Denervation supersensitivity as a model for the neural control of muscle

Nerve-dependent distribution of subsynaptic type 1 inositol 1,4,5-trisphosphate receptor at the neuromuscular junction

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are enriched at postsynaptic membrane compartments of the neuromuscular junction (NMJ), surrounding the subsynaptic nuclei and close to nicotinic acetylcholine receptors (nAChRs) of the motor endplate. At the endplate level, it has been proposed that nerve-dependent electrical activity might trigger IP3-associated, local Ca2+ signals not only involved in excitation-transcription (ET) coupling but also crucial to the development and stabilization of the NMJ itself. The present study was undertaken to examine whether denervation affects the subsynaptic IP3R distribution in skeletal muscles and which are the underlying mechanisms. Fluorescence microscopy, carried out on in vivo denervated muscles (following sciatectomy) and in vitro denervated skeletal muscle fibers from flexor digitorum brevis (FDB), indicates that denervation causes a reduction in the subsynaptic IP3R1-stained region, and such a decrease appears to be determined by the lack of muscle electrical activity, as judged by partial reversal upon field electrical stimulation of in vitro denervated skeletal muscle fibers.

RELATIONSHIP between prandial drinking behavior and supersensitivity of salivary glands after superior salivatory nucleus lesions in RATS

Prandial drinking, an increase in the number of drinking responses and secondary or non-homeostatic polydipsia in the presence of dry food, is typically associated with a deficit in salivary secretion. This study investigates the degree of salivary gland supersensitivity to pilocarpine administration after lesions to the superior salivatory nucleus (SSN), the site of origin of the parasympathetic preganglionic neurons that innervate the submandibular-sublingual (S-S) salivary glands. The main aim was to determine if there is a relationship between the degree of glandular supersensitivity, as an index of secretory deficit, and the development of prandial drinking in lesioned rats. Results showed that following SSN lesions two subgroups of rats were obtained. One subgroup exhibited prandial drinking but the other was similar to the control group. The SSN-lesioned prandial drinking subgroup presented significantly greater supersensitivity than the SSN-lesioned non-prandial drinking rats; the non-prandial drinking subgroup, in turn, presented significantly more supersensitivity than controls. Additionally, S-S supersensitivity observed in rats that exhibited prandial drinking due to the sectioning of chorda tympani efferent axons was compared to that observed in rats exhibiting prandial drinking due to SSN lesions. It was found that both groups presented the same S-S supersensitivity curve. These results indicate that SSN lesions produce a gradation of S-S supersensitivity values that appear to run parallel to the degree of glandular secretory deficit caused by the lesions. Thus, only the rats with greater secretory deficit (greater supersensitivity) develop prandial drinking. These data support the idea that there is in fact a functional link between the lateral reticular formation of the brainstem (the region associated with the SSN) and S-S salivary glands.

Selective Upregulation by Theanine of Slc38a1 Expression in Neural Stem Cell for Brain Wellness

Theanine is an amino acid abundant in green tea with an amide moiety analogous to glutamine (GLN) rather than glutamic acid (Glu) and GABA, which are both well-known as amino acid neurotransmitters in the brain. Theanine has no polyphenol and flavonoid structures required for an anti-oxidative property as seen with catechins and tannins, which are more enriched in green tea. We have shown marked inhibition by this exogenous amino acid theanine of the uptake of [³H]GLN, but not of [³H]Glu, in rat brain synaptosomes. Beside a ubiquitous role as an endogenous amino acid, GLN has been believed to be a main precursor for the neurotransmitter Glu sequestered in a neurotransmitter pool at glutamatergic neurons in the brain. The GLN transporter solute carrier 38a1 (Slc38a1) plays a crucial role in the incorporation of extracellular GLN for the intracellular conversion to Glu by glutaminase and subsequent sequestration at synaptic vesicles in neurons. However, Slc38a1 is also expressed by undifferentiated neural progenitor cells (NPCs) not featuring a neuronal phenotype. NPCs are derived from a primitive stem cell endowed to proliferate for self-renewal and to commit differentiation to several daughter cell lineages such as neurons, astrocytes, and oligodendrocytes. In vitro culture with theanine leads to the marked promotion of the generation of new neurons together with selective upregulation of Slc38a1 transcript expression in NPCs. In this review, we will refer to a possible novel neurogenic role of theanine for brain wellness through a molecular mechanism relevant to facilitated neurogenesis with a focus on Slc38a1 expressed by undifferentiated NPCs on the basis of our accumulating findings to date.

Abnormalities of Dopamine D3 Receptor Signaling in the Diseased Brain

Dopamine D₃ receptors (D₃R) modulate neuronal activity in several brain regions including cortex, striatum, cerebellum, and hippocampus. A growing body of evidence suggests that aberrant D₃R signaling contributes to multiple brain diseases, such as Parkinson’s disease, essential tremor, schizophrenia, and addiction. In line with these findings, D₃R has emerged as a potential target in the treatment of neurological disorders. However, the mechanisms underlying neuronal D₃R signaling are poorly understood, either in healthy or diseased brain. Here, I review the molecular mechanisms involved in D₃R signaling via monomeric D₃R and heteromeric receptor complexes (e.g., D₃R-D₁R, D₃R-D₂R, D₃R-A_2aR, and D₃R-D₃nf). I focus on D₃R signaling pathways that, according to recent reports, contribute to pathological brain states. In particular, I describe evidence on both quantitative (e.g., increased number or affinity) and qualitative (e.g., switched signaling) changes in D₃R that has been associated with brain dysfunction. I conclude with a description of basic mechanisms that modulate D₃R signaling such as desensitization, as disruption of these mechanisms may underlie pathological changes in D₃R signaling. Because several lines of evidence support the idea that imbalances in D₃R signaling alter neural function, a better understanding of downstream D₃R pathways is likely to reveal novel therapeutic strategies toward dopamine-related brain disorders.

