Neurotropic action of MSH/ACTH 4-10 on neuromuscular function in hypophysectomized rats

Myopathies of endocrine origin: A review for physicians

Myopathies are a common manifestation of endocrine disorders. Endocrine myopathies are often overlooked while considering differential diagnoses in patients with musculoskeletal symptoms. The hindrance to mobility and the musculoskeletal discomfort owing to these myopathies are important causes of disability and depreciated quality of life in these patients. Endocrine myopathies occur due to the effects of endogenous or iatrogenic hormonal imbalance on skeletal muscle protein and glucose metabolism, disrupting the excitation-contraction coupling. Abnormalities of the pituitary, thyroid, parathyroid, adrenal, and gonadal hormones have all been associated with myopathies and musculoskeletal symptoms. Endocrine myopathies can either be the complication of a secondary endocrine disorder or a presenting symptom of a missed underlying disorder. Therefore, an underlying endocrine abnormality must always be excluded in all patients with musculoskeletal symptoms. This review presents a compilation of various endocrine myopathies, their etiopathogenesis, clinical presentation, diagnostic modalities, and treatment protocols.

Melanocortins regulate the electric waveforms of gymnotiform electric fish

The hypothalamic-pituitary-adrenal/interrenal axis couples serotonergic activity in the brain to the peripheral regulators of energy balance and response to stress. The regulation of peripheral systems occurs largely through the release of peptide hormones, especially the melanocortins (adrenocorticotropic hormone [ACTH] and alpha melanocyte stimulating hormone [alpha-MSH]), and beta-endorphin. Once in circulation, these peptides regulate a wide range of processes; alpha-MSH in particular regulates behaviors and physiologies with sexual and social functions. We investigated the role of the HPI and melanocortin peptides in regulation of electric social signals in the gymnotiform electric fish, Brachyhypopomus pinnicaudatus. We found that corticotropin releasing factor, thyrotropin-releasing hormone, and alpha-MSH, three peptide hormones of the HPI/HPA, increased electric signal waveform amplitude and duration when injected into free-swimming fish. A fourth peptide, a synthetic cyclic-alpha-MSH analog attenuated the normal circadian and socially-induced EOD enhancements in vivo. When applied to the electrogenic cells (electrocytes) in vitro, only alpha-MSH increased the amplitude and duration of the electrocyte discharge similar to the waveform enhancements seen in vivo. The cyclic-alpha-MSH analog had no effect on its own, but blocked or attenuated alpha-MSH-induced enhancements in the single-cell discharge parameters, demonstrating that this compound functions as a silent antagonist at the electrocyte. Overall, these results strongly suggest that the HPI regulates the EOD communication signal, and demonstrate that circulating melanocortin peptides enhance the electrocyte discharge waveform.

David and Goliath - the slingshot that started the neuropeptide revolution

This review in honor of David de Wied summarizes the work done in my laboratory that first indicated that adrenocorticotropic hormone (ACTH) has a direct effect on the neuromuscular system. Cold stress or ACTH and its related peptides alpha-melanocyte-stimulating hormone (alpha-MSH ) and beta-lipotropin improve the electromechanical characteristics of adrenalectomized and hypophysectomized rats. ACTH-(1-39) accelerates the return of motor and sensory function and improves the morphological characteristics of the motor endplate after peripheral nerve crush. The non-corticotropic fragments ACTH-(4-10), alpha-MSH, the ACTH-(4-9) analogue Organon 2766 (Org 2766) or the ACTH-(4-10) analogue Biomeasure 22015 (BIM 22015) improve electrophysiological and morphological parameters of the regenerating neuromuscular system. ACTH-(4-10) immunoreactivity, present in ventral horn motor neurons in low levels, is decreased ipsilaterally following ipsilateral nerve crush but increases both ipsilaterally and contralaterally if injured animals are treated with ACTH-(4-10) indicating a neuroprotective action. Similarly, Org 2766 appears to have a protective action in the brain following nigrostriatal lesions. In developmental studies, perinatal exposure to ACTH peptides improves the structure of the neuromuscular junction, accelerates the maturation of electromechanical properties and enhances nerve-muscle integration and nerve regeneration. Perinatal exposure to these peptides decreases adult male sexual behavior, a change correlated with increased serotinergic input within the medial preoptic area. Similar changes occur in female rats and appear to be long-lasting. In tissue culture studies, both Org 2766 and BIM 22015 promote neurite outgrowth in the absence of nerve growth factor, indicating a neurotrophic role for these peptides.

