Apparent block of K+ currents in mouse motor nerve terminals by tetrodotoxin, mu-conotoxin and reduced external sodium

Antagonistic postsynaptic and presynaptic actions of cyclohexanol on neuromuscular synaptic transmission and function

Intentional ingestion of agricultural organophosphorus insecticides is a significant public health issue in rural Asia, causing thousands of deaths annually. Some survivors develop a severe, acute or delayed myasthenic syndrome. In animal models, similar myasthenia has been associated with increasing plasma concentration of one insecticide solvent metabolite, cyclohexanol. We investigated possible mechanisms using voltage and current recordings from mouse neuromuscular junctions (NMJs) and transfected human cell lines. Cyclohexanol (10-25 mM) reduced endplate potential (EPP) amplitudes by 10-40% and enhanced depression during repetitive (2-20 Hz) stimulation by up to 60%. EPP decay was prolonged more than twofold. Miniature EPPs were attenuated by more than 50%. Cyclohexanol inhibited whole-cell currents recorded from CN21 cells expressing human postjunctional acetylcholine receptors (hnAChR) with an IC₅₀ of 3.74 mM. Cyclohexanol (10-20 mM) also caused prolonged episodes of reduced-current, multi-channel bursting in outside-out patch recordings from hnAChRs expressed in transfected HEK293T cells, reducing charge transfer by more than 50%. Molecular modelling indicated cyclohexanol binding (-6 kcal/mol) to a previously identified alcohol binding site on nicotinic AChR α-subunits. Cyclohexanol also increased quantal content of evoked transmitter release by ∼50%. In perineurial recordings, cyclohexanol selectively inhibited presynaptic K⁺ currents. Modelling indicated cyclohexanol binding (-3.8 kcal/mol) to voltage-sensitive K⁺ channels at the same site as tetraethylammonium (TEA). TEA (10 mM) blocked K⁺ channels more effectively than cyclohexanol but EPPs were more prolonged in 20 mM cyclohexanol. The results explain the pattern of neuromuscular dysfunction following ingestion of organophosphorus insecticides containing cyclohexanol precursors and suggest that cyclohexanol may facilitate investigation of mechanisms regulating synaptic strength at NMJs. KEY POINTS: Intentional ingestion of agricultural organophosphorus insecticides is a significant public health issue in rural Asia, causing thousands of deaths annually. Survivors may develop a severe myasthenic syndrome or paralysis, associated with increased plasma levels of cyclohexanol, an insecticide solvent metabolite. Analysis of synaptic transmission at neuromuscular junctions in isolated mouse skeletal muscle, using isometric tension recording and microelectrode recording of endplate voltages and currents, showed that cyclohexanol reduced postsynaptic sensitivity to acetylcholine neurotransmitter (reduced quantal size) while simultaneously enhancing evoked transmitter release (increased quantal content). Patch recording from transfected cell lines, together with molecular modelling, indicated that cyclohexanol causes selective, allosteric antagonism of postsynaptic nicotinic acetylcholine receptors and block of presynaptic K⁺ -channel function. The data provide insight into the cellular and molecular mechanisms of neuromuscular weakness following intentional ingestion of agricultural organophosphorus insecticides. Our findings also extend understanding of the effects of alcohols on synaptic transmission and homeostatic synaptic function.

Structural and Functional Abnormalities of the Neuromuscular Junction in the Trembler-J Homozygote Mouse Model of Congenital Hypomyelinating Neuropathy

