An in vitro mature spinal cord preparation from the rat

Decerebrate mouse model for studies of the spinal cord circuits

The adult decerebrate mouse model (a mouse with the cerebrum removed) enables the study of sensory-motor integration and motor output from the spinal cord for several hours without compromising these functions with anesthesia. For example, the decerebrate mouse is ideal for examining locomotor behavior using intracellular recording approaches, which would not be possible using current anesthetized preparations. This protocol describes the steps required to achieve a low-blood-loss decerebration in the mouse and approaches for recording signals from spinal cord neurons with a focus on motoneurons. The protocol also describes an example application for the protocol: the evocation of spontaneous and actively driven stepping, including optimization of these behaviors in decerebrate mice. The time taken to prepare the animal and perform a decerebration takes ∼2 h, and the mice are viable for up to 3-8 h, which is ample time to perform most short-term procedures. These protocols can be modified for those interested in cardiovascular or respiratory function in addition to motor function and can be performed by trainees with some previous experience in animal surgery.

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.

An ex vivo preparation of mature mice spinal cord to study synaptic transmission on motoneurons

Mammalian spinal cord motoneurons are highly susceptible to chemical and mechanical disturbances, which imposes substantial difficulties for electrophysiological investigation in acute in vitro preparations. The aim of the present study was to establish an isolated spinal cord preparation from adult mice and to examine the synaptic activities of motoneurons in vitro. We removed the lumbo-sacral cord from the vertebral canal by hydraulic extrusion and maintained the isolated cord in vitro for extracellular recordings. Population spikes of motoneurons were evoked by electrical stimulation of dorsal roots (orthodromic) or ventral roots (antidromic) and these evoked responses could be continuously monitored for 5-6 h. The orthodromic population spikes were reversibly suppressed by the AMPA/kainate receptor antagonist 2,3-dihyro-6-nitro-7-sulfamoylbenzo quinoxaline (NBQX, 10 microM) but they persisted in the presence of the NMDA receptor antagonist D(-)-2-amino-5-phosphonovaleric acid (AP5, 50 microM). The antidromic population spikes exhibited evident paired pulse inhibition when evoked at inter-stimulus intervals of pound 6 ms. Histological examination revealed that basic morphological features of the lumbo-sacral motoneurons were preserved after 3-4 h of in vitro maintenance. This in vitro preparation is ideally suited for the electrophysiological study of synaptic transmission on adult mouse spinal motoneurons.

H-Reflex Operant Conditioning in Mice

Rats, monkeys, and humans can alter the size of their spinal stretch reflex and its electrically induced analog, the H-reflex (HR), when exposed to an operant conditioning paradigm. Because this conditioning induces plasticity in the spinal cord, it offers a unique opportunity to identify the neuronal sites and mechanisms that underlie a well-defined change in a simple behavior. To facilitate these studies, we developed an HR operant conditioning protocol in mice, which are better suited to genetic manipulation and electrophysiological spinal cord study in vitro than rats or primates. Eleven mice under deep surgical anesthesia were implanted with tibial nerve stimulating electrodes and soleus and gastrocnemius intramuscular electrodes for recording ongoing and stimulus-evoked EMG activity. During the 24-h/day computer-controlled experiment, mice received a liquid reward for either increasing (up-conditioning) or decreasing (down-conditioning) HR amplitude while maintaining target levels of ongoing EMG and directly evoked EMG (M-responses). After 3–7 wk of conditioning, the HR amplitude was 133 ± 7% (SE) of control for up-conditioning and 71 ± 8% of control for down-conditioning. HR conditioning was successful (i.e., ≥20% change in HR amplitude in the appropriate direction) in five of six up-conditioned animals (mean final HR amplitude = 139 ± 5% of control HR for successful mice) and in four of five down-conditioned animals (mean final HR amplitude = 63 ± 8% of control HR for successful mice). These effects were not attributable to differences in the net level of motoneuron pool excitation, stimulation strength, or distribution of HR trials throughout the day. Thus mice exhibit HR operant conditioning comparable with that observed in rats and monkeys.