Immunohistochemical evidence of alpha-, beta- and gamma 3-melanocyte stimulating hormone expression in cutaneous malignant melanoma of nodular type

Opiomelanocortins are formed after cleavage of the larger precursor molecule, proopiomelanocortin (POMC), which contains several peptide residues, sharing certain amino acid homology, including adrenocorticotrophic hormone (ACTH) and alpha-, beta- and gamma-melanocyte stimulating hormone (MSH). The expressions of alpha-, beta- and gamma 3-MSH in human cutaneous malignant melanoma of nodular type are demonstrated. For the MSHs, the immunolabelling was concentrated mostly in the tumour cellular cytoplasm, with occasional cells displaying a nuclear staining. Labelled tumour cells were dispersed throughout the epidermis and dermis as individual cells or in so-called 'pearl-like nests', most of which consisted mainly of round or oval shaped cells as well as a few pleomorphic or spindle-shaped cells. The fluorescence intensity seemed to increase in accordance with the development of the tumours. All cases examined were clearly stained with protein S-100, which provided us with a definite diagnosis. Considering the overall MSHs-related staining intensity of each section, the general perception we got was that the closer to the centre of the tumour parenchyma, the stronger was the staining and, furthermore the larger/more poorly differentiated the cells, the stronger was the staining. We also found the MSHs expressions to appear in the peripheral part of the tumour and the perilesional tissues including epidermis, sweat glands, sebaceous glands as well as hair follicles. Neurohypertrophic features were encountered including increases in both the number of nerve fibres and their diameter. Our results presented here strongly support the viewpoint earlier proposed that MSH peptides, by an autocrine and/or paracrine production from melanoma cells, are engaged in the regulation of melanogenesis, growth and proliferation of the tumour cells. We also conclude that, although alpha-, beta- and gamma 3-MSH peptides do not provide as high a sensitivity for diagnosis as protein S-100, they appear as useful markers for supportive diagnosis and assessment of malignant melanoma.

Melanocortins as factors in somatic neuromuscular growth and regrowth

Melanocortins, non-corticotropic fragments of adrenocorticotropic hormone, accelerate growth of the developing neuromuscular system and regrowth of damaged neurons, both in the adult and neonatal rat. Morphological, electrophysiological and behavioral characteristics are all improved by melanocortins, which, however, vary in potency, with alpha-MSH being the most effective. Tissue substrate, dosage, critical time periods and pattern of neuropeptide administration are all important variables. Melanocortins protect central neurons affecting motor behavior during development or following neuronal damage in the adult brain. Possible mechanisms of melanocortin action are discussed.

