Intracellular studies showing modulation of facial motoneurone excitability by serotonin

Morphology and distribution of serotoninergic and oculomotor internuclear neurons in the cat midbrain

Serotoninergic fibers have been reported in both the abducens and facial nuclei of the cat. Furthermore, serotoninergic dorsal raphe and oculomotor internuclear neurons occupy similar locations in the periaqueductal gray overlying the oculomotor and trochlear motor nuclei. To resolve the issue of whether these two populations of neurons overlap, serotoninergic fibers were assayed in the abducens and facial nucleus; then the morphologies and distributions of identified serotoninergic neurons and oculomotor internuclear neurons were determined. Both the abducens and facial nuclei contained varicosities labelled with antibody to serotonin, but a much higher density of immunoreactive fibers was present in the latter, especially in its medial aspect. Distinct synaptic profiles labelled with antibodies to serotonin were observed in both nuclei. In both cases, terminal profiles contained numerous small, predominantly spheroidal, synaptic vesicles as well as a few, large, dense-core vesicles. These profiles made synaptic contacts onto dendritic and, in the facial nucleus, somatic profiles that occasionally displayed asymmetric, postsynaptic, membrane densifications. Following injection of horseradish peroxidase into either the abducens or facial nuclei, double-label immunohistochemical techniques demonstrated that the serotoninergic and oculomotor internuclear neurons form two distinct cell populations. The immunoreactive serotoninergic cells were distributed within the dorsal raphe nucleus, predominantly caudal to the retrogradely labelled oculomotor internuclear neurons. The latter were located in the oculomotor nucleus along its dorsal border and in the adjacent supraoculomotor area. Intracellular injection of horseradish peroxidase revealed that oculomotor internuclear neurons have multipolar somata with up to ten long, tapering dendrites that bifurcate approximately five times. Their dendritic fields were generally contained within the nucleus and adjacent supraoculomotor area. In contrast, putative serotoninergic neurons were often spindle-shaped and exhibited far fewer primary dendrites. Many of these long, narrow, sparsely branched dendrites crossed the midline and extended to the surface of the cerebral aqueduct. In the vicinity of the aqueduct they branched repeatedly to form a dendritic thicket. The axons of the intracellularly stained serotoninergic neurons emerged either from the somata or the end of a process with dendritic morphology, and in some cases they produced axon collaterals within the periaqueductal gray. Thus the oculomotor internuclear and serotoninergic populations differ in both distribution and morphology.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacologically distinct actions of serotonin on single pyramidal neurones of the rat hippocampus recorded in vitro

1. The actions of serotonin (5-HT) on pyramidal cells of the CA1 region of the rat hippocampus were characterized using intracellular recording in in vitro brain slices. 2. 5-HT typically evokes a biphasic response consisting of a hyperpolarization which is followed by a longer-lasting depolarization. These effects on membrane potential are accompanied by a decrease in the calcium-activated after-hyperpolarization (a.h.p). 3. Detailed analysis using 5-HT antagonists and agonists indicates that the hyperpolarization is mediated by a 5-HT1A receptor. Spiperone is the most effective antagonist of the response and the selective 5-HT1A agonist, 8-OHDPAT, behaves as a partial agonist at this receptor. In agreement with the distribution of 5-HT1A binding sites, responses to 5-HT were most prominent in the stratum radiatum. 4. The hyperpolarizing response is associated with a decrease in input resistance, is blocked by extracellular barium and intracellular caesium, is unaffected by the chloride gradient, and its reversal potential shifts with the extracellular concentration of potassium as predicted for a response mediated by a selective increase in potassium permeability. 5. The depolarizing response and reduction in the a.h.p. could be studied in isolation by blocking the hyperpolarizing response with either pertussis toxin or spiperone. The pharmacology of these responses did not correspond to that of any of the 5-HT binding sites reported in C.N.S. tissue. Although the depolarization and blockade of the a.h.p. have the same time course it is unclear if they are mediated by the same or different receptors. 6. The depolarization most likely results from a decrease in resting potassium conductance. However, neither a blockade of the M current nor the a.h.p. current can account for the depolarization. 7. Blockade of phosphodiesterase activity by 3-isobutyl-1-methylxanthine (IBMX) did not enhance the depressant action of 5-HT on the a.h.p., making it unlikely that this action is mediated by cyclic AMP. 8. Blockade of the a.h.p. by 5-HT reduces spike frequency adaptation and counteracts the inhibitory action of 5-HT on 5-HT1A receptors. This excitatory action outlasts the hyperpolarizing action. 9. In summary 5-HT acts on at least two distinct receptors on hippocampal pyramidal cells, one coupled to the opening of potassium channels and a second coupled to a decrease in a resting potassium conductance and a decrease in the a.h.p.

