Immunohistochemical studies on the serotonergic innervation of the pia mater

Local treatments of dorsal raphe nucleus induce changes in serotonergic activity in rat major cerebral arteries

BACKGROUND AND PURPOSE

Rat major cerebral arteries seem to receive serotonergic fibers originating from the dorsal raphe nucleus (DRN), but little is known about their function. The aim of our present work was to establish a functional relationship between this brain stem nucleus and the cerebral blood vessels by studying the effects of several treatments in the DRN on cerebrovascular serotonergic activity.

METHODS

Serotonin, clomipramine, 8-OH-DPAT, and WAY-100635 were administered in DRN. A stereotaxically localized electrode allowed the electrical stimulation of this brain stem nucleus. Serotonergic activity was appraised in major cerebral arteries, striatum, and hippocampus from 5-hydroxytryptophan accumulation after aromatic L-amino acid decarboxylase inhibition with NSD-1015.

RESULTS

Serotonin significantly decreased serotonergic activity in major cerebral arteries and striatum without affecting it in hippocampus. This reduction was blocked by previous injection of WAY-100635 in DRN. Local administration of 8-OH-DPAT or clomipramine elicited an effect similar to that of serotonin, whereas that of WAY-100635 did not modify serotonergic activity in either of the tissues. Electrical stimulation of DRN significantly increased serotonergic activity in major cerebral arteries and striatum but not in hippocampus.

CONCLUSIONS

These results confirm the presence of a serotonergic innervation in rat major cerebral arteries functionally related to DRN. 5-HT(1A) receptor activation partly mediates the action of serotonin in DRN. A serotonergic tone acting on these somatodendritic receptors was not clearly found.

Immunohistochemical examination of intraspinal serotonin neurons and fibers in the chicken lumbar spinal cord and coexistence with Leu-enkephalin

Intraspinal serotonin-positive cells and fibers were examined in the chicken lumbar spinal cord following removal of descending serotonin fibers by spinal transection. Co-localization of Leu-enkephalin immunoreactivity in intraspinal serotonin cells was also examined using a double immunofluorescence labeling technique. By one or two weeks after spinal transection, virtually all supraspinal serotonin fibers were eliminated. Intraspinal serotonin cells were located ventral or ventrolateral to the central canal corresponding to laminae VII, VIII, and IX, and the anterior funiculus. Intraspinal serotonin cells sent fibers to (1) the pia mater on the ventral or ventrolateral surface of the spinal cord; (2) vessels in the spinal cord; (3) sympathetic preganglionic column of Terni; (4) other intraspinal serotonin neurons; (5) the central canal. Some 30%-50% of the intraspinal serotonin cells co-localized with Leu-enkephalin. Intraspinal serotonin fibers co-containing Leu-enkephalin were observed in the pia mater located on the most lateral surface of the spinal cord.

Innervation of cerebral arteries by nerves containing 5-hydroxytryptamine and noradrenaline

Noradrenaline (NA)-containing nerves, mainly originating in the sympathetic superior cervical ganglia, supply large and small cerebral arteries. In large cerebral arteries, nerves containing serotonin (5-hydroxytryptamine, 5-HT) may represent neuronal uptake of circulating 5-HT by sympathetic nerves. 5-HT-containing nerves supplying small pial vessels probably have a central origin in the dorsal raphe nucleus. In most species, NA is a weak vasoconstrictor (alpha 1- or alpha 2-adrenoceptors), while 5-HT is a potent vasoconstrictor (5-HT2 or 5-HT1-like receptors) of large cerebral arteries. In contrast, both NA and 5-HT tend to cause vasodilatation in small pial vessels and arterioles. Adrenergic and serotonergic transmission can be modulated by pH, a range of putative neurotransmitters and neuromodulators, and by the endothelium. Sumatriptan, a 5-HT1-like receptor agonist, has been shown to be effective in the treatment of migraine. Changes in NA- or 5-HT-containing nerves and/or in the responses of cerebral vessels to NA and 5-HT have been observed in a variety of vascular disorders, including cerebral vasospasm following subarachnoid haemorrhage, hypertension, and atherosclerosis.

Serotonin (5-HT) fibers of the rat dura mater: 5-HT-positive, but not authentic serotoninergic, tryptophan hydroxylase-like fibers

Serotonin (5-HT)-positive, but not tryptophan-5-hydroxylase (TPOH)-positive, authentic serotoninergic fibers were shown in the rat dura mater. 5-HT immunoreactive fibers in the dura are postulated to result from 5-HT uptake from circulating blood elements (e.g. platelets, mast cells) by perivascular sympathetic nerve fibers. A robust TPOH-immunoreactive mast cell population was identified in the dura; this result confirms the TPOH antibody specificity to cells known to synthesize 5-HT. While these results indicate that there are no authentic serotoninergic fibers in the dura mater, the mast cells, platelets and cerebrospinal fluid can serve as a source of 5-HT activating 5-HT receptors known to be present in this tissue.