Melanotropins as growth factors

Peptides that regulate the growth of tissues, whether in a positive or negative manner, are termed growth factors. The melanocortins, neurotrophic sequences that correspond to peptide fragments contained within ACTH-(1-13), beneficially affect neural growth during development and regeneration. Analogues of ACTH-(4-9) (Org 2766) and ACTH-(4-10) (BIM 22015) are capable of sustaining neurite outgrowth from cultured dorsal root ganglion and spinal cord cells in the absence of nerve growth factor. The development of sexually dimorphic behavior in both male and female rats is influenced by perinatal administration of ACTH. This change appears to be correlated with changes in the growth and metabolism of developing serotonergic and dopaminergic systems in the hypothalamic nuclei associated with male and female sexual behavior. Similar melanotropic influences are found in the developing neuromuscular system. Neuromuscular development is accelerated by perinatal administration of melanocortins, provoking both nerve and muscle to attain early maturation. However, the responding tissue varies pivotally with age: early in gestation, embryonic muscle is acutely sensitive to peptide exposure; but once innervation has occurred, only the developing nerve reacts to melanocortin treatment. Melanocortins have little if any effect on the normal, adult neuromuscular system. Following peripheral nerve injury or pathology, melanotropins once again become effective growth factors, accelerating and enhancing nerve regeneration and muscle reinnervation. Electrophysiological, morphological, biochemical, and functional tests all indicate that ACTH-(4-10), Org 2766, BIM 22015, and alpha-MSH improve various facets of nerve regeneration, the degree to which the specific parameter is improved being dependent on the peptide fragment, its dosage, and pattern of administration. BIM 22015, while less effective as a neurotrophic factor, has potent myotrophic effects that the other peptides lack. Org 2766 may provide some protective action to the injured CNS as demonstrated by tests of cognitive function following brain lesions, although evaluation of recovery is sometimes enigmatic. Recovery from destruction of the nigrostriatal system is more easily measured through tests of motor function and open field behavior, both of which support a protective role for Org 2766. Compensatory mechanisms, including the presence of increased tyrosine hydroxylase and greater density of dopaminergic fibers, may be involved. Melanocortins are effective growth factors in sciatic nerve regeneration in neonatal rats. Both alpha-MSH and ACTH-(4-10) favor the formation of morphologically normal end plates despite the trauma following nerve crush at postnatal day 2.(ABSTRACT TRUNCATED AT 400 WORDS)

Pro-opiomelanocortin neuropeptide receptors on developing and dystrophic muscle fibers

Autoradiography was used to investigate the presence of corticotropin and beta-endorphin receptors on soleus, extensor digitorum longus, and diaphragm muscles of normal and dystrophic adult mice and developing mice. In both adult and developing mice, specific binding sites for both [125I]ACTH and [125I]beta-endorphin were present in some fibers in all of the muscles examined. The specific binding sites appeared to be distributed over the length of the surface membrane in the fibers that expressed them. There were significantly higher proportions of fibers exhibiting the specific beta-endorphin and the specific ACTH binding sites in the three muscle types in mice of 5 d of age compared to the muscles of the normal adult. There was also a higher proportion of fibers with the putative ACTH and beta-endorphin receptors in the three muscle types in dystrophic mice.

ACTH/MSH(4-10) analog BIM 22015 aids regeneration via neurotrophic and myotrophic attributes

Structural differences between noncorticotropic ACTH peptides result in marked differences in their effects on regenerating nerve and muscle in rats. The ACTH/MSH(4-10) analog BIM 22015 was administered IP in dosages from 0.1 to 40 micrograms/kg/48 h for 5, 7, or 11 days after peroneal nerve crush, and characteristics of extensor digitorum longus (EDL) muscle were studied and compared with ACTH/MSH(4-10). Eleven days postcrush 40 micrograms/kg BIM 22015 increases rate of development of tetanic tension and amplitude of contraction of indirectly stimulated EDL. In a 21-day study, reinnervated BIM 22015-treated muscles retain tetanic strength, whereas ACTH/MSH(4-10)-treated muscles are significantly weakened. Both peptides show neurotrophic characteristics in their stimulation of endplate nerve terminal branching. However, in contrast to ACTH/MSH(4-10), BIM 22015 also prevents denervation atrophy of the EDL. This dual neurotrophic and myotrophic role for BIM 22015 accords it a clinical potential for degenerative myopathies of either pure or mixed origin, such as muscular dystrophy, infantile spinal atrophy, and hypotonia.

