A comparison of the effects of serotonin, substance P and thyrotropin-releasing hormone on excitability of rat spinal motoneurons in vivo

Differential activating effects of thyrotropin-releasing hormone and its analog taltirelin on motor output to the tongue musculature in vivo

Thyrotropin-releasing hormone (TRH) is produced by the hypothalamus but most brain TRH is located elsewhere where it acts as a neuromodulator. TRH-positive neurons project to the hypoglossal motoneuron pool where TRH receptor RNA shows a high degree of differential expression compared with the rest of the brain. Strategies to modulate hypoglossal motor activity are of physiological and clinical interest given the potential for pharmacotherapy for obstructive sleep apnea (OSA), a common and serious respiratory disorder. Here, we identified the effects on tongue motor activity of TRH and a specific analog (taltirelin) applied locally to the hypoglossal motoneuron pool and systemically in vivo. Studies were performed under isoflurane anesthesia and across sleep–wake states in rats. In anesthetized rats, microperfusion of TRH (n = 8) or taltirelin (n = 9) into the hypoglossal motoneuron pool caused dose-dependent increases in tonic and phasic tongue motor activity (both p < 0.001). However, the motor responses to TRH were biphasic, being significantly larger “early” in the response versus at the end of the intervention (p ≤ 0.022). In contrast, responses to taltirelin were similar “early” versus “late” (p ≥ 0.107); i.e. once elicited, the motor responses to taltirelin were sustained and maintained. In freely behaving conscious rats (n = 10), microperfusion of 10 μM taltirelin into the hypoglossal motoneuron pool increased tonic and phasic tongue motor activity in non-rapid-eye-movement (REM) sleep (p ≤ 0.038). Intraperitoneal injection of taltirelin (1 mg/kg, n = 16 rats) also increased tonic tongue motor activity across sleep–wake states (p = 0.010). These findings inform the studies in humans to identify the potential beneficial effects of taltirelin for breathing during sleep and OSA.

Removal of supraspinal input reveals a difference in the flexor and extensor monosynaptic reflex response to quipazine independent of motoneuron excitation

The purpose of this study was to determine if quipazine, a serotonergic agonist, differentially modulates flexor and extensor motor output. This was achieved by examining the monosynaptic reflex (MSR) of the tibial (extensor) and peroneal (flexor) nerves, by determining the basic and rhythmic properties of extensor and flexor motoneurons, and by recording extracellular Ia field potentials of the tibial and peroneal nerves in the in vivo adult decerebrate rat in both spinal intact and acute spinalized preparations. In the spinal intact preparation, the tibial and peroneal MSR amplitude significantly increased compared with baseline in response to quipazine, with no difference between nerves ( P < 0.05). In the spinalized preparation, the MSR was significantly increased in both the tibial and peroneal nerves with the latter increasing more than the former (5.7 vs. 3.6 times; P < 0.05). Intracellular motoneuron experiments demonstrated that rheobase decreased, while input resistance, afterhyperpolarization amplitude, and the firing rate at a given current injection increased in motoneurons following quipazine administration with no differences between extensor and flexor motoneurons. Both the tibial and peroneal nerve extracellular Ia field potentials increased with the peroneal demonstrating a significantly greater increase (7 vs. 38%; P < 0.05) following quipazine. It is concluded that in the spinal intact preparation quipazine does not have a differential effect on flexor or extensor motor output. However, in the acute spinalized preparation, quipazine preferentially affects the flexor MSR compared with the extensor MSR, likely due to the removal of a descending tonic inhibition on flexor Ia afferents.

SEROTONERGIC MECHANISMS IN AMYOTROPHIC LATERAL SCLEROSIS

Serotonin (5-HT) has been intimately linked with global regulation of motor behavior, local control of motoneuron excitability, functional recovery of spinal motoneurons as well as neuronal maturation and aging. Selective degeneration of motoneurons is the pathological hallmark of amyotrophic lateral sclerosis (ALS). Motoneurons that are preferentially affected in ALS are also densely innervated by 5-HT neurons (e.g., trigeminal, facial, ambiguus, and hypoglossal brainstem nuclei as well as ventral horn and motor cortex). Conversely, motoneuron groups that appear more resistant to the process of neurodegeneration in ALS (e.g., oculomotor, trochlear, and abducens nuclei) as well as the cerebellum receive only sparse 5-HT input. The glutamate excitotoxicity theory maintains that in ALS degeneration of motoneurons is caused by excessive glutamate neurotransmission, which is neurotoxic. Because of its facilitatory effects on glutaminergic motoneuron excitation, 5-HT may be pivotal to the pathogenesis and therapy of ALS. 5-HT levels as well as the concentrations 5-hydroxyindole acetic acid (5-HIAA), the major metabolite of 5-HT, are reduced in postmortem spinal cord tissue of ALS patients indicating decreased 5-HT release. Furthermore, cerebrospinal fluid levels of tryptophan, a precursor of 5-HT, are decreased in patients with ALS and plasma concentrations of tryptophan are also decreased with the lowest levels found in the most severely affected patients. In ALS progressive degeneration of 5-HT neurons would result in a compensatory increase in glutamate excitation of motoneurons. Additionally, because 5-HT, acting through presynaptic 5-HT1B receptors, inhibits glutamatergic synaptic transmission, lowered 5-HT activity would lead to increased synaptic glutamate release. Furthermore, 5-HT is a precursor of melatonin, which inhibits glutamate release and glutamate-induced neurotoxicity. Thus, progressive degeneration of 5-HT neurons affecting motoneuron activity constitutes the prime mover of the disease and its progression and treatment of ALS needs to be focused primarily on boosting 5-HT functions (e.g., pharmacologically via its precursors, reuptake inhibitors, selective 5-HT1A receptor agonists/5-HT2 receptor antagonists, and electrically through transcranial administration of AC pulsed picotesla electromagnetic fields) to prevent excessive glutamate activity in the motoneurons. In fact, 5HT1A and 5HT2 receptor agonists have been shown to prevent glutamate-induced neurotoxicity in primary cortical cell cultures and the 5-HT precursor 5-hydroxytryptophan (5-HTP) improved locomotor function and survival of transgenic SOD1 G93A mice, an animal model of ALS.

Postnatal development of brainstem serotonin-containing neurons projecting to lumbar spinal cord in rats

We quantified postnatal changes in brainstem serotonin (5-hydroxytryptamine, 5-HT)-containing neurons projecting to lumbar spinal cord. The medulla-spinal cord descending neurons were identified by a retrograde neurotracer, choleratoxin B subunit (CTb), and 5-HT neurons were stained by immunohistochemistry. Double-labeled neurons were assumed to be 5-HT neurons projecting to the lumbar spinal cord, and were quantitatively analyzed in each raphe nucleus in the medulla. The following results were obtained: (1) At PND 3, numerous CTb-labeled neurons (CTLN) were already present in the raphe pallidus (B1), while few CTLN were seen in raphe obscurus (B2) and raphe magnus (B3). CTLN then rapidly increased in number and were separately distributed after PND 7 in B3 and after PND 14 in B2. (2) At PND 3, numerous 5-HT-containing neurons were already present in B1-B3, with 23.4% and 14.0% of them labeled with CTb in B1 and B2, respectively, while there were few double-labeled neurons in B3. From PND 3 to 28, although the proportion of double-labeled to 5-HT neurons remained unchanged in B1 and B2, that in B3 rapidly increased from 5.8% at PND 7 to 28.8% at PND 14. Previous studies have shown that the 5-HT neurons in B3 send fibers mainly to the dorsal horn, while those in B1 and B2 send fibers mainly to the ventral horn at all spinal cord levels. Taken together, the present findings suggest that the brainstem 5-HT systems influence the ventral horn of the spinal cord, where spinal motoneurons exist earlier than in the dorsal horn. The functional significance of these early 5-HT systems in motor development and/or disabilities is discussed.

