Effects of serotonin on dorsal horn dorsal spinocerebellar tract neurons

Serotonin shifts first-spike latencies of inferior colliculus neurons

Many studies of neuromodulators have focused on changes in the magnitudes of neural responses, but fewer studies have examined neuromodulator effects on response latency. Across sensory systems, response latency is important for encoding not only the temporal structure but also the identity of stimuli. In the auditory system, latency is a fundamental response property that varies with many features of sound, including intensity, frequency, and duration. To determine the extent of neuromodulatory regulation of latency within the inferior colliculus (IC), a midbrain auditory nexus, the effects of iontophoretically applied serotonin on first-spike latencies were characterized in the IC of the Mexican free-tailed bat. Serotonin significantly altered the first-spike latencies in response to tones in 24% of IC neurons, usually increasing, but sometimes decreasing, latency. Serotonin-evoked changes in latency and spike count were not always correlated but sometimes occurred independently within individual neurons. Furthermore, in some neurons, the size of serotonin-evoked latency shifts depended on the frequency or intensity of the stimulus, as reported previously for serotonin-evoked changes in spike count. These results support the general conclusion that changes in latency are an important part of the neuromodulatory repertoire of serotonin within the auditory system and show that serotonin can change latency either in conjunction with broad changes in other aspects of neuronal excitability or in highly specific ways.

Serotonin in the inferior colliculus

It has been recognized for some time that serotonin fibers originating in raphe nuclei are present in the inferior colliculi of all mammalian species studied. More recently, serotonin has been found to modulate the responses of single inferior colliculus neurons to many types of auditory stimuli, ranging from simple tone bursts to complex species-specific vocalizations. The effects of serotonin are often quite strong, and for some neurons are also highly specific. A dramatic illustration of this is that serotonin can change the selectivity of some neurons for sounds, including species-specific vocalizations. These results are discussed in light of several theories on the function of serotonin in the IC, and of outstanding issues that remain to be addressed.

Modulation of responses of feline ventral spinocerebellar tract neurons by monoamines

Ventral spinocerebellar tract neurons located in laminae V-VII of cat lumbar spinal cord were tested for the effects of ionophoretically applied monoamines and receptor selective agonists. Extracellularly recorded responses, monosynaptically evoked by group I afferents in a muscle nerve, were compared before, during, and after ionophoresis. They were analyzed with respect to changes in the number of evoked spikes and in the latency. Both serotonin (5-HT) and noradrenaline (NA) were found to facilitate responses of all neurons tested. Ionophoresis of three serotonin subtype receptor agonists (5-carboxamidotryptamine maleate, 5 methoxytryptamine HCl, and alpha-methyl 5-hydroxytryptamine) and of two NA receptor agonists (phenylephrine and isoproterenol) likewise had a facilitatory effect. However, three other 5-HT receptor agonists (8-hydroxy-dipropylaminotetraline hydrobromide), 2-methyl 5-hydroxytryptamine, and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl and two NA receptor agonists (tizanidine and clonidine) had the opposite effect because they depressed responses of the tested neurons. These results show that information forwarded by means of the ventral spinocerebellar tract may be modulated by monoamines and that several receptor subtypes, located pre- or postsynaptically, may be involved. The results also demonstrate that transmission by means of group I muscle afferents may not only be facilitated by monoamines but also depressed by selective receptor subtype activation.

Serotonin alters multi-segmental convergence patterns in spinal cord deep dorsal horn and intermediate laminae neurons in an in vitro young rat preparation

Each spinal neuron has a receptive field that corresponds to stimulation of a specific area of skin or subcutaneous tissue. Receptive fields are plastic and can be altered during development and injury but the actions of neuromodulators, such as serotonin (5-hydroxytryptamine, 5-HT) on receptive field properties are not well known. We used stimulation of multiple adjacent dorsal root spinal segments as a measure of "receptive field size" to determine the effects of 5-HT on multi-segmental convergent input onto neurons in laminae IV-VII. Whole-cell patch-clamp recordings were undertaken in the in vitro hemisected thoracolumbar spinal cord of rats aged 8-10 days old. Based on synaptic responses, neurons could be divided into two predominant groups and 5-HT exerted different effects on these groups. The first group received excitatory post-synaptic potentials (EPSPs) from the homonymous dorsal root but inhibitory post-synaptic potentials (IPSPs) with increasing amplitude from more distant dorsal roots. In this group, 5-HT preferentially depressed the IPSPs from adjacent nerve roots while leaving the EPSP intact. The second group received short-latency EPSPs from all segments stimulated and 5-HT potently depressed all synaptic input. In both populations the depressant actions of 5-HT increased with dose (0.1-10.0 microM). Bicuculline and strychnine did not affect the 5-HT induced short-latency synaptic depression. These results suggest that descending serotonergic systems depress spinal sensory convergence in a graded and differentiated manner. The findings are discussed in relation to the modulation of nociceptive signaling.

