Motoneuron Somata Research Articles

Androgenic Regulation of Expression of β‐Tubulin Messenger Ribonucleic Acid in Motoneurons of the Spinal Nucleus of the Bulbocavernosus

Expression of beta-tubulin mRNA was examined in androgen-sensitive motoneurons of the spinal nucleus of the bulbocavernosus (SNB) in adult male rats by in situ hybridization histochemistry using cDNA encoding mouse beta-tubulin. Hybridizable beta-tubulin mRNA was localized in the somata and proximal dendrites of SNB motoneurons. Removal of androgen by castration significantly reduced the expression level of beta-tubulin mRNA in the SNB motoneurons, whereas the change was prevented by testosterone treatment. On the contrary, castration or testosterone treatment did not induce any changes in the expression level of beta-tubulin mRNA in the androgen-insensitive motoneurons of the retrodorsolateral nucleus. These results suggest that androgen regulates the expression of beta-tubulin gene in the SNB motoneurons and may provide evidence for the molecular mechanisms of hormonally-induced neuronal plasticity in the SNB motoneurons.

Ornithine decarboxylase-like immunoreactivity in rat spinal motoneurons and motoric nerves

Ornithine decarboxylase (ODC) is the rate-limiting enzyme in the formation of the polyamines putrescine, spermidine and spermine. In the present study ornithine decarboxylase-like immunoreactivity (ODC-LI) was localized immunocytochemically in rat spinal motoneurons, motoric nerves and myoneural junctions in several muscles. In the spinal cord ODC-LI was expressed in most of the large multipolar neurons located in the ventral horn at cervical and lumbar levels. ODC-LI was localized in the cytoplasm, dendrites and axons of the labelled neurons. The nuclei of motoneurons were unlabelled; however, the nuclear membranes and the surrounding cytoplasm were strongly stained. ODC-immunoreactive (IR) axons could be traced through the white matter entering the ventral roots. The myelinated axons in the ventral roots and in the nerve bundles among the muscles were intensely stained with ODC antiserum. The myoneural junctions apposing individual muscle fibers showed ODC-LI with slightly less intensity. Some ODC-IR nerve fibers were seen in the muscle spindles. The present results show that motoneurons in adult rat spinal cord express ODC-LI and that ODC-LI is transported to motoric nerves and myoneural junctions. This suggests that polyamines can be synthesized both in the motoneuron somata and in their peripheral projections. Polyamines may thus regulate cellular functions in all parts of motoneurons. In addition, polyamines may be secreted from their distal projections and have trophic effects on Schwann cells and/or muscular tissue.

Changes in serotonin-induced potentials during spinal cord development.

1. Motoneuron responses to serotonin (5-hydroxytryptamine, 5-HT), and the growth pattern of 5-HT projections into the ventral horn were studied in the isolated spinal cord of embryonic and neonatal rats. 2. 5-HT projections first appeared in lumbar spinal cord at days 16-17 of gestation (E16-E17) and were localized in the lateral and ventral funiculi. By E18, the projections had grown into the ventral horn, and at 1-2 days after birth they were in close apposition to motoneuron somata. 3. At E16-E17, slow-rising depolarizing potentials of 1-4 mV were recorded intracellularly in lumbar motoneurons in response to bath application of 5-HT. These potentials were not apparent after E18; at that time 5-HT generated long-lasting depolarizations with an average amplitude of 6 mV, and an increase of 11% in membrane resistance. Starting at E18, 5-HT also induced high-frequency fast-rising potentials that were blocked by antagonists of glutamate, gamma-aminobutyric acid, and glycine. 4. Motoneuron responses to 5-HT increased significantly after birth, when 5-HT produced an average depolarization of 19 mV and repetitive firing of action potentials. 5. Tetrodotoxin and high Mg2+ did not reduce the amplitude of the long-lasting depolarizations, which suggested that they were produced by direct action of 5-HT on motoneuron membrane. 6. At all developmental ages, 5-HT reduced the amplitude of dorsal root-evoked potentials. The suppressed responses were neither due to 5-HT-induced depolarization nor the result of a decrease in motoneuron excitability. 7. The pharmacological profile of 5-HT-induced potentials was studied with the use of various agonists and antagonists of 5-HT. The findings indicated that the actions of 5-HT on spinal neurons were mediated via multiple 5-HT receptor subtypes. 8. Our results suggested that 5-HT excited spinal neurons before 5-HT projections grew into the ventral horn. The characteristics of 5-HT-induced potentials changed, however, at the time when the density of 5-HT projections increased in the motor nuclei.

