Actions of snake venom toxins on neuronal nicotinic receptors and other neuronal receptors

Bungarus multicinctus multicinctus Snakebite in Taiwan

AbstractAlthough specific antivenom is available in Taiwan, respiratory failure and general pain frequently accompany Bungarus multicinctus envenomation and there have been few reports on the management of B. multicinctus envenomation. We retrospectively analyzed 44 cases of B. multicinctus bite admitted to Taichung Veterans General Hospital (VGH) or to Taipei VGH. Demographic data, treatment, and outcome of patients with and without respiratory failure were compared. In this study, 20.5% patients had bites without noticeable signs or symptoms of significant envenoming, 27.3% developed respiratory failure, and 27.3% experienced general pain. Bivalent specific antivenom for B. multicinctus and N. atra was administered in all envenomed cases. Respiratory failure occurred 1.5-6.5 hours post-bite and general pain occurred 1-12 hours post-bite. Specific antivenom for B. multicinctus and N. atra at the recommended dose (i.e., 2-4 vials) might not effectively prevent respiratory failure and pain. Respiratory failure, general pain, and autonomic effects after B. multicinctus bite were probably caused, at least partly, by β-bungarotoxin. Although general weakness, ptosis, dysarthria, and dilated pupils were significantly associated with respiratory failure, their predictive value could not be accurately determined in such a retrospective study. Due to the rapid onset of respiratory failure, every suspected envenomed case thus should be closely monitored in the first few hours. We recommend the initial administration of four vials of antivenom in all envenomation cases, and a subsequent four vials be considered if the patient's condition is deteriorating. Prospective evaluation of the antivenom dosing regimen is urgently needed to improve B. multicinctus envenomation treatment.

Role of pudendal afferents in voiding efficiency in the rat

The reciprocal activities of the bladder and external urethral sphincter (EUS) are coordinated by descending projections from the pontine micturition center but are subjected to modulation by peripheral afferent inputs. Transection of the somatic pudendal nerve innervating the striated EUS decreases voiding efficiency and increases residual urine in the rat. The reduction in voiding efficiency was attributed to the lack of phasic bursting activity of the EUS following denervation. However, transection of the pudendal nerve also eliminates somatic sensory feedback that may play a role in voiding. We hypothesized that feedback from pudendal afferents is required for efficient voiding and that the loss of pudendal sensory activity contributes to the observed reduction in voiding efficiency following pudendal nerve transection. Quantitative cystometry in urethane anesthetized female rats following selective transection of pudendal nerve branches, following chemical modulation of urethral afferent activity, and following neuromuscular blockade revealed that pudendal nerve afferents contributed to efficient voiding. Sensory feedback augmented bladder contraction amplitude and duration, thereby increasing the driving force for urine expulsion. Second, sensory feedback was necessary to pattern appropriately the EUS activity into alternating bursts and quiescence during the bladder contraction. These findings demonstrate that the loss of pudendal sensory activity contributes to the reduction in voiding efficiency observed following pudendal nerve transection, and illustrate the importance of urethral sensory feedback in regulating bladder function.

Presynaptic nicotinic receptors and the modulation of transmitter release

Nicotine is increasingly recognized to promote transmitter release in the brain by a direct action on presynaptic terminals. Pharmacological evidence indicates that this action is mediated by nicotinic receptors. From their sensitivity to mecamylamine, neosurugatoxin and neuronal bungarotoxin these presynaptic receptors can be distinguished from alpha-bungarotoxin-sensitive muscle-type nicotinic receptors, and can be correlated with [3H] nicotine binding sites in the brain. The release of many transmitters in different brain regions is susceptible to stimulation by nicotine, but this effect is not ubiquitous. However, lesioning and subcellular fractionation studies suggest that the majority of brain nicotine receptors are located presynaptically, so that a direct influence of nicotine on transmitter release assumes considerable importance. Although the sensitivity of presynaptic receptors is such that they are likely to be partially activated by doses of nicotine obtained by smoking, the desensitization-induced up-regulation of nicotinic binding sites that follows chronic nicotine treatment raises questions about their functional status during tobacco usage. Chronic administration of the agonist (+)anatoxin-a also up-regulated [3H] nicotine binding sites, and led to increased nicotine-evoked transmitter release in vitro. This could have implications for the involvement of these receptors during withdrawal.

Effect of bilateral hypogastric nerve transection on voiding dysfunction in rats with spinal cord injury

We determined if bilateral section of the hypogastric nerves (HGN), which provide the major sympathetic input to the urinary bladder neck/proximal urethra, could improve voiding by reducing urethral resistance in conscious, female spinal-cord-injured (SCI) rats 2-3 weeks after T(7-9) transection of the spinal cord. Cystometry was performed in animals with HGN intact and with HGN sectioned bilaterally 1-2 h before the experiment. Residual volume (RV), volume threshold for inducing micturition (VT), maximal voiding pressure, and bladder compliance were significantly lower (71, 35, 33, and 31%, respectively) in SCI rats with sectioned HGN than in rats with intact HGN, whereas voided volume (VV), pressure threshold for micturition, and bladder contraction duration (BCD) in the two groups were similar. Voiding efficiency (VE) in the HGN-sectioned group was 36% greater than that in the HGN-intact group. Antagonists for AMPA and NMDA glutamatergic receptors (LY215490 and MK-801, respectively) were administered to rats with sectioned HGN, to determine if activity in the HGN contributes to the previously reported inhibitory effects of these drugs, on voiding function after SCI. MK-801 (3 mg/kg iv) significantly reduced VV (75%) and VE (85%) and increased RV (8-fold), VT (87%), and bladder compliance (60%), whereas LY215490 (10 mg/kg iv) significantly increased VT and BCD by 15 and 19%, respectively. It is concluded that bilateral section of HGN reduces voiding dysfunction in the SCI rat but does not alter the effects of AMPA and/or NMDA glutamatergic receptor antagonists on the micturition reflex in the SCI rat. Thus the effects of these drugs are not dependent on changes in activity of sympathetic axons in the HGN.

Influences of external urethral sphincter relaxation induced by alpha-bungarotoxin, a neuromuscular junction blocking agent, on voiding dysfunction in the rat with spinal cord injury

OBJECTIVES

To determine whether external urethral sphincter (EUS) relaxation induced by alpha-bungarotoxin, a highly selective neuromuscular junction blocking agent, could ameliorate voiding dysfunction after spinal cord injury (SCI) in rats.