Spastic long-lasting reflexes of the chronic spinal rat studied in vitro

Over the months following sacral spinal cord transection in adult rats, a pronounced spasticity syndrome emerges in the affected tail musculature, where long-lasting muscle spasms can be evoked by low-threshold afferent stimulation (termed long-lasting reflex). To develop an in vitro preparation to examine the neuronal mechanisms underlying spasticity, we removed the whole sacrocaudal spinal cord of these spastic chronic spinal rats (>1 mo after S(2) sacral spinal transection) and maintained it in artificial cerebral spinal fluid in a recording chamber. The ventral roots were mounted on monopolar recording electrodes in grease, and the reflex responses to dorsal root stimulation were recorded and compared with the reflexes seen in the awake chronic spinal rat. When the dorsal roots were stimulated with a single pulse, a long-lasting reflex occurred in the ventral roots, with identical characteristics to the long-lasting reflex in the awake spastic rat tail. The reflex response was low threshold (T), short latency, long duration ( approximately 2 s), and enhanced by repeated stimulation. Brief high-frequency stimulation trains (0.5 s, 100 Hz, 1.5 x T) evoked even longer duration responses (5-10 s), with repeated bursts of activity that were similar to the repeated muscle spasms evoked in awake rats with stimulation trains or manual skin stimulation. Stimulation of a given dorsal root evoked long-lasting reflexes in both the ipsilateral and contralateral ventral roots. Long-lasting reflexes did not occur in the sacrocaudal spinal cord of acute spinal rats (S(2) transection), which is similar to the areflexia seen in awake acute spinal rats. However, long-lasting reflexes could be made to occur in the acute spinal rat by altering K(+) (7 mM) or Mg(2+) (0 mM) concentrations, or by application of high doses of the neuromodulators norepinephrine (NE, >20 microM) or serotonin (5-HT, >20 microM). In chronic spinal rats, much lower doses of these neuromodulators (0.1 microM) enhanced the long-lasting reflexes, suggesting a denervation supersensitivity to 5-HT and NE following injury. Higher doses of NE or 5-HT produced a paradoxical inhibition of the long-lasting reflexes. The high dose inhibition by NE was mimicked by the alpha(2)-adrenergic receptor agonist clonidine but not the alpha(1)-adrenergic receptor agonist methoxamine. In summary, the sacral spinal in vitro preparation offers a new approach to the study of spinal cord injury and analysis of antispastic drugs.

Participation of AMPA- and NMDA-type excitatory amino acid receptors in the spinal reflex transmission, in rat

Classical in vitro and in vivo models and electrophysiological techniques were used to investigate the role of AMPA- and NMDA-type glutamate receptors in various components of spinal segmental reflex potentials. In the rat hemisected spinal cord preparation, the AMPA antagonists NBQX and GYKI 52466 abolished the monosynaptic reflex (MSR) potential but caused only partial inhibition of the motoneuronal population EPSP. NMDA antagonists had no noticeable effect on the MSR in normal medium, but markedly depressed the late part of EPSP. However, an NMDA receptor antagonist sensitive monosynaptic response was recorded in magnesium-free medium at complete blockade of the AMPA receptors. In spinalized rats, the AMPA antagonists completely blocked all components of the dorsal root stimulation evoked potential. MK-801 (2mg/kg, i.v.) reduced monosynaptic responses in a frequency dependent way, with no effect at 0.03 Hz and 22% inhibition at 0.25 Hz. The reduction of the di- and polysynaptic reflex components was about 30% and did not depend on stimulation frequency. Long-latency reflex discharge responses, especially when evoked by train stimulation, were more sensitive to MK-801 than the polysynaptic reflex. These results suggest that glutamate activates MSR pathways through AMPA receptors. However, under certain conditions, NMDA receptors can modulate this transmission through plastic changes in the underlying neuronal circuits. AMPA and NMDA receptors play comparable roles in the mediation of longer latency reflex components.

The novel antagonist 3-CBW discriminates between kainate receptors expressed on neonatal rat motoneurones and those on dorsal root C-fibres

1. The natural product willardiine is a selective AMPA receptor agonist. We report that an N(3)-substituted analogue of willardiine, (S)-3-(4-carboxybenzyl)willardiine 3-CBW, antagonizes AMPA and kainate receptors expressed on motoneurones and dorsal root C-fibres, respectively. 2. Reduction of the fast component of the dorsal root-evoked ventral root potential (fDR-VRP) has been used as a novel method to compare AMPA receptor antagonists. 3-CBW, NBQX and GYKI53655 depressed the fDR-VRP with IC(50) values of 10.3+/-2.4, 0.214+/-0.043 and 4.03+/-0.31 micro M, respectively. That 3-CBW depressed the fDR-VRP by acting at AMPA and not metabotropic glutamate receptors was demonstrated by the lack of effect of LY341495 (100 micro M). 3. The Schild plot for antagonism of responses to (S)-5-fluorowillardiine on motoneurones by 3-CBW had a slope of 1.11+/-0.13 giving a pA(2) value of 4.48. The Schild plot for antagonism of kainate responses on the dorsal root by 3-CBW had a slope of 1.05+/-0.05 giving a pA(2) value of 4.96. 4. On neonatal rat motoneurones 3-CBW (200 micro M) almost completely abolished responses to AMPA while responses to NMDA, kainate and DHPG were 101.6+/-11.6%, 39.4+/-5.8% and 110.5+/-9.0% of control, respectively. 3-CBW can therefore be used to isolate kainate receptor responses from those mediated by AMPA receptors. 5 3-CBW antagonized kainate-induced responses on dorsal root C-fibres with a pA(2) value of 4.96 whereas kainate receptor mediated responses (isolated by including GYKI53655 in the medium) on motoneurones were not completely blocked by 200 micro M 3-CBW, substantiating evidence that kainate receptors on neonatal rat motoneurones differ from those on dorsal root C-fibres.

