The effects of blocking nerve conduction on retrograde HRP labeling of rat motoneuron

Locations of the motor endplate band and motoneurons innervating the sternomastoid muscle in the rat

Sternocleidomastoid (SCM) is a long muscle with two bellies, sternomastoid (SM) and cleidomastoid (CM) in the lateral side of the neck. It has been widely used as muscle and myocutaneous flap for reconstruction of oral cavity and facial defects and as a candidate for reinnervation studies. Therefore, exact neuroanatomy of the SCM is critical for guiding reinnervation procedures. In this study, SM in rats were investigated to document banding pattern of motor endplates (MEPs) using whole-mount acetylcholinesterase (AChE) staining and to determine locations of the motoneurons innervating the muscle using retrograde horseradish peroxidase (HRP) tracing technique. The results showed that the MEPs in the SM and CM were organized into a single band which was located in the middle portion of the muscle. After HRP injections into the MEP band of the SM, ipsilaterally labeled motoneurons were identified in the caudal medulla oblongata (MO), C1, and C2. The SM motoneurons were found to form a single column in lower MO and dorsomedial (DM) nucleus in C1. In contrast, the labeled SM motoneurons in C2 formed either one (DM nucleus), two [DM and ventrolateral (VL) nuclei], or three [DM, VL, and ventromedial (VM)] columns. These findings are important not only for understanding the neural control of the muscle but also for evaluating the success rate of a given reinnervation procedure when the SM is chosen as a target muscle.

Relationship of presympathetic-premotor neurons to the serotonergic transmitter system in the rat brainstem

Numerous physiological conditions and emotionally motivated behaviors require concomitant activation of somatomotor and sympathetic efferents. Using a virally mediated retrograde transsynaptic tract-tracing approach, we have previously determined locations of presympathetic-premotor neurons (PSPMNs) in the rat brainstem. These putative dual-function neurons send projections to somatomotor and sympathetic targets and likely participate in sympatho-somatomotor integration. A significant portion of these neurons is found within brainstem areas known to contain serotonergic neurons. Thus, we hypothesized that some of the PSPMNs utilize serotonin as their neurotransmitter. To test this hypothesis we first produced an antibody against TPH2, a brain-specific isoform of tryptophan hydroxylase (serotonin synthetic enzyme). We identified PSPMNs by using recombinant strains of the pseudorabies virus (PRV) for transsynaptic tract-tracing. PRV-152, a strain that expresses enhanced green fluorescent protein, was injected into sympathectomized gastrocnemius muscle, while PRV-BaBlu, which expresses beta-galactosidase, was injected into the adrenal gland in the same animals. Using immunofluorescent methods we determined whether coinfected neurons expressed TPH2. Our findings demonstrate that TPH2-positive PSPMNs are present at different rostrocaudal levels of the brainstem. Just over half of them are found at the pontomedullary junction within raphe obscurus, raphe magnus, and gigantocellular nucleus pars alpha. These cells may play a role in mediating responses to acute pain stimuli and/or participate in the central control of exercise. Overactivity of these serotonergic sympatho-somatomotor circuits may also play a role in the pathophysiology of serotonin syndrome.

Brainstem substrates of sympatho-motor circuitry identified using trans-synaptic tracing with pseudorabies virus recombinants

Previous physiological investigations have suggested the existence of a neural circuit that coordinates activation of motor and autonomic efferents before or at the onset of exercise. Traditionally these circuits have been postulated to involve forebrain areas. However, overlapping populations of medullary reticular formation neurons that participate in motor or autonomic control have been described previously, suggesting that individual pontomedullary reticular formation neurons may coordinate both motor and autonomic responses. We tested this hypothesis by conducting transneuronal retrograde tracing of motor and sympathetic nervous system pathways in rats using recombinant strains of pseudorabies virus (PRV). A PRV strain expressing the green fluorescent protein (PRV-152) was injected into the left gastrocnemius muscle, which was surgically sympathectomized, whereas another recombinant (PRV-BaBlu) was injected into the left adrenal gland. Immunofluorescence methods using monospecific antisera and distinct fluorophores identified neurons infected with one or both of the recombinants. Brainstem neurons coinfected with both PRV recombinants, which presumably had collateralized projections to both adrenal sympathetic preganglionic neurons and gastrocnemius motoneurons, were observed in several areas of the pontomedullary reticular formation. The largest number of such neurons was located in the rostral ventromedial medulla within the ventral gigantocellular nucleus, gigantocellular nucleus pars alpha, raphe obscurus, and raphe magnus. These neurons are candidates for relaying central command signals to the spinal cord.

Factors regulating AMPA-type glutamate receptor subunit changes induced by sciatic nerve injury in rats

Excitatory glutamatergic neurotransmission at Ia afferent-motoneuron synapses is enhanced shortly after physically severing or blocking impulse propagation of the afferent and/or motoneuron axons. We considered the possibility that these synaptic changes occur because of alterations in the number or properties of motoneuron alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors. Therefore, we quantitatively analyzed glutamate receptor (GluR)1, GluR2/3, and GluR4 AMPA subunit immunoreactivity (ir) in motoneurons 3, 7, or 14 days after axotomy or continuous tetrodotoxin (TTX) block of the sciatic nerve. GluR1-ir remained low in experimental and control motoneurons with either treatment and at any date. However, there was a large reduction of GluR2/3-ir (peak at 7 days >60% reduced) and a smaller, but statistically significant, reduction of GluR4-ir (around 10% reduction at days 3, 7, and 14) in axotomized motoneurons. TTX sciatic blockade did not affect AMPA subunit immunostainings. Axonal injury or interruption of the trophic interaction between muscle and spinal cord, but not activity disruption, appears therefore more likely responsible for altering AMPA subunit immunoreactivity in motoneurons. These findings also suggest that synaptic plasticity induced by axotomy or TTX block, although similar in the first week, could be related to different mechanisms. The effects of axotomy or TTX block on motoneuron expression of the metabotropic glutamate receptor mGluR1a were also studied. mGluR1a-ir was also strongly decreased after axotomy but not after TTX treatment. The time course of the known stripping of synapses from the cell somas of axotomized motoneurons was studied by using synaptophysin antibodies and compared with AMPA and mGluR1a receptor changes. Coverage by synaptophysin-ir boutons was only clearly decreased 14 days post axotomy and not at shorter intervals or after TTX block.

