Calcitonin gene-related peptides and neuromuscular interactions

Ten open questions in migraine prophylaxis with monoclonal antibodies blocking the calcitonin-gene related peptide pathway: a narrative review

The monoclonal antibodies (mAbs) blocking the calcitonin-gene related peptide (CGRP) pathway, collectively called here "anti-CGRP/rec mAbs", have dramatically improved preventive migraine treatment. Although their efficacy and tolerability were proven in a number of randomized controlled trials (RCTs) and, maybe even more convincingly, in real world settings, a number of open questions remain. In this narrative review, we will analyze published data allowing insight in some of the uncertainties related to the use of anti-CGRP/rec mAbs in clinical practice: their differential efficacy in migraine subtypes, outcome predictors, switching between molecules, use in children and adolescents, long-term treatment adherence and persistence, effect persistence after discontinuation, combined treatment with botulinum toxin or gepants, added-value and cost effectiveness, effectiveness in other headache types, and potential contraindications based on known physiological effects of CGRP. While recent studies have already provided hints for some of these questions, many of them will not find reliable and definitive answers before larger studies, registries or dedicated RCTs are available.

Motoneuron deafferentation and gliosis occur in association with neuromuscular regressive changes during ageing in mice

BACKGROUND

The cellular mechanisms underlying the age-associated loss of muscle mass and function (sarcopenia) are poorly understood, hampering the development of effective treatment strategies. Here, we performed a detailed characterization of age-related pathophysiological changes in the mouse neuromuscular system.

METHODS

Young, adult, middle-aged, and old (1, 4, 14, and 24-30 months old, respectively) C57BL/6J mice were used. Motor behavioural and electrophysiological tests and histological and immunocytochemical procedures were carried out to simultaneously analyse structural, molecular, and functional age-related changes in distinct cellular components of the neuromuscular system.

RESULTS

Ageing was not accompanied by a significant loss of spinal motoneurons (MNs), although a proportion (~15%) of them in old mice exhibited an abnormally dark appearance. Dark MNs were also observed in adult (~9%) and young (~4%) animals, suggesting that during ageing, some MNs undergo early deleterious changes, which may not lead to MN death. Old MNs were depleted of cholinergic and glutamatergic inputs (~40% and ~45%, respectively, P < 0.01), suggestive of age-associated alterations in MN excitability. Prominent microgliosis and astrogliosis [~93% (P < 0.001) and ~100% (P < 0.0001) increase vs. adults, respectively] were found in old spinal cords, with increased density of pro-inflammatory M1 microglia and A1 astroglia (25-fold and 4-fold increase, respectively, P < 0.0001). Ageing resulted in significant reductions in the nerve conduction velocity and the compound muscle action potential amplitude (~30%, P < 0.05, vs. adults) in old distal plantar muscles. Compared with adult muscles, old muscles exhibited significantly higher numbers of both denervated and polyinnervated neuromuscular junctions, changes in fibre type composition, higher proportion of fibres showing central nuclei and lipofuscin aggregates, depletion of satellite cells, and augmented expression of different molecules related to development, plasticity, and maintenance of neuromuscular junctions, including calcitonin gene-related peptide, growth associated protein 43, agrin, fibroblast growth factor binding protein 1, and transforming growth factor-β1. Overall, these alterations occurred at varying degrees in all the muscles analysed, with no correlation between the age-related changes observed and myofiber type composition or muscle topography.

CONCLUSIONS

Our data provide a global view of age-associated neuromuscular changes in a mouse model of ageing and help to advance understanding of contributing pathways leading to development of sarcopenia.

Monoclonal antibodies blocking CGRP transmission: An update on their added value in migraine prevention

The avenue of effective migraine therapies blocking calcitonin gene-related peptide (CGRP) transmission is the successful outcome of 35 years of translational research. Developed after short-acting, the small antagonists of the CGRP receptor (the "gepants"), the monoclonal antibodies blocking CGRP or its receptor (CGRP/rec mAbs) have changed the paradigm in migraine treatment. Contrary to the classical acute medications like triptans or nonsteroidal anti-inflammatory drugs (NSAIDs) with a transient effect, they act for long durations exclusively in the peripheral portion of the trigeminovascular system and can thus be assimilated to a durable attack treatment, unlike the classical preventives that chiefly act upstream on the central facets of migraine pathophysiology. Randomized controlled trials (RCT) of eptinezumab, erenumab, fremanezumab and galcanezumab have included collectively several thousands of patients, making them the most extensively studied class of preventive migraine treatments. Their results clearly indicate that CGRP/rec mAbs are significantly superior to placebo and have been comprehensively reviewed by Dodick [Cephalalgia 2019;39(3):445-458]. In this review we will briefly summarize the placebo-subtracted outcomes and number-needed-to-treat (NNT) of these pivotal RCTs and analyze new and post-hoc studies published afterwards focusing on effect size, effect onset and sustainability, response in subgroups of patients, safety and tolerability, and cost-effectiveness. We will also summarize our limited real-world experience with one of the CGRP/rec mAbs. Although methodological differences and lack of direct comparative trials preclude any reliable comparison, the overall impression is that there are only minor differences in efficacy and tolerability profiles between the four monoclonals: the average placebo-subtracted 50% responder rates for reduction in migraine headaches are 21.4% in episodic migraine (NNTs: 4-5), 17.4% in chronic migraine (NNTs: 4-8). Patients with an improvement exceeding 50% are rare, chronic migraineurs with continuous headache are unlikely to be responders and migraine auras are not improved. The effect starts within the first week after administration and is quasi maximal at one month. It is sustained for long time periods and may last for several months after treatment termination. CGRP/rec mAbs are effective even after prior preventive treatment failures and in patients with medication overuse, but the effect size might be smaller. They significantly reduce disability and health care resource utilization. The adverse effect profile of CGRP/rec mAbs is close to that of placebo with few minor exceptions and despite concerns related to the safeguarding role of CGRP in ischemia, no treatment-related vascular adverse events have been reported to date. Putting the CGRP/rec mAbs in perspective with available preventive migraine drug treatments, their major advantage seems not to be chiefly their superior efficacy but their unprecedented efficacy over adverse event ratio. Regarding cost-effectiveness, preliminary pharmaco-economic analyses of erenumab suggest that it is cost-effective for chronic migraine compared to no treatment or to onabotulinumtoxinA, but likely not for episodic migraine unless attack frequency is high, indirect costs are considered and its price is lowered.

Ryanodine- and CaMKII-dependent release of endogenous CGRP induces an increase in acetylcholine quantal size in neuromuscular junctions of mice

OBJECTIVE

The aim of this study was to identify the mechanism responsible for an increase in miniature endplate potentials (MEPPs) amplitude, induced by ryanodine as an agonist of ryanodine receptors in mouse motor nerve terminals.

