Catecholaminergic innervation of the spinal dorsal horn: a correlated light and electron microscopic analysis of tyrosine hydroxylase-immunoreactive fibres in the cat

The posterior hypothalamus exerts opposing effects on nociception via the A7 catecholamine cell group in rats

Stimulation of the posterior hypothalamic area (PH) produces antinociception in rats and humans, but the precise mechanisms are unknown. The PH forms anatomical connections with the parabrachial area, which contains the pontine A7 catecholamine cell group, a group of spinally projecting noradrenergic neurons known to produce antinociception in the dorsal horn. The aim of the present study was to determine whether PH-induced antinociception is mediated in part through connections with the A7 cell group in female Sprague-Dawley rats, as measured by the tail flick and foot withdrawal latency. Stimulation of the PH with the cholinergic agonist carbachol (125 nmol) produced antinociception that was blocked by pretreatment with atropine sulfate. Intrathecal injection of the α(2)-adrenoceptor antagonist yohimbine reversed PH-induced antinociception, but the α(1)-adrenoceptor antagonist WB4101 facilitated antinociception. Intrathecal injection of normal saline had no effect. In a separate experiment, cobalt chloride, which reversibly arrests synaptic activity, was microinjected into the A7 cell group and blocked PH-induced antinociception. These findings provide evidence that the PH modulates nociception in part through connections with the A7 catecholamine cell group through opposing effects. Antinociception occurs from actions at α(2)-adrenoceptors in the dorsal horn, while concurrent hyperalgesia occurs from actions of norepinephrine at α(1)-adrenoceptors. This hyperalgesic response likely attenuates antinociception from PH stimulation.

Ultrastructural analysis of rat ventrolateral periaqueductal gray projections to the A5 cell group

Stimulation of neurons in the ventrolateral periaqueductal gray (PAG) produces antinociception as well as cardiovascular depressor responses that are mediated in part by pontine noradrenergic neurons. A previous report using light microscopy has described a pathway from neurons in the ventrolateral PAG to noradrenergic neurons in the A5 cell group that may mediate these effects. The present study used anterograde tracing and electron microscopic analysis to provide more definitive evidence that neurons in the ventrolateral PAG form synapses with noradrenergic and non-catecholaminergic A5 neurons in Sasco Sprague-Dawley rats. Deposits of anterograde tracer, biotinylated dextran amine, into the rat ventrolateral PAG labeled a significant number of axons in the region of the rostral subdivision of the A5 cell group, and a relatively lower number in the caudal A5 cell group. Electron microscopic analysis of anterogradely-labeled terminals in both rostral (n=127) and caudal (n=70) regions of the A5 cell group indicated that approximately 10% of these form synapses with noradrenergic dendrites. In rostral sections, about 31% of these were symmetric synapses, 19% were asymmetric synapses, and 50% were membrane appositions without clear synaptic specializations. In caudal sections, about 22% were symmetric synapses, and the remaining 78% were appositions. In both rostral and caudal subdivisions of the A5, nearly 40% of the anterogradely-labeled terminals formed synapses with non-catecholaminergic dendrites, and about 45% formed axoaxonic synapses. These results provide direct evidence for a monosynaptic pathway from neurons in the ventrolateral PAG to noradrenergic and non-catecholaminergic neurons in the A5 cell group. Further studies should evaluate if this established monosynaptic pathway may contribute to the cardiovascular depressor effects or the analgesia produced by the activation of neurons in the ventrolateral PAG.

Postnatal development of noradrenergic terminals in the rat trigeminal motor nucleus: A light and electron microscopic immunocytochemical analysis

The noradrenergic (NA) innervation in the trigeminal motor nucleus (Vmot) of postnatal and adult rats was examined by light and electron microscopic immunocytochemistry using antibodies against dopamine-beta-hydroxylase or tyrosine hydroxylase. NA fibers were identified in the Vmot as early as the day of birth (postnatal day 0; P0). A continuous increase in the density of labeled fibers was observed during development up to P20, with a slight decrease at P30 and in the adult. Electron microscopic analysis of serial ultrathin sections revealed that, at P5, nearly half (46%) of the examined NA terminals made synaptic contact with other neuronal elements with membrane specializations. The percentage of examined NA varicosities engaged in synaptic contacts increased at P15 (74%), then decreased in the adult (64%). At all developmental ages, the majority of contacts made by these boutons were symmetrical, the postsynaptic elements being mainly dendrites and occasionally somata. Interestingly, some of the NA terminals made axo-axon contacts with other unidentified boutons. These results show that, although the density of NA fibers increases during postnatal development, functional NA boutons are present in the Vmot at early postnatal ages. Some of these fibers might exert their effects via nonsynaptic release of noradrenaline, the so-called volume transmission, but, in the main, they form conventional synaptic contacts with dendrites, somata, and other axonal terminals in the Vmot. These results are consistent with previous electrophysiological studies that propose an important role for the NA system in modulating mastication.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

The therapeutic potential of neuropeptide Y in central nervous system disorders with special reference to pain and sympathetically maintained pain

Neuropeptide Y (NPY), a widely distributed peptide, has been shown to have numerous effects in both the central and peripheral nervous systems. In particular, NPY has an important role in mediating analgesia and hyperalgesia by distinct central and peripheral mechanisms. At least six NPY receptor subtypes are known to exist and the development of subtype-specific ligands targeted at NPY receptors will offer novel therapeutic agents. This article will review the involvement of NPY in diverse pathologies of the nervous system, including pain, and will propose a role for NPY in the maintenance of sympathetically maintained pain.

