Effects of urethane and ketamine on substance P- and excitatory amino acid-induced behavior in mice

The anesthetic urethane blocks excitatory amino acid responses but not GABA responses in isolated frog spinal cords

PURPOSE

The anesthetic urethane is commonly used in physiological experiments. We tested urethane's actions on GABA receptors on the primary afferents in the spinal cord, which are one of the few areas in the adult central nervous system (CNS) that are depolarized by GABA, and on ligand-gated excitatory amino acid (EAA) receptors located on motoneurons. Both receptor types are critically important during anesthetic immobilization.

METHODS

We used the isolated hemisected spinal cord of the frog in a sucrose gap chamber to record glutamate-, N-methyl-D-aspartate (NMDA)-, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-, kainate-, and gamma-aminobutyric acid (GABA)-induced depolarizations of the dorsal root (DR) and ventral root (VR). DR potentials (DRPs) and VR potentials (VRPs) evoked by single supramaximal afferent stimuli were also studied. Urethane (10-80 mM) was applied for 10-30 min.

RESULTS

Urethane depressed EAA responses on the motoneurons in a dose-dependent manner. At a clinical anesthetic concentration (10 mM), EAA-induced depolarizations were reduced by 8.1 ± 2.2 % (n = 7, P = 0.025), but increasing the concentration to 40 mM revealed a larger, 24.7 ± 3 % (n = 53, P = 0.0001) depressing effect of urethane on all EAA responses in the motoneurons. However, GABA and K(+) responses recorded in the DR were not altered by the presence of 10 or 40 mM urethane. Evoked DRPs and VRPs were reduced by urethane and spontaneous DR and VR potentials were suppressed by 10 or blocked by 40 mM urethane.

CONCLUSION

Urethane appears to be selective for EAA-, sparing GABA responses at a clinical anesthetic concentration. Only a 10 % reduction of EAA activity seems to be necessary to induce anesthesia.

Associative cortex features in the first olfactory brain relay station

Synchronized firing of mitral cells (MCs) in the olfactory bulb (OB) has been hypothesized to help bind information together in olfactory cortex (OC). In this survey of synchronized firing by suspected MCs in awake, behaving vertebrates, we find the surprising result that synchronized firing conveys information on odor value ("Is it rewarded?") rather than odor identity ("What is the odor?"). We observed that as mice learned to discriminate between odors, synchronous firing responses to the rewarded and unrewarded odors became divergent. Furthermore, adrenergic blockage decreases the magnitude of odor divergence of synchronous trains, suggesting that MCs contribute to decision-making through adrenergic-modulated synchronized firing. Thus, in the olfactory system information on stimulus reward is found in MCs one synapse away from the sensory neuron.

Persistent facial pain increases superoxide anion production in the spinal trigeminal nucleus

Previous studies have demonstrated that there is an increase in oxidative stress in the cerebral cortex of rats after repeated painful stimulation and that long-lasting pain increases the production of superoxide ion (O(2) (-)), nitric oxide and peroxynitrite due to the activation of AMPA and NMDA receptors. The purpose of the present study was to evaluate the possible role of O(2) (-) in the transmission of oro-facial pain. Formaldehyde 1% was injected subcutaneously into one vibrissal pad of adult male Sprague-Dawley rats as a model of persistent pain, then O(2) (-) production and superoxide dismutase (SOD) activity were evaluated in the left and right spinal trigeminal nuclei. O(2) (-) production was revealed using dihidroetidium (DHE) injected at 10 or 45 min after the formalin injection in conscious or anaesthetized rats. A histochemical assay for SOD was performed to evaluate the activity of SOD at 10 min after the formalin injection. The results showed a significant increase in O(2) (-) production in the homolateral nucleus at 45 min. However, there was no significant difference between the two sides at 10 min after the formalin injection. No significant difference was observed in SOD activity between the two sides of the spinal trigeminal nucleus. This study demonstrated that there is an increased production of O(2) (-) in the second phase but not in the first phase of the formalin test; thus O(2) (-) is involved in pain induced by inflammation, but not in acute pain.

