Neuromedin N decreases self-stimulation of the medial prefrontal cortex.

Intracerebral microinjections of neurotensin (NT) decrease intracranial self-stimulation (ICSS) of the medial prefrontal cortex (MPC) in the rat. This effect could be due to the ability of NT to bind dopamine. To test this hypothesis we studied the effects of intracerebral microinjections of neuromedin N, a natural NT analogue that does not bind dopamine, on ICSS of the rat MPC. Unilateral microinjections of neuromedin N into the MPC at doses of 2.5, 5, 10, 20 and 40 nmol produced a dose-related decrease in ICSS of the ipsilateral MPC. ICSS of the contralateral MPC, used as a control, was not affected by the microinjections. These results suggest that the inhibitory effect of NT on ICSS is independent of NT-dopamine binding. Because neuromedin N is also present in the MPC, these results also suggest a possible neuromodulatory role of this neuropeptide on ICSS of the prefrontal cortex.     

Turning behavior induced by injections of glutamate receptor antagonists into the substantia nigra of the rat

We have found recently that muscimol microinjections into the subthalamic nucleus produce contralateral turning activity [Murer and Pazo (1993) NeuroReport, 4:1219–1222]. To test the hypothesis that a reduced glutamate action on substantia nigra pars reticulata neurons mediates this turning response, we examined the effect of unilateral intranigral microinjections of the AMPA/kainate receptor antagonist 6,7-dinitro-quinoxaline-2,3-dione (DNQX) and the competitive N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP-5). DNQX and AP-5 both produced a dose-dependent contralateral turning response, while vehicle administration did not induce turning activity. Application of glutamate receptor antagonist at adjacent regions of the mesencephalic tegmentum were also ineffective. Coadministration of NMDA or AMPA significantly reduced the turning response induced by AP-5 or DNQX, respectively. Lesions of the nigrostriatal pathway by 6-hydroxydopamine did not modify the response to DNQX or AP-5 administration into the nigra. However, their behavioral effects were significantly reduced by a lesion of the ipsilateral subthalamic nucleus. Our results show that the blockade of a tonic input acting on AMPA/kainate and NMDA receptors located at the substantia nigra produces contralateral turning behavior. The effect seems to involve pars reticulata cells since this area remains unchanged after destruction of dopaminergic neurons. The subthalamic nucleus seems to be the endogenous source of the agonist acting on the nigral glutamate receptors related to turning behavior. © 1996 Wiley-Liss, Inc.     

Excitatory amino acid pathways in brain-stimulation reward

A range of agonists and antagonists active at different glutamate/aspartate (Glu/Asp) receptor subtypes were injected into rat ventral tegmental (VTA) sites downstream from self-stimulation electrodes in the medial forebrain bundle. Control injections were made into the contralateral tegmentum. Variable-interval (VI 10 s) self-stimulation was not significantly affected by a specific antagonist of N-methyl-D-aspartate (NMDA)-type receptors (D,L-2-amino-5-phosphonovaleric acid (2-AP5), 10 and 50 nmol). Broad-spectrum excitatory amino acid (EAA) antagonists viz cis-2,3-piperidine dicarboxylate (cPDA) (10 and 50 nmol), gamma-D-glutamylaminomethyl sulphonic acid (GAMS) (10 nmol) and p-chlorobenzoyl-2,3-piperazine dicarboxylic acid (pCB PzDA) (2.0 and 10 nmol), active at kainate, quisqualate, as well as NMDA receptors, all produced significant depression of responding when injected into the ipsilateral, but not the contralateral, tegmentum. Compounds inhibiting Glu/Asp reuptake had variable effects: strong depression with dihydrokainic acid (7.5 nmol), or no significant effect (L-threo-3-hydroxyaspartic acid, 2.0 and 10 nmol). The receptor agonist, NMDA (10 nmol), depressed responding regardless of injection side; kainic and responding regardless of injection side; kainic and quisqualic acid elicited myoclonic and other non-specific responses in preliminary tests, and were not examined further; enhanced responding was not seen. The side-specific blockade of responding by non-NMDA antagonists indicates the existence of non-NMDA EAA terminals in the VTA, signalling the receipt of hypothalamic brain-stimulation reward. Caudally directed EAA projections terminating on A10 dopamine cell bodies may account for depression of self-stimulation by EAA antagonists.     