Substance P release in the cat spinal cord upon afferent C-fibre stimulation is not attenuated by clonidine at analgesic doses

In anaesthetized cats, antibody microprobes were used to measure the release of immunoreactive substance P (irSP) in the lumbar dorsal horn during electrical stimulation of primary afferent fibres at intensities suprathreshold for unmyelinated fibres. Release of irSP was detected in the region of the superficial dorsal horn. This evoked release was not reduced by clonidine hydrochloride, administered intravenously or by superfusion of the dorsal cord surface. Microprobes inserted during cord superfusion with lignocaine hydrochloride detected less irSP along their entire length, including in the region of evoked release. The results suggest that the analgesic action of clonidine does not involve reduced release of SP from the central terminals of nociceptors in the spinal cord.

The actions of monoamines and distribution of noradrenergic and serotoninergic contacts on different subpopulations of commissural interneurons in the cat spinal cord

Modulatory actions of monoamines were investigated on spinal commissural interneurons which coordinate left-right hindlimb muscle activity through direct projections to the contralateral motor nuclei. Commissural interneurons located in Rexed lamina VIII, with identified projections to the contralateral gastrocnemius-soleus motor nuclei, were investigated in deeply anaesthetized cats. Most interneurons had dominant input from either the reticular formation or from group II muscle afferents; a small proportion of neurons had input from both. Actions of ionophoretically applied serotonin and noradrenaline were examined on extracellularly recorded spikes evoked monosynaptically by group II muscle afferents or reticulospinal tract fibres. Activation by reticulospinal fibres was facilitated by both serotonin and noradrenaline. Activation by group II afferents was also facilitated by serotonin but was strongly depressed by noradrenaline. To investigate the possible morphological substrates of this differential modulation, seven representative commissural interneurons were labelled intracellularly with tetramethylrhodamine-dextran and neurobiotin. Contacts from noradrenergic and serotoninergic fibres were revealed by immunohistochemistry and analysed with confocal microscopy. There were no major differences in the numbers and distributions of contacts among the interneurons studied. The findings suggest that differences in modulatory actions of monoamines, and subsequent changes in the recruitment of subpopulations of commissural interneurons in various behavioural situations, depend on intrinsic interneuron properties rather than on the patterns of innervation by monoaminergic fibres. The different actions of noradrenaline on different populations of interneurons might permit reconfiguration of the actions of the commissural neurons according to behavioural context.

Kappa opioid receptors in rat spinal cord: sex-linked distribution differences

Activation of kappa opioid receptors (KORs) in the spinal cord can diminish nociception. Humans and rodents show sex differences in the analgesia produced by KOR agonists, and female rats show fluctuations in KOR density and sensitivity across the estrous cycle. However, it is unclear whether there are sex differences in the amount and/or distribution of spinal KORs. In the present study, immunocytochemically labeled KORs were examined in laminae I and II of the lumbosacral spinal dorsal horn of male and normally cycling female Sprague-Dawley rats. The basic pattern of KOR labeling was determined in both sexes using qualitative electron microscopy (EM), and sex-linked differences in the density and subcellular distribution of KOR immunoreactivity were determined with quantitative EM and light microscopy. KOR labeling was visualized with immunoperoxidase for optimally sensitive detection, or with immunogold for precise subcellular localization. By EM, the general pattern of KOR immunoreactivity was similar in males and females. KOR immunoreactivity was common in dendrites, axons, and axon terminals, and was in a few glia and neuronal somata. Most KOR-immunoreactive (-ir) axons were fine-diameter and unmyelinated. Most KOR-ir terminals were small or medium-sized, and a minority formed asymmetric or symmetric synapses with unlabeled dendrites. KOR immunoreactivity was associated both with the plasma membrane and with cytoplasmic organelles, notably including dense core vesicles in terminals. Light microscopic densitometry revealed that KOR immunoreactivity was significantly denser in estrus and proestrus females than in males. By EM, the distribution of KOR-immunogold labeling within axon terminals differed, with a greater proportion of cytoplasmic KOR labeling in estrus females compared with males. In contrast, the abundance and types of KOR-immunoperoxidase-labeled profiles did not show sex-linked differences. We conclude that in both sexes, KORs are positioned to influence both pre- and postsynaptic neurotransmission and are present in morphologically heterogeneous neuron populations. These findings are consistent with complex consequences of KOR activation in the spinal cord. In addition, the presence of increased KOR density and proportionally elevated intracellular KORs in proestrus/estrus females suggests a basis for sex-linked differences in KOR-mediated antinociception.