Plateau potentials in sacrocaudal motoneurons of chronic spinal rats, recorded in vitro

Intracellular recordings were made from sacrocaudal tail motoneurons of acute and chronic spinal rats to examine whether plateau potentials contribute to spasticity associated with chronic injury. The spinal cord was transected at the S2 level, causing, over time, exaggerated long-lasting reflexes (hyperreflexia) associated with a general spasticity syndrome in the tail muscles of chronic spinal rats (1-5 mo postinjury). The whole sacrocaudal spinal cord of chronic or acute spinal rats was removed and maintained in vitro in normal artificial cerebral spinal fluid (ACSF). Hyperreflexia in chronic spinal rats was verified by recording the long-lasting ventral root responses to dorsal root stimulation in vitro. The intrinsic properties of sacrocaudal motoneurons were studied using intracellular injections of slow triangular current ramps or graded current pulses. In chronic spinal rats, the current injection triggered sustained firing and an associated sustained depolarization (plateau potential; 34/35 cells; mean, 5.5 mV; duration >5 s; normal ACSF). The threshold for plateau initiation was low and usually corresponded to an acceleration in the membrane potential just before recruitment. After recruitment and plateau activation, the firing rate changed linearly with current during the slow ramps [63% of cells had a linear frequency-current (F-I) relation] despite the presence of the plateau. The persistent inward current (I(PIC)) producing the plateau and sustained firing was estimated to be on average 0.8 nA as determined by the reduction in injected current needed to stop the sustained firing [DeltaI = -0.8 +/- 0.6 (SD) nA], compared with the current needed to start firing (I = 1.7 +/- 1.5 nA; 47% reduction). In motoneurons of acute spinal rats, plateaus were rarely seen (3/22), although they could be made to occur with bath application of serotonin. In motoneurons of chronic spinal rats there were no significant changes in the mean passive input resistance, rheobase or amplitude of the spike afterhyperpolarization (AHP) as compared with acute spinal rats. However, there were significant increases in AHP duration and initial firing rate at recruitment and decreases in minimum firing rate and F-I slope. We suggest that the higher initial firing rate resulted from the plateau activation at recruitment and the lower F-I slope resulted from an increase in active conductance during firing, due to I(PIC). Brief dorsal root stimulation also triggered a plateau and sustained discharge (long-lasting reflexes; 2-5 s) in motoneurons of chronic (but not acute) spinal rats. When the plateau was eliminated by a hyperpolarizing current bias, the reflex response was significantly shortened (to 1 s). Thus plateaus contributed substantially to the long-lasting reflexes in vitro and therefore should contribute significantly to the corresponding exaggerated reflexes and spasticity in awake chronic spinal rats.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

An in vitro functionally mature mouse spinal cord preparation for the study of spinal motor networks

An in vitro isolated whole spinal cord preparation has been developed in 'motor functionally mature' mice; that is mice of developmental maturity sufficient to weight-bear and walk. In balb/c mice this stage occurs at around postnatal day 10 (P10). Administration of strychnine elicited synchronous activity bilaterally in lumbar ventral roots. Rhythmic alternating locomotor-like activity could be produced by application of a combination of serotonin (5-HT), N-methyl-d-aspartate (NMDA), and dopamine in animals up to P12. Using a live cell-dead cell assay, it is demonstrated that there are primarily viable cells throughout the lumbar spinal cord. The viability of descending pathways was demonstrated with stimulation of the mid-thoracic white matter tracts. In addition, polysynaptic segmental reflexes could be elicited. Although usually absent in whole cord preparations, monosynaptic reflexes could invariably be elicited following longitudinal midline hemisection, leading to the possible explanation that there might be an active crossed pathway producing presynaptic inhibition of primary afferent terminals. The data demonstrate that this functionally mature spinal cord preparation can be used for the study of spinal cord physiology including locomotion.

