Receptor mechanisms underlying heterogenic reflexes among the triceps surae muscles of the cat. (1/582)

The soleus (S), medial gastrocnemius (MG), and lateral gastrocnemius (LG) muscles of the cat are interlinked by rapid spinal reflex pathways. In the decerebrate state, these heterogenic reflexes are either excitatory and length dependent or inhibitory and force dependent. Mechanographic analysis was used to obtain additional evidence that the muscle spindle primary ending and the Golgi tendon organ provide the major contributions to these reflexes, respectively. The tendons of the triceps surae muscles were separated and connected to independent force transducers and servo-controlled torque motors in unanesthetized, decerebrate cats. The muscles were activated as a group using crossed-extension reflexes. Electrical stimulation of the caudal cutaneous sural nerve was used to provide a particularly strong activation of MG and decouple the forces of the triceps surae muscles. During either form of activation, the muscles were stretched either individually or in various combinations to determine the strength and characteristics of autogenic and heterogenic feedback. The corresponding force responses, including both active and passive components, were measured during the changing background tension. During activation of the entire group, the excitatory, heterogenic feedback linking the three muscles was found to be strongest onto LG and weakest onto MG, in agreement with previous results concerning the strengths of heteronymous Ia excitatory postsynaptic potentials among the triceps surae muscles. The inhibition, which is known to affect only the soleus muscle, was dependent on active contractile force and was detected essentially as rapidly as length dependent excitation. The inhibition outlasted the excitation and was blocked by intravenous strychnine. These results indicate that the excitatory and inhibitory effects are dominated by feedback from primary spindle receptors and Golgi tendon organs. The interactions between these two feedback pathways potentially can influence both the mechanical coupling between ankle and knee.  (+info)

Immunomodulatory effects of glycine on LPS-treated monocytes: reduced TNF-alpha production and accelerated IL-10 expression. (2/582)

Cytokines play a pivotal role in the pathogenesis of septic shock. Proinflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) stimulate the progression of septic shock whereas the anti-inflammatory cytokine IL-10 has counterregulative potency. The amino acid glycine (GLY) has been shown to protect against endotoxin shock in the rat by inhibiting TNF-alpha production. In the current study we investigated the role of GLY on lipopolysaccharide (LPS) -induced cell surface marker expression, phagocytosis, and cytokine production on purified monocytes from healthy donors. GLY did not modulate the expression of HLA-DR and CD64 on monocytes, whereas CD11b/CD18 expression (P<0.05) and E. coli phagocytosis (P<0.05) decreased significantly. GLY decreased LPS-induced TNF-alpha production (P<0.01) and increased IL-10 expression of purified monocytes. Similarly, in a whole blood assay, GLY reduced TNF-alpha (P<0.0001) and IL-1beta (P<0.0001) synthesis and increased IL-10 expression (P<0.05) in a dose-dependent manner. The inhibitory effects of GLY were neutralized by strychnine, and the production of IL-10 and TNF-alpha was augmented by anti-IL-10 antibodies. Furthermore, GLY decreased the amount of IL-1beta and TNF-alpha-specific mRNA. Our data indicate that GLY has a potential to be used as an additional immunomodulatory tool in the early phase of sepsis and in different pathophysiological situations related to hypoxia and reperfusion.  (+info)

Modulation of nicotinic acetylcholine receptors by strychnine. (3/582)

Strychnine, a potent and selective antagonist at glycine receptors, was found to inhibit muscle (alpha1beta1gammadelta, alpha1beta1gamma, and alpha1beta1delta) and neuronal (alpha2beta2 and alpha2beta4) nicotinic acetylcholine receptors (AcChoRs) expressed in Xenopus oocytes. Strychnine alone (up to 500 microM) did not elicit membrane currents in oocytes expressing AcChoRs, but, when applied before, concomitantly, or during superfusion of acetylcholine (AcCho), it rapidly and reversibly inhibited the current elicited by AcCho (AcCho-current). Although in the three cases the AcCho-current was reduced to the same level, its recovery was slower when the oocytes were preincubated with strychnine. The amount of AcCho-current inhibition depended on the receptor subtype, and the order of blocking potency by strychnine was alpha1beta1gammadelta > alpha2beta4 > alpha2beta2. With the three forms of drug application, the Hill coefficient was close to one, suggesting a single site for the receptor interaction with strychnine, and this interaction appears to be noncompetitive. The inhibitory effects on muscle AcChoRs were voltage-independent, and the apparent dissociation constant for AcCho was not appreciably changed by strychnine. In contrast, the inhibitory effects on neuronal AcChoRs were voltage-dependent, with an electrical distance of approximately 0.35. We conclude that strychnine regulates reversibly and noncompetitively the embryonic type of muscle AcChoR and some forms of neuronal AcChoRs. In the former case, strychnine presumably inhibits allosterically the receptor by binding at an external domain whereas, in the latter case, it blocks the open receptor-channel complex.  (+info)