Responses to noxious stimuli in mice lacking alpha(1d)-adrenergic receptors

Nociceptive behaviors were examined in the mice lacking alpha1d-adrenergic receptor (alpha1d-AR) and wild type littermates using tail-flick, hot-plate (hindpaw-licking and jumping), tail-pinch and formalin tests. The distribution of alpha1d-AR was studied using in situ hybridization in the wild type mice. Mutant mice showed longer tail-flick and hindpaw-licking latencies while their jumping latency was shorter. Mechanical and chemical nociception was not altered in alpha1d-knockout mice. In situ hybridization study revealed dense alpha1d-AR mRNA expression in the reticular thalamic nucleus, the hippocampus, the cingulate cortex and the spinal cord. These results suggest that alpha1d-AR in the spinal cord contributes to thermal pronociception; and that the jump behavior seen when escaping from heat is inhibited via the supraspinal alpha1d-AR.

Neurokinin-1 projection cells in the rat dorsal horn receive synaptic contacts from axons that possess alpha2C-adrenergic receptors

Thealpha2C subclass of adrenergic receptor (alpha2C-AR) mediates some of the antinociceptive actions of norepinephrine in the spinal cord. Axon terminals, which possess this receptor, are concentrated in the superficial dorsal horn and originate from spinal interneurons. We performed a series of combined tract-tracing and immunocytochemical studies to determine whether alpha2C-AR-immunoreactive axons target projection neurons that possess the neurokinin-1 (NK-1) receptor because such cells are likely to transmit nociceptive information to the brain. Spinomedullary neurons were labeled by stereotaxic injection of the B-subunit of cholera toxin (CTb) into the caudal ventrolateral medulla of three anesthetized adult rats. After 3 d, the animals were anesthetized again and fixed by perfusion. Sections were cut from midlumbar segments and reacted with antibodies to reveal alpha2C-ARs, CTb, and NK-1 receptors. Retrogradely labeled neurons possessing the NK-1 receptor (n = 45) were examined with confocal microscopy to investigate their relationship with alpha2C-AR-immunoreactive axons. Numerous alpha2C-AR axons were apposed to cell bodies and proximal dendrites of cells in lamina I and also to distal dendrites that originate from labeled cell bodies in lamina III/IV. A combined confocal and electron microscopic method confirmed that these appositions were synaptic. Additional experiments showed that virtually all alpha2C-AR terminals in contact with labeled cells are also immunoreactive for the vesicular glutamate transporter 2 and therefore are glutamatergic. These data suggest that norepinephrine can modulate excitatory synaptic transmission from spinal interneurons to projection cells by acting at alpha2C-ARs. This could be one of the mechanisms that underlie the antinociceptive actions of norepinephrine.

An investigation of neurones that possess the alpha 2C-adrenergic receptor in the rat dorsal horn

The function of the alpha(2C) subclass of adrenergic receptor in the spinal cord is unclear at present. Immunoreactivity for this receptor is found predominantly on axon terminals of the superficial dorsal horn but limited information is available about the properties and origin of these axons. The aim of this study was to determine which classes of neurone give rise to axons that possess this receptor and to investigate the synaptic organisation of these terminals. A series of double-labelling experiments was performed to investigate the relationship between the alpha(2C) receptor and each one of 14 chemical markers that label various types of axon terminal in the dorsal horn. Tissue was examined with two-colour confocal laser scanning microscopy. Quantitative analysis revealed that alpha(2C)-adrenergic receptors are not present on terminals of unmyelinated or peptidergic primary afferents and descending noradrenergic or serotoninergic axons. They were found on a proportion of terminals belonging to a mixed population of excitatory and inhibitory spinal interneurones, including those that contain neurotensin, somatostatin, enkephalin, GABA and neuropeptide Y. However, a greater proportion of terminals originating from excitatory interneurones were found to possess the receptor. Electron microscopic analysis revealed that alpha(2C)-adrenergic receptor immunoreactivity is predominantly associated with axon terminals that are presynaptic to dendrites but a small proportion of immunoreactive terminals formed axo-axonic synaptic arrangements. These studies indicate that noradrenaline can modulate transmission in the dorsal horn by acting through alpha(2C)-adrenergic receptors on terminals of spinal interneurones.

