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

Serotonergic depression of spinal monosynaptic transmission is mediated by 5-HT1B receptors

In the spinal cord, various subtypes of serotonin (5-hydroxytryptamine; 5-HT) receptors are involved in the modulation of motor output. Although the excitatory role of 5-HT(2) receptors is known, the receptor subtypes mediating the inhibitory effect of 5-HT on monosynaptic reflex transmission remain unclear. In this study, segmental spinal reflexes were recorded to examine the receptor subtypes underlying 5-HT-mediated inhibition of monosynaptic reflex transmission in spinalized rats. Under conditions of monoamine oxidase blockade with clorgyline, the 5-HT precursor L-5-hydroxytryptophan depressed the monosynaptic reflex. 3-Hydroxybenzylhydrazine dihydrochloride (NSD-1015), a centrally active decarboxylase inhibitor, abolished this inhibition, confirming that the depression of the monosynaptic reflex by L-5-hydroxytryptophan was due to 5-HT. In the presence of GR127935 or isamoltane, which show high affinity for 5-HT(1B) receptors, L-5-hydroxytryptophan did not suppress the monosynaptic reflex, whereas 5-HT(1A), 5-HT(1D), 5-HT(2) and 5-HT(7) receptor antagonists did not alter the inhibitory effect of L-5-hydroxytryptophan. These results suggest that serotonergic depression of monosynaptic reflex transmission is mediated by 5-HT(1B) receptors.

Modulatory actions of serotonin, norepinephrine, dopamine, and acetylcholine in spinal cord deep dorsal horn neurons

The deep dorsal horn represents a major site for the integration of spinal sensory information. The bulbospinal monoamine transmitters, released from serotonergic, noradrenergic, and dopaminergic systems, exert modulatory control over spinal sensory systems as does acetylcholine, an intrinsic spinal cord biogenic amine transmitter. Whole cell recordings of deep dorsal horn neurons in the rat spinal cord slice preparation were used to compare the cellular actions of serotonin, norepinephrine, dopamine, and acetylcholine on dorsal root stimulation-evoked afferent input and membrane cellular properties. In the majority of neurons, evoked excitatory postsynaptic potentials were depressed by the bulbospinal transmitters serotonin, norepinephrine, and dopamine. Although, the three descending transmitters could evoke common actions, in some neurons, individual transmitters evoked opposing actions. In comparison, acetylcholine generally facilitated the evoked responses, particularly the late, presumably N-methyl-D-aspartate receptor-mediated component. None of the transmitters modified neuronal passive membrane properties. In contrast, in response to depolarizing current steps, the biogenic amines significantly increased the number of spikes in 14/19 neurons that originally fired phasically (P < 0.01). Together, these results demonstrate that even though the deep dorsal horn contains many functionally distinct subpopulations of neurons, the bulbospinal monoamine transmitters can act at both synaptic and cellular sites to alter neuronal sensory integrative properties in a rather predictable manner, and clearly distinct from the actions of acetylcholine.

Serotonergic neurotransmission in the spinal cord and motor cortex of patients with motor neuron disease and controls: quantitative autoradiography for 5-HT1a and 5-HT2 receptors

Serotonin 5-HT is a potent modulator of motor neuron excitability in the spinal cord. Serotonergic neurotransmission, because of its effects on glutamatergic excitation, may be relevant to the pathogenesis and therapy of motor neuron disease (MND). The human motor system was studied at two levels, spinal cord and motor cortex, by autoradiography for the 5-HT1A and 5-HT2 receptor subclasses. In addition, biochemical estimations of indole metabolites were performed in the spinal cord. Post mortem tissue from control cases and MND patients showed a reduction in 5-HT1A receptor binding in the cervical (p < 0.01) but not lumbar ventral horn in MND. 5-HT2 receptors were preserved in the ventral horn at both levels and were focally abundant around motor neuron somata. Tissue levels of 5-HT were unchanged in the spinal cord in MND. The metabolite 5-HIAA was increased in the cervical spinal cord in MND as was the molar ratio of 5HIAA:5-HT, implying that there may be an increased turnover of 5HT. In the motor cortex and premotor cortex the 5-HT1A receptor remained unchanged in MND. There was a 20% reduction in 5-HT2 receptor binding sites (p < 0.05) across all the cortical laminae with preservation of the normal pattern of laminar binding. These changes in two levels of the motor system in MND most likely represent physiological adaptations in the spinal cord and motor cortex rather than primary involvement of the serotonergic system in the pathogenesis of the disease.