Putative neurotrophic factors and functional recovery from peripheral nerve damage in the rat

1. In rats, recovery of sensory-motor function following a crush lesion of the sciatic or tibial nerve was monitored by measuring foot reflex withdrawal from a local noxious stimulation of the foot sole. 2. Putative neurotrophic compounds were tested on this functional recovery model: melanocortins (peptides derived from ACTH (corticotropin) and alpha-MSH (melanotropin], gangliosides and nimodipine were effective whereas isaxonine and TRH (thyrotropin releasing hormone) were not. 3. Structure-activity studies with melanocortins revealed a similar effectiveness of alpha-MSH, [N-Leu4, D-Phe7]-alpha-MSH, desacetyl-alpha-MSH and the ACTH analogue ORG 2766, questioning the validity of the previously suggested notion that the melanotrophic properties of these peptides are responsible for their neurotrophic effect. 4. As recovery of function after peripheral nerve damage follows a similar time course in hypophysectomized (five days post operation) and sham-operated rats, effective melanocortin therapy does not mimic an endogenous peptide signal in the repair process from pituitary origin. 5. Subcutaneous treatment with ORG 2766 (7.5 micrograms kg-1 48 h-1) facilitates recovery of function following peripheral nerve damage in young (6-7 weeks old), mature (5 month old) and old (20 month old) rats. 6. In view of the diversity in structure of the effective neurotrophic factors and the complexity of nerve repair, the present data support the notion that peripheral nerve repair may be facilitated by different humoral factors likely to be active on different aspects of the recovery process.

Melanocortins and fast axonal transport in intact and regenerating sciatic nerve

The effect of ACTH/MSH peptides on fast axonal transport along intact or regenerating sciatic nerve was examined following injection of tritiated leucine into the rat lumbar spinal cord. The rate of fast axonal transport was not significantly changed by treatment with ACTH/MSH(4-10), the ACTH(4-9) analog ORG 2766, hypophysectomy, or adrenalectomy. Fast axonal transport was unchanged in regenerating nerves and in regenerating, ACTH(4-10)-treated nerves. However, treatment with ORG 2766 in dosages of either 1 or 10 micrograms/kg/day IP for seven days significantly reduced (62% and 64%, respectively) the crest height of the fast axonal transport curve of intact sciatic nerve. The results suggest that the reported peptide-induced enhancement of nerve regeneration is not due to changes in the rate of fast axonal transport.

Presence of alpha-melanocyte-stimulating hormone-like immunoreactivity in the innervation of amphibian skeletal muscle

Amphibian motor nerve terminals are sensitive to a wide variety of peptides, including alpha-melanocyte-stimulating hormone (alpha-MSH). We determined the presence and distribution of alpha-MSH-like immunoreactivity (alpha-MSHLI) in the innervation of the cutaneous pectoris muscle from bullfrog (Rana catesbeiana) tadpoles and postmetamorphic froglets, and adult frogs (R. catesbeiana and R. pipiens). alpha-MSHLI was found in unmyelinated, noncholinergic axons, in motor axons, and in motor nerve terminals. In motor axons, alpha-MSHLI was predominantly associated with neurofilaments. The distribution of this form of alpha-MSHLI changed during development and seasonally in adult frogs. The possible functional roles of this alpha-MSHLI are discussed.

