Responses to 5-hydroxytryptamine evoked in the hemisected spinal cord of the neonate rat

The excitability of lumbar motoneurones in the neonatal rat is increased by a hyperpolarization of their voltage threshold for activation by descending serotonergic fibres

Previous work has shown there is an increase in motoneurone excitability produced by hyperpolarization of the threshold potential at which an action potential is elicited (Vth) at the onset, and throughout brainstem-induced fictive locomotion in the decerebrate cat. This represents a transient facilitation in the membrane potential for activation dependent on the presence of fictive locomotion. The present study tests the hypothesis that a similar neuromodulatory mechanism facilitating neuronal recruitment also exists in the neonatal rat, and the endogenous pathway mediating the Vth hyperpolarization can be activated by electrical stimulation of the neonatal brainstem. Isolated brainstem-spinal cord preparations from 1- to 5-day-old neonatal rats, and whole-cell recording techniques were used to examine the patterns of ventral root (VR) activity produced, and the effect of electrical stimulation of the ventromedial medulla on lumbar spinal neurones. Hyperpolarization of Vth was seen in 10/11 (range -2 to -18 mV) neurones recorded during locomotor-like VR activity, and appeared analogous to the locomotor-dependent Vth hyperpolarization previously described in the cat. However, in the present study, Vth hyperpolarization was also seen during electrical brainstem stimulation that evoked alternating, rhythmic, or tonic VR activity, or failed to evoke VR activity. Thirty-six of 71 neurones were antidromically identified as lumbar motoneurones and 33/36 showed a hyperpolarization of Vth (-2 to -14 mV) during electrical brainstem stimulation. Of the unidentified lumbar ventral horn neurones, 31/35 also showed hyperpolarization of Vth (-2 to -20 mV) during brainstem stimulation. The hyperpolarization of Vth and VR activity induced by brainstem stimulation was reversibly blocked by cooling of the cervical cord, indicating it is mediated by descending fibres, and application of the serotonergic antagonist ketanserin to the spinal cord was effectively able to block the brainstem-evoked hyperpolarization of Vth. These results demonstrate a previously unknown action of the endogenous descending serotonergic system to facilitate spinal motoneuronal recruitment and firing by inducing a hyperpolarization of Vth. This modulatory process can be examined in the neonatal rat brainstem-spinal cord preparation without the requirement for ongoing locomotor activity.

Monoamines increase the excitability of spinal neurones in the neonatal rat by hyperpolarizing the threshold for action potential production

During fictive locomotion in the adult decerebrate cat, motoneurone excitability is increased by a hyperpolarization of the threshold potential at which an action potential is elicited (V(th)). This lowering of V(th) occurs at the onset of fictive locomotion, is evident for the first action potential elicited and is presumably caused by a neuromodulatory process. The present study tests the hypothesis that the monoamines serotonin (5-HT) and noradrenaline (NA) can hyperpolarize neuronal V(th). The neonatal rat isolated spinal cord preparation and whole-cell recording techniques were used to examine the effects of bath-applied 5-HT and NA on the V(th) of spinal ventral horn neurones. In the majority of lumbar ventral horn neurones, 5-HT (13/26) and NA (10/16) induced a hyperpolarization of V(th) ranging from -2 to -8 mV. 5-HT and NA had similar effects on V(th) for individual neurones. This hyperpolarization of V(th) was not due to a reduction of an accommodative process, and could be seen without changes in membrane potential or membrane resistance. These data reveal a previously unknown action of 5-HT and NA, hyperpolarization of V(th) of spinal neurones, a process that would facilitate both neuronal recruitment and firing.

The role of serotonin in reflex modulation and locomotor rhythm production in the mammalian spinal cord

Over the past 40 years, much has been learned about the role of serotonin in spinal cord reflex modulation and locomotor pattern generation. This review presents an historical overview and current perspective of this literature. The primary focus is on the mammalian nervous system. However, where relevant, major insights provided by lower vertebrate models are presented. Recent studies suggest that serotonin-sensitive locomotor network components are distributed throughout the spinal cord and the supralumbar regions are of particular importance. In addition, different serotonin receptor subtypes appear to have different rostrocaudal distributions within the locomotor network. It is speculated that serotonin may influence pattern generation at the cellular level through modulation of plateau properties, an interplay with N-methyl-D-aspartate receptor actions, and afterhyperpolarization regulation. This review also summarizes the origin and maturation of bulbospinal serotonergic projections, serotonin receptor distribution in the spinal cord, the complex actions of serotonin on segmental neurons and reflex pathways, the potential role of serotonergic systems in promoting spinal cord maturation, and evidence suggesting serotonin may influence functional recovery after spinal cord injury.