Mutations in peripheral myelin protein 22 (PMP22) result in the most common form of Charcot-Marie-Tooth (CMT) disease, CMT1A. This hereditary peripheral neuropathy is characterized by dysmyelination of peripheral nerves, reduced nerve conduction velocity, and muscle weakness. APMP22 point mutation in L16P (leucine 16 to proline) underlies a form of human CMT1A as well as the Trembler-J mouse model of CMT1A. Homozygote Trembler-J mice (Tr(J)) die early postnatally, fail to make peripheral myelin, and, therefore, are more similar to patients with congenital hypomyelinating neuropathy than those with CMT1A. Because recent studies of inherited neuropathies in humans and mice have demonstrated that dysfunction and degeneration of neuromuscular synapses or junctions (NMJs) often precede impairments in axonal conduction, we examined the structure and function of NMJs in Tr(J)mice. Although synapses appeared to be normally innervated even in end-stage Tr(J)mice, the growth and maturation of the NMJs were altered. In addition, the amplitudes of nerve-evoked muscle endplate potentials were reduced and there was transmission failure during sustained nerve stimulation. These results suggest that the severe congenital hypomyelinating neuropathy that characterizes Tr(J)mice results in structural and functional deficits of the developing NMJ.

Quantal Analysis of Endplate Potentials in Mouse Flexor Digitorum Brevis Muscle

The isolated flexor digitorum brevis (FDB) muscle from mice is extremely well suited to rapid acquisition of data and analysis of neurotransmitter release and action at neuromuscular junctions, because the muscle and its tibial nerve supply are simple to dissect and its constituent muscle fibers are short (<1 mm) and isopotential along their length. Methods are described here for dissection of FDB, stimulation of the tibial nerve, microelectrode recording from individual muscle fibers, and quantal analysis of endplate potentials (EPPs) and miniature endplate potentials (MEPPs). Curr. Protoc. Mouse Biol. 1:429-444 © 2011 by John Wiley & Sons, Inc.

Anti-GD1a antibodies activate complement and calpain to injure distal motor nodes of Ranvier in mice

The motor axonal variant of Guillain-Barré syndrome is associated with anti-GD1a immunoglobulin antibodies, which are believed to be the pathogenic factor. In previous studies we have demonstrated the motor terminal to be a vulnerable site. Here we show both in vivo and ex vivo, that nodes of Ranvier in intramuscular motor nerve bundles are also targeted by anti-GD1a antibody in a gradient-dependent manner, with greatest vulnerability at distal nodes. Complement deposition is associated with prominent nodal injury as monitored with electrophysiological recordings and fluorescence microscopy. Complete loss of nodal protein staining, including voltage-gated sodium channels and ankyrin G, occurs and is completely protected by both complement and calpain inhibition, although the latter provides no protection against electrophysiological dysfunction. In ex vivo motor and sensory nerve trunk preparations, antibody deposits are only observed in experimentally desheathed nerves, which are thereby rendered susceptible to complement-dependent morphological disruption, nodal protein loss and reduced electrical activity of the axon. These studies provide a detailed mechanism by which loss of axonal conduction can occur in a distal dominant pattern as observed in a proportion of patients with motor axonal Guillain-Barré syndrome, and also provide an explanation for the occurrence of rapid recovery from complete paralysis and electrophysiological in-excitability. The study also identifies therapeutic approaches in which nodal architecture can be preserved.

Progressive abnormalities in skeletal muscle and neuromuscular junctions of transgenic mice expressing the Huntington's disease mutation

Huntington's disease (HD) is a neurodegenerative disorder with complex symptoms dominated by progressive motor dysfunction. Skeletal muscle atrophy is common in HD patients. Because the HD mutation is expressed in skeletal muscle as well as brain, we wondered whether the muscle changes arise from primary pathology. We used R6/2 transgenic mice for our studies. Unlike denervation atrophy, skeletal muscle atrophy in R6/2 mice occurs uniformly. Paradoxically however, skeletal muscles show age-dependent denervation-like abnormalities, including supersensitivity to acetylcholine, decreased sensitivity to mu-conotoxin, and anode-break action potentials. Morphological abnormalities of neuromuscular junctions are also present, particularly in older R6/2 mice. Severely affected R6/2 mice show a progressive increase in the number of motor endplates that fail to respond to nerve stimulation. Surprisingly, there was no constitutive sprouting of motor neurons in R6/2 muscles, even in severely atrophic muscles that showed other denervation-like characteristics. In fact, there was an age-dependent loss of regenerative capacity of motor neurons in R6/2 mice. Because muscle fibers appear to be released from the activity-dependent cues that regulate membrane properties and muscle size, and motor axons and nerve terminals become impaired in their capacity to release neurotransmitter and to respond to stimuli that normally evoke sprouting and adaptive reinnervation, we speculate that in these mice there is a progressive dissociation of trophic signalling between motor neurons and skeletal muscle. However, irrespective of the cause, the abnormalities at neuromuscular junctions we report here are likely to contribute to the pathological phenotype in R6/2 mice, particularly in late stages of the disease.