Serotonin as a modulator of glutamate- and GABA-mediated neurotransmission: implications in physiological functions and in pathology

The neurotransmitter serotonin (5-HT), widely distributed in the central nervous system (CNS), is involved in a large variety of physiological functions. In several brain regions 5-HT is diffusely released by volume transmission and behaves as a neuromodulator rather than as a "classical" neurotransmitter. In some cases 5-HT is co-localized in the same nerve terminal with other neurotransmitters and reciprocal interactions take place. This review will focus on the modulatory action of 5-HT on the effects of glutamate and gamma-amino-butyric acid (GABA), which are the principal neurotransmitters mediating respectively excitatory and inhibitory signals in the CNS. Examples of interaction at pre-and/or post-synaptic levels will be illustrated, as well as the receptors involved and their mechanisms of action. Finally, the physiological meaning of neuromodulatory effects of 5-HT will be briefly discussed with respect to pathologies deriving from malfunctioning of serotonin system.

The crossed phrenic phenomenon: a model for plasticity in the respiratory pathways following spinal cord injury

Hemisection of the cervical spinal cord rostral to the level of the phrenic nucleus interrupts descending bulbospinal respiratory pathways, which results in a paralysis of the ipsilateral hemidiaphragm. In several mammalian species, functional recovery of the paretic hemidiaphragm can be achieved by transecting the contralateral phrenic nerve. The recovery of the paralyzed hemidiaphragm has been termed the "crossed phrenic phenomenon." The physiological basis for the crossed phrenic phenomenon is as follows: asphyxia induced by spinal hemisection and contralateral phrenicotomy increases central respiratory drive, which activates a latent crossed respiratory pathway. The uninjured, initially latent pathway mediates the hemidiaphragm recovery by descending into the spinal cord contralateral to the hemisection and then crossing the midline of the spinal cord before terminating on phrenic motoneurons ipsilateral and caudal to the hemisection. The purpose of this study is to review work conducted on the crossed phrenic phenomenon and to review closely related studies focusing particularly on the plasticity associated with the response. Because the review deals with recovery of respiratory muscles paralyzed by spinal cord injury, the clinical relevance of the reviewed studies is highlighted.

The neuropharmacology of upper airway motor control in the awake and asleep states: implications for obstructive sleep apnoea

Obstructive sleep apnoea is a common and serious breathing problem that is caused by effects of sleep on pharyngeal muscle tone in individuals with narrow upper airways. There has been increasing focus on delineating the brain mechanisms that modulate pharyngeal muscle activity in the awake and asleep states in order to understand the pathogenesis of obstructive apnoeas and to develop novel neurochemical treatments. Although initial clinical studies have met with only limited success, it is proposed that more rational and realistic approaches may be devised for neurochemical modulation of pharyngeal muscle tone as the relevant neurotransmitters and receptors that are involved in sleep-dependent modulation are identified following basic experiments.

Inhibition of pudendal reflexes in spinal rats. Reassessing the role of serotonin

The effects of serotonin (5-HT) and thyrotropin-releasing hormone (TRH) on penile reflexes were investigated in intact and spinally transected male rats. Doses of intrathecal 5-HT (0.0, 1.13, 2.26, 11.3, 22.6, and 113.0 nmol), in a range previously shown to inhibit pudendal reflexes in anesthetized spinal preparations, prolonged the latency to the first penile erection in awake intact rats. However, these doses also provoked hyperreactivity and vocalization. Doses of intrathecal TRH (100 and 500 pmol) that effectively inhibited penile erection in intact animals were less effective in spinalized animals. Finally, a combination of subthreshold doses of TRH (100 pmol) and 5-HT (4.0 nmol) at a ratio known to affect other TRH/5-HT-mediated circuits significantly extended erection latency in animals with spinal transections. These data suggest that 5-HT and TRH are both involved in the inhibitory circuits regulating penile erection, either through corelease onto the same population of cells or through independent release onto different populations of neurons.

Modulation of hypoglossal motoneuron excitability by NK1 receptor activation in neonatal mice in vitro

1. The effects of substance P (SP), acting at NK1 receptors, on the excitability and inspiratory activity of hypoglossal (XII) motoneurons (MNs) were investigated using rhythmically active medullary-slice preparations from neonatal mice (postnatal day 0-3). 2. Local application of the NK1 agonist [SAR(9),Met (O(2))(11)]-SP (SP(NK1)) produced a dose-dependent, spantide- (a non-specific NK receptor antagonist) and GR82334-(an NK1 antagonist) sensitive increase in inspiratory burst amplitude recorded from XII nerves. 3. Under current clamp, SP(NK1) significantly depolarized XII MNs, potentiated repetitive firing responses to injected currents and produced a leftward shift in the firing frequency-current relationships without affecting slope. 4. Under voltage clamp, SP(NK1) evoked an inward current and increased input resistance, but had no effect on inspiratory synaptic currents. SP(NK1) currents persisted in the presence of TTX, were GR82334 sensitive, were reduced with hyperpolarization and reversed near the expected E(K). 5. Effects of the alpha(1)-noradrenergic receptor agonist phenylephrine (PE) on repetitive firing behaviour were virtually identical to those of SP(NK1). Moreover, SP(NK1) currents were completely occluded by PE, suggesting that common intracellular pathways mediate the actions of NK1 and alpha(1)-noradrenergic receptors. In spite of the similar actions of SP(NK1) and PE on XII MN responses to somally injected current, alpha(1)-noradrenergic receptor activation potentiated inspiratory synaptic currents and was more than twice as effective in potentiating XII nerve inspiratory burst amplitude. 6. GR82334 reduced XII nerve inspiratory burst amplitude and generated a small outward current in XII MNs. These observations, together with the first immunohistochemical evidence in the newborn for SP immunopositive terminals in the vicinity of SP(NK1)-sensitive inspiratory XII MNs, support the endogenous modulation of XII MN excitability by SP. 7. In contrast to phrenic MNs (Ptak et al. 2000), blocking NMDA receptors with AP5 had no effect on the modulation of XII nerve activity by SP(NK1). 8. In conclusion, SP(NK1) modulates XII motoneuron responses to inspiratory drive primarily through inhibition of a resting, postsynaptic K+ leak conductance. The results establish the functional significance of SP in controlling upper airway tone during early postnatal life and indicate differential modulation of motoneurons controlling airway and pump muscles by SP.

Non-associative learning and serotonin induce similar bi-directional changes in excitability of a neuron critical for learning in the medicinal leech

In studies of the cellular basis of learning, much attention has focused on plasticity in synaptic transmission in terms of transmitter release and the number or responsiveness of neurotransmitter receptors. However, changes in postsynaptic excitability independent of receptors may also play an important role. Changes in excitability of a single interneuron in the leech, the S-cell, were measured during non-associative learning of the whole-body shortening reflex. This interneuron was chosen because it is known to be necessary for sensitization and full dishabituation of the shortening response. During sensitization, S-cell excitability increased, and this enhancement corresponded to facilitation of the shortening reflex and increased S-cell activity during the elicited response. During habituation training, there was a decrement in both the shortening reflex and the elicited S-cell activity, along with decreased S-cell excitability. Conversely, dishabituation facilitated both the shortening response and S-cell activity during shortening, with an accompanying increase in S-cell excitability. Bath application of 1-10 micrometer serotonin (5HT), a modulatory neurotransmitter that is critical for sensitization, for full dishabituation, and for associative learning, increased S-cell excitability. S-cell excitability also increased after stimulation of the serotonergic Retzius cells. However, focal application of serotonin onto the S-cell soma hyperpolarized the interneuron, and bath application of a lower dose of serotonin (0.1 micrometer) decreased excitability. The observed changes in postsynaptic excitability appear to contribute to non-associative learning, and modulatory neurotransmitters, such as serotonin, evidently help regulate excitability. Such changes in S-cell excitability may also be relevant for more complex, associative forms of learning.