Electrophysiological studies on the role of the NMDA receptor in nociception in the developing rat spinal cord

The present study investigated the effects of spinally applied N-methyl-D-aspartate (NMDA) antagonists 2-amino-5-phosphonovaleric acid (AP5) and ketamine on convergent neurones in the deep dorsal horn of rats, in vivo at different postnatal ages (P) 14, 21, 28, and 56 days. AP5 inhibited the primary afferent fibre input, the C fibre, post-discharge and windup evoked responses in a dose-dependent manner at each age, and was significantly more effective in the pups than adult rats (P<0.03 at 100-microg dose). AP5 100 microg abolished windup almost completely in the pups, whilst the adults required 10-fold higher doses. In contrast there was no difference in ketamine potency between age groups. Windup in the ketamine groups was reduced in a dose-dependent manner equally across all the age groups. The differential inhibitory effects of AP5 and ketamine may be due to postnatal changes in density, localisation and receptor subunit composition, altering receptor affinity and kinetics.

Single-unit and polygraphic recordings associated with systemic or local pharmacology: a multi-purpose stereotaxic approach for the awake, anaesthetic-free, and head-restrained rat

In order to avoid any artifactual pharmacological interferences with anaesthetic agents, a procedure has been developed for working on the awake, anaesthetic-free rat in a head-restrained condition. It allows, on the same animal and over several consecutive days, single-unit recordings in combination with systemic or local pharmacology (microiontophoresis or micropressure ejections), as well as monitoring vigilance states via the electroencephalogram and the electromyogram. After the cementing of a special "U"-shaped device on its skull under general anaesthesia, the animal is progressively habituated to stay daily, for several hours, under a painless corresponding stereotaxic restraint. This system can be easily adapted to different stereotaxic frames and, because of its spatial flexibility for targetting the desired rostrocaudal or lateral positions, allows access to a large number of cerebral structures. Experiments performed on Globus Pallidus, Substantia Nigra, and Locus Coeruleus neurons, combining the different possibilities of this system, are reported. They demonstrate, on the awake anaesthetic-free head-restrained rat, and under suitable ethical conditions, the feasibility of single-unit recordings of identified neurons associated with the study of their pharmacological reactivity after systemic or local drug administrations without any other drug interferences, and in physiologically relevant conditions such as the spontaneous alternance of vigilance states.

Effect of anesthetics on reflex micturition in the chronic cannula-implanted rat

It is well known that urethane is a suitable anesthetic for acute studies and has been extensively recommended for investigations related to micturition physiology. This is mainly because of the capability of urethane anesthesia to spare reflex micturition as well as its easily established long-lasting and stable anesthetic level. However, urethane anesthesia is usually restricted to acute experiments due to its potential toxicity. This study searched for an alternative to urethane that would be suitable for studies in which recovery from anesthesia was needed. The list of administered drugs was as follows: pentobarbital, thiobutabarbital, ketamine-acepromazine, ketamine-diazepam, tiletamine-zolazepam, fentanyl-droperidol, alphaxalone-alphadolone, propofol, isoflurane, methoxyflurane, azaperone, tribromoethanol, and buprenorphine. Among these drugs, only tiletamine-zolazepam spared the reflex micturition contractions. However, the duration of this anesthesia was too short (approximately 30 minutes) to complete the necessary testing and additional dosing of the anesthetic generally obliterated the micturition reflex. On the other hand, rats given i.v. urethane infusion (10% solution in 0.9% saline, 3.2-4.0 mg/kg/min, total dose 0.56-1.03 g/kg) maintained a stable anesthesia that permitted both reflex micturition and stereotaxic procedures. Rats moved spontaneously 3-16 hours after cessation of i.v. urethane anesthesia and completely recovered in 2 days without significant after-effects. Bladder function was normal. No pathological changes were seen 1 week later. The present results suggest that urethane is the most suitable anesthetic for acute and chronic physiological experiments that require demonstration of reflex micturition. Neurourol. Urodynam. 19:87-99, 2000.

