Antioxidant effects of a single dose of acetylsalicylic acid and salicylic acid in rat brain slices subjected to oxygen-glucose deprivation in relation with its antiplatelet effect

The pineal gland of birds, in contrast to its mammalian counterpart, is a directly photosensitive organ. It has recently been demonstrated that light also acting via the retina acutely suppresses melatonin synthesis in the chicken pineal gland. The present study was aimed to investigate whether retinal illumination alone was capable of resetting the biological oscillator generating the circadian rhythm of pineal serotonin N-acetyltransferase (AA-NAT) activity in the chicken. Ocular exposure of chickens to 6 h low intensity white light (4 lux) potently suppressed AA-NAT activity (the penultimate and key regulatory enzyme in the melatonin biosynthetic pathway) in the pineal gland. In addition, this light pulse produced phase-dependent shifts in the circadian AA-NAT rhythm. Exposure to light early in the subjective night (circadian time (CT) 12–18) caused a phase delay in the circadian rhythm of pineal AA-NAT activity by 3.5±0.4 h compared to non-exposed controls. When the light pulse was applied during the second half of the subjective night (CT18–24), it produced a large phase advance of the circadian rhythm of pineal AA-NAT activity by 10.9±0.4 h. The advancing effect of light was more pronounced than the phase-delaying effect. Our results suggest that in the chicken retinally perceived light provides a powerful and important signal for synchronization of circadian rhythmicity in the pineal gland.     

Transduction of light in the suprachiasmatic nucleus: evidence for two different neurochemical cascades regulating the levels of Per1 mRNA and pineal melatonin

The suprachiasmatic nucleus (SCN) contains a circadian clock and regulates melatonin synthesis in the pineal gland. Light exposure during the subjective night acutely increases the mRNA levels of the Period (Per)1 gene in the SCN and acutely suppresses melatonin levels in the pineal gland. Activation of N-methyl-D-aspartate (NMDA) receptors in the SCN has been demonstrated to phase-shift the circadian clock in a manner similar to light. We tested the hypothesis that activation of excitatory amino acid (EAA) receptors in the SCN mediates the acute effects of light on Per1 mRNA levels and pineal melatonin. NMDA, injected into the SCN of Syrian hamsters during the night, acutely suppressed melatonin levels in the pineal gland. Both the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (AP5) and the alpha-amino-3-hydroxy-5-methylisoxazoleproprionic acid (AMPA)/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) inhibited the light-induced increase of Per1 mRNA levels in the SCN. In the same animals, however, these antagonists had no effect on the ability of light to suppress pineal melatonin. These results support the hypothesis that EAA receptor activation in the SCN is necessary for the acute effects of light on Per1 mRNA levels. They also indicate that NMDA receptor activation in the SCN is sufficient but may not be necessary for the acute effects of light on pineal melatonin. These data suggest that there may be at least two different neurochemical cascades that transduce the effects of light in the SCN     

Differential effects of the excitatory amino acid antagonists, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 3-((+-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), on spinal reflex activity in mice

Intrathecal administration of the preferential quisqualate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in anesthetized mice depressed Hoffmann (H)-reflexes, while flexor reflexes remained unaffected. The depressant effect of CNQX on H-reflexes was dose-dependent (range 0.1-10 nmol). The intrathecal administration of the selective N-methyl-d-aspartate (NMDA) antagonist 3-[(+-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonate (CPP) reduced flexor reflexes (range 10-100 nmol) and had no effect on H-reflexes. These results suggest that H-reflexes in mice are mediated by spinal non-NMDA receptors, while flexor reflexes involve NMDA receptors.     