Thyrotropin-releasing hormone and its receptor in glia

The presence of thyrotropin-releasing hormone (Thyroliberin, TRH) and its receptor (TRH-R) in frozen coronal sections of the adult rat spinal cord and neonatal rat astroglial cultures was investigated by means of immunocytochemistry and Western blot using polyclonal antibodies generated against the hormone and monoclonal antibodies originated against discrete sequences of the type 1 rat TRH receptor (TRH-R1). TRH-R1 and TRH are present both in astroglial cells from adult rats and in cultured cells from newborn animals. The localization of TRH and TRH-R1 in nonneuronal cells in the central nervous system may reflect that some of the neurotrophic actions of TRH upon the central nervous system are mediated by glial cells.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Development of spinal motoneurons in rats after a neonatal hypoxic insult

To determine the developmental changes of cervical and lumbar motoneurons (MNs) during normal development and after a neonatal hypoxic insult, cervical and lumbar MNs were studied in rats of various postnatal ages using a retrograde neurotracing technique combined with immunohistochemistry. The results regarding normal development could be summarized as follows: (1) the dendrites elongated mainly during the first 5 postnatal days (PNDs), being longer and more extensive in cervical MNs than in lumbar MNs; (2) the average cell body area increased from PND 5 to 14; and (3) the distribution of cell body areas changed from a unimodal to a bimodal pattern between PND 5 and 14. The temporal differences in morphologic development between cervical and lumbar MNs may influence the motor development in a rostrocaudal manner. The dendrites of lumbar MNs were shorter and less extensive in rats with a neonatal hypoxic insult than in rats without one; no significant difference was observed in cervical MNs between the two groups. The developmental difference between cervical and lumbar MNs after a neonatal hypoxic insult may contribute to motor deficits, with greater effect on the lower than the upper limbs.

Ultrastructural evidence that substance P neurons form synapses with noradrenergic neurons in the A7 catecholamine cell group that modulate nociception

Potent antinociception can be produced by electrical stimulation of spinally projecting noradrenergic neurons in the A7 catecholamine cell group and this effect is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. Microinjection of substance P near A7 neurons also produces antinociception that is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. These observations suggest that substance P produces antinociception by activating noradrenergic A7 neurons. However, it is not known whether this effect of substance P is produced by a direct or an indirect action on A7 neurons. Although light microscopic studies have demonstrated the existence of both substance P-containing axon terminals and neurokinin-1 receptors in the region of the A7 cell group, it is not known whether substance P terminals form synapses with noradrenergic A7 neurons. These experiments used double-labeling immunocytochemical methods and electron microscopic analysis to determine whether substance P-containing axons form synapses with noradrenergic neurons in the A7 cell group. Pre-embedding immunocytochemistry, combined with light and electron microscopic analysis, was used to provide ultrastructural evidence for synaptic connections between substance P-immunoreactive terminals labeled with immunoperoxidase and tyrosine hydroxylase-immunoreactive A7 neurons labeled with silver-enhanced immunogold. Tyrosine hydroxylase labeling was found in perikarya and dendrites in the A7 region, and substance P labeling was found in axons and synaptic terminals. Substance P-labeled terminals formed asymmetric synapses with tyrosine hydroxylase-labeled dendrites, but only a few of these were present on tyrosine hydroxylase-labeled somata. Substance P-labeled terminals also formed asymmetric synapses with unlabeled dendrites, and many unlabeled terminals formed both symmetric and asymmetric synapses with tyrosine hydroxylase-labeled dendrites. These results demonstrate that substance P neurons form a significant number of synapses with the dendrites of noradrenergic A7 neurons and support the conclusion that microinjection of substance P in the A7 cell group produces antinociception by direct activation of spinally projecting noradrenergic neurons.

Decreased neuropeptide content in the spinal cord of aged rats: the effect of GM1 ganglioside

This study investigated the status of substance P (SP), methionine-enkephalin (Met-Enk) and dynorphin A(1-13) (Dyn A) in the spinal cord of aged Sprague-Dawley rats and the effect of GM1 ganglioside on these neuropeptides. SP and Met-Enk, but not Dyn A, were decreased in both dorsal and ventral horns of the aged spinal cord. Treatment with GM1 ganglioside (30 mg/kg i.p., daily for 30 days) restored, in part, the neuropeptide deficits in the ventral horns, but not in the dorsal horns. This information might be important for understanding the sensory and motor deficits associated with ageing, and how the spinal cord neuropeptides might be amplified in the aged spinal cord.

Substance P modulates NMDA responses and causes long-term protein synthesis-dependent modulation of the lamprey locomotor network

Tachykinin immunoreactivity is found in a ventromedial spinal plexus in the lamprey. Neurons in this plexus project bilaterally and are thus in a position to modulate locomotor networks on both sides of the spinal cord. We have examined the effects of the tachykinin substance P on NMDA-evoked locomotor activity. Brief (10 min) application of tachykinin neuropeptides results in a prolonged concentration-dependent (>24 hr) modulation of locomotor activity, shown by the increased burst frequency and more regular burst activity. These effects are blocked by the tachykinin antagonist spantide II. There are at least two phases to the burst frequency modulation. An initial phase (approximately 2 hr) is associated with the protein kinase C-dependent potentiation of cellular responses to NMDA. The long-lasting phase (>2 hr) appears to be protein synthesis-dependent, with protein synthesis inhibitors causing the increased burst frequency to recover after washing for 2-3 hr. The modulation of the burst regularity is caused by a separate effect of tachykinins, because unlike the burst frequency modulation it does not require the modulation of NMDA receptors for its induction and is blocked by H8, an inhibitor of cAMP- and cGMP-dependent protein kinases. The effects of substance P were mimicked by the dopamine D2 receptor antagonist eticlopride. The effects of eticlopride were blocked by the tachykinin antagonist spantide II, suggesting that eticlopride may endogenously release tachykinins. Because locomotor activity in vitro corresponds to that during swimming in intact animals, we suggest that endogenously released tachykinins will result in prolonged modulation of locomotor behavior.

The cellular localization of 5-HT2A receptors in the spinal cord and spinal ganglia of the adult rat

The localization of serotonin2A (5-HT2A) receptors in the adult rat spinal cord and dorsal root ganglia was examined by using a polyclonal antibody that recognizes the C-terminus peptides of the mouse 5-HT2A receptor. Positive cell bodies of 5-HT2A receptor were found in several regions of the spinal cord. Generally, large-to-intermediate sized neuronal cell bodies were intensely immunolabeled. Motoneurons in the ventral horn were the most intensely labeled. Dot-like immunoreactive profiles were located beneath the cell membrane of motoneurons. Neuronal somata in the intermediolateral nucleus of the thoracic spinal cord were moderately labeled. The immunoreactivity in the dorsal horn was weak. A considerable number of glial cell bodies in the white matter were immunostained. The majority of both small and large sized neurons were 5-HT2A immunopositive in the dorsal root ganglion.

Serotonergic axonal contacts on identified cat trigeminal motoneurons and their correlation with medullary raphe nucleus stimulation

The innervation of the trigeminal motor nucleus by serotonergic fibers with cell bodies in the raphe nuclei pallidus and obscurus suggests that activation of this pathway may alter the excitability of trigeminal motoneurons. Thus, we recorded intracellular responses from cat jaw-closing (JC) andjaw-opening (JO) alpha-motoneurons evoked by raphe stimulation and used a combination of intracellular staining of horseradish peroxidase (HRP) and immunohistochemistry at the light and electron microscopic levels to examine the distribution of contacts made by serotonin (5-HT)-immunoreactive boutons on the two motoneurons types. Electrical stimulation applied to the nucleus raphe pallidus-obscurus complex induced a monosynaptic excitatory postsynaptic potential (EPSP) in JC (masseter) alpha-motoneurons and an EPSP with an action potential in JO (mylohyoid) alpha-motoneurons. The EPSP rise-times (time to peak) and half widths were significantly longer in the JC than in the JO motoneurons. The EPSPs were suppressed by systemic administration of methysergide (2 mg/kg). Six JC and seven JO alpha-motoneurons were well stained with HRP. Contacts were seen between 5-HT-immunoreactive boutons and the motoneurons. The JC motoneurons received a significantly larger number of the contacts than did the JO motoneurons. The contacts were distributed widely in the proximal three-fourths of the dendritic tree of JC motoneurons but were distributed on more proximal dendrites in the JO motoneurons. At the electron microscopic level, synaptic contacts made by 5-HT-immunoreactive boutons on motoneurons were identified. The present study demonstrated that JC motoneurons receive stronger 5-HT innervation, and this correlates with the fact that raphe stimulation caused larger EPSPs among these neurons than among JO motoneurons.