Two distinct effects of 5-hydroxytryptamine on single cortical neurons

The ability of the indoleamine serotonin (5-hydroxytryptamine; 5-HT) to alter membrane characteristics of neocortical neurons was analyzed using intracellular recording techniques. The present study demonstrates that 5-HT primarily depolarized 68% of cortical neurons probably by decreasing a resting K+ conductance, an effect blocked by the antagonists ritanserin and cinanserin and apparently mediated by 5-HT2 receptors. A hyperpolarization associated with an increased conductance state and insensitive to 5-HT2 antagonists was observed in 26% of the neurons and could be mimicked by the selective 5-HT1A agonist (+/-)-8-hydroxy-2-(di-N-propyl-amino)tetralin (8-OH-DPAT). Therefore cortical pyramidal neurons contain at least two distinct functional 5-HT receptors whose activation produces opposing effects on membrane potential and conductance.

Modulation of the vestibulo-ocular reflex by serotonin in the rat

The effects of intraventricular injection of serotonin (5-HT) and its agonists and antagonists on the amplitude of the vestibulo-ocular reflex were studied in chronic implanted rats. 5-HT (10(-5) M) triggers an increase of the amplitude of the reflex which lasts 30 min. Similar results are obtained when N,N-dimethyl-5-methoxytryptamine (10(-3) M) is introduced into the ventricular cannula. The increasing effects observed both with 5-HT and N,N-dimethyl-5-methoxy-tryptamine are abolished by methiothepin, a potent antagonist of 5-HT receptors. Injection of indirect agonists like pargyline, a monoamine oxidase inhibitor, or fluoxetine, a potent inhibitor of 5-HT reuptake, is followed by an increase of the amplitude of the vestibulo-ocular reflex. These results indicate that 5-HT can modulate the activity of the vestibulo-ocular pathway and muscular tone of extraocular muscles. Location and involvement of various modulating 5-HT sites are discussed.

Inter-male aggressive signals in weakly electric fish are modulated by monoamines

Apteronotus leptorhynchus is a gymnotid fish producing a constant high frequency electric organ discharge (EOD). Males of this species use transient increases in EOD frequency (chirps) as aggressive signals. They will also shift the frequency of their EOD away from the similar frequency of a nearby conspecific in order to protect their ability to electrolocate (jamming avoidance response, JAR). Monoamines have been implicated as modulatory agents for various sensorimotor and affective systems, including aggressive behaviour. Since these monoamines are present in the brain of this fish (unpublished observation), we have used these simple and quantifiable behaviours to study the role of monoamines, with special emphasis on possible specific effects on aggressive signalling (chirps). When serotonin (0.1 microgram) is injected directly into the ventricle of these fish it briefly inhibits chirping (aggression) without inhibiting the JAR; this is consistent with the hypothesis that, in mammals, serotonin inhibits aggressive behaviour. Noradrenaline (0.1 microgram) enhances both chirping and the JAR. Dopamine (0.1 microgram) enhances the JAR; it has powerful but inconsistent effects on chirping (inhibition or excitation).