Peripheral glutamate receptors contribute to mechanical hyperalgesia in a neuropathic pain model of the rat

We hypothesized that glutamate (Glu) released from the peripheral terminals of primary afferents contributes to the generation of mechanical hyperalgesia following peripheral nerve injury. Nerve injury was performed on rats with a lumbar 5 spinal nerve lesion (L5 SNL), which was preceded by L5 dorsal rhizotomy (L5 DR) to avoid the potential central effects induced by L5 SNL through the L5 dorsal root. Mechanical hyperalgesia, as evidenced by a reduction in paw withdrawal threshold (PWT), was short-lasting (<6 days) after L5 DR, but persistent (>42 days) after L5 SNL preceded by L5 DR. When an intraplantar injection into the affected hind paw was given immediately before L5 SNL, non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (20 nmol), group-I metabotropic Glu (mGlu) receptor antagonist DL-amino-3-phosphonopropionic acid (DL-AP3; 70 nmol), and selective group-II mGlu receptor agonist 4-aminopyrrolidine-2,4-dicarboxylate (APDC; 20 nmol) delayed the onset of PWT reduction for 1-4 days. However, this onset was not affected by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4,-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX; 100 nmol). When the same injection was given after L5 SNL-induced mechanical hyperalgesia had been established, MK-801 reversed the PWT reduction for 30-75 min, whereas NBQX, DL-AP3, or APDC had no effect. These results suggest that the manipulation of the peripheral Glu receptors reduces neuropathic pain, by blocking NMDA and group-I mGlu receptors and by stimulating group-II mGlu receptor during the induction phase of neuropathic pain, but only by blocking the NMDA receptor during its maintenance phase.     

Contribution of endogenous glycine site NMDA agonists to excitotoxic retinal damage in vivo

N-Methyl-d-aspartate (NMDA) receptors, which play an important role in neuronal excitotoxicity, require not only agonists at the glutamate-binding site but also co-agonists at the glycine site for their activation. Here we examined the role of endogenous agonists at the glycine site of NMDA receptors in excitotoxic retinal damage in vivo. To quantify the number of surviving retinal ganglion cells (RGCs), we injected a retrograde tracer, fluoro-gold, into the superior colliculus bilaterally and subsequently counted RGCs on whole-mounted retinas. Co-injection of 5,7-dichlorokynurenic acid (300 nmol), a competitive antagonist at the glycine site of NMDA receptors, rescued RGCs from damage induced by 200 nmol NMDA. On the other hand, RGC death induced by 20 nmol NMDA was enhanced by addition of glycine (10 nmol), D-serine (10 nmol) or a competitive glycine transporter-1 inhibitor, sarcosine (0.3 or 3 nmol). Moreover, application of d-serine-degrading enzyme, D-amino acid oxidase (30 mU), partially suppressed RGC death induced by 20 nmol NMDA. These results suggest that the severity of excitotoxic retinal damage in vivo depends on the levels of both glycine and D-serine.     

Respiratory activation of the genioglossus muscle involves both non-NMDA and NMDA glutamate receptors at the hypoglossal motor nucleus in vivo

Brainstem respiratory neurons innervate the hypoglossal motor nucleus which in turn transmits this respiratory drive signal to the genioglossus muscle of the tongue. The mechanism of this transmission is important to help maintain an open airspace for effective breathing, and is thought to rely almost exclusively on non-N-methyl-d-aspartate (non-NMDA) glutamate receptor activation during respiration. However those studies were performed in slices of medulla from neonatal animals in vitro which may have led to an underestimation of the contribution of NMDA glutamate receptors that may normally operate in intact preparations. The current study tests the hypothesis that both NMDA and non-NMDA receptors contribute to respiratory drive transmission at the hypoglossal motor nucleus in vivo. Experiments were performed in urethane-anesthetized and tracheotomized adult Wistar rats in which vagus nerves were either intact or sectioned. In the presence of augmented genioglossus activity produced by vagotomy, microdialysis perfusion of either an NMDA receptor antagonist (D-2-amino-5-phosphonovaleric acid, 0.001-10 mM) or a non-NMDA receptor antagonist (6-cyano-7-nitroquinoxaline-2, 3-dione disodium salt, 0.001-1 mM) to the hypoglossal motor nucleus reduced respiratory-related genioglossus activity in a dose-dependent manner (P < 0.001) indicating that both NMDA and non-NMDA glutamate receptors are necessary for transmission of the respiratory drive signal to genioglossus muscle in vivo. Similar effects were observed in the vagus nerve intact rats. Further experiments demonstrated that each delivered antagonist had effects that were specific to its respective receptor. Regression analysis also revealed that the activity of both NMDA and non-NMDA receptors at the hypoglossal motor nucleus is related to levels of the prevailing respiratory drive. These results show that both NMDA and non-NMDA glutamate receptors at the hypoglossal motor nucleus are involved in transmission of the respiratory drive signal to genioglossus muscle in vivo.     