Immunocytochemistry of GABA and glutamic acid decarboxylase in the thoracic ganglion of the crab Eriphia spinifrons

We have used specific antisera against protein-conjugated γ-aminobutyric acid (GABA) and rat-brain glutamic acid decarboxylase (GAD) in immunocytochemical preparations to study the distribution of putatively GABAergic neurons in the fused thoracic ganglion of the crab Eriphia spinifrons. In the thoracic neuromeres, about 2000 neurons with somata arranged in clusters or located singly in the cell cortex exhibited both GABA-like and GAD-like immunoreactivity. In addition, more than a hundred cells showed only GABA-like immunoreactivity. Fibrous immunoreactive staining to GAD and GABA was distributed throughout the neuropil of the thoracic ganglion, and several fiber tracts contained immunoreactive processes. Sets of serially homologous neurons exhibited GABA-like and GAD-like immunoreactivity in the thoracic neuromeres. Especially prominent were one medial and four ventro-lateral clusters of somata, together with thirteen individually recognized cells in each neuromere. Six of these cells in the ventro-medial cell cortex may be the somata of inhibitory motoneurons. The leg nerves contained three immunoreactive fibers, corresponding to the previously described common inhibitory motoneuron and the two specific inhibitors. The results present further evidence for GABA being the neurotransmitter of all inhibitory leg motorneurons, and suggest its presence and role as a neurotransmitter in a considerable number of interneurons in the thoracic ganglion of the crab.

Role of GABA in the extraocular motor nuclei of the cat: A postembedding immunocytochemical study

The GABAergic innervation of the extraocular motor nuclei in the cat was evaluated using postembedding immunocytochemical techniques. The characterization of GABA-immunoreactive terminals in the oculomotor nucleus was carried out at the light and electron microscopic levels. GABA immunopositive puncta suggestive of boutons were abundant in semithin sections throughout the oculomotor nucleus, and were found in close apposition to somata and dendrites. Ultrathin sections revealed an extensive and dense distribution of GABA-immunoreactive synaptic endings that established contacts with the perikarya and proximal dendrites of motoneurons and were also abundant in the surrounding neuropil. GABAergic boutons were characterized by the presence of numerous mitochondria, pleiomorphic vesicles and multiple small symmetrical synaptic contacts. The trochlear nucleus exhibited the highest density of GABAergic terminations. In contrast, scarce GABA immunostaining was associated with the motoneurons and internuclear neurons of the abducens nucleus.In order to further elucidate the role of this neurotransmitter in the oculomotor system, retrograde tracing of horseradish peroxidase was used in combination with the GABA immunostaining. First, medial rectus motoneurons were identified following horseradish peroxidase injection into the corresponding muscle. This was carried out because of the peculiar afferent organization of medial reclus motoneurons that contrasts with the remaining extraocular motoneurons, especially their lack of direct vestibular inhibition. Semithin sections of the oculomotor nucleus containing retrogradely labeled medial rectus motoneurons and immunostained for GABA revealed numerous immunoreaclive puncta in close apposition to horseradish peroxidase-labeled somata and in the surrounding neuropil. At the ultrastructural level, GABAergic terminals established synaptic contacts with the somata and proximal dendrites of medial rectus motoneurons. Their features and density were similar to those found in the remaining motoneuronal subgroups of the oculomotor nucleus.Second, oculomotor inlernuclear neurons were identified following the injection of horseradish peroxidase into the abducens nucleus to determine whether they could give rise to GABAergic terminations in the abducens nucleus. About 20% of the oculomotor internuclear neurons were doubly labeled by retrograde horseradish peroxidase and GABA immunostaining. A high percentage (80%) of the oculomotor internuclear neurons projecting to the abducens nucleus showed immunonegative perikarya.It was concluded that the oculomotor internuclear pathway to the abducens nucleus comprises both GABAergic and non-GABAergic neurons and, at least in part, the GABA input to the abducens nucleus originates from this source. It is suggested that this pathway might carry excitatory and inhibitory influences on abducens neurons arising bilaterally.