METHODS

The effects of intravenous alpha-bungarotoxin (333 microg/kg) were evaluated during cystometry in decerebrate, unanesthetized female Sprague-Dawley rats (250 to 300 g) with spinal cords chronically transected at T7-9 (n = 7) or with normal spinal cords (NSC) (n = 7). Parameters measured included voided volume (VV), residual volume (RV), volume threshold for inducing micturition (VT), voiding efficiency (VE), micturition pressure (MP), pressure threshold for inducing micturition (PT), bladder contraction duration (BCD), and compliance (CP).

RESULTS

In SCI rats, treatment with alpha-bungarotoxin improved voiding. The toxin increased VE (31%) and reduced RV (42%), MP (52%), BCD (14%), and VT (31%). VV, PT, and CP were not altered. In NSC rats, alpha-bungarotoxin decreased VE (23%), increased RV (63%), and decreased MP (36%), VV (38%), and VT (20%) but did not change BCD and CP.

CONCLUSIONS

The results of our study demonstrated that alpha-bungarotoxin improved voiding in SCI rats by reducing urethral outlet resistance. However, in NSC rats, the toxin reduced voiding, probably by suppressing high-frequency phasic sphincter activity, necessary for efficient urine elimination in normal animals. The present results provide further support for the view that drugs that depress striated muscle activity can be useful in the treatment of voiding dysfunction after SCI.

Urethral afferent nerve activity affects the micturition reflex; implication for the relationship between stress incontinence and detrusor instability

PURPOSE

A causative relationship between stress urinary incontinence (SUI) and detrusor instability has been suspected but never proven. Many women with mixed incontinence have resolution of detrusor instability after surgical correction of SUI. We sought experimental support that stimulation of urethral afferent nerves can induce or change reflex detrusor contractions.

MATERIALS AND METHODS

Urethral perfusion pressure and isovolumetric bladder pressure were measured with catheters inserted through the bladder dome in urethane anesthetized female S.D. rats (250 to 300 grams; n = 12). The catheter assembly was seated securely in the bladder neck to block passage of fluid between the bladder and urethra without affecting the nerve supply to the organs. The external urethra was not catheterized. Responses were examined in the control state at a urethral saline perfusion speed of 0.075 ml. per minute. Intraurethral drugs were administered following blockade of striated sphincter activity with intravenous alpha-bungarotoxin (0.1 mg./kg.).

RESULTS

Stopping the urethral saline infusion caused a significant decrease in micturition frequency in approximately 50% of the animals studied (n = 12). Intraurethral lidocaine (1%) infused at 0.075 ml. per minute caused a slight decrease in urethral perfusion pressure but no change in detrusor contraction amplitude. However, intraurethral lidocaine caused a significant (45%) decrease in the bladder contraction frequency (n = 5). The micturition frequency returned to baseline 30 minutes after stopping lidocaine infusion. Intraurethral infusion of nitric oxide (NO) donors (S-nitroso-N-acetylpenicillamine [SNAP] (2 mM) or nitroprusside (1 mM) immediately decreased urethral perfusion pressure by 30 to 37% (n = 5). A 45 to 75% decrease (n = 5) in bladder contraction frequency was also seen, which was similar to that observed following lidocaine. Neither NO donor changed the amplitude of bladder contractions.

CONCLUSIONS

These results indicate that in the anesthetized rat activation of urethral afferents by urethral perfusion can modulate the micturition reflex. Thus in patients with stress urinary incontinence, leakage of urine into the proximal urethra may stimulate urethral afferents and facilitate voiding reflexes. This implies that stress incontinence can induce and/or increase detrusor instability. These findings have significant implications for the treatment of patients with mixed urge and stress incontinence. Correction of stress incontinence by surgery or pelvic floor exercise in patients with mixed incontinence may resolve the detrusor instability.

Presence of nicotinic acetylcholine receptors in cat carotid body afferent system

With immunocytochemical techniques using a monoclonal antibody for alpha7 subunits of neuronal nicotinic acetylcholine receptors, we have found these subunits to be exclusively expressed in nerve fibers in the carotid body. Double-immunostaining showed that alpha7 subunit-positive nerve endings enveloped tyrosine hydroxylase-positive glomus cells. Some carotid sinus nerve fibers and tyrosine hydroxylase-positive petrosal ganglion neurons also expressed alpha7 subunits. These data support a role for acetylcholine in carotid body neurotransmission.

The cholinergic system in Alzheimer's disease

The past decade has witnessed an enormous increase in our knowledge of the variety and complexity of neuropathological and neurochemical changes in Alzheimer's disease. Although the disease is characterized by multiple deficits of neurotransmitters in the brain, this overview emphasizes the structural and neurochemical localization of the elements of the acetylcholine system (choline acetyltransferase, acetylcholinesterase, and muscarinic and nicotinic acetylcholine receptors) in the non-demented brain and in Alzheimer's disease brain samples. The results demonstrate a great variation in the distribution of acetylcholinesterase, choline acetyltransferase, and the nicotinic and muscarinic acetylcholine receptors in the different brain areas, nuclei and subnuclei. When stratification is present in certain brain regions (olfactory bulb, cortex, hippocampus, etc.), differences can be detected as regards the laminar distribution of the elements of the acetylcholine system. Alzheimer's disease involves a substantial loss of the elements of the cholinergic system. There is evidence that the most affected areas include the cortex, the entorhinal area, the hippocampus, the ventral striatum and the basal part of the forebrain. Other brain areas are less affected. The fact that the acetylcholine system, which plays a significant role in the memory function, is seriously impaired in Alzheimer's disease has accelerated work on the development of new drugs for treatment of the disease of the 20th century.