ACTH modulation of nerve development and regeneration

(1) The availability of short amino acid sequences of the naturally occurring ACTH 1-39 molecule has made it possible to separate the corticotropic characteristics of the parent molecule from its neurotrophic effects. Potent neurotrophic fragments are ACTH 4-10, an analog of ACTH 4-9 (Org 2766), and alpha-MSH (ACTH 1-13), peptide fragments that do not evoke corticosteroid secretion, yet clearly affect both the development and regeneration of peripheral nerve. (2) Early postnatal administration of either ACTH 4-10 or Org 2766 accelerates the neuromuscular development of the immature rat, increasing the contractile strength of the EDL muscle and inducing more rapid muscle contractions. Grasping strength and motor activity are increased; these are all changes indicative of more rapid neuromuscular maturation. Prenatal peptide treatment elicits a more complex pattern of response since administration early in gestation (GD 3-12) accelerates neuromuscular development whereas later administration (GD 13-21) decelerates maturation. (3) ACTH peptides have a similar accelerating effect on the morphology of the developing neuromuscular junction. At two weeks of age, nerve arborization is conspicuously increased by postnatal administration of either ACTH 4-10 or Org 2766, as is nerve terminal branching within the endplate itself. However, this is preceded by an initial depression of nerve branching in the 7-day-old rat pup. We conclude that while the developing neuromuscular system is sensitive to ACTH peptides, this susceptibility is age-related. The crucial role of these peptides may be limited to very brief, defined periods during which the peptides may interact with trophic or growth-associated substances, each of which may have its own decisive, circumscribed time frame of influence. (4) Perinatal administration of ACTH peptides affects CNS development. One measurable indication of this is an acceleration of eye opening. Early exposure to ACTH peptides has long-lasting effects on behavior, apparent when these animals are tested as adults. Increased spontaneous motor activity, heightened states of arousal and agitation, and changes in social behavior have been reported. Certain avoidance responses and tests of visual discrimination in male rats are improved by neonatal treatment with alpha-MSH. Overall motor activity is increased and the normal period of hyperactivity is initiated earlier. Male sexual behavior is decreased and sexually dimorphic behaviors in males are eliminated. alpha-MSH may alter the development of its own dopaminergic feedback circuitry while ACTH affects serotonin levels in the preoptic nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

ACTH peptides stimulate motor nerve sprouting in development

Maturational changes at the neuromuscular junction (nmj) of rat neonates were studied using scanning electron microscopy and light microscopy that permitted quantification of muscle fiber diameter, length of nerve terminal branching, end-plate area, and perimeter. Administration of ACTH 4-10 (10 micrograms/kg s.c. daily from day of birth) stimulated nerve terminal branching, an effect most evident in 14-day-old pups. The trisubstituted derivative of ACTH 4-9 (Org 2766) when administered at 0.01 microgram/kg/daily, had a more potent effect, increasing end-plate perimeter and nerve terminal branching on the first postnatal week and markedly increasing only nerve terminal branching at 14 days of age. This is a dose-responsive action since 10 micrograms/kg/daily severely inhibits nerve sprouting. By 21 days, there were no differences between peptide- and saline-treated neonates. Peptide-induced sprouting was elicited only in the first 2 weeks of postnatal life. This time course corresponds with the critical period for nmj maturation and ceases when polyneuronal innervation of muscle fibers also terminates. It is suggested that ACTH peptides may exert a physiological role on nerve sprouting during development.

Neonatal neuromuscular parameters vary in susceptibility to postnatal ACTH/MSH 4-10 administration

Neonatal Sprague-Dawley rats administered the fragment of adrenocorticotropic hormone ACTH/MSH 4-10 (10 micrograms/kg/daily, SC) postnatally, show marked differences in the plasticity of the functional and morphological parameters of their neuromuscular system. Initial contraction durations of the immature fast muscle, extensor digitorum longus (EDL), are shorter than saline-treated controls indicating accelerated development. Qualitative studies of the developing EDL neuromuscular junctions as viewed by the scanning electron microscope and quantitative analysis permitted by light microscopy confirms that ACTH/MSH 4-10 affects the maturation of the endplate region. Motor behavior of rat pups demonstrates an age-related difference in the susceptibility to this peptide fragment; one week old neonates showing no response to ACTH/MSH 4-10, two week old pups showing an increase in motor activity. The results indicate that while the developing neuromuscular system is sensitive to the input of ACTH/MSH peptide treatment, this susceptibility is age-related.