Absence of serotonergic innervation from raphe nuclei in rat cerebral blood vessels--I. Histological evidence

Anterograde tracing from dorsal raphe neurons by Phaseolus vulgaris leucoagglutinin and serotonin immunocytochemistry revealed no serotonergic projections from raphe nuclei to cerebral pial vessels in the rat. However, cerebrovascular nerve fibres, mainly located in major pial arteries, were immunoreactive to tryptophan-5-hydroxylase antibodies as previously shown by others. It thus seems that the rate-limiting enzyme catalysing the biosynthesis of serotonin, tryptophan-5-hydroxylase, is present in cerebrovascular nerve fibres which do not originate in the dorsal raphe nucleus and which do not contain enough serotonin to be labelled by serotonin immunocytochemistry. We also observed tryptophan hydroxylase-immunoreactive but no serotonin-immunoreactive nerve fibres in the femoral artery and, occasionally, in the dura mater. The femoral artery, like the dura mater, contained numerous mast cells reacting positively to both tryptophan hydroxylase and to serotonin immunocytochemistry. The colocalization of the enzyme and its final product thus appears to be a general feature, since it has already been demonstrated within the central nervous system. The only exception appears to be the tryptophan hydroxylase-immunoreactive nerves present in cerebral and peripheral vessels. These results suggest that there is not a true serotonergic (i.e. serotonin-containing) innervation in cerebral blood vessels. They also strongly suggest that the cerebrovascular nerve fibres which appear to contain tryptophan hydroxylase do not originate in the raphe nuclei.

Immunohistochemical study of tyrosine-hydroxylase-positive cells and fibers in the chicken spinal cord

Tyrosine hydroxylase (TH)-positive cells and fibers were examined by immunohistochemistry in the chick spinal cord. TH-positive cells, which were located in laminae I, V and X, were most frequently found in the rostral part of the cervical spinal cord, with fewer cells being found in more caudal levels of the spinal cord. TH-positive cells located in lamina X, which were bipolar in shape, were mainly found in regions lateral as well as just ventral to the central canal. They had processes reaching to the central canal. The terminals of these cerebrospinal-fluid-contacting cells were oval in shape, and were most frequently found at the ventral wall of the central canal. There were dense clusters of TH-positive fibers in lamina X. A meshwork-like structure of TH-positive fibers was found over the lateral wall of the central canal. A high density of TH-positive fibers was also found in the medial part of laminae V-VII. In lamina IX, small numbers of TH-positive fibers were observed in the lateral motor column of the brachial spinal cord, and in the medial and lateral motor columns of the lumbosacral spinal cord. However, within the medial motor column of the brachial spinal cord TH-positive fibers were densely distributed around somal as well as dendritic profiles. Similar to our previous observations on serotoninergic fibers. TH-positive fibers were also differentially distributed in the ventral horn of the chicken spinal cord: a high density of TH-positive fibers was localized to specific regions of the spinal motor nucleus.

Lesion of the dorsal raphe nucleus induces supersensitivity to serotonin in isolated cat middle cerebral artery

Simultaneous lesions of dorsal and median raphe nuclei were induced after 15 days postjunctional supersensitivity to serotonin in isolated segments of cat middle cerebral artery. The same result was obtained when only the dorsal raphe nucleus was destroyed. The lesion of the median raphe nucleus brought about an increased contractile response to serotonin only at the three first doses used. The contractile response to noradrenaline was unaffected by these treatments. These results suggest the existence of a serotonergic innervation of the cat middle cerebral artery whose main origin might be the dorsal raphe nucleus.

Serotonin uptake into cerebrovascular nerve fibers of rat, visualization by immunohistochemistry, disappearance following sympathectomy, and release during electrical stimulation

Immunohistochemistry as well as in vitro uptake and release of [3H]5-HT were performed on pial arteries of rat to investigate the nature of 5-HT containing nerve fibers. Immunoreactive fibers were constantly found only in the basilar, vertebral and superior cerebellar arteries, while in the other parts of the circle of Willis, 5-HT immunofluorescent fibers were absent. After systemic treatment with tryptophan following inhibition of monoamine oxidase with nialamide the immunofluorescence intensity was markedly enhanced. The 5-HT immunoreactive fibers disappeared after superior cervical ganglionectomy or intraventricular administration of 6-hydroxydopamine, but persisted after administration of 5,6-dihydroxytryptamine. When isolated vessels were incubated in low concentration of 5-HT (1 nM) together with nialamide, a very dense plexus of 5-HT immunoreactive fibers appeared in all branches of the circle of Willis. Uptake and release experiments were carried out by incubation of arterial preparations with 3 nM [3H]5-HT (together with nialamide), followed by electrical field stimulation, or by exposure to tyramine or 124 mM potassium, all of which induced a 100%-350% increase in the tritium release over prestimulation values. Preincubation with cocaine and bilateral superior cervical ganglionectomy abolished or markedly attenuated the release upon all modes of stimulation. The results suggested that the 5-HT observed by immunohistochemistry in pial arteries is located in sympathetic nerve terminals where it may serve as a neuromodulator that is released during nerve activation.