Pharmacology of neuronal nicotinic acetylcholine receptor subtypes

The search for the physiological function of nicotinic receptors on neurons in the brain began with their discovery. It was initially assumed that, as in ganglia and at the neuromuscular junction, nicotinic receptors would gate fast synaptic transmission in the brain. The best functional evidence now, however, points to a role in modifying the release of other transmitters. This does not preclude a postsynaptic role in transmission for nicotinic receptors in the brain, but attempts to locate such a synapse have not been successful. If fast nicotinic synapses are present in the brain, they are probably low in number and may be masked by other more prevalent synapses (such as glutamatergic) so identification will not be easy. The extent of diversity of nicotinic receptors is substantial. At the molecular level this is reflected in the number of different genes that encode receptor subunits and the multiple possible combinations of subunits that function in expression systems. From the cellular level there is a broad diversity of properties of native receptors in neurons. Some useful pharmacological tools allow the limited identification of subunits in native receptors. For example, block by alpha-bungarotoxin identifies alpha 7, alpha 8, or alpha 9 subunits; activation of a receptor by cytisine indicates an alpha 7 or beta 4 subunit; and neuronal bungarotoxin block identifies a beta 2 subunit. Despite the clues to identity gained by careful use of these agents, we have not been able to identify all the components of any native receptor based on pharmacological properties assessed from expression studies. When both pharmacological and biophysical properties of a receptor are taken into consideration, none of the combinations tested in oocytes mimics native receptors exactly. The reason for this discrepancy has been debated at length; it is possible that oocytes do not faithfully manufacture neuronal nicotinic receptors. For example, they may not correctly modify the protein after translation or they may allow a combination of subunits that do not occur in vivo. Another possibility is that correct combinations of subunits have not yet been tested in oocytes. Data from immunoprecipitation experiments suggest that many receptors contain three or more different subunits. Results from further experiments injecting combinations of three or more subunits into oocytes may be enlightening. The diversity of receptors may allow targeting of subtypes to specific locations. Nicotinic receptors are located presynaptically, preterminally, and on the cell soma. The function of the nicotinic receptors located on innervating axons is presumably to modify the release of other neurotransmitters. It is an attractive hypothesis that nicotinic receptors might be involved in modifying the weight of central synapses; however, in none of the regions where this phenomenon has been described is there any evidence for axoaxonal contacts. The presynaptic receptors described so far are pharmacologically unique; therefore, if there are different subtypes of nicotinic receptors modifying the release of different transmitters, they may provide a means of exogenously modifying the release of a particular transmitter with drugs. There are still many basic unanswered questions about nicotinic receptors in the brain. What are the compositions of native nicotinic receptors? What is their purpose on neurons? Although there is clearly a role presynaptically, what is the function of those located on the soma? Neuronal nicotinic receptors are highly permeable to calcium, unlike muscle nicotinic receptors, and this may have important implications for roles in synaptic plasticity and development. Finally, why is there such diversity? (ABSTRACT TRANCATED)

Synaptic currents generated by neuronal acetylcholine receptors sensitive to alpha-bungarotoxin

Nicotinic acetylcholine receptors are widely distributed throughout the nervous system, but their functions remain largely unknown. One of the most abundant is a class of receptors that contains the alpha 7 gene product, has a high relative permeability to calcium, and binds alpha-bungarotoxin. Here, we report that receptors sensitive to alpha-bungarotoxin, though concentrated in perisynaptic clusters on neurons, can generate a large amount of the synaptic current. Residual currents through other nicotinic receptors are sufficient to elicit action potentials, but with slower rise times. This demonstrates a postsynaptic response for alpha-bungarotoxin-sensitive receptors on neurons and suggests that the functional domain of the postsynaptic membrane is broader than previously recognized.

Blockade of cardiac nicotinic responses by anticholinesterases

1. Tacrine (10 microM) and physostigmine (10 microM) completely inhibited the positive chronotropic and inotropic actions of acetylcholine (ACh) or nicotine in the atropinized guinea pig right atria. 2. Edrophonium (6 microM) and soman (0.1 microM) completely inhibited these nicotinic responses, as well as the associated increase in pyridine nucleotide fluorescence and vasodilation induced by ACh in the atropinized guinea pig perfused heart. 3. The 200-fold increase in noradrenaline release induced by ACh in the perfused heart was blocked by 10 microM tacrine and 6 microM edrophonium. 4. Tacrine (10 microM) significantly (16-32%) reduced the basal heart rate of both preparations. 5. Edrophonium (6 microM) induced a five- to sixfold increase in basal 3,4-dihydroxyphenyl-(ethylene) glycol (DOPEG) release. 6. The inhibition of nicotinic receptor activation in the atria by the anticholinesterases appears mainly non-competitive. IC50 values range from 0.1 to 10 microM in the perfused heart to 1 to 100 microM in atria (in either case tacrine about 2 microM). 7. The possibility that these compounds have a direct action at nicotinic receptors is discussed.

Neuronal bungarotoxin displaces (125I) alpha-bungarotoxin binding at the neuromuscular junction as well as to the spinal cord during embryogenesis

Alpha-Bungarotoxin (alpha BTX) administration in ovo prevents motoneuron apoptosis during development. This process may be mediated by alpha BTX-sensitive nicotinic cholinoceptors in the spinal cord, at the neuromuscular junction or at both sites. In order to differentiate between these possibilities, neuronal bungarotoxin binding (NBTX) binding to embryonic muscle and spinal cord was investigated in the chick.

Neuronal nicotinic alpha 7 receptor expressed in Xenopus oocytes presents five putative binding sites for methyllycaconitine

1. The recently isolated compound methyllycaconitine (MLA) is a plant toxin which is a competitive inhibitor of nicotinic acetylcholine receptors (nAChRs). We found that homomeric alpha 7 receptors display a very high sensitivity to MLA with an IC50 in the picomolar range. 2. The competitive nature of the alpha 7 MLA blockade was reinforced by the observation that this compound has no action on wild-type serotoninergic receptors (5-HT3), whereas it is a powerful antagonist of chimaeric receptors alpha 7-5-HT3. 3. The time course of MLA inhibition of the wild-type (WT) alpha 7 follows a monotonic exponential decay whose time constant is proportional to the MLA concentration and could be described by a bimolecular mechanism with a forward rate constant (k+) of 2.7 x 10(7) S-1 M-1. In contrast, recovery from MLA inhibition displays an S-shaped time course that is incompatible with a simple bimolecular reaction. 4. Given the pentameric nature of the neuronal nicotinic receptors, a linear chain model, including five putative MLA binding sites corresponding to the homomeric nature of alpha 7, is proposed. 5. Both onset and recovery data obtained on the alpha 7 wild-type receptor are adequately described by this model assuming that a single MLA molecule is sufficient to block receptor function. 6. Analysis of MLA blockade and recovery of reconstituted heteromeric alpha 4 beta 2 receptors reveals, as expected, a time course compatible with only two binding sites for the toxin and, thus, further supports the validity of our model.