Different effects of toosendanin on perineurially recorded Ca(2+) currents in mouse and frog motor nerve terminals

By perineurial recording, the effects of toosendanin (TSN), a presynaptic blocker, on nerve terminal calcium currents (I(Ca)) were observed in innervated triangularis sterni of the mouse and cutaneous pectoris of the frog. It was found that TSN blocked the slow component of I(Ca) insensitive to nifedipine and omega-conotoxin-GVIA, and increasing the extracellular Ca(2+) concentration partially antagonized the inhibitory effect in mouse motor nerve terminals. However, in the frog, TSN increased the slow component of I(Ca) and this effect disappeared in the presence of nifedipine in perfusion solution. Based on previous data showing that the slow component of I(Ca) were mediated by different subtypes of calcium channels in mouse and frog motor nerve terminals, we presume that TSN could exercise different effects on various subtypes of calcium channels.

Safety factor at the neuromuscular junction

Reliable transmission of activity from nerve to muscle is necessary for the normal function of the body. The term 'safety factor' refers to the ability of neuromuscular transmission to remain effective under various physiological conditions and stresses. This is a result of the amount of transmitter released per nerve impulse being greater than that required to trigger an action potential in the muscle fibre. The safety factor is a measure of this excess of released transmitter. In this review we discuss the practical difficulties involved in estimating the safety factor in vitro. We then consider the factors that influence the safety factor in vivo. While presynaptic transmitter release may be modulated on a moment to moment basis, the postsynaptic features that determine the effect of released transmitter are not so readily altered to meet changing demands. Different strategies are used by different species to ensure reliable neuromuscular transmission. Some, like frogs, rely on releasing a large amount of transmitter while others, like man, rely on elaborate postsynaptic specialisations to enhance the response to transmitter. In normal adult mammals, the safety factor is generally 3-5. Both pre- and postsynaptic components change during development and may show plasticity in response to injury or disease. Thus, both acquired autoimmune and inherited congenital diseases of the neuromuscular junction (NMJ) can significantly reduce, or even transiently increase, safety factor.

Beta-agkistrodotoxin inhibits fast and Ca2+-activated K+ currents recorded from mouse motor nerve terminals

Beta-agkistrodotoxin (beta-AgTx), a polypeptide purified from the venom of Agkistrodon blomhoffii brevicaudus, is a presynaptic blocker acting on neurotransmitter release. In this work, perineural recording technique was employed to study the effects of beta-AgTx on sodium, potassium and calcium currents of mouse motor nerve terminals. The results showed that beta-AgTx selectively inhibited Ca2+-dependent (I(K,Ca)) and fast (I(K,f) K+ currents, but did not affect slow K+ current (I(K,s)), sodium and calcium currents. However there are other components in A. blomhoffii brevicaudus venom which inhibit perineural sodium current. The present data have provided additional evidence that the site of action of beta-AgTx is different from that of beta-bungarotoxin.