Subtype-selective positive cooperative interactions between brucine analogs and acetylcholine at muscarinic receptors: functional studies. (4/582)

In radioligand binding studies, it has been reported that brucine, N-chloromethyl brucine, and brucine N-oxide increased the affinity of acetylcholine for M1, M3, and M4 muscarinic receptors, respectively, in a manner consistent with the predictions of the ternary complex allosteric model. We now demonstrate an equivalent ability of these three allosteric agents to modulate the actions of acetylcholine in functional studies in membranes and in whole cells. The enhancing actions of brucine and brucine N-oxide on acetylcholine (ACh) potency at M1 and M4 receptors respectively have been confirmed in guanosine-5'-O-(3-[35S]thio)triphosphate, GTPase, cAMP, and intracellular Ca2+ mobilization assays of function. In general, neither the basal nor the maximally stimulated response to ACh is affected. The subtype-selective allosteric effects of N-chloromethyl brucine on M2 and M3 receptors were shown to be qualitatively and quantitatively the same in guanosine-5'-O-(3-[35S]thio)triphosphate functional assays, in terms of both its affinity and cooperativity with ACh, as those found in binding assays. Neutral cooperativity of N-chloromethyl brucine with ACh on M4 receptor function was also observed, thereby demonstrating its "absolute subtype selectivity": a lack of action at any concentration at M4 receptors and an action at M2 and M3 receptors. The enhancing action of N-chloromethyl brucine on neurogenically released ACh binding at M3 receptors was also detected in whole tissue as an increased contraction of the isolated guinea pig ileum to submaximal electrical stimulation. In conclusion, these functional studies confirm that brucine analogs are allosteric enhancers of ACh affinity at certain muscarinic receptor subtypes.  (+info)

Contribution of single-channel properties to the time course and amplitude variance of quantal glycine currents recorded in rat motoneurons. (5/582)

The amplitude of spontaneous, glycinergic miniature inhibitory postsynaptic currents (mIPSCs) recorded in hypoglossal motoneurons (HMs) in an in vitro brain stem slice preparation increased over the first 3 postnatal weeks, from 42 +/- 6 pA in neonate (P0-3) to 77 +/- 11 pA in juvenile (P11-18) HMs. Additionally, mIPSC amplitude distributions were highly variable: CV 0.68 +/- 0.05 (means +/- SE) for neonates and 0.83 +/- 0.06 for juveniles. We wished to ascertain the contribution of glycine receptor (GlyR)-channel properties to this change in quantal amplitude and to the amplitude variability and time course of mIPSCs. To determine whether a postnatal increase in GlyR-channel conductance accounted for the postnatal change in quantal amplitude, the conductance of synaptic GlyR channels was determined by nonstationary variance analysis of mIPSCs. It was 48 +/- 8 pS in neonate and 46 +/- 10 pS in juvenile HMs, suggesting that developmental changes in mIPSC amplitude do not result from a postnatal alteration of GlyR-channel conductance. Next we determined the open probability (Popen) of GlyR channels in outside-out patches excised from HMs to estimate the contribution of stochastic channel behavior to quantal amplitude variability. Brief (1 ms) pulses of glycine (1 mM) elicited patch currents that closely resembled mIPSCs. The GlyR channels' Popen, calculated by nonstationary variance analysis of these currents, was approximately 0.70 (0.66 +/- 0.09 in neonates and 0.72 +/- 0.05 in juveniles). The decay rate of patch currents elicited by brief application of saturating concentrations of glycine (10 mM) increased postnatally, mimicking previously documented changes in mIPSC time course. Paired pulses of glycine (10 mM) were used to determine if rapid GlyR-channel desensitization contributed to either patch current time course or quantal amplitude variability. Because we did not observe any fast desensitization of patch currents, we believe that fast desensitization of GlyRs underlies neither phenomenon. From our analysis of glycinergic patch currents and mIPSCs, we draw three conclusions. First, channel deactivation is the primary determinant of glycinergic mIPSC time course, and postnatal changes in channel deactivation rate account for observed developmental changes in mIPSC decay rate. Second, because GlyR-channel Popen is high, differences in receptor number between synapses rather than stochastic channel behavior are likely to underlie the majority of quantal variability seen at glycinergic synapses throughout postnatal development. We estimate the number of GlyRs available at a synapse to be on average 27 in neonate neurons and 39 in juvenile neurons. Third, this change in the calculated number of GlyRs at each synapse may account for the postnatal increase in mIPSC amplitude.  (+info)