Intrathecal noradrenaline has a dose-dependent inhibitory or facilitatory effect on the flexion reflex in the rat

The effect of intrathecal noradrenaline (NA) on the hamstring flexion reflex to subcutaneous electrical shocks was examined in unanaesthetized, decerebrate, spinalized rats. Low doses of NA (200 ng and 2 micrograms) depressed and high doses (20 micrograms and 200 micrograms) facilitated the reflex. It is suggested that the primary effect of NA in the dorsal horn is inhibitory and in the ventral horn excitatory. Furthermore, the neurons mediating these effects are more sensitive to NA in the dorsal horn than in the ventral horn.

Effect of spinal cord TRH deficiency on lower motorneuron function in the rat

Thyrotropin-releasing hormone (TRH), present in high concentrations in the mammalian spinal cord, exerts excitatory effects on the alpha-motorneuron (AMN) via axodendritic contacts. We used the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) to deplete TRH from the ventral horn of the spinal cord of adult rats to determine whether the tripeptide may be trophic to the AMN. The rats were studied blindly and sequentially for 11 weeks. Motor performance remained normal by clinical and electrophysiologic assessments. AMN counts were not reduced in the lumbar cord, and gastrocnemius muscle showed no evidence of denervation in treated rats. We conclude that in the adult rat chronic ventral horn TRH deficiency does not lead to AMN degeneration and is not associated with a significant alteration of AMN function.

Excitatory action of 5-HT on deglutitive substrates in the rat solitary complex

The excitatory effect of serotonin (5-HT) on the pharyngeal stage of swallowing was investigated in urethane anaesthetised rats with respect to the involvement of neural substrates located in the central and intermediolateral regions of the nucleus tractus solitarii (NTS). Micropneumophoretic ejection of 5-HT 5-50 pmol either produced deglutitory responses or selectively facilitated the S-glutamate-evoked pharyngeal responses when applied in 1-10 pmol prepulses. The excitatory/facilitatory effect of 5-HT was enhanced by intravenous threshold doses of the 5-HT-mimetic, quipazine (0.3-1 mumol/kg) and reversibly blocked by the 5-HT2-receptor antagonists, methysergide, metergoline and ketanserin. 5-HT doses exceeding 10-60 pmol gave rise to a non-selective reversible inhibition of glutamate- and acetylcholine (ACh)-evoked pharyngeal or oesophageal responses which was not prevented or reversed by 5-HT2-receptor antagonists, but was readily overcome by increasing the amount of glutamate or ACh ejected. Non-selective deglutitive inhibition after high doses of 5-HT could, therefore, result from neuronal desensitization secondary to excessive stimulation or activation of a different type of 5-HT receptor. These results corroborate an excitatory role of 5-HT in both reflex and automatic swallowing and demonstrate that the NTS is a major site of serotoninergic facilitation of swallowing.

Membrane excitability changes in hindlimb motoneurons induced by stimulation of the locus coeruleus in cats

The present analysis describes the cellular mechanisms underlying the heightened membrane excitability of hindlimb flexor and extensor motoneurons upon stimulation of the locus coeruleus (LC) in unanesthetized, decerebrate cats. In a total of 73 cells, brief train stimuli to the LC at 50-300 microA intensity evoked one of 4 patterns of motoneuronal responses: a simple excitatory postsynaptic potential (EPSP) with weak trailing depolarization, a double-peak EPSP, an EPSP succeeded by a weak hyperpolarization, or a slow rising EPSP. As the initial dominant EPSP was a consistent finding among all cells and the ensuing potentials were variable in polarity, quantitative characterization was focused on the initial EPSP only. In all cells tested (n = 11), the LC-EPSP was accompanied by a decrease in input resistance. The excitatory LC action was further demonstrated by the consistent (n = 25 cells) motoneuron rheobase decrease when the latter was measured coincident with the summit of an LC-EPSP. Furthermore, the time course of the single-spike afterhyperpolarization became shortened during the LC conditioning stimuli (n = 16 motoneurons). Our data show that the descending LC action on motoneurons is typified by an EPSP accompanied by a net decrease in input resistance as well as a concurrent increase in motoneuron electrical excitability.