METHODS

Using intracellular microelectrode recordings of MEPPs and evoked endplate potentials (EPPs), the changes in spontaneous and evoked acetylcholine release in motor synapses of mouse diaphragm neuromuscular preparations were studied.

RESULTS

Ryanodine (0.1 μM) increased both the amplitudes of MEPPs and EPPs to a similar extent (up to 130% compared to control). The ryanodine effect was prevented by blockage of receptors of calcitonin gene-related peptide (CGRP) by a truncated peptide CGRP_8-37 . Endogenous CGRP is stored in large dense-core vesicles in motor nerve terminals and may be released as a co-transmitter. The ryanodine-induced increase in MEPPs amplitude may be fully prevented by inhibition of vesicular acetylcholine transporter by vesamicol or by blocking the activity of protein kinase A with H-89, suggesting that endogenous CGRP is released in response to the activation of ryanodine receptors. Activation of CGRP receptors can, in turn, upregulate the loading of acetylcholine into synaptic vesicles, which will increase the quantal size. This new feature of endogenous CGRP activity looks similar to recently described action of exogenous CGRP in motor synapses of mice. The ryanodine effect was prevented by inhibitors of Ca/Calmodulin-dependent kinase II (CaMKII) KN-62 or KN-93. Inhibition of CaMKII did not prevent the increase in MEPPs amplitude, which was caused by exogenous CGRP.

CONCLUSIONS

We propose that the activity of presynaptic CaMKII is necessary for the ryanodine-stimulated release of endogenous CGRP from motor nerve terminals, but CaMKII does not participate in signaling downstream the activation of CGRP-receptors followed by quantal size increase.

Adenosine Receptors in Developing and Adult Mouse Neuromuscular Junctions and Functional Links With Other Metabotropic Receptor Pathways

In the last few years, we have studied the presence and involvement in synaptogenesis and mature transmitter release of the adenosine autoreceptors (AR) in the mammalian neuromuscular junction (NMJ). Here, we review and bring together the previously published data to emphasize the relevance of these receptors for developmental axonal competition, synaptic loss and mature NMJ functional modulation. However, in addition to AR, activity-dependent mediators originating from any of the three cells that make the synapse (nerve, muscle, and glial cells) cross the extracellular cleft to generate signals in target metabotropic receptors. Thus, the integrated interpretation of the complementary function of all these receptors is needed. We previously studied, in the NMJ, the links of AR with mAChR and the neurotrophin receptor TrkB in the control of synapse elimination and transmitter release. We conclude that AR cooperate with these receptors through synergistic and antagonistic effects in the developmental synapse elimination process. In the adult NMJ, this cooperation is manifested so as that the functional integrity of a given receptor group depends on the other receptors operating normally (i.e., the functional integrity of mAChR depends on AR operating normally). These observations underlie the relevance of AR in the NMJ function.

Abnormal response of dopaminergic neurons to nicotine without perturbation of nicotinic receptors in alphaCGRP knock-out mice

Alpha-calcitonin gene-related peptide (alphaCGRP) is a neuropeptide with multiple biological properties, including the regulation of nicotinic acetylcholine receptors (nAChRs). We have previously reported a reduction of somatic withdrawal symptoms in alphaCGRP knock-out mice exposed to chronic nicotine, leading us to investigate the contribution of alphaCGRP to the regulations of ventral tegmental area (VTA) neurons and their response to nicotine. The electrophysiological activity of VTA dopaminergic (DA) neurons was recorded in vivo, under anesthesia. These neurons displayed identical spontaneous electrophysiogical activities in wild-type and alphaCGRP-/- mice. However, we found that intravenous administration of nicotine (30 microg/kg) had no significant effect on the activity of DA neurons in alphaCGRP-/- mice, whereas it induced a doubling of the firing rate in wild-type animals. A higher dose (90 microg/kg) produced a significant excitation in both strains, but this effect remained smaller in the mutants. To investigate this difference, we have studied the functional state of nAChRs in wild-type and alphaCGRP-/- mice. Both strains exhibited identical expression of alpha(7) and alpha(4)beta(2) nAChRs as revealed by autoradiographical studies in the VTA. In addition, focal application of acetylcholine on DA neurons recorded by patch-clamp revealed identical currents mediated by nAChRs in mutant animals, as compared to wild-type mice. These data outline the possibility of a contribution of alphaCGRP to the effects of nicotine on DA neurons, by a physiological pathway independent of VTA nicotinic receptors.

Localization of the urotensin II receptor in the rat central nervous system

The vasoactive peptide urotensin II (UII) is primarily expressed in motoneurons of the brainstem and spinal cord. Intracerebroventricular injection of UII provokes various behavioral, cardiovascular, motor, and endocrine responses in the rat, but the distribution of the UII receptor in the central nervous system (CNS) has not yet been determined. In the present study, we have investigated the localization of UII receptor (GPR14) mRNA and UII binding sites in the rat CNS. RT-PCR analysis revealed that the highest density of GPR14 mRNA occurred in the pontine nuclei. In situ hybridization histochemistry showed that the GPR14 gene is widely expressed in the brain and spinal cord. In particular, a strong hybridization signal was observed in the olfactory system, hippocampus, olfactory and medial amygdala, hypothalamus, epithalamus, several tegmental nuclei, locus coeruleus, pontine nuclei, motor nuclei, nucleus of the solitary tract, dorsal motor nucleus of the vagus, inferior olive, cerebellum, and spinal cord. Autoradiographic labeling of brain slices with radioiodinated UII showed the presence of UII-binding sites in the lateral septum, bed nucleus of the stria terminalis, medial amygdaloid nucleus, anteroventral thalamus, anterior pretectal nucleus, pedunculopontine tegmental nucleus, pontine nuclei, geniculate nuclei, parabigeminal nucleus, dorsal endopiriform nucleus, and cerebellar cortex. Intense expression of the GPR14 gene in some hypothalamic nuclei (supraoptic, paraventricular, ventromedian, and arcuate nuclei), in limbic structures (amygdala and hippocampus), in medullary nuclei (solitary tract, dorsal motor nucleus of the vagus), and in motor control regions (cerebral and cerebellar cortex, substantia nigra, pontine nuclei) provides the anatomical substrate for the central effects of UII on behavioral, cardiovascular, neuroendocrine, and motor functions. The occurrence of GPR14 mRNA in cranial and spinal motoneurons is consistent with the reported autocrine/paracrine action of UII on motoneurons.