Spasticity in rats with sacral spinal cord injury

We have investigated sacral spinal cord lesions in rats with the goal of developing a rat model of muscular spasticity that is minimally disruptive, not interfering with bladder, bowel, or hindlimb locomotor function. Spinal transections were made at the S2 sacral level and, thus, only affected the tail musculature. After spinal transection, the muscles of the tail were inactive for 2 weeks. Following this initial period, hypertonia, hyperreflexia, and clonus developed in the tail, and grew more pronounced with time. These changes were assessed in the awake rat, since the tail is readily accessible and easy to manipulate. Muscle stretch or cutaneous stimulation of the tail produced muscle spasms and marked increases in muscle tone, as measured with force and electromyographic recordings. When the tail was unconstrained, spontaneous or reflex induced flexor and extensor spasms coiled the tail. Movement during the spasms often triggered clonus in the end of the tail. The tail hair and skin were extremely hyperreflexive to light touch, withdrawing quickly at contact, and at times clonus could be entrained by repeated contact of the tail on a surface. Segmental tail muscle reflexes, e.g., Hoffman reflexes (H-reflexes), were measured before and after spinalization, and increased significantly 2 weeks after transection. These results suggest that sacral spinal rats develop symptoms of spasticity in tail muscles with similar characteristics to those seen in limb muscles of humans with spinal cord injury, and thus provide a convenient preparation for studying this condition.

Thyrotropin-releasing hormone (TRH) has independent excitatory and modulatory actions on lamina IX neurons of lumbosacral spinal cord slices from adult rats

Coronal and horizontal slices of the lumbar and sacral spinal cord, respectively, of ovariectomized adult rats, either treated with estrogen (OVX+E) or untreated (OVX), were used to test the neuronal actions of TRH and its metabolite, cyclo(His-Pro) (or cHP). Both coronal slices, which possess only short stumps of ventral roots (VRs), and horizontal slices, in which long sections of VRs were preserved, were used for extracellular recording of single motor and other types of neurons. Methodological comparisons between these two types of slices showed that the length of VRs preserved had no significant effect on the characteristics of motoneurons (MNs). In coronal slices, MNs in medial and lateral lamina IX (MNM and MNL, respectively) were identified by antidromic activation. Of these lumbar MNs, estrogen treatment lowered the antidromic activation threshold for MNM but not MNL. Because MNM innervate the back muscles crucial for the execution of the estrogen-dependent lordosis, the observed estrogen effect may contribute to the hormone's induction of the sexual behavior. The recorded MNs and other types of neurons were subjected to bath applications of TRH, cHP, and neurotransmitters. TRH was found to be capable of evoking an early, shorter-lasting neuronal excitation and/or a late, longer-lasting modulation of neuronal responses to transmitters. Each neuronal action could occur with or without the other, and the occurrence of the excitation did not affect the probability of whether a modulation would occur later. The modulatory, but not the excitatory, action appeared to be shared by cHP, because cHP could also modulate neuronal responses in similar, if not identical, ways as TRH did, but could neither stimulate neurons nor mimic TRH in desensitizing TRH-evoked excitation. The modulatory actions of the two peptides were not affected by estrogen. Although the excitatory action was desensitized by repeated TRH applications, the modulatory action did not appear to be attenuated but instead was often enhanced by repeated administrations of TRH and/or cHP. These results, together with the essentially identical findings from our previous study on hypothalamic neurons, indicate that the excitatory and the modulatory actions of TRH are independent of each other and, hence, are mediated by different subcellular mechanisms.

The isolated mammalian spinal cord

This review considers: spinal cord slices; isolated spinal cord sagitally or transversely hemisected; whole spinal cord; respiration control--[brain-stem spinal cord; brain-stem spinal cord with attached lungs]; nociception--[spinal cord with tail]; fictive locomotion--[spinal cord with one hind limb; spinal cord with two hind limbs]. Much of the functional circuitry of the CNS can be studied in the isolated spinal cord with the additional advantage that the isolated spinal cord can be perfused with known concentrations of ions, neurotransmitters, agonists, antagonists, and anaesthetics. These can be washed away, the circuitry allowed to recover and other drugs or different concentrations applied. Future preparations including the complete spinal cord, the two hind limbs, and a sagittal section of the complete brain will allow greater understanding of the multiple sensory and motor pathways and their interactions in the CNS.

Pharmacology of descending noradrenergic systems in relation to motor function

The function of descending noradrenergic systems in the spinal ventral horn has not been fully elucidated. We have reviewed our own findings and those of others relating to motor function of these noradrenergic systems. We studied the effects of adrenergic drugs on spinal reflexes, decerebrate rigidity, and noradrenaline release from the spinal cord in rats, and motoneuron activity in spinal cord slices isolated from adult rats. It was shown that the descending noradrenergic systems were facilitatory to the motor system, and that alpha 1-antagonistic action at the spinal cord and alpha 2-agonistic action at the brainstem inhibited spinal motor activity by blocking spinal alpha 1-receptors and by reducing the release of noradrenaline in the spinal cord, respectively.