Rat whisker movement after facial nerve lesion: evidence for autonomic contraction of skeletal muscle

Vibrissal whisking is often employed to track facial nerve regeneration in rats; however, we have observed similar degrees of whisking recovery after facial nerve transection with or without repair. We hypothesized that the source of non-facial nerve-mediated whisker movement after chronic denervation was from autonomic, cholinergic axons traveling within the infraorbital branch of the trigeminal nerve (ION). Rats underwent unilateral facial nerve transection with repair (N=7) or resection without repair (N=11). Post-operative whisking amplitude was measured weekly across 10weeks, and during intraoperative stimulation of the ION and facial nerves at ⩾18weeks. Whisking was also measured after subsequent ION transection (N=6) or pharmacologic blocking of the autonomic ganglia using hexamethonium (N=3), and after snout cooling intended to elicit a vasodilation reflex (N=3). Whisking recovered more quickly and with greater amplitude in rats that underwent facial nerve repair compared to resection (P<0.05), but individual rats overlapped in whisking amplitude across both groups. In the resected rats, non-facial-nerve-mediated whisking was elicited by electrical stimulation of the ION, temporarily diminished following hexamethonium injection, abolished by transection of the ION, and rapidly and significantly (P<0.05) increased by snout cooling. Moreover, fibrillation-related whisker movements decreased in all rats during the initial recovery period (indicative of reinnervation), but re-appeared in the resected rats after undergoing ION transection (indicative of motor denervation). Cholinergic, parasympathetic axons traveling within the ION innervate whisker pad vasculature, and immunohistochemistry for vasoactive intestinal peptide revealed these axons branching extensively over whisker pad muscles and contacting neuromuscular junctions after facial nerve resection. This study provides the first behavioral and anatomical evidence of spontaneous autonomic innervation of skeletal muscle after motor nerve lesion, which not only has implications for interpreting facial nerve reinnervation results, but also calls into question whether autonomic-mediated innervation of striated muscle occurs naturally in other forms of neuropathy.

Upregulation of D2-class signaling in dopamine-denervated striatum is in part mediated by D3 receptors acting on Ca V 2.1 channels via PIP2 depletion

The loss of dopaminergic neurons in the substantia nigra compacta followed by striatal dopamine depletion is a hallmark of Parkinson's disease. After dopamine depletion, dopaminergic D(2) receptor (D(2)R)-class supersensitivity develops in striatal neurons. The supersensitivity results in an enhanced modulation of Ca(2+) currents by D(2)R-class receptors. However, the relative contribution of D(2)R, D(3)R, and D(4)R types to the supersensitivity, as well as the mechanisms involved, have not been elucidated. In this study, whole cell voltage-clamp recordings were performed to study Ca(2+) current modulation in acutely dissociated striatal neurons obtained from rodents with unilateral 6-hydroxydopamine lesions in the substantia nigra compacta. Selective antagonists for D(2)R, D(3)R, and D(4)R types were used to identify whether the modulation by one of these receptors experiences a selective change after dopaminergic denervation. It was found that D(3)R-mediated modulation was particularly enhanced. Increased modulation targeted Ca(V)2.1 (P/Q) Ca(2+) channels via the depletion of phosphatidylinositol 4,5-bisphosphate, an intracellular signaling cascade hard to detect in control neurons and hypothesized as being amplified by dopamine depletion. An imbalance in the striatal expression of D(3)R and its splice variant, D(3)nf, accompanied enhanced D(3)R activity. Because Ca(V)2.1 Ca(2+) channels mediate synaptic GABA release from the terminals of striatal neurons, reinforcement of their inhibition by D(3)R may explain in part the profound decrease in synaptic strength in the connections among striatal projection neurons observed in the dopamine-depleted striatum.

Sodium channel Na(V)1.5 expression is enhanced in cultured adult rat skeletal muscle fibers

This study analyzes changes in the distribution, electrophysiological properties, and proteic composition of voltage-gated sodium channels (Na(V)) in cultured adult rat skeletal muscle fibers. Patch clamp and molecular biology techniques were carried out in flexor digitorum brevis (FDB) adult rat skeletal muscle fibers maintained in vitro after cell dissociation with collagenase. After 4 days of culture, an increase of the Na(V)1.5 channel type was observed. This was confirmed by an increase in TTX-resistant channels and by Western blot test. These channels exhibited increased activation time constant (tau(m)) and reduced conductance, similar to what has been observed in denervated muscles in vivo, where the density of Na(V)1.5 was increasing progressively after denervation. By real-time polymerase chain reaction, we found that the expression of beta subunits was also modified, but only after 7 days of culture: increase in beta(1) without beta(4) modifications. beta(1) subunit is known to induce a negative shift of the inactivation curve, thus reducing current amplitude and duration. At day 7, tau(h) was back to normal and tau(m) still increased, in agreement with a decrease in sodium current and conductance at day 4 and normalization at day 7. Our model is a useful tool to study the effects of denervation in adult muscle fibers in vitro and the expression of sodium channels. Our data evidenced an increase in Na(V)1.5 channels and the involvement of beta subunits in the regulation of sodium current and fiber excitability.