Modulatory effects of alpha1-,alpha2-, and beta -receptor agonists on feline spinal interneurons with monosynaptic input from group I muscle afferents

Previous studies have shown that monoamines may modulate operation of spinal neuronal networks by depressing or facilitating responses of the involved neurons. Recently, activation of interneurons mediating reciprocal inhibition from muscle spindle (Ia) afferents and nonreciprocal inhibition from muscle spindle and tendon organ (Ia/Ib) afferents in the cat was found to be facilitated by noradrenaline (NA). However, which subclass membrane receptors are involved in mediating this facilitation was not established; the aim of the present experiments was to investigate this. Individual Ia- and Ia/Ib-inhibitory interneurons were identified in the cat lumbar spinal cord, and NA agonists were applied close to these neurons by ionophoresis. The agonists included the alpha1-receptor agonist phenylephrine, the alpha2-receptor agonists clonidine and tizanidine, and the beta-receptor agonist isoproterenol. Effects were measured by comparing changes in the number of extracellularly recorded spike potentials evoked by electrical stimulation of muscle nerves and changes in the latency of these potentials before, during, and after application of the tested compounds. Results show that the facilitatory effect of phenylephrine is as strong as that of NA, whereas the facilitatory effect of isoproterenol is weaker. Clonidine depressed activity of both Ia- and Ia/Ib-inhibitory interneurons, whereas tizanidine had no effect. These findings lead to the conclusion that beneficial antispastic effects of clonidine and tizanidine in humans are unlikely to be associated with an enhancement of the actions of Ia- and Ia/Ib-inhibitory interneurons, and the findings also support previous proposals that these compounds exert their antispastic actions via effects on other neuronal populations.

Ultrastructural analysis of ventrolateral periaqueductal gray projections to the A7 catecholamine cell group

Stimulation of neurons in the ventrolateral periaqueductal gray produces antinociception that is mediated in part by pontine noradrenergic neurons. Previous light microscopic analysis provided suggestive evidence for a direct projection from neurons in the ventrolateral periaqueductal gray to noradrenergic neurons in the A7 cell group that innervate the spinal cord dorsal horn. Therefore, the present ultrastructural study used anterograde tracing combined with tyrosine hydroxylase immunoreactivity to provide definitive evidence that neurons in the ventrolateral periaqueductal gray form synapses with the somata and dendrites of noradrenergic neurons of the A7 cell group. Injections of the anterograde tracers biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin into the ventrolateral periaqueductal gray of Sasco Sprague-Dawley rats yielded a dense innervation in the region of the lateral pons containing the A7 cell group. Electron microscopic analysis of anterogradely labeled terminals (n=401) in the region of the A7 cell group indicated that approximately 10% of these formed plasmalemmal appositions to tyrosine hydroxylase-immunoreactive dendrites with no intervening astrocytic processes. About 23% of these were asymmetric synapses, 10% were symmetric synapses, and 67% did not exhibit clearly differentiated synaptic specializations. The majority of anterogradely labeled terminals (60%) formed plasmalemmal appositions with dendrites and somata that lacked detectable tyrosine hydroxylase immunoreactivity. About 35% of these were symmetric synapses, 9% were asymmetric synapses and 56% did not form synaptic specializations. Approximately 30% of all anterogradely labeled terminals displayed features characteristic of axo-axonic synapses.The present results provide direct ultrastructural evidence to support the hypothesis that the analgesia produced by stimulation of neurons in the ventrolateral periaqueductal gray is mediated, in part, by activation of spinally projecting noradrenergic neurons in the A7 catecholamine cell group.

Bidirectional modulation of nociception by GABA neurons in the dorsolateral pontine tegmentum that tonically inhibit spinally projecting noradrenergic A7 neurons

The A7 catecholamine cell group in the dorsolateral pontine tegmentum constitutes an important part of the descending pathways that modulate nociception. Evidence from immunocytochemical studies demonstrate that noradrenergic A7 neurons are densely innervated by GABA terminals arising from GABA neurons that are located in the dorsolateral pontine tegmentum medial to the A7 cell group. GABA(A) receptors are also located on the somata and dendrites of noradrenergic A7 neurons. These findings suggest that noradrenergic neurons in the A7 cell group may be under tonic inhibitory control by GABA neurons. To test this hypothesis, the GABA(A) antagonist bicuculline methiodide in doses of 0.2 or 1.0nmol was microinjected into sites located dorsal to the A7 cell group and the resulting effects on tail flick and nociceptive foot withdrawal responses were measured. Both doses of bicuculline produced significant increases in tail flick latencies and small, but significant, increases in foot withdrawal latencies. Intrathecal injection of the alpha(2)-adrenoceptor antagonist yohimbine, in a dose of 76.7nmol (30microg), attenuated the antinociceptive effect of bicuculline on both the tail and the feet. In contrast, the alpha(1)-adrenoceptor antagonist WB4101, in a nearly equimolar dose of 78.6nmol (30microg), increased the antinociceptive effect of bicuculline on both the tail and the feet. Intrathecal injection of the antagonists alone did not consistently alter nociceptive responses of either the feet or the tail. These findings suggest that noradrenergic neurons in the A7 cell group are tonically inhibited by local GABA neurons. Furthermore, these findings suggest that inhibition of GABA(A) receptors located on spinally-projecting A7 noradrenergic neurons disinhibits, or activates, two populations of A7 neurons that have opposing effects on nociception. One of these populations facilitates nociception by an action mediated by alpha(1)-adrenoceptors in the spinal cord dorsal horn and the other population inhibits nociception by an action mediated by alpha(2)-adrenoceptors.