Non-NMDA glutamate receptors modulate capsaicin induced c-fos expression within trigeminal nucleus caudalis

1. We examined the effects of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzol[f]quinoxaline-7-sulpho namide (NBQX), the kainate receptor antagonists gamma-(R-)-glutamylaminomethanesulphonic acid (GAMS) and 6,7,8,9-tetrahydro-5-nitro-1H-benz[g]indole-2,3-dione-3-oxime (NS-102), and the group III metabotropic glutamate receptor (mGluR) agonist 2-amino-4-phosphono-S-butanoic acid (L-AP4) on c-fos-like immunoreactivity (c-fos LI) in trigeminal caudalis (Sp5C), lateral reticular (LRt), medullary reticular (Md) and solitary tract (Sol) nuclei, after intracisternal injection of capsaicin in urethane anaesthetized Sprague-Dawley rats. 2. Few c-fos labelled cells were observed within Sp5C in capsaicin-vehicle treated animals. The number of positive c-fos cells increased by 17 fold after intracisternal capsaicin (5 nmol) administration. 3. Pretreatment with CNQX (0.02, 0.1, 0.6, 3 and 15 mg kg-1) or NBQX (0.01, 0.1 and 1 mg kg-1), administered intraperitoneally 15 min before capsaicin, significantly reduced labelled cells within Sp5C by a maximum of 45 and 34%, respectively. The number of c-fox LI cells within LRt, Md and Sol was not affected. Pretreatment with L-AP4 (1, 3 and 10 mg kg-1) decreased the number of Sp5C c-fos LI cells by a maximum of 30%, whereas GAMS (1 and 10 mg kg-1) and NS-102 (1 and 5 mg kg-1) did not show any significant effect. 4. These results suggest that blockade of AMPA receptors, but not kainate receptors, or the activation of group III mGluRs, decrease the response of Sp5C neurons to trigeminovascular activation. Thus, in addition to NMDA receptors, mGluRs and AMPA receptors may modulate cephalic pain and may provide a potential therapeutic target for antimigraine drugs.

Effects of MK-801 on bladder overactivity in rats with cerebral infarction

PURPOSE

Our objective was to evaluate the underlying mechanisms of neurogenic voiding dysfunction following cerebral infarction.

MATERIALS AND METHODS

The left middle cerebral artery (MCA) was occluded using 4-0 monofilament nylon thread in male S-D rats. Cystometric examination was performed in unanesthetized and urethane-anesthetized rats through a catheter chronically implanted in the dome of the bladder.

RESULTS

Bladder capacity of unanesthetized or urethane anesthetized rats was significantly reduced just after occlusion of the left MCA; 2 weeks after the occlusion, the capacity was less than half that in sham-operated rats. Intravenous administration of N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 to the unanesthetized sham-operated rats led to a marked dose-dependent decrease in bladder capacity. Its administration to unanesthetized rats with cerebral infarction resulted in a slight decrease in bladder capacity. In the urethane-anesthetized state, the bladder capacity of the rats with cerebral infarction was significantly increased by MK-801, 0.1 mg./kg., without inhibiting the contraction pressure or increasing the amount of residual urine. A high dose (1 mg./kg.) of MK-801 was required to increase the bladder capacity of sham-operated rats. This led to an inhibition of contraction pressure and an increase in residual urine.

CONCLUSION

Results in urethane anesthetized rats indicate that NMDA glutamatergic transmission is important in the overactivity of the bladder following a cerebral infarction. This model is useful in studying the neurogenic voiding dysfunction observed in patients with cerebrovascular disease.

Influence of anesthesia on bladder hyperactivity induced by middle cerebral artery occlusion in the rat

The effect of anesthesia or an N-methyl-D-aspartate (NMDA) glutamatergic antagonist (MK-801) on bladder hyperactivity induced by unilateral middle cerebral artery (MCA) occlusion was examined in female rats. Before infarction, control bladder contractions were monitored in two groups of rats: 1) awake and 2) urethan anesthetized. The awake rats were then anesthetized with halothane, and MCA occlusion was performed in both groups. After recovery from halothane, bladder capacity in awake rats was significantly reduced (60.8 +/- 1.3%) 0.5-4.5 h after MCA occlusion but not changed in urethan-anesthetized rats. MK-801 (0.1 mg/kg i.v.) administered before MCA occlusion blocked the reduction in bladder capacity in awake rats 1.5-4.5 h after MCA occlusion. Bladder capacity was not changed by sham operation in either the awake or urethan-anesthetized rats. Urethan administered after recovery from halothane anesthesia increased bladder capacity in MCA-occluded awake rats but not in sham-operated awake rats. Infarct areas in halothane, urethan-anesthetized, or MK-801-treated rats were not significantly different. These results indicate that cerebral infarction induces bladder hyperactivity in awake rats and that urethan or MK-801 inhibits the development of this hyperactivity, most likely by blocking glutamatergic transmission in the brain.