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.

Effects of 8-OH-DPAT, a 5-HT1A receptor agonist, and DOI, a 5-HT2A/2C agonist, on monosynaptic transmission in spinalized rats

We investigated the effect of 5-HT1A receptor agonist (-/+)-8-hydroxy-2-(di-N-propylamino)tetraline (8-OH-DPAT) and the 5-HT2A/2C receptor agonist (-/+)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on monosynaptic transmission in spinalized rats. 8-OH-DPAT significantly inhibited the excitation of alpha-motoneurons evoked by monosynaptic transmission without a direct effect on alpha-motoneuron excitation. DOI potentiated the excitation of alpha-motoneurons by both direct stimulation and monosynaptic transmission. These results indicate that activation of 5-HT1A receptors inhibits monosynaptic transmission, whereas activation of 5-HT2A/2C receptors enhances it.

Effect of (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide on spinal motor systems in anesthetized intact and spinalized rats

In the present study, we examined the effect of the 5-HT1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT), on the mono- and polysynaptic reflexes in intact and spinalized rats. 8-OH-DPAT (10 micrograms/kg i.v.) significantly potentiated the amplitude of the monosynaptic reflex in intact rats. In contrast, 8-OH-DPAT (30 and 100 micrograms/kg i.v.) produced a significant dose-related inhibition of the amplitude of the monosynaptic reflex in spinalized rats. These results suggest that 8-OH-DPAT predominantly excites spinal motor systems at the supraspinal site, and inhibits such systems at a spinal cord site.

Voltage-dependent block of NMDA responses by 5-HT agonists in ventral spinal cord neurones

1. Modulation by 5-hydroxytryptamine receptor agonists of the NMDA responses of ventral spinal cord neurones was studied by use of the whole-cell patch-clamp technique. 2. In a Mg-free solution containing tetrodotoxin and glycine, 5-hydroxytryptamine (5-HT, 10-100 microM) reduced the NMDA response, the block increasing with hyperpolarization. Kainate responses were little affected. 3. Some classical agonists of 5-HT receptors induced similar blocking effects. At 10 microM, both a selective agonist of 5-HT2 receptors, (+/-)-2,5-dimethoxy-4 iodo amphetamine (DOI), and a selective agonist of some 5-HT1 receptors, (+/-)-8-hydroxy-2(n-dipropyl amino) tetralin (8-OH-DPAT), induced pronounced blocking effects, of 48% and 33% respectively at -100 mV, whereas another 5-HT1 agonist, 5-carboxamidotryptamine (5-CT) was ineffective. At 100 microM, 5-methoxytryptamine (5-MeOT) induced a complete block of the NMDA responses recorded at -100 mV. The order of potency was: 5-MeOT congruent to DOI > 8-OH-DPAT > 5-HT > 5-CT. 4. Neither spiperone nor ketanserin (1 microM) prevented the blocking effect of 5-HT or DOI. 5. Prolonged preincubations with 5-HT did not block the response if NMDA was applied without 5-HT. When 5-HT agonists were applied both by preincubation and with NMDA, the degree of block increased during the NMDA application. 6. Lowering the NMDA concentration (from 100 to 20 microM) slightly decreased the blocking effect of 5-MeOT. 7. External Mg2+ ions (1 mM) also reduced the blocking effects of 5-HT and 5-MeOT. 8. The blocking effects described appear to be independent of classical 5-HT receptors. Their voltage-dependence suggests a mechanism of open channel block consistent with all the results obtained.

Modulation of frog spinal cord interneuronal activity by activation of 5-HT3 receptors