Androgenic down-regulation of urotensin II precursor, urotensin II-related peptide precursor and androgen receptor mRNA in the mouse spinal cord

We had previously shown that in rat spinal cord motoneurons urotensin II (UII) precursor mRNA was down-regulated by androgens. Very recently, a gene encoding the precursor of a UII analog, termed UII-related peptide (URP), has been identified. Using in situ hybridization, we studied the localization of UII and URP precursor as well as androgen receptor (AR) mRNA in the male mouse thoracic spinal cord. We also evaluated the androgenic regulation of the two peptide precursor and AR mRNA expression in the ventral horn of the mouse thoracic spinal cord. The results revealed that URP precursor mRNA was localized in motoneurons and that the vast majority of the motoneurons expressed both peptide precursor as well as AR mRNA. Seven-day castration induced an increase in UII and URP precursor and AR mRNA levels. Short term (3-24 h) administration of dihydrotestosterone to castrated animals restored the three protein mRNA levels to the levels observed in intact animals. These results suggest that in the ventral horn of the mouse spinal cord both UII and URP precursor and AR mRNA are expressed by the same neurons and that circulating androgens are exerting a down-regulation of the three protein mRNA expression, possibly by a direct action on motoneurons.

Differential expression of calcitonin gene-related peptide (CGRP) and choline acetyltransferase (ChAT) in the axotomized motoneurons of normoxic and hypoxic rats

We employed a double injury model (axotomy along with hypoxia) to determine how nerve injury and hypoxic insult would affect the expression of calcitonin gene-related peptide (CGRP) and choline acetyltransferase (ChAT) in the hypoglossal nucleus (HN) and nucleus ambiguus (NA). Adult rats were subjected to unilateral vagus and hypoglossal nerve transection, following which half of the animals were kept in an altitude chamber (PO2=380 Torr). The immunoexpression of CGRP and ChAT (CGRP-IR/ChAT-IR) were examined by quantitative immunohistochemistry at 3, 7, 14, 30 and 60 days post-axotomy. The results revealed that CGRP-IR in the HN was increased at 3 days but decreased to basal levels at 7 days following nerve injury. The decline was followed by a second rise in CGRP-IR at 30 days post-axotomy, followed again by a return to basal levels at 60 days. In the NA, CGRP-IR was up-regulated at 3 days and remained increased for up to 60 days after nerve injury. Animals treated with a double injury showed a greater CGRP-IR than normoxic group in both nuclei at all post-axtomized periods. In contrast to CGRP, ChAT-IR was markedly reduced in the HN and NA at 3 days reaching its nadir at 14 days following nerve injury. Hypoxic animals showed a stronger reduction of ChAT-IR in both nuclei at all post-axtomized periods. Results of cell counting showed that neuronal loss was somewhat obvious in hypoxic HN than that of normoxic ones. The present results suggest that up-regulation of CGRP-IR may exert its trophic effects while down-regulation of ChAT-IR may correlate with the poor neurotransmission within the injured neurons. It is speculated that the enhanced expression of CGRP-IR and the pronounced reduction of ChAT-IR in hypoxic rats may result from a drastic shift of intracellular metabolic pathways, which in turn could lead to more metabolic loading to the severely damaged neurons following the double insult.

Diabetes mellitus influences the degree of colocalization of calcitonin gene-related peptide with insulin and somatostatin in the rat pancreas

OBJECTIVES

Although calcitonin-gene related peptide (CGRP) (C162H262N50O50S2) has been shown to be present in pancreatic islet cells, no data have been reported on the pattern of its distribution in the islets of Langerhans of diabetic rats.

METHODS

The present study used immunohistochemical and immunofluorescence techniques to examine the pattern of distribution of CGRP-like immunoreactive (CGRP-LIR) cells and nerves in the pancreata of rats with streptozotocin-induced diabetes. The effect of CGRP on insulin secretion from rat pancreatic tissue fragments was also investigated using a radioimmunoassay technique.

RESULTS

Numerous CGRP-LIR cells were observed in both the peripheral and central regions of the islets of Langerhans of normal pancreata where they colocalized with large and small subsets of insulin-LIR and somatostatin-LIR cells, respectively. By contrast, the islets of diabetic rat pancreata contained significantly (P < 0.0001) fewer CGRP-LIR cells compared with normal rats. In diabetic rat pancreata, CGRP was colocalized with larger and smaller subsets of somatostatin-LIR and insulin-LIR cells, respectively. CGRP-LIR nerve fibers were discerned in the perivascular and periacinar regions of the pancreata of both normal and diabetic rats. CGRP (10 M) induced a significant (P < 0.02) increase in insulin secretion from the pancreas of normal rat.

CONCLUSION

CGRP is colocalized with insulin and somatostatin in the pancreata of normal and diabetic rats and may play an important role in the humoral and neural regulation of the endocrine pancreas.

Biochemical characterization and immunohistochemical localization of urotensin II in the human brainstem and spinal cord

The human urotensin II (UII) precursor encompasses several potential cleavage sites and thus, processing of pro-UII may generate various forms of mature UII including the peptides of 11 (UII11), 16 (UII16) and 19 (UII19) residues. Until now, the native form of human UII had not been characterized. Here, we show that the major UII peptide occurring in the human spinal cord corresponds to UII11. In contrast, neither the UII16 nor the UII19 forms could be detected. In 50% of the brainstem and in all the spinal cord extracts analysed, a second minor UII-immunoreactive peptide was resolved. Immunohistochemical labelling of the cervical segment of the human spinal cord revealed that the UII-immunoreactive material was confined to a subset of ventral horn motoneurones. These data provide the first evidence that in the human, the UII precursor, expressed in motoneurones, is processed at the tribasic KKR93 cleavage site to generate a mature form of UII of 11 amino acids. The absence of N-terminally elongated forms of UII of 16 and 19 residues indicates that pro-UII is not cleaved at the R85 or K88 monobasic sites. Finally, the minor UII-immunoreactive peptide detected in several tissue extracts might correspond to an extended form of UII resulting from the processing of the UII precursor at the basic RK50 or RK66 doublets.