Complex temporal changes in 5-hydroxytryptamine synthesis in the central nervous system induced by experimental polyarthritis in the rat

The purpose of this study was to investigate modifications of 5-HT synthesis in a chronic pain model, the arthritic rat, at different times after the inoculation with Freund's adjuvant. This study confirms our previous findings that experimental induced polyarthritis is associated with a marked increase in free tryptophan levels in serum. During the acute phase of the disease (15-21 days after the adjuvant), the general increase in 5-HT synthesis observed in the CNS appeared to be related to an increase in tryptophan availability due to the elevation of free tryptophan in serum. During the post-acute phase of the disease (28-42 days after the adjuvant), the level of free tryptophan in the serum remained markedly increased but the levels in the CNS tended to return to normal values in all areas examined. At 42 days, 5-HT synthesis in the brain had also returned to normal values but was further increased at the spinal level. In addition, although 5-HT levels and 5-HT synthesis were increased in the dorsal as well as in the ventral part of the cord, an increase in the rate of disappearance of the amine after blockade of the decarboxylase (benserazide) was only observed in the dorsal part. This result tends to suggest that the descending serotonergic system projecting to the dorsal horn is preferentially activated during chronic pain.

Epinephrine and norepinephrine modulate neuronal responses to excitatory amino acids and agonists in frog spinal cord

The interaction of the catecholamines epinephrine (E) and norepinephrine (NE) (1.0-100 microM) and excitatory amino acids on motoneurons of the isolated superfused frog spinal cord was investigated by sucrose gap recordings from ventral roots. Exposure of the cord to E or NE 30 sec prior to application of L-aspartate or L-glutamate reduced the motoneuron depolarizations produced by the amino acids. The reduction of responses to the mixed receptor agonists L-glutamate and L-aspartate may be the result of opposite actions of the catecholamines on the activation of specific excitatory receptors by the amino acids. Thus, E and NE facilitated depolarizations caused by application of N-methyl-D-aspartate (NMDA) and depressed those produced by quisqualate. The effect on NMDA responses appeared to be beta-adrenoceptor mediated because it was mimicked by the beta-agonist isoproterenol and blocked by propranolol. The effect on quisqualate depolarizations appeared to require activation of alpha 2-adrenoceptors; it was mimicked by the alpha 2-agonists clonidine and alpha-methylnorepinephrine and antagonized by yohimbine and piperoxan. These results are important in understanding the actions of catecholamines on reflex transmission in spinal pathways which use excitatory amino acids as transmitters.

Raphe dorsalis-spinal cord cografts in oculo: electrophysiological evidence for an excitatory serotonergic innervation of transplanted spinal neurons

Intraocular replicas of descending serotonergic bulbospinal pathways were constructed by means of sequential intraocular grafting of nucleus raphe dorsalis and spinal cord. Using extracellular recordings we have studied the functional connections between such double grafts. Superfusion of single spinal cord grafts with serotonin causes an increase in spontaneous activity. This excitation is reversibly blocked by the specific 5-hydroxytryptamine (5-HT) antagonist metergoline. Stimulation of the raphe part of nucleus raphe dorsalis-spinal cord double grafts causes a long-lasting excitation of the spinal neurons similar to that seen in single spinal cord grafts given serotonin. The electrically induced excitation could also be reversibly blocked with metergoline. It is concluded that serotonin-containing nerves from grafts of nucleus raphe dorsalis are not only morphologically organotypic, but also form functional contacts with neurons in cografted spinal cord. The results further support an excitatory or modulatory role of the descending spinal serotonergic pathways and demonstrate that functional contacts can be established between isolated CNS grafts when 5-HT fibers invade immature or mature spinal cord tissue.