alpha-Bungarotoxin, kappa-bungarotoxin, alpha-cobratoxin and erabutoxin-b do not affect [3H]acetylcholine release from the rat isolated left hemidiaphragm

Endplate preparations of the rat left hemidiaphragm were incubated with [3H]choline to label neuronal transmitter stores. Nerve evoked release of newly-synthesized [3H]acetylcholine was measured in the absence of cholinesterase inhibitors to investigate whether snake venom neurotoxins by blocking presynaptic nicotinic autoreceptors affect evoked transmitter release. Contractions of the indirectly stimulated hemidiaphragm were recorded to characterize the blocking effect of alpha-neurotoxins at the post-synaptic nicotinic receptors. Neither the long chain neurotoxins alpha-cobratoxin (1 microgram ml-1) and alpha-bungarotoxin (5 microgram ml-1) nor the short chain neurotoxin erabutoxin-b (0.1, 1 and 10 micrograms ml-1) affected the nerve-evoked release of [3H]acetylcholine. kappa-Bungarotoxin (1 and 5 micrograms ml-1), a toxin preferentially blocking neuronal nicotinic receptors, did also not affect evoked [3H]acetylcholine release, whereas (+)-tubocurarine (1 microM) under identical conditions reduced the release by about 50%. alpha-Bungarotoxin, alpha-cobratoxin and erabutoxin-b concentration-dependently (0.01-0.6 micrograms ml-1) inhibited nerve-evoked contractions of the hemidiaphragm. All neurotoxins except erabutoxin-b enhanced the basal tritium efflux immediately when applied to the endplate preparation or to a non-innervated muscle strip labelled with [3H]choline. This effect was attributed to an enhanced efflux of [3H]phosphorylcholine, whereas the efflux of [3H]choline and [3H]acetylcholine was not affected. It is concluded that the alpha-neurotoxins and kappa-bungarotoxin do not block presynaptic nicotinic receptors of motor nerves. These nicotinic autoreceptors differ from nicotinic receptors localized at the muscle membrane and at autonomic ganglia.

Acetylcholine-induced currents in acutely dissociated sympathetic neurons from adult hypertensive and normotensive rats have similar properties

Whole-cell patch-clamp recordings were used to compare the amplitude and kinetics of acetylcholine-induced currents (IACh) in acutely isolated superior cervical ganglion (SCG) neurons from spontaneously hypertensive (SHR) rats and Wistar-Kyoto (WKY) rats, to determine if altered postsynaptic transmitter responsiveness underlies the increased sympathetic nerve activity in SHR rat neurons. Rapidly activating and slowly inactivating inward currents were recorded in response to rapid application of ACh (5 microM to 2 mM). Concentration/response relationships for SCG neurons isolated for SHR and WKY rats had dissociation constants of 161 microM and 169 microM, maximum responses of 26 nS/pF and 24 nS/pF, and Hill coefficients of 1.8 and 1.9, respectively. Activation of the currents was fitted well by a single exponential function with concentration-dependent time constants, whereas inactivation was fitted well by a double exponential function also with concentration-dependent time constants. The time constants of both activation and inactivation for SHR and WKY rats were not significantly different at any concentration tested. The results demonstrate that the postsynaptic effects of ACh are similar between SHR and WKY rat postganglionic neurons and, therefore, probably do not contribute to the observed differences in ganglionic transmission between SHR and WKY rat nerves.

Actions of neurotoxins (bungarotoxins, neosurugatoxin and lophotoxins) on insect and nematode nicotinic acetylcholine receptors

Neurotoxins of natural origin have proved to be of considerable value in the isolation and characterization of vertebrate muscle and neuronal nicotinic acetylcholine receptors (nAChRs). To date, they have been used less extensively in studies of invertebrate nAChRs. Here we examine how a variety of neurotoxins (the snake toxins alpha-bungarotoxin, alpha-BGT, and kappa-bungarotoxin, kappa-BGT, the molluscan toxin, neosurugatoxin, and the soft coral toxins, lophotoxin and bipinnatin-B) can be used to characterize nAChRs in an insect, Periplaneta americana, and in a parasitic nematode, Ascaris suum. The agonist profiles of these nAChRs are distinct, but the most striking differences are in the actions of antagonists. Whereas the insect nAChR is blocked by both alpha- and kappa-bungarotoxins, the nematode receptor is only blocked by kappa-BGT. Neosurugatoxin blocks nAChRs in both species, but the lophotoxins which block all nAChRs investigated to date are much less effective on the Ascaris muscle receptor.

The effect of cholinoceptor agonists and neurotoxins on the release of vasopressin in the rat in relation to the subunit composition of the cholinoceptor

The effects of cholinoceptor agonists and neurotoxins on the release of vasopressin and oxytocin have been investigated in water-loaded rats under ethanol anaesthesia. Release of vasopressin was monitored by antidiuretic responses accompanied by increased urinary excretion of vasopressin. The rate of excretion of oxytocin-like radioimmunoreactivity was measured as an indicator of oxytocin release. Both nicotine and cytisine caused a preferential release of vasopressin. The release by nicotine was not inhibited by alpha- or neuronal-bungarotoxin. Neosurugatoxin blocked the release by cytisine. Comparison with the effects of these agents on combinations of alpha and beta subunits expressed in oocytes suggests that the central cholinoceptors mediating release of vasopressin are similar to those at autonomic ganglia and may contain a beta 4 subunit.

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.

Neuronal acetylcholine receptors that bind alpha-bungarotoxin with high affinity function as ligand-gated ion channels

Neuronal membrane components that bind alpha-bungarotoxin with high affinity can increase intracellular levels of free calcium, demonstrating the components function as nicotinic receptors. Though such receptors often contain the alpha 7 gene product, which by itself can produce ionotropic receptors in Xenopus oocytes, numerous attempts have failed to demonstrate an ion channel function for the native receptors on neurons. Using rapid application of agonist, we show here that the native receptors are ligand-gated ion channels which are cation selective, prefer nicotine over acetylcholine, and rapidly desensitize. Much of the calcium increase caused in neurons by the receptors under physiological conditions appears to result from their depolarizing the membrane sufficiently to trigger calcium influx through voltage-gated channels.