Localization of the calcitonin gene-related peptide receptor complex at the vertebrate neuromuscular junction and its role in regulating acetylcholinesterase expression

The calcitonin gene-related peptide (CGRP) is released by motor neurons where it exerts both short and long term effects on skeletal muscle fibers. In addition, sensory neurons release CGRP on the surrounding vasculature where it is in part responsible for local vasodilation following muscle contraction. Although CGRP-binding sites have been demonstrated in whole muscle tissue, the type of CGRP receptor and its associated proteins or its cellular localization within the tissue have not been described. Here we show that the CGRP-binding protein referred to as the calcitonin receptor-like receptor is highly concentrated at the avian neuromuscular junction together with its two accessory proteins, receptor activity modifying protein 1 and CGRP-receptor component protein, required for ligand specificity and signal transduction. Using tissue-cultured skeletal muscle we show that CGRP stimulates an increase in intracellular cAMP that in turn initiates down-regulation of acetylcholinesterase expression at the transcriptional level, and, more specifically, inhibits expression of the synaptically localized collagen-tailed form of the enzyme. Together, these studies suggest a specific role for CGRP released by spinal cord motoneurons in modulating synaptic transmission at the neuromuscular junction by locally inhibiting the expression of acetylcholinesterase, the enzyme responsible for terminating acetylcholine neurotransmission.

Modulation of neuronal nicotinic receptor function by the neuropeptides CGRP and substance P on autonomic nerve cells

1. One classical example of how neuropeptides can affect the function of ligand-gated receptors is the modulation of neuronal nicotinic receptors (nAChRs) by substance P. The present review updates current understanding of this action by substance P and compares it with other neuropeptides more recently found to modulate nAChRs in the autonomic nervous system. 2. Calcitonin gene-related peptide (CGRP) and its N-terminal fragments have been shown to exert complex inhibitory as well facilitatory actions on nAChRs. Fragments such as CGRP(1-4), CGRP(1-5) and CGRP(1-6) rapidly and reversibly enhance agonist sensitivity of nAChRs without directly activating those receptors. Longer fragments or the full-length peptide potently inhibit responses mediated by nAChRs via an apparently competitive-type antagonism. This phenomenon differs from the substance P-induced block, which is agonist use-dependent and preferential towards large nicotinic responses. 3. It is argued that the full-length peptides CGRP and substance P might play distinct roles in the activity-dependent modulation of cholinergic neurotransmission, by inhibiting background noise in the case of CGRP or by reducing excessive excitation in the case of substance P. Hence, multiple neuropeptide mechanisms may represent a wide array of fine-tuning processes to regulate nicotinic synaptic transmission. 4. The availability of novel CGRP derivatives with a strong enhancing action on nAChRs may offer new leads for the drug design targeted for potentiation of nAChRs in the autonomic nervous system as well as in the brain, a subject of interest to counteract the deficit of the nAChR function associated with neurodegenerative diseases like Alzheimer's and Parkinson's diseases.

Role of androgens in the regulation of urotensin II precursor mRNA expression in the rat brainstem and spinal cord

It has been reported that both urotensin II precursor (pro UII) mRNA and androgen receptors (ARs) are highly expressed in rat brainstem motor nuclei and ventral horn of the spinal cord. In order to determine the possible involvement of androgens in regulation of pro UII mRNA expression, we have studied the co-localization of pro UII mRNA and AR immunoreactivity and the effect of castration and dihydrotestosterone (DHT) replacement therapy on pro UII mRNA in the rat facial nucleus and ventral horn of the spinal cord. By in situ hybridization, pro UII mRNA was only detected in motoneurons in both the facial nucleus and ventral horn of the spinal cord. Double-labelling studies revealed that the vast majority (over 95%) of motoneurons immunostained for AR also expressed pro UII mRNA in both areas examined. Three weeks after castration, pro UII mRNA expression, as measured by semi-quantitative in situ hybridization, was increased by 17% and 58% in the ventral horn of the spinal cord and the facial nucleus, respectively. The administration of DHT completely prevented the stimulating effect of castration. These results indicate that circulating androgens are exerting a down-regulation of pro UII expression possibly by a direct action at the level of motoneurons. The physiological relevance of these new findings remains to be fully explored.

Botulinum neurotoxins: from paralysis to recovery of functional neuromuscular transmission

The neuromuscular junction is one of the most accessible mammalian synapses which offers a useful model to study long-term synaptic modifications occurring throughout life. It is also the natural target of botulinum neurotoxins (BoNTs) causing a selective blockade of the regulated exocytosis of acetylcholine thereby triggering a profound albeit transitory muscular paralysis. The scope of this review is to describe the principal steps implicated in botulinum toxin intoxication from the early events leading to a paralysis to the cellular response implementing an impressive synaptic remodelling culminating in the functional recovery of neuromuscular transmission. BoNT/A treatment promotes extensive sprouting emanating from intoxicated motor nerve terminals and the distal portion of motor axons. The current view is that sprouts have the ability to form functional synapses as they display a number of key proteins required for exocytosis: SNAP-25, VAMP/synaptobrevin, syntaxin-I, synaptotagmin-II, synaptophysin, and voltage-activated Na+, Ca2+ and Ca2+-activated K+ channels. Exo-endocytosis was demonstrated (using the styryl dye FM1-43) to occur only in the sprouts in vivo, at the time of functional recovery emphasising the direct role of nerve terminal outgrowth in implementing the restoration of functional neurotransmitter release (at a time when nerve stimulation again elicited muscle contraction). Interestingly, sprouts are only transitory since a second distinct phase of the rehabilitation process occurs with a return of synaptic activity to the original nerve terminals. This is accompanied by the elimination of the dispensable sprouts. The growth or elimination of these nerve processes appears to be strongly correlated with the level of synaptic activity at the parent terminal. The BoNT/A-induced extension and later removal of "functional" sprouts indicate their fundamental importance in the rehabilitation of paralysed endplates, a finding with ramifications for the vital process of nerve regeneration.

GDNF gene delivery to injured adult CNS motor neurons promotes axonal growth, expression of the trophic neuropeptide CGRP, and cellular protection

Glial-cell-line--derived neurotrophic factor (GDNF) has been identified as a potent survival and differentiation factor for several neuronal populations in the central nervous system (CNS), but to date, distinct effects of GDNF on motor axon growth and regeneration in the adult have not been demonstrated. In the present study, ex vivo gene delivery was used to directly examine whether GDNF can influence axonal growth, expression of neuronal regeneration-related genes, and sustain the motor neuronal phenotype after adult CNS injury. Adult Fischer 344 rats underwent unilateral transections of the hypoglossal nerve, followed by intramedullary grafts of fibroblasts genetically modified to secrete GDNF. Control animals received lesions and grafts of cells expressing a reporter gene. Two weeks later, GDNF gene delivery (1) robustly promoted the growth of lesioned hypoglossal motor axons, (2) altered the expression and intracellular trafficking of the growth-related protein calcitonin gene-related peptide (CGRP), and (3) significantly sustained the cholinergic phenotype in 84 +/- 6% of hypoglossal neurons compared with 39 +/- 6% in control animals (P < 0.001). This is the first neurotrophic factor identified to increase the in vivo expression of the trophic peptide CGRP and the first report that GDNF promotes motor axonal growth in vivo in the adult CNS. Taken together with previous in vitro studies, these findings serve as the foundation for a model wherein GDNF and CGRP interact in a paracrine manner to regulate neuromuscular development and regeneration.