Engineering neuronal nicotinic acetylcholine receptors with functional sensitivity to alpha-bungarotoxin: a novel alpha3-knock-in mouse

We report here the construction of a novel knock-in mouse expressing chimeric alpha3 nicotinic acetylcholine receptor (nAChR) subunits with pharmacological sensitivity to alpha-bungarotoxin (alphaBTX). Sensitivity was generated by substituting five amino acids in the loop C (beta9-beta10) region of the mouse alpha3 subunit with the corresponding residues from the alpha1 subunit of the muscle type receptor from Torpedo californica. To demonstrate the utility of the underlying concept, expressed alpha3[5] subunits were characterized in the superior cervical ganglia (SCG) of homozygous knock-in mice, where the synaptic architecture of postsynaptic alpha3-containing nAChR clusters could now, for the first time, be directly visualized and interrogated by live-staining with rhodamine-conjugated alphaBTX. Consistent with the postsynaptic localization of ganglionic nAChRs, the alphaBTX-labeled puncta colocalized with a marker for synaptic varicosities. Following in vivo deafferentation, these puncta persisted but with significant changes in intensity and distribution that varied with the length of the recovery period. Compound action potentials and excitatory postsynaptic potentials recorded from SCG of mice homozygous for alpha3[5] were abolished by 100 nmalphaBTX, even in an alpha7 null background, demonstrating that synaptic throughput in the SCG is completely dependent on the alpha3-subunit. In addition, we observed that the genetic background of various inbred and outbred mouse lines greatly affects the functional expression of alpha3[5]-nAChRs, suggesting a powerful new approach for exploring the molecular mechanisms underlying receptor assembly and trafficking. As alphaBTX-sensitive sequences can be readily introduced into other nicotinic receptor subunits normally insensitive to alphaBTX, the findings described here should be applicable to many other receptors.

Function of neuromuscular synapses in the zebrafish choline-acetyltransferase mutant bajan

We have identified a zebrafish mutant line, bajan, in which compromised motility and fatigue result from a point mutation in the gene coding choline acetyltransferase (ChAT), the enzyme responsible for acetylcholine (ACh) synthesis. Although the mutation predicts loss of ChAT function, bajan inexplicably retains low levels of neuromuscular transmission. We exploited this residual activity and determined the consequences for synaptic function. The attenuated synaptic responses were a direct consequence of a decrease in both resting mean quantal size and quantal content. To replicate behavioral fatigue in swimming, motorneurons were stimulated at high frequencies. A prominent reduction in quantal content, reflecting vesicle depletion, was coincident with a small additional reduction in quantal size. In humans, defective ChAT leads to episodic apnea, a form of congenital myasthenic syndrome characterized by use-dependent fatigue. In contrast to bajan, however, afflicted individuals exhibit a normal resting quantal size and quantal content. The fatigue in humans results from a pronounced long-lasting drop in quantal size with little or no change in quantal content. These differences have important implications for interpreting fatigue as well as on understanding the impact of ACh availability on vesicle filling and recycling.

Neuromuscular expression of the BTB/POZ and zinc finger protein myoneurin

Myoneurin belongs to the BTB/POZ and zinc finger protein family whose members have been implicated in regulatory functions of gene expression. Myoneurin has been identified in various tissues, but muscle is a privileged site of myoneurin gene transcription. We examined the neuromuscular expression of myoneurin during development and after axotomy. Myoneurin expression is developmentally regulated in mouse muscle and appeared to be associated with neuromuscular junctions during the late embryonic period. Myoneurin is located in and around synaptic myonuclei in mouse and human adult muscle. The expression of myoneurin is dysregulated after nerve section. Thus, the restricted myoneurin expression in synaptic myonuclei appeared to be controlled by muscle electrical activity. Myoneurin is identified within the peripheral condensed chromatin and the euchromatin/heterochromatin regions, and thus fulfills structural and expression criteria to represent a synaptic gene regulator.

Mouse muscle denervation increases expression of an alpha7 nicotinic receptor with unusual pharmacology

Neuronal nicotinic alpha7 subunits have been found in chick and rat skeletal muscle during development and denervation. In the present study, reverse transcriptase-polymerase chain reaction was used to detect alpha7 subunit mRNA in denervated mouse muscle. To determine whether the alpha7 subunit forms functional nicotinic acetylcholine receptors (nAChRs) in muscle, choline was used to induce a membrane depolarization because choline has been considered a specific agonist of alpha7-containing (alpha7*) nAChRs. We found, however, that choline (3-10 mM) also weakly activates muscle nAChRs. After inhibiting muscle nAChRs with a specific muscle nAChR inhibitor, alpha-conotoxin GI (alphaCTxGI), choline was used to activate the alpha7* nAChRs on muscle selectively. Four weeks after denervation, rapid application of choline (10 mM) elicited a substantial depolarization in the presence of alphaCTxGI (0.1 microM). This component of the depolarization was never present in denervated muscles obtained from mutant mice lacking the alpha7 subunit (i.e. alpha7-null mice). The depolarization component that is resistant to alphaCTxGI was antagonized by pancuronium (3-10 microM) and by a 4-oxystilbene derivative (F3, 0.1-0.5 microM) at concentrations considered highly specific for alpha7* nAChRs. Another selective alpha7 antagonist, methyllycaconitine (0.05-5 microM), did not strongly inhibit this choline-induced depolarization. Furthermore, the choline-sensitive nAChRs showed little desensitization over 10 s of application with choline (10-30 mM). These results indicate that functional alpha7* nAChRs are significantly present on denervated muscle, and that these receptors display unusual functional and pharmacological characteristics.