Increases in thyrotropin-releasing hormone messenger RNA expression induced by a model of human temporal lobe epilepsy: effect of partial and complete kindling

Thyrotropin-releasing hormone and its receptor are differentially distributed throughout the limbic forebrain. In addition to its neuroendocrine function, several non-endocrine central nervous system effects of thyrotropin-releasing hormone and its analogs have been reported, including anticonvulsant effects in animals and humans. Kindling, as a model of temporal lobe epilepsy, produces elevations of endogenous thyrotropin-releasing hormone specifically in seizure-prone limbic regions. The present study used semi-quantitative in situ hybridization to characterize changes in thyrotropin-releasing hormone messenger RNA that occurred during the kindling process (partial kindling), as well as after fully kindled seizures. No significant change in thyrotropin-releasing hormone messenger RNA was detected 1 h postictally, whereas significant elevations were detected in the granule cell layer of the hippocampal dentate gyrus, diffuse nuclei of the amygdala and in layers II and III of piriform and entorhinal cortices from 3 to 48 h after a single generalized seizure in fully kindled rats. Peak messenger RNA expression occurred from 6 to 12 h postictally, with a decline at 24 h, followed by a precipitous return to undetectable levels by 48 h, except in the dentate gyrus. In marked contrast, partial kindling produced no detectable change in thyrotropin-releasing hormone messenger RNA by 6 h after the first occurrence of stage 1-5 seizures. Electrode placement, a single afterdischarge, or a 20-microA stimulation of the amygdala was not associated with accumulation of thyrotropin-releasing hormone messenger RNA. Thus, only full kindled generalized seizures increased thyrotropin-releasing hormone messenger RNA expression in identical limbic regions which also showed postictal elevations in thyrotropin-releasing hormone. However, this enhancement followed a more immediate and shorter lasting time-course than previously demonstrated increases in the tripeptide. These results support the hypothesis that thyrotropin-releasing hormone is an important neuromodulator in epileptic foci.

Distribution of dopamine immunoreactivity in the rat, cat and monkey spinal cord

In the present study, the distribution of dopamine (DA) was identified light microscopically in all segments of the rat, cat, and monkey spinal cord by using immunocytochemistry with antibodies directed against dopamine. Only fibers and (presumed) terminals were found to be immunoreactive for DA. Strongest DA labeling was present in the sympathetic intermediolateral cell column (IML). Strong DA labeling, consisting of many varicose fibers, was found in all laminae of the dorsal horn, including the central canal area (region X), but with the exception of the substantia gelatinosa, which was only sparsely labeled, especially in rat and monkey. In the motoneuronal cell groups DA labeling was also strong and showed a fine granular appearance. The sexually dimorphic cremaster nucleus and Onuf's nucleus (or its homologue) showed a much stronger labeling than the surrounding somatic motoneurons. In the parasympathetic area at sacral levels, labeling was moderate. The remaining areas, like the intermediate zone (laminae VI-VIII), were only sparsely innervated. The dorsal nucleus (column of Clarke) showed the fewest DA fibers, as did the central cervical nucleus, suggesting that cerebellar projecting cells were avoided by the DA projection. In all species, the descending fibers were located mostly in the dorsolateral funiculus, but laminae I and III also contained many rostrocaudally oriented fibers. It is concluded that DA is widely distributed within the spinal cord, with few differences between species, emphasizing that DA plays an important role as one of the monoamines that influences sensory input as well as autonomic and motor output at the spinal level.

Changes in receptor levels for thyrotropin releasing hormone, serotonin, and substance P in cervical spinal cord of Wobbler mouse: a quantitative autoradiography study during early and late stages of the motoneuron disease

Receptor levels for thyrotropin releasing hormone (TRH) measured by quantitative autoradiography in the Wobbler mouse cervical spinal cord show receptor losses that may relate to the inherited loss of motoneurons, most pronounced late (at Stage 4) in the motoneuron disease. An age-related decrease of TRH and serotonin (5-HT) receptors can be seen in the ventral horn of the control specimens (normal phenotype littermate and wild-type alike). However, this pattern is missing for substance P (SP) receptors from the wild-type specimens. Therefore the age-related decrease of SP receptors detected in the Wobbler mouse strain may identify a strain-related defect in SP neuronal/receptor developmental patterns. A higher level of TRH receptors was measured in the Wobbler dorsal horn at an early stage (Stage 1) in the motoneuron disease compared with the control specimens. The data are discussed in relation to an aberrant neuronal sprouting that occurs around the degenerating motoneurons in the ventral horn during the course of the motoneuron disease.

Effect of muscle denervation on the expression of substance P in the ventral raphe-spinal pathway of the rat

The medullary raphe nuclei, wherein serotonin (5-HT) coexists with substance P (SP) and thyrotropin-releasing hormone (TRH), innervate lower motor neurons in the spinal cord ventral horn by means of the ventral raphe-spinal pathway. Destruction of the ventral raphe-spinal pathway is associated with deficient recovery of denervated muscle, indicating that it may exert a trophic effect upon lower motor neurons. To determine whether SP could be a trophic factor for lower motor neurons within the ventral raphe-spinal pathway, the effect of muscle denervation with botulinum toxin type A on SP-encoding beta-preprotachykinin mRNA in the rat medullary raphe was examined by in situ hybridization histochemistry. Silver grain density over hybridized medullary raphe neurons was increased by up to 11%, although the number of hybridized neurons did not change in denervated as compared to control rats. Increased SP gene expression in the medullary raphe in response to motor unit lesioning suggests that raphe-spinal SP may be trophic to lower motor neurons.

Thyrotropin-releasing hormone inputs are preferentially directed towards respiratory motoneurons in rat nucleus ambiguus

In the present study, we assessed the extent of the thyrotropin-releasing hormone (TRH) input to motoneurons in the ambigual, facial, and hypoglossal nuclei of the rat using a combination of intracellular recording, dye filling, and immunohistochemistry. Twelve motoneurons in the rostral nucleus ambiguus were labelled by intracellular injection in vivo of Neurobiotin (Vector). Seven out of 12 ambigual motoneurons displayed rhythmic fluctuations of their membrane potential in phase with phrenic nerve discharge, whereas the other five had no modulations of any kind. Seven facial motoneurons and seven hypoglossal motoneurons were also filled with Neurobiotin. All three motor nuclei contained TRH-immunoreactive varicosities, with the largest numbers found in the nucleus ambiguus. Close appositions were seen between TRH-immunoreactive boutons and every labelled motoneuron. Respiratory-related motoneurons in the nucleus ambiguus received the largest number of TRH appositions with 74 +/- 38 appositions/neuron (mean +/- S.D.; n = 7). In contrast, nonrespiratory ambigual motoneurons received significantly fewer TRH appositions (11 +/- 5; n = 5; P < 0.05; Mann-Whitney U test). Facial motoneurons received about the same number of TRH appositions as nonrespiratory ambigual motoneurons, with 13 +/- 4 (n = 7). Hypoglossal motoneurons received the fewest appositions from TRH-containing boutons, with 8 +/- 2 (n = 7). There were no differences in the TRH inputs to respiratory and nonrespiratory motoneurons in the facial and hypoglossal nuclei. These results demonstrate that, among motoneurons in the medulla, respiratory motoneurons in the rostral nucleus ambiguus are preferentially innervated by the TRH-immunoreactive boutons.