Nonadrenergic innervation of the rat laryngeal vasculature

In order to gain a better understanding of the central and local control of laryngeal blood flow, the vascular innervation to the rat laryngeal muscles was examined. To visualize the vascular network, the animals were perfused with a gelatin/India ink solution. The larynges were removed and fixed. The superior laryngeal, cricothyroid, and inferior laryngeal arteries (all branch off the superior thyroid artery) were dissected in continuity into their respective muscles. Specimens were reacted in toto using immunohistochemical techniques for the presence of neuropeptide-Y (NPY), vasoactive intestinal peptide (VIP), calcitonin gene related peptide (CGRP), and neuronal nitric oxide synthase (NOS-1). Results show that all of the laryngeal vasculature is richly innervated by fibers containing these peptides. Qualitatively, the most prominent of these is NPY in association with the superior and the inferior laryngeal arteries, followed by VIP and NOS-1, and finally CGRP distributed equally on all the vessels. Immunopositive fibers are found along the entire course of the feeding arteries, beginning with the superior thyroid artery and continuing down to small arterioles into the terminal vascular beds. These peptides can act as vasodilators, vasoconstrictors, and/or neuromodulators and may work synergistically or antagonistically with other transmitters in controlling laryngeal blood flow. Their effects are dependent on the specific vascular bed in question, that is, in some areas they are vasodilators, in others vasoconstrictors, and in other neuromodulators. What effects they have on the laryngeal vasculature and how they interact within the larynx have yet to be determined.

Differential effects of NT-3 on reinnervation of the fast extensor digitorum longus (EDL) and the slow soleus muscle of rat

Previous studies of gastrocnemius muscle reinnervation showed specific normalization of the proportion and diameter of fast type 2b muscle fibres following NT-3 delivery to the proximal stump of the cut sciatic nerve. Here, we investigate if normalization was related to greater improvement of muscle reinnervation of fast (extensor digitorum longus; EDL) than slow (soleus) motor units. NT-3-impregnated (NT-3 group) or plain fibronectin (FN group) mats were inserted into a sciatic nerve gap. Neuromuscular junctions (NMJs) labelled with TRITC-alpha-bungarotoxin were colabelled with calcitonin gene-related peptide (CGRP) or 4E2 antisera and imaged using confocal microscopy. CGRP and 4E2 were used as markers for newly reinnervated and structurally mature NMJs, respectively. At 40 days postsurgery, denervated NMJs in EDL and soleus muscles of both groups presented a 50% decrease of surface area due to decreased width. At day 80 in EDL, more NMJs were reinnervated by CGRP-immunoreactive terminals in the NT-3 (7.1%) than in the FN group (4.2%); there was no difference between groups for soleus. At 120 days, 4E2-immunoreactive NMJs were more numerous in EDL of the NT-3 (40.0%) than in the FN group (7.3%), unlike in soleus (NT-3, 1. 6%; FN, 1.8%), and presented a partial size recovery. These results indicate that NT-3 preferentially improves reinnervation of fast muscles over slow muscle, although the mechanism of this improvement is still unclear.

Comparison of the vascular innervation of the rat cochlea and vestibular system

In order to gain a better understanding of the neuronal and local control of inner ear blood flow, the vascular innervation to the rat cochlea and vestibular system was examined. Specimens were removed in toto beginning at the basilar artery extending to the anterior inferior cerebellar artery, labyrinthine artery, common cochlear artery, modiolar artery and anterior vestibular artery. When possible the vessels were dissected in continuity through the cribrose area. The vestibular endorgans were also removed. Specimens were examined using immunohistochemical techniques for the presence of vasoactive intestinal peptide, neuronal nitric oxide synthase, neuropeptide-Y, substance P and calcitonin gene related peptide. Results show that the vasculature to the cochlea and vestibular portion of the inner ear receive similar types of nonadrenergic innervation, that within the vestibular endorgans, only CGRP and SP were found in the neuroepithelium or in association with vessels, and that within the vestibular system, the majority of the vascular innervation appears to stop at or near the cribrose area. In the cochlea however, it extends to include the radiating arterioles. These findings suggest that cochlear blood flow is under finer control and that neuronally induced changes in blood flow may have a more global effect in the vestibular periphery.

Cloning, sequence analysis and tissue distribution of the mouse and rat urotensin II precursors

Urotensin II (UII) is a cyclic neuropeptide initially isolated from the caudal neurosecretory system of teleost fish. The recent cloning of the UII precursor in frog and human has demonstrated that the peptide is not restricted to the fish urophysis but that it is also expressed in the central nervous system of tetrapods. Here, we describe the characterization of the cDNAs encoding prepro-UII in mouse and rat. A comparison of the primary structures of mouse and rat UII with those of other vertebrate UII reveals that the sequence of the cyclic region of the molecule (CFWKYC) has been fully conserved. In contrast, the N-terminal flanking domain of prepro-UII has markedly diverged with only 48% sequence identity between the mouse or rat and the human precursors. In situ hybridization histochemistry showed that the prepro-UII gene is predominantly expressed in motoneurons of the brainstem and spinal cord, suggesting that UII may play a role in the control of neuromuscular functions.

Parabrachial origin of calcitonin gene-related peptide-immunoreactive axons innervating Meynert's basal nucleus

Meynert's basal nucleus is innervated by calcitonin gene-related peptide (CGRP)-immunoreactive axons synapsing with cholinergic principal cells. Origin of CGRP-immunopositive axons was studied in the albino rat. Since beaded axons containing the nicotinic acetylcholine receptor (nAChR) are also present in the basal nucleus, the microstructural arrangement raises the question whether or not an interaction between CGRP and nAChR exists like in the neuromuscular junction. We found that electrolytic lesion of the parabrachial nucleus results in degeneration of CGRP-immunoreactive axons in the ipsilateral nucleus basalis and induces shrinkage of principal cholinergic neurons while the contralateral nucleus basalis remains intact. Electrolytic lesions in the thalamus, caudate-putamen, and hippocampus did not induce alterations in Meynert's basal nucleus. Disappearance of CGRP after lesions of the parabrachial nucleus does not impair presynaptic nAChR in the basal nucleus, suggesting that, unlike in the neuromuscular junction, CGRP is not involved in the maintenance of nAChR in the basal forebrain. It is concluded that the parabrachial nucleus is involved in the activation of the nucleus basalis-prefrontal cortex system, essential in gnostic and mnemonic functions.