Physiological and pharmacological characterization of the spinal tachykinin NK2 receptor

The goal of these investigations was to study the role of tachykinin NK2 receptors in neonatal spinal cords using the selective NK2 receptor agonist [beta-Ala8]neurokinin A-(4-10) and the new NK2 receptor antagonist GR 94800. Experiments were performed with superfused hemisected rat and gerbil spinal cords. Dorsal roots were electrically stimulated and the synaptically elicited responses and the DC-potentials were recorded extracellularly from the corresponding ventral roots. [beta-Ala8]neurokinin A-(4-10) depolarized ventral roots (0.01-10 microM) and increased their spontaneous activity in a concentration-dependent manner. These effects of [beta-Ala8]neurokinin A-(4-10) were reduced by GR 94800. The action of GR 94800 was selective because the depolarizing effects of similar magnitude evoked by the NK1 receptor agonist [Sar9,Met(O2)11]substance P were not affected by GR 94800. The pA2 values of GR 94800 amounted to 6.0 +/- 0.4 in the rat and 5.4 +/- 0.3 in the gerbil. The NK2 receptor agonist was more potent in the rat than in the gerbil. The estimated EC50 (mean +/- S.E.M.) was found to be 3.9 + 6.0/-1.3 microM in the rat and 2.4 + 2.9/-1.3 microM in the gerbil spinal cord. The NK2 receptor agonist [beta-Ala8]neurokinin A-(4-10) potentiated the monosynaptic reflex evoked by dorsal root stimulation. The potentiation manifested itself as an increase in the amplitude of the early component of the response. The receptor type mediating this effect could not be elucidated. The potentiation ranged between 30 +/- 27 and 110 +/- 36% (0.3 and 10 microM), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The modulation of the monosynaptic reflex by substance P in the hemisected spinal cord preparation of the rat and gerbil

The effects of substance P and the selective neurokinin-1 receptor antagonist (+/-)-CP-96,345 have been compared on in vitro spinal cord preparations from the rat and the gerbil. Substance P produced a concentration-dependent depolarization of motoneurons recorded from ventral roots of both species. The EC50 values (microM mean +/- S.E.M.) obtained in rat (0.95 + 1.0/-0.49) and gerbil (0.47 + 0.26/-0.17) preparations were comparable. The mean maximal depolarization (mV mean +/- S.E.M.) evoked in rat (2.07 + 0.26/-0.25) was approximately two-fold greater than that evoked in gerbil (1.21 + 0.15/-0.14) preparations. In the rat substance P had a biphasic effect (depression followed by potentiation) on the short latency probably monosynaptic reflex evoked by electrical stimulation of a dorsal root. In gerbil preparations substance P produced only potentiation of the monosynaptic reflex. The EC50 values (microM) mean +/- S.E.M.) for this potentiating action in rat (0.97 + 0.75/-0.43) and gerbil (0.46 + 3.6/-0.4) preparations were similar. This potentiation demonstrates a positive modulation of an endogenous excitatory probably glutamatergic transmission by substance P in the ventral horn of the spinal cord. The depressant phase observed in rat preparations may be related to the relative immaturity of myelination in rat ventral root fibres compared to the gerbil. The selective neurokinin-1 antagonist (+/-)-CP-96,345 was one hundred-fold less potent as an antagonist of substance P-induced depolarizations in the rat (pA2 4.69 +/- 0.18, n = 7) than in the gerbil (pA2 6.79 +/- 0.16, n = 5) spinal cord. This finding suggests that (+/-)-CP-96,345 may not act solely at the neurokinin-1 recognition site. In conclusion this study demonstrates that substance P modulates the monosynaptic reflex in the spinal cord presumably via activation of neurokinin-1 receptors.

The action of new potent GABAB receptor antagonists in the hemisected spinal cord preparation of the rat

CGP 52432 (3-N-(3,4-dichlorobenzyl)aminopropyl-P-diethoxymethylphosphinic acid), CGP 54062 (3-N[1-(R,S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-P-benzy l- phosphinic acid), CGP 54626 (3-N[[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)- hydroxypropyl-P-cyclohexylmethylphosphinic acid) and CGP 55845 (3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)- hydroxypropyl-P-benzyl-phosphinic acid) are novel selective GABAB receptor antagonist. The apparent Kd values for the complex formed between the GABAB receptor and these compounds were determined using the monosynaptic reflex in the hemisected rat spinal cord preparation in vitro. CGP 55845 was found to be the most potent GABAB receptor antagonist tested (apparent Kd = 30 nM). On the same preparation 0.3 microM CGP 55845 was equipotent with 100 microM of CGP 35348 (P-(3-aminopropyl)-P-diethoxymethyl-phosphinic acid) for reversal of the depressant action of (R)-(-)-baclofen.

