Influence of serotonin antagonists on bulbospinal systems

Cortical reorganization after spinal cord injury: always for good?

Plasticity constitutes the basis of behavioral changes as a result of experience. It refers to neural network shaping and re-shaping at the global level and to synaptic contacts remodeling at the local level, either during learning or memory encoding, or as a result of acute or chronic pathological conditions. 'Plastic' brain reorganization after central nervous system lesions has a pivotal role in the recovery and rehabilitation of sensory and motor dysfunction, but can also be "maladaptive". Moreover, it is clear that brain reorganization is not a "static" phenomenon but rather a very dynamic process. Spinal cord injury immediately initiates a change in brain state and starts cortical reorganization. In the long term, the impact of injury - with or without accompanying therapy - on the brain is a complex balance between supraspinal reorganization and spinal recovery. The degree of cortical reorganization after spinal cord injury is highly variable, and can range from no reorganization (i.e. "silencing") to massive cortical remapping. This variability critically depends on the species, the age of the animal when the injury occurs, the time after the injury has occurred, and the behavioral activity and possible therapy regimes after the injury. We will briefly discuss these dependencies, trying to highlight their translational value. Overall, it is not only necessary to better understand how the brain can reorganize after injury with or without therapy, it is also necessary to clarify when and why brain reorganization can be either "good" or "bad" in terms of its clinical consequences. This information is critical in order to develop and optimize cost-effective therapies to maximize functional recovery while minimizing maladaptive states after spinal cord injury.

Serotonergic pharmacotherapy promotes cortical reorganization after spinal cord injury

Cortical reorganization plays a significant role in recovery of function after injury of the central nervous system. The neural mechanisms that underlie this reorganization may be the same as those normally responsible for skilled behaviors that accompany extended sensory experience and, if better understood, could provide a basis for further promoting recovery of function after injury. The work presented here extends studies of spontaneous cortical reorganization after spinal cord injury to the role of rehabilitative strategies on cortical reorganization. We use a complete spinal transection model to focus on cortical reorganization in response to serotonergic (5-HT) pharmacotherapy without any confounding effects from spared fibers left after partial lesions. 5-HT pharmacotherapy has previously been shown to improve behavioral outcome after SCI but the effect on cortical organization is unknown. After a complete spinal transection in the adult rat, 5-HT pharmacotherapy produced more reorganization in the sensorimotor cortex than would be expected by transection alone. This reorganization was dose dependent, extended into intact (forelimb) motor cortex, and, at least in the hindlimb sensorimotor cortex, followed a somatotopic arrangement. Animals with the greatest behavioral outcome showed the greatest extent of cortical reorganization suggesting that the reorganization is likely to be in response to both direct effects of 5-HT on cortical circuits and indirect effects in response to the behavioral improvement below the level of the lesion.

Serotonergic fibers induce a long-lasting inhibition of monosynaptic reflex in the neonatal rat spinal cord

