallenged with the kindling stimulus (PTZ 25 mg/kg, s.c.) 15 and 30 days after the last injection of the chronic treatment with PTZ (30 mg/kg, s.c.) and the behavioural changes in the kindled mice were compared with the control ones (chronically treated with vehicle). The present data demonstrated that kindling produced long-lasting alterations, substantiating that epileptogenesis initiated by kindling renders the brain more susceptible to central nervous system (CNS) side effects of quinolones. An interaction between PTZ and quinolone derivatives which involves either an inhibition of γ-aminobutyric acid (GABA) neurotransmission or/and an increase in the function of the excitatory amino acid (EAA) system is suggested.