Serotoninergic and noradrenergic axonal contacts associated with premotor interneurons in spinal pathways from group II muscle afferents

We investigated the possibility that monoaminergic axons make contacts with spinal interneurons which project to motor nuclei and are monosynaptically activated by group II muscle afferents. Interneurons in midlumbar spinal segments of adult cats were characterized electrophysiologically and intracellularly labelled with tetramethylrhodamine dextran. Serotoninergic and noradrenergic axons were identified with immunofluorescence in sections containing labelled cells. Contacts between monoaminergic axons and interneurons were investigated with three-colour confocal laser scanning microscopy and analysed with a computer reconstruction program. Cell bodies and dendritic trees of five cells were reconstructed and putative contacts were plotted. The average number of contacts formed by serotoninergic axons was 140 and the average number of noradrenergic contacts was 38. The majority (95%) of contacts were formed with dendrites; these were distributed over the entire dendritic tree, even on the most distal branches. These findings provide a morphological basis for the modulatory actions of monoamines on premotor spinal interneurons in pathways from group II muscle afferents.

Microinjection of morphine in the A7 catecholamine cell group produces opposing effects on nociception that are mediated by alpha1- and alpha2-adrenoceptors

Stimulation of neurons in the ventromedial medulla produces antinociception in part by inhibiting nociceptive dorsal horn neurons. This antinociceptive effect is mediated in part by spinally projecting noradrenergic neurons located in the A7 catecholamine cell group. Methionine-enkephalin-immunoreactive neurons in the ventromedial medulla project to an area that includes the A7 cell group, and these enkephalin neurons may mediate part of the antinociception produced by stimulation of sites in the ventromedial medulla. This possibility was tested by determining the effects of microinjecting morphine near the A7 cell group on nociceptive foot and tail responses. Microinjection of a 3.75 nmol dose of morphine in the A7 region did not alter nociceptive responses, but a higher dose of 7.5 nmol facilitated these responses. In contrast, a higher dose of 15 nmol of morphine did not alter nociceptive responses. Selective alpha-adrenoceptor antagonists were injected intrathecally to determine whether the hyperalgesia produced by morphine is mediated by spinally projecting noradrenergic A7 neurons. Intrathecal injection of the alpha2-adrenoceptor antagonist yohimbine did not alter the hyperalgesic effect produced by the 7.5 nmol dose of morphine, but the alpha1 antagonist WB4101 reversed the hyperalgesia and produced antinociception that lasted for nearly 30 min. Although the 15 nmol dose of morphine did not alter nociceptive responses, intrathecal injection of yohimbine after the microinjection of morphine produced a significant facilitation of nociception, and intrathecal injection of WB401 produced a significant antinociceptive effect. Intrathecal injection of the antagonists alone did not consistently alter nociception. These findings, and those of published reports, suggest that morphine indirectly activates two populations of spinally projecting A7 noradrenergic neurons that have opposing effects on nociception. One of these populations facilitates nociception by an action mediated by alpha1-adrenoceptors in the spinal cord dorsal horn and the other population inhibits nociception by an action mediated by alpha2-adrenoceptors. These results suggest that some of the methionine-enkephalin neurons located in the ventromedial medulla that project to the A7 cell group can exert bidirectional control of nociceptive responses.

Ultrastructural evidence that substance P neurons form synapses with noradrenergic neurons in the A7 catecholamine cell group that modulate nociception

Potent antinociception can be produced by electrical stimulation of spinally projecting noradrenergic neurons in the A7 catecholamine cell group and this effect is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. Microinjection of substance P near A7 neurons also produces antinociception that is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. These observations suggest that substance P produces antinociception by activating noradrenergic A7 neurons. However, it is not known whether this effect of substance P is produced by a direct or an indirect action on A7 neurons. Although light microscopic studies have demonstrated the existence of both substance P-containing axon terminals and neurokinin-1 receptors in the region of the A7 cell group, it is not known whether substance P terminals form synapses with noradrenergic A7 neurons. These experiments used double-labeling immunocytochemical methods and electron microscopic analysis to determine whether substance P-containing axons form synapses with noradrenergic neurons in the A7 cell group. Pre-embedding immunocytochemistry, combined with light and electron microscopic analysis, was used to provide ultrastructural evidence for synaptic connections between substance P-immunoreactive terminals labeled with immunoperoxidase and tyrosine hydroxylase-immunoreactive A7 neurons labeled with silver-enhanced immunogold. Tyrosine hydroxylase labeling was found in perikarya and dendrites in the A7 region, and substance P labeling was found in axons and synaptic terminals. Substance P-labeled terminals formed asymmetric synapses with tyrosine hydroxylase-labeled dendrites, but only a few of these were present on tyrosine hydroxylase-labeled somata. Substance P-labeled terminals also formed asymmetric synapses with unlabeled dendrites, and many unlabeled terminals formed both symmetric and asymmetric synapses with tyrosine hydroxylase-labeled dendrites. These results demonstrate that substance P neurons form a significant number of synapses with the dendrites of noradrenergic A7 neurons and support the conclusion that microinjection of substance P in the A7 cell group produces antinociception by direct activation of spinally projecting noradrenergic neurons.