Calcium and Potassium Currents in leg Motoneurons During Postembryonic Development in the Hawkmoth Manduca Sexta

During insect metamorphosis the nervous system is reorganized to accommodate changes in behavior. In the hawkmoth, Manduca sexta, many identified larval motoneurons persist to innervate new adult muscles, while undergoing changes in dendritic morphology and synaptic connections. The thoracic leg motoneurons, for example, innervate different sets of muscles in the larva and adult and participate in distinct types of locomotor behavior in the two stages of life. To determine whether changes in the biophysical properties of these motoneurons accompany the structural and functional modifications that have been described, we used the whole-cell voltage-clamp technique to compare the Ca2+ and K+ currents expressed by leg motoneurons isolated from the larval, pupal and adult stages of Manduca. After 24 h in culture, the somata of leg motoneurons isolated from all three stages expressed voltage-sensitive Ca2+ currents that could be blocked by Cd2+, Co2+ or Ni2+. The currents were larger with Ba2+ as the charge carrier. The Ca2+ current density was significantly lower in these motoneurons during the early pupal stage than in either the larva or adult. Similar experiments revealed both transient and sustained K+ currents in the leg motoneurons that could be blocked with Cs+. There was a significant decrease in the density of the transient, inactivating outward current in leg motoneurons isolated from the early pupal stage. Thus, the levels of some types of ionic currents are modulated during metamorphosis. These changes may be important for the developmental or behavioral changes that accompany metamorphosis.

Spinal commissural neurons mediating vestibular input to neck motoneurons in the cat upper cervical spinal cord

Spinal commissural neurons (CNs) activated di- or trisynaptically by stimulation of ipsilateral vestibular afferents were stained with intraaxonal injection of horseradish peroxidase in the cat upper cervical spinal cord. Stem axons of CNs in lamina VIII or VII, after crossing the midline, had ascending and/or descending main branches that gave off multiple axon collaterals to laminae IX and VIII over a few cervical segments. Terminal boutons appeared to make contact with proximal dendrites and somata of retrogradely-labelled neck motoneurons. Therefore, these CNs were regarded as mediating vestibular afferent input to contralateral neck motoneurons trisynaptically at the shortest.

Interactions between dorsal root axons and their target motor neurons in developing mammalian spinal cord

We have utilized the lipid-soluble tracers Dil and DiA to investigate interactions between group la dorsal root afferent axons and their target motor neurons in developing rat spinal cord. We show here that la axons project toward motor pools in fascicles that exhibit a considerable degree of spatial order. A rough topography is present in that axons that innervate medially located axial motor neurons cross over others in the intermediate zone and follow a separate path along the midline toward their appropriate targets. Surprisingly, we have also found that motor neuron dendritic projections are well established in the transverse plane prior to the arrival of la afferents. Although dendrites from motor pools innervating limb muscles project directly in the path of incoming la afferents, they do not guide afferents to appropriate motor pools. The la afferents pass over the distal dendrites and grow all the way to the border between gray and developing white matter. A significant amount of terminal branching and bouton formation is in the vicinity of motor neuron somata and proximal regions of the dendritic arbors. Few boutons are found near dendrites that project dorsal to the motor pools, and virtually no boutons are found on dendrites in white matter. Our results show that la afferent axons are not guided to appropriate motor pools by random encounters with motor dendrites, and raise the possibility that mechanisms exist that promote an orderly projection of la afferents to particular regions of the ventral horn. The striking lack of innervation of white matter and dorsally directed dendrites by la afferents raises the question of whether descending and intersegmental systems have their initial interactions with these regions of the motor neuron dendritic arbor.