Glutamate-associated plasticity in the ventral tegmental area is necessary for conditioning environmental stimuli with morphine

We sought to determine if plasticity in the ventral tegmental area (VTA) of the midbrain is involved in learning to associate morphine exposure with a specific environment. For this, we tested whether activation of glutamate receptors and protein kinase A is needed for the acquisition and expression of a morphine-conditioned place preference (CPP). Rats received bilateral microinjections of either the NMDA antagonist AP5 (0.48 nmol/0.3 microl), the AMPA antagonist CNQX (0.21 nmol/0.3 microl), or vehicle into the VTA prior to each of three morphine-conditioning sessions. Both the AMPA and NMDA receptor antagonists blocked the development of morphine CPP when given into the VTA but not when given outside the VTA. In similar studies the protein kinase A (PKA) inhibitor, Rp-cAMPS (13 nmol/0.3 microl), blocked the acquisition of morphine CPP when given into the VTA immediately after morphine conditioning. In separate experiments, glutamate antagonists, or Rp-cAMPS, immediately prior to the preference test blocked the expression of morphine CPP when microinjected into the VTA. These data indicate that the VTA is an important site for synaptic modifications involved in the learning and memory of environmental cues predicting reward, and that glutamate input and PKA activation are crucial to this process.     

Cocaine increases endoplasmic reticulum stress protein expression in striatal neurons

Cocaine administration upregulates the levels of extracellular glutamate and dopamine in the striatum. Activation of the receptors alters calcium homeostasis in striatal neurons leading to the expression of the endoplasmic reticulum (ER) stress proteins. It was therefore hypothesized that cocaine upregulates the expression of the ER stress proteins, immunoglobulin heavy chain binding protein (BiP), Ire1alpha and perk via glutamate and dopamine receptor activation. A novel glutamate microbiosensor and Western immunoblot analyses were mainly performed to test the hypothesis in the rat dorsal striatum. The results showed that i.p. injection of repeated cocaine (20 mg/kg) for nine consecutive days significantly increased extracellular glutamate levels while acute cocaine injection did not. However, the immunoreactivities (IR) of the ER stress proteins in the dorsal striatum were significantly increased by either acute or repeated cocaine injections as compared with saline controls. Intrastriatal injection (i.s.) of the selective group I metabotropic glutamate receptor (mGluR) antagonist N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC; 25 nmol) or the mGluR5 subtype antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP; 2 and 25 nmol) significantly decreased repeated cocaine-induced increases in the IR of the ER stress proteins in the injected dorsal striatum. Similarly, the selective D1 antagonist (R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390; 0.1 mg/kg, i.p.) or the N-methyl-d-aspartate antagonist dizocilpine/(5S,10R)-(+)-5-methyl-10,11-dihydro-5H-ibenzo[a,d]cyclohepten-5,10-imine maleate (MK801; 2 nmol, i.s.) decreased acute or repeated cocaine-induced the IR of the ER stress proteins in the dorsal striatum. These data suggest that cocaine upregulates expression of the ER stress proteins in striatal neurons via a mechanism involving activation of glutamate and dopamine receptors.     