Neurotoxic effects of excitatory amino acids in the mouse spinal cord: quisqualate and kainate but not N-methyl-D-aspartate induce permanent neural damage

Despite extensive evidence for the neurotoxic effects of excitatory amino acids (EAA) in the brain little is known about their neurotoxic action in the spinal cord. In this study we attempted to produce differential lesions of spinal neurons by pretreating mice, intrathecally, with high concentrations of the EAA: N-methyl-D-aspartate (NMDA), quisqualate and kainate. Pharmacological, behavioral and histological consequences were examined 1, 3, 7 and, in some cases, 30 days after pretreatment. A single, intrathecal, injection of high concentrations of quisqualate and kainate but not NMDA, resulted in damage to spinal cord neurons. The highest concentrations of these agonists produced, in some animals, a massive, non-selective destruction of neurons within the lumbar spinal cord, accompanied by complete paralysis of the hindlimbs. Pretreatment with lower concentrations of intrathecal kainate or quisqualate produced damage to spinal interneurons, as well as more limited damage to motor neurons. No detectable motor deficit could be detected but a decrease in responsiveness to noxious stimuli was observed. Such damage also manifest as a permanent decrease in the sensitivity of the spinal interneurons, as well as more limited damage to motor neurons. No detectable motor deficit could be detected but a decrease in responsiveness to noxious stimuli was observed. Such damage also manifest as a permanent decrease in the sensitivity of the spinal cord to EAA, as seen from the decrease in biting behavior elicited by intrathecal EAA. The neurotoxic effects of quisqualate were completely blocked by the quisqualate/kainate receptor antagonist glutamylaminomethylsulphonate (GAMS), but not the NMDA antagonist 2-amino-5-phosphovalerate. GAMS attenuated the effects of kainate only partially.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for spinal N-methyl-D-aspartate receptor involvement in prolonged chemical nociception in the rat

Subcutaneous injection of formalin into the hindpaw peripheral receptive field of deep dorsal horn multireceptive (convergent) nociceptive neurones was used to produce a prolonged (1 h) activation of the cells. This chemical noxious stimulus produced a first peak of firing which lasted 10 min followed by a second peak of prolonged activity which was monitored for 50 min. gamma-D-glutamylglycine (DGG), a non-selective N-methyl-D-aspartate (NMDA) and quisqualate/kainate (non-NMDA) receptor antagonist was applied intrathecally both as a pretreatment and after the formalin. A complete abolition of both peaks of the formalin response was produced by DGG pretreatment (1000 micrograms) (n = 4). This dose produced profound inhibition of the acute C-fibre evoked responses of the same cells. However, no inhibitions were produced when the antagonist was applied once the formalin response had developed (n = 4). The selective NMDA receptor antagonist 5-amino-phosphonovaleric acid (AP5) was administered intrathecally (250 and 500 micrograms) as a 40 min pretreatment and caused a small inhibition of the first peak but a marked dose-related reduction in the second prolonged phase (n =7). AP5 did not influence the C-fibre inputs onto the cells. The non-competitive NMDA receptor channel blockers, ketamine and MK801, were administered i.v. during the second phase of firing. Ketamine (1-8 mg/kg) caused a short-lasting but marked and dose-related inhibition of the neuronal responses to formalin (n = 11). MK801 (0.5-1 mg/kg) resulted in a prolonged inhibition of cell firing during the second phase of the response (n = 11).(ABSTRACT TRUNCATED AT 250 WORDS)     

Non-NMDA but not NMDA blockade at deep prepiriform cortex protects against hippocampal cell death in status epilepticus

The present study investigates the role of pharmacologic blockade of NMDA (N-methyl-d-aspartate) and non-NMDA receptors at deep prepiriform cortex (area tempestas, AT) in neuronal injury during prolonged seizures in rat. Status epilepticus was induced by intravenous kainate (15 mg/kg) and neuronal death was assessed in hippocampal CA3 sector 72 h following status epilepticus. Unilateral equimolar microinjections of 2-amino-7-phosphonoheptanoic acid (AP-7), an NMDA receptor antagonist, or 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX), a non-NMDA receptor antagonist, into AT were given prior to kainate administration. Counts of surviving cells in CA3 ipsilateral to NBQX-injected AT were significantly greater than on the contralateral control-side, but no significant difference between the AP-7-injected and saline-injected side was found. These results indicate that neurotransmission via non-NMDA receptors is more important than that via NMDA receptors at AT in the genesis of neuronal injury in hippocampus during kainate-induced status epilepticus.     