Exogenously administered alpha-bungarotoxin binds to embryonic chick spinal cord: implications for the toxin-induced arrest of naturally occurring motoneuron death

Administration of alpha-bungarotoxin and other curare-like drugs during embryogenesis arrests motoneuron death which normally occurs in the spinal cord from day 6 to day 10 of embryogenesis. The accepted explanation is that such motoneuron rescue is mediated by inhibition of neuromuscular transmission following the blockade of nicotinic cholinoceptors at the neuromuscular junction. In this study we investigated a further possibility, namely that motoneuron rescue might also involve the blockade of alpha-bungarotoxin-sensitive sites within the spinal cord. The kinetic profile of [125I]alpha-bungarotoxin binding was examined in the brachial and lumbar regions of chick spinal cord at embryonic day 15. Binding was specific and apparently saturable within the range 1-34 nM reaching a maximum after 45 min. Specific binding involved a single class of non-interacting sites with a KD of 8.0 nM and a Bmax of 106 +/- 12 fmol/mg of protein. Nicotine displaced specific [125I]alpha-bungarotoxin binding in a concentration-dependent manner. Furthermore, specific binding dissociated slowly in the absence of nicotine. Autoradiographs localizing [125I]alpha-bungarotoxin binding in embryonic spinal cord revealed that, at embryonic day 15, specific toxin binding sites could be detected throughout the gray matter. In contrast, at embryonic day 6, the ventral horn contained the majority of specific binding sites. Exogenously administered [125I]alpha-bungarotoxin reached and bound to nicotine-sensitive sites in the spinal cord at embryonic day 7. To conclude, these data demonstrate that central nicotine-sensitive sites which bind [125I]alpha-bungarotoxin in a saturable and specific manner were present at the beginning of the critical motoneuron death phase of neurogenesis and that they were accessible to exogenously administered toxin. It is proposed that the [125I]alpha-bungarotoxin binding characterized here is to a class of putative alpha-bungarotoxin-sensitive nicotinic cholinoceptors. These studies raise the possibility that alpha-bungarotoxin blockade of such putative nicotinic cholinoceptors within the spinal cord may contribute to toxin-induced arrest of naturally occurring motoneuron death.

The fall and rise of neuronal alpha-bungarotoxin binding proteins

Although neuronal [125I]-alpha-bungarotoxin binding proteins are similar in many respects to muscle nicotinic acetylcholine receptors, their functional significance has eluded researchers for the past fifteen years. Over this period, their status became increasingly doubtful, as almost all attempts failed to demonstrate that alpha-bungarotoxin could block neuronal nicotinic responses. Recently, these enigmatic proteins have been cloned and expressed in oocytes, and have been examined afresh in their native state. As Paul Clarke explains, it is time to recognize neuronal alpha-bungarotoxin binding proteins as distinct members of the nicotinic acetylcholine receptor gene family, even if perhaps they do not function quite like other members.

A study of the bovine adrenal chromaffin nicotinic receptor using patch clamp and concentration-jump techniques

1. Voltage clamp records have been obtained from bovine adrenal chromaffin cells in the outside-out and whole-cell configurations, in response to step changes of acetylcholine (ACh) concentration. The concentrations used ranged from 50 nM to 20 mM. 2. At high acetylcholine concentrations, the activation and desensitization kinetics of the nicotinic receptor, as observed in outside-out patches, may be described by a model incorporating a single, fast agonist binding step, and relatively slow isomerization to the open state. The affinity of the closed receptor for ACh is 310 microM, the channel opening rate constant is 460 s-1, and the closing rate constant is 29 s-1. 3. Single channel events, observed when nanomolar ACh concentrations are applied to whole cells, have two distinct channel lifetimes: 0.6 ms and 11-15 ms. The variation of the frequencies of the events with ACh concentration, suggests that the short lifetimes are openings of a singly liganded receptor and the longer lifetimes are openings of a doubly liganded receptor. 4. Only a single exponential associated with receptor desensitization is seen with outside-out patches, but two are seen with whole cells. It is postulated that there are two nicotinic receptor types present on adrenal chromaffin cells. 5. The rate of desensitization (9 s-1 and 26 s-1, whole cells; 24 s-1, patches), is fast enough to be significant in determining the open channel lifetime. 6. A sudden increase in current (rebound) is observed when a high concentration of ACh is abruptly removed from outside-out patches. This is evidence for a blocked state. The affinity of the blocking site for ACh is 1400 microM (outside-out patches). 7. The total number of activatable nicotinic channels per whole cell is estimated to be 2600.

Immunohistochemical localization of nicotinic acetylcholine receptor subunits in the mesencephalon and diencephalon of the chick (Gallus gallus)

Monoclonal antibodies against two alpha-bungarotoxin-binding subunits (alpha 7 and alpha 8) of the nicotinic acetylcholine receptors (nAChRs) were used as immunohistochemical probes to map their distribution in the chick diencephalon and mesencephalon. The distribution of the alpha 7 and alpha 8 nAChR subunits was compared to the distribution of immunoreactivity produced by a monoclonal antibody against the beta 2 structural subunit of the nAChRs. Structures that contained high numbers of alpha 7-like immunoreactive (LI) somata included the intergeniculate leaflet, nucleus intercalatus thalami, nucleus ovoidalis, organum paraventricularis, nucleus rotundus, isthmic nuclei, nucleus trochlearis, oculomotor complex, nucleus interstitio-pretecto-subpretectalis, stratum griseum centrale of the optic tectum, and nucleus semilunaris. Neuropil staining for alpha 7-LI was intense in the nucleus dorsomedialis hypothalami, nucleus geniculatus lateralis ventralis, griseum tecti, isthmic nuclei, nucleus lentiformis mesencephali, nucleus of the basal optic root, and stratum griseum et fibrosum superficiale of the tectum. High numbers of alpha 8-LI somata were found in the stratum griseum et fibrosum superficiale of the tectum and the nucleus interstitio-pretecto-subpretectalis, and intense neuropil staining for alpha 8-LI was found in the dorsal thalamus, nucleus geniculatus lateralis ventralis, lateral hypothalamus, griseum et fibrosum superficiale of the tectum. High numbers of beta 2-LI somata were found only in the nucleus spiriformis lateralis, whereas neuropil staining for beta 2-LI was intense in the nucleus geniculatus lateralis ventralis, nucleus suprachiasmaticus, nucleus lateralis anterior, nucleus habenularis lateralis, area pretectalis, griseum tecti, nucleus lentiformis mesencephalis, nucleus externus, and nucleus interpeduncularis, and in the stratum griseum centrale, stratum griseum et fibrosum superficiale, and stratum opticum of the tectum. These results indicate that there are major disparities in the localization of the alpha-bungarotoxin-binding alpha 7 and alpha 8 nAChR subunits and the beta 2 structural nAChR subunit in the chick diencephalon and mesencephalon. These nAChR subunits appear, however, to coexist in several regions of the chick brain.