Thyrotropin-releasing hormone-immunoreactive varicosities synapse on rat phrenic motoneurons

The relationship between retrogradely labelled or intracellularly filled phrenic motoneurons and varicosities containing thyrotropin-releasing hormone immunoreactivity was investigated in rats by light and electron microscopy. Phrenic motoneurons were identified via retrograde tracing from the diaphragm with cholera toxin B subunit, which was followed by immunocytochemistry to visualise retrogradely labelled motoneurons and thyrotropin-releasing hormone-immunoreactive nerve fibres in their vicinity. At the light microscopic level, varicose thyrotropin-releasing hormone-immunoreactive nerve fibres were distributed sparsely in the phrenic motor nucleus, with some axons surrounding retrogradely labelled motoneurons. In separate intracellular experiments, four phrenic motoneurons identified by antidromic activation from the C5 phrenic nerve root were subsequently filled with Neurobiotin, and nerve fibres that contained thyrotropin-releasing hormone immunoreactivity were identified by immunocytochemistry. The numbers and locations of thyrotropin-releasing hormone-immunoreactive varicosities that were closely appeared to the intracellularly labelled motoneurons were mapped using a camera lucida technique. Close appositions by thyrotropin-releasing hormone-immunoreactive varicosities were seen on somata as well as on proximal and distal dendrites. The closely apposed varicosities were usually present in tight clusters, which were formed by single varicose axons. However, the distribution was nonuniform, in that some dendrites did not receive any close appositions. Ultrastructural analysis of random ultrathin sections through retrogradely labelled neurons showed that varicosities with thyrotropin-releasing hormone immunoreactivity made 1.8% of all synapses and direct contacts on somata and 2.3% of synapses and contacts with dendrites of the retrogradely labelled phrenic motoneurons. The results of these experiments suggest that thyrotropin-releasing hormone-immunoreactive varicosities provide similar numbers of inputs to both the somata and dendrites of phrenic motoneurons. These thyrotropin-releasing hormone-containing inputs seen via light and electron microscopy could modulate the excitability of phrenic motoneurons.

Synaptic contact of neuropeptide-and amine-containing axons on parasympathetic preganglionic neurons in the superior salivatory nucleus of the rat

Parasympathetic preganglionic neurons in the superior salivatory nucleus (SSNNs) projecting to the pterygopalatine ganglion were labeled by retrograde transport of cholera toxin B subunit (CTB) in the rat. Morphological interactions between SSNNs and afferent fibers immunoreactive (IR) for neuropeptide and amine were examined with light and electron microscopes by double-immunostaining techniques. SSNNs were found in the ipsilateral ventrolateral part of the rostral medulla oblongata. Around SSNNs, substance P-, enkephalin-, neuropeptide Y-and somatostatin-IR nerve fibers were very rich and tyrosine hydroxylase (TH)-, serotonin (5-HT)-, vasoactive intestinal polypeptide- and calcitonin gene-related peptide (CGRP)-IR axons showed moderate density. Thyrotropin-releasing hormone-containing axons were scarce in this region. The electron microscopic examinations revealed that CTB-IR structures directly received synaptic input from axon varicosities IR for TH, 5-HT and all neuropeptides except for CGRP. These findings suggest that catecholamine, 5-HT and the neuropeptides directly influence the activity of SSNNs and are concerned with the autonomic regulation of nasal and palatal mucosa, lacrimal glands and cerebral blood vessels of the rat.

Serotoninergic, peptidergic and GABAergic innervation of the ventrolateral and dorsolateral motor nuclei in the cat S1/S2 segments: an immunofluorescence study

Indirect single- and double-staining immunofluorescence techniques were used to study the serotoninergic, peptidergic and GABAergic innervation of the ventrolateral (Onuf's nucleus) and dorsolateral (innervating intrinsic foot sole muscles) nuclei, located in the S1/S2 segments of the cat spinal cord. The relative density of 5-hydroxytryptamine-, thyrotropin-releasing hormone-, substance P- and gamma-aminobutyric acid-immunoreactive axonal varicosities was similar in both nuclei. The highest relative density was recorded for varicosities immunoreactive to gamma-aminobutyric acid, while those immunoreactive to 5-hydroxytryptamine or thyrotropin-releasing hormone yielded the lowest values. The density of enkephalin-immunoreactive varicosities was higher in the ventrolateral than in the dorsolateral nucleus. Calcitonin gene-related peptide-like immunoreactivity could be seen in neurons of the ventrolateral and dorsolateral nuclei. Occasionally, calcitonin gene-related peptide-immunoreactive axonal fibers were also encountered in these nuclei. Virtually all thyrotropin-releasing hormone-immunoreactive varicosities in the ventrolateral and dorsolateral nuclei also contained 5-hydroxytryptamine-like immunoreactivity, while a somewhat smaller number of them were co-localized with substance P. About 5-10% of the 5-hydroxytryptamine-immunoreactive varicosities were devoid of peptide-like immunoreactivity, and the number of 5-hydroxytryptamine-immunoreactive varicosities lacking thyrotropin-releasing hormone-like immunoreactivity was higher in the dorsolateral than in the ventrolateral nucleus. Finally, the free fraction of substance P-immunoreactive varicosities, i.e., those lacking both 5-hydroxytryptamine and thyrotropin-releasing hormone, was about 39% in the ventrolateral and 26% in the dorsolateral nucleus. Spinal cord transection at the lower thoracic level induced a depletion of 5-hydroxytryptamine and thyrotropin-releasing hormone-immunoreactive fibers from the ventrolateral and dorsolateral nuclei, indicating an exclusive supraspinal origin for these fibers. A reduction in substance P-like immunoreactivity following spinal cord transection alone or spinal cord transection combined with unilateral dorsal rhizotomy was also detected in both nuclei, suggesting a dual origin for substance P-immunoreactive fibers, i.e., both supra- and intraspinal. The decrease in number of substance P-immunoreactive fibers was however smaller than expected from the analysis of the fraction of substance P-immunoreactive fibers co-localized with 5-hydroxytryptamine, indicating thus that the experimental lesions may have triggered a sprouting of substance P-immunoreactive axons originating from spinal cord sources. The distribution of gamma-aminobutyric acid in the ventrolateral and dorsolateral nuclei was not affected by the different lesion paradigms. It is therefore assumed that these inputs are intrinsic to the spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical analysis of the relation between 5-hydroxytryptamine- and neuropeptide-immunoreactive elements in the spinal cord of an amphibian (Xenopus laevis)

In mammals, a large proportion of the bulbospinal 5-hydroxytryptamine (5-HT) neurons also contain neuropeptides, such as substance P (SP) and galanin (GAL). To examine whether a similar coexistence occurs in an amphibian, an immunofluorescence double-labelling technique was employed on sections of the Xenopus laevis spinal cord. Antisera raised against SP, GAL, enkephalin (ENK), corticotropin-releasing factor (CRF), calcitonin gene-related peptide (CGRP), and cholecystokinin (CCK) produced a labelling of fibers at all rostrocaudal levels of the spinal cord, with the highest fiber densities for SP and ENK and intermediate densities for GAL, CCK, and CGRP, while CRF-immunoreactive fibers were barely detectable in intact animals. 5-HT-immunoreactive fibers were widely distributed in the spinal cord, and they often occurred in the vicinity of different types of peptide-immunoreactive fibers. However, no coexistence between 5-HT and the different peptide immunoreactivities could be detected, although SP and GAL immunoreactivities were sometimes found to be colocalized in the same fiber. Similar negative results were obtained when 5-HT+SP- and 5-HT+GAL-labelled sections were examined in single focal planes with a confocal microscope. After a spinal transection, (survival period 6 weeks to 4 months), almost all 5-HT-immunoreactive fibers below the lesion were lost, and a build-up of immunoreactive material occurred in fibers just rostral to the cut. In contrast, no significant loss of peptide-immunoreactive fibers occurred, although some swollen SP-, GAL-, ENK-, CRF-, and CCK-immunoreactive fibers were present rostral to the cut. The distribution of swollen peptide-immunoreactive fibers did not overlap with that of the swollen 5-HT-immunoreactive fibers. Although negative immunohistochemical data must be interpreted with caution, in conjunction with previous studies (Brodin et al. [1988] J. Comp. Neurol. 271:1-18; Sakamoto and Atsumi [1991] Cell Tissue Res. 264:221-230), the present results indicate that bulbospinal 5-HT neurons in nonmammalian vertebrates cocontain neuropeptides to a lesser extent than in mammals.