Calcitonin gene-related peptide rapidly downregulates nicotinic receptor function and slowly raises intracellular Ca2+ in rat chromaffin cells in vitro

Although calcitonin gene-related peptide (CGRP) modulates muscle-type nicotinic acetylcholine receptors (nAChRs) via intracellular second messenger-mediated phosphorylation, the action of this peptide on neuronal-type nAChRs remains unknown. Using neuronal nAChRs of rat chromaffin cells in vitro we studied the effect of CGRP, which is physiologically present in adrenal medulla, on membrane currents and [Ca2+]i transients elicited by nicotine. Our main novel observation was that CGRP (either bath-applied or focally applied for a few seconds or even co-applied with nicotine for a few milliseconds) selectively and rapidly blocked nAChRs (a phenomenon unlikely caused by intracellular messengers in view of its speed) without affecting GABA receptors. The inhibitory effect of CGRP was independent of [Ca2+]i or membrane potential and not accompanied by baseline current changes. Like the competitive antagonist N,N,N-trimethyl-1-(4-trans-stilbenoxy)-2-propilammonium, CGRP induced a rightward, parallel shift of the nicotine dose-response curve; during co-application of these blockers the nicotine dose-ratio value was the sum of the values obtained with each antagonist alone. The block by CGRP was insensitive to the receptor antagonist hCGRP8-37 but mimicked by CGRP1-7. Persistent application of CGRP slowly increased [Ca2+]i, a phenomenon independent from external Ca2+, thus implying Ca2+ release from internal stores, and suppressed by hCGRP8-37. CGRP1-7 had no significant effect on [Ca2+]i. We propose that the 1-7 amino acid sequence of CGRP was responsible for the direct, rapid block of nAChRs, whereas the full-length peptide molecule was necessary for the delayed rise in internal Ca2+ potentially able to trigger phosphorylation-dependent modulation of nicotinic receptor function.

Development of the vertebrate neuromuscular junction

We describe the formation, maturation, elimination, maintenance, and regeneration of vertebrate neuromuscular junctions (NMJs), the best studied of all synapses. The NMJ forms in a series of steps that involve the exchange of signals among its three cellular components--nerve terminal, muscle fiber, and Schwann cell. Although essentially any motor axon can form NMJs with any muscle fiber, an additional set of cues biases synapse formation in favor of appropriate partners. The NMJ is functional at birth but undergoes numerous alterations postnatally. One step in maturation is the elimination of excess inputs, a competitive process in which the muscle is an intermediary. Once elimination is complete, the NMJ is maintained stably in a dynamic equilibrium that can be perturbed to initiate remodeling. NMJs regenerate following damage to nerve or muscle, but this process differs in fundamental ways from embryonic synaptogenesis. Finally, we consider the extent to which the NMJ is a suitable model for development of neuron-neuron synapses.

Cloning of the cDNA encoding the urotensin II precursor in frog and human reveals intense expression of the urotensin II gene in motoneurons of the spinal cord

Urotensin II (UII) is a cyclic peptide initially isolated from the caudal neurosecretory system of teleost fish. Subsequently, UII has been characterized from a frog brain extract, indicating that a gene encoding a UII precursor is also present in the genome of a tetrapod. Here, we report the characterization of the cDNAs encoding frog and human UII precursors and the localization of the corresponding mRNAs. In both frog and human, the UII sequence is located at the C-terminal position of the precursor. Human UII is composed of only 11 amino acid residues, while fish and frog UII possess 12 and 13 amino acid residues, respectively. The cyclic region of UII, which is responsible for the biological activity of the peptide, has been fully conserved from fish to human. Northern blot and dot blot analysis revealed that UII precursor mRNAs are found predominantly in the frog and human spinal cord. In situ hybridization studies showed that the UII precursor gene is actively expressed in motoneurons. The present study demonstrates that UII, which has long been regarded as a peptide exclusively produced by the urophysis of teleost fish, is actually present in the brain of amphibians and mammals. The fact that evolutionary pressure has acted to conserve fully the biologically active sequence of UII suggests that the peptide may exert important physiological functions in humans.

Regulation of ion channel expression in neural cells by hormones and growth factors

Voltage-and ligand-gated ion channels are key players in synaptic transmission and neuron-glia communication in the nervous system. Expression of these proteins can be regulated at several levels (transcriptional, translational, or posttranslational) and by multiple extracellular factors in the developing and mature nervous system. A wide variety of hormones and growth factors have been identified as important in neural cell differentiation, which is a complex process involving the acquisition of cell-type-specific ion channel phenotypes. Much literature has already accumulated describing the structural and functional characteristics of ion channels, but relatively little is known about the factors that influence their synthesis and cell surface expression, although this area has generated considerable interest in the context of neural cell development. This article reviews several examples of regulated expression of these channels by cellular factors, namely peptide growth factors and steroid hormones, and discusses, where applicable, current understanding of molecular mechanisms underlying such regulation of voltage-and neurotransmitter-gated ion channels.

Calcitonin gene-related peptide: distribution and effects on spontaneous rhythmic activity in embryonic chick spinal cord

Immunohistochemical and in vitro electrophysiological techniques were utilized to examine the distribution and possible role of calcitonin gene-related peptide (CGRP) in the spinal cord of the developing chick. CGRP-like immunoreactivity first appeared in the lateral motor column of the lumbosacral spinal cord at embryonic day 6 followed by the emergence of fiber immunoreactivity in the dorsal horn at embryonic day 11. A rostrocaudal survey of the cervical to lumbosacral spinal cord in embryonic day 18 chick demonstrated robust CGRP-like immunoreactivity at all levels in both putative motor neurons and dorsal horn fibers. Additionally, small immunoreactive lamina VII neurons were observed in sections of lumbosacral cord. In the embryonic day 10 (E10) in vitro reduced spinal cord preparation, bath application of the calcitonin gene-related peptide antagonist human alpha-CGRP fragment 8-37 decreased the frequency and increased the duration of episodes of spontaneously occurring rhythmic activity. Conversely, application of alpha or beta forms of calcitonin gene-related peptide increased the frequency of the rhythmic episodes. The electrophysiological results suggest there is a constitutive release of calcitonin gene-related peptide contributing to the spontaneous rhythmic activity. Immunohistochemical results from E10 animals suggest that CGRP-like immunoreactive putative motoneurons may be the source of the released CGRP.

