Phaclofen antagonizes the depressant effect of baclofen on spinal reflex transmission in 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.

Dissociation of (-) baclofen-induced effects on the tail withdrawal and hindlimb flexor reflexes of chronic spinal rats

We previously reported that the antinociceptive effect of the GABA(B) receptor agonist, (-)baclofen, in chronic spinal rats depended on the route of administration. That is, subcutaneous (SC) injections significantly increased the latency of the thermally elicited tail withdrawal (tail flick, TF) reflex, whereas spinal (intrathecal, IT) injections did not. The present studies attempted to determine the reason for this differential response. The possible contribution of a peripheral component to the systemic effect was evaluated, but was not supported by negative results of intradermal (-)baclofen injections (50 and 500 microg) into the tail skin of chronic spinal rats. A spinal site of action was indicated when pretreatment with 30 microg, IT of the GABA(B) receptor antagonist, phaclofen, significantly reduced the antinociceptive effect of SC (-)baclofen in both chronic spinal (5 mg/kg) and intact rats (2 mg/kg). Moreover, direct IT injections of (-)baclofen in chronic spinal rats produced a modest, but statistically significant increase in TF latency at doses of 0.06, 0.12, 0.3, and 0.6 microg, but not 1.2 microg. In the same spinal preparation, the flexor response was significantly reduced by IT injection of 0.6 and 1.2 microg, but not lower doses of 0.3 and 0.12 microg. These results provide the first quantitative, electrophysiological evidence of an antispastic effect of IT (-)baclofen in an in vivo, unanesthetized animal model. Second, the data show a separation between an antinociceptive effect of low spinal doses and an antispastic/muscle relaxant effect at higher doses, which may account for the results of our prior report. Finally, the data are also consistent with behavioral reports of antiallodynic/analgesic effects of low-dose baclofen, and may be relevant to the electrophysiological evidence of a preferential presynaptic action of low-dose (-)baclofen at the primary afferent synapse.

N-methyl-D-aspartate and alpha 2-adrenergic mechanisms are involved in the depressant action of flupirtine on spinal reflexes in rats

In urethane-chloralose anesthetised rats the muscle relaxant activity of flupirtine was investigated on the monosynaptic Hoffmann reflex recorded from plantar foot muscles and on the polysynaptic flexor reflex recorded from tibialis muscle. Intraperitoneal (i.p.; 2.5-25 mumol/kg) and intrathecal (i.t.; 33-330 nmol) administration of flupirtine depressed the polysynaptic flexor reflex in anesthetised rats in a dose-dependent manner without affecting the monosynaptic Hoffmann reflex. Flupirtine produced a similar pattern on spinal reflexes as NMDA receptor antagonists, such as (-)-2-amino-7-phosphonoheptanoic acid (500 nmol i.t.) and memantine (125 mumol/kg i.p.), the benzodiazepines diazepam (18 mumol/kg i.p.) and midazolam (80 nmol i.t.), and the alpha 2-adrenoceptor agonist tizanidine (2 mumol/kg). In contrast, the GABAA receptor agonist muscimol (21 mumol/kg i.p.; 20 nmol i.t.) and the GABAB receptor agonist baclofen (47 mumol/kg i.p.; 2 nmol i.t.) reduced the magnitude of both the flexor and the Hoffmann reflex, whereas the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX; 10 nmol i.t.) depressed the Hoffmann reflex without affecting the flexor reflex. The effect of i.t. injection of flupirtine was prevented by coadministration of the mixed alpha 1/alpha 2-adrenoceptor antagonist yohimbine (10 nmol) and the excitatory amino acid N-methyl-D-aspartate (NMDA; 0.1 nmol), but neither by coadministration of the alpha 1-adrenoceptor antagonist prazosine (10 nmol), the GABAA receptor antagonist bicuculline (1 nmol), the GABAB receptor antagonist phaclofen (100 nmol), the non-NMDA receptor agonist alpha-amino-3-hydroxy-5-tertbutyl-4-isoxazolepropionic acid (ATPA; 0.1 pmol) nor by pre-treatment with the benzodiazepine receptor antagonist flumazenil (16 mumol/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

The depressant effect of GYKI 52466 on spinal reflex transmission in rats is mediated via non-NMDA and benzodiazepine receptors

The present study examined the mechanisms by which GYKI 52466 (1-(amino-phenyl)-4-methyl-7,8-methyldioxy-5H-2,3-benzodiazepine) exerts its muscle relaxant effects. Intrathecal injection of the specific N-methyl-D-aspartate (NMDA) receptor antagonist (-)-2-amino-7-phosphonoheptanoate (AP7, 50-500 nmol) and systemic application of the benzodiazepine diazepam (0.2-5 mg/kg) dose dependently reduced the integrated area of the polysynaptic flexor reflex without affecting the monoxynaptic H-reflex. In contrast, intrathecal administration of the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX, 0.1-10 nmol) depressed the H-reflex in a dose-dependent manner without affecting the flexor reflex. The depressant effect of GYKI 52466 on the flexor reflex was reduced by coadministration with flumazenil (5 mg/kg i.p.), an antagonist at the benzodiazepine receptor, whereas coadministration of the non-NMDA receptor agonist alpha-amino-3-hydroxy-5-tertbutyl-4-isoxazole-propionic acid (ATPA, 0.1 pmol) with GYKI 52466 attenuated the reduction of the H-reflex induced by GYKI 52466. The chosen doses of flumazenil and ATPA did not affect spinal reflex transmission when given alone. These data suggest that GYKI 52466 depresses spinal reflex transmission via an action on non-NMDA receptors and on benzodiazepine receptors.