The transmitter mechanism of a long-lasting descending inhibition of the monosynaptic reflex was investigated in the isolated spinal cord of the neonatal rat. The monosynaptic reflex elicited by dorsal root stimulation was recorded extracellularly from a lumbar ventral root (L3-L5). Electrical stimulation of the upper thoracic part of the hemisected cord caused an inhibition lasting about 40 s of the monosynaptic reflex. This descending inhibition was markedly attenuated by perfusing the spinal cord with reserpine (1 microM) or 5,7-dihydroxytryptamine (10 microM) for 2-6 h. The perfusion with reserpine (1 microM) for 4 h significantly decreased the contents of 5-hydroxytryptamine, dopamine, and norepinephrine of the neonatal rat spinal cord, whereas the perfusion with 5,7-dihydroxytryptamine (10 microM) for 4 h decreased the contents of 5-hydroxytryptamine and dopamine. The descending inhibition was markedly potentiated by a 5-hydroxytryptamine uptake blocker, citalopram (10 nM), and was blocked by a 5-hydroxytryptamine antagonist, ketanserin (10-100 nM). Application of 5-hydroxytryptamine to the spinal cord induced an inhibition of the monosynaptic reflex, a later part of which was blocked by ketanserin. Ketanserin also moderately blocked inhibitions of the monosynaptic reflex caused by norepinephrine and dopamine. Phentolamine (10 microM) abolished the depressant actions of norepinephrine and dopamine, but did not affect that of 5-hydroxytryptamine or the descending inhibition. These results strongly suggest the involvement of 5-hydroxytryptamine, but not dopamine nor norepinephrine, in the descending inhibition. Besides ketanserin, the descending inhibition was blocked by ritanserin, haloperidol, and pipamperone, which have affinities to 5-hydroxytryptamine2 receptors, and also by spiperone and methiothepin, which are antagonists at both 5-hydroxytryptamine1 and 5-hydroxytryptamine2 receptors (all 1 microM). On the other hand, a 5-hydroxytryptamine1C and 5-hydroxytryptamine2 antagonist, mesulergine (1 microM), and 5-hydroxytryptamine3 antagonists, ICS 205-930 and quipazine (both 1 microM), did not depress either the descending inhibition or the 5-hydroxytryptamine-evoked inhibition of the monosynaptic reflex. The results with these antagonists favor the involvement of 5-hydroxytryptamine2 receptors although the results with mesulergine disagree with this notion. 5-Hydroxytryptamine1 agonists, such as 8-hydroxy-2-(di-n-propylamino)tetralin, buspirone, and 5-carboxyamidotryptamine, and a 5-hydroxytryptamine3 agonist, 2-methyl-5-hydroxytryptamine, induced a long-lasting inhibition of the monosynaptic reflex, which was blocked by ketanserin whereas a 5-hydroxytryptamine2 agonist, S-(+)-alpha-methyl-5-hydroxytryptamine, evoked a biphasic inhibition, in which only the later component was blocked by ketanserin.(ABSTRACT TRUNCATED AT 400 WORDS)

Evaluation of traumatic spinal cord edema using evoked potentials recorded from the spinal epidural space. An experimental study in the rat

Spinal cord evoked potentials (SCEP) elicited by simultaneous distal tibial and sural nerve stimulation were continuously recorded from the epidural space at the T9 and T12 levels of urethane anaesthetized rats before and after a unilateral incision (about 3 mm deep and 5 mm long) in the right dorsal horn of the T10-11 segments. The changes in SCEP were correlated with the increase in spinal cord water content measured 5 h after injury. In addition, the influence of serotonin (5-HT) in mediating such changes was explored using a pharmacological approach. The changes in SCEP immediately after injury correlated well with development of spinal cord edema measured 5 h after injury. Thus, the maximal negative peak (MNP) amplitude of SCEP decreased by an average of 64.0% immediately after injury and the water content of the spinal cord was increased from 71.6% (controls) to 77.6% 5 h after injury. Pretreatment with p-CPA (a serotonin synthesis inhibitor) prevented the initial decrease of the MNP amplitude and also the increase of water content (72.5%). On the other hand, pretreatment with cyproheptadine (a 5-HT2 receptor antagonist) enhanced both the initial decrease of the MNP amplitude as well as the increase of water content (81.3%). The results show a good correlation between changes of SCEP immediately after injury and the magnitude of spinal cord edema (r = 0.9) measured 5 h after injury. The findings reveal a major role of serotonin in mediating early changes of SCEP and later development of spinal cord edema and demonstrate a prognostic value of early SCEP recordings in predicting the final outcome of traumatic spinal cord injuries.

A biphasic excitability change in hindlimb motoneurons evoked by stimulation of the nucleus raphes magnus in the cat

1. The influence of electrical stimulation of the nucleus raphes magnus (RM) on spinal segmental systems were examined. 2. RM stimulation produced an initial increase and a subsequent suppression of the amplitude of both fiextor and extensor lumbar monosynaptic reflex potentials (MSRs). 3. Intracellular recordings were made from alpha-motoneurons of the common peroneal nerve (flexor) and the tibial nerve (extensor). RM stimulation evoked postsynaptic potentials with a time course similar to that of MSR facilitation. 4. RM stimulation inhibited the aggregate excitatory synaptic potential (EPSP) evoked by stimulation of group I afferent fibers without apparent changes in the motoneuronal membrane potential. 5. These data suggest that the RM-evoked biphasic effect on MSR consists of early facilitation due to EPSP, and late inhibition possibly due to presynaptic inhibition of group I afferent fibers.