Synaptic vesicle dynamics in rat fast and slow motor nerve terminals

We have investigated whether rat motor nerve terminals with different in vivo activity patterns also have different vesicle trafficking characteristics. To do this, we monitored, using combined optical and electrical techniques, the rate of exocytosis (during different frequencies and patterns of activity), the releasable pool size, and the recycle time of synaptic vesicles in terminals on soleus (slow-twitch) and extensor digitorum longus [(EDL); fast-twitch] muscle fibers. EDL terminals had a higher initial quantal content (QC) than soleus, but during tonic or phasic stimulation at 20-80 Hz, EDL QC ran down to a greater extent than soleus QC. By recording loss of fluorescence from exocytosing vesicles labeled with the dye FM1-43, EDL terminals were found to destain faster than those in soleus. Simultaneous intracellular recording of end plate potentials, to count the number of vesicles released, permitted estimation of the total vesicle pool (VP) size and the recycle time by combining the optical and electrophysiological data. Soleus vesicle pool was larger than EDL, but recycle time was not significantly different. These terminals, therefore, are adapted to their in vivo activity patterns by alterations in QC and VP size but not recycle time.

Synaptic vesicle dynamics in rat fast and slow motor nerve terminals

We have investigated whether rat motor nerve terminals with different in vivo activity patterns also have different vesicle trafficking characteristics. To do this, we monitored, using combined optical and electrical techniques, the rate of exocytosis (during different frequencies and patterns of activity), the releasable pool size, and the recycle time of synaptic vesicles in terminals on soleus (slow-twitch) and extensor digitorum longus [(EDL); fast-twitch] muscle fibers. EDL terminals had a higher initial quantal content (QC) than soleus, but during tonic or phasic stimulation at 20-80 Hz, EDL QC ran down to a greater extent than soleus QC. By recording loss of fluorescence from exocytosing vesicles labeled with the dye FM1-43, EDL terminals were found to destain faster than those in soleus. Simultaneous intracellular recording of end plate potentials, to count the number of vesicles released, permitted estimation of the total vesicle pool (VP) size and the recycle time by combining the optical and electrophysiological data. Soleus vesicle pool was larger than EDL, but recycle time was not significantly different. These terminals, therefore, are adapted to their in vivo activity patterns by alterations in QC and VP size but not recycle time.

Characterisation of the effects of depolarising toxins on nerve terminal action potentials: apparent block of presynaptic potassium currents

Previous studies showed that toxic phospholipases A2 (Pa-8 and Pa-10F) from the venom of Pseudechis australis, the Australian king brown snake, reduced acetylcholine release at mouse neuromuscular junctions and depressed motor nerve terminal action potentials [Fatehi et al. (1994a), Toxicon 32, 1559-1572], and it was postulated that these toxins induced their effect on the action potential waveforms through nerve terminal depolarisation. To test this hypothesis, the effects of Pa-11 (another phospholipase A2 from the venom of Pseudechis australis), and the known depolarising agents. myotoxin a, from the venom of the rattlesnake. Crotalus viridis viridis, and ouabain on these waveforms were compared with the changes induced in the nerve terminal action potentials by Pa-8 and Pa-10F. The experiments were performed on the isolated mouse triangularis sterni preparation, using extracellular recordings. Pa-11 (0.1 microM) decreased the component of nerve terminal action potential related to Na+ and K+ currents to about 80% and 40% of control, respectively, after 60 min. Myotoxin alpha (5 microM) and ouabain (50 microM) produced similar, time-dependent changes in the nerve terminal action potential. These effects are similar to those produced by Pa-8 and Pa-10F, and are consistent with a slow but partial loss of membrane potential at the nerve terminal. In addition, whole-cell patch-clamp recording was employed to investigate possible direct actions of Pa-8. Pa10F and Pa-11 on Na+ and K+ currents in NG108 and PC12 cells in culture. None of these toxins (0.8 microM) reduced the Na+ and K+ currents in these cells. There was also no displacement of [125I]alpha-dendrotoxin bound to voltage-sensitive potassium channels on rat synaptosomal membranes induced by Pa-8, Pa-10F and Pa-11 (up to 100 microM). These results support the hypothesis that the alteration of nerve terminal waveforms by these toxic phospholipases A2 is mediated by nerve terminal depolarisation.