Mammalian neuromuscular development accelerated with early but slowed with late gestational administration of ACTH peptide

The neuropeptide ACTH 4-10, a nonsteroidogenic fragment of adrenocorticotropic hormone, has two distinct and opposite effects on developing nerve and muscle. Muscle is positively influenced by ACTH during the first part of gestation (G days 3-12) before innervation occurs. Subsequent effects on innervation are largely depressive and exerted only during G13-21. Treatment during G3-12 increases twitch amplitude, rise time and speed of contraction of directly and indirectly stimulated extensor digitorum longus (EDL) muscle of two wk old rats. Treatment during G13-21 slows contractions of indirectly stimulated EDL, whereas treatment throughout gestation (G3-G21) shows little effect. Thus, ACTH first accelerates muscle development then modulates this development through neuronal depression.

Pharmacological aspects of the influence of melanocortins on the formation of regenerative peripheral nerve sprouts

Adrenocorticotropin (ACTH) and alpha-melanocyte-stimulating hormone (alpha-MSH) stimulate the initial sprouting response in the crushed rat sciatic nerve. In this report a detailed analysis of the neurotrophic action of Org.2766 [a degradation resistant ACTH(4-9) analog] and alpha-MSH is described. Org.2766 treatment results in enhanced numbers of outgrowing sprouts in the damaged nerve. The growth velocity of the sprouts is not affected. The peptide effect is dose-dependent. A single peptide injection administered immediately following the crush stimulates the formation of sprouts significantly. Continued high blood levels of Org.2766 are probably not critical for the neurotrophic effect of these peptides, since a more moderate dosing protocol (injections given every 48 hr) was more effective than more frequent injections (injections given every 12 hr). The present results further the understanding of the mode of action of ACTH/alpha-MSH-like peptides and underscore the necessity to test a wide range of doses and injection protocols to avoid false negative results in clinical work being planned to start in the near future.

Org.2766 improves functional and electrophysiological aspects of regenerating sciatic nerve in the rat

The beneficial effect of short-term (8 days) melanocortin therapy on regenerating peripheral nerves is demonstrated using functional and electrophysiological tests. Following a crush lesion of the rat sciatic nerve, recovery of sensory function is monitored by assessing the responsiveness of the rat to a small electric current applied to the footsole. Recovery of motor function is assessed by means of an analysis of walking patterns. Normalization of the walking pattern reflects reinnervation of different muscle groups. The motor and H-reflex related sensory nerve conduction velocity of the regenerated nerves are longitudinally investigated in the same rats in which the recovery of motor and sensory function had been assessed previously. Functional tests show an enhanced recovery under melanocortin therapy, but in the end both saline- and melanocortin-treated rats show 100% recovery. However, when compared to the contralateral sciatic nerve, in the peptide-treated animals motor nerve conduction in the regenerated nerves has fully recovered after about 90 days following the crush lesion and the sensory conduction after about 120 days, whereas in the saline-treated rats a deficit of 20-40% in both motor and sensory conduction remains. This difference is observed even 214 days following crush.

Development and regeneration of motor systems under the influence of ACTH peptides

ACTH peptides exert quantitative and qualitative influences on the formation and maturation of motor units in developing and regenerating neuromuscular systems. ACTH 4-10, administered daily (10 micrograms/kg. s.c.) from the day of birth, accelerated the rate at which muscle strength developed in the immature rat, the effect of this peptide being most marked in animals 11-15 days old. A similar increase in grasping time occurred in ACTH 4-10 treated animals, indicating that the peptide affects neuronal maturation at a time in development when organization and maturation of the neuromuscular system is most active. The synthetic analogue of ACTH 4-9 (Org 2766), administered in the same dosage, had little effect on these parameters, indicating a differential sensitivity to these similar peptides. Elevated circulating titers of ACTH, whether exogenous (0.2 U ACTH 1-39 IP daily), or endogenous (adrenalectomy), stimulated the formation of more functional motor units, as indicated by increased amplitude of muscle action potentials and tetanic tension following nerve stimulation. ACTH appears to favor the recovery of high threshold, small-size motor units. Fine control of muscle function in peptide-treated animals is partially restored, as indicated by the return of stepwise recruitment to an extent not seen in the reinnervated, saline-treated controls.