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 modulation of spinal ascending activity and sacral reflex activity evoked by pelvic nerve stimulation in cats

Serotonin (5-HT) may be inhibitory to micturition at a spinal level. A potential mechanism of action for serotonergic inhibition of bladder function is a depression of the ascending limb of the supraspinal reflex mediating micturition. Ascending activity evoked by pelvic nerve stimulation was recorded in the thoracic spinal cord of anesthetized cats. For comparison, spinal reflex activity evoked by pelvic nerve stimulation was recorded on the pudendal nerve. The effects of intrathecal administration of serotonergic agents were examined to determine whether spinal and supraspinal responses to bladder afferent activation were modulated by 5-HT. Methysergide (60 nmol), a non-selective serotonergic antagonist, increased ascending activity by 61+/-7% and depressed spinal reflex activity by 38+/-6%. Zatosetron (10 nmol), a 5-HT3 antagonist had a similar effect on both activities (increased by 93+/-24% and decreased by 77+/-7%, respectively). The effect on ascending activity of blocking 5-HT3 receptors was also confirmed with ICS 205930 and MDL 72222. 2-Methyl-5-HT (800 nmol), a 5-HT3 agonist, depressed ascending activity to 46+/-9% of control, but enhanced spinal reflex activity by 73+/-92%. These results demonstrate that stimulation of 5-HT3 and methysergide-sensitive 5-HT receptors can inhibit ascending activity and facilitate spinal reflex activity elicited by activation of bladder afferents. It is suggested that descending serotonergic pathways may participate in the spinal coordination of urinary continence.

Modulation of responses of four types of feline ascending tract neurons by serotonin and noradrenaline

Modulation of responses of four types of ascending tract cells by noradrenaline and serotonin was compared in order to investigate how information forwarded by these cells may be gated by monoaminergic tract neurons. Spinocervical tract, postsynaptic dorsal column and dorsal spinocerebellar tract neurons located in Clarke's column and in the dorsal horn were identified by their axonal projections. Noradrenaline and serotonin were applied ionophoretically close to a selected neuron, and their effects were tested on extracellularly recorded responses of this neuron to electrical stimulation of low-threshold skin afferents and group II muscle spindle afferents. The modulatory actions of noradrenaline and serotonin were estimated from changes in the number of responses evoked by 30 successive stimuli, the minimal latencies of these responses, and their firing frequency. All four populations of ascending tract neurons investigated were modulated by serotonin and noradrenaline, but not in the same way. The responses were most often depressed by noradrenaline and facilitated by serotonin, but in some types of neuron they were affected in the same direction. Transmission from low-threshold skin and group II muscle afferents changed in the same direction in some types of neuron but in the opposite direction in other types. The results indicate that transfer of information from skin and group II muscle afferents to supraspinal centres may be gated by descending monoaminergic pathways in a highly differentiated manner, and is adjusted to the requirements of various behavioural situations.

Synaptic relationships between serotonin-immunoreactive axons and dorsal horn spinocerebellar tract cells in the cat spinal cord

Dorsal horn spinocerebellar tract cells were identified according to electrophysiological criteria in adult cats and labelled intracellularly with horseradish peroxidase. Sections containing labelled neurons were processed to reveal serotonin immunoreactivity and examined with light and electron microscopy. Numerous contacts were observed on cell bodies, and on proximal and intermediate parts of dendrites. Electron microscopic examination of contacts revealed that synaptic junctions were usually present at the region of apposition. It is concluded that serotonin has a postsynaptic action on dorsal horn spinocerebellar tract cells and that this action is mediated through conventional synapses.

Contacts between serotoninergic fibres and dorsal horn spinocerebellar tract neurons in the cat and rat: a confocal microscopic study

Contacts between serotoninergic nerve fibres and dorsal horn dorsal spinocerebellar tract neurons were analysed in order to investigate the morphological basis of actions of serotonin upon dorsal spinocerebellar tract neurons. In a series of experiments dorsal spinocerebellar tract neurons were labelled with intracellularly injected rhodamine-dextran in the cat. The neurons were monosynaptically excited by group II muscle afferents and cutaneous afferents and were identified by antidromic activation following stimuli applied in the cerebellum. In the second series of experiments dorsal spinocerebellar tract neurons were labelled by retrograde transport of Fluorogold injected into the cerebellum in the rat. In both series, serotoninergic fibres were labelled by using a specific anti-serotonin antiserum and were revealed by immunofluorescence. Appositions between the serotoninergic fibres and the cells were inspected with a dual channel confocal microscope. The merged images obtained with the two channels of the microscope were viewed in single optical planes 2 microns apart and in rotated three-dimensional reconstructions. Serotoninergic nerve fibres were found in apposition to cell bodies of all feline dorsal spinocerebellar tract neurons (n = 7) and of 75% of rat dorsal spinocerebellar tract neurons (n = 90). The numbers of putative contacts on cell bodies varied between less than 100 and nearly 300 (mean 160) in the cat and between about five and 30 in the rat. Contacts with dendrites of feline neurons were seen on 96% of 72 dendrites within 300 microns from soma and on 91% of 23 dendrites at distances of 300-500 microns. The number of such contacts varied from less than five to 150 on a single dendrite within these ranges of distances. Their total number within 100 microns from the soma was comparable or exceeded the number of contacts on the soma.