Effect of an NMDA receptor antagonist on the wind-up of neurons in the trigeminal oralis subnucleus

Extracellular recordings of convergent neurons of the oralis subnucleus of the trigeminal sensory complex were performed in paralysed rats under halothane-N2O-O2 anesthesia using glass micropipettes. The effects of MK801, a noncompetitive NMDA receptor antagonist, were observed on the increased cell activity (wind-up), triggered by the repetition, at a low frequency (0.66 Hz) and high intensity (3 times the threshold of C-fiber response), of electrical stimulation of the receptive field. Successive cumulative doses (up to 1 mg/kg) of MK801 i.v. resulted in a dose-dependent decrease in the responses related to C-fiber input (11 cells). A single dose of 1 mg/kg applied in four cells had effects similar to the 1 mg/kg dose given cumulatively. Three units were either weakly or not modified by MK801. In a second experiment, recordings were performed in 12 cells for 80 min after an injection of a small dose of MK801 (0.15 mg/kg). C input was not significantly modified by the antagonist. The effects of MK801 on the first part of the wind-up response (wind-up proper) peaked between 15 and 50 min and returned to control values at about 80 min. The effects on the postdischarge followed approximately the same time course. It is concluded that despite being devoid of substantia gelatinosa, the oralis subnucleus contains neurons that display an NMDA receptor-linked wind-up similar to the phenomenon described in the dorsal horn of the spinal cord.

Interactions between NMDA and AMPA/kainate receptors in the control of micturition in the rat

In unanesthetized decerebrate rats, GYKI 52466 (1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride), an AMPA/kainate receptor antagonist, and MK-801 (dizocilpine), an NMDA receptor antagonist, acted synergistically to depress the micturition reflex. MK-801 (1 mg/kg i.v.) and GYKI 52466 (4 mg/kg i.v.) administered separately had no or only a small depressant effect on reflex bladder contractions but markedly depressed external urethral sphincter activity. However, in MK-801-treated rats, GYKI 52466 decreased the amplitude, frequency and duration of reflex bladder contractions. These results suggest that both AMPA/kainate and NMDA glutamate receptors are important in the micturition reflex pathway and that these receptors may be activated in parallel at some site in the pathway so that excitatory transmission via only one receptor type is sufficient to mediate reflex activation of the bladder.

Effects of GYKI 52466 and CNQX, AMPA/kainate receptor antagonists, on the micturition reflex in the rat

GYKI 52466, a non-competitive AMPA/kainate glutamate receptor antagonist, administered in graded doses (0.5-8 mg/kg, i.v.) at 10 min intervals, decreased the amplitude and duration of reflex bladder contractions (maximal inhibition = 63%), the intercontraction interval (maximal inhibition = 83%) and external urethral sphincter activity (maximal inhibition = 91%) in urethane-anesthetized (1.2 g/kg, s.c.) intact rats during continuous transurethral cystometrograms. On the other hand, in unanesthetized decerebrate rats, the drug did not alter reflex bladder activity but did produce a significant depression of sphincter activity (maximal inhibition = 59%). The depressant effects of single doses of GYKI 52466 (4 mg/kg, i.v.) on external urethral sphincter EMG activity occurred with similar time courses in both urethane-anesthetized (1.2 g/kg, s.c.) intact and unanesthetized decerebrate rats during continuous transurethral cystometrograms. In urethane-treated (0.6 g/kg, i.p.) decerebrate rats, GYKI 52466 (0.5-4 mg/kg, i.v.) depressed bladder contraction amplitude and sphincter EMG activity, similar to the effects in urethane-anesthetized (1.2 g/kg, s.c.) intact rats. CNQX (0.01-1 mg/kg, i.v.), a competitive AMPA/kainate receptor antagonist, administered to urethane-anesthetized (1.2 g/kg, s.c.) intact or unanesthetized decerebrate rats did not alter the bladder or the external urethral sphincter activity during continuous transurethral cystometrograms, possibly due to the inability of the drug to readily cross the blood-brain barrier. The present results indicate that glutamatergic excitatory transmission mediated by AMPA/kainate receptors is essential for the activation of external urethral sphincter activity during micturition in anesthetized and unanesthetized preparation. However, the depressant effect of GYKI 52466 on reflex bladder activity is only unmasked by urethane anesthesia, raising the possibility that urethane interacts with AMPA/kainate glutamate receptors in the spinobulbospinal micturition reflex pathway.