Thyrotropin-releasing hormone (TRH)-induced facilitation of spinal neurotransmission: a role for NMDA receptors

Thyrotropin-releasing hormone (TRH) is known to enhance spinal reflexes and modulate NMDA receptors in supraspinal areas. We have investigated the relationship between TRH and NMDA receptors in the spinal cord of alpha-chloralose-anaesthetized spinalized rats. TRH was tested (a) on dorsal horn neurone responses to iontophoretic NMDA, AMPA and kainate and (b) on spinal reflexes evoked by noxious pinch and electrical stimulation (2 Hz) shown to involve NMDA receptor activation. TRH given i.v. (0.5 mg/kg) or iontophoretically selectively potentiated neuronal responses to NMDA. TRH (0.5-2 mg/kg) also dose-dependently increased single motor unit reflex responses. The NMDA antagonist ketamine (2 mg/kg i.v.) abolished these TRH effects; ketamine reduced single motor unit (SMU) reflex responses more effectively when administered after TRH than in pre-TRH control tests. These results indicate that TRH-induced facilitation of spinal sensory transmission involves NMDA receptor activation.

Distribution and co-localization of 5-hydroxytryptamine, thyrotropin-releasing hormone and substance P in the cat medulla

This study demonstrates the co-existence of three neurochemicals in ventral medullary neurons of the cat utilizing fluorescence immunohistochemistry. Neurons containing 5-hydroxytryptamine, thyrotropin-releasing hormone and substance P were identified within the rostrocaudal extent of the medulla, specifically within the raphe pallidus and raphe magnus and in the reticular formation of the ventrolateral medulla in the nucleus paragigantocellularis lateralis. Within the raphe pallidus the majority of 5-hydroxytryptamine-containing neurons were co-localized with thyrotropin-releasing hormone and substance P. However, in the raphe magnus the majority of stained neurons contained 5-hydroxytryptamine and thyrotropin-releasing hormone but were devoid of substance P. In the ventrolateral medulla two major populations of neurons were identified rostral to the inferior olivary nuclei, one containing 5-hydroxytryptamine and thyrotropin-releasing hormone, while a more lateral group contained substance P alone. More caudally, at the level of the inferior olives, the majority of 5-hydroxytryptamine-containing cells also displayed immunoreactivity for thyrotropin-releasing hormone and substance P. A consistent finding in both the ventromedial and ventrolateral regions of the medulla was a population of 5-hydroxytryptamine-containing cells which did not stain for either thyrotropin-releasing hormone or substance P. The functional role of co-localized neurochemicals remains unknown but co-existence of neurotransmitter substances in medullary neurons may allow for specific and multiple actions in the spinal cord.

Thyrotropin-releasing hormone (TRH) neurons sprout in cervical spinal cord of Wobbler mouse

The present study was undertaken to quantify the immunocytochemical changes for thyrotropin-releasing hormone (TRH) within the ventral horn of the cervical spinal cord from Wobbler (wr/wr) mice selected at postnatal ages 3 weeks to 5 months compared with the normal phenotype (NFR/wr) littermates as well as mice from two related normal mouse strains: the NFR/N parent strain, and the closely related C57B1/6N mouse strain. The immunoreactive (IR) neuronal processes containing TRH appeared in all specimens within Rexed's laminae VIII, IX, and X. Compared with the normal (C57B1/6N, NFR/N) specimens, the pair-matched normal phenotype (NFR/wr) and Wobbler (wr/wr) specimens possessed significantly greater numbers of IR-TRH containing processes at every age studied. Compared with the normal phenotype (NFR/wr) specimens, greater numbers of IR-TRH containing processes appeared in the ventral horn region studied from the Wobbler (wr/wr) specimens taken early (Stage 1) as well as later (Stages 3 and 4) in the motoneuron disease. An age-related decline in the number of IR-TRH processes was apparent among the specimens from the Wobbler mouse strain (NFR/wr, wr/wr), but not the normal (NFR/N, C57B1/6N) mouse strains. The data suggest that TRH may play a significant role in the Wobbler disease, possibly even before the symptoms become apparent. In addition strain-related differences exist which may be important to the etiology of the Wobbler disorder.

A study of the barium-sensitive and -insensitive components of the action of thyrotropin-releasing hormone on lumbar motoneurons of the rat isolated spinal cord

The electrophysiological action of thyrotropin-releasing hormone (TRH) on rat spinal motoneurons was studied in vitro using single-electrode voltage- and current-clamp techniques. In current-clamp conditions TRH elicited a slowly developing depolarization, associated with a large input resistance increase and sustained neuronal firing; the primary metabolites of TRH were ineffective. Under voltage-clamp conditions in the presence of tetrodotoxin, TRH evoked a large inward current (ITRH; peaking at approximately -40 mV) associated with a large input conductance fall. Only 44% of cells displayed ITRH reversal; when the chord conductance values of these cells were plotted against membrane potential, a bell-shaped relation occurred, indicating voltage-dependent block by TRH of a persistent conductance active over a wide range of membrane potentials. ITRH reversal values were shifted to more positive levels in high K+ solution in Nernstian fashion; hence a large proportion of the TRH response is suggested to be mediated by the block of a K+ conductance (IK(T)). IK(T) (and its voltage-dependent block by TRH) was resistant to certain K+ channel antagonists (tetraethylammonium, Cs+, 4-aminopyridine or apamin), but was depressed by Ba2+. The Ba(2+)-resistant fraction of ITRH was attenuated by Cd2+, Mn2+ or Co2+, indicating that it probably involved a Ca(2+)-sensitive inward current. Concomitant application of Ba2+ and Cd2+ induced a near-total block of the response to TRH. It is suggested that suppression of IK(T), associated with the onset of a Ca(2+)-sensitive current, can explain the excitatory effect of TRH on rat spinal motoneurons.

Response of neurons in the dorsal motor nucleus of the vagus to thyrotropin-releasing hormone

Autonomic motoneurons in the dorsal motor nucleus of the vagus (DMX) were recorded intracellularly in an in vitro slice preparation of the guinea pig brainstem. Bath-applied thyrotropin releasing hormone (TRH) (1-10 microM) induced a reversible depolarization of neurons that was typically accompanied by an increase in the spontaneous firing of the cells. In some cells, TRH induced rhythmic bursting activity. The TRH-induced depolarization occurred also in the presence of reduced Ca2+ and TTX. The response was dose-dependent over TRH concentrations of 0.1-10 microM. The TRH-induced depolarization was accompanied by an increase in input resistance. The reversal potential of this effect corresponded to that of K+. Our results indicate that TRH increases the excitability of DMX neurons by reducing a resting K+ conductance.

The sites of origin of serotoninergic afferent fibers in the trigeminal motor, facial, and hypoglossal nuclei in the rat

The sites of origin of serotoninergic afferents in the trigeminal motor (Vm), facial (VII), and hypoglossal nuclei (XII) were studied in the rat by fluorescent retrograde labeling with Fluoro-Gold, in combination with immunofluorescence histochemistry for serotonin (5-HT). The results indicated: (1) The nucleus raphe magnus, nucleus raphe pallidus, and nucleus raphe obscurus contained 5-HT neurons projecting to the Vm, VII or XII. (2) The nucleus raphe dorsalis sends 5-HT fibres to the Vm and VII, but not to the XII. (3) The gigantocellular reticular nucleus pars alpha contained 5-HT neurons projecting to the VII.

Glutamatergic and non-glutamatergic responses evoked in neonatal rat lumbar motoneurons on stimulation of the lateroventral spinal cord surface

The effects on lumbar motoneurons of thoracic cord stimulation were investigated in the neonatal rat hemisected spinal cord in vitro using intracellular recording. Four responses were evoked--a fast, excitatory postsynaptic potential, a second component to the fast excitatory postsynaptic potential, a fast inhibitory postsynaptic potential and a slow excitatory postsynaptic potential. The fast (CNQX-sensitive) excitatory postsynaptic potential was probably monosynaptic, was blocked by CNQX, (10 microM) and showed a frequency-dependent run-down at stimulation frequencies between 0.1 and 1 Hz. A slower component to the fast excitatory postsynaptic potential ((+-)-2-amino-5- phosphono-valeric acid-sensitive excitatory postsynaptic potential) was blocked by (+-)-2-amino-5-phosphonovaleric acid (50 microM). Following fast excitatory postsynaptic potential blockade with both CNQX and (+-)-2-amino-5-phosphonovaleric acid, a fast inhibitory postsynaptic potential was revealed. This reversed at a membrane potential close to resting and was incompletely blocked by either bicuculline (30 microM) or strychnine (10 microM). The slow excitatory postsynaptic potential was a delayed depolarization associated with a small increase in input resistance (20%) and was insensitive to block by CNQX and/or (+/-)-2-amino-5-phosphonovaleric acid. It increased in amplitude on membrane depolarization and decreased on hyperpolarization and was potentiated by cocaine (3 microM) and citalopram (0.1 microM), but not by desipramine (5 microM). The slow excitatory postsynaptic potential was blocked by ketanserin (1 microM) and by LY 53857 (1 microM). It is concluded that a non-glutamatergic transmitter is involved in generating the slow excitatory postsynaptic potential possibly 5-hydroxytryptamine acting at 5-hydroxytryptamine 2 receptors.