Motoneuron membrane potentials were recorded from the ventral roots of isolated, hemisected frog spinal cords using sucrose gap techniques. The effects of the selective 5-HT3 agonist 2-methyl-serotonin (2-Me-5HT) on the changes in motoneuron membrane potential produced by dorsal root stimulation and by superfusion of excitatory amino acid agonists were evaluated. Application of 2-Me-5HT (100 microM) did not alter motoneuron membrane potential, but did substantially reduce (approximately 20%) the polysynaptic ventral root potentials evoked by dorsal root stimulation. 2-Me-5HT did not change motoneuron depolarizations generated by addition to the Ringer's solution of the excitatory amino acid agonists AMPA (10-30 microM), kainate (30 microM), or t-ACPD (100 microM), but NMDA-induced motoneuron depolarizations (100 microM) were significantly and reversibly reduced (approximately 20%) by exposure to 2-Me-5HT (100 microM). 2-Me-5HT-evoked decreases of NMDA depolarizations were blocked by the 5-HT3 antagonists ICS 205 930 (50-100 microM) and D-tubocurarine (3-10 microM), but not by MDL 72222 (20-100 microM), the 5-HT2 receptor antagonist ketanserin (10 microM), or the 5-HT1A/5-HT2A antagonist spiperone (10 microM). Two lines of evidence support the hypothesis that the effects of 2-Me-5HT are generated by an indirect mechanism involving interneurons: (1) TTX (0.781 microM) eliminated the effect of 2-Me-5HT on NMDA-induced motoneuron depolarizations, and (2) 2-Me-5HT reduced spontaneous ventral root potentials that result from interneuronal discharges. We attempted to establish the identity of a putative transmitter released by interneurons responsible for the effects on NMDA-depolarizations produced by 2-Me-5HT, but the AMPA receptor antagonist, CNQX (10 microM), the GABAA receptor antagonist, bicuculline (50 microM), the GABAB receptor antagonist, saclofen (100 microM), the opioid antagonist, naloxone (100 microM), and the adenosine antagonists, CPT (20-100 microM) and CSC (10-100 microM) did not alter 2-Me-5HT-induced reductions of NMDA-depolarizations. In sum, the site of interaction between 2-Me-5HT and NMDA appears to be at interneuronal locus, but the mechanism remains unclear.

Activation of the central pattern generators for locomotion by serotonin and excitatory amino acids in neonatal rat

1. The role of serotonin (5-HT) and excitatory amino-acids (EAAs) in the activation of the neural networks (i.e. the central pattern generators) that organize locomotion in mammals was investigated in an isolated brainstem-spinal cord preparation from the newborn rat. 2. The neuroactive substances were bath applied and the activity of fictive locomotion was recorded in the ventral roots. 3. Serotonin initiated an alternating pattern of right and left action potential bursts. The period of this rhythm was dose dependent, i.e. it decreased from around 10 s at 2 x 10(-5) M to 5 s at 10(-4) M. The effects of serotonin were blocked by a 5-HT1 antagonist (propranolol) and by 5-HT2 antagonists (ketanserin, cyproheptadine, mianserin). 5-HT3 antagonists were ineffective. The effects of methoxytryptamine, a non-selective 5-HT agonist, mimicked the 5-HT effects. 4. The endogenous EAAs, glutamate and aspartate, also triggered an alternating rhythmic pattern. Their effects were blocked by 2-amino-5-phosphonovaleric acid (AP-5; a N-methyl-D-aspartate (NMDA) receptor blocker) and 6,7-dinitro-quinoxaline-2,3-dione (a non-NMDA receptor blocker). 5. Several EAA agonists (N-methyl-D,L-aspartate (NMA) and kainate) initiated rhythmic activity. The period of the induced rhythm (from 3 to 1 s) was similar with both of these substances but in a range of concentrations which was ten times lower in the case of kainate (10(-6) to 5 x 10(-6) M) than in that of NMA (10(-5) to 4 x 10(-5) M). alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate and quisqualate occasionally triggered some episodes of fictive locomotion with a threshold at 6 x 10(-7) and 10(-5) M, respectively.

5-HT2/5-HT1C receptor-mediated facilitatory action on unit activity of ventral horn cells in rat spinal cord slices

5-Methoxy-N,N-dimethyltryptamine (5-MeODMT) and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) facilitate motoneuron excitability through 5-HT1C/5-HT2 receptors in rats. Using spinal cord slices prepared from adult rats, we recorded unitary cell discharges, evoked by local stimulation of the adjacent site, extracellularly in the motor nuclei of the ventral horn. 5-MeODMT, DOI, 5-hydroxytryptamine (5-HT), 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) and tandospirone facilitated the probability of firing in the motor nuclei, with 5-MeODMT and DOI being the most potent. The effect of 5-MeODMT was significantly suppressed by ketanserin (a 5-HT2 receptor-selective antagonist), spiperone (a 5-HT1A/5-HT2 receptor antagonist) and cyproheptadine (a 5-HT1C/5-HT2 receptor antagonist), but not by 3-tropanyl-3,5-dichlorobenzoate (MDL 72222, a 5-HT3 receptor-selective antagonist) or pindolol (a 5-HT1A/5-HT1B receptor antagonist). This suggests that 5-HT2 and/or 5-HT1C receptors are involved in the facilitatory effects of 5-HT receptor agonists on the synaptic activity of ventral horn cells.