ACTH/MSH 4-10 improves motor unit reorganization during peripheral nerve regeneration in the rat

ACTH/MSH 4-10 administration (10 micrograms/48 hr IP for 7 days) enhances neuromuscular activity following crush denervation of rat extensor digitorum longus muscle. The number of regenerated functional motor units is greater in peptide treated rats than in saline treated denervated controls. Selective activation of responding motor units indicates that ACTH/MSH 4-10 preferentially accelerates the reformation and stabilization of small size motor units. These observed effects may be beneficial since they contribute to the early reestablishment of more organized motor units, thereby restoring fine control of motor functions, in contrast to the disorderly reorganization of untreated regenerating systems. Possible mechanisms of peptide action (neurotransmitter synthesis and release, excitability changes, etc.) are discussed.

Central nervous system and peripheral effects of ACTH, MSH, and related neuropeptides

Adrenocorticotropic Hormone (ACTH), Melanocyte-Stimulating Hormone (MSH), and related peptides have been shown to have several neurogenic effects: alteration of cerebral protein synthesis, RNA synthesis, protein phosphorylation, and neurotransmitter turnover. Furthermore, there appears to be an ACTH containing circuit in the CNS which originates in the arcuate nucleus. Changes in concentration of the peptides in this family have been shown to alter electrophysiology, neuromuscular function, and behavior (e.g., grooming, learning) in infrahuman subjects. These findings suggest that the neuropeptides MSH and ACTH influence the capacity of an organism to efficiently evaluate information and influence the affective functioning of humans.

Peptide influences on the development and regeneration of motor performance

ACTH 1-39 (0.2 U IP daily for up to 18 days) has a beneficial effect on the functional reorganization of regenerating motor units of the extensor digitorum longus (EDL) in the adrenalectomized adult rat following crushing of the peroneal nerve. Motor unit activity (maximum twitch tension amplitude/mean increment in twitch tension as voltage is increased by 0.1 V gradations) and nerve-muscle efficiency (tetanic tension from indirect stimulation/tetanic tension from direct stimulation of EDL) were enhanced by ACTH 1-39. Other electrophysiological and contractile parameters were unaffected by the peptide. Spontaneous motor activity in cold stressed 13 day old rats was prolonged by Org 2766, a substituted analogue of ACTH/MSH 4-9, (0.1 microgram/kg daily) but unaffected by the same dosage of ACTH/MSH 4-10. The responsiveness of developing and regenerating motor systems to neuropeptides indicates a plasticity of neuronal connections, which depends on peptide sequence, dosage and the physiological state of the animal (normal, depressed, regenerating or developing, at rest or stressed).

Neuromuscular response of the immature rat of ACTH/MSH 4--10

The adrenocorticotropin fragment ACTH/MSH 4--10 (0.1 ug/kg IP) effectively modulates the neuromuscular responses of 9 to 15 day old rats. Muscle (extensor digitorum longus) contraction amplitude is increased, fatigue is delayed and muscle half-relaxation time is shortened during 20 min of continuous in situ stimulation of a branch of the deep peroneal nerve (square wave shocks 10 Hz, duration 0.5 msec, strength supermaximal). No effect on contraction time is seen. There is no facilitation or change in any contraction parameter in rats older than two weeks (16 to 40 days) indicating that these older animals, like normal adult rats, are unaffected by the peptide. Immature rats, however, are even more sensitive than hypophysectomized adult rats [29] to the ameliorative action of ACTH/MSH 4-10. This early sensitivity to ACTH/MSH 4--10 corresponds to important developmental changes occurring in nerve and muscle during the most critical period in postnatal development, the first two weeks.