The effect of urethane anesthesia on evoked potentials in dentate gyrus

We examined the effect of urethane (1000 mg/kg, followed by 50 mg/kg per h, i.v.), an anesthetic commonly used by physiologists, on evoked potentials recorded in dentate gyrus in adult Wistar rats by stimulating the ipsilateral perforant path, via chronically implanted electrodes. Urethane decreased paired-pulse inhibition. Under urethane, with paired-pulse stimulation, the ratio of the second population spike amplitude to the first increased by 11.1-20.7% at 25-60 ms interstimulus interval (n = 18, P < 0.05). At 25 ms, the proportion was 3.6 +/- 1.6 while awake, and 14.7 +/- 3.5 under urethane. Urethane depressed granule cell excitability and strength of synaptic responses. Under urethane, the ratio of the population spike amplitude obtained at 250 microA stimulation to the maximal response in the same input/output response examination decreased by 20%, and the ratio of the excitatory postsynaptic response slopes fell by 10%. These results indicate that urethane affects neurotransmission in the hippocampus, and suggest that its effect may be exerted in part on excitatory neurotransmission.

Urethane reduces contraction to 5-hydroxytryptamine (5-HT) and enhances the action of the 5-HT antagonist ketanserin on the rat thoracic aortic ring

The general anesthetic urethane (ethyl carbamate) is widely used in electrophysiological in vivo experiments. However, its pharmacological effects are poorly understood. Here, the effects of urethane on in vitro contractile responses of the rat thoracic aortic ring preparation were investigated. Bath application of 5-HT produced a concentration-dependent contractile response (EC50 = 4.3 x 10(-6) M). Urethane (11.2 mM = 1 mg/ml) shifted the concentration-response curve (CRC) for 5-HT to the right (EC50 = 1.7 x 10(-5) M) and decreased the maximal contraction by 30.8%. The CRC for NA (EC50 = 7.2 X 10(-9)M) was also shifted to the right by urethane (EC50 = 1.4 X 10(-8)M), but the shift of the 5-HT-CRC was twice that of the NA-CRC (3.95 vs. 1.95). The CRC to KCl was shifted rightwards only slightly by urethane (ratio 1.27) and the maximal contraction to KCl was not affected. The CRC to replacement of CaCl2 (0.1-10 mM) to KCl-depolarized vessels in a Ca(2+)-free Krebs solution was unaffected by urethane. Ketanserin (10(-9)M) antagonized the contraction to 5-HT, and a combination of ketanserin and urethane was markedly more effective than either drug alone, decreasing the maximal contraction by 58%. Antagonism of NA contraction by prazosin (5 X 10(-8)M) was not increased by addition of urethane. The urethane dose used here approximates blood and brain concentrations required to produce anesthetic effects in mammals. It is possible that reductions in 5-HT transmission and, to a lesser extent, in NA transmission, but not blockade of Ca2+ or K+ channels, may contribute to the anesthetic effect of urethane. In addition, the action of the selective 5-HT2 antagonist ketanserin is clearly altered by urethane. These findings are important to consider when urethane is used for in vivo neurophysiological investigations, particularly when 5-HT mechanisms are involved.