Effect of androgen on the expression of gap junction and β-actin mRNAs in adult rat motoneurons

Expression of gap junction and β-actin mRNAs was examined in androgen-sensitive motoneurons of the spinal nucleus of the bulbocavernosus (SNB) in adult male rats by in situ hybridization histochemistry using complementary DNAs encoding rat liver gap junction protein (connexin 32) and chick β-actin. Hybridizable gap junction and β-actin mRNAs were localised on the somata and proximal dendrites of SNB motoneurons. Removal of androgen by castration significantly reduced the expression levels of both gap junction and β-actin mRNAs in the SNB motoneurons, whereas these changes were prevented by testosterone treatment. On the contrary, castration or testosterone treatment did not induce any changes in the expression levels of gap junction and β-actin mRNAs in the motoneurons of the retrodorsolateral nucleus (RDLN), which accumulate androgen less frequently and sparsely than those in the SNB. These results suggest that androgen regulates the expression of both gap junction and β-actin genes in the SNB motoneurons and may provide evidence for the molecular mechanisms of hormonally induced neuronal plasticity in the SNB motoneurons.

Differential effects of testosterone metabolites upon the size of sexually dimorphic motoneurons in adulthood

This study examined the effect of testosterone and two of its metabolites on the size of motoneurons in the sexually dimorphic spinal nucleus of the bulbocavernosus (SNB) in adult male rats. Treatment of castrates with either testosterone or dihydrotestosterone maintained SNB cell size, although testosterone was more effective in this regard. However, estradiol, either alone or in conjunction with dihydrotestosterone treatment, had no effect on the size of the somata or nuclei of SNB motoneurons. These results indicate that testosterone affects SNB cell size by interacting with androgen receptors and that aromatized metabolites of testosterone are not involved in this aspect of motoneuronal plasticity in adulthood. Because the penile reflexes mediated by the SNB neuromuscular system are also sensitive to androgen but not estrogen treatment, morphological changes in SNB cells may contribute to the androgenic modulation of these reflexes.

Androgen regulates gap junction mRNA expression in androgen-sensitive motoneurons in the rat spinal cord

Expression of gap junction mRNA was examined in the androgen-sensitive motoneurons of the spinal nucleus of the bulbocavernosus (SNB) by in situ hybridization histochemistry using cDNA encoding rat liver gap junction protein (connexin 32). Hybridizable gap junction mRNA was localized on the somata and proximal dendrites of SNB motoneurons. The removal of androgen by castration dramatically reduced the expression level of gap junction mRNA in the SNB motoneurons, whereas this change was prevented by testosterone treatment. These results are the first demonstration of hormonal regulation of gap junction mRNA expression in the central nervous system.

Corticomotoneuronal connections in the rat: Evidence from double‐labeling of motoneurons and corticospinal axon arborizations

In order to investigate the possibility of direct corticomotoneuronal (CM) connections in the rat, an anterograde-retrograde double-labeling method was developed. Phaseolus vulgaris-leucoagglutinin (PHA-L) anterograde tracing of corticospinal axons was combined with retrograde labeling of spinal motoneurons either by a conjugate of choleragen subunit B with horseradish peroxidase (CB-HRP) or by wheat germ agglutinin (WGA). The location of PHA-L injection unilaterally in the forelimb area of sensorimotor cortex and the CB-HRP or WGA injections in corresponding contralateral wrist or digit extensors or flexors were determined and matched on the basis of movement responses elicited by intracortical microstimulation. Light microscopic observation showed, in addition to the main contralateral dorsal corticospinal tract (CST), the presence of four other CST minor components in the contralateral lateral, ipsilateral ventral, and ipsilateral dorsal funiculi of the cervical spinal white matter and at the base of contralateral dorsal horn of the gray matter, respectively. PHA-L-labeled CST axonal arbors were observed from Rexed's lamina I through lamina X of contralateral spinal gray matter, most extensively in laminae VI and VII; some CST axons reached the zone of motoneuronal somata in lamina IX and a few of them also entered the lateral and occasionally the ventral funiculi, ramifying in the white matter. Between the zones of PHA-L-labeled CST axonal arbors on the one hand and CB-HRP/WGA labeled spinal motoneuronal somata with their extensive dendritic trees on the other, there was a large overlap, covering partly both the gray and the white matter. PHA-L-labeled axonal boutons (en passant or terminaux) were seen to contact the dendrites or even the somata of motoneurons in the gray matter, according to light-microscopic criteria for identification of synaptic contacts. Axodendritic CM contacts were occasionally observed in the lateral funiculus of the white matter as well. In general, only a single contact was observed between an individual PHA-L-labeled CST axon and a given retrogradely labeled motoneuron. In contrast to the common notion that direct CM connections are a specialty of primates, the present morphological data support the presence of direct CM connections also in some other mammals, such as the rat.