Afferent pathways to points of self-stimulation in the medial prefrontal cortex of the rat as revealed by the horseradish peroxidase technique

Afferent projections to points of self-stimulation (SS) in the medial prefrontal cortex (MPC) of the rat were studied using the horseradish peroxidase (HRP) technique. Intracranial microinjections of HRP (30%) were delivered at the same stereotaxic points at which the electrodes eliciting SS were located. Retrogradely transported HRP labeled neurons in different thalamic, hypothalamic, mesencephalic and pontine areas. In the thalamus, labeled neurons were found in the dorsomedial, anteromedial, anteroventral, ventral, ventromedial, posteromedial, paratenial, parafascicular nuclei and n. reuniens. Labeled neurons in mesencephalic areas were found in the n. interpeduncularis, ventral tegmental area (AVT) and substantia nigra (SN). In the pons, labeled neurons were found in the locus coeruleus and in the periaqueductal gray. Other nuclei in which labeled neurons were also found were: lateral hypothalamus (LH), periventricular gray and zona incerta (ZI). Theoretically it is possible that all these afferent areas contribute to SS of MPC. This assumption is discussed and criticized in connection with previous literature on SS. It is suggested that only specific areas and their projections are good candidates for the neural mechanisms involved in the reward produced by electrical stimulation of the prefrontal cortex.     

NMDA and AMPA/kainate glutamatergic agonists increase the extracellular concentrations of GABA in the prefrontal cortex of the freely moving rat: modulation by endogenous dopamine

Using microdialysis in the prefrontal cortex, this study investigated first the effects of the ionotropic glutamatergic agonists NMDA and AMPA on extracellular concentrations of GABA, and second, the modulation of these effects by increasing endogenous dopamine. NMDA (20, 100, and 500 microM) and AMPA (1, 20, and 100 microM), perfused through the microdialysis probe for 60 min, produced a dose-related increase of extracellular concentrations of GABA in the prefrontal cortex of the awake rat. NMDA 100 and 500 microM produced a maximal increase of extracellular GABA of 150 +/- 38% and 245 +/- 75% of baseline, respectively. AMPA 20 and 100 microM produced a maximal increase of extracellular GABA of 140 +/- 17% and 195 +/- 41% of baseline, respectively. NMDA and AMPA also increased extracellular concentrations of glutamate. Increases of extracellular GABA, and also of glutamate, produced by NMDA (500 microM) and AMPA (100 microM) were significantly blocked by the NMDA antagonist CPP (100 microM) and the AMPA/kainate antagonist DNQX (100 microM), respectively. To investigate whether dopamine modulates the increases of GABA produced by NMDA and AMPA, endogenous dopamine was increased with the dopamine uptake inhibitor nomifensine. Nomifensine (1, 100, and 1000 microM) produced a dose-related increase of dialysate dopamine (from 0.1 to 1.0 nM) but did not modify basal extracellular concentrations of GABA in the prefrontal cortex. However, increases of endogenous dopamine at 0.5-0.7 nM did potentiate the increases of extracellular GABA produced by AMPA (20 microM) (from 140% to 240% of baseline), but not by NMDA (100 microM), in this area of the brain. These effects were attenuated by the perfusion of (-)sulpiride (D2 antagonist), but not by the perfusion of SCH-23390 (D1 antagonist). These results suggest that glutamate, through the activation of both NMDA and AMPA/kainate ionotropic receptors, facilitates GABAergic transmission in the prefrontal cortex, and that dopamine can modulate the effects of glutamate through AMPA/kainate receptors on GABA transmission in this area of the brain.     

Suppression of self-stimulation of the medial prefrontal cortex after local micro-injection of kainic acid in the rat

The question of whether neurons versus fibers of passage in the medial prefrontal cortex (MPC) are essential in maintaining self-stimulation of this same area of the brain was examined. Rats were prepared with electrode-guide cannulae implanted stereotaxically to rest within MPC. A micro-injection of (KA), 10 nmol/1.0 microliter, into the right MPC produced a clear degeneration of neuronal cell bodies characterized by picnocytosis and glial invasion of the tissue surrounding the tip of the electrode. These histopathological changes were correlated with a permanent abolition of self-stimulation of the right MPC. In contrast, self-stimulation of the contralateral side of the MPC, micro-injected with 0.9% NaCl vehicle as a control, was unaffected. These results suggest that neurons of the MPC are part of the neural substrate underlying self-stimulation behavior in this cortical area of the rat.     