Organization of the serotonergic innervation of spinal neurons in rats--III. Differential serotonergic innervation of somatic and parasympathetic preganglionic motoneurons as determined by patterns of co-existing peptides

The spinal cord is innervated by brainstem serotonergic neurons, some of which contain substance P and/or thyrotropin-releasing hormone in addition to serotonin. These neurons project at least three types of axons to the spinal cord: those containing both substance P and thyrotropin-releasing hormone, those containing thyrotropin-releasing hormone but not substance P, and those containing neither substance P nor thyrotropin-releasing hormone. However, the organization of the different types of serotonergic processes is unclear. In the present studies, the types of serotonergic axons projecting to two kinds of spinal neurons were examined. Somatic and parasympathetic preganglionic motoneurons were labeled retrogradely from the pelvic or sciatic nerve, respectively. Sections containing these neurons were stained either for serotonin and substance P, or for serotonin and thyrotropin-releasing hormone. Of a total of 428 profiles examined that were retrogradely labeled from the sciatic nerve, 425 (99%) were apposed by serotonin-immunoreactive varicosities; similarly, of a total of 382 profiles examined that were retrogradely labeled from pelvic nerve, 353 (92%) were apposed by serotonin-immunoreactive varicosities. However, differences appeared to exist between the types of serotonergic varicosities innervating these two groups of neurons. Among the profiles labeled from the sciatic nerve, it was estimated that over 97% were apposed by serotonin-immunoreactive varicosities in which serotonin co-existed with substance P and thyrotropin-releasing hormone. In contrast, among the profiles labeled from pelvic nerve that were apposed by serotonin-immunoreactive varicosities, it was estimated that less than 1% were apposed by serotonin-immunoreactive varicosities containing both thyrotropin-releasing hormone and substance P. We estimate that most of the remainder (about 80%) were apposed by serotonin-immunoreactive varicosities containing thyrotropin-releasing hormone but not substance P. We conclude that both the cell bodies of neurons retrogradely labeled from the pelvic nerve and those labeled from the sciatic nerve were apposed by serotonin varicosities. However, these two systems of neurons appear to be innervated largely by two different populations of serotonergic cells. This suggests that the raphe-spinal serotonergic system may independently modulate the activities of somatic motoneurons and parasympathetic preganglionic motoneurons.

6-Hydroxydopamine treatment of neonatal rats. II. Effects on the development of the hindlimb flexor reflex

The flexor reflex (FR) of the hindlimb was measured in rats at various ages between postnatal days (PND) 12-45, in vehicle-treated control rats and in rats treated with 6-hydroxydopamine (6-OHDA) (100 mg/kg, i.p.) on the first and second days after birth. The hindlimb FR was elicited by graded electrical stimulation of the footpad (0.1-2.0 mA) and quantified by the amplitude of the flexion in grams. The half maximal FR response was evoked by 1.2 mA and was 2-3 g in animals at PND 12-17 and 9-10 g in animals on PND 30 and 45. A similar age dependency was evident in the maximum hindlimb FR evoked by 2.0 mA; the maximum FR was 4.7 +/- 0.5 g on PND 12 and 24 +/- 2 g on PND 45. In rats treated with 6-OHDA, the strength of the FR was similar to that of the controls up to PND 15. However, the FR was increased 25% by PND 17 and 200% by PND 45 in the 6-OHDA-treated animals, versus controls. Pretreatment with clonidine (100 micrograms/kg, i.p.), which activates alpha 1 receptors under the current experimental conditions, did not enhance the FR in control animals. However clonidine pretreatment caused an increase of 400-800% in the FR in the 6-OHDA-treated animals at PND 15 and beyond. Our companion paper demonstrated that as early as PND 5, there was a significant increase (P < 0.05) in alpha 1 receptors in the spinal cord of 6-OHDA-treated animals, versus the controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Substance P and TRH share a common effector pathway in rat spinal motoneurones: an in vitro electrophysiological investigation

The actions of substance P and thyrotropin-releasing hormone (TRH) on neonatal rat spinal motoneurones in vitro were compared using intracellular current and voltage clamp techniques. Like TRH, substance P evoked a slowly-developing, persistent depolarisation plus an increase in input resistance under current clamp conditions. Under voltage clamp conditions, substance P elicited an inward current (mainly due to a conductance block) which peaked near -40 mV and reversed polarity close to the estimated EK. A distinct conductance increase (with a reversal potential near zero) also appeared to contribute to this response. The response to substance P at resting potential was suppressed by 1.5 mM Ba2+, but not by 20 mM tetraethylammonium, 2 mM 4-aminopyridine, 2 mM Cs+ and 0.2 mM Cd2+. In addition, co-application of TRH and substance P mutually occluded each other. Thus, it is suggested that substance P and TRH share a common effector mechanism, which primarily involves the suppression of IK(T), a persistent K+ current recently discovered in these neurones.

Quantitative synaptology of functionally different types of cat medial gastrocnemius alpha-motoneurons

The aim of this ultrastructural investigation was to study quantitatively the synaptology of the cell bodies and dendrites of cat medial gastrocnemius (MG) alpha-motoneurons of functionally different types. In electrophysiologically classified and intracellularly HRP-labelled MG alpha-motoneurons of the FF (fast twitch, fatigable), FR (fast twitch, fatigue resistant) and S (slow twitch, very fatigue resistant) types, the synaptic covering of the soma as well as that of dendritic segments located within 100 microns and at 300, 700, and 1,000 microns distance, respectively from the soma, was analyzed. The synaptic boutons were classified into the L-(apposition length > 4 microns) and S-types (< 4 microns) with spherical synaptic vesicles, and the F-type with flat or pleomorphic synaptic vesicles. The length of apposition towards the motoneuron membrane was measured for each bouton profile. Approximately 1,000 boutons contacted the soma and a similar number of boutons contacted the proximal dendrites within 50 microns from the soma. The number of dendritic boutons was larger at the 300 microns distance than at the 100 and 700 microns distances. The three types of motoneurons showed similar values for percentage synaptic covering and synaptic packing density in the proximal dendrites, while in the most distal dendritic regions the S motoneurons had more than 50% higher values for percentage covering, packing density and total number of boutons. The S motoneurons also exhibited a larger preponderance of F-type boutons on the soma. The ratio between the F- and S-types of boutons decreased somatofugally along the dendrites in the type FF and FR motoneurons, while in the S motoneurons it remained fairly constant.

Modulatory effects of achatin-I, an Achatina endogenous neuroactive peptide, on responses to 5-hydroxytryptamine

Achatin-I (Gly-D-Phe-L-Ala-L-Asp) was found in the ganglia of an African giant snail (Achatina fulica Férussac), and proposed as an excitatory neurotransmitter of Achatina neurones. At 3 x 10(-6) the peptide markedly enhanced the fast inward current (Iin) of an Achatina neurone type, TAN (tonically autoactive neurone), produced by the pneumatic pressure ejection of 5-HT. This Iin was facilitated immediately by the achatin-I perfusion, and the facilitation decreased gradually even with the peptide present. The dose (duration)-response curves of the TAN fast Iin on pressure ejection in the absence (control) and presence of achatin-I at 3 x 10(-6) M (n = 8) were analyzed as follows. The ED50 (95% confidence limit) were 59.3 ms (13.9-95.3 ms) for the control, and 36.5 ms (19.5-52.6 ms) for achatin-I. The Emax were 1.06 +/- 0.11 nA for the control, and 1.74 +/- 0.26 nA for achatin-I (P < 0.01 for paired data). Among achatin-I derivatives, achatin-II (Gly-L-Phe-L-Ala-L-Asp) enhanced the TAN fast response to 5-HT, but was ten times weaker than achatin-I. [L-Glu4]achatin-I (Gly-D-Phe-L-Ala-L-Glu) and achatin-I amide (Gly-D-Phe-Ala-L-Asp-NH2) had no facilitatory effect. We propose that achatin-I is a neuromodulator as well as a neurotransmitter for Achatina giant neurones.