Fast rhythms in phrenic motoneuron and nerve discharges

1. Fast rhythms in discharges of individual phrenic (PHR) motoneurons were studied by spectral and interval analyses; and they were compared, using coherence analysis, with similar rhythms in whole-PHR nerve discharge. The purpose of this study was to ascertain the origin of the two rhythms, manifested as distinct spectral peaks, in PHR motoneuron and nerve discharge: medium-frequency oscillations (MFO, usual range 20-50 Hz); and high-frequency oscillations (HFO, usual range 50-100 Hz). 2. In paralyzed artificially ventilated cats, unit recordings were taken from 1) 26 isolated single PHR fibers (in 8 sodium pentobarbital-anesthetized cats) and 2) 27 identified PHR motoneuron somata in the spinal cord (in 5 decerebrate cats). Simultaneous whole-PHR activity was monophasically recorded from the contralateral PHR nerve for 1 and from both PHR nerves for 2. 3. The signals were subjected to time- and frequency-domain analyses. The latter included a novel application of coherence analysis to the study of population synchrony. 4. The autospectra of all PHR units showed prominent MFO peaks in the frequency range of the nerve MFO spectral peaks, as well as harmonic peaks, indicating the presence of this type of fast rhythm in the units' discharges. Spectral analysis of the augmenting PHR activities in different segments of the inspiratory (I) phase showed that the frequency of unit MFO and of nerve MFO rose during the course of I. Further, cycle-triggered histogram and interval analysis indicated that the frequencies of unit MFO autospectral peaks were very close to the peak firing rates of the units during the portion of I analyzed. Thus unit MFO spectral peaks reflected the rhythmic and augmenting discharges of the motoneurons, and similar nerve MFO peaks reflected the superposition of individual motoneuron discharges. 5. The coherences of motoneurons' MFOs to nerve MFOs were low or zero, indicating that only partial and weak MFO correlations occurred within the PHR motoneuron population. 6. In those cats (n = 11) that had clear PHR nerve HFO spectral peaks, about one-half of the recorded PHR motoneurons had HFO, as indicated by HFO peaks in the unit autospectra and/or the unit-nerve coherences. 7. For motoneurons having HFO, the coherence between unit and nerve HFOs was substantial, particularly when the latter were strong, indicating HFO correlations among a number of PHR motoneurons. 8. In the light of theoretical considerations on the generation of aggregate rhythms from superposition of unitary rhythms, these observations indicate the following.(ABSTRACT TRUNCATED AT 400 WORDS)

Phrenic motoneuron morphology in the neonatal rat

The morphology of neonatal rat phrenic motoneurons was studied following retrograde labeling with horseradish peroxidase, which resulted in Golgi-like fills of phrenic motoneuron somata and dendrites. At birth, these neurons have well-developed dendritic trees with many characteristics described for phrenic motoneurons in the adult rat. The dendrites form tightly fasciculated bundles that emerge from the phrenic nucleus primarily along four axes: ventromedial, ventrolateral, dorsolateral, and rostral/caudal, with smaller and more variable projections directly lateral and ventral. Although sparse, some dendritic appendages were also present, and in a few animals, somata clustering was apparent. The most significant difference between adult and neonatal rat phrenic motoneurons is in the extent to which medially and laterally projecting dendrites extend beyond the borders of the ipsilateral gray matter. In the neonate, unlike the adult, these dendrites project extensively past the gray/white border to the edge of the hemicord. Ventromedial dendrites occasionally cross to the contralateral ventral horn in the ventral white commissure and laterally projecting dendrites could be seen reaching the edge of the cord, turning and traveling rostrally or caudally for up to 100 microns. Phrenic motoneurons are not unique in having long dendrites at birth. A brief comparative study showed that neonatal cervical, thoracic, and lumbar motoneurons also have long dendrites that project to the medial and lateral borders of the hemicord.