Alteration by urethane of glutamatergic control of micturition

The i.v. administration of MK-801 (0.001-3 mg/kg), a non-competitive NMDA receptor antagonist, did not alter reflex bladder activity in unanesthetized decerebrate rat recorded during fast infusion (0.21 ml/min) cystometry or under isovolumetric conditions, but did depress reflex bladder contractions in doses between 0.1 and 3 mg/kg i.v. in the urethane-anesthetized (1.2 g/kg s.c.) intact rat during fast infusion cystometry. The ED50 and the dose to produce maximal inhibition in urethane-anesthetized intact rats were 0.25 mg/kg and 3 mg/kg i.v., respectively. During slow infusion (0.04 ml/min) cystometry, in unanesthetized decerebrate rats, MK-801 (0.1-1 mg/kg i.v. or 6-60 micrograms i.t.) decreased by 12-44% the micturition volume threshold (VT) but did not change the amplitude and duration of the bladder contractions. The administration of a larger i.t. dose (60 micrograms) of MK-801 produced no further decrease in VT but decreased the amplitude of bladder contractions by 24%. External urethral sphincter electromyogram activity was reduced or abolished by MK-801 (0.01-3 mg/kg i.v.) in both unanesthetized decerebrate and urethane-anesthetized intact rats with ED50 of 0.12 mg/kg and 0.05 mg/kg, respectively. These results indicate that NMDA receptors play an important role in both facilitatory and inhibitory central neural control of voiding function and that there is a significant interaction between urethane anesthesia and NMDA glutamatergic transmission. Thus, even though urethane anesthesia has been useful for studying the physiological characteristics of the micturition reflex, it seems inappropriate for analyzing the normal transmitter role of glutamic acid in reflex voiding.

Inhibitory effects of salmon calcitonin on the tail-biting and scratching behavior induced by substance P and three excitatory amino acids

We have examined the effects of salmon calcitonin (SCT), injected into the cerebral ventricle (i.c.v.), on the tail-biting and scratching behavior induced by the intrathecal injection of different types of nociceptive agents, i.e., substance P, N-methyl-D-aspartate (NMDA), kainate (KA), and quisqualate (Quis). Tail-biting and scratching behavior induced by the 4 substances was significantly inhibited by SCT (i.c.v.) in the same manner: the dose-response curves were U-shaped, and the most effective dose was 0.1 IU/mouse in all cases. SCT did not, however, completely inhibit tail-biting and scratching behavior. At its most effective dose, the percent inhibition of substance P-, NMDA-, KA- and Quis-induced behavior were 77.9%, 40.2%, 49.4%, and 52.9%, respectively. These results suggest that SCT has the inhibitory effects of substance P- and glutamate receptor agonists-induced nociceptive response in vivo.

Urethane anesthesia reverses the protective effect of noncompetitive NMDA receptor antagonists against cocaine intoxication

The present experiments examined whether pretreatment with the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists, MK-801 and dextrorphan, could antagonize cocaine-induced convulsions and lethality in conscious Sprague-Dawley (SD) rats and whether urethane anesthesia alters the observed interactions. Conscious, restrained male SD rats received continuous i.v. infusions of cocaine hydrochloride (1.25 mg/kg.min) until convulsions and death occurred. Cocaine doses of 21.2 +/- 1.8 and 29.5 +/- 2.5 mg/kg caused convulsions and death, respectively, in saline treated rats (n = 8). Convulsions were absent in MK-801 (1 mg/kg, i.v.; n = 8) pretreated rats; the lethal cocaine dose was 44.0 +/- 2.7 mg/kg (p < 0.05). In contrast, urethane anesthesia (1.2 g/kg, i.p.) decreased the dose of cocaine required to cause toxicity, compared to that in saline controls (24.8 +/- 0.8 mg/kg, n = 13), in MK-801 (2.0 +/- 0.3, n = 7; p < 0.01) and in dextrorphan mg/kg, n = 13), in MK-801 (2.0 +/- 0.3, n = 7; p < 0.01) and in dextrorphan (25 mg/kg, i.v.; 13.1 +/- 1.4, n = 6; p < 0.01) pretreated rats. Pressor responses with little change in heart rate were evident during cocaine infusion in vehicle pretreated rats. Bradycardiac responses were noted to cocaine in groups following NMDA receptor blockade. Reversal of the pressor response to cocaine was noted in MK-801 pretreated animals, while dextrorphan pretreatment moderated cocaine-induced increases in blood pressure. Ventilatory support protected against cocaine lethality in urethane anesthetized rats, indicating that respiratory failure is the proximate cause of death with cocaine infusion. However, artificially ventilated rats, pretreated with MK-801, were more sensitive (lethal cocaine dose, 76.6 +/- 8.0 mg/kg, n = 5) than vehicle pretreated rats (129.4 +/- 15.8 mg/kg, n = 6), indicating that MK-801 may increase both the respiratory and the cardiac toxicity of cocaine in urethane anesthetized rats. Interactions between NMDA receptors and cocaine are modified by urethane anesthesia.