Nicotinic and muscarinic modulation of 5-hydroxytryptamine (5-HT) release from porcine and canine small intestine

Strips of porcine and canine small intestine were incubated in vitro and the release of 5-hydroxytryptamine (5-HT) was determined by HPLC with electrochemical detection. The spontaneous outflow of 5-HT from the porcine and canine small intestine largely reflects calcium-dependent 5-HT secretion from enterochromaffin cells which are under a spontaneous neuronal, excitatory input as indicated by the inhibitory effect (30-40%) of tetrodotoxin. In both species, nicotine enhanced the release of 5-HT in a concentration-dependent manner by a maximum of about 50% at 100 microM. This effect was blocked by the nicotine receptor antagonist hexamethonium, but not by the subtype-selective nicotine receptor antagonist alpha-bungarotoxin. The effect of nicotine was rapidly desensitized. The presence of tetrodotoxin abolished the effect of nicotine on 5-HT release in canine tissue but not in porcine tissue. The presence of the muscarine receptor antagonist scopolamine prevented the effect of nicotine on 5-HT release from canine tissue, but significantly enhanced 5-HT release from porcine tissue. The muscarine receptor agonist oxotremorine inhibited 5-HT release from porcine tissue, but increased 5-HT release from canine tissue. However, in the presence of tetrodotoxin, oxotremorine enhanced 5-HT release in tissue from both species. In conclusion, activation of nicotine receptors induce the release of 5-HT from porcine and canine small intestine. In the dog, the effect of nicotine is mediated via the release of acetylcholine which then stimulates 5-HT release via muscarine receptors on the enterochromaffin cells. In the pig, the stimulatory effect of nicotine appears to be located directly on the enterochromaffin cells. In addition, activation of neuronal muscarine receptors in the porcine small intestine induced the release of a previously unidentified neurotransmitter which inhibited 5-HT release. Nicotine, via cholinergic interneurons, also appears to induce the release of this inhibitory neurotransmitter which opposes the direct stimulatory action of nicotine on 5-HT release.

The interaction of kappa-bungarotoxin with the nicotinic receptor of bovine chromaffin cells

Whole-cell recording techniques were used to examine acetylcholine-induced nicotinic currents in isolated bovine chromaffin cells. The effects on these currents of kappa-bungarotoxin, a snake venom kappa-neurotoxin, were tested. Exposure of cells to kappa-bungarotoxin (600 nM for 40 min) produced a prolonged blockade of nicotinic currents. The mechanism of this blockade was examined in several ways. Firstly, the pre-exposure of cells to trimetaphan, a competitive nicotinic antagonist, protected against the action of subsequent additions of kappa-bungarotoxin. Secondly, voltage-clamp measurements indicated that the degree of blockade produced by kappa-bungarotoxin was independent of cell membrane potential. Unlike (+)-tubocurarine, kappa-bungarotoxin had no direct agonist effects on nicotinic receptors. It is concluded from the present functional studies and from previously reported binding studies that kappa-bungarotoxin blocks nicotinic responses in bovine chromaffin cells by binding to regions overlying acetylcholine sites on nicotinic receptors.

Substitution of Torpedo acetylcholine receptor alpha 1-subunit residues with snake alpha 1- and rat nerve alpha 3-subunit residues in recombinant fusion proteins: effect on alpha-bungarotoxin binding

A fusion protein consisting of the TrpE protein and residues 166-211 of the Torpedo acetylcholine receptor alpha 1 subunit was produced in Escherichia coli using a pATH10 expression vector. Residues in the Torpedo sequence were changed by means of oligonucleotide-directed mutagenesis to residues present in snake alpha 1 subunit and rat nerve alpha 3 subunit which do not bind alpha-bungarotoxin. The fusion protein of the Torpedo sequence bound 125I-alpha-bungarotoxin with high affinity (IC50 = 2.5 x 10(-8) M from competition with unlabeled toxin, KD = 2.3 x 10(-8) M from equilibrium saturation binding data). Mutation of three Torpedo residues to snake residues, W184F, K185W, and W187S, had no effect on binding. Conversion of two additional Torpedo residues to snake, T191S and P194L, reduced alpha-bungarotoxin binding to undetectable levels. The P194L mutation alone abolished toxin binding. Mutation of three Torpedo alpha 1 residues to neuronal alpha 3-subunit residues, W187E, Y189K, and T191N, also abolished detectable alpha-bungarotoxin binding. Conversion of Try-189 to Asn which is present in the snake sequence (Y189N) abolished toxin binding. It is concluded that in the sequence of the alpha subunit of Torpedo encompassing Cys-192 and Cys-193, Try-189 and Pro-194 are important determinants of alpha-bungarotoxin binding. Tyr-189 may interact directly with cationic groups or participate in aromatic-aromatic interactions while Pro-194 may be necessary to maintain a conformation conductive to neurotoxin binding.

Nicotinic receptors that bind alpha-bungarotoxin on neurons raise intracellular free Ca2+

Many populations of vertebrate neurons have a membrane component that binds alpha-bungarotoxin and cholinergic ligands. Despite the abundance of this component and its similarities to nicotinic receptors, its function has remained controversial. Using a fluorescence assay, we show here that activation of the component elevates the intracellular concentration of free Ca2+, demonstrating a receptor function for the toxin-binding component. Whole-cell voltage-clamp and intracellular recordings did not detect a significant current resulting from receptor activation, possibly because the currents were small or the receptors rapidly desensitized. The rise in intracellular free Ca2+ caused by the receptor was prevented by Ca2+ channel blockers. This suggests a signaling cascade likely to have important regulatory consequences for the neuron.