Serotonin1A facilitation of frog motoneuron responses to afferent stimuli and to N-methyl-D-aspartate

The effects of serotonin and excitatory amino acids on motoneurons were examined by sucrose gap recordings from the ventral root of the isolated, hemisected frog spinal cord superfused with magnesium-free, carbonate-buffered Ringer solution. Low concentrations of serotonin (0.1 microM) and the serotonin1A agonist 8-hydroxy-2-(n-dipropylamino)tetralin (8-OH-DPAT; 0.01 microM) significantly increased the duration and amplitude of the polysynaptic components of ventral root potentials produced by dorsal root stimulation. The facilitations of the ventral root potentials were blocked by the serotonin1A antagonist spiroxatrine, but were unaffected by the serotonin2 antagonist ketanserin or the serotonin3 antagonist 1 alpha H,3 alpha,5 alpha H-tropan-3-yl-3,-dichlorobenzoate (MDL 72222). The actions of 0.1 microM serotonin on motoneuron depolarizations evoked by the putative excitatory amino acid transmitters L-glutamate and L-aspartate were quite variable, but in the presence of ketanserin (20 microM), small consistent increases in amino acid-induced motoneuron depolarizations were observed. 8-OH-DPAT significantly enhanced motoneuron depolarizations elicited by the selective excitatory amino acid agonist N-methyl-D-aspartate in both normal and tetrodotoxin-containing Ringer solution. Quisqualate-induced motoneuron depolarizations were also facilitated by 8-OH-DPAT in normal Ringer solution, but these increases were eliminated by addition of either tetrodotoxin or the N-methyl-D-aspartate antagonist D(-)-2-amino-5-phosphonovalerate to the Ringer superfusate. Kainate-depolarizations were not altered by low concentrations of serotonin or 8-OH-DPAT. Prior exposure of the cord to spiperone, but not ketanserin or MDL 72222 blocked the enhancement of N-methyl-D-aspartate-induced motoneuron depolarizations by 8-OH-DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Serotonin and L-norepinephrine as mediators of altered excitability in neonatal rat motoneurons studied in vitro

The actions of serotonin on the membrane properties of motoneurons and on the synaptic responses evoked by stimulating the segmental dorsal root have been investigated using intracellular recording in a neonatal rat hemisected spinal cord preparation in vitro. Superfusion with serotonin produced concentration-dependent depolarizations (EC50 32.1 microM) with an apparent increase in input resistance and increase in motoneuron excitability. During serotonin depolarizations an increase in membrane noise was seen. At higher serotonin concentrations repetitive firing was induced. Sensitivity to serotonin was enhanced by blockade of neuronal uptake with citalopram, when the EC50 was 1.4 microM. The depolarization was mimicked by alpha-methyl-5-hydroxytryptamine (EC50 11.7 microM). Serotonin depolarizations were blocked by ketanserin (0.1 and 1 microM), ritanserin (1 microM), spiperone (0.1 and 1 microM) and LY 53857 (1 microM). A norepinephrine-induced depolarization of motoneurons, which was mimicked by L-phenylephrine and antagonized by prazosin, is probably mediated by an alpha 1-adrenoceptor. An inhibitory action of serotonin was also apparent. The frequency and amplitude of spontaneous postsynaptic potentials and the response following dorsal root stimulation were markedly reduced. This action was mimicked by 5-carboxamidotryptamine and 8-hydroxy-2-(n-dipropylamino)tetralin, but was not antagonized by ketanserin (1 microM), ritanserin (1 microM), methiothepin (1 microM), metergoline (1 microM), spiperone (1-10 microM) or 21-009 (1-10 microM). It is proposed that the depolarization and increase in excitability of spinal motoneurons is mediated by a serotonin (5-HT2) receptor subtype.

Effects of 5-hydroxytryptophan on extracellular serotonin in the spinal cord of rats with experimental allergic encephalomyelitis

Serotonin (5-HT) and the serotonin metabolite, 5-hydroxyindoleacetic acid (5-HIAA) were collected by in vivo dialysis in the lumbar spinal cord of control rats and rats with hindlimb paralysis induced by experimental allergic encephalomyelitis (EAE). Both 5-HT and 5-HIAA were significantly decreased in baseline samples from EAE rats compared to controls. This decrease in extracellular 5-HT and 5-HIAA in the EAE rats was accompanied by marked morphological changes in spinal cord axons and axon terminal plexuses that were stained for 5-HT-like immunoreactivity. The 5-HT precursor, 5-hydroxytryptophan (5-HTP)-increased 5-HT and 5-HIAA levels in dialysate samples from both control and EAE animals. However, the 5-HTP-induced increase in extracellular 5-HT was significantly greater in the EAE rats than in the controls, despite a lower baseline 5-HT level in the EAE animals. In contrast to 5-HT, both baseline and post-5-HTP levels of 5-HIAA were significantly higher in control animals than in EAE animals. The decreased extracellular 5-HT and 5-HIAA in baseline samples from the EAE rats compared to controls is probably a consequence of the damage to descending 5-HT axons and axon terminals that occurs during the disease. The larger increase in extracellular 5-HT in EAE animals after precursor injection may reflect both decreased 5-HT reuptake from the extracellular space by damaged 5-HT terminals and disruption of the blood-brain barrier that allows entry into the central nervous system of 5-HT that was synthesized from 5-HTP in the periphery.