Glutamate motivational ensembles in nucleus accumbens: rostrocaudal shell gradients of fear and feeding

This study demonstrates that microinjection of an AMPA/kainate glutamate antagonist elicits motivated fear and feeding behaviour mapped along rostrocaudal gradients of positive-to-negative valence in nucleus accumbens shell (similar to rostrocaudal shell gradients recently reported for GABA agonist microinjections). Rats received rostral or caudal microinjections of the glutamate AMPA/kainate receptor antagonist DNQX (0, 50, 450 or 850 ng in 0.5 micro L) or the NMDA receptor antagonist MK-801 (0, 0.5, 1 or 2 micro g in 0.5 micro L), into medial accumbens shell prior to behavioural tests for fear, feeding or conditioning of place preference or avoidance. Another group received rostral or caudal microinjections of DNQX in nucleus accumbens core. Rostral shell DNQX microinjections potently increased appetitive food intake and established only weak conditioned place avoidance. Caudal shell DNQX microinjections elicited defensive treading behaviour, caused rats to defensively bite the experimenter and emit fearful distress vocalizations when handled, and established strong conditioned place avoidance. By contrast, no rostrocaudal gradients of motivational bivalence were produced by microinjections of the glutamate AMPA/kainate receptor antagonist DNQX into the core, or by microinjections of the NMDA antagonist MK-801 into the shell. Our results indicate that appetitive and aversive motivation is carried in anatomically differentiated channels by mesocorticolimbic glutamate signals to microcircuits in the medial shell. Hyperpolarization of local shell ensembles by AMPA/kainate glutamate receptor blockade elicits fear and feeding behaviours mapped along distinct positive-to-negative rostrocaudal gradients.     

The role of the lateral cortico-cortical prefrontal pathway in self-stimulation of the medial prefrontal cortex in the rat

Effects of electrolytic and kainic acid lesions at several stereotaxic planes of the lateral cortico-cortical prefrontal efferent pathway on self-stimulation of the medial prefrontal cortex were investigated. Electrolytic bilateral lesion of the sulcal prefrontal cortex, the first terminal area of this pathway, produced no effects on self-stimulation of the medial prefrontal cortex. However, bilateral electrolytic lesion of this pathway at the rostral part of the external capsule produced a permanent abolition of self-stimulation of the medial prefrontal cortex. These effects seemed selective since operant behaviour to obtain water, similar to that performed for self-stimulation and used as a control, was not affected by the lesion except on the 1st, 3rd (P less than 0.01) and 5th (P less than 0.05) days postlesion. Interestingly, bilateral microinjections of kainic acid (10 nmol in 0.8 microliters) at the same stereotaxic planes of the external capsule where electrolytic lesion was produced, had no effects on self-stimulation. These results suggest that fibres-of-passage through the external capsule are responsible for the abolition of self-stimulation. Bilateral electrolytic lesion of the entorhinal cortex, one of the caudal terminal areas of this descending set of fibres, produced a short transient decrease of self-stimulation of the medial prefrontal cortex. These results are discussed on the basis that complex, rather than single circuits are involved in maintaining self-stimulation in this neocortical area.     

The effects of excitatory amino acids on proenkephalin and prodynorphin mRNA levels in the hippocampal dentate gyrus of the rat; an in situ hybridization study.

Injection of N-methyl-D-aspartate (NMDA, 7.5 micrograms) kainate (1 microgram) or quisqualate (2 micrograms) into the rat dorsal hippocampus induced wet-dog shakes and convulsions. As shown by an in situ immunohistochemical analysis, 3 h after the excitatory amino acids injections the rats displayed a bilateral profound elevation of the proenkephalin and prodynorphin mRNA levels in dentate gyrus granule cells (2-3 or 1.5-2 fold higher than control levels, respectively). Pretreatment of rats with D-amino-phosphonovalerate (D-APV, 10 micrograms), a selective antagonist of NMDA receptor, prevented the behavioral and biochemical changes evoked by NMDA. The changes in the behavior and gene expression evoked by kainate or quisqualate were diminished in rats which received 6-cyano-7-nitroquinoxaline-2,3-dion (CNQX, 2 micrograms), a putative antagonist of quisqualate and kainate receptors. The study demonstrated that activation of NMDA, quisqualate or kainate receptors in the hippocampus induced seizures associated with a marked increase in the proenkephalin (PENK) and the prodynorphin (PDYN) gene expression in the rat dentate gyrus.     