Central role of microglia in neonatal excitotoxic lesions of the murine periventricular white matter

Periventricular leukomalacia (PVL) is the main cause of neurologic handicap in pre-term infants. The understanding of cellular and molecular mechanisms leading to white matter damage is critical for development of innovative therapeutic strategies for PVL.The pathogenesis of PVL remains unclear but possibly involves glutamate excitotoxicity as an important molecular pathway. We previously described a neonatal mouse model of excitotoxic white matter lesion mimicking human PVL. In the present study, we used this experimental tool to investigate the cellular populations and the glutamate receptor subtypes involved in excitotoxic white matter lesions. Combined immunohistochemical, electron microscopic, and cell death detection data revealed that microglial activation and astrocytic death were the primary responses of white matter to excitotoxic insult. In vitro experiments suggested that microglia activated by ibotenate released soluble factors that kill astrocytes. The use of selective agonists and antagonists of glutamate receptors revealed that N-methyl-D-aspartate (NMDA) receptor activation was essential and sufficient to produce cystic white matter lesions. NMDA receptor immunohistochemistry labeled microglial cells in the neonatal periventricular white matter. The developing white matter displayed a window of sensitivity to excitotoxic damage that was paralleled by the transient presence of NMDA receptor-expressing white matter cells. Assuming that similar pathophysiologic mechanisms are present in human pre- term infants, microglia and NMDA receptors could represent key targets for treatment of PVL.     

The ventrolateral periaqueductal gray is involved in the cardiovascular response evoked by l-glutamate microinjection into the lateral hypothalamus of anesthetized rats

Microinjection of l-glutamate (l-glu: 1, 3, 10 and 30nmol/100nL) into the lateral hypothalamus (LH) caused dose-related depressor and bradycardiac responses. The cardiovascular response to l-glu stimulation of the LH was blocked by pretreatment of the ventrolateral portion of the periaqueductal gray matter (vlPAG) with CoCl2 (1mM/100nL), indicating the existence of a synaptic relay of the hypotensive pathway in that area. Furthermore, the response to l-glu was blocked by pretreatment of the vlPAG with 2nmol/100nL of the selective NMDA-receptor antagonist LY235959 and was not affected by pretreatment with 2nmol/100nL of the selective non-NMDA-receptor antagonist NBQX, suggesting a mediation of the hypotensive response by NMDA receptors in the vlPAG. In conclusion, our results indicate that the hypotensive pathway activated by microinjection of l-glu into the LH involves a NMDA synaptic relay in the vlPAG.     

Quinolinate induces selective loss of melanin-concentrating hormone neurons,rather than orexin neurons,in the hypothalamus of mice and young rats

Orexins are neuropeptides produced in the lateral hypothalamus and implicated in regulation of sleep-wake cycle. Selective loss of orexin neurons is found in the brain of patients with narcolepsy, but the mechanisms of this pathological change are unclear. A previous study showed that excessive stimulation of N-methyl-d-aspartate (NMDA) receptors by quinolinic acid (QA) caused selective loss of orexin neurons in rat hypothalamic slice culture. Here we examined QA toxicity on orexin neurons and melanin-concentrating hormone (MCH) neurons in vivo. Contrary to the expectation, injection of QA (60 and 120 nmol) into the lateral hypothalamus of male C57BL/6 mice caused selective loss of MCH neurons rather than orexin neurons, and this toxicity of QA was attenuated by MK-801, an NMDA receptor antagonist. Selective loss of MCH neurons with preserved orexin neurons was observed even when GABA_A receptor antagonists such as bicuculline and picrotoxin were injected with QA. A significant decrease in the number of orexin neurons was induced when QA injection was performed in the dark phase of diurnal cycle, but the degree of the decrease was still lower than that in the number of MCH neurons. Finally, QA (60 nmol) induced selective loss of MCH neurons also in young rats at 3–4 weeks of age. These results do not support the hypothesis that acute excitotoxicity mediated by NMDA receptors is responsible for the pathogenesis of narcolepsy.     