Characterization of the 5-HT receptor subtypes involved in the motor behaviours produced by intrathecal administration of 5-HT agonists in rats

1. The motor behavioural effects of intrathecal injections of 5-hydroxytryptamine (5-HT) and a variety of 5-HT receptor agonists were examined in adult Wistar rats to establish; (a) which 5-HT receptor subtype/s elicit each behaviour and (b) whether these receptors are located within the spinal cord. 2. Intrathecal injection of 5-methoxy-N,N'-dimethyltryptamine (5-MeODMT), (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) or 2,5-dimethoxy-alpha,4-dimethylbenzene ethamine hydrochloride (DOM) produced dose-related back muscle contractions (BMC) and wet dog shakes (WDS) which were both markedly attenuated by intraperitoneal pretreatment with either ritanserin (1 mg kg-1), ketanserin (0.16 mg kg-1) or mianserin (0.6 mg kg-1) indicating the involvement of 5-HT2 receptors in both these motor behaviours. Both fluoxetine (1-20 mg kg-1, i.p.) and high doses of 5-HT (50 micrograms) following fluoxetine (5 mg kg-1, i.p.) also elicited BMC, further confirming the involvement of 5-HT in this behaviour. 3. Intrathecal 5-carboxamidotryptamine (5-CT) evoked a marked wet-dog shake response without producing any BMC. Intrathecal pretreatment with 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT) enhanced, while in contrast 2-methyl-5-HT pretreatment attenuated, 5-HT agonist-induced BMC without affecting WDS. These data suggest that the spinal 5-HT2 receptors mediating BMC are positively modulated by 5-HT1A but negatively influenced by 5-HT3 receptor activation and may be of a different subtype to the supra-spinal 5-HT2 receptors which elicit WDS. 4. A contrast, reciprocal forepaw treading, lateral head weaving, flat body posture and Straub-tail were evoked by 5-MeODMT, 8-OH-DPAT or 5-CT but not by DOI or DOM indicating that these behaviours were not produced by 5-HT2 receptor activation alone. Ritanserin (1 mg kg- 1, i.p.) or ketanserin (0.16mgkg-1, i.p.) pretreatment reduced the reciprocal forepaw treading induced by high intrathecal doses of either 5-MeODMT (25.pg) or 5-CT (50,ug) suggesting that this behaviour may be facilitated by 5-HT2 receptor activation. 5. Intrathecal injection of 5-HT (0.05-50pg, after systemic fluoxetine, 5mg kg 1, i.p.), or 1-(3-chlorophenyl) piperazine (mCPP) produced dose-related forepaw-licking and grooming, neither of which were attenuated by ketanserin (0.16 mgkg-1, i.p.) pretreatment suggesting these behaviours may be mediated by 5-HT1c receptors. In contrast, 2-methyl-5-HT (50 and 100pg) produced sideward tail-flicks, not evoked by any other 5-HT agonist and could therefore be mediated by spinal 5-HT3 receptor activation. 6. These data provide behavioural evidence for the existence of spinal 5-HT2 receptors which produce a novel motor behaviour, BMC. Ligand binding studies and dose-response studies with a range of selective 5-HT antagonists are required to establish whether BMC and WDS are mediated by different subtypes of 5-HT2 receptors.

