Epinephrine and norepinephrine modulate neuronal responses to excitatory amino acids and agonists in frog spinal cord

Chemoarchitecture of the anuran auditory midbrain

The anuran torus semicircularis consists of several subnuclei that are part of the ascending auditory pathway as well as audiomotor interface structures. Additionally, recent anatomical studies suggest that the midbrain tegmentum is an integral part of the audiomotor network. To describe the chemoarchitecture of these nuclei, taking into account the toral subdivisions, we investigated the distribution of serotonin, leucine-enkephalin, substance P, tyrosine-hydroxylase, dopamine D2-receptor, parvalbumin, aspartate, GABA, and estrogen-binding protein-immunoreactivity in the midbrain of Bombina orientalis, Discoglossus pictus and Xenopus laevis. In the torus semicircularis, the highest density of immunoreactive fibers and terminals for all transmitters was found in the laminar nucleus. Parvalbumin-like immunoreactivity was highest in the principal nucleus, and D2-receptor-like immunoreactivity was uniformly distributed throughout the torus. In the tegmentum, axons and/or dendrites were stained with all antibodies except estrogen-binding protein. Additionally, heavily stained enkephalin and substance P-immunopositive fiber plexus were found in the lateral and dorsal tegmentum. The immunostainings revealed no qualitative differences between the three species. Immunopositive cell bodies were labeled in several brain areas, the connectivity of which with torus and tegmentum is discussed on the background of functional questions. The putative neuromodulatory innervation of both the laminar nucleus of the torus semicircularis and the tegmentum may be the anatomical basis for the influence of the animal's endogenous state on the behavioral reaction to sensory stimuli. These data corroborate earlier anatomical and physiological findings that the neurons of these nuclei are key elements in the audio-motor interface.

Excessive release of [3H]noradrenaline and glutamate in response to simulation of ischemic conditions in rat spinal cord slice preparation: effect of NMDA and AMPA receptor antagonists

In the present study we investigated the effects of NMDA and non-NMDA glutamate receptor antagonists on the ischemia-evoked release of [3H]noradrenaline from rat spinal cord slices. An in vitro ischemia model (oxygen and glucose deprivation) was used to simulate the ischemic conditions known to cause neuronal injury. Spinal cord slices were loaded with [3H]noradrenaline and superfused with Krebs solution in a micro-organ bath. Both axonal stimulation and ischemia increased the release of [3H]noradrenaline, but the release in response to glucose and oxygen deprivation was [Ca2+]o independent. Dizocilpine (MK-801), an NMDA receptor antagonist, suppressed the release of [3H]noradrenaline produced by ischemia, while it enhanced the release of [3H]noradrenaline evoked by electrical field stimulation. In contrast, LY300168 (GYKI-53655) [(+/-)-3-N-methylcarbamyde-1-(4-aminophenyl)-4-methyl-1.8-methylen e-dioxy-5H-2.3-benzodiazepine] and its (-)isomer LY303070 (GYKI-53784) [(-)-3-N-methylcarbamyde-1-(4-aminophenyl)-4-methyl-1.8-methylene- dioxy-5H-2.3-benzodiazepine] AMPA receptor antagonists, had no effect on the release of [3H]noradrenaline evoked by either electrical stimulation or ischemia. Desipramine, a noradrenaline uptake inhibitor, potentiated the release of [3H]noradrenaline evoked by ischemia, while in the absence of [Ca2+]o but under conditions when [3H]noradrenaline release was further increased, it reduced the release. Dizocilpine also decreased glutamate and aspartate release, measured by high performance liquid chromatography, during ischemia. It is concluded that glutamate release and NMDA receptors, but not AMPA receptors, are involved in the acute effect of oxygen and glucose deprivation on the excessive release of noradrenaline and that this release is not related to physiological axonal conduction.

NMDA and AMPA receptors evoke transmitter release from noradrenergic axon terminals in the rat spinal cord

N-methyl-D-aspartate (NMDA) stimulated release of [3H]noradrenaline (NA) from prelabelled rat spinal cord slices. The release was partially insensitive to tetrodotoxin (TTX) and was inhibited by the NMDA antagonist MK-801. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) also evoked release of [3H]NA, which was enhanced by blocking AMPA receptor desensitization with cyclothiazide. AMPA-evoked release was inhibited by the non-NMDA antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)-quinoxaline (NBQX) but was not affected by TTX. NMDA and AMPA showed synergistic effects, indicating co-existence of NMDA and AMPA receptors on noradrenergic terminals. Kainate evoked [3H]NA release only at high concentrations and the release was not potentiated by blocking kainate receptor desensitization with concanavalin A. Thus, the results indicate that there are stimulatory presynaptic NMDA and AMPA receptors on noradrenergic axon terminals in the spinal cord and that they interact synergistically to evoke release of [3H]NA.