Excitation of rat substantia nigra pars reticulata neurons by 5-hydroxytryptamine in vitro: evidence for a direct action mediated by 5-hydroxytryptamine2C receptors

Single-unit extracellular and whole-cell patch clamp recording were used to study the actions of exogenously applied 5-hydroxytryptamine on substantia nigra pars reticulata neurons in parasaggital slices of rat midbrain. Seventy-six per cent of substantia nigra pars reticulata cells (254/334) recorded extracellularly were excited by 5-hydroxytryptamine (EC50 = 9.56 microM); in the remainder, inhibitions (13.5%), biphasic responses (4.2%) or lack of response (6.3%) were observed. Using whole-cell patch recording, 5-hydroxytryptamine (10 microM) caused either an inward current (9/9 cells) or a depolarization (3/3 cells) at membrane potentials in the range -50 to -90 mV, which was resistant to tetrodotoxin (4/4 cells), indicating that the predominant, excitatory action of 5-hydroxytryptamine was due to a direct action on substantia nigra pars reticulata neurons. The 5-hydroxytryptamine excitation (recorded extracellularly) was reduced to 24 +/- 6% of control values by methysergide (0.1 microM) and to 17 +/- 5% of control by ketanserin (10 microM), but was unaffected by the 5-hydroxytryptamine antagonists spiperone (0.1 microM), yohimbine (0.1 microM), pindolol (1 microM), GR113808A (1 microM) or ICS 205930 (10 microM). In addition, the 5-hydroxytryptamine excitation was mimicked by the 5-hydroxytryptamine2C receptor--preferring agonist alpha-methyl 5-hydroxytryptamine (10 microM), but the agonists CP93, 129 (0.1-1 microM) and (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (0.1-1 microM) were without effect. Taken together, this pharmacology indicated involvement of the 5-hydroxytryptamine2C receptor in the 5-hydroxytryptamine excitation, while other candidate receptors known to be present in rat substantia nigra pars reticulata (5-hydroxytryptamine1B, 5-hydroxytryptamine2A and 5-hydroxytryptamine4) could be excluded from consideration. While in accord with current information on the location of 5-hydroxytryptamine receptor subtypes in substantia nigra pars reticulata, and the consequence of activation of neuronal 5-hydroxytryptamine2C receptors, these results contrast with data from in vivo experiments which suggest that the net effect of 5-hydroxytryptamine is to inhibit substantia nigra pars reticulata neurons. The reason for this apparent discrepancy may lie in detailed consideration of the microcircuitry of the substantia nigra pars reticulata. This may lead to a re-evaluation of the influence of 5-hydroxytryptamine on this basal ganglia output relay nucleus, and its role in motor control and the gating of generalized seizure activity.

Ketanserin-sensitive depressant actions of 5-HT receptor agonists in the neonatal rat spinal cord

1. The monosynaptic reflex (MSR), recorded in vitro from the neonatal rat spinal cord, was depressed by 5-hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT), methysergide and R(+)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), and also by the selective 5-HT1D agonists, sumatriptan and N-methyl-3-(1-methyl-1-piperidinyl)-1H-indole-5-ethane sulphonamide (GR 85548). 2. Ketanserin (1 microM) and methiothepin (1 microM) reduced the duration of depressions elicited by 5-CT, but not those produced by 5-HT, sumatriptan, GR 85548, methysergide or 8-OH-DPAT. 3. The IC50 for MSR depression by 5-CT was 3.6, 2.1-6.2 nM (n = 4), by sumatriptan was 15.2, 12.9-18.0 nM (n = 32), by GR 85548 was 18.4, 11.7-29.1 nM (n = 12), by methysergide was 29.8, 10.2-87.1 nM (n = 4) and by 8-OH-DPAT was 0.21, 0.11-0.43 microM (n = 3) (geometric means and 95% confidence limits). 4. Ketanserin (0.1 or 1 microM) antagonized competitively responses to sumatriptan (apparent pA2 7.8 +/- 0.1, n = 5), GR 85548 (apparent pA2 7.6, unpaired data, n = 5), methysergide (apparent pA2 7.9 +/- 0.12, n = 4) and 8-OH-DPAT (apparent pA2 8.3 +/- 0.1, n = 3). Concentration-response curves to 5-CT showed a smaller, parallel shift to the right (apparent pA2 6.8 +/- 0.1, n = 4), but responses to 5-HT were unaffected by ketanserin (1 microM) (n = 4). 5. Methiothepin (1 microM) antagonized competitively responses to GR 85548 (apparent pA2 7.7, unpaired data, n = 5). 6. Mianserin (0.3 microM), a concentration sufficient to cause substantial block of 5-HT2C-mediated responses but have only a small effect on 5-HT1D-mediated actions, caused a small, non-parallel shift of the concentration-response curve to sumatriptan. 7. Depression of the MSR by sumatriptan was not blocked by (+/-)-cyanopindolol (0.1 microM), (+/-)-propranolol (0.5 or 1 microM) or spiroxatrine (0.1 microM), and depression of MSR by 8-OH-DPAT was not blocked by spiroxatrine (0.1 microM). (+/-)-Cyanopindolol (0.1 and 1 microM) itself induced a slow depression of the MSR. 8. The novel 5-HT1D antagonist, N-[4-methyl-1-piperazinyl) phenyl]2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl) [1,1-biphenyl]-4-carboxamide (GR 127935, 30 nM to 1 microM) caused a concentration-related depression of the reflex (up to 50%) usually slow in onset. Neither with these concentrations nor with concentrations in the range 1-3 nM was there any unequivocal blockade of responses to sumatriptan. 9. It is concluded that sumatriptan, GR 85548, methysergide and 8-OH-DPAT depress the MSR in the neonate rat spinal cord via ketanserin-sensitive receptors, which have some similarities to 5-HT1D alpha receptors but which are not blocked by GR 127935. 5-HT released by tryptaminergic pathways may act via the same receptors to depress the MSR. 5-HT applied to the cord probably acts via a different, possibly novel 5-HT receptor to depress the MSR.