Further studies on the action of 5-hydroxytryptamine on lumbar motoneurones in the rat isolated spinal cord

Using the hemisected spinal cord of the neonate rat, the effects of altered external Ca, thyrotrophin-releasing hormone (TRH) and a number of antagonists were tested on depolarizations evoked by 5-hydroxytryptamine (5-HT). Responses of populations of motoneurones were recorded via a ventral root. 5-Hydroxytryptamine depolarizations were not Ca-dependent but were enhanced in amplitude in Ca-free solutions. Raised Mg reversed this enhancement. 5-Hydroxytryptamine depolarizations persisted in the presence of Mn (1.5-3 mmol/l). TRH depolarized motoneurones; there was no evidence of modulation of 5-HT responses on concurrent application of TRH. Ritanserin (0.1 mumol/l) had a modest blocking action on 5-hydroxytryptamine depolarizations reducing the maximum; 1 mumol/l ritanserin caused a greater antagonism which was unsurmountable (pIC50 5.2). Ritanserin (0.1 or 1 mumol/l) did not depress responses to noradrenaline (NA). Ketanserin (0.1 mumol/l) caused a blockade of slow onset, equilibrium with the receptors requiring 1 h. Blockade by 0.01, 0.1 and 1 mumol/l ketanserin was concentration-dependent (pIC50 6.2). Ketanserin 1 mumol/l, but not at lower concentrations, depressed noradrenaline responses. Mianserin (0.1 mumol/l) also caused a blockade of slow onset; 0.1 or 1 mumol/l produced a flattening of the 5-hydroxytryptamine concentration-response curve but did not depress noradrenaline responses (pIC50 4.7). The pIC50 for spiperone was 8.0. DOI (10-100 mumol/l) had no detectable agonist action but at concentrations of 0.01 and 0.1 mumol/l it acted as an antagonist. Equilibration with the receptors occurred over 2 h. DOI (0.01 mumol/l) depressed 5-hydroxytryptamine but not noradrenaline responses; higher concentrations of DOI also depressed noradrenaline responses.(ABSTRACT TRUNCATED AT 250 WORDS)

5-Hydroxytryptamine responses in neonate rat motoneurones in vitro

1. Current and voltage recordings were made from antidromically identified motoneurones (MNs) in transverse thoracolumbar spinal cord slices of neonatal rats. 2. Applied by superfusion (10-100 microM) or pressure ejection, 5-hydroxytryptamine (5-HT) elicited a slow depolarization (or inward current) in 81% and a hyperpolarization (or outward current) in 9% of responsive MNs; the responses persisted in a low-Ca2+, high-Mg2+ or tetrodotoxin (TTX)-containing solution. 3. 5-HT induced the occurrence in some MNs of excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs), which were reversibly eliminated by TTX, low-Ca2+, high-Mg2+ solution or by the 5-HT2 receptor antagonists ketanserin and spiperone. Also, kynurenic acid and strychnine abolished, respectively, the 5-HT-induced EPSPs and IPSPs. 4. The 5-HT depolarization was associated with increased membrane resistance, was reduced by hyperpolarization and nullified near -100 mV. The extrapolated reversal potential was shifted to a positive direction in elevated [K+]o. 5. The depolarizing response was mimicked by the 5-HT2 receptor agonist (+2-)-1(2,5-dimethyoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) and blocked by 5-HT antagonists methysergide and cyproheptadine and by 5-HT2 antagonists ketanserin and spiperone; methiothepin and MDL 72222 were without effect. 6. The 5-HT hyperpolarization was associated with decreased membrane resistance. The 5-HT1A agonist 8-hydroxy-2-(di-N-propylamino) tetralin hydrobromide (8-OH-DPAT) mimicked the hyperpolarizing response. 7. Single or repetitive (10-30 Hz) electrical stimuli elicited in about 30% of MNs, in addition to a fast EPSP, a slow EPSP with electrophysiological characteristics similar to that of 5-HT induced depolarization. Methysergide and spiperone abolished the slow EPSPs evoked in some of these MNs. 8. It is suggested that 5-HT, acting on 5-HT2 and 5-HT1A receptors, depolarizes and hyperpolarizes the MNs by decreasing and increasing K+ conductance. Additionally, 5-HT activates, via 5-HT2 receptors, excitatory and inhibitory interneurones, thereby indirectly affecting the activity of MNs. More importantly, 5-HT released from intraspinal nerves appears to be the mediator of a slow EPSP in a population of MNs.

Receptor subtypes mediating facilitation by serotonin of excitability of spinal motoneurons