Na+ channel block prevents the ischemia-induced release of norepinephrine from spinal cord slices

The principal finding of the present study with rat spinal cord slices was the novel demonstration of the [Ca2+]o-independent effect of ischemia on norepinephrine release and its antagonism by tetrodotoxin and low temperature (10 degrees C). Our finding that tetrodotoxin antagonized the effects of glucose deprivation on norepinephrine release in a [Ca2+]o-independent way suggests that Na+ channel block alone, i.e., the prevention of Na+ accumulation, may account for the protective action. Low temperature completely prevented the effect of ischemia on norepinephrine release but did not change the release associated with axonal activity. This finding is in good agreement with the observation that small changes in brain temperature critically determine the extent of neuronal injury from ischemia and suggests that both [Ca2+]o-independent release and cell injury are associated with the norepinephrine membrane carrier. It is suggested, therefore, that drugs able to attenuate the increase in [Na+]i during ischemia may be useful agents to protect against ischemic damage if given before the insult.

Clonidine antagonizes pressor effect of N-methyl-D-aspartate in the rostral ventrolateral medulla of rats

The purpose of the present study was to investigate the modulatory actions of adrenoreceptor agonists on N-methyl-D-aspartate (NMDA)-induced pressor effect in rostral ventrolateral medulla (RVLM). These drugs were locally applied into RVLM of urethane-anesthetized Sprague-Dawley rats through multibarrel pipettes. Microinjection of NMDA increased the arterial pressure, an effect which was abolished by pretreatment with clonidine, whereas neither the beta-adrenergic agonist isoproterenol nor the alpha 1-adrenergic agonist phenylephrine did alter this pressor response. Previous experiments demonstrated that clonidine binds to noradrenergic alpha 2 and imidazoline receptors in the RVLM. Norepinephrine, which has high affinity for the alpha 2 receptor and low affinity to the imidazoline receptor, partially antagonized NMDA-induced hypertension. On the other hand, administration of selective imidazoline receptor antagonist idazoxan partially reversed clonidine-mediated antagonism of NMDA. Taken together, these results suggest that clonidine may modulate the excitatory amino acid-induced pressor response through noradrenergic alpha 2 and imidazoline receptors in the RVLM.

Changes in neurotransmitter sensitivity in the mouse neocortical slice following propranolol and theophylline administration

1. The mouse neocortical slice has been used to examine the sensitivity of neurones to isoprenaline, 5-hydroxytryptamine (5-HT) and adenosine acutely and following chronic treatment of animals with propranolol or theophylline. 2. While having little effect alone, all three agonists enhanced the d.c. depolarizing potential produced by N-methyl-D-aspartate (NMDA). The effect of (-)-isoprenaline (0.2 microM) was shared by (+)-isoprenaline at the much higher concentration of 10 microM. 3. Superfusion of slices with theophylline or 8-phenyltheophylline blocked responses to adenosine with evidence of selectivity. A single injection of theophylline 24 h before slice preparation did not alter agonist sensitivity, but when administered daily at 100 mg kg-1 for 14 days, the xanthine caused a loss of sensitivity to adenosine and (-)-isoprenaline but not 5-HT. The lower dose of theophylline, 10 mg kg-1 daily, also led to a loss of adenosine responses but no change of sensitivity to the amines. 4. Following the 14 day treatment with theophylline at 100 mg kg-1 daily in two groups of mice, responses to adenosine recovered to control levels after 20 days. 5. Propranolol superfusion blocked responses to both isomers of isoprenaline and 5-HT but did not affect sensitivity to adenosine. 6. Chronic treatment with propranolol at 25 mg kg-1 daily for 14 days induced a loss of sensitivity to (-)-isoprenaline and 5-HT but not adenosine. A lower dose of 5 mg kg-1 daily caused no change in responses to adenosine or 5-HT, but yielded an increased sensitivity to (-)-isoprenaline. 7. The results are discussed with respect to reports of receptor up-regulation in binding studies; caution is clearly required in extrapolating from such work to receptor activity in a functional system, especially in the case of theophylline and adenosine.

The alpha 2 adrenergic agonist, dexmedetomidine, selectively attenuates ischemia-induced increases in striatal norepinephrine concentrations

This study was designed to evaluate the ability of a selective alpha 2 adrenergic agonist (dexmedetomidine) to attenuate ischemia-induced increases in striatal norepinephrine, 3 methoxy-4-hydroxyphenethyleneglycol (MHPG), dopamine, and 5-hydroxyindoleacetic acid (5-HIAA). Following the induction of anesthesia with halothane and oxygen, microdialysis catheters were stereotactically inserted into the striatum of 9 New Zealand white rabbits. Monitored variables included epidural temperature, arterial blood gases and pH, mean arterial pressure, blood glucose concentrations and the electroencephalogram. Following collection of baseline samples of dialysate, animals were randomized to receive a continuous infusion of saline (n = 4) or dexmedetomidine (n = 5). Cerebral ischemia was produced by the inflation of a neck tourniquet and induction of deliberate hypotension. Dialysate collection continued during the ischemic period and for the ensuing 140 min of reperfusion. All dialysate was frozen at -80 degrees C prior to its analysis by liquid chromatography for catecholamine content. There were no significant differences between the two groups for temperature, arterial blood gases, or mean arterial pressure. Blood glucose concentrations increased in the dexmedetomidine group. The electroencephalogram became isoelectric within 30 s of tourniquet inflation in all animals. Analysis of the norepinephrine and MHPG levels revealed significantly lower values for the dexmedetomidine-treated group during and following the ischemic period. There were no differences between groups for extracellular dopamine or 5-HIAA concentrations. These results suggest that the alpha 2 agonist dexmedetomidine can selectively attenuate ischemia-induced increases in striatal norepinephrine concentrations.