The contribution of postsynaptic folds to the safety factor for neuromuscular transmission in rat fast- and slow-twitch muscles

1. At the rat neuromuscular junction, the postsynaptic folds and the voltage-gated sodium channels (VGSCs) within them are thought to amplify the effects of postsynaptic currents. In this study, the contribution of this effect to the safety factor for neuromuscular transmission, the ratio of the normal quantal content to the number of quanta required to reach threshold, has been estimated. 2. Normal quantal content was determined in isolated nerve-muscle preparations of rat soleus and extensor digitorum longus (EDL) muscles in which muscle action potentials were blocked by mu-conotoxin. The quantal content estimated from voltage recordings was 61.8 and 79.4 in soleus and EDL, respectively, and from charge measurements derived from current recordings was 46.3 (soleus) and 65.1 (EDL). 3. The threshold for action potential generation in response to nerve stimulation was determined from endplate potentials (EPPs) and endplate currents (EPCs) in preparations partially blocked with d-tubocurarine. The number of quanta required to reach threshold was estimated from voltage recordings to be 19.7 (soleus) and 23.2 (EDL) and from charge measurements derived from current recordings to be 13.3 (soleus) and 13.0 (EDL). 4. When intracellular electrodes were used to inject current into the muscle fibre, the total charge required to reach threshold was approximately twice that of the nerve-evoked threshold EPC. 5. The safety factor for nerve-evoked responses at the junction was 3.5 (soleus) and 5.0 (EDL). In the extrajunctional region the safety factor estimated from injected currents was 1.7 (soleus) and 2.5 (EDL). 6. It is concluded that the effect of the postsynaptic folds and the VGSCs within them is to double the safety factor. At normal frequencies of nerve impulse activity in vivo, this effect is likely to be crucial for ensuring effective neuromuscular transmission.

Polyamine FTX-3.3 and polyamine amide sFTX-3.3 inhibit presynaptic calcium currents and acetylcholine release at mouse motor nerve terminals

FTX-3.3 is the proposed structure of a calcium-channel blocking toxin that has been isolated from the funnel web spider (Agelenopsis aperta). The effects of FTX-3.3 and one of its analogues, sFTX-3.3, on acetylcholine release, on presynaptic currents at mouse motor nerve terminals and on whole-cell sodium currents in SK.N.SH cells (a human neuroblastoma cell line) have been studied. FTX-3.3 (10-30 microM) and sFTX-3.3 (100-300 microM) reversibly reduced release of acetylcholine by approximately 70-90% and 40-60%, respectively. FTX-3.3 (10 microM) blocked the fast component of presynaptic calcium currents by approximately 60%. sFTX-3.3 (100 microM) reduced the duration of the slow component of presynaptic calcium currents by about 50% of the control and also reduced presynaptic sodium current by approximately 20% of the control. sFTX-3.3 (100 microM) reduced whole-cell sodium current recorded from SK.N.SH cells by approximately 15%, whereas FTX-3.3, even at 200 microM, did not affect this current. Since the only difference in chemical structures of these toxins is that sFTX-3.3 has an amide function which is absent in FTX-3.3, the amide function may be responsible for the reduced potency and selectivity of sFTX-3.3. This study also provides further support for the existence of P-type calcium channels at mouse motor nerve terminals.

Niflumic acid-induced increase in potassium currents in frog motor nerve terminals: effects on transmitter release