Action of hallucinogens on raphe-evoked dorsal root potentials (DRPs) in the cat

The dorsal root potential (DRP) evoked by stimulation of the inferior central nucleus (ICN) of the cat is affected by administration of a variety of hallucinogenic agents. It has been previously shown that a single low dose of LSD is unique in that it potentiates this DRP, while injections of 5-methoxy-N,N- dimethyltryptamine (5-MeODMT), ketamine or phencyclidine (PCP) inhibit its production. Tolerance develops to the facilitatory effect of low doses of LSD on the DRP, but not to the inhibitory action of 5-MeODMT. Repeated injections of ketamine every 30 minutes also fail to produce tachyphylaxis to the inhibitory effect of this dissociative anesthetic. The raphe-evoked DRP is a long latency potential that is inhibited by a wide variety of putative serotonin antagonists and has therefore been traditionally thought to be mediated by serotonin. However, in light of the inability of either tryptophan or fluoxetine to potentiate this DRP, and the resistance of this DRP to blockade by parachlorophenylalanine, reserpine or intrathecally administered 5,7-dihydroxytryptamine, it appears that this potential may in fact be mediated, at least in part, by a non-serotonergic transmitter.

Effects of acute administration of 5-methoxy-N,N-dimethyltryptamine upon the latency and duration of post-decapitation convulsions

The effect of acute administration of rats with the 5-hydroxytryptamine (5-HT) agonist drug 5-methoxy-N,N-dimethyltryptamine on the convulsions released by decapitation was examined. The postsynaptic agonist, 5-methoxy-N,N-dimethyltryptamine, prolonged the latency and duration from the 0.5 mg/kg dose upwards. Methergoline, 2.0 mg/kg intraperitoneally injected immediately prior to 5-methoxy-N,N-dimethyltryptamine, caused some considerable blockade of the effects of the 5-HT agonist on post-decapitation convulsions (PDG's). Long-term p-chloroamphetamine (2x10 mg/kg) and p-chlorophenylalanine (1 x 300 mg/kg) did not antagonise the 5-methoxy-N-N-dimethyltryptamine induced changes of PDC's but, by themselves, prolonged PDC duration. The utility of the 5-methoxy-N,N-dimethyltryptamine-PDC method for studying 5-HT receptor mechanisms may be worth considering.

Collateralization of brainstem pathways in the spinal ventral horn in rat as demonstrated with the retrograde fluorescent double-labeling technique

The collateralization of brainstem pathways to the spinal ventral horn was studied in rat by means of injections of True Blue (TB) and Diamidino Yellow Dihydrochloride (DY .2HCl) at different levels in the spinal cord. TB (or DY .2HCl) was injected in the cervical gray and DY .2HCl (or TB) was injected ipsilaterally either at mid-thoracic or at lumbar levels. The retrogradely single- and double-labeled neurons were studied in the interstitial nucleus of Cajal, the lateral vestibular nucleus of Deiters, the nucleus (sub) coeruleus and the nucleus raphe pallidus, including the adjoining ventral medullary reticular formation. In all those brainstem nuclei many double-labeled neurons were present after both mid-thoracic and lumbar injections. This indicates that these brainstem spinal pathways to the ventral horn probably give off many collaterals along their trajectory in the spinal cord.

Acute and chronic effects of LSD and 5-MeODMT on raphe-evoked dorsal root potentials in the cat

Both acute and chronic effects of lysergic acid diethylamide (LSD) and 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) on the dorsal root potential (DRP), evoked by stimulation of the nucleus raphe magnus of the cat, were examined. Single injections of LSD potentiated while those of 5-MeODMT inhibited the raphe-evoked DRP. The electrophysiologic response produced by each drug correlates well in dosage and time-course with their reported behavioral effects. Following four consecutive daily injections of LSD, complete tolerance developed to the potentiating effect of LSD on this potential. A similar pretreatment schedule with 5-MeODMT failed to alter its acute inhibitory effect on the DRP. These results correlate well with the development of tolerance to the behavioral effects of LSD and 5-MeODMT. This system may thus provide a unique electrophysiological model to examine the effects of these drugs.