Peptidergic innervation and the nicotinic acetylcholine receptor in the primate basal nucleus

Peptidergic innervation and localization of the neuronal nicotinic acetylcholine receptor (nAChR) was studied in the basal forebrain of Macaca fascicularis in order to provide microstructural proofs for the theory (Changeux et al., 1992) that calcitonin gene-related peptide (CGRP) is responsible for the maintenance of the acetylcholine receptor. Distribution and localization of five neuropeptides, namely substance P (SP), CGRP, neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP) neurotensin (NT), and the neuropeptides parvalbumin (PV) and the alpha-bungarotoxin- (alpha-BTX-) binding protein was studied by means of light- and electron microscopic pre-embedding immunocytochemistry. Immunohistochemical double staining revealed that large cholinergic principal nerve cells in the basal forebrain, corresponding to cell group Ch4 constituting Meynert's basal nucleus (BNM), and exerting intense choline acetyltransferase (ChAT) immunoreactivity, are synaptically innervated by axons displaying CGRP immunoreactivity. While SP, NPY, PV and CGRP establish dense networks in BNM, innervation by NT and VIP is sparse. Biotinylated alpha-BTX visualizes beaded axons that surround dendrites and perikarya of cholinergic principal cells. Electron microscopic organization of the neuropil in BNM is characterized by a glomerular (or rather cartridge-like) arrangement of axons surrounding dendrites of non-cholinergic principal nerve cells. At least one of the axons establishing the glomerulus (cartridge) exerts CGRP immunopositivity while alpha-BTX-immunopositive axons, presynaptic to dendrites of principal cells, are attached to the glomeruli (cartridges) from outside. As alpha-BTX-binding indicates localization of the alpha7 subunit of the neuronal nAChR, the microtopographical arrangement supports the idea that, in a manner similar to that in the neuromuscular junction, CGRP might contribute to the maintenance of nAChR also in BNM. Our results suggest that presynaptic nAChR-s are involved in the regulation of acetylcholine release from a feed-forward amplification mechanism of cholinergic principal cells of BNM.

Mutual dependence of calcitonin-gene related peptide and acetylcholine release in neuromuscular preparations

To investigate the mutual dependence of calcitonin gene-related peptide (CGRP) and acetylcholine release, we examined the effect of a cholinesterase inhibitor neostigmine on the release of CGRP-like immunoreactivity in rat phrenic nerve-hemidiaphragm muscle preparation, and conversely, the effect of CGRP on [3H]acetylcholine release from motor nerve terminals loaded with [3H]choline in the same preparations of mice. Release of CGRP-like immunoreactivity was increased by electrical nerve stimulation (train of 40 pulses of 200 micros pulse duration and frequency of 50 Hz applied every 10 s) in the whole preparation but not in the segmental preparation containing the endplate region. Neostigmine (0.1-0.3 microM) enhanced the resting release of CGRP-like immunoreactivity in a concentration-dependent manner, whereas it depressed the nerve-evoked release of CGRP-like immunoreactivity. CGRP (1 microM) added to perfusate decreased nerve-evoked [3H]acetylcholine release. These results suggest that CGRP, which is released by electrical nerve stimulation or a cholinesterase inhibitor in intact skeletal muscles, negatively modulates nerve-evoked acetylcholine release.

Upregulation of calcitonin gene-related peptide at mouse motor nerve terminals poisoned with botulinum type-A toxin

Calcitonin gene-related peptide (CGRP)-like immunoreactivity of motor nerve terminals was investigated at different times after local in vivo injection of botulinum type-A toxin (BoNT/A) close to the mouse levator auris longus muscle. CGRP expression in most of control nerve terminals was undetectable, but markedly increased during muscle paralysis and synaptic remodelling and, declined once functional recovery occurred.

The cellular localization of the neuropeptides substance P, neurokinin A, calcitonin gene-related peptide and neuropeptide Y in guinea-pig vestibular sensory organs: a high-resolution confocal microscopy study

Four neuropeptides, substance P, neurokinin A, calcitonin gene-related peptide and neuropeptide Y, were detected by radioimmunoassay in guinea-pig vestibular end-organs. High-resolution confocal microscopy visualization of immunofluorescence staining was used to determine the cellular localization of these peptides. Substance P- and neurokinin A-like immunoreactivities were found to co-exist in afferent fibers innervating the peripheral regions of both the utricular and ampullar sensory organs. The immunoreactivity was more concentrated in the distal ends of the calyceal-shaped nerve endings that innervate type I sensory cells. While in the guinea-pig, nerve calyces and type I cells are distributed in both the central and peripheral regions of the sensory epithelia, immunoreactive calyces were found only in the peripheral regions. Calcitonin gene-related peptide-like immunoreactivity was localized in small bouton endings situated at the level of the base of the hair cells. These boutons were in a position to make axosomatic contacts with type II sensory cells and axodendritic contacts with afferent nerve endings. Calcitonin gene-related peptide immunoreactivity co-existed with choline acetyltransferase immunoreactivity. The localization and shape of these boutons identified them as the axonal endings of efferent vestibular fibers. Neuropeptide Y-like immunoreactivity was not observed in the actual sensory epithelium but in the underlying connective tissue, where it was located in varicose fibers along blood vessels. The synaptic position of the tachykinins is clearly distinct from that of calcitonin gene-related peptide. This segregation distinguishes the vestibular end-organs from most peripheral tissues where these peptides are co-localized. The tachykinin-immunoreactive afferent fibers are postsynaptic to the hair cells. If, as in somatic sensory endings, these fibers can be triggered to release the neuropeptides by an axon reflex type of activation, then the tachykinins could interfere directly with the function of type I and type II vestibular hair cells. Calcitonin gene-related peptide co-exists with acetylcholine in the efferent axonal endings that are presynaptic to type II hair cells and to afferent fibers. Calcitonin gene-related peptide can thus interfere by direct synaptic action with type II hair cells only. It may also regulate the activity of the tachykinin-containing afferents.