5-Hydoxytryptamine evokes depolarizations and membrane potential oscillations in rat sympathetic preganglionic neurones

1. Whole-cell recordings were made from seventy-seven identified rat sympathetic preganglionic neurones (SPN) in spinal cord slices. Perfusion of 5-HT (0.5-30 microM) strongly depolarized 90% of neurones. The response was slow in onset, could last over 10 min and was associated with an increase in input resistance. 5-HT could also evoke rhythmical membrane potential oscillations in a population of previously quiescent neurones. 2. The 5-HT response persisted in TTX and also in low-Ca(2+)-high-Mg2+ artificial cerebrospinal fluid (ACSF), suggesting that the receptors are on SPN. The 5-HT uptake inhibitor 6-nitroquipazine potentiated the 5-HT-induced depolarization. 3. The 5-HT-induced depolarization was reduced and then abolished by membrane hyperpolarization to potentials of about -100 mV, but was not reversed in sign by further hyperpolarization. In voltage clamp, 5-HT evoked inward currents associated with the reduction of an outwardly rectifying potassium conductance. 4. The 5-HT2 receptor agonist alpha-methyl-5-HT mimicked the 5-HT response on all neurones, as did the 5-HT1 receptor agonist 5-carboxamidotryptamine (5-CT) on 71% of SPN. The responses to 5-HT, alpha-methyl-5-HT and 5-CT were inhibited by the 5-HT2 antagonists ketanserin and ritanserin. 5. Pressure ejection of 5-HT over the central canal region could evoke a biphasic inhibitory-excitatory response. This response persisted in TTX, suggesting that an inhibitory 5-HT receptor may be located on the medial dendrites. 6. SPN are powerfully depolarized by 5-HT acting at 5-HT2 receptors, via the closure of an outwardly rectifying potassium conductance. The long duration of the response and the ability of 5-HT to induce rhythmical oscillations suggest that 5-HT may have an important role in regulating SPN excitability.

5-HT receptors involved in initiation or modulation of motor patterns: opportunities for drug development

A clearer understanding of the role of descending systems in motor control can be achieved by using in vitro preparations of mammalian spinal cord that display patterned motor output, together with the use of selective pharmacological agents. It has been suggested that 5-HT is involved in either the initiation or the modulation of certain motor behaviours, and that it acts to enhance or regulate the motor pattern. Most attention has been paid to the locomotor rhythms underlying walking or swimming, and in respiratory pattern generation. In this article, David Wallis discusses the involvement of 5-HT1 and 5-HT2 receptors in these processes and the possible therapeutic relevance.