The use of in vivo direct drug application to assess neural regulation of muscle properties

Skeletal muscle is a convenient model system for studying basic questions on the neural regulation of synaptogenesis and of many properties of sarcolemma and contractile apparatus. The study of the neural signals involved in a particular regulation and of the mediating intracellular pathways, requires the chronic application of drugs, second messengers, antibodies, trophic factors and the like. The most common way of application is in vitro treatment of muscle cell lines or primary myotube cultures. As an alternative to tissue culture, we developed a technique for in vivo application of the agents under study directly on skeletal muscle. An initial surgical step secures the tip of a fine polyethylene catheter (<or=250 microm) onto the proximal surface of the adult rat soleus muscle. The onset of perfusion of the solution containing the agent, however, starts only 10-15 days later, pushed by an implanted Alzet pump connected to the catheter. After chronic in vivo treatment for the appropriate number of days, the muscle is processed as needed. We used various agents which are known to affect muscle fibre acetylcholine receptor regulation, namely, CGRP, tetrodotoxin, and KCl and obtained results which demonstrate the full effectiveness of this way of application.

Nicotinic acetylcholine receptor subunit proteins alpha7 and beta4 decrease in the superior cervical ganglion after axotomy

Synaptic transmission in the superior cervical ganglion (SCG) is mediated by nicotinic acetylcholine receptors (nAChR). After transection of the postganglionic nerves of the SCG in the adult rat, the transcript levels of four of the five nAChR subunits present in the ganglion, alpha3, alpha5, alpha7, and beta4, decrease dramatically. In the present study, the effect of axotomy on nAChR subunit expression was examined at the protein level, focusing on the alpha7 and beta4 subunits. Immunohistochemistry with monoclonal antibody mAb306 (for the alpha7 subunit) and polyclonal antibody 4886 (for the beta4 subunit) showed that immunoreactivities for both alpha7 and beta4 subunits were concentrated in neurons in the intact ganglion. Results from double staining with antibodies to these subunits and to tyrosine hydroxylase, the enzyme that catalyzes the rate-limiting step in the biosynthesis of the sympathetic neurotransmitter norepinephrine, demonstrated that most neurons in the SCG express both the alpha7 and beta4 subunits. Three days after axotomy, the number of immunolabeled neurons and the intensity of the immunostaining per labeled neuron were decreased for both subunits. Decreases in subunit levels were also observed by Western blot analysis. Observing changes in these subunits over time after surgery revealed that, while the protein level of the alpha7 subunit recovered substantially within 2 weeks after the lesion, that of the beta4 subunit stayed low. These data demonstrate that decreases in nicotinic receptor subunits are among the changes in proteins that occur in axotomized sympathetic neurons, and suggest that these decreases may contribute to the depression in ganglionic synaptic transmission observed in axotomized ganglia.

Dopamine-deficient mice are hypersensitive to dopamine receptor agonists

Dopamine-deficient (DA-/-) mice were created by targeted inactivation of the tyrosine hydroxylase gene in dopaminergic neurons. The locomotor activity response of these mutants to dopamine D1 or D2 receptor agonists and l-3,4-dihydroxyphenylalanine (l-DOPA) was 3- to 13-fold greater than the response elicited from wild-type mice. The enhanced sensitivity of DA-/- mice to agonists was independent of changes in steady-state levels of dopamine receptors and the presynaptic dopamine transporter as measured by ligand binding. The acute behavioral response of DA-/- mice to a dopamine D1 receptor agonist was correlated with c-fos induction in the striatum, a brain nucleus that receives dense dopaminergic input. Chronic replacement of dopamine to DA-/- mice by repeated l-DOPA administration over 4 d relieved the hypersensitivity of DA-/- mutants in terms of induction of both locomotion and striatal c-fos expression. The results suggest that the chronic presence of dopaminergic neurotransmission is required to dampen the intracellular signaling response of striatal neurons.

Differential regulation of levels of nicotinic receptor subunit transcripts in adult sympathetic neurons after axotomy

Axotomy of adult peripheral neurons produces decreases in the levels of transcripts for a number of proteins involved in synaptic transmission. For example, tyrosine hydroxylase and neuropeptide Y mRNA decrease in axotomized sympathetic neurons in the superior cervical ganglion (SCG). In the present study, the effects of axotomy on the expression of nicotinic receptor subunit transcripts were examined in the SCG and the results were compared to those produced by deafferentation and explantation. Normally, neurons in the SCG express five different nicotinic subunits: alpha3, alpha5, alpha7, beta2, and beta4. Forty-eight hours after axotomy in vivo or explantation, dramatic decreases in these transcripts were seen, except for beta2, which increased. In contrast, deafferentation of the SCG had negligible effects on any of these transcripts. Both leukemia inhibitory factor (LIF) and nerve growth factor (NGF) have been shown to play a role in the decrease in neuropeptide Y mRNA expression after axotomy. In the cases of these nicotinic receptor transcripts, however, similar decreases were seen in wild-type and LIF knockout animals. Furthermore, administration of an antiserum to NGF in intact animals produced no changes in transcript levels. On the other hand, providing exogenous NGF to axotomized SCG in vivo or in explant cultures partially prevented the decreases in the transcripts for alpha3, alpha5, alpha7, and beta4. These data indicate that axotomy produces dramatic decreases in the expression of several nicotinic receptor subunit transcripts, and that the molecular signals underlying these changes differ from those previously shown to mediate the decrease in neuropeptide Y expression.

Hyperreactivity to nitrovasodilators in forearm vasculature is related to autonomic dysfunction in insulin-dependent diabetes mellitus

BACKGROUND

The link between diabetes and vascular disease is poorly understood. Data regarding endothelial function in vivo in patients with insulin-dependent diabetes mellitus (IDDM) have been inconsistent with in vitro studies demonstrating hyperglycemia-induced impairments in endothelium-dependent vasodilation.