Antagonism of synaptic potentials in ventral horn neurones by 6-cyano-7-nitroquinoxaline-2,3-dione: a study in the rat spinal cord in vitro

1. The rat spinal cord in vitro has been used to assess the effect of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on the dorsal root evoked extracellular ventral root reflex (DR-VRR) and the intracellular excitatory postsynaptic potential (e.p.s.p.) in ventral horn neurones and motoneurones. 2. CNQX (1-5 microM) produces a selective and dose-dependent reduction in the amplitude of the monosynaptic component of the DR-VRR recorded from lumbar spinal segments. 3. With low intensity dorsal root stimulation CNQX selectively attenuates the amplitude of the short latency intracellular e.p.s.p. (70% reduction, P < 0.005) and its rise-time (75%, P < 0.01) without affecting the half-time to decay. 4. When high intensity stimulation is used CNQX significantly attenuates the amplitude of the e.p.s.p. (56%, P < 0.005), rise-time (76%, P < 0.01) and abolishes the short latency spike. In addition longer latency synaptic components are attenuated and the half-time to decay significantly reduced (47%, P < 0.005). 5. The results with CNQX are compared to D-aminophosphonovalerate and discussed in relation to the recruitment of low versus high threshold afferents. The data supports an involvement of non-NMDA receptors in transmission through both mono- and polysynaptic pathways in the ventral horn.

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.

CGP 35348: a centrally active blocker of GABAB receptors

The biochemical, electrophysiological and pharmacological properties of the new GABAB receptor blocker CGP 35348 are described. In a variety of receptor binding assays CGP 35348 showed affinity for the GABAB receptor only. CGP 35348 had an IC50 of 34 microM at the GABAB receptor. The compound antagonized (100, 300, 1000 microM) the potentiating effect of L-baclofen on noradrenaline-induced stimulation of adenylate cyclase in rat cortex slices. In electrophysiological studies CGP 35348 (10, 100 microM) antagonized the effect of L-baclofen in the isolated rat spinal cord. In the hippocampal slice preparation CGP 35348 (10, 30, 100 microM) blocked the membrane hyperpolarization induced by D/L-baclofen (10 microM) and the late inhibitory postsynaptic potential. CGP 35348 appeared to be 10-30 times more potent than the GABAB receptor blocker phaclofen. Ionophoretic and behavioural experiments showed that GABAB receptors in the brain were blocked after i.p. administration of CGP 35348. This compound may be of considerable value in elucidating the roles of brain GABAB receptors.

Effect of 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX) on dorsal root-, NMDA-, kainate- and quisqualate-mediated depolarization of rat motoneurones in vitro

1. Mature in vitro rat spinal cord preparations have been used to compare the depressant effects of 6-cyano-2,3-dihydroxy-7-nitroquinoxalinedione (CNQX) and kynurenate on transmission from low threshold myelinated primary afferents in dorsal roots. EC50 values +/- s.e.mean (number of preparations in parentheses) for depression of the monosynaptic ventral root reflex were respectively 1.0 +/- 0.3 microM (5) and 135 +/- 15 microM (3) for CNQX and kynurenate. Transmission through superior cervical ganglia was not significantly affected by concentrations of CNQX up to 100 microM or kynurenate up to 5 mM. 2. Immature in vitro rat spinal cord preparations were used to measure dose-ratios for antagonism of depolarizations induced by N-methyl-D-aspartate (NMDA), kainate or quisqualate by 4, 10 and 25 microM CNQX. In the presence of 0.75 mM Mg2+ pA2 values +/- s.e.mean were respectively 4.62 +/- 0.05 (16), 5.79 +/- 0.01 (4) and 5.59 +/- 0.05 (16) for each agonist. These values were not significantly altered in the absence of added Mg2+. The mean pA2 values for kainate were significantly higher than those for quisqualate (P less than 0.01). 3. Antagonism of NMDA-induced depolarizations was evident at 10 and 25 but not 4 microM CNQX. The antagonism of NMDA was reversed by D-serine (100 and 200 microM). 4. A similarity between the relative potencies of both CNQX and kynurenate for depression of synaptic transmission and antagonism of amino acid-induced depolarizations indicates that monosynaptic transmission from myelinated primary afferents to motoneurones is mediated by kainate and/or quisqualate sub-types of non-NMDA receptors.