Involvement of the nucleus accumbens in the myorelaxant effect of baclofen in rats

The study was designed to search for brain structures responsible for the myorelaxant action of baclofen. Rats were chronically implanted with cannulae in the medial nucleus accumbens (NAS). The muscle tone was increased by reserpine (10 mg/kg i.p.) and measured as a resistance of the hind foot, developed in response to successive passive bendings and straightenings in the ankle joint. (+/-)Baclofen (1 or 2.5 mg/0.5 microliter), injected bilaterally into the NAS 1.5 h after pretreatment with reserpine, decreased the muscle tone of flexors and extensors enhanced by that compound. It is suggested that GABAB receptors of the NAS are involved in the myorelaxant action of baclofen.

Antagonism of L-baclofen-induced antinociception by CGP 35348 in the spinal cord of the rat

This study examined the potency and selectivity of the GABAB receptor antagonist CGP 35348 (3-amino-propyl(diethoxy-methyl)phosphinic acid) in the spinal cord of the rat. Intrathecal (i.t.) administration of 3-30 micrograms CGP 35348 produced a dose-dependent antagonism of the antinociception produced by i.t. administered L-baclofen. Increasing doses of CGP 35348 produced progressive, rightward parallel shifts in the dose-effect relationship of L-baclofen in both the tail flick and hot plate tests. However, in the tail flick test, the magnitude of the shift was not proportional to the dose of CGP 35348 such that doses greater than 10 micrograms i.t. produced no further antagonism. Schild analysis using all three doses of CGP 35348 yielded a slope of -0.45. However, if the Schild analysis was confined to the two lowest doses of CGP 35348 at which progressive shifts were obtained, a slope of -0.91 and an apparent pA2 value of 9.3 were obtained. An apparent pA2 value of 9.0 was also obtained in the hot plate test using the two lowest doses of CGP 35348. These data suggest that CGP 35348 is a competitive GABAB receptor antagonist at low concentrations. However, the failure to observe greater antagonism at higher doses of CGP 35348 suggests that this drug may exhibit additional properties at higher concentrations that can mask or prevent the occurrence of further antagonism. In contrast, the antinociception produced by i.t. administration of the GABAA receptor agonist isoguvacine was not antagonized by 30 micrograms i.t. CGP 35348, a dose that shifted the dose-effect relationship of L-baclofen 10-fold to the right.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA receptor subtypes involved in the neuronal mechanisms of baroreceptor reflex in the nucleus tractus solitarii of rabbits

The role of GABAA and GABAB receptors in neuronal mechanisms of the baroreceptor reflex in the nucleus tractus solitarii (NTS) of the anesthetized and immobilized rabbits were investigated using a microiontophoretic technique. Baroreceptive neurons (BRNs) activated or depressed by baroreceptor stimulation (phenylephrine, 10 micrograms/kg, i.v.) were identified in the NTS (activated BRN (A-BRN) and depressed BRN (D-BRN), respectively). The GABAA antagonist bicuculline (40-80 nA) increased spontaneous activities of these neurons, but the GABAB antagonist phaclofen (80-160 nA) did not. Evoked responses of A-BRNs were potentiated by bicuculline and phaclofen, while the responses of D-BRNs were not clearly affected by these drugs. These results suggest that most of A- and D-BRNs are tonically inhibited by endogenous GABA acting on GABAA receptors, but not on GABAB receptors, and that GABAergic mechanisms suppressively modulate the baroreceptor reflex acting on GABAA and GABAB receptors of A-BRNs, but not of D-BRNs.

Actions of 4-amino-3-(5-methoxybenzo(b)furan-2-yl) butanoic acid and 4-amino-3-benzo(b)furan-2-yl butanoic acid in the rat spinal cord