The actions of the nonsteroidal antiinflammatory drug niflumic acid were studied on frog neuromuscular preparations by conventional electrophysiological techniques. Niflumic acid reduced the amplitude and increased the latency of endplate potentials in a concentration-dependent manner. Neuromuscular junctions pretreated with niflumic acid (0.05-0.5 mM) showed much less depression than control when they were stimulated with trains of impulses. Inhibition of acetylcholine release was reverted by raising the extracellular Ca(2+) concentration but not by simply washing out the preparations with niflumic acid-free solutions. Pretreatment with indomethacin (0.1 mM), another nonsteroidal antiinflammatory drug, did not affect the niflumic acid-induced inhibition of evoked responses. Niflumic acid (0.1 mM) did not change the amplitude of miniature endplate potentials and had a dual action on the frequency of miniatures: it decreased their frequency at 0.1 mM whereas it produced an enormous increase in the rate of spontaneous discharge at 0.5 mM. Niflumic acid (0.1 - 1 mM) reversibly increased the amplitude and affected the kinetics of presynaptic voltage-activated K+ current and Ca(2+)-activated K(+) current in a concentration-dependent manner. Niflumic acid (0.1 - 1 mM) irreversibly decreased the amplitude and reversibly affected the kinetics of the nodal Na(+) current. Indomethacin (0.1 mM) had no effect on presynaptic currents. In conclusion, niflumic acid reduces acetylcholine release by increasing presynaptic K+ currents. This may shorten the depolarizing phase of the presynaptic action potential and may reduce the entry of Ca(2+) with each impulse.

On the simultaneous electrophysiological measurements of neurotransmitter release and perineural calcium currents from frog motor nerve endings

Ca2+ currents from the perineural region of motor nerve endings were measured together with evoked acetylcholine (ACh) release (i.e., end-plate potentials EPPs) in frog skeletal muscle in an attempt to define experimental conditions in which simultaneous measurements of both phenomena were feasible. In a solution containing low Ca2+ (0.9 mM), high Mg2+ (10 mM) and modest concentrations of K+ channel blockers (250 microM tetraethylammonium, 100 microM 3,4,-diaminopyridine), reliable measurements of perineural Ca2+ currents were possible. For convenience, this solution will be termed 'Ca2+ current' Ringer. The mean number of ACh quanta released in Ca2+ current Ringer was near the midpoint of the relationship between extracellular [Ca2+] and evoked ACh release observed previously in normal Ringer solutions. Consequently, ACh release in response to low-frequency motor nerve stimulation (0.05 Hz) was well maintained, allowing simultaneous measurements of Ca2+ currents and evoked ACh release to be made. Ca2+ currents and EPPs measured simultaneously in Ca2+ current Ringer were increased or decreased in parallel by increasing or decreasing the extracellular Ca2+ concentrations. Ca2+ channel blockers (Cd2+, 500 microM; omega-conotoxin, 3 microM) eliminated both EPPs and the Ca2+ component of the perineural current. NaF (10 mM), which stimulates ACh release, produced parallel increases in EPPs and perineural Ca2+ currents. NG-cyclohexyladenosine (CHA), an A1 adenosine receptor agonist, inhibits ACh release without effects on perineural currents. The results suggest that the concurrent electrophysiological recording of Ca2+ currents and ACh release in Ca2+ current Ringer is a reliable experimental approach for determining whether drugs or disease states affect ACh release by acting on Ca2+ channels in the presynaptic membrane.

The effects of five phospholipases A2 from the venom of king brown snake, Pseudechis australis, on nerve and muscle