Effects of N-methyl-D-aspartate antagonists and spantide on spinal reflexes and responses to substance P and capsaicin in isolated spinal cord preparations from mouse and rat

Electrical stimulus intensity, capsaicin, excitatory amino acid antagonists and the substance P antagonist, spantide, have been used to investigate the roles of primary afferent C fibres and excitatory amino acid receptors in the generation of long duration (half time 9.1 s +/- 1.1 S.E.M., N = 24) contralateral reflexes recorded in ventral roots of immature rat spinal cords in vitro. The relationship between C fibre compound action potentials recorded in the dorsal root and duration of the dorsal root-evoked contralateral ventral root potential appeared to be coincidental rather than causal. Dorsal root-evoked contralateral ventral root potentials of greater than 2 s in duration could not be evoked in mature mouse spinal preparations. Application of capsaicin (1 microM for 15-120 min) produced a long lasting increase in spontaneous activity of ventral roots as well as blockade of C fibre conduction in dorsal roots. The dorsal root potential evoked following stimulation of adjacent dorsal roots at intensities insufficient for activation of C fibres was depressed by capsaicin. Dorsal root-evoked contralateral ventral root potentials were abolished by kynurenate (EC50 56 +/- 13 microM, N = 3) and depressed to 38.2 +/- 6.9% S.E.M. (N = 7) of pre-drug levels by the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonopentanoate (20 microM) or to 51.8 +/- 9.0% (N = 7) by the substance P analogue spantide (33 microM). Spantide consistently antagonised substance P-induced, but not capsaicin-induced, depolarizations recorded in ventral roots (+-)-2-Amino-5-phosphonopentanoic acid (10-50 microM) depressed both substance P- and capsaicin-induced depolarizations. The depressant effect of spantide, unlike that of (+/-)-2-amino-5-phosphonopentanoic acid, was associated with a long lasting excitatory action. In the presence of tetrodotoxin (0.1 microM), spantide (33 microM) failed to antagonize substance P-induced depolarizations. It is suggested that long duration of the dorsal root-evoked contralateral ventral root potential is a consequence of the activation of the N-methyl-D-aspartate receptor operated ion channels by excitatory amino acid transmitters.

Tubocurarine and strychnine block Renshaw cell inhibition in the isolated mammalian spinal cord

1. Stimulating a lumbar dorsal root (A) of an isolated mammalian spinal cord preparation sets up a reflex (DR-VR) in the corresponding ventral root (A) of that segment. 2. If an adjacent motor root (B) is antidromically stimulated 100 msec before the dorsal root A is stimulated, it inhibits the ventral root A response. (i.e. Renshaw cell inhibition of A through antidromic stimulation of recurrent collateral of B). 3. Stimulation of the lateral side of the spinal cord 5 mm rostral to the DR-VR reflex sets up a powerful inhibition of the reflex. 4. The inhibition is abolished by tubocurarine, or by strychnine indicating that the lateral stimulation is activating a cholinergic and a glycinergic pathway. 5. Addition of tubocurarine increases the steady "spontaneous" activity in the motor roots. 6. Addition of tubocurarine and 10(-3) M magnesium produces long lasting rhythmic bursting activity in the motor roots that persists even after 45 min of washing. 7. The isolated mammalian spinal cord preparation in addition to having active sensory, motor, and interneurones, has active Renshaw cells that can be studied using this preparation free from any blood-brain barrier and so are easily permeated by tubocurarine, etc.

Spontaneous activity induced in isolated mouse spinal cord by high extracellular calcium and by low extracellular magnesium

Spontaneous discharges were observed in recordings from dorsal and ventral roots (DRs and VRs) of hemisected mouse spinal cords in vitro when bath [Ca2+] was raised from the control level of 1.2 to 1.8 or 2.4 mmol/l, and when bath [Mg2+] was lowered resembling drug-induced 'interictal' discharges described earlier. Maximum discharge frequency was reached at 2.4 or 3.6 mmol/l [Ca2+] while at higher concentrations mean frequency diminished but mean amplitude still increased somewhat. The frequency distribution of wave amplitudes suggests recruitment of neurons by groups or assemblies. The pacemaker is located in the dorsal spinal quadrant. The GABAA receptor antagonists picrotoxin and bicuculline in low concentration blocked the spontaneous activity, indicating an obligatory GABAergic link in the pacemaker circuit. The NMDA antagonists D.L-APV and D-APV in high concentration variably depressed but did not abolish spontaneous activity. The propensity of spinal neuron assemblies for self-paced discharge may contribute to the pathologic irritability of injured spinal cords.

Effects of depressant amino acids and antagonists on an in vitro spinal cord preparation from the adult rat