METHODS AND RESULTS

We determined whether alterations in neural control of the vascular tone might contribute to blood flow responses to intrabrachial infusions of acetylcholine (ACh), sodium nitroprusside (SNP), and L-N-monomethyl-arginine (L-NMMA) in 22 men with IDDM (12 with normoalbuminuria. HbA1c = 8.6 +/- 0.3%; 10 with macroalbuminuria, HbA1c = 8.6 +/- 0.3%) and 11 matched normal men. Autonomic function was assessed from reflex vasoconstriction to cold, the blood pressure response to standing and hand grip, and heart rate variation, including spectral analysis, during controlled breathing, and the Valsalva maneuver. IDDM with macroalbuminuria exhibited hyperresponsiveness to both ACh and SNP compared with the patients with normoalbuminuria or normal subjects. Reflex sympathetic vasoconstriction to cold was severely impaired in the IDDM patients with macroalbuminuria (-19 +/- 6%) compared with normoalbuminuric patients (-39 +/- 5%, P < .05) and normal subjects (-54 +/- 7%, P < .001). The macroalbuminuric patients also had evidence of autonomic dysfunction during controlled and deep breathing tests and during the Valsalva maneuver. Within the group of IDDM patients, neither the urinary albumin excretion rate nor other parameters such as HbA1c or serum cholesterol correlated with forearm blood flow during the vasoactive drug infusions. There were, however, significant inverse correlations between several measures of both sympathetic and parasympathetic autonomic functions and vascular hyperresponsiveness to SNP and ACh. For example, the Valsalva ratio was inversely correlated with the increase in blood flow in response to infusion of 3 (r = -.74, P < .001) and 10 (r = -.73, P < .001) micrograms/min SNP and 7.5 (r = -.73, P < .001) and 15 (r = -.75, P < .001) micrograms/min ACh.

CONCLUSIONS

These data are consistent with idea that altered neurotransmission is an important determinant of vascular reactivity of diabetic blood vessels to nitrovasodilators in vivo.

Effects of long-term conduction block on membrane properties of reinnervated and normally innervated rat skeletal muscle

1. Do motoneurons regulate muscle extrajunctional membrane properties through chemical (trophic) factors in addition to evoked activity? We addressed this question by comparing the effects of denervation and nerve conduction block by tetrodotoxin (TTX) on extrajunctional acetylcholine (ACh) sensitivity and action potential resistance to TTX in adult rats. 2. We applied TTX to sciatic or tibial nerves for up to 5 weeks using an improved blocking technique which completely suppresses conduction but avoids nerve damage. 3. Reinnervation by TTX-blocked axons had no effect on the high ACh sensitivity and TTX resistance induced by nerve crush. 4. Long-lasting block of intact nerves (up to 38 days) induced extrajunctional changes as pronounced as after denervation. At shorter times (3 days), however, denervation induced much larger changes than TTX block; such a difference is thus only transiently present in muscle. 5. The effects of long-lasting block were dose dependent. Dose levels (6.6 micrograms day-1) corresponding to those used in the literature to block the rat sciatic nerve induced muscle effects much smaller than those induced by denervation, confirming published data. Our novel finding is that equal effects are obtained using doses substantially higher (up to 10.5 micrograms day-1). For the soleus it was necessary in addition to apply the TTX directly to the smaller tibial nerve. 6. The TTX-blocked nerves were normal in their histological appearance and capacity to transport anterogradely 3H-labelled proteins, to release ACh in quantal and non-quantal form or cluster ACh receptors and induce functional ectopic junctions on denervated soleus muscles. 7. We conclude that muscle evoked activity is the physiological regulator of extrajunctional membrane properties. Chemical factors from the nerve do not appear to participate in this regulation. The stronger response to denervation at short times only is best accounted for by factors produced by degenerating nerves.

Number of junctional acetylcholine receptors: control by neural and muscular influences in the rat

1. The number of acetylcholine receptors (AChRs) per neuromuscular junction in soleus muscles of adult rats was estimated from counts of 125I-alpha-bungarotoxin binding sites. The muscles were either denervated, denervated and electrically stimulated, paralysed by botulinum toxin (BoTX), or paralysed by tetrodotoxin (TTX). 2. After denervation, the number of junctional AChRs was normal after 18 days and then fell to 54 and 35% of normal after 33 and 57 days, respectively. 3. Direct high frequency muscle stimulation (100 Hz) maintained a normal number of junctional AChRs for at least 2 months when the stimulation started on the day of denervation. When the stimulation was started progressively later, the effect of the stimulation on AChR number disappeared within about a week. The disappearance was gradual and appeared to affect all the muscle fibres equally. 4. Stimulation at 100 Hz, starting on the day of denervation and stopping after 18 days, did not prevent the endplates from losing AChRs during the subsequent 15 days without stimulation. Thus 100 Hz stimulation and innervation are not equivalent in their effects on junctional AChR number. 5. Direct low frequency muscle stimulation from the day of denervation did not maintain a normal number of junctional AChRs, as the number of AChRs fell to 70 and 62% of normal after 33 days of stimulation at 20 and 10 Hz, respectively. 6. Endplates paralysed by BoTX or TTX for 33 days lost about as many junctional AChRs (54 and 55%) as endplates denervated for 33 days (46%). Direct stimulation at 100 Hz during the last 15 days of BoTX treatment reduced but did not prevent this AChR loss (36% loss at 33 days). 7. The results show that when motor nerve terminals in rat soleus muscles are removed by axotomy, they leave a 'trace' which, in conjunction with appropriate muscle stimulation, can maintain a normal number of AChRs in the postsynaptic region. In non-stimulated muscles the trace responsible for this maintenance disappears within about a week. In stimulated muscles it persists for at least 2 months. From indirect evidence it appears that the trace is a factor, or the postsynaptic effect of a factor, released by impulse activity in the nerve, and that its degradation after denervation is accelerated by the acute effects of nerve degeneration.