alpha2-adrenoceptors modulate NMDA-evoked responses of neurons in superficial and deeper dorsal horn of the medulla

Extracellular single unit recordings were made from neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in 21 male rats anesthetized with urethan. NMDA produced an antagonist-reversible excitation of 46 nociceptive as well as nonnociceptive neurons. Microiontophoretic application of a preferential alpha2-adrenoceptor (alpha2AR) agonist, (2-[2, 6-dichloroaniline]-2-imidazoline) hydrochloride (clonidine), reduced the NMDA-evoked responses of 86% (6/7) of nociceptive-specific (NS) neurons, 82% (9/11) of wide dynamic range (WDR) neurons, and 67% (4/6) of low-threshold (LT) neurons in the superficial dorsal horn. In the deeper dorsal horn, clonidine inhibited the NMDA-evoked responses of 94% (16/17) of NS and WDR neurons and 60% (3/5) of LT neurons. Clonidine facilitated the NMDA-evoked responses in 14% (1/17) of NS, 9% (1/11) of WDR, and 33% (2/6) of LT neurons in the superficial dorsal horn. Idazoxan, an alpha2AR antagonist, reversed the inhibitory effect of clonidine in 90% (9/10) of neurons, whereas prazosin, an alpha1-adrenoceptor antagonist with affinity for alpha2BAR, and alpha2CAR, were ineffective. We suggest that activation of alpha2ARs produces a predominantly inhibitory modulation of the NMDA-evoked responses of nociceptive neurons in the medullary dorsal horn.

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.

Dopaminergic and GABAergic cerebrospinal fluid-contacting neurons along the central canal of the spinal cord of the eel and trout

In anamniote vertebrates the central region of the spinal cord has been implicated in its regeneration. This is a complex region and so as a first step in understanding its possible regenerative role we have examined the organization of the cells that contact the lumen of the spinal cord in two teleost fishes, eel and trout, using immunohistochemical procedures and light and electron microscopy. Cell bodies immunoreacting positively with antibodies for tyrosine hydroxylase and for dopamine were located at the ventral rim of the central canal, whereas cell bodies reacting for an antibody for gamma-aminobutyric acid were more laterally located. None of the canal-contacting cells were positively immunoreactive for choline acetyltransferase. All immunopositive cells have a similar morphology: the amphora-shaped perikaryon is bipolar and has a single process that extends to the lumen of the canal, and another that branches and forms extensive lateral and ventral plexuses. Electron microscopic investigations of the ventral dopaminergic cells showed that the apical processes bear one or more cilia, which protrude into the canal lumen and which originate from within a superficial rosette of nonciliated processes. The ventral process was occasionally seen to form synapses; the cell body was also the target of synapses.

Lack of effect of microinjection of noradrenaline or medetomidine on stimulus-evoked release of substance P in the spinal cord of the cat: a study with antibody microprobes

1. Experiments were performed on barbiturate anaesthetized, spinalized cats to investigate the effect of microinjected noradrenaline or medetomidine on the release of immunoreactive substance P in the dorsal spinal cord following peripheral nerve stimulation. The presence of immunoreactive substance P was assessed with microprobes bearing C-terminus-directed antibodies to substance P. 2. Noradrenaline or medetomidine were microinjected into the grey matter of the spinal cord, near microprobe insertion sites, at depths of 2.5, 2.0, 1.5 and 1.0 mm below the spinal cord surface with volumes of approximately 0.125 microliters and a concentration of 10(-3) M. 3. In the untreated spinal cord, electrical stimulation of the ipsilateral tibial nerve (suprathreshold for C-fibres) elicited release of immunoreactive substance P which was centred in and around lamina II. Neither noradrenaline nor medetomidine administration in the manner described produced significant alterations in this pattern of nerve stimulus-evoked release. 4. In agreement with recent ultrastructural studies these results do not support a control of substance P release by catecholamines released from sites near to the central terminals of small diameter primary afferent fibres.

Light- and electron-microscopic analysis of neuropeptide Y-immunoreactive profiles in the cat spinal dorsal horn

The organization of neuropeptide Y-containing profiles in the dorsal horn of cat lumbosacral spinal cord was examined in an immunocytochemical study employing a specific antiserum against neuropeptide Y. Light-microscopic inspection revealed heavy concentrations of immunoreactive axons and varicosities within the superficial layers of the dorsal horn (laminae I and II) and only low to moderate numbers of positive terminals in the deeper layers (laminae III-VI). Neuropeptide-Y immunoreactivity in the superficial laminae occurred primarily as single punctate terminals, although in sagittal sections long rostrocaudally orientated fibres were also found. Immunoreactive fibres in the deeper layers were usually long and beaded. Two-hundred and eight neuropeptide Y-immunoreactive profiles throughout laminae I-VI were examined through serial sections with the electron microscope, and the overwhelming majority (n = 194) was confirmed to be axon terminals, most of which (95%) formed synaptic junctions. These terminals were packed with small irregularly shaped agranular vesicles, together with a number of large dense-core vesicles. Immunoreactivity was homogeneously scattered throughout the cytoplasm, and was also associated with the dense-core vesicles. A few neuropeptide Y-containing profiles (n = 14) were difficult to classify but they could have been vesicle-containing dendrites. The postsynaptic targets of neuropeptide Y-positive terminals were similar throughout each dorsal horn lamina. Most frequently, neuropeptide Y-positive boutons formed axodendritic and axosomatic synaptic junctions (range = 64% of synapses in laminae V/VI to 83% in lamina III). A smaller proportion of synapses were found upon other axon terminals and in laminae I-III the postsynaptic axon terminals were sometimes the central boutons of glomeruli. A number of terminals, especially those in lamina II, formed multiple synapses which often comprised a triadic arrangement. These findings suggest that neuropeptide Y regulates spinal sensory transmission through both a postsynaptic action upon dorsal horn neurons and a presynaptic action upon primary afferent terminals.