NMDA and group I metabotropic glutamate receptors activation modulates substance P release from the arcuate nucleus and median eminence

Glutamate participates in the regulation of secretion of several neuropeptides, including substance P (SP). Glutamate acts through ionotropic (iGluR) and metabotropic (mGluR) receptors. We have investigated whether glutamate receptor agonists and antagonists could affect SP release from the arcuate nucleus and the median eminence (ARC/ME). An increase in SP-like immunoreactivity (SP-LI) release from ARC/ME was induced by glutamate and N-methyl-D-aspartate (NMDA). This increase was prevented by D-(-)-2-amino-5-phosphono pentanoic acid (DAP5) (0.1mM), a specific NMDA antagonist and by (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) (0.1 mM), a selective antagonist of group I mGluR. The selective non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3(1H-4H)-dione (DNQX) (0.1mM) and (RS)-alpha-methyl-4-tetrazolylphenylglycine (MTPG) (0.1 mM), a group II and III mGluRs antagonist, did not affect the stimulatory effect of glutamate. A group I selective agonist, (S)-3,5-dihydroxyphenylglycine (DHPG) induced a significant increase in SP-LI release. Supporting the participation of nitric oxide (NO) in the effect of glutamate on SP-LI release, NAME (0.5 mM), a NO synthase inhibitor, reduced the glutamate-induced increase in SP-LI release from ARC/ME. Similarly, glutamate did not induce an increase in SP-LI release in the presence of meloxicam (0.1 mM) (a cyclooxygenase-2 (COX-2) specific inhibitor) indicating that prostaglandins production may also be involved in the glutamate effect. These data indicate that glutamate increases SP-LI release from the ARC/ME by acting through NMDA and group I mGluRs in the male rat. This stimulatory effect could be mediated by nitric oxide and prostaglandin production.     

NMDA receptor antagonism blocks the cardiovascular responses to microinjection of trans-ACPD into the NTS of awake rats

The possible interaction of glutamatergic metabotropic agonists and N-methyl- d -aspartate (NMDA) receptors was investigated in the nucleus tractus solitarii (NTS) of awake rats. The cardiovascular responses to unilateral microinjection of trans -1-amino-1,3-cyclopentanediocarboxylic acid ( trans -ACPD; 250 pmol/50 nL) into the NTS ( n = 8) produced hypotension (−64 ± 4 mmHg) and bradycardic (−206 ± 11 bpm) responses, which were blocked by previous microinjection of 2-amino-5-phosphonovaleric acid (AP-5; 10 nmol/50 nL), a selective antagonist of NMDA ionotropic receptors, into the same site. Intravenous injection of methyl-atropine blocked both the bradycardic and hypotensive responses to microinjection of trans -ACPD into the NTS, indicating that the hypotension was secondary to the intense bradycardic response. The data also showed that the bradycardic and hypotensive responses to microinjection of an NMDA agonist (10 pmol/50 nL) into the NTS were not affected by previous microinjection of α-methyl-4-carboxyphenylglycine (MCPG; 5 nmol/50 nL), a non-selective antagonist of metabotropic receptors. The results showing that the cardiovascular responses to microinjection of trans -ACPD into the NTS were blocked by AP-5 indicate that the responses to metabotropic agonists in the NTS involves NMDA receptors.     

Fast-spiking interneurons and gamma oscillations may be involved in the antidepressant effects of ketamine

Accumulating lines of evidence have indicated that a non-selective N-methyl-d-aspartate (NMDA) receptor antagonist ketamine exerts fast and robust antidepressant effects via stimulating glutamate transmission and activating the glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Moreover, NMDA receptor antagonist has the ability to reduce the activity of fast-spiking (FS) interneurons which results in the disinhibition of pyramidal neurons and increases the glutamate transmission. We therefore hypothesize that FS interneurons may play an important role in the antidepressant effects of ketamine. Quantification of FS interneurons function via analyzing gamma oscillations may guide the antidepressant therapy of ketamine in clinical practice.     