A mature sacrococcygeal in vitro spinal preparation from the rat has been used to demonstrate effects of neutral amino acids and their antagonists. gamma-Aminobutanoate (GABA), glycine and taurine (0.5-5 mM) produced dose-dependent depression of spontaneous paroxysmal activity generated in Mg2+ -free medium. The depressant effect of GABA was antagonised selectively by picrotoxin (25-50 microM) and the depressant effects of glycine and taurine were antagonised selectively by strychnine (0.2 microM). Glycine (0.5-5 mM) had a dose-dependent depolarizing action which was present at the central ends of isolated ventral roots. gamma-Aminobutanoate and taurine, had only weak depolarizing actions on ventral root fibres. Depolarizing responses to glycine showed a marked fading. Reduction in the fading appeared to be responsible for a paradoxical potentiation of glycine-induced depolarizations, which occurred in the presence of strychnine (0.2-2 microM). Strychnine (2-10 microM), picrotoxin (10-50 microM) or bicuculline (10 microM) had little or no effect on the amplitude, duration or latency of the monosynaptic component of ventral root reflexes evoked by supramaximal stimulation of dorsal roots (DR-VRP). However all three antagonists introduced slow, NMDA receptor mediated, components to these ventral root potentials. Picrotoxin and bicuculline, but not strychnine, reversibly depressed the dorsal root potential evoked from an adjacent dorsal root (DR-DRP). The depressant actions of 2-amino-5-phosphonopentanoate (AP5), kynurenate and 3-((+/-)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) revealed both NMDA and non-NMDA receptor mediated components in the dorsal root potential.

Primary afferent depolarization in the rat spinal cord is mediated by pathways utilising NMDA and non-NMDA receptors

In the present experiments the dorsal root-evoked dorsal root potential (DR-DRP) has been measured in vitro from a mature rat sacrococcygeal preparation. The DR-DRP is an index of presynaptic inhibition since it represents the depolarization of primary afferent terminals by gamma-aminobutyric acid (GABA) released synaptically from interneurones. The present study shows that the synaptic excitation of the GABAergic interneurons contains a large component resistant to the selective N-methyl-D-aspartate (NMDA) receptor antagonists 2-amino-5-phosphonopentanoate (AP5) (100 microM) and 3((+)-2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP) 20-100 microM. This non-NMDA receptor mediated component reflected in the DR-DRP was depressed markedly by the non-selective excitatory amino acid receptor antagonists kynurenate (1-2 mM) and 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX) (10-20 microM). Because previous reports show non-cholinergic activation of Renshaw cells to be blocked by NMDA receptor antagonists the present observations suggest that pre- and postsynaptic inhibition in the spinal cord are mediated by different types of excitatory amino acid receptor.

Differential sensitivity of dorsal and ventral root activity to magnesium and 2-amino-5-phosphonovalerate (APV) in an isolated mammalian spinal cord preparation

Spontaneous activity emerging from the isolated hamster spinal cord simultaneously along the ventral and dorsal lumbar roots was shown by inter-spike interval analysis to have different firing patterns. Ventral root firing was reduced in solutions containing 1 mM magnesium or sub-micromolar concentrations of 2-amino-5-phosphonovalerate (APV), whereas the activity in the dorsal roots was unchanged. Differences in sensitivity to magnesium and APV were also shown on the evoked ventral and dorsal root reflexes, with the dorsal root reflex being unaffected, and the ventral root reflex exhibiting a long latency component that was sensitive to magnesium and APV. These results indicate that NMDA receptors are involved in the generation of part of the ventral root responses in the spinal cord, but not in the generation of the dorsal root reflex and spontaneous dorsal root activity.

Amino acid pharmacology in a mature rat spinal cord preparation in vitro

1. It has been shown that a relatively large intact piece of mature mammalian spinal cord can be maintained in vitro if suitable experimental conditions are employed. 2. The preparation as described gives robust and reproduceable reflex responses in both dorsal and ventral horns (ventral horn activity has been maintained for over 36 hr in vitro). 3. The action of antagonists at both excitatory and inhibitory amino acid receptors give qualitatively predictable results from known in vivo experiments with the added advantage of fine quantitative control. 4. A significant advantage of such an adult preparation over more common immature spinal cord preparation is the ability to clearly separate low threshold reflexes, such as those described in this article, from reflexes evoked by small diameter non-myelinated afferents.

The dorsal root reflex in isolated mammalian spinal cord

1. The dorsal root reflex has been investigated in an isolated preparation of adult mammalian spinal cord. 2. Both evoked and spontaneous activity can be recorded from the cord in the dorsal spinal roots. 3. The spontaneous activity has a characteristic pattern of firing in bursts of action potentials. Spontaneous and evoked activity are optimum at temperatures between 25 and 27 degrees C; little activity can be detected above 35 degrees C. 4. The spontaneous dorsal root activity has been shown to be correlated with negative potentials in the dorsal horn of the cord, and intracellular recordings made from primary afferent fibres have shown spontaneous primary afferent depolarizations (PAD) which underlie the generation of the spontaneous dorsal root activity. 5. The evoked dorsal root reflex has been shown to spread up to 16 spinal segments both rostrally and caudally from the stimulated dorsal root, and to the contralateral side of the cord. 6. The spontaneous dorsal root activity in widely separated segments has been shown by cross-correlation analysis to be linked both ipsi- and contra-laterally. 7. The significance of such a widespread system for the generation of PAD is discussed.