Cholinergic activation of a population of corneal afferent nerves

The in vivo rabbit cornea was used to investigate the effect of acetylcholine on "free" nerve endings from A-delta and C-fibers in the long-ciliary (corneal) nerve. Extracellular electrophysiologic recordings were obtained from 67 corneal nerve fibers. Acetylcholine in concentrations of 10(-5) to 10(-3) M stimulated a specific population of these corneal afferents that were not activated by mechanical or thermal stimuli. Their conduction velocity was determined to be 1.14 +/- 0.34 m/s (mean +/- SD). The other three previously characterized corneal nerve populations (mechanical, mechano-heat, and cold) were not stimulated by the cholinergic agonists or antagonists. Acetylcholine sensitive afferents were also stimulated by carbachol (10(-5) to 10(-3) M) and nicotine (10(-6) to 10(-4) M) but not by bethanecol (10(-5) to 10(-3) M). Acetylcholine-induced activity was abolished by d-tubocurare (10(-4) M) and kappa-bungarotoxin (10(-6) M). The cAMP analog 8-bromoadenosine 3'5'-cyclic monophosphate activated the same population of chemosensitive C-fibers as acetylcholine. It is concluded that a specific population of C-fiber afferents exist in the rabbit cornea which are stimulated by acetylcholine possibly acting via a neuronal nicotinic receptor. Physiologically, these nerves may be involved in the production of pain following tissue injury or ischemia.

Pharmacology of nicotinic receptor-mediated inhibition in rat dorsolateral septal neurones

1. Intracellular electrophysiological techniques were employed to investigate the effects of nicotinic receptor stimulation on rat dorsolateral septal nucleus (DLSN) neurones in a submerged rat brain slice preparation. 2. Acetylcholine (in the presence of the muscarinic antagonist, atropine), nicotine or dimethylphenylpiperazinium (DMPP), applied either by pressure ejection or superfusion, produced predominantly a membrane potential hyperpolarization. 3. Following concentration-response comparisons, DMPP appeared to exhibit fewer desensitizing properties and greater efficacy than nicotine with half-maximal hyperpolarizing responses attainable at 3 and 10 microM, respectively. 4. Pharmacological analyses revealed that the agonist-induced membrane hyperpolarization was sensitive to antagonism by mecamylamine (50-100 microM) and neuronal bungarotoxin (0.2-0.3 microM), but not alpha-bungarotoxin (0.5-1.0 microM), curare (10-50 microM) or dihydro-beta-erythroidine (50-100 microM). 5. Hyperpolarizing responses to DMPP were found to reverse near the equilibrium potential for potassium and were sensitive to changes in extracellular potassium concentration as predicted by the Nernst equation. Under single-electrode voltage clamp, application of DMPP produced an outward current (75-100 pA) which approached reversal at around -88 mV. These findings indicated that the hyperpolarizing response to nicotinic receptor stimulation was mediated by changes in membrane permeability to potassium. 6. DMPP-induced membrane hyperpolarization resulted from a direct action on postsynaptic DLSN neurones since the response persisted under conditions of superfusion with calcium-free/high-magnesium media or tetrodotoxin; both conditions blocked orthodromically induced neurotransmission. The hyperpolarizing response remained unaltered in TTX but was diminished in calcium-free/high-magnesium media. Further studies revealed blockade of the DMPP response following intracellular injection of EGTA. This response was also sensitive to antagonism by various calcium-dependent potassium channel blockers including apamin, barium and tetraethylammonium. 7. Our studies reveal a novel class of CNS nicotinic receptor whose action upon stimulation by an agonist results in a membrane hyperpolarization via a calcium-dependent increase in potassium ion conductance.

Amino acid residues forming the interface of a neuronal nicotinic acetylcholine receptor with kappa-bungarotoxin: a study using single residue substituted peptide analogs

kappa-Bungarotoxin is a high affinity antagonist of neuronal nicotinic acetylcholine receptors of the alpha 3 subtype. Three sequence segments of the alpha 3 subunit that contribute to forming the binding site for kappa-bungarotoxin were previously located using synthetic peptides corresponding to the complete alpha 3 subunit, i.e., alpha 3(1-18), alpha 3(50-71) and alpha 3(180-201). Here we use single residue substituted peptide analogs of the alpha 3(50-71) sequence, in which amino acids are sequentially replaced by Gly, to determine which residues are important for kappa-bungarotoxin binding activity. Although no single substitution obliterated kappa-bungarotoxin binding, several amino acid substitutions lowered the affinity for kappa-bungarotoxin--i.e., two negatively charged residues (Glu51 and Asp62), and several aliphatic and aromatic residues (Leu54, Leu56, and Tyr63). These results indicate that the interface of the alpha 3 subunit with kappa-bungarotoxin involves primarily hydrophobic interactions, and a few negatively charged residues.

Initial characterization of the nicotinic acetylcholine receptors in rat hippocampal neurons

The properties of the neuronal nicotinic acetylcholine receptor in primary cultures of hippocampal cells from fetal rats (17-18 days gestation) were studied using the whole-cell patch-clamp technique in Na(+)-external, Cs(+)-internal and nominally Mg(2+)-free solutions. The nicotinic agonists acetylcholine, (+)anatoxin-a, and (-) and (+)nicotine all evoked inward whole-cell currents in hippocampal neurons that were voltage clamped near their resting potentials. Sensitivity to (+)anatoxin-a was first detected at around day 6, and thereafter the magnitude of the response increased as a function of number of days in culture up to about 40 days. The whole-cell current waveforms consisted of more than one peak whose relative amplitude depended on the agonist concentration. These currents were reversibly blocked by micromolar concentrations of d-tubocurarine, mecamylamine, and dihydro-beta-erythroidine. At nanomolar concentrations, neuronal bungarotoxin, alpha-bungarotoxin and alpha-cobratoxin caused an irreversible blockade of the currents but they were unaffected by tetrodotoxin, atropine, DL-2-amino-5-phosphonovaleric acid, Mg2+, and 6,7-dinitroquinoxaline-2,3-dione. In addition, the currents were also blocked in a reversible manner by methyllycaconitine at picomolar concentration. The current-voltage plots elicited by both (+)anatoxin-a and acetylcholine revealed larger inward currents and smaller or no outward currents. The present results demonstrate the existence of an inwardly rectifying, snake neurotoxin-sensitive functional nicotinic acetylcholine receptor ion channel in rat hippocampal neurons.

Cholinergic receptors in human brain: effects of aging and Alzheimer disease

A general review of cholinergic receptors in human brain is presented. The paper focuses upon changes in normal aging brain and in Alzheimer disease. Studies from five different approaches are reported: 1) molecular biology; 2) receptor binding studies; 3) studies with specific neurotoxins; 4) immunocytochemistry; and 5) PET scan. These studies document profound and characteristic differences between the normal aging and the pathological Alzheimer brain with regard to cholinergic receptor localization, distribution, and function.