Monoamine innervation of the oculomotor nucleus in the rat. A radioautographic study

The serotonin and noradrenaline innervations of the rat oculomotor nucleus were examined by high resolution radioautography after in vivo labeling with tritiated 5-hydroxytryptamine and dopamine, respectively. Noradrenaline as well as serotonin endings (axonal varicosities) pervaded the entire nucleus, but the latter were at least six times more numerous (1.3 X 10(6) per mm3 of tissue) and were often found in the immediate vicinity of neuronal somata and proximal dendrites. The axon terminals of both types were of similar size and exhibited some large dense-cored vesicles in association with aggregated small and clear vesicles. The dense-cored vesicles were, however, more frequent and the content in clear vesicles more pleomorphic in serotonin than noradrenaline endings. In single thin sections, the proportion of noradrenaline and serotonin profiles exhibiting a synaptic junction was relatively small (15%). These were either symmetrical or asymmetrical when made on dendritic branches but invariably symmetrical on spines. In addition, a significant number of serotonin terminals were seen in close apposition or synaptic contact with neuronal perikarya and large dendrites, allowing for a direct, "proximal" action of serotonin. Moreover, many such terminals appeared to be coupled with unlabeled endings of another category, characterized by dispersed, uniformly round and clear synaptic vesicles, providing an alternate route for a proximal effect of serotonin in the oculomotor nucleus. In line with previous investigations on other motor nuclei, these data support the likelihood of a close involvement of both noradrenaline and serotonin in the control of motoneuronal activity.

Norepinephrine depolarizes lateral horn cells of neonatal rat spinal cord in vitro

Superfusion of norepinephrine (NE) (1-50 microM) onto lateral horn cells, including antidromically identified sympathetic preganglionic neurons (SPNs), situated in thin transverse neonatal rat thoracolumbar spinal cord slices caused a membrane depolarization and repetitive cell discharges. The NE depolarization was associated with an increase in membrane resistance, and the response became smaller upon conditioning hyperpolarization; a clear reversal of polarity, however, was not observed. Pretreating the slices with phentolamine and prazosin but not yohimbine or propranolol prevented the depolarizing effect of NE. This finding, in conjunction with the evidence of the presence of noradrenergic fibers in the spinal cord, suggests that NE may serve as an excitatory neurotransmitter to neurons of the lateral horn.

Modulation of a transient outward current in serotonergic neurones by alpha 1-adrenoceptors

The excitability of various neurones in the mammalian central nervous system (CNS), ranging from motoneurones to serotonergic neurones, is enhanced by alpha 1-adrenoceptor agonists. Excitations mediated via alpha 1-adrenoceptors are associated with a slow depolarization and an increase in input resistance, probably resulting from a decrease in resting potassium conductance. However, the involvement of voltage-dependent transient currents in mediating alpha 1 excitatory effects has not been evaluated. An early transient outward current has been described which is important in regulating the frequency of repetitive firing; it is activated by depolarizing voltage steps from potentials more negative than rest and blocked by 4-aminopyridine. This current, which has been termed 'IA', was found originally in invertebrates and subsequently in various vertebrate neurones. The present single-electrode voltage-clamp study demonstrates an early transient outward current (IA) in serotonergic neurones which is suppressed by noradrenaline and the alpha 1-agonist phenylephrine; a suppression of IA may account in part for the acceleration of pacemaker activity induced by alpha 1-agonists in serotonergic neurones.

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

Lumbar spinal motoneurons of urethane-anesthetized rats were driven at stable low firing rates by automatically cycled iontophoretic applications of glutamate or aspartate. The effects of iontophoretically applied serotonin, substance P or thyrotropin-releasing hormone (TRH) on glutamate or aspartate-evoked activity were then tested. All 3 substances were found to enhance both glutamate- and aspartate-induced excitation of the motoneurons. This enhancement of excitability was usually preceded by a brief period of inhibition at current onset. Although the effects of serotonin and substance P were qualitatively remarkably similar, TRH differed in that TRH occasionally inhibited motoneuron excitability without subsequent facilitation, and tachyphylaxis developed for the facilitatory effects of TRH. After TRH desensitization, serotonin could still enhance spinal motoneuron excitability.