Serotonin receptor ligands, with differential affinity for subtypes of serotonin (5-HT) receptors, were administered intravenously or iontophoretically to urethane-anesthetized rats and the effects of these compounds on glutamate-evoked firing of spinal motoneurons were tested. The excitability of spinal motoneurons was markedly enhanced after intravenous administration of the selective 5-HT1A ligand 8-hydroxy-2-(di-n-propylamino) tetralin (DPAT) in rats with acute spinal transections at C1. However, local application of DPAT, directly into the ventral horn by microiontophoresis, inhibited the glutamate-evoked firing of motoneurons in direct contrast to the facilitatory effects of iontophoretically applied 5-HT. The DPAT-induced inhibition may have been nonspecific, since it was not antagonized by methysergide. Other 5-HT agonists, with relatively selective affinity for 5-HT1B, 5-HT1C and 5-HT2 receptors, increased the excitability of spinal motoneurons when applied iontophoretically or intravenously. The excitatory effect of iontophoretically applied 5-HT was antagonized by the nonselective 5-HT antagonist, methysergide and by ketanserin and ritanserin, which have relatively selective affinity for 5-HT1C and 5-HT2 receptors. These results indicate that 5-HT1A receptors do not mediate facilitation of excitability of motoneurons produced by local application of 5-HT directly into the vicinity of the motoneurons. However, the marked increase in firing of motoneurons that was caused by intravenous administration of DPAT in spinal transected rats, suggests that 5-HT1A receptors in the spinal cord may participate in 5-HT-induced enhancement of somatomotor outflow, at sites presynaptic to the motoneurons. The iontophoretic results suggest that 5-HT1B, 5-HT1C and 5-HT2 receptors may all play a role in facilitation of the excitability of spinal motoneurons by locally applied 5-HT. Differentiation between these subtypes of receptor awaits the development of more completely selective agonists and antagonists.

Analysis of the 5-HT receptor in rabbit saphenous vein exemplifies the problems of using exclusion criteria for receptor classification

5-Hydroxytryptamine (5-HT) contracts ring preparations of rabbit saphenous vein via direct and indirect components, the latter being compatible with a "tyramine-like" action at sympathetic nerve terminals. Here an attempt was made to establish the identity of the receptor mediating contraction directly, in terms of the currently accepted proposals (Bradley et al. 1986). Results with agonists suggested 5-HT1-like receptor activation: methylsergide behaved as a partial agonist with microcolar affinity and 5-HT effects were mimicked by 5-carboxamidotryptamine (5-CT) and GR43175. The agonist potency order was 5-CT greater than 5-HT greater than methysergide greater than or equal to GR43175, the same as that reported at the 5-HT1-like receptor in dog saphenous vein (Feniuk et al. 1985; Humphrey et al. 1988). Consistent with this, 5-HT effects were resistant to blockade by the selective 5-HT3 receptor antagonist MDL72222 (1.0 mumol/l). In contrast, methiothepin (0.01-0.3 mumol/l), ketanserin (0.3-30.0 mumol/l) and spiperone (0.3-30.0 mumol/l) each produced surmountable antagonism which, although competitive in nature only for methiothepin (pKB = 9.45 +/- 0.09, 17 d.f.), implied 5-HT2 receptor involvement. The possibility that these discrepancies resulted from mixed populations of 5-HT1-like and 5-HT2 receptors can be excluded because; 1). Ketanserin and spiperone blocked the actions of 5-HT and the selective 5-HT1-like receptor agonist GR43175 with equal facility and 2). Responses to all of the agonists studied were similarly antagonised by flesinoxan (pKB approximately 6.4), a simple competitive antagonist at the receptor in rabbit saphenous vein.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of 5-hydroxytryptamine agonists and antagonists on identified sympathetic preganglionic neurones in the rat, in vivo

1. 5-Hydroxytryptamine (5-HT) was applied by microiontophoresis in the vicinity of identified sympathetic preganglionic neurones in the upper thoracic spinal cord of the rat, in vivo. 2. Sympathetic preganglionic neurones responded in one of three ways to 5-HT: by (a) excitation (76%), (b) inhibition (4%) or (c) in a biphasic manner (5%). 3. The excitatory responses evoked by 5-HT were mimicked by 5-carboxamidotryptamine (5-CT) and alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT). The inhibitory and biphasic responses evoked by 5-HT were mimicked by 2-methyl-5-hydroxytryptamine (2-Me-5-HT). The observed responses evoked by 5-HT and selective agonists may be different on the same cell. In several instances a single neurone excited by one agonist was inhibited by another agonist. 4. The 5-HT2-receptor antagonists, ketanserin and LY 53857, failed to abolish selectively the excitatory responses evoked by 5-HT and alpha-Me-5-HT, when applied by microiontophoresis. The antagonists non-selectively reduced the excitatory responses evoked by 5-HT, 5-CT, alpha-Me-5-HT, D,L-homocysteic acid (DLH) and noradrenaline (NA). A reduction in synaptically evoked activity was also observed. 5. The 5-HT3-receptor antagonist, ICS 205-930, failed to abolish the inhibitory responses evoked by 5-HT. 6. It was concluded that the excitatory responses evoked by 5-HT are mediated by a receptor that is neither 5-HT2 or 5-HT3, but shows similarities to the 5-HT1-like receptor profile. The inhibitory actions of 5-HT are mimicked by 2-Me-5-HT, but the receptor is not 5-HT3, or 5-HT1-like or 5-HT2.