Nicotinic acetylcholine receptor subtypes in human frontal cortex: changes in Alzheimer's disease

Molecular genetic and pharmacological studies have suggested that several subtypes of nicotinic acetylcholine receptors exist in the mammalian and avian brain. Combining 3H-(-)-nicotine, 125I-alpha-bungarotoxin, and 125I-kappa-bungarotoxin as ligands, we report here the first evidence for the existence in human frontal cortex of at least three different subtypes of nicotinic receptors. Autoradiographic analysis shows that specific 125I-kappa-bungarotoxin binding sites are concentrated mainly in several cortical layers. We also show that kappa-bungarotoxin, but not alpha-bungarotoxin decreases the evoked release of 3H-acetylcholine in rat cortical slices, indicating a likely presynaptic localization for some of the alpha-bungarotoxin-insensitive kappa-bungarotoxin sites in mammalian brain. The brains of patients with Alzheimer's disease show marked decreases in Bmax values for low-affinity 125I-kappa-bungarotoxin sites and both high- and low-affinity 3H-nicotine sites, whereas 125I-alpha-bungarotoxin sites are not significantly different in number from age-matched control brains. We conclude that Alzheimer's disease does not affect all subtypes of nicotinic receptors in the frontal cortex to the same extent.

Intracellular recording in avian brain of a nicotinic response that is insensitive to K-bungarotoxin

We examined nicotinic acetylcholine receptors in the avian brain using a combination of autoradiographic and intracellular electrophysiological techniques. We found that the lateral spiriform nucleus (SPL) in the mesencephalon has a very high density of 3H-nicotine binding sites but no detectable 125I-K-bungarotoxin (125I-K-BuTx) or 125I-alpha-bungarotoxin (125I-alpha-BuTx) bindings sites. Intracellular recordings in brain slices revealed that SPL neurons depolarize in response to nicotine and carbachol (in the presence of atropine). These depolarizations were blocked by the classic nicotinic antagonists d-tubocurarine and dihydro-beta-erythroidine. As predicted for nicotinic receptors with a high affinity for nicotine, neither K-BuTx nor alpha-BuTx blocked these nicotinic responses. Thus, although the existence of high-affinity 3H-nicotine binding sites has been known for some time, we now report the in situ detection of a functional nicotinic receptor that has a high affinity for nicotine and is K-BuTx-insensitive.

Rectification of currents activated by nicotinic acetylcholine receptors in rat sympathetic ganglion neurones

1. The inward rectification of the whole-cell current evoked by acetylcholine (ACh) and other nicotinic agonists in rat sympathetic ganglion neurones has been studied using patch-clamp recording techniques. The selective nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) (20 microM) induced an average peak current of -367 pA at -50 mV but no detectable outward current at +50 mV. Similar observations were made with ACh and carbachol. 2. The current-voltage relation of the whole-cell response induced by DMPP was linear in the negative voltage range; however, there was no detectable outward current in the voltage range 0 to about +70 mV. Above +70 mV an outward current became clearly detectable. Rapid depolarizing jumps in the holding potential failed to reveal any rapidly decaying outward current. 3. The rectification was not alleviated by changing the main permeant cation, by removal of divalent cations from the intracellular or extracellular solutions or by altering the pH buffer in the extracellular solution from HEPES to Tris. 4. Intracellular magnesium ions can block the channel. This effect increases with depolarization, but dissociation outwards (i.e. permeation by Mg2+) appears to relieve the block at more extreme positive potentials. This effect alone, or in combination with the voltage dependence of the burst length, is unlikely to be able to account for the whole-cell rectification in intact cells, much less that seen in cells perfused with Mg2(+)-free intracellular medium. 5. When the reversal potential was shifted to approximately -50 mV (by the use of impermeant cations) nicotinic agonists produced small outward currents in the membrane potential range -20 to +10 mV while shifting it to about +40 mV produced small inward currents in the potential range 0 to +20 mV. The rectification therefore appears to be independent of the direction of current flow and is maximum at a potential positive to 0 mV. 6. At positive potentials the receptors desensitized much less than at negative potentials in the continued presence of agonist. Thus, exposure of the cells to a steady application of 30 microM-ACh produced no detectable response if the cell was at a positive potential, but when the cell was stepped to a negative potential in the continued presence of ACh (at a time when much of the ACh current would be expected to have desensitized), ACh induced a large inward current. The onset of the ACh current had a time constant of 10 ms. It then decayed with a time constant of 790 ms as desensitization developed.(ABSTRACT TRUNCATED AT 400 WORDS)

Brain alpha-bungarotoxin binding protein cDNAs and MAbs reveal subtypes of this branch of the ligand-gated ion channel gene superfamily

alpha-Bungarotoxin (alpha Bgt) is a potent, high-affinity antagonist for nicotinic acetylcholine receptors (AChRs) from muscle, but not for AChRs from neurons. Both muscle and neuronal AChRs are thought to be formed from multiple homologous subunits aligned around a central cation channel whose opening is regulated by ACh binding. In contrast, the exact structure and function of high-affinity alpha Bgt binding proteins (alpha BgtBPs) found in avian and mammalian neurons remain unknown. Here we show that cDNA clones encoding alpha BgtBP alpha 1 and alpha 2 subunits define alpha BgtBPs as members of a gene family within the ligand-gated ion channel gene superfamily, but distinct from the gene families of AChRs from muscles and nerves. Subunit-specific monoclonal antibodies raised against bacterially expressed alpha BgtBP alpha 1 and alpha 2 subunit fragments reveal the existence of at least two different alpha BgtBP subtypes in embryonic day 18 chicken brains. More than 75% of all alpha BgtBPs have the alpha 1 subunit, but no alpha 2 subunit, and a minor alpha BgtBP subtype (approximately 15%) has both the alpha 1 and alpha 2 subunits.

Effects of Eperisone on Renshaw cells of cats

Actions of Eperisone (4'-ethyl-2-methyl-3-piperidinopropiophenone; EMPP) on Renshaw cells were investigated on spinal cats. The characteristic high-frequency discharges of Renshaw cells within 20 ms following antidromic ventral root stimulation were depressed while the later responses which follow the earlier responses for 80 ms were enhanced following administration of EMPP. These effects suggest that EMPP has differential effects on nicotinic acetylcholine receptors of the Renshaw cells.