Morphology and frequency of axon terminals on the somata, proximal dendrites, and distal dendrites of dorsal neck motoneurons in the cat

The purpose of the present study was to compare the frequency of different classes of axon terminals on selected regions of the somatodendritic surface of dorsal neck motoneurons. Single motoneurons supplying neck extensor muscles were antidromically identified and intracellularly stained with horseradish peroxidase. By using light microscopic reconstructions as a guide, axon terminals on the somata, proximal dendrites (within 250 microns of the soma), and distal dendrites (more than 540 microns from the soma) were examined at the electron microscopic level. Axon terminals were divided into several classes based on the shape, density, and distribution of their synaptic vesicles. The proportion of axon terminals belonging to each axon terminal class was similar on the somata and proximal dendrites. However, there were major shifts in the relative frequency of most classes of axon terminals on the distal dendrites. The most common classes of axon terminals on the somata and proximal dendrites contained clumps of either spherical or pleomorphic vesicles. These types of axon terminals accounted for more than 60% of the axon terminals on these regions. In contrast, only 11% of the axon terminals found on distal dendrites belonged to these types of axon terminals. The most commonly encountered axon terminal on distal dendrites contained a dense collection of uniformly distributed spherical vesicles. These types of axon terminals accounted for 40% of all terminals on the distal dendrites, but only 5-7% of the axon terminals on the somata and proximal dendrites. Total synaptic density on each of the three regions examined was similar. However, the percentage of membrane in contract with axon terminals was approximately four times smaller on distal dendrites than somata or proximal dendrites. Axon terminals (regardless of type) were usually larger on somata and proximal dendrites than distal dendrites. These results indicate that there are major differences in the types and arrangement of axon terminals on the proximal and distal regions of dorsal neck motoneurons and suggest that afferents from different sources may preferentially contact proximal or distal regions of the dendritic trees of these cells.

Location and connectivity of abdominal motoneurons in the embryo and larva of Drosophila melanogaster

The soma location and peripheral connectivity of motoneurons in abdominal segments of the embryo and larva of the fruitfly, Drosophila melanogaster are described as an initial step in determining the mechanisms by which motoneurons make connections with their target muscles in a genetically accessible organism. Embryonic motoneuron somata were retrogradely labelled by application of the fluorescent dye, DiI, to the whole peripheral nerve or to its separate anterior or posterior fascicles in segments A5-A7 of late stage 15/early stage 16 embryos. This technique reveals a stereotyped, segmentally repeated population of 34 motoneurons per hemisegment, several of which can be individually identified from their soma position. The same set of motoneurons was revealed in third instar larvae of D. melanogaster by cobalt backfilling of abdominal peripheral nerves, although the positions of some of these neurons change during larval development. The peripheral connectivity and axon morphology of several of the abdominal motoneurons was determined by intracellular injection with Lucifer Yellow in stage 16 embryos. For the motoneurons with axons in the anterior fascicle there is no clear relationship between somata groupings and the muscle targets innervated: contrary to earlier claims, these motoneurons arborize over both ventral and dorsal muscles. Individual motoneurons possess a stereotyped pattern of terminal arborization.