LSD but not methysergide reduces the inhibitory effect of the medullary raphe stimulation on the spinal reflex in rats

Conditioning stimulation of the nucleus raphe in the rat medulla enhanced the monosynaptic reflex (MSR) and depressed the polysynaptic reflex (PSR) in separate time courses. The PSR decreasing effect was reduced in preparations pretreated with reserpine, PCPA, or 5,6-DHT. The MSR increasing effect was not altered by these three pretreatments. Intravenously administered LSD but not methysergide reduced the PSR decreasing effect with no influence on the MSR increasing effect. These two drugs had similar effects on the unconditioned responses; the MSR was decreased and the PSR was increased. Thus, the PSR decreasing effect seems to be mediated via a serotonergic pathway probably descending from the nucleus raphe.

Motoneurons innervating the cremaster muscle of the rat are characteristically densely innervated by serotoninergic fibers as revealed by combined immunohistochemistry and retrograde fluorescence DAPI-labelling

The lumbar spinal cord of the rat was studied by combined retrograde fluorescent labelling with 4',6-diamidino-2-phenylindole-2HCl (DAPI) and immunoperoxidase procedure using serotonin antiserum. A peculiar small neuronal group endowed more densely than other anterior horn neurons with serotonin-like immunoreactive fibers was recognized in the anterior column of lumbar segments L1-L2. At the same time, this small nucleus was shown to contain the motoneurons innervating the cremaster muscle by means of retrograde labelling with DAPI. It is tentatively suggested that the bulbospinal descending serotonin system is particularly intimately connected with the function of the cremaster muscle.

Selective inhibition by methysergide of the monosynaptic reflex discharge in the isolated spinal cord of the newborn rat

In the isolated spinal cord of the newborn rat, methysergide and LSD-25 depressed the monosynaptic reflex discharge selectively. Cyproheptadine and dimethothiazine did not inhibit the monosynaptic reflex. The selective inhibitory effect of methysergide on the monosynaptic reflex was not due to a presumptive low safety factor of this reflex. The inhibition was restored under a condition such as the compound action potential in the dorsal root was enhanced by 4-aminopyridine. Methysergide did not decrease the sensitivity of the motoneuron to substance P and L-glutamic acid. It is suggested that methysergide acts at the presynaptic terminal of Ia afferent fibers and depresses evoked transmitter release.

Quantitative differences in collateralization of the descending spinal pathways from red nucleus and other brain stem cell groups in rat as demonstrated with the multiple fluorescent retrograde tracer technique

In 8 rats 'True Blue' was injected into dorsal half of C5-C8 spinal grey, 5 days later 'Nuclear Yellow' was injected in midthoracic, upper lumbar, lumbosacral and sacral cord respectively. The animals were sacrificed about 43 hours after NY injections. The distribution of retrogradely labeled neurons was studied in Red Nucleus, in Ventrolateral Pontine Tegmentum and in Nucleus Raphe Magnus, all of which project to spinal dorsal grey. In Red Nucleus large populations of single TB-labeled neurons and single NY-labeled ones occurred in the dorsomedial and ventrolateral part, respectively. In addition, about 8% of the neurons labeled with TB from C5-C8 were double labeled with NY from L5-S1, and 35% from T7-8, which percentages resemble those of electrophysiological studies. However, in ipsilateral Nucleus Raphe Magnus about 40% of the TB-labeled neurons were double labeled from L5-S1. This percentage resembles the 66% obtained in electrophysiological studies of reticulospinal collaterals. These findings in rat support electrophysiological findings in cat and show, that rubrospinal neurons distribute their fibers primarily to the grey matter of specific groups of spinal segments, while many of the raphe spinal neurons distribute fibers throughout the spinal cord.