This study examined whether two putative GABAB receptor antagonists, 4-amino-3-(5-methoxybenzo (b)furan-2-yl) butanoic acid (MBFG) and 4-amino-3-benzo(b)furan-2-yl butanoic acid (BFG), antagonized the antinociception produced by intrathecal (i.t) administration of the GABAB receptor agonist baclofen in the rat. In rats pretreated with 30 micrograms i.t. MBFG, the dose-effect relationship of D,L-baclofen was shifted approximately 2-fold and 4-fold to the right in the tail flick and hot plate tests, respectively. No further shift was obtained in the presence of 60 micrograms i.t. MBFG. I.t. injection of MBFG by itself did not alter either tail flick or hot plate latency. These data suggest that MBFG is a GABAB receptor antagonist in the spinal cord in vivo, although of marginal utility. Contrary to expectations, i.t. administration of 30-60 micrograms BFG alone increased tail flick and hot plate latencies; this increase was partially attenuated by coadministration of the GABAB receptor antagonist phaclofen. Pretreatment with 10 micrograms i.t. BFG, which was itself without effect on nociceptive threshold, antagonized the antinociceptive effects of 0.3 microgram i.t. L-baclofen, but interacted with higher and lower doses of baclofen in a complex manner. These results suggest that BFG acts as weak, partial agonist at GABAB receptors and that it may have additional, non-specific effects in the spinal cord of the rat. The pharmacological properties of BFG, therefore, resemble those of the GABAB receptor partial agonist/antagonist beta-phenyl-GABA, to which it bears a strong structural resemblance.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA receptor-mediated modification of reticulo-ruminal myoelectrical activity in sheep

The involvement of central and peripheral GABA receptors in the control of the forestomach periodic motor activity was examined in five conscious ewes at rest, chronically fitted with electrodes implanted in the reticular and ruminal wall and a cannula placed in a cerebral lateral ventricle. Intravenous (IV) administration of the GABAA receptor agonist muscimol (5-50 micrograms/kg) did not affect reticulo-ruminal myoelectrical activity, while its intracerebroventricular (ICV) injection (0.05-0.10 micrograms/kg) dose-dependently increased the frequency of reticular and ruminal motor cycles and provoked rumination in 11 trials out of 15. Forestomach responses to ICV muscimol were abolished by a previous ICV treatment of the animals with the GABAA receptor antagonist bicuculline (0.5-4 micrograms/kg), which did not in itself influence reticular or ruminal motility. The GABAB receptor agonist baclofen, given either IV (300-800 micrograms/kg) or ICV (1-3 micrograms/kg), inhibited in a dose-dependent manner both reticular and ruminal activity, with the effects of its ICV administration appearing earlier and lasting longer. The GABAB receptor blocker baclofen given ICV alone (50-300 micrograms/kg) was ineffective, but such a treatment before IV or ICV administration of baclofen in part antagonized the inhibition of the forestomach myoelectrical cyclic activity. These pharmacological data suggest a possible excitatory role of strictly central GABAA and an inhibitory one of mainly central GABAB receptors in the regulation of reticulo-ruminal extrinsic motility in sheep. In this regulation, however, a probable peripheral component of inhibitory GABAB receptors should not be excluded.

Effects of phaclofen and the enantiomers of baclofen on cardiovascular responses to intrathecal administration of L- and D-baclofen in the rat

In a previous study it was found that i.t. administration of L-baclofen decreased arterial pressure and heart rate while D-baclofen differentially increased arterial pressure. The objective of the present study was to determine which of these effects was blocked by prior administration of the GABAB receptor antagonist, phaclofen, and whether the effect of one enantiomer of baclofen could be blocked by prior administration of the other. The decreases in systolic and diastolic arterial pressures and in heart rate produced by i.t. administration of 70 nmol of L-baclofen were unaffected by i.t. administration of 7, 70 or 700 nmol of D-baclofen 10 min prior to administration of L-baclofen, but were blocked by administration of 5 mumol of phaclofen given 3-5 min prior to L-baclofen. On the other hand, the increases in systolic and diastolic arterial pressures induced by i.t. administration of 700 nmol of D-baclofen were blocked by 70 nmol but not by 7 nmol of L-baclofen, as well as by 2.5 mumol of phaclofen; the effect of L-baclofen cannot be attributed to a desensitization of D-baclofen-sensitive receptors as two successive doses of D-baclofen given 7 min apart had quantitatively similar effects. Phaclofen alone increased systolic and diastolic arterial pressures and heart rate. The results are interpreted as indicating that D-baclofen is not an antagonist of L-baclofen in this paradigm; rather, they suggest that L-baclofen reduces the effects of D-baclofen.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of the cardiovascular effects of GABAB receptor activation in the nucleus tractus solitarii of the rat

The effects of baclofen microinjected into the nucleus tractus solitarii (NTS) on blood pressure, heart rate and baroreflex bradycardia were studied in urethane-anesthetized rats. Baclofen caused dose-dependent pressor and tachycardic effects and inhibited the reflex bradycardia elicited by i.v. phenylephrine. The effects of baclofen were inhibited by similarly administered GABAB receptor antagonists, phaclofen and 2-OH-saclofen, or the non-NMDA glutamate receptor antagonist, DNQX, or by pretreatment of rats with intracisternally administered pertussis toxin. DNQX and pertussis toxin, but not the NMDA antagonist, MK-801, also inhibited baroreflex bradycardia. Intra-NTS injections of glutamate caused hypotension and bradycardia, which were potentiated by baclofen, and were not affected by either DNQX or MK-801 or by pretreatment with pertussis toxin. These findings indicate that the cardiovascular effects of stimulation of GABAB receptors in the NTS are due, at least in part, to inhibition of the depressor baroreflex response. Inhibition of the release and/or postsynaptic action of an excitatory amino acid transmitter other than glutamate is the most likely mechanism.