Catecholaminergic innervation of the lateral cervical nucleus: a correlated light and electron microscopic analysis of tyrosine hydroxylase-immunoreactive axons in the cat

The organization of catecholamine-containing axons in the cat lateral cervical nucleus was examined by immunocytochemical methods using a specific tyrosine hydroxylase antiserum. Light microscopic examination revealed numerous tyrosine hydroxylase-immunoreactive axons and varicosities throughout this nucleus, and some of these structures were found in contact with neuronal cell bodies. Correlated ultrastructural analysis showed that these varicosities were synaptic boutons which formed symmetric synaptic junctions with dendrites and somata. This evidence suggests that catecholamines exert a postsynaptic action upon neurons within the lateral cervical nucleus.

Relationships between spinocervical tract neurons and descending catecholamine-containing axons in the cat

Lumbosacral (L6-S1) spinal cord neurons in the cat were retrogradely labelled after uptake of horseradish peroxidase by their severed axons in the upper cervical (C3-C4) dorsolateral funiculus. Sections of L6-S1 containing labelled neurons were then processed immunocytochemically using antibodies against dopamine-beta-hydroxylase or tyrosine hydroxylase, two enzymes responsible for the synthesis of catecholamines. Two hundred and ninety eight retrogradely-labelled cells within laminae III-V of the dorsal horn were examined under high power (x 1000) with the light microscope. In Triton X-100-treated material, only 13% of these cells had catecholamine-containing varicosities closely apposed to their somata and proximal dendrites, which suggests that in comparison with the postsynaptic dorsal column pathway, spinocervical tract neurons are only sparsely innervated by descending catecholaminergic axons.

5-hydroxydopamine-labeled dopaminergic axons: three-dimensional reconstructions of axons, synapses and postsynaptic targets in rat neostriatum

Previous studies employing 5-hydroxydopamine to identify nigrostriatal dopaminergic axons and their synapses found that labeled axons made few synapses or that asymmetric contacts predominated. In contrast, recent studies using tyrosine hydroxylase or dopamine antibody techniques indicate that presumed dopaminergic axons form small symmetric contacts. We re-examined 5-hydroxydopamine-labeled material from the rat neostriatum using serial three-dimensional reconstruction techniques to characterize the morphology of labeled axons, synapses and postsynaptic targets. This ultrastructural analysis revealed a class of heavily labeled axons that are small (0.06-1.5 microns in diameter) and lack large varicosities. These axons form small (0.011-0.09 microns 2), en passant, symmetric synapses, mainly onto dendritic spines and spiny dendritic shafts and, in some cases, onto aspiny dendritic segments near branch points. The sites of these synapses along the axon appeared unrelated to the locations of axonal enlargements, suggesting that counting varicosities may not be an accurate indication of the extent of dopaminergic innervation in the neostriatum. The characteristics of these 5-hydroxydopamine-labeled elements correspond in all respects to axons and synapses identified as dopaminergic by immunohistochemistry in previous studies. In tissue in which all labeled and unlabeled synapses were classified, approximately 9% of all synapses were identified as dopaminergic by this type of label. Three-dimensional reconstructions provided additional insight concerning the interaction of dopaminergic afferents with postsynaptic striatal targets and their relation to other afferents to these neurons. They reveal that a short, unbranched dopaminergic axonal segment can make multiple synapses onto dendritic spines, shafts and branch points of one or more dendrites. In addition, one dendrite can receive contacts from several labeled axons. Dopamine synapses onto spines are always associated with unlabeled, asymmetric synapses onto the same spine. Synapses of various morphologies with a distinctly different, lighter form of labeling were much rarer, and may represent other aminergic afferents to the neostriatum. The presence of this second form of label in earlier 5-hydroxydopamine studies may have contributed to the long-standing controversy over the appearance of dopaminergic synapses examined by different techniques. Our results help to resolve this controversy and confirm that the nigrostriatal projection makes small symmetric synapses with a variety of striatal targets.