Effects of raphe magnus and raphe pallidus lesions on morphine-induced analgesia and spinal cord monoamines

These studies examined the role of bulbospinal serotonin-containing neurons found in the nucleus raphe magnus and nucleus raphe pallidus in the mediation of morphine-induced antinociception. Lesions were made using both electrolytic coagulation and the axon-sparing technique of monosodium-L-glutamate injection to ascertain whether the effects following lesions in the area of the medullary raphe nuclei are due to destruction of neuronal perikarya or fibers passing near these nuclei. These studies revealed that lesions of both the raphe magnus and raphe pallidus resulted in decreased nociceptive thresholds and attenuation of morphine-induced analgesia. Such effects were observed regardless of the lesioning method used, which suggests that destruction of neurons in these nuclei was responsible for lesion-induced effects. In addition, lesion-induced changes in spinal cord serotonin content and morphine analgesia were significantly correlated which lends support to the conclusion that the bulbospinal serotonin systems are necessary for the mediation of morphine effects. Furthermore, no correlation was observed between changes in spinal cord norepinephrine content and morphine analgesia. This observation suggests that lesion-induced damage to bulbospinal noradrenergic fibers which pass near the midline does not contribute to the attenuation of morphine analgesia resulting from raphe lesions.

The role of GABA and serotonin in the mediation of raphe-evoked spinal cord dorsal root potentials

The possible involvement of bulbospinal serotonergic systems in the mediation of analgesia has created a need for a better understanding of the influence this system has on neuronal mechanisms in the spinal cord. Therefore, these studies were designed to examine the effects of caudal raphe stimulation on primary afferent depolarization and to determine the role of serotonin (5-HT) and GABA in the mediation of these stimulation-produced effects. Stimulation of the raphe evoked two electrotonically conducted dorsal root potentials (DRP-1 and DRP-2) and two compound action potentials (VRP-1 and VRP-2) which were recorded from the dorsal and ventral roots, respectively. Length constant measurements indicated that DRP-1 was generated in group II and DRP-2 in group I primary afferent fibers. Histological determination of stimulation sites revealed that short-latency potentials (DRP-1 and VRP-1) were evoked from many sites within the caudal brain stem, while the long-latency potentials (DRP-2 and VRP-2) were evoked primarily from sites within the caudal raphe nuclei. The role of serotonin in mediating these evoked potentials was assessed by administering various antagonists of serotonin (cinanserin, methysergide and D-lysergic acid diethylamide). These agents consistently attenuated the long-latency potentials (DRP-2 and VRP-2) but increased the magnitude of DRP-1. The possibility of a GABAergic neuron in the descending systems projecting to primary afferent terminals was studied. Depletion of GABA by semicarbazide blocked DRP-1, but had only a modest effect of DRP-2. However, the putative GABA antagonist, bicuculline, inhibited both DRP-1, and DRP-2. These results suggest that a GABA interneuron is not involved in the bulbospinal serotonergic depolarization of primary afferent terminals. This system appears to constitute a presynaptic filter of afferent input, with the capacity to inhibit different fiber groups.

The bulbo-spinal indoleaminergic pathway in the frog

The role of the bulbo-spinal indoleaminergic pathway of the frog was investigated. Using the isolated spinal cord preparation, responses of motoneurons of segments 9 and 10 to lateral column stimulation were recorded from ventral roots (LC-VRP) and by intracellularly placed microelectrodes. Control responses were compared to those obtained after addition of substances known to alter indoleaminergic synthesis or receptor activation. Responses from spinal cords of animals that were pretreated with indoleamine-depleting agents were compared to mean control responses. Procedures that inhibited indoleamine synthesis or blocked indoleamine receptors reduced motoneuronal activity. This was manifested as an increase in mean latency and a decrease in amplitude of the monosynaptic LC-VRP, an increased duration of suppression following an LC conditioning stimulus, and a decrease in spontaneous activity. In contrast to intracellularly recorded responses from control cords, those recorded under these conditions typically showed single spikes with longer latencies. Additions of indoleamine precursors to normal cords or to cords depleted of monoamines by reserpine shortened mean latencies, increased amplitudes of LC-elicited responses and caused an increase in spontaneous activity. These observations were recorded both extracellularly and intracellularly. Our results suggest that the bulbo-spinal indoleaminergic pathway modulates the output of motoneurons of the frog spinal cord.

The effect of morphine and some other narcotic analgesics on brain tryptophan concentrations

An acute dose of morphine increased brain tryptophan in mice. This effect was not prevented by naloxone nor was it produced by other narcotic analgesics. Dextrorphan, but not levorphanol, had a similar effect to morphine. A large dose of tryptophan had no effect on the antinociceptive action of morphine in mice. Morphine increased brain tryptophan in rats. This effect was prevented by naloxone. A large dose of tryptophan antagonised the antinociceptive action of morphine in the rat.