Pharmacology of spinal adrenergic systems which modulate spinal nociceptive processing

Spinopetal pathways may be activated by a variety of brainstem manipulations including microinjections of morphine which are known to modulate spinal nociceptive processing. Based on the ability of these manipulations to release spinal noradrenalin; the ability to reverse the antinociceptive effects by intrathecal adrenergic antagonists and the fact that intrathecal injections of noradrenalin mimic the antinociceptive effect, it appears that the descending modulation may be mediated by descending noradrenergic systems. Examination of the spinal receptor systems with intrathecally administered agents indicates that spinal alpha, but not beta adrenergic receptor agonists produce a powerful analgesia as measured on a variety of reflex and operant measures in mouse, rat, cat, primate and man. On the basis of agonist and antagonist structure-activity relationships it appears that a significant effect can be produced in the absence of any detectable effect on motor function by the occupation of spinal alpha 2 receptors. Distinguishable alpha 1 receptors also appear "analgetically-coupled," but their effects are uniformly contaminated by signs of cutaneous hyperreflexia at doses required to produce analgesia. The ordering of potency with which intrathecal adrenergic antagonists reverse the effects of intrathecal noradrenalin is indistinguishable from that of the reversal by these intrathecal agents of the antinociceptive effects evoked by brainstem morphine. This suggests that the population of spinal receptors acted upon by exogenously administered adrenergic agonists and endogenously released noradrenaline have indistinguishable characteristics.

Serotonin depolarizes type A and C primary afferents: an intracellular study in bullfrog dorsal root ganglion

Intracellular recordings were obtained from the somata of type A and C primary afferents in the isolated bullfrog dorsal root ganglion (DRG) preparation. Bath application of serotonin (5-HT) in concentrations of 0.25-1.0 mM led to slow and fast depolarizing responses. Slow, maintained 5-HT depolarizations were observed in 47% of type A and 70% of type C neurons. These slow depolarizations were associated with an underlying increase in input resistance (Rin). In some type A neurons, the Rin increase was masked by a decrease in Rin due to depolarization-induced rectification. The slow 5-HT depolarization of type A, but not type C neurons showed pronounced tachyphylaxis to repeated 5-HT applications. In type C afferents, serotonin's slow action was often accompanied by spontaneous firing. Manganese decreased slow 5-HT depolarizations of both cell types. A slow depolarization and excitation of type C afferents by methysergide and cinanserin was also observed. Fast transient 5-HT depolarizations accompanied by a rapid decrease in Rin were observed in 7% of type A and 24% of type C neurons. In some DRG cells the fast and slow depolarizations combined to form a biphasic response. The actions of 5-HT reported here resemble in some ways 5-HT responses recorded extracellularly from the spinal terminations of primary afferents.

Serotonin and co-localized peptides: effects on spinal motoneuron excitability

Iontophoretically applied serotonin (5HT), substance P (SP), thyrotropin releasing hormone (TRH) and proctolin all enhanced spinal motoneuron activity evoked by glutamate in urethane-anesthetized rats. 5HT and SP produced more consistent facilitation of excitability both within and between motoneurons than did TRH and proctolin which failed to facilitate several motoneurons. It is concluded that 5TH, SP, TRH and proctolin, which appear to co-exist in various combinations in ventral horn terminals near spinal motoneurons, have complimentary effects on motoneuron excitability.

Hyperpolarization of serotonergic neurons by serotonin and LSD: studies in brain slices showing increased K+ conductance

Serotonin and LSD hyperpolarized serotonergic dorsal raphe neurons in rat midbrain slices; the hyperpolarizations were accompanied by a decrease in input resistance, suggesting an increase in potassium conductance as one possible mechanism. Reversal potentials for serotonin and LSD-induced hyperpolarizations showed a shift of approximately 18 mV for a two-fold change in extracellular potassium concentration; this shift was close to that predicted by the Nernst equation for a potassium-dependent conductance.