The effects on vertebrate neuromuscular function of five homologous phospholipases A2 (PLA2) (Pa-3, Pa-8, Pa-9C, Pa-10F and Pa-12B) from the venom of the Australian king brown snake, Pseudechis australis, were determined. These isoenzymes (0.2-1.6 microM) reduced, with different potencies, responses of chick biventer cervicis preparations to nerve stimulation and to exogenously applied acetylcholine, carbachol and KCl in a time- and concentration-dependent way but with different potencies. They also blocked twitches of mouse hemidiaphragm preparations evoked by nerve and by direct muscle stimulation. Pa-8 was the most active and Pa-9C was the least potent. There was a strong correlation between the enzymatic activity and the effect of toxins on the responses of mouse hemidiaphragm to direct muscle stimulation, but weak correlation between the effects on indirect responses and enzymatic activity. Intracellular recording from endplate regions of mouse triangularis sterni nerve-muscle preparations showed that Pa-10F and Pa-12B at 0.2 microM significantly reduced quantal content after 10 min. Pa-8 (0.2 microM) reduced the amplitude of endplate potentials by about 25% and abolished miniature endplate potentials within 15 min. Pa-3 (0.2 microM) and Pa-9C (0.8 microM) also significantly reduced quantal content by about 30% of control after 30 min. Among these toxins, Pa-3 and Pa-8 at 0.2 microM depolarised mouse muscle fibres after 30 min. Extracellular recording of action potentials at motor nerve terminals of mouse triangularis sterni preparations indicated that these isoenzymes reduced the waveforms associated with both Na+ and K+ conductances. Since no facilitatory effect on the release process has been observed, the apparent blockade of K+ conductance by some of these toxins may not be a selective action on K+ channels, but may be secondary to membrane depolarisation. An in vivo study with Pa-8 and Pa-10F demonstrated myotoxic effects. Light microscopic examination showed a degeneration of mouse and rat skeletal muscle fibres caused by Pa-8 and Pa-10F. For the in vivo study, rats received 80 micrograms/kg of the toxins s.c. and mice were injected i.m. with the toxins (40 micrograms/kg). Myotoxicity appears to be the predominant effect of these five toxins.

A pharmacological study of NK1 and NK2 tachykinin receptor characteristics in the rat isolated urinary bladder

1. We have estimated potencies of tachykinin receptor agonist and antagonist analogues in order to determine the recognition characteristics of tachykinin receptors mediating phasic contractile responses of the rat isolated urinary bladder in vitro. 2. The NK1-selective synthetic agonists, substance P methyl ester and GR73632, the synthetic NK2-selective agonists [beta-Ala8]-NKA(4-10) and GR64349, and the mammalian tachykinins, neurokinin A and neurokinin B, were assayed relative to substance P and were found to be approximately equipotent. The NK3-selective agonist, senktide, was inactive (10 microM). 3. Potencies of all these agonists were not significantly different (P > 0.05) when experiments were carried out in the presence of the neutral endopeptidase inhibitor, phosphoramidon, and the kininase II inhibitor, enalaprilat (both 1 microM). 4. The NK1-selective antagonist, GR82334, inhibited responses to substance P methyl ester in a competitive manner in the rat urinary bladder and the rat ileum, and also in the guinea-pig ileum. Markedly different pKB estimates were obtained in the rat bladder (6.38) and rat ileum (6.56) compared to the guinea-pig ileum (7.42). GR82334 (3 microM) was inactive against responses of the rat bladder to [beta-Ala8]-NKA(4-10). 5. The NK1-selective antagonist (+/-)-CP-96,345 also inhibited responses of the rat bladder and guinea-pig ileum to substance P methyl ester; however, in the rat bladder at 1 microM, this antagonist reversibly inhibited responses both to the NK2-selective agonist [beta-Ala8]-NKA(4-10) and to the muscarinic agonist carbachol (P < or = 0.01), thus showing evidence of some non-selective depressant actions. 6. The NK2-selective antagonists, MEN10207 and L-659,874, competitively inhibited responses of the rat bladder to the NK2-selective agonist [P-Ala5]-NKA(4-10) giving pKB estimates of 5.75 and 6.68,respectively. Both antagonists (1O microM) were inactive against responses to the NKI-selective agonist substance P methyl ester.7. These results support the proposal of a mixed population of NKI and NK2 receptors mediating contraction of the rat isolated urinary bladder. The NK2 receptor is characterized by a relatively low affinity for the NK2-selective antagonist MEN10207 but a high affinity for L-659,874. The NKImediated responses are inhibited by (+/-)-CP-96,345: this compound however, has non-specific depressant effects in the rat bladder at high concentration (1 microM). In contrast, the NK,-receptor peptide antagonist GR82334, did not have non-specific depressant effects and competitively inhibited NK, responses in the rat bladder and rat ileum with an affinity significantly lower than at the NK,-receptors in the guinea-pigileum.