Some effects of excitatory amino acid receptor antagonists on synaptic transmission in the rat olfactory cortex slice

A study has been made of the effects of a series of excitatory amino acid receptor antagonists on the field potentials evoked on electrical stimulation of the lateral olfactory tracts of olfactory cortex slices perfused in vitro. The antagonists studied included (+/-)-2-amino-5-phosphonovaleric acid, a potent, specific antagonist of N-methyl-D-aspartate (NMDA) receptors, gamma-D-glutamylglycine, an antagonist of NMDA and kainate receptors and (+/-)-cis-2,3-piperidine dicarboxylic acid and 2-amino-4-phosphonobutyric acid, drugs which in addition to antagonizing NMDA and kainate receptors also block responses to quisqualic acid. From the patterns of effects of the drugs it is proposed that quisqualate and NMDA but not kainate receptors are involved in mediating excitatory transmission in the olfactory cortex; quisqualate receptors are located at the lateral olfactory tract - superficial pyramidal cell synapse whereas NMDA receptors are present at the synapses of the superficial pyramidal cell collaterals with the deep pyramidal cell dendrites and/or at the synapses of the pyramidal cell collaterals and inhibitory interneurones. The results are discussed in terms of possible presynaptic and/or postsynaptic sites of antagonist action.     

Neurotransmitters, pathways and circuits as the neural substrates of self-stimulation of the prefrontal cortex: facts and speculations

Through a multidisciplinary approach considerable progress has been made in understanding the neural substrates of self-stimulation (SS) of the medial prefrontal cortex (MPC). Thus, neuroanatomical studies have revealed that intrinsic neurones in the MPC seem to be the central elements responsible for initiating and maintaining this phenomenon in this area of the brain. Complementary to this central finding are the electrophysiological and neurohistological data reviewed here, showing that neurones in the MPC are directly activated and have monosynaptic feed-back connections with neurones located in areas which also support SS. These findings have given rise to the hypothesis that several single feed-back pathways or single circuits exist between points of SS in the MPC and points of SS in other areas of the brain. This hypothesis implies that SS in a particular area would depend not only on the intrinsic local activity induced by the electrical stimulation but on the functional and specific activity of other nuclei in the brain. The fact that lesions of single circuits, which are apparently involved in SS of the MPC such as the medial prefrontal cortex-ventrotegmental area-medial prefrontal cortex and medial prefrontal cortex-n. dorsomedialis of the thalamus-medial prefrontal cortex, do not produce a permanent decrease of SS, together with the finding that transynaptic connections seem to exist between MPC and other areas of the brain, suggests further that a complex rather than several single independent circuits could be at the neural basis of SS of the MPC. If that were the case, then SS of the MPC would not only depend upon local and single feed-back activity but upon specific functional feed-back activity among the nuclei, which in turn have single feed-back connections with the MPC (see the concept of 'complex circuit' outlined in the section of Behavioural studies). On the basis of this hypothesis no permanent changes should be expected after lesions of single pathways since physiological and even anatomical compensation could be reached through the rest of the undamaged circuit. That terminals containing specific neurotransmitters exist in layers of the PC where electrodes for SS are located has been reviewed in this paper. Some of these neurotransmitters have been suggested to be part of the local substrates activated by SS.(ABSTRACT TRUNCATED AT 400 WORDS)     

Prefrontal cortex and neostriatum self-stimulation in the rat: Differential effects produced by apomorphine

In a dose-response experiment, the effects of intraperitoneal injections of the dopamine receptor agonist, apomorphine (0.075, 0.15, 0.3, 0.6 and 1.2 mg/kg) were studied on self-stimulation elicited from electrodes implanted in the medial and sulcal prefrontal cortex and caudate-putamen in the rat. From the medial and sulcal prefrontal cortex electrodes, apomorphine produced a dose-related decrease of self-stimulation rate which was consistent across animals. From the caudate-putamen electrodes, on the contrary, apomorphine produced a facilitatory effect in the majority of the animals at one or more doses, however, at other doses a decreased self-stimulation rate was observed. The clear and consistent effects of apomorphine on self-stimulation of the prefrontal cortex, together with other experimental evidence in the same line, suggest that dopamine is mediating self-stimulation of this cortical area.     