Excitatory amino acid receptor mediation of sensory inputs to functionally identified dorsal horn neurons in cat spinal cord

As excitatory amino acid receptors have been implicated in nociceptive sensory transmission, the principal objective of the present study was to investigate the effects of various excitatory amino acid antagonists on naturally evoked responses in spinal dorsal horn neurons. Extracellular single unit activity was recorded from functionally identified, spinal dorsal horn neurons in unanesthetized, decerebrated cats and in alpha-chloralose-anesthetized cats. The tests included iontophoretic application of the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV), the non-N-methyl-D-aspartate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and kynurenate, and also the intravenous administration of the N-methyl-D-aspartate receptor antagonist, ketamine. In addition, attempts were made to determine the effects on these neurons of iontophoretic application of the excitatory amino acid agonists, L-glutamate, N-methyl-D-aspartate, quisqualate, (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and domoate. Marked differences were noted in the actions of agonists and antagonists between the responses observed in the unanesthetized, decerebrated and the anesthetized animals. In decerebrated cats, responses to hair afferent stimulation were blocked by kynurenate, 6-cyano-7-nitroquinoxaline-2,3-dione and 2-amino-5-phosphonovaleric acid. Responses to noxious thermal stimulation were attenuated by 2-amino-5-phosphonovaleric acid and in one unit also by ketamine. Neither 6-cyano-7-nitroquinoxaline-2,3-dione nor kynurenate affected the responses to noxious thermal stimulation. The proportion of cells responding to the agonists were: N-methyl-D-aspartate 24/27 (89%), quisqualate 12/13 (92%) and domoate 6/7 (86%). In chloralose-anesthetized cats, responses to hair afferent stimulation were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and kynurenate but not by 2-amino-5-phosphonovaleric acid. Responses to noxious thermal stimulation were not affected by any of these antagonists, while the response to non-noxious thermal stimulation was blocked by 2-amino-5-phosphonovaleric acid, ketamine and kynurenate in the one neuron studied. The proportion of cells excited by the agonists differed from those observed in decerebrated cats: N-methyl-D-aspartate 9/32 (28%), quisqualate 50/54 (93%), (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate 19/23 (83%) and domoate 17/38 (45%). Application of the putative endogenous excitatory amino acid precursor N-acetyl-aspartyl-glutamate (NAAG) did not elicit a response in any of the neurons studied.(ABSTRACT TRUNCATED AT 400 WORDS)

Potentiation by choline of basal and electrically evoked acetylcholine release, as studied using a novel device which both stimulates and perfuses rat corpus striatum

We examined the release of acetylcholine (ACh) and dopamine (DA) using a novel probe through which striatal neurons could be both superfused and stimulated electrically in both anesthetized and freely moving awake animals. Optimal stimulation parameters for eliciting ACh release from cholinergic neurons differed from those required for eliciting DA release from dopaminergic terminals: at 0.6 ms pulse duration, 20 Hz and 200 microA, ACh release increased to 357 +/- 30% (P < 0.01) of baseline and was blocked by the addition of tetrodotoxin (TTX). Pulse durations of 2.0 ms or greater were required to increase DA release. Unlike ACh release, DA release showed no frequency dependence above 5 Hz. The maximal evoked releases of ACh and DA were 556 +/- 94% (P < 0.01) and 254 +/- 38% (P < 0.05) of baseline, respectively. Peripheral administration of choline (Ch) chloride (30-120 mg/kg) to anesthetized animals caused dose-related (r = 0.994, P < 0.01) increases in ACh release; basal release rose from 117 +/- 7% to 141 +/- 5% of initial baseline levels (P < 0.05) and electrically evoked ACh release rose from 386 +/- 38% to 600 +/- 34% (P < 0.01) in rats given 120 mg/kg. However, Ch failed to affect basal or evoked DA release although neostigmine (10 microM) significantly elevated basal DA release (from 36.7 fmol/10 min to 71.5 fmol/10 min; P < 0.05). In awake animals, Ch (120 mg/kg) also elevated both basal (from 106 +/- 7% to 154 +/- 17%; P < 0.05) and electrically evoked (from 146 +/- 13 to 262 +/- 16%; P < 0.01) ACh release.(ABSTRACT TRUNCATED AT 250 WORDS)