The role of different glutamate receptors in the mediation of glutamate-evoked excitation of red nucleus neurons after simulated microgravity in rat

The present study investigates changes in red nucleus (RN) neuronal activity and the role of glutamate receptors (GluRs) after simulated microgravity (tail-suspension) in the rat using single-unit recording and microinjection. The results showed that tail-suspension for 3, 7, and 14 days could induce a significant decrease in spontaneous firing rate of RN neurons in a time-dependent manner. Unilateral microinjection of glutamate into the RN significantly increased the firing rate of RN neurons, but the increased firing rate was significantly reduced following tail-suspension time. Microinjection of the NMDA receptor antagonist MK-801 or the non-NMDA receptor antagonist DNQX into the RN blocked this excitatory effect induced by glutamate. However, microinjection of the metabotropic glutamate receptor (mGluR) antagonist (±)-MCPG into the RN had no effect. These results suggest that simulated microgravity can reduce excitability of RN neurons following a functional impairment of glutamate receptors. NMDA and non-NMDA receptors, but not mGluRs, are involved in the mediation of glutamate-evoked excitation of RN neurons. The decrease in excitability of RN neurons may be involved in simulated microgravity-induced muscle atrophy.     

Brain-derived neurotrophic factor activation of extracellular signal-regulated kinase is autonomous from the dominant extrasynaptic NMDA receptor extracellular signal-regulated kinase shutoff pathway

NMDA receptors bidirectionally modulate extracellular signal-regulated kinase (ERK) through the coupling of synaptic NMDA receptors to an ERK activation pathway that is opposed by a dominant ERK shutoff pathway thought to be coupled to extrasynaptic NMDA receptors. In the present study, synaptic NMDA receptor activation of ERK in rat cortical cultures was partially inhibited by the highly selective NR2B antagonist Ro25-6981 (Ro) and the less selective NR2A antagonist NVP-AAM077 (NVP). When Ro and NVP were added together, inhibition appeared additive and equal to that observed with the NMDA open-channel blocker MK-801. Consistent with a selective coupling of extrasynaptic NMDA receptors to the dominant ERK shutoff pathway, pre-block of synaptic NMDA receptors with MK-801 did not alter the inhibitory effect of bath-applied NMDA on ERK activity. Lastly, in contrast to a complete block of synaptic NMDA receptor activation of ERK by extrasynaptic NMDA receptors, activation of extrasynaptic NMDA receptors had no effect upon ERK activation by brain-derived neurotrophic factor. These results suggest that the synaptic NMDA receptor ERK activation pathway is coupled to both NR2A and NR2B containing receptors, and that the extrasynaptic NMDA receptor ERK inhibitory pathway is not a non-selective global ERK shutoff.     

Glutamate release upon purinergic action in the dorsal facial area of the medulla increases blood flow in the common carotid artery in cats

Interactions of glutamatergic and purinergic actions in the medulla regulate important cardiovascular functions. The glutamatergic action in dorsal facial area (DFA) of the medulla increases blood flow of common carotid artery (CCA) in cats. We hypothesized that interactions of glutamatergic and purinergic actions in the DFA may regulate the CCA blood flow. Purinergic and glutamatergic agonists and antagonists were microinjected into the DFA through a four-barrel tubing in anesthetized cats. Drug effects were evaluated by changes in the CCA blood flow. Microinjection with 20 nmol ATP or α,β-methyleneATP (α,β-MeATP, a P2 purinergic receptor agonist) induced an increase of the CCA blood flow. This increase was dose-dependently reduced by prior administration with 1,3-dipropyl-8-p-sulfophenylxanthine (DPSPX, a specific P1 purinergic receptor antagonist), or pyridoxalphosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS, a selective P2 purinergic receptor antagonist) as well as with MK-801 (a non-competitive NMDA receptor antagonist) or glutamate diethyl ester (GDEE, a competitive AMPA/kainate receptor antagonist). It was almost completely blocked by administrations with combined maximal doses of P1 and P2 receptor antagonists as well as NMDA and AMPA receptor antagonists. Nevertheless, P1 receptor agonist induced only mild and poorly reproducible increase in the CCA blood flow. In conclusion, prominent P2 and minor P1 purinergic receptors appear to be present in the DFA; the purinergic activation can mediate a release of glutamate that stimulates NMDA and AMPA to induce the increase of the CCA blood flows. These findings may provide important information for developing therapeutic strategy for diseases involving the CCA blood flow, such as hypertensive disease and cerebral ischemia.     