Excitatory and inhibitory effects of serotonin on spinal axons

We studied the effects of serotonin on compound action potentials in dorsal columns isolated from young (nine to 13 days old) rats. Conducting action potentials were activated by submaximal (50%) and supramaximal constant current electrical stimuli and recorded with glass micropipettes. At 10 microM and 100 microM concentrations, serotonin significantly increased mean action potential amplitudes by 9.6 +/- 6.5% (+/- S.D., P < 0.05) and 16.6 +/- 12.2% (+/- S.D., P < 0.005), respectively. Likewise, 10 microM and 100 microM of quipazine (a serotonin2A agonist) increased the amplitudes by 9.6 +/- 2.5% (+/- S.D., P < 0.0005) and 37.7 +/- 8.7% (+/- S.D., P < 0.0005), respectively. In contrast, 10 microM and 100 microM concentrations of 8-hydroxy-dipropylaminotetralin-hydrobromide (a serotonin 1A agonist) reduced axonal excitability by -9.4 +/- 5.5% (+/- S.D., P < 0.05) and -32.9 +/- 10.6% (+/- S.D., P < 0.0005), respectively. At 50 microM concentration, mianserin (a serotonin2A and serotonin2C antagonist) eliminated the excitatory effects of 100 microM quipazine dimaleate. The combination of 50 microM mianserin and 100 microM serotonin reduced action potential amplitudes by -5.6 +/- 4.9% (+/- S.D., P < 0.05). These results suggest that serotonin1A and serotonin2A receptor subtypes are present on spinal dorsal column axons. These two receptor subtypes have opposing effects on axonal excitability. The ratios and sensitivities of these two axonal receptor subtypes may modulate axonal excitability in rat dorsal column axons and have important implications for both development and injury of axons.

Direct immunocytochemical localization of 5-hydroxytryptamine receptors in the adult rat spinal cord: a light and electron microscopic study using an anti-idiotypic antiserum

In the present study, we performed the immunodetection of serotonergic (5-HT) receptor subtypes in the spinal cord by using an anti-idiotypic antiserum (TH8) at light and electron microscopic levels. This antibody has been shown to recognize 5-HT1B, 5-HT1C, and 5-HT2 receptor subtypes (Tamir et al.: J Neurochem 57:930-942, 1991). The TH8 immunoreactivity was observed in the dorsal and ventral horns of the gray matter. Light microscopy revealed that small cell bodies located in laminae I-III of the dorsal horn were intensely immunolabeled. A more homogenous and discrete staining was also observed throughout the entire dorsal horn. In the ventral horn, motoneurons were also immunoreactive (IR). Peroxidase deposits were observed as numerous patches covering the motoneuronal surface. Numerous interneurons were moderately and homogeneously immunostained. With the electron microscope, most of the labeled structures were identified as neurons (dendrites and perikarya) in both the dorsal and ventral horns. In the dorsal horn, immunoreactivity was present in dendrites and neuronal perikarya. A large majority of the immunoreactivity found in dendrites was not associated with synaptic differentiations. Indeed, the dendrites, in which peroxidase deposit was seen, were not locally involved in synapses. Very scarce synaptic varicosities were observed in close apposition with IR dendrites. In the ventral horn, TH8 immunoreactivity was present in dendrites, with an accumulation of peroxidase deposit on the active zone of synapses, facing presynaptic membranes. Both the postsynaptic membrane and the submembrane area were IR. In addition, a few astroglial fine processes were immunostained; most of them were observed in the dorsal horn. Scarce IR astroglial profiles were observed in the ventral horn. These observations show that such an antiserum constitutes a useful tool for the ultrastructural analysis of 5-HT receptor distribution. Finally, correlation between the immunocytochemical localization of 5-HT receptor subtypes and the modes of 5-HT transmission in the spinal cord (wiring and volume transmissions) is discussed in the present report.

Modulation of respiratory activity of neonatal rat phrenic motoneurones by serotonin

1. The effects of serotonin on phrenic motoneurones were studied in an in vitro preparation of the isolated brainstem and spinal cord from neonatal rats. 2. Serotonin (5-HT; > or = 5-10 microM) increased inspiratory-modulated phrenic nerve activity and produced a small amount of tonic activity during expiration. Inspiratory-modulated activity of the fourth cervical ventral root also increased, but was accompanied by robust tonic activity, which often obscured the rhythmic activity. 3. Serotonin, in both normal and tetrodotoxin-containing medium, depolarized phrenic motoneurones and increased cell input resistance. Serotonin also increased inspriatory-modulated firing as well as the response of phrenic motoneurones to injected current. The y-intercept of the relationship between firing frequency and injected current (f-I) was increased, but the slope was not affected. There was no bistable firing behaviour. 4. Under voltage clamp conditions, 5-HT produced a tonic inward current of 0.07-0.37 nA. This current increased with less negative holding potentials and decreased with more negative holding potentials (-75 to -90 mV) but did not reverse. 5. In addition, 5-HT decreased inspiratory-modulated synaptic current by 23 +/- 6%. The degree of attenuation was not affected by holding potential. The time course of the decrease in inspiratory-modulated synaptic current was similar to the changes seen in tonic inward current and input resistance. 6. Depolarization, tonic inward current, and shift in the f-I relationship produced by 5-HT were antagonized by the 5-HT2/1C receptor antagonist ketanserin and mimicked by the 5-HT2/1C agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI). However, the 5-HT induced decrease in inspiratory-modulated synaptic current was not reduced by ketanserin nor mimicked by DOI. 7. We conclude that exogenously applied 5-HT simultaneously increases cell excitability and decreases inspiratory-modulated synaptic current in phrenic motoneurones via different types of receptors. When these responses occurred simultaneously, the increase in excitability predominated and the net effect was an augmentation of inspiratory-modulated phrenic motoneurone activity.