Pre- and post-natal ontogeny of thyrotropin-releasing-hormone in the rat spinal cord: an immunocytochemical study

This work aimed at providing by means of immunocytochemical techniques a detailed study of the ontogeny of thyrotropin-releasing hormone (TRH) in the spinal cord of the rat. We report the first appearance of TRH-immunoreactive fibers in the ventral funiculus of thoracic and lumbar levels at embryonic day 17. At embryonic day 18, fibers penetrated the ventral gray matter towards the central canal. At embryonic day 19, the first immunoreactive fibers were seen in the intermediolateral cell column at upper thoracic levels. This region was invaded at lower thoracic levels on the day of birth. At this time, TRH-immunoreactive axodendritic synapses were observed in the ventral horn and in the intermediolateral cell column. Immunoreactivity increased in these regions until post-natal day 21 when the adult pattern of TRH immunoreactivity was established in the sympathetic nuclei and in the ventral horn. However, a transient TRH-like immunoreactivity was detected in lamina IIi of the dorsal horn between post-natal days 14 and 30: at ultrastructural level, immunoreactive varicosities were seen to establish axodendritic synapses. In conclusion, TRH is one of the earliest peptidergic systems established in the spinal cord and it presents extensive temporal and topographical similarities with the serotonergic system with which it could be colocalized.

Distribution of thyrotropin-releasing hormone receptor messenger RNA in the rat brain: an in situ hybridization study

Based on the recent cloning of the mouse thyrotropin-releasing hormone receptor, oligonucleotide probes complementary to the DNA sequence were constructed and used for in situ hybridization studies on the rat brain. Thyrotropin-releasing hormone receptor messenger RNA was found in many areas of the brain, mostly showing high degree of overlap with the distribution thyrotropin-releasing hormone binding sites as previously revealed in autoradiographic studies. Thus, a strong signal was observed in the accessory olfactory bulb, the perirhinal sulcus, the ventral aspects of the hippocampal formation, some amygdaloid nuclei, the diagonal band nucleus, parts of nucleus accumbens, the bed nucleus of the stria terminalis, dorsomedial, lateral and perifornical hypothalamic regions, the septohippocampal nucleus, parts of the vestibular complex, as well as many bulbar motoneurons including the facial, dorsal vagal, ambiguus and hypoglossal nuclei, the superficial layer of the spinal trigeminal nucleus, and motoneurons and dorsal horn neurons in the spinal cord. Cells within one and the same nucleus expressed varying levels of thyrotropin releasing hormone receptor messenger RNA suggesting marked differences in rate of receptor synthesis. Most of these areas receive an input by thyrotropin-releasing hormone-positive nerve endings. Taken together these results suggest that thyrotropin-releasing hormone receptors are mostly localized in the vicinity of the cell bodies which express thyrotropin-releasing hormone receptor messenger RNA and mediate the wide range of actions that have been recorded after administration of exogenous thyrotropin-releasing hormone.

Alteration in the levels of thyrotropin releasing hormone, substance P and enkephalins in the spinal cord, brainstem, hypothalamus and midbrain of the Wobbler mouse at different stages of the motoneuron disease

The present study was undertaken to quantify selected neuropeptides (thyrotropin releasing hormone, substance P, methionine and leucine enkephalin) in the cervical spinal cord and other regions of the central nervous system of Wobbler mice by radioimmunoassays during several stages of the motoneuron disease compared with age- and sex-matched normal phenotype littermates. In Wobbler spinal cord, thyrotropin releasing hormone is higher early in the disease, whereas in the brainstem it is higher at a later stage. Substance P in spinal cord is also higher late in the disease. Leucine enkephalin levels are greater at all stages in diseased spinal cord and brainstem, but methionine enkephalin increases only late in the disease. Highly significant increases of the peptides (except thyrotropin releasing hormone) appear in hypothalamus and midbrain only late in the motoneuron disease. Regression analyses show that thyrotropin releasing hormone in spinal cord and brainstem decreases normally with age in the control mice and at a faster rate related to the extent of motor impairment in Wobbler mice. Thyrotropin releasing hormone and methionine enkephalin in the Wobbler brainstem correlate (P less than 0.05) with the progress of the motoneuron disease. Methionine enkephalin increases faster in Wobbler brainstem and decreases faster in control spinal cord with age. The increase of leucine enkephalin in the Wobbler spinal cord correlates significantly with age and with the progress of the disease, but leucine enkephalin declines slightly with age in the controls. The changes of substance P in spinal cord and brainstem do not correlate significantly with the progress of the disease. In the hypothalamus, increasing values for substance P in control specimens and enkephalins in Wobbler specimens are significantly correlated with age. However, in the midbrain, higher methionine and leucine enkephalin levels are significantly associated with age only in the control mice. Alterations of neuropeptides in the Wobbler mouse spinal cord and brainstem may result from the degeneration of bulbospinal raphe neurons projecting to the ventral spinal cord, or from primary afferent or interneuronal nerve terminals. The data imply that the neuronal degeneration process in the Wobbler motoneuron disease is not limited to motoneurons. In the spinal cord, the data support our previous hypothesis that neuronal sprouting presynaptic to the motoneurons may account for increased neuropeptide concentrations. Alternatively, synthesis and/or degradation of these peptides may be altered. In addition, it is proposed that enkephalinergic neurons may develop abnormally in Wobbler mice. The early increase of leucine enkephalin in the Wobbler spinal cord possibly indicates its importance in the etiology of the motoneuron disease.

Identification of neurons expressing thyrotropin releasing-hormone receptor mRNA in spinal cord and lower brainstem of rat

The distribution of mRNA coding for a pituitary thyrotropin releasing-hormone (TRH) receptor was examined on sections of spinal cord and lower brainstem of rat using in situ hybridization. Hybridization signals were observed over large neurons in the ventral horn in cervical, thoracic, and lumbar segments of spinal cord, and over neurons in the motor nuclei of the lower brainstem. Although significant thyrotropin-releasing hormone binding has been reported in the superficial dorsal horn, only background levels of hybridization were observed over neurons in this region. These findings suggest that mRNA coding for thyrotropin-releasing hormone receptor is expressed in some spinal and brainstem motor neurons. Since many of these neurons are innervated by TRH-containing afferents, TRH may exert a direct effect upon at least some of these cells.

Antinociception induced by microinjection of substance P into the A7 catecholamine cell group in the rat

Stimulation of neurons in the ventromedial medulla produces antinociception that is mediated in part by indirect activation of pontospinal noradrenergic neurons. Substance P-containing neurons located in the ventromedial medulla project to the A7 catecholamine cell group and may serve as an excitatory link between these two cell groups. Thus, the antinociception induced by stimulation of the neurons in ventromedial medulla may be mediated by substance P released from these projections which activates spinally projecting noradrenergic neurons in the A7 cell group. This hypothesis was tested by determining whether microinjection of various doses of substance P into the A7 cell group of the rat could induce antinociception. The results indicated that substance P induced dose-dependent antinociception that was more pronounced in the hindlimb ipsilateral to the microinjections. This observation is consistent with anatomical observations that noradrenergic A7 neurons project predominantly to the ipsilateral spinal cord dorsal horn. Moreover, the antinociceptive effects of substance P microinjection appear to be mediated at least in part by activation of spinally projecting noradrenergic neurons in the A7 cell group, because intrathecal injections of the alpha-2 noradrenergic antagonists yohimbine and idazoxan blocked these antinociceptive effects. The results of these experiments support the hypothesis that the antinociception induced by stimulation of neurons in the ventromedial medulla is mediated in part by activation of substance P-containing neurons that project to, and activate, spinally projecting noradrenergic neurons located in the A7 catecholamine cell group.