Arousal-dependent properties of medial septal neurons in the unanesthetized rat

We have performed a qualitative and quantitative analysis of the electrophysiological properties of medial septal neurons in the unanesthetized rat. The rat's head was held in a stereotaxic apparatus by a painless head-restrained system that was implanted seven days prior to the recording sessions. Extracellular recordings were made in a mixed population of antidromically identified septohippocampal neurons and unidentified medial septal neurons in different states of arousal and in response to peripheral and reticular stimulations. The spontaneous activity as well as the percentage of rhythmically bursting septal neurons varied significantly according to the state of arousal. Higher values were noted in paradoxical sleep (28 imp/s and 94% of bursting neurons) as compared with wakefulness with hippocampal theta rhythm (17.4 imp/s and 64.2% of bursting neurons) and slow wave sleep (12.3 imp/s and 8% of bursting neurons). The frequency of the bursts was significantly higher during paradoxical sleep. In individual medial septal neurons, arousing stimuli and paradoxical sleep could induce rhythmic bursting activity in previously non-bursting neurons provided that they were fast-firing neurons. No differences were noted in the functional characteristics of neurons in the medial septal nucleus as compared with the diagonal band of Broca. When the unanesthetized rats were compared with a group of urethane-anesthetized rats, the spontaneous activity was higher and more irregular in the absence of anesthesia. The percentage of the bursting neurons was significantly lower in the unanesthetized rats (32.3% vs 43.3%). However, the frequency of the bursts was higher (5.9 +/- 0.1 Hz vs 3.5 +/- 0.1 Hz). Since the patterns of activity of medial septal neurons fluctuate in different physiologically relevant states, previous classifications of these neurons made by ourselves and other authors, in urethane-anesthetized rats, may not be appropriate.

Low doses of urethane effectively inhibit spinal seizures evoked by sudden cooling of toad isolated spinal cord

The effect of low doses of urethane on three phases of spinal seizures evoked by sudden cooling (SSSC) of toad isolated spinal cord was studied. In control toads, SSSC began with a latency of 91 +/- 3 sec (mean +/- S.E.M.) exhibiting brief tremors, followed by clonic muscle contractions and finally reaching a tonic contraction (tonic phase). The latency of onset of seizures was significantly enhanced. The tonic phase was markedly abolished in toads pretreated intralymphatically with 0.15 g/kg of urethane. Tremors were the only phase observed in 55% of toads that received doses of 0.2 g/kg, and a total blockade of seizures was seen after doses of 0.25 g/kg of urethane in 50% of the preparations. A possible depressant effect of urethane on transmission mediated by excitatory amino acids is suggested.

Evidence that activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors within the nucleus tractus solitarii triggers swallowing

Swallowing is a patterned motor activity generated by neurons located within the nucleus tractus solitarii (NTS). Previous experiments have shown that administration of excitatory amino acids within the NTS induces swallowing. The present study was undertaken to identify the receptor subtypes involved in this effect. Pressure microinjections of L-glutamate (10-100 pmol), quisqualate (0.1-10 pmol) and N-methyl-D-aspartate (NMDA, 0.1-10 pmol) were performed into the NTS of decerebrate rats. Glutamate and quisqualate microinjections elicited short series of swallows while NMDA microinjections induced long-lasting, rhythmic swallowing. Pretreatment with the selective NMDA antagonist, DL-2-amino-5-phosphonovalerate (50 pmol), almost completely suppressed the response elicited by NMDA (10 pmol) but did not induce a significant modification of swallowing triggered by either glutamate (25 pmol) or quisqualate (10 pmol). Pretreatment with 6-cyano-7-nitroquinoxaline-2,3-dione (50 pmol), a selective blocker of non-NMDA receptors, suppressed the swallows elicited by glutamate and strongly inhibited the response elicited by quisqualate microinjections. The same pretreatment induced only a slight modification of the swallowing elicited by NMDA. These data demonstrate that deglutition can be triggered by activating either NMDA or non-NMDA receptors localized within the NTS, and therefore suggest that both receptor subtypes may be involved in swallowing elicited under physiological conditions.