Differential effects of spinal cord lesions on narcotic and non-narcotic suppression of nociceptive reflexes: further evidence for the physiologic multiplicity of pain modulation

These studies examined the effects of bilateral lesions of the dorsolateral funiculus (DLF) of the rat spinal cord on the inhibition of a nociceptive reflex produced either by a systemic injection of 4 mg/kg of morphine or by a 20 sec exposure to 1.0 mA of transcutaneous electric shock. Reflex inhibition was quantified and analgesia inferred by use of a modified version of the D'Amour-Smith tail-flick test. Lesions which included only the DLF reduced morphine-produced analgesia (MA) by 73% but had no effect on shock-produced analgesia (SA) observed in the same rats. Baseline tail-flick latencies of this group were not affected by the lesions. Control lesions restricted to the dorsal columns attenuated neither MA nor SA. Lesions which included both the dorsal columns and DLF did not affect SA and produced no greater reduction in MA than lesions of the DLF alone. Previous work indicates that both MA and SA result, at least in part, from supraspinal activity. The current data indicate that: (1) supraspinal modulation participating in two different types of analgesic induction involves separate descending spinal pathways and (2) the maximal expression of analgesia produced by administration of narcotics requires the integrity of a supraspinal neural system projecting in the DLF.

The differential effects of 5-hydroxytryptamine, noradrenaline and raphe stimulation on nociceptive and non-nociceptive dorsal horn interneurones in the cat

The effects of 5-hydroxytryptamine (5-HT), noradrenaline (NA) and stimulation of the inferior central nucleus of the raphe (RN) were examined on nociceptive and non-nociceptive spinal neurones in anaesthetized cats. 5-HT reduced excitation evoked by noxious stimulation, but increased spontaneous firing and firing evoked by DL-homocysteic acid (DLH) on both nociceptive and non-nociceptive cells. NA reduced spontaneous activity, DLH-evoked excitation and excitation evoked by a noxious stimulus on nociceptive neurones, but had little action on non-nociceptive units. RN inhibited spontaneous, stimulus-evoked and DLH-evoked firing of nociceptive cells and caused briefer inhibitions of non-nociceptive cells. Excitatory effects were also observed. Strychnine antagonized short-duration inhibitions from RN of non-nociceptive cells responding to hair movement, but failed to antagonize any of the other effects of RN. No antagonism of the inhibitory effect of RN was observed with phenoxybenzamine, phentolamine, sotalol, bicuculline or methysergide. However, methysergide antagonized some excitatory effects of 5-HT and RN, but also produced non-specific actions on some cells. It was concluded that, although glycine may mediate some of the brief duration inhibitions evoked by RN, the longer duration inhibitions were unlikely to have been mediated by either glycine or GABA. 5-HT may be a mediator of raphe-spinal actions but may have presynaptic inhibitory actions coupled with postsynaptic excitatory effects. NA could mediate some descending inhibition of nociceptive neurones.

Serotonin involvement in the blockade of bulbospinal inhibition of the spinal monosynaptic reflex

Bulbospinal inhibition of the extensor quadriceps monosynaptic reflex (MSR) was antagonized by the serotonin precursor, 5-hydroxytryptophan (5-HTP, 75 mg/kg), in unanesthetized, mid-collicular, decerebrate cats. Fluoxetine HCl (Lilly 110140, 0.25 - 6 mg/kg), a specific serotonin neuronal uptake blocker, also blocked this inhibition as well as bulbospinal inhibition of the flexor posterior biceps-semi-tendinosus MSR. The serotonin antagonist, cyproheptadine HCl (5 mg/kg), partially reversed the above blocking actions of 5-HTP and fluoxetine and enhanced bulbospinal inhibition when administered alone in doses of 2.5-5 mg/kg. Imipramine HCl (0.125 - 4 mg/kg) was more potent in antagonizing bulbospinal inhibition of the dorsal root-ventral root MSR when administered intra-arterially to the spinal cord than when injected intra-arterially to the brain stem or intravenously, indicating that the spinal cord is the site of imipramine's action. These results support our earlier proposal that a 5-HT system antagonizes bulbospinal inhibition of the MSR. They also indicate that the 5-HT system is tonically active and exerts its blocking action in the spinal cord.