Possible novel mechanism for bitter taste mediated through cGMP. (6/582)

Taste is the least understood among sensory systems, and bitter taste mechanisms pose a special challenge because they are elicited by a large variety of compounds. We studied bitter taste signal transduction with the quench-flow method and monitored the rapid kinetics of the second messenger guanosine 3',5'-cyclic monophosphate (cGMP) production and degradation in mouse taste tissue. In response to the bitter stimulants, caffeine and theophylline but not strychnine or denatonium cGMP levels demonstrated a rapid and transient increase that peaked at 50 ms and gradually declined throughout the following 4.5 s. The theophylline- and caffeine-induced effect was rapid, transient, concentration dependent and gustatory tissue-specific. The effect could be partially suppressed in the presence of the soluble guanylyl cyclase (GC) inhibitor 10 microM ODQ and 30 microM methylene blue but not 50 microM LY 83583 and boosted by nitric oxide donors 25 microM NOR-3 or 100 microM sodium nitroprusside. The proposed mechanism for this novel cGMP-mediated bitter taste signal transduction is cGMP production partially by the soluble GC and caffeine-induced inhibition of one or several phosphodiesterases.  (+info)

Actions of 3-[2-phosphonomethyl[1,1-biphenyl]-3-yl]alanine (PMBA) on cloned glycine receptors. (7/582)

1. PMBA is a novel antagonist of strychnine-sensitive glycine receptors in the rat spinal cord, however, its mode of action is unknown. The actions of PMBA on rat glycine receptor alpha1 and alpha2 homomers in Xenopus oocytes were studied under two-electrode voltage-clamp. 2. Co-application of PMBA and glycine to both alpha1 and alpha2 homomers yielded inward currents which decayed to a steady-state. Responses rose slowly to the same steady-state amplitude following a 2 min pre-incubation in PMBA. Strychnine, but not picrotoxinin, showed similar antagonism to PMBA. The potency of PMBA was independent of membrane potential between -100 and 0 mV. 3. When tested against EC50 concentrations of glycine, PMBA was almost equally potent on alpha1 (IC50, 406+/-41 nM: Hill coefficient, 1.5+/-0.2) and alpha2 (IC50, 539+/-56 nM; Hill coefficient, 1.4+/-0.2) homomers. 4. PMBA (1-I0 microM) and strychnine (200 nM) reduced the potency of glycine and the amplitude of the maximal agonist response of alpha1 and alpha2 homomers. In 10 microM PMBA, two distinct classes of glycine response were observed on alpha2, only a single class of responses were observed on alpha1. 5. There are similarities in PMBA and strychnine antagonism, although these compounds are structurally distinct. The possibility that PMBA interacts at two binding sites which differ in alpha1 and alpha2 subunits is discussed. PMBA may provide a lead structure for novel antagonists with which to investigate structural differences in glycine receptor at alpha1 and alpha2 subunits.  (+info)

Rostrocaudal progression in the development of periodic spontaneous activity in fetal rat spinal motor circuits in vitro. (8/582)

Rostrocaudal progression in the development of periodic spontaneous activity in fetal rat spinal motor circuits in vitro. Developmental changes in the periodic spontaneous bursts in cervical and lumbar ventral roots (VRs) were investigated using isolated spinal cord preparations obtained from rat fetuses at embryonic days (E) 13.5-18. 5. Spontaneous bursts were observed in the cervical VR at E13.5-17.5, and in the lumbar VR at E14.5-17.5. Bursts occurrence in the cervical and lumbar VRs was correlated in a 1:1 fashion at E14.5-16. 5. The bursts in the cervical VR preceded those in the lumbar VR at E14.5, but the latter came to precede the former by E16.5. The interval between spontaneous bursts in the lumbar VR was greatly prolonged after spinal cord transection at the midthoracic level at E14.5, whereas that in the cervical VR became significantly longer at E14.5-16.5. These results suggest that the dominant neuronal circuit initiating the spontaneous bursts shifts from cervical to lumbar region during this period. Bath application of a glutamate receptor antagonist, kynurenate (4 mM), had little effect on the spontaneous bursts in either cervical or lumbar VRs at E14.5-15.5. At E16.5, kynurenate abolished the spontaneous bursts in the cervical VR. Concomitant application of kynurenate and strychnine (5 microM), a glycine receptor antagonist, abolished all spontaneous bursts, suggesting that the major transmitter mediating the spontaneous bursts changes from glycine to glutamate in the cervical region by E16.5, but not in the lumbar region during this period.  (+info)