The role of tonic preganglionic neuron firing in the turnover of the large dense-cored vesicle store in sympathetic preganglionic nerve terminals

Large dense-cored vesicles are transported centrifugally in the cervical sympathetic trunk and are depleted in a calcium-dependent manner from synaptic boutons of the cat superior cervical ganglion during orthodromic stimulation at 20-40 Hz [P. Weldon et al. (1993) Neuroscience 55, 1045-1054]. In the present study, we tested in awake cats whether the normal tonic firing of the sympathetic preganglionic neuron contributes to the turnover of large dense-cored vesicles in synaptic boutons of the superior cervical ganglion. Tetrodotoxin was applied with a mini-osmotic pump to one cervical sympathetic trunk, while vehicle alone was applied to the contralateral cervical sympathetic trunk, for two, four or seven days. The appearance of Horner syndrome ipsilateral to the tetrodotoxin application demonstrated block of action potential propagation. Both superior cervical ganglia were excised and processed for electron microscopy. The number of large dense-cored vesicles per bouton cross-section was higher in the ganglion with tetrodotoxin-treated input than in the control. The content at four days was higher than at two days; the content at seven days was similar to that at four days. The number of lysosomes per bouton profile also increased in the ganglion with tetrodotoxin-treated input. No changes were observed in size of bouton profiles, number of boutons or of synapses per grid square and length of the presynaptic densities in the ganglion with tetrodotoxin-treated input.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of neuron soma firing in the restoration of neurotensin store in sympathetic preganglionic neuron terminals after stimulus-evoked depletion

We have previously shown that prolonged preganglionic stimulation (e.g. 40 Hz 20 min) depletes the neurotensin (NT) store of preganglionic axon terminals in the stellate ganglion (SG) of the cat and that replenishment of the store requires several days. The present study investigates the mechanisms which control turnover of the NT store in sympathetic preganglionic axon terminals. NT content of the SG and of the preganglionic axons which innervate it was determined by radioimmunoassay in the anesthetized cat. This study shows that, during the 24 h after 40-Hz 20-min stimulation of the preganglionic input to the SG, the rate of NT accumulation proximal to a ligature on the stimulated input is three times that observed in the control. The poststimulus increase in NT accumulation rate is prevented by treatment with protein-synthesis inhibitors. When the centripetal propagation of action potentials from the stimulus site on the preganglionic axons is prevented by a tetrodotoxin block applied locally during the stimulation period, the poststimulus increase in NT accumulation rate and the replenishment of the store are both prevented. These data suggest that the level of activity of the neuron regulates NT supply to the axon terminals, presumably by regulating NT synthesis. Thus, in the sympathetic preganglionic neuron, the action potential provides a mechanism for matching peptide synthesis to release.

In vitro and in vivo evidence of neurotensin release from preganglionic axon terminals in the stellate ganglion of the cat

We have previously shown that the neurotensin (NT) store in preganglionic axon terminals of the cat stellate ganglion (SG) is reversibly depleted by prolonged preganglionic stimulation. The present study addresses the questions of whether the preganglionic axon terminals release NT in response to depolarizing stimuli in vitro and whether in vivo NT is released by the tonic firing of the sympathetic preganglionic neurons. Slices of the SG of the anaesthetized cat, maintained in oxygenated Ringer solution, released NT. The efflux increased when the K concentration was increased from 5 to 25 or 45 mM or when veratridine was added to the medium. In Ca-free medium, efflux was suppressed. The effect of veratridine was blocked by tetrodotoxin (TTX). In awake, freely moving cats, in which TTX was applied for 4 days to the preganglionic input of the right SG, the NT content of this ganglion doubled by comparison with the left SG. Since NT accumulates proximal to a ligature on the preganglionic input of the SG, the increased NT content is likely to result from suppression of action potential-dependent release while influx into the terminals persists. This result suggests that the steady state of the NT store in sympathetic preganglionic terminals is the result of a steady influx from the soma balanced by action potential-dependent loss, presumably release.

Does androgen affect axonal transport of cholera toxin HRP in spinal motoneurons?

We examined the effect of systemic androgen levels upon the rate at which lumbosacral motoneurons are labeled with cholera toxin-conjugated horseradish peroxidase (CT-HRP) injected into target muscles. CT-HRP first reaches the spinal nucleus of the bulbocavernosus between 8 and 10 h after injection into the bulbocavernosus muscle of adult male rats, but the number of motoneurons filled with CT-HRP does not differ between androgen-treated and control castrates at any of the time points examined. Thus, contrary to current speculation, we found no evidence that androgen can affect retrograde transport of CT-HRP by rat motoneurons.

Nerve stimulation does not necessarily enhance retrograde HRP labeling of rat motoneuron

The retrograde horseradish peroxidase (HRP) labeling of medial gastrocnemius (MG) motoneurons was not enhanced by electrical stimulation of the sciatic nerve after HRP injection into the MG muscle of the rat. When pulse trains with short intermissions were applied immediately after the HRP injection, the number of labeled MG motoneurons on the stimulated side was even smaller than that on the control side.