Changes in membrane potential of frog motoneurons induced by activation of serotonin receptor subtypes

Application of serotonin to the isolated, hemisected frog spinal cord resulted in two distinctive changes in motoneuron membrane potential: hyperpolarizations were produced by low concentrations (0.01-1.0 microM) and depolarizations by higher concentrations (3.0-100 microM). The hyperpolarizations appeared to be caused by a direct action of the amine upon motoneurons since exposure of spinal cord tetrodotoxin or magnesium ions in concentrations which blocked interneuronal firing and synaptic transmission, respectively did not reduce these responses. In contrast, depolarizations were significantly reduced by tetrodotoxin or magnesium indicating a large indirect component. The use of agonists and antagonists known to discriminate among different subtypes of serotonin receptors indicated that the hyperpolarizations were produced by activation of 5-HT1A receptors and the depolarizations were generated by activation of 5-HT2 and/or 5-HT1C receptors. Accordingly, the selective 5-HT1A agonists 8-hydroxy-2-(n-dipropylamino)tetralin and ipsapirone directly hyperpolarized motoneurons. The changes in potential produced by low concentrations of serotonin and by these agonists were blocked by the 5-HT1A receptor antagonists spiperone and spiroxatrine. In contrast, application of high concentrations of alpha-methyl-5-hydroxytryptamine, a serotonin analog which activates 5-HT1C and 5-HT2 receptor subtypes, depolarized motoneurons. These depolarizations, and those produced by high concentrations of serotonin, were blocked by the 5-HT1C/5-HT2 antagonists ketanserin, methysergide and mianserin. These observations indicate that serotonin can alter the membrane potential of motoneurons directly and indirectly by activation of both 5-HT1 and 5-HT2 receptor subtypes. Activation of different receptor subtypes depends upon the concentration of the amine.

5-Hydroxytryptamine depolarizes neonatal rat motorneurones through a receptor unrelated to an identified binding site

Superfusion of hemisected lumbar spinal cord of the neonatal rat with solutions containing 10(-6) to 10(-3) M 5-hydroxytryptamine (5-HT) elicited depolarizations of graded amplitude which were recorded from motorneurons through a ventral root. Maximum responses (amplitude 1.0 +/- 0.1 mV, mean +/- SEM, n = 30) were evoked by 10(-4) M 5-HT. Repeated concentration-response curves could be determined from the same preparation. There was no involvement of 5-HT2 receptors in the depolarizing response to 5-HT, since neither ritanserin nor ICI 169, 369 showed any antagonist action. Amongst agents with activity at 5-HT1A sites, the selective 5-HT1A receptor agonist, 8-hydyroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT), neither mimicked the action of 5-HT nor antagonised it, while spiperone (10(-8)-10(-7 M) antagonised responses to 5-HT in a concentration-related manner. Responses to 10(-4) M noradrenaline, used as a control depolarizing agent, were unaffected by spiperone. The onset of blockade by spiperone was slow, 1 hr being required for equilibration of the tissue with antagonist. The blockade was surmountable by larger concentrations of 5-HT. Concentration-response curves to 5-HT were shifted to the right in an approximately parallel manner by spiperone. The dose ratios measured from these curves at the EC50 level, yielded an apparent pA2 of 8.24 +/- 0.14 (mean +/- SEM, n = 15), although the Schild plot of the data had a slope less than unity. The lack of activity of the selective 5-HT1B receptor agonist, RU 24969, and the 5-HT1B receptor antagonists, (+/-) cyanopindolol and quipazine, indicated that 5-HT1B receptors were not involved in the 5-HT response of motorneurones to 5-HT. Mesulergine, metergoline and cyproheptadine also antagonised responses of motorneurones to 5-HT, producing a surmountable blockade. Mesulergine (10(-8), 3 x 10(-8) and 10(-7) M caused a progressive rightward shift of the concentration-response curves, but 10(-7) M depressed the maximum response to 5-HT. Responses to noradrenaline were not affected by these concentrations of mesulergine. The apparent pA2 for blockade of 5-HT responses by mesulergine, calculated from experiments in which there was a parallel displacement of the concentration-response curves, was 8.75 +/- 0.11 (mean +/- SEM, n = 10).(ABSTRACT TRUNCATED AT 400 WORDS)