Morphological relationships among extensor digitorum longus, tibialis anterior, and semitendinosus motor nuclei of the cat: An investigation employing the retrograde transport of multiple fluorescent tracers

To determine the morphological relationships among extensor digitorum longus (EDL), tibialis anterior (TA), and semitendinosus (St) motor nuclei in the spinal cord of the cat, these nuclei were retrogradely labeled with three different fluorescent tracers. The fluorochromes--bisbenzimide, nuclear yellow, and propidium iodide--were applied by intramuscular injection or soaking the muscle nerve. The positions of the labeled motor nuclei were bilaterally symmetrical. The EDL and TA motoneurons were located in close proximity to one another, in the lateral regions of lamina IX in spinal segments L6 and L7. Although the boundaries of each nucleus were tightly opposed, the EDL and TA motor nuclei overlapped minimally, with the somata of EDL motoneurons positioned dorsal to those of TA. The St motor nucleus was located ventromedial to that of EDL and extended from the caudal portion of L6 through S1. Supplemental studies of the reflex effects evoked in EDL, TA, and St muscles by cutaneous nerve stimulation provided physiological observations that may be related to these anatomical results.

Physiological and morphological analysis of cat digastric motoneurons after intracellular staining with horseradish peroxidase.

Horseradish peroxidase was injected into the somata of cat digastric motoneurons. Twelve motoneurons for which the spatial distribution of dendritic arborization and axon trajectory were iden-tified physiologically were studied with reference to the location of cell bodies in the trigeminal motor nucleus. It was revealed that, irrespective of the rostrocaudal level of the soma, dendrities of digastric motoneurons were present mainly in the medial half of the nucleus as well as in extranuclear regions. Interestingly very few dendritic branches were observed in the lateral half of the motor nucleus, this “unoccupied” region corresponding to the masseter subdivision of the nucleus. The axons ran a dorsomedial course and descended to the motor root without giving rise to collaterals. The average size of the soma was 44.2μm with a range of 36.9-50.1μm. A linear correlation was observed between the total stem dendrite diameter and the mean soma diameter, giving a coefficient of 0.534. No excitatory postsynaptic potential was recorded from the soma upon stimulation of the digastric nerve with a current below the threshold, indicating a lack of muscle spindles.

Modulatory action of proctolin in the locust ( Locusta migratoria) antennal motor system

Previous investigations of locust antennal muscles suggested the involvement of a peptidergic cotransmitter in the excitation-contraction coupling. The presence and action of proctolin in the antennal motor system were therefore investigated with physiological and immunohistochemical techniques. In experiments with the scape muscles, application of synthetic proctolin enhanced electrically-induced single contractions as well as tetanic contractures. In the presence of proctolin, electrical nerve stimulation additionally evoked an underlying slow contraction with a refractory period of several seconds. Neither the membrane potential of scape muscle fibres nor the amplitude of intracellularly recorded junction potentials were altered by proctolin indicating a predominantly postsynaptic action of the pentapeptide. Even in the absence of motoneuronal activity, proctolin was found to induce concentration-dependent increases in muscle tension of the scape and the tentorio-scapal muscles. Proctolin-like immunoreactivity was found to be present on all four antennal muscles and in the nerves containing antennal motoneurones. Up to 12 proctolin-like immunoreactive somata were found on each side of the deutocerebrum within the same region where the somata of antennal motoneurones are located. These results suggest that proctolin is an endogenous modulator within the locust antennal motor system.

Evidence for two kinds of serotoninergic fibers in the ventral horn of spinal cord of the newly hatched chick

We observed two types of serotonin-positive (5-HT) fibers in the ventral horn of the lumbar spinal cord of the newly hatched chicken: the first was composed of fine 5-HT fibers, which were increased transiently in the neuropil at 1 week after hatching (transient type); the second type consisted of thick fibers, which were densely localized around motoneuron somata in the motoneuron pools innervating extensor muscles of the hip joint (adult type). Pharmacological perturbation experiments demonstrated that these two types of 5-HT fibers may have different functions: the adult type of fibers may act to maintain an erect standing posture, whereas the transient type may act to induce or facilitate dendrogenesis (or dendritic elongation) of motoneurons. Thus, we concluded that 5-HT fibers may modulate neuronal transmission and serve as a kind of inductive agent for dendritic development in the chick spinal cord.