Putative effects of nerve extract on carbonic anhydrase III expression in rat muscles

Carbonic anhydrase III (CA III), the predominant CA isoform in skeletal muscle is very sensitive to neuronal influences. We aimed to determine whether CA III expression could be influenced by neurotrophic factor(s) present in sciatic nerve extract (SNE). Intact muscles were thus compared with denervated soleus (SOL), extensor digitorum longus (EDL), and tibialis anterior (TA) muscles injected daily for 7 days with saline solution (SS) or with SNE. CA III activity was significantly increased in SS-treated EDL and TA muscles compared to control (CTR), while SNE injections partially prevented this increase. There was no significant difference for CA III activity in the SOL between CTR, SS, and SNE groups. The CA III mRNA increase observed in response to denervation was reduced by 40% in SNE-treated EDL and TA muscles. While SOL CA III mRNA level was not affected by denervation, a 52% decrease was observed with SNE. We concluded that neuronal modulation of CA III expression in type II fibers may involve a neurotrophic component.

Involvement of peripheral sensory fibers in amyotrophic lateral sclerosis: electrophysiological study of 64 cases

We report electrophysiological findings of conduction along peripheral sensory fibers in 64 patients with amyotrophic lateral sclerosis. Distribution of the values of action potential amplitudes and conduction velocities of peripheral afferent fibers were significantly lower than in normal age-matched controls. Sensory action potential amplitudes (SAPas) were more affected than sensory conduction velocities (SCVs). When single patients were considered, SAPas were slightly but significantly reduced in 22% of the cases (median nerve 17%, ulnar nerve 11%, and sural nerve 22%). A parallel decrease in SCVs and MCVs in 14 patients in whom the study was repeated over a period of time was also found. All these electrophysiological findings are due to progressive neuronopathy of peripheral sensory fibers. A pathogenetic mechanism is proposed.

Nerve stump effects in muscle are independent of synaptic connections and are temporally correlated with nerve degeneration phenomena

Close or distant denervation of the rat soleus muscle indicated that (1) longer soleus nerve stumps delay the onset of axon terminal degeneration and of muscle membrane changes (spike resistance to TTX) by strictly comparable times, and (2) the stump-induced delay of the muscle effect is independent of synaptic connections, because it is also obtained (RMP fall and TTX-resistance development) when sectioning a foreign nerve previously transplanted on the soleus surface but not making synaptic contacts. Both lines of evidence are consistent with the interpretation that, as far as the extrajunctional membrane properties are concerned, the effect of the length of the nerve stump on muscle is mediated by nerve terminal breakdown.

The effect of vasoactive intestinal polypeptide on the lower esophageal sphincter in achalasia

Vasoactive intestinal polypeptide (VIP) is one of the main neurotransmitters implicated in the relaxation of the lower esophageal sphincter (LES). The effect of exogenous VIP on LES motor activity was determined by esophageal manometry. LES pressure (LESP) and LES relaxation were compared in four healthy volunteers and in six patients with achalasia. The effects of intravenous doses of 1.5, 3, and 5 pmol.kg-1.min-1 of VIP were compared with placebo. Neither placebo nor 3 and 5 pmol.kg-1.min-1 of VIP produced any effect on esophageal motility in healthy volunteers. In achalasia the three doses of VIP caused a dose-dependent decrease in LESP with a significant improvement in LES relaxation. A dose of 5 pmol.kg-1.min-1 produced a maximal decrease of 51% in LESP. A beta-adrenergic agonist, isoproterenol, caused a decrease in LESP both in healthy volunteers and in patients with achalasia without improving LES relaxation. In summary, intravenous VIP improved LES relaxation and caused a decrease in LESP in patients with achalasia without affecting LESP in healthy volunteers, indicating that the LES muscle in achalasia is supersensitive to VIP. The current study suggests that a selective damage in the noncholinergic nonadrenergic innervation of the esophagus is in part responsible for the motor alteration seen in these patients. The findings and the inability of isoproterenol to improve LES relaxation despite decreasing LESP support a role in VIP as a indicator of LES relaxation.

Neural factors regulate AChR subunit mRNAs at rat neuromuscular synapses

To elucidate the nature of signals that control the level and spatial distribution of mRNAs encoding acetylcholine receptor (AChR), alpha-, beta-, gamma-, delta- and epsilon-subunits in muscle fibers chronic paralysis was induced in rat leg muscles either by surgical denervation or by different neurotoxins that cause disuse of the muscle or selectively block neuromuscular transmission pre- or postsynaptically and cause an increase of AChRs in muscle membrane. After paralysis, the levels and the spatial distributions of the different subunit-specific mRNAs change discoordinately and seem to follow one of three different patterns depending on the subunit mRNA examined. The level of epsilon-subunit mRNA and its accumulation at the end-plate are largely independent on the presence of the nerve or electrical muscle activity. In contrast, the gamma-subunit mRNA level is tightly coupled to innervation. It is undetectable or low in innervated normally active muscle and in innervated but disused muscle, whereas it is abundant along the whole fiber length in denervated muscle or in muscle in which the neuromuscular contact is intact but the release of transmitter is blocked. The alpha-, beta-, and delta-subunit mRNA levels show a different pattern. Highest amounts are always found at end-plate nuclei irrespective of whether the muscle is innervated, denervated, active, or inactive, whereas in extrasynaptic regions they are tightly controlled by innervation partially through electrical muscle activity. The changes in the levels and distribution of gamma- and epsilon-subunit-specific mRNAs in toxin-paralyzed muscle correlate well with the spatial appearance of functional fetal and adult AChR channel subtypes along the muscle fiber. The results suggest that the focal accumulation at the synaptic region of mRNAs encoding the alpha-, beta-, delta-, and epsilon-subunits, which constitute the adult type end-plate channel, is largely determined by at least two different neural factors that act on AChR subunit gene expression of subsynaptic nuclei.