Serotonin and NMDA glutamate receptor antagonists selectively impair the reactivation of associative memory in the common snail

Experiments on the common snail were performed to study the influences of serotonin and glutamate receptor antagonists on the processes of reactivation of an associative habit consisting of refusing a particular type of food. Twenty-four hours after training, animals were injected with the non-selective serotonin receptor antagonist methiothepin (0.1 mg/snail) or the NMDA glutamate receptor antagonist MK-801 (0.005 mg/snail), after which they were presented with a "reminder" stimulus (the "conditioned reflex" foodstuff, a banana) and tested for retention of the habit. Three hours after antagonist injections and the "reminding" procedure, snails showed impairments in the reproduction of the acquired habit, which persisted for more than two weeks. Furthermore, animals with amnesia after treatment with methiothepin/reminding showed facilitation of repeated acquisition of the aversive habit to banana. Repeat training of animals which had shown amnesia after MK-801/reminding did not result in acquisition of the habit. It is suggested that serotonin receptors are involved in the mechanisms underlying extraction of the memory trace of the aversive habit to the foodstuff in snails, while NMDA glutamate receptors are involved in memory trace storage processes.     

Swallowing responses induced by microinjection of glutamate and glutamate agonists into the nucleus tractus solitarius of ketamine-anesthetized rats

Summary Swallowing is a patterned motor activity generated by neurons located within the nucleus tractus solitarius (NTS). An excitatory amino acid (EAA) neurotransmitter, such as glutamate (GLU), is suspected of being involved in the initiation of swallowing by NTS neuronal components. However, swallowing can still be elicited in animals anesthetized with ketamine, an antagonist of the N-methyl-D-aspartate (NMDA) subclass of EAA receptors. The present experiments were therefore designed to investigate the influence of EAA administration within the NTS on the swallowing motor acitivity of rats anesthetized with ketamine. Pressure microinjections of GLU in doses ranging from 25 to 500 pmol elicited swallowing. This effect was dose-dependent and was not reproduced when control injections of the vehicle solution were performed. Microinjections of the GLU agonists, quisqualate (QUIS) and NMDA, in doses ranging between 2.5 and 50 pmol, also induced swallowing motor activities. QUIS, like GLU, elicited a short series of swallows at a brief latency while NMDA generated long-lasting rhythmic swallowing with a longer latency. Swallowing induced by GLU microinjections (100 pmol) was suppressed almost completely by local pretreatment with either the broad spectrum EAA receptor antagonist, gamma-D-glutamylglycine (250 pmol), or the more selective non-NMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (50–100 pmol), but not by pretreatment with the selective NMDA antagonist, DL-2-amino-5-phosponovalerate (250 pmol). On the other hand, pretreatment with DL-2-amino-5-phosphonovalerate (50 pmol) suppressed the deglutitions induced by NMDA microinjections (10 pmol) but not those elicited by QUIS microinjections (10 pmol). These results provide evidence that swallowing can be induced by activation of EAA receptors of both the NMDA and the non-NMDA subclasses located within the NTS. Furthermore they indicate that both subclasses may still be active in ketamine-anesthetized animals.