Selective depletion of clear synaptic vesicles and enhanced quantal transmitter release at frog motor nerve endings produced by trachynilysin, a protein toxin isolated from stonefish (Synanceia trachynis) venom

Our previous observation that low concentrations of stonefish (Synanceia trachynis) venom elicit spontaneous quantal acetylcholine release from vertebrate motor nerve terminals prompted our present study to purify the quantal transmitter-releasing toxin present in the venom and to characterize the toxin's ability to alter the ultrastructure and immunoreactivity of frog motor nerve terminals. Fractionation of S. trachynis venom by sequential anion exchange fast protein-liquid chromatography (FPLC) and size-exclusion FPLC yielded a highly purified preparation of a membrane-perturbing (haemolytic) protein toxin, named trachynilysin. Trachynilysin (2-20 micrograms/ml) significantly increased spontaneous quantal acetylcholine release from motor endings, as detected by recording miniature endplate potentials from isolated frog cutaneous pectoris neuromuscular preparations. Ultrastructural analysis of nerve terminals in which quantal acetylcholine release was stimulated to exhaustion by 3 h exposure to trachynilysin revealed swelling of nerve terminals and marked depletion of small clear synaptic vesicles. However, trachynilysin did not induce a parallel depletion of large dense-core vesicles. Large dense core vesicles contained calcitonin gene-related peptide (CGRP), as revealed by colloidal gold immunostaining, and trachynilysin-treated nerve endings exhibited CGRP-like immunofluorescence similar to that of untreated terminals. Our results indicate that the ability of stonefish venom to elicit spontaneous quantal acetylcholine release from vertebrate motor nerve terminals is a function of trachynilysin, which selectively stimulates the release of small clear synaptic vesicles and impairs the recycling of small clear synaptic vesicles but does not affect the release of large dense-core vesicles. Trachynilysin may be a valuable tool for use in other secretory terminals to discriminate between neurotransmitter and neuropeptide release.

A rat skeletal muscle cell line (L6) expresses specific adrenomedullin binding sites but activates adenylate cyclase via calcitonin gene-related peptide receptors

We have previously demonstrated specific binding sites for adrenomedullin, a novel member of the calcitonin family of peptides, in rat muscles. It is unclear whether these receptors are vascular or muscular. Receptors for the structurally similar calcitonin gene-related peptide (CGRP) are present on myocytes and might be involved in the regulation of myocyte glucose metabolism and control by motor neurons. We investigated whether adrenomedullin binding sites were present on L6 myocytes. Specific [125I]adrenomedullin binding sites were demonstrated where adrenomedullin competed with an IC50 of 0.22 +/- 0.04 nM (mean +/- S.E.M.) and a concentration of binding sites (Bmax) of 0.95 +/- 0.19 pmol/mg of protein (mean +/- S.E.M.). CGRP and the specific CGRP receptor antagonist CGRP(8-37) competed weakly at this site (IC50 > 10 and 601 +/- 298 nM respectively). Binding studies with [125I]CGRP revealed a binding site for CGRP (IC50 = 0.13 +/- 0.01 nM; Bmax = 0.83 +/- 0.10 pmol/mg of protein) where both CGRP(8-37) and adrenomedullin competed with [125I]CGRP with IC50 values of 1.15 +/- 0.12 and 8.68 +/- 0.98 nM respectively. Chemical cross-linking showed the CGRP and adrenomedullin binding site-ligand complexes to have approximate molecular masses of 82 and 76 kDa respectively. Both CGRP and adrenomedullin increased adenylate cyclase activity with similar potencies. In both cases adenylate cyclase activation was blocked by CGRP(8-37). Stimulation with 10 nM adrenomedullin or CGRP caused an increase in the percentage of total activated cellular cAMP-dependent protein kinase from 38% in resting cells to 100% and 98% respectively. Therefore in L6 cells adrenomedullin can bind to CGRP receptors, activating adenylate cyclase and cAMP-dependent protein kinase.

Modulation of acetylcholine receptor function in TE671 (rhabdomyosarcoma) cells by non-AChR ligands: possible relevance to seronegative myasthenia gravis

The acetylcholine receptor (AChR) is the main target antigen in myasthenia gravis (MG), but about 15% of patients with typical immunologically mediated MG do not have detectable anti-AChR antibodies. Previous studies showed that plasma from these 'seronegative' patients (SNMG) reduced AChR function in the human AChR-expressing TE671 cell line, and it was proposed that SNMG plasmas may act indirectly via phosphorylation of AChR. We show here that substances such as the beta 2-adrenergic agonist, salbutamol, calcitonin-gene-related-peptide (CGRP), and cholera toxin, that increase intracellular cAMP via binding to specific cell-surface receptors, reduced AChR function in TE671 cells. Moreover, non-specific activation of cell surface proteins by lectins achieved similar results. These observations lead us to hypothesise that SNMG immunoglobulins act in TE671 cells by cross-linking of specific cell surface antigen(s) resulting in generation of intracellular cAMP and/or other second messengers. The role of such antibodies at the neuromuscular junction in vivo could be reduction in AChR function by desensitization and/or damage to the postsynaptic membrane following complement activation.

Antidromic vasodilatation in the striated muscle and its sensitivity to resiniferatoxin in the rat

Antidromic stimulation at the L4-L5 dorsal roots elicited a blood flow increase in ipsilateral muscles of lower extremities in rats measured by laser-Doppler flowmetry. Stimulation with 0.5 Hz; 20 V; 0.5 ms; 50 impulses was much less effective in muscle (18.9 +/- 6.4 area under the curve (%); mean +/- S.E.) than in the glabrous skin (80.5 +/- 8.25; P < 0.001). No significant difference was seen at 10 Hz (51.6 +/- 10.6 muscle; 60.6 +/- 17.3 skin). In the muscle the latency period of the response was long (37.4 +/- 3.1 s; mean +/- S.E.) at 0.5 Hz stimulation and was much shorter (8.8 +/- 0.8 s) at the higher frequency of 10 Hz, unlike in the skin where latency values at both frequencies were similar (9.7 +/- 0.8 s and 8.9 +/- 0.9 s, respectively). Antidromic vasodilatation in the muscle and the skin was abolished by resiniferatoxin (RTX) in an i.v. dose of 1.0 microgram/kg. These results provide a direct evidence for the existence of antidromic vasodilatation in striated muscle and suggest a mediating role for capsaicin/RTX sensitive afferents.

Calcitonin gene-related peptide-like immunoreactivity, in botulinum toxin-paralysed rat muscles

Changes in calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) at the motor endplates of botulinum toxin-paralysed rat muscles were investigated using immunohistochemistry. One day following toxin injection, a dramatic increase in CGRP-LI was detected at the motor endplates and within preterminal axons of the soleus and gastrocnemius muscles. The upregulation of CGRP-LI persisted throughout the period during which muscle fibres were paralysed and new neuromuscular junctions were being formed by the growing sprouts. Decline of CGRP-LI at the motor endplates coincided with clinical recovery. Both up- and down-regulation of CGRP-LI took place earlier in the soleus than in the gastrocnemius muscle. Up-regulation of CGRP-LI was also detected in a subpopulation of motor axons in the sciatic nerves and in the spinal motor neurons innervating the paralysed muscles. These results indicate that levels of CGRP are regulated, at least partly, by changes in the target innervation. They also suggest an important role for CGRP in the regenerative processes following muscle paralysis.