Differential regulation of MyoD and myogenin mRNA levels by nerve induced muscle activity

The levels of mRNAs coding for the myogenic factors MyoD and myogenin were measured during synapse formation in developing muscle and in adult muscle, after denervation and reinnervation and after muscle paralysis induced by blocking of neuromuscular transmission by neurotoxins known to alter the density and localization of synaptic proteins such as the acetylcholine receptor (AChR). The mRNA levels of both factors depend on usage of the neuromuscular synapses, but they change to different extents. Myogenin mRNA levels decrease drastically with innervation and increase strongly following blocking of transmission whereas the level of MyoD mRNA showed only a small decrease in response to innervation, denervation or muscle paralysis by neurotoxins. Neither mRNA showed a synapse-related cellular distribution. The results suggest that nerve-induced electrical muscle activity determines the cellular ratio of MyoD and myogenin mRNAs in adult muscle.

Delayed response to denervation in muscles of C57BL/Ola mice

Muscle fibre areas and numbers, tetanic force, resident macrophage numbers and acetylcholine sensitivity were measured in normal and denervated soleus muscles of C57BL/Ola mice for comparison with data from outbred and other inbred strains. Serum creatine kinase levels were also measured. The muscles of normal C57BL/Ola mice had more fibres, generated more tension, had fewer resident macrophages and had a lower acetylcholine sensitivity than muscles of other strains. The normal level of serum creatine kinase was lower in C57BL/Ola mice than in Balb/c mice. Following denervation, the mean cross-sectional area of muscle fibres in Balb/c mice started to fall from day 1 but a fall was not seen in muscles from C57BL/Ola mice until day 5. The development of increased acetylcholine sensitivity was slower in the C57BL/Ola mice although the values in all mouse strains had converged by five days. The levels of serum creatine kinase also rose more slowly following denervation in C57BL/Ola mice. The differences between the data from C57BL/Ola mice and others are ascribed to the slow rate of Wallerian degeneration in that strain. The results support the view that the effects of denervation on muscle are due to both inactivity and an inflammatory effect contributed by nerve degeneration. The mutation that causes slow nerve degeneration may, however, also affect muscle directly, making it more resistant to catabolism.

Nerve growth factor induces functional nicotinic acetylcholine receptors on rat sensory neurons in culture

Neonatal sensory neurons from rat nodose ganglia express nicotinic acetylcholine receptors when grown in tissue culture without other cell types. The present study investigates the role of nerve growth factor in inducing these receptors. Nerve growth factor has little effect on the growth and survival of nodose neurons in culture, although most neurons were found by quantitative radioautography to have high-affinity nerve growth factor receptors. Nerve growth factor strongly influenced the expression of nicotinic receptors on these neurons: the proportion of acetylcholine-sensitive neurons was approximately 60% in cultures with nerve growth factor compared with 15% in cultures grown without nerve growth factor. The proportion of acetylcholine-sensitive neurons increased over the first week, plateaued by day 12 and remained high for at least three weeks. In contrast, without NGF, the proportion of acetylcholine-sensitive neurons was low throughout the three-week period. The results indicate that nerve growth factor is an important factor in promoting nicotinic receptors on these neurons in culture.

On the effect of muscle activity on the end-plate membrane in denervated mouse muscle

1. Mouse soleus muscles were denervated and some of them were chronically stimulated. Sixteen to twenty-one days later, the number of junctional acetylcholine receptors (AChR) and their metabolic stability were examined by measuring binding of 125I-alpha-bungarotoxin, their gating properties by analysis of acetylcholine-induced current fluctuations and the ultrastructure of the end-plate membrane by electron microscopy. 2. In agreement with other studies on inactive muscles, no effect of denervation on junctional AChR number could be resolved. However, some of the fast-gating 'adult' AChR channels had been replaced by slowly gating fetal AChRs, their half-life was lowered to 38 h and the folding of the end-plate membrane was reduced. 3. These changes were prevented in denervated but stimulated active muscles: the junctional AChR population remained homogeneously 'adult', the half-life of junctional AChRs was 13 days and folding of the end-plate membrane remained comparable to that in control muscles. 4. The significance of these results is discussed with respect to the role of muscle activity in end-plate development.

Mechanism of action of lithium on acetylcholine receptor metabolism in skeletal muscle

Changes in the levels of cations within skeletal muscle are thought to mediate the neural regulation of turnover of extrajunctional acetylcholine receptors (AChRs). We have used lithium as a probe of these cation influences because of its resemblance to calcium and other ions. In the present experiments we studied the mechanism of action of lithium on AChR metabolism in cultured mammalian skeletal muscle. We measured the effects of lithium on AChR turnover (using [125I]alpha-bungarotoxin binding), and evaluated the resemblance of lithium and calcium in producing their effects on AChR metabolism. Our results provide insight into the mechanisms of action of lithium and the cellular processes controlling AChR metabolism in muscle. Lithium reduces the number of AChRs in skeletal muscle in vitro to a degree similar to that which we previously reported in vivo. Lithium appears to enter cells via both sodium and calcium channels. It then produces its effect on levels of AChRs primarily by selectively reducing AChR synthesis and insertion into the surface membrane. Lithium induces this change in AChR metabolism in a manner resembling neural and calcium-mediated effects on AChRs. Phosphoinositide pathways may be involved in the lithium-induced effects. Further analysis of the effects of lithium on AChR turnover should provide new information about the mechanisms underlying the cellular control of receptor metabolism.