Characteristic structures of serotonin neuron system. An immunohistochemical study

Since 1982, the authors have carried out intensive immunohistochemical studies on the distribution of serotonin neurons in the central nervous system and have reported the distinctive structures of these nerve cells. In the present study, two characteristic findings are described with special regard to the histophysiological significance: (1) direct innervation of the cerebral arteries by central serotonin neurons and (2) the perineuronal ivylike serotonin plexus.

Measurement of 5-hydroxytryptamine turnover rate in rat cerebral arteries

Two methods for assessing 5-hydroxytryptamine (5-HT) turnover rate have been tested in the cerebral vessels of the rat. The pretreatment of the animals with benserazide 45 min before death brought about an increase in the levels of 5-hydroxytryptophan (5-HTP). When the MAO inhibitor pargyline was injected and the animals killed at different times, there was an exponential decrease in the concentration of 5-hydroxyindole acetic acid (5-HIAA) with time, whereas the increase in 5-hydroxytryptamine was linear only during the first 30 min, thereafter reaching a plateau. This pattern was similar to that obtained in the caudate nucleus after MAO blockade. In the hippocampus, pargyline induced a lineal accumulation of 5-HT throughout the experiment as well as an exponential decay of the 5-HIAA concentration. These results indicate that the turnover rate of 5-HT can be appraised in the rat cerebral arteries either after the rate of disappearance of 5-HIAA or from the accumulation rate of 5-HTP.

Evidence for coexistence of serotonin and noradrenaline in sympathetic nerves supplying brain vessels of guinea pig

Nerve fibers containing 5-hydroxytryptamine (5-HT) were demonstrated immunohistochemically in the wall of pial vessels associated with the circle of Willis in the guinea pig. The fibers formed a network structure which was more dense in the rostral part of the arterial circle and its branches than in the caudal part. The 5-HT immunoreactive fibers disappeared in all arteries studied after bilateral superior cervical ganglionectomy, and unilateral ganglionectomy eliminated the 5-HT immunoreactivity in the ipsilateral part of the middle cerebral, posterior cerebral and superior cerebellar arteries. Decentralization of the superior cervical ganglion had no effect on the perivascular nerve plexus. Subsequent staining with dopamine-beta-hydroxylase (DBH) antiserum following elution of the first antibody revealed that 5-HT was present in the noradrenergic nerve fibers. Small intensive fluorescent cells with positive immunoreaction for 5-HT and DBH, respectively, were located in clusters within the ganglion, which showed no immunohistochemical evidence for the presence of serotonergic neurons. It is concluded that 5-HT is probably not synthesized in truly serotonergic fibers but rather taken up and stored together with noradrenaline in cerebrovascular sympathetic nerves originating in the superior cervical ganglia.

Serotonin neurons on the ventral brain surface

Serotonin neurons and fibers on the subpial surface of the ventral medulla oblongata in the rat are described by immunohistochemistry and autoradiography. The neurons are concentrated in the area encompassed by the origins of the abducens, hypoglossal, glossopharyngeal, and vagus nerves. The highest number of serotonin surface neurons appears along the median medullary fissure or basilar sulcus, where they may represent the most ventral extensions of the raphe pallidus group. As these cells lie on the surface of the brain, they could be directly affected by alterations in the chemical composition of the cerebrospinal fluid and, depending on their connections, could influence important medullary functions.

Serotonergic innervation of cat cerebral arteries

5-HT and 5-HIAA were measured in cat cerebral arteries by HPLC. Removal of both superior cervical ganglia or simultaneous lesions of dorsal and central raphe nuclei significantly decreased 5-HT levels but not those of 5-HIAA. This suggests that cat cerebral blood vessels are innervated by serotonergic fibers of different origin.

Effects of cholecystokinin octapeptide on pial arteriolar diameter

The effects of the perivascular microapplication of sulphated cholecystokinin octapeptide (CCK-8) on pial arteriolar calibre have been examined in anaesthetised cats using a video-display image-splitting technique. The microinjection of CCK-8 (10(-7) M) in the injectate did not modify the constriction or dilatation of pial arterioles provoked by the perivascular microinjection of alkalotic (pH 7.45) or acidic (pH 6.8) artificial CSFs, respectively. Thus, although CCK-8-immunoreactive neuronal processes are known to be associated with cortical blood vessels, CCK-8 appears to have minimal vasomotor effects on pial vessels.