FAST and SLOW ipsilateral and contralateral spinal reflexes in the neonate rat are modulated by 5-HT

1. Three ipsilateral (MSR, PSR, IPSI SLOW) and two contralateral segmental reflexes (CON FAST, CON SLOW) were recorded from L4 or L5 ventral roots of the neonate rat spinal cord in vitro. MSR, PSR and CON FAST were evoked from lower threshold afferents; more intense stimulation evoked IPSI SLOW and CON SLOW. 2. Kainate/AMPA receptors were involved in mediation of MSR, PSR, CON FAST, IPSI SLOW and CON SLOW and NMDA receptors in mediation of CON FAST, IPSI SLOW and CON SLOW. 3. All five reflexes were depressed by 5-HT (IC50 1.2-7.9 microM; order of sensitivity, CON SLOW > CON FAST = IPSI SLOW > MSR = PSR); and by 5-CT (IC50 1.9-8.8 nM; order of sensitivity, MSR > IPSI SLOW = CON FAST = CON SLOW > PSR). alpha-Me-5-HT also depressed all five reflexes. 4. Dipropyl-5-CT selectively depressed MSR and CON SLOW (IC50 90-170 nM) but was less potent than 5-CT. 8-OH-DPAT selectively depressed MSR (IC50 1.1 microM), IPSI SLOW and CON SLOW (IC50 5.7-7.6 microM), while methylsergide depressed only MSR (IC50 26 nM). 5. Phenyl biguanide and m-chlorophenyl biguanide (5-HT3 receptor agonists) had no significant effects on any reflex. 6. It is concluded that a 5-HT1-like receptor mediates depression of the MSR. A different receptor or a mixed population of receptors, but not 5-HT3 receptors, mediate inhibition of PSR, CON FAST, IPSI SLOW and CON SLOW.

Ionic mechanisms mediating 5-hydroxytryptamine- and noradrenaline-evoked depolarization of adult rat facial motoneurones

1. The actions of 5-hydroxytryptamine (5-HT) and noradrenaline (NA) on the membrane properties of facial motoneurones in slices from the adult rat brainstem in vitro were examined using intracellular recording techniques. 2. In voltage clamp recording, hyperpolarizing voltage steps (> 20 mV), from holding potentials at or close to the resting potential, induced a slowly activating, voltage-dependent inward current possessing properties similar to the hyperpolarization-activated current (Ih) seen in other cell types. From tail current analysis two groups of facial motoneurones can be distinguished in terms of the activation range for Ih, one with a half-maximal activation at -81 mV and the other at -94 mV but with similar shapes. 3. 5-HT (120/126) and NA (21/21) depolarized facial motoneurones. The reversal potentials (Em) obtained from peak voltage amplitude I-V plots in varying extracellular potassium concentrations suggested mechanisms involving a decrease in K+ conductance. 4. Under voltage clamp, close to the resting potential, both 5-HT (39/41) and NA (13/13) evoked inward currents. 5. I-V plots and plots of 5-HT-sensitive current at different membrane potentials, obtained from currents evoked by voltage steps and measured before the development of Ih (instantaneous current), indicated that the 5-HT-evoked inward current was predominately associated with a decrease in conductance but with a range of reversal potentials for 5-HT (E5-HT) from close to, to much more negative than the reversal potential for a potassium conductance (EK). In some cases no change or increases in instantaneous conductance were observed. 6. Steady-state I-V relationships and plots of 5-HT-sensitive current, measured after development of Ih, indicated a 5-HT-associated conductance increase with a time and voltage dependence close to that of Ih, which could be abolished by extracellular caesium (2-5 mM). 7. The NA-evoked inward current was always associated with a decrease in conductance. Instantaneous and steady-state I-V relationships as well as plots of NA-sensitive current indicated a reversal potential at EK. 8. The activation curve for Ih was shifted to more positive potentials in the presence of 5-HT. The time constant for activation of Ih showed a similar shift. 9. 5-Carboxamidotryptamine (5-CT), a 5-HT receptor agonist, was selective for the enhancement of Ih and only evoked an inward current when the holding potential was within the activation range of Ih.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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.