Effects of chronic stimulation on the size and speed of long-term denervated and innervated rat fast and slow skeletal muscles

This study seeks to identify the mechanisms which motoneurones use to control the contractile force and speed of skeletal muscles. We have stimulated directly slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles of adult rats intermittently at 100 Hz for 1-9 months. The muscles were either chronically denervated, denervated and reinnervated, or normally innervated. The stimulation started either immediately, or more commonly, after 1-9 months of denervation. Stimulation starting several months after denervation increased the mean maximum tetanic tension 37 times in SOL and eight times in EDL. These values represented 40 and 12% of the increases obtained by reinnervation after comparable periods of time. In denervated SOL and EDL muscles stimulated directly for more than 2 months, the mean isometric twitch contraction times were 13 and 12.7 ms, as in normal EDL muscles (13 ms). In innervated SOL muscles stimulated directly for 1-4 months, the mean twitch contraction times were 23.6 ms (normally innervated) and 19.2 ms (reinnervated), which were considerably shorter than in normal control SOL muscles (39.2 ms). Single motor unit recordings revealed that the natural (background) nerve impulse activity was essentially unaffected by the stimulation. Twitch contraction time and percentage of type II fibres in SOL muscles were related. The fastest muscles (denervated and stimulated) consisted of 100% type II fibres (with one exception), the second fastest (reinnervated and stimulated) of 70-50%, the third fastest (normally innervated and stimulated) of 45-0%, the second slowest (reinnervated) of 15-0%, and the slowest muscles (innervated controls) of 5-0% type II fibres.

Role of cholinergic neuromuscular transmission in the neuroregulation of the autophosphorylatable regulatory subunit of cyclic AMP-dependent protein kinase type II and the acetylcholine receptor content in skeletal muscle

Previously, we found that the in vitro [32P]-autophosphorylation of the regulatory subunit of cyclic AMP-dependent protein kinase type II in rat soleus muscles is subject to a nerve-stump-length-dependent neuroregulation which indicates that this event is dependent upon some neural signal other than the impulse-directed release of acetylcholine. In this investigation, tetrodotoxin and alpha-bungarotoxin were also administered to further differentiate the effect of impulse-directed and spontaneously released acetylcholine upon this event and also upon the appearance of new acetylcholine receptors as measured by the binding of radioiodinated bungarotoxin. A 24 h blockade of cholinergic transmission with either neurotoxin did not change the phosphorylation level of the regulatory subunit, while a significant increase is observed when solei are surgically denervated for this period. The phosphorylation level and also the acetylcholine receptor content were increased only after more prolonged (48-96 h) muscle inactivity was produced with the neurotoxins. However, then their effects may not be solely related to alterations in cholinergic transmission. Taken together, our results do not support a trophic role for spontaneously released acetylcholine with respect to the two neurotrophic events studied.

Muscle-specific beta-enolase concentrations after cross- and random innervation of soleus and extensor digitorum longus in rats

The concentration of beta-enolase, a highly specific marker of the skeletal muscle of rats, was determined in a slow-twitch muscle, the soleus (SOL) and a fast-twitch muscle, the extensor digitorum longus (EDL) after cross-innervation, random reinnervation, or denervation. The beta-enolase concentration is normally high in EDL and low in SOL. When the nerves entering into these muscles were cross-sutured, the beta-enolase concentration in EDL decreased and that in SOL increased to reach an almost equal value in 20 weeks and by the 35th week the SOL ultimately had a higher beta-enolase concentration than the EDL. When the sciatic nerve trunk was completely transected and sutured immediately, the beta-enolase concentration in EDL decreased and that of SOL increased; in 20 weeks SOL had a beta-enolase concentration similar to that of the EDL. When these muscles were denervated by cutting the sciatic nerve trunk, their beta-enolase concentrations were markedly lowered, but EDL still retained on the 12th week a beta-enolase value comparable to the normal SOL. Possible mechanisms behind the observed changes in beta-enolase concentration are discussed.

Nerve dependent regulation of neural cell adhesion molecule expression in skeletal muscle

The expression of neural cell adhesion molecule was analysed by indirect immunofluorescence on adult mouse skeletal muscle subjected to a variety of experimental lesions. Adult mouse muscle does not express neural cell adhesion molecule at the sarcolemma. However, following denervation there is a rapid rise in neural cell adhesion molecule levels; this is initially in the cytoplasm of the myofibres but by 18 days there is intense reactivity at the sarcolemma. A nerve crush lesion was used to show that the increase in neural cell adhesion molecule levels following denervation is accompanied by a switch-off of neural cell adhesion molecule expression following reinnervation. Paralysis of skeletal muscle by botulinum toxin injection is sufficient to activate neural cell adhesion molecule expression. As paralysis of skeletal muscle by botulinum toxin is not accompanied by activation of muscle satellite cells or degeneration products of nerve or myelin, it suggests that the observed levels of neural cell adhesion molecule are synthesised by myofibres. As the expression of neural cell adhesion molecule in these lesions parallels the ability of skeletal muscle to accept innervation it is possible that neural cell adhesion molecule acts as a molecular cue at the sarcolemma in regulating synaptogenesis.