NMDA receptors in the midbrain periaqueductal gray mediate hypothalamically evoked hissing behavior in the cat

The present study was designed to test the hypothesis that the descending pathway from the medial hypothalamus to the dorsal periaqueductal gray (PAG) is critical for the expression of defensive rage behavior in the cat and utilizes excitatory amino acids as a neurotransmitter. In the first phase of the study, monopolar stimulating electrodes were implanted into the medial hypothalamus from which defensive rage behavior could be elicited by electrical stimulation. For the entire study, the hissing response was used as a measure of defensive rage behavior. Cannula electrodes were implanted into the PAG from which defensive rage sites could be identified and were later used for microinfusion of the NMDA receptor antagonist,dl-2-amino-7-phosphoheptanoic acid (AP-7), into behaviorally identified sites within the PAG. Initially, intracerbral microinjections of the NMDA receptor antagonist, AP-7 (0.2, 2.0 nmol), which were placed directly into sites within the PAG from which defensive rage had been elicited, blocked the occurrence of hypothalamic hissing. Microinjections of similar doses of AP-7 into the PAG also blocked the facilitatory effects of medial hypothalamic stimulation upon hissing behavior elicited from the PAG. However, microinjections of 2 nmol into the PAG had no effect upon hissing that was also elicited from the region of the injection site. This finding indicates that AP-7 selectively blocks hissing elicited from the medial hypothalamus and that the suppressive effects of AP-7 cannot be the result of anesthetic or other nonselective properties of the drug. The next phase of the study, which employed immunohistochemical, receptor autoradiographic techniques, identified NMDA receptors to be present in highest concentrations in the dorsolateral aspect of the PAG where defensive rage is typically elicited. The final phase of the study, which employed a combination of retrograde labeling procedures following microinjections of Fluoro-Gold into defensive rage sites in the dorsal PAG and the immunocytochemical labeling of glutamatergic neurons, identified large numbers of neurons in the medial hypothalamus that were labeled positively for both Fluoro-Gold and glutamate. The overall findings of this study support the hypothesis that descending fibers of the medial hypothalamus that supply the dorsal aspect of the PAG mediate defensive rage behavior and utilize excitatory amino acids that act upon NMDA receptors within the dorsal PAG.     

Role of NMDA receptors in hypothalamic facilitation of feline defensive rage elicited from the midbrain periaqueductal gray

The present study tested the hypothesis that the pathway from the medial hypothalamus to the midbrain periaqueductal gray (PAG) subserving defensive rage behavior in the cat facilitates the occurrence of this response when elicited from the PAG by utilizing excitatory amino acids as a neurotransmitter or neuromodulator. Cannula electrodes were implanted into the PAG for the elicitation of defensive rage behavior as well as for microinjections of excitatory amino acid antagonists and N-methyl-D-aspartic acid (NMDA). Monopolar stimulating electrodes were also implanted into the medial hypothalamus from which this response could also be elicited and, when stimulated at subthreshold levels for elicitation of behavior, could also facilitate the occurrence of PAG elicited defensive rage. Initially, dual stimulation of the PAG and medial hypothalamus facilitated the occurrence of defensive rage elicited from the PAG. Then, the identical dual stimulation paradigm was repeated with the same current parameters following the infusion of various antagonists for different receptors into the PAG defensive rage sites. The results indicate that infusion of either kynurenic acid [(0.1-2.0 nmol), a non-selective excitatory amino acid receptor antagonist] or D-2-amino-7-phosphonoheptanoic acid (AP7) [(0.1-2.0 nmol), a specific NMDA receptor antagonist], produced a dose and time dependent blockade of the facilitatory effects of medial hypothalamic stimulation. In contrast, microinjections of relatively larger doses of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) [(4 nmol), a non-NMDA receptor (quisqualate and kainate) antagonist] or atropine [(4.4 nmol), a muscarinic receptor antagonist] had little effect upon medial hypothalamically elicited facilitation of the PAG response. In a second experiment, NMDA [0.1-1.0 nmol] was microinjected directly into PAG defensive rage sites in the absence of medial hypothalamic stimulation. In these animals, drug infusion mimicked the effects of dual stimulation by producing a dose and time dependent decrease in response latencies. A third experiment was designed to further test the hypothesis by neuroanatomical methods. Here, the retrograde label, Fluoro-Gold, was microinjected into defensive rage sites within the PAG and following a survival time of 5-6 days, the animals were sacrificed. The brains were then processed for immunocytochemical analysis of cells that immunoreact positively for aspartate and glutamate. The results indicated the presence of many retrogradely labelled and immunocytochemically positive cells within the rostro-caudal extent of the medial hypothalamus as well as others that were double labelled.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cardiovascular changes induced by the local application of glutamate-related drugs in the rat nucleus tractus solitarii