Assessment of the role of adrenoceptor function in ischemia-induced impairment of 2-deoxyglucose uptake and CA1 field potential in rat hippocampal slices

The release of catecholamines, dopamine and noradrenaline has been suggested to play a role in mediating ischemic damage in susceptible brain regions, the hippocampus and striatum. We now provide evidence that suggests a role for adrenoceptors in the deficit of 2-deoxyglucose uptake and CA1 field potential induced in hippocampal slices by hypoxia/hypoglycemia (ischemia). Treatment with alpha 1- or beta-adrenoceptor agonists or cAMP potentiated an ischemia-induced decline of both 2-deoxyglucose uptake and CA1 field potential in hippocampal slices, whereas alpha 1- or beta-adrenoceptor antagonists, or alpha 2-adrenoceptor agonists produced a remarkable neuroprotective action against these deficits. The results indicate that stimulation of adrenoceptors may play a detrimental role in the development of ischemic damage, and suggest a neuroprotective action for adrenoceptor antagonists, which may lessen the functional deficits induced by ischemia.

Electrolytic lesions of the locus coeruleus or 6-hydroxydopamine lesions of the medial forebrain bundle protect against excitotoxic damage in rat hippocampus

Unilateral lesioning of the rat locus coeruleus by an electric current (0.3 mA/10 s) reduces the ipsilateral hippocampal tissue content of dopamine (DA) and noradrenaline (NA) by 80% and 60% respectively, and protects against the excitotoxic action of bilateral kainate injections (1.1 nmol/microliters) in the hippocampus. On the ipsilateral side hippocampal CA1, CA2 and DG cells were protected; lesser protection of CA1 cells was seen on the contralateral side. The unilateral injection of 6-hydroxydopamine (8.8 micrograms/2 microliters/5 min) into the medial forebrain bundle abolishes the increase in extracellular NA concentration induced by kainate in the ipsilateral hippocampus without altering the low extracellular DA levels. This medial forebrain bundle lesion protects (on the ipsilateral side) against kainate toxicity (500 microM through the probe for 20 min) in the hippocampus (CA1, CA2 and DG cells). The noradrenergic innervation of the hippocampus apparently potentiates the excitotoxic effect of focal injection of kainate.

Direct evidence for acute and massive norepinephrine release in the hippocampus during transient ischemia

Recent studies suggest the norepinephrine (NE) may play a regulatory role in neuronal cell death in the hippocampus after transient ischemia. However, ischemia-induced changes in extracellular NE release have not been demonstrated. In the present study, we utilized the microdialysis technique to measure extracellular NE levels in the hippocampus before, during, and after 20 min of global ischemia induced by two-vessel occlusion combined with systemic hypotension in the rat. Stable basal concentrations of extracellular NE were detected in three consecutive samples collected prior to ischemia (1.86 +/- 1.21 pmol/ml of perfusate mean +/- SEM). During ischemia, NE levels increased to 30.1 +/- 5.5 pmol/ml, representing an 18-fold increase. The levels gradually returned to baseline by 40 min of reperfusion. These results are the first to demonstrate that acute and massive extracellular release of NE occurs in the hippocampus during ischemia and early recirculation. These results support the hypothesis that the activation of the noradrenergic system may play a significant role in modulating the development of ischemic neuronal damage.

Effect of ischemia on the in vivo release of striatal dopamine, glutamate, and gamma-aminobutyric acid studied by intracerebral microdialysis

We have previously described a marked attenuation of postischemic striatal neuronal death by prior substantia nigra (SN) lesioning. The present study was carried out to evaluate whether the protective effect of the lesion involves changes in the degree of local cerebral blood flow (ICBF) reduction, energy metabolite depletion, or alterations in the extracellular release of striatal dopamine (DA), glutamate (Glu), or gamma-aminobutyric acid (GABA). Control and SN-lesioned rats were subjected to 20 min of forebrain ischemia by four-vessel occlusion combined with systemic hypotension. Levels of ICBF, as measured by the autoradiographic method, and energy metabolites were uniformly reduced in both the ipsi- and contralateral striata at the end of the ischemic period, a finding implying that the lesion did not affect the severity of the ischemic insult itself. Extracellular neurotransmitter levels were measured by microdialysis; the perfusate was collected before, during, and after ischemia. An approximately 500-fold increase in DA content, a 7-fold increase in Glu content, and a 5-fold increase in GABA content were observed during ischemia in nonlesioned animals. These levels gradually returned to baseline by 30 min of reperfusion. In SN-lesioned rats, the release of DA was completely prevented, the release of GABA was not affected, and the release of Glu was partially attenuated. However, excessive extracellular Glu concentrations were still attained, which are potentially toxic. This, taken together with the previous neuropathological findings, suggests that excessive release of DA is important for the development of ischemic cell damage in the striatum.