The effects of the local application of drugs acting on glutamatergic receptors in the nucleus tractus solitarii (NTS) were investigated in anesthetized rats. Unilateral microinjection of agonists (L-glutamate, L-aspartate, N-methyl-D-aspartate (NMDA) and quisqualate) produced a dose-dependent hypotension and bradycardia. The effects of NMDA were prevented by low doses of the selective NMDA-receptor antagonist, 2-amino-5-phosphonovalerate (2-APV), or by the mixed NMDA/kainate antagonist, gamma-D-glutamylglycine. The response to all agonists and the bradycardia which was elicited in response to the intravenous administration of phenylephrine (vagal reflex response) could be prevented by the local microinjection of the glutamate antagonists kynurenic acid (3 nmol) and 2-APV (10 nmol) into the NTS. The present data suggest that in the NTS, NMDA and quisqualate receptors are implicated in blood pressure reflex regulation.     

N-methyl-D-aspartate, quisqualate and kainate receptors are all involved in transmission of photic stimulation in the suprachiasmatic nucleus in rats.

In order to clarify the neuronal transmission mechanism of photic stimulation in the suprachiasmatic nucleus (SCN), the effects of agonists and antagonists for excitatory amino acid receptors on N-acetyltransferase (NAT) activity in the pineal gland were observed following the microinjection of drugs into both sides of the nuclei. N-Methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, and kainate (which are selective agonists for three different subtypes, i.e. NMDA, quisqualate and kainate receptors, respectively) significantly decreased NAT activity similarly to the suppressive effect of light. Moreover, compared with a control group, all the groups pretreated with a selective competitive antagonist for NMDA receptor (D-2-amino-5-phosphonovalerate or 3-((+-)-2-carboxypiperazine-4-yl)-propyl-1-phosphonate), or a selective non-competitive antagonist for non-NMDA receptors (Joro spider toxin-3 or 1-naphthylacetyl spermine) partially blocked the suppressive effect of photic stimulation on NAT activity. These results suggest that NMDA, quisqualate and kainate receptors are all involved in mediating photic stimulation in the SCN.     

A quantitative estimate of the contribution made by various receptor categories to the depolarizations evoked by some excitatory amino acids in the olfactory cortex

A study has been undertaken to assess the percentage contributions made by N-methyl-D-aspartate (NMDA), kainate and quisqualate receptors to the composite depolarizations evoked by L-cysteate, L-cysteinesulphinate, L-homocysteate and S-sulpho-L-cysteine in the rat olfactory cortex slice. The percentage contribution made by NMDA receptors, which was quantified by measuring the reduction in agonist responses in the presence of the highly selective NMDA receptor antagonist 2-amino-5-phosphonopentanoate (0.1 mM), was: L-homocysteate, 73%; S-sulpho-L-cysteine, 65%; L-cysteate, 42% and L-cysteinesulphinate, 30%. Responses mediated by NMDA, kainate and quisqualate receptors were abolished by a 'desensitization' procedure involving repeated application of a mixture containing high concentrations of the selective agonists followed by perfusion of the non-selective receptor antagonist cis-2,3-piperidine dicarboxylate (5 mM). Following this procedure, responses to L-homocysteate and S-sulpho-L-cysteine were almost abolished and simple calculation gave the contribution of kainate plus quisqualate receptors to the agonist responses as: L-cysteinesulphinate, 46%; L-cysteate, 34%; S-sulpho-L-cysteine, 28% and L-homocysteate, 23%. However, approximately 24% of the composite depolarizations evoked by L-cysteate and L-cysteinesulphinate was mediated by a mechanism not involving NMDA, kainate or quisqualate receptors, neither did it reflect possible electrogenic uptake of the amino acids nor an interaction with 2-amino-4-phosphonobutyrate receptors. It is suggested that this fraction of the depolarizations evoked by L-cysteate and L-cysteinesulphinate might be due to a non-receptor-mediated release of K+ or, perhaps, to activation of an as yet unidentified receptor category.