Sympathetic innervation of the rat's eye and peripheral ganglia: an electron microscopic autoradiographic tracing study

The sympathetic innervation of the rat anterior eye segment and related peripheral ganglia was studied using light and electron microscopic autoradiography after anterograde tracing with 3H-leucine from the superior cervical ganglion. In the trigeminal and pterygopalatine ganglia, unmyelinated nerve fibers were labeled. Some fibers contained accumulations of small vesicles. Close apposition of these labeled sympathetic fibers with other unmyelinated fibers was common, and was also observed at sites where accumulations of vesicles were found. In the iris, ciliary body and trabeculum, numerous fibers and vesicle-containing varicosities were labeled, which all had a similar morphology. No labeling was found in the cornea. Sympathetic fibers traversing the trigeminal and pterygopalatine ganglion closely appose other unmyelinated fibers and contain clusters of vesicles without forming classical synaptic contacts. However, non-synaptic information transfer needs further investigation. The anterior eye segment, except for the cornea, is densely innervated by sympathetic nerve terminals.

Direct catecholaminergic innervation of spinal dorsal horn neurons with axons ascending the dorsal columns in cat

Previous ultrastructural studies have shown that catecholamine-containing nerve terminals in the spinal dorsal horn form synaptic junctions with dendrites and somata, but the identity of the neurons giving rise to these structures is largely unknown. In this study we have investigated the possibility that spinomedullary neurons, which project through the dorsal columns to the dorsal column nuclei, are synaptic targets for descending catecholaminergic axons. Neurons with axons ascending the dorsal columns were retrogradely labelled after uptake of horseradish peroxidase by their severed axons in the thoracic (T10-T12) or cervical (C2-C3) dorsal columns. After the retrogradely labelled neurons were visualized, the tissue was immunocytochemically stained with antisera raised against tyrosine hydroxylase or dopamine-beta-hydroxylase. Three hundred forty-three retrogradely labelled neurons within laminae III-V of the lumbosacral dorsal horn were examined under high power with the light microscope. In Triton X-100 treated material, over 60% of cells were found to have dopamine-beta-hydroxylase-immunoreactive varicosities closely apposed to their somata and proximal dendrites. The number of contacts per cell varied from 1 to 22, with a mean number of 4.5. Fewer cells (34%) received contacts from axons immunoreactive for tyrosine hydroxylase as a consequence of the weaker immunoreaction produced by this antiserum. Correlated light and electron microscopic analysis confirmed that many of these contacts were regions of synaptic specialization and that immunostained boutons contained pleomorphic (round to oval) agranular vesicles together with several dense core vesicles. These observations suggest that catecholamines regulate sensory transmission through this spinomedullary pathway by a direct postsynaptic action upon its cells of origin. Such an action would be predicted to suppress transmission generally through this pathway.

Neuropeptide Y-immunoreactive terminals form axo-axonic synaptic arrangements in the substantia gelatinosa (lamina II) of the cat spinal dorsal horn

The ultrastructural organization of nerve terminals containing neuropeptide Y-immunoreactivity was studied in the substantia gelatinosa of the cat spinal dorsal horn. Seventy immunoreactive boutons were examined through serial sections and 67 of them were found to form between one and five synaptic junctions with dendrites (59.5% of synapses), somata (3% of synapses) and other axon terminals (37.5% of synapses). The postsynaptic axon terminals were often the central boutons of glomeruli. These findings suggest that neuropeptide Y regulates spinal sensory transmission through both a postsynaptic action upon dorsal horn neurons and a presynaptic action upon primary afferent terminals.

Depolarization of group II muscle afferents by stimuli applied in the locus coeruleus and raphe nuclei of the cat

1. Electrical stimuli applied in the locus coeruleus/subcoeruleus (LC/SC) and raphe nuclei produce a profound depression of transmission in reflex pathways from group II muscle afferents. The present experiments were performed to determine whether presynaptic inhibitory mechanisms contribute to these effects. 2. Changes in the excitability of afferent terminals to electrical stimuli have been used as an indication of primary afferent depolarization (PAD) produced by conditioning stimuli applied within the LC/SC and raphe nuclei and, for comparison, in the nucleus ruber. Group II afferents originating from ankle flexor muscles and terminating in the midlumbar segments were used for testing. 3. Clear changes in excitability were observed in fourteen of nineteen group II fibres in which the effects of conditioning stimuli applied in the LC/SC were tested and in twelve of seventeen fibres in which the effects of stimuli applied within the raphe nuclei were tested. By comparison, only one of the twelve fibres tested with conditioning stimuli applied to the nucleus ruber was found to be influenced. These effects matched those of the same conditioning stimuli on field potentials evoked by group II afferents at the location at which the terminals of group II fibres were stimulated. 4. Stimuli applied in the LC/SC and in the raphe nuclei both produced a mean decrease in threshold stimulus current of 19%. These effects are comparable to those produced by the most effective volleys in peripheral afferent which, in the same fibres, produced a mean decrease in threshold stimulus current of 24%. 5. In all cases (twelve) in which the effects of stimuli applied in the LC/SC and raphe nuclei were tested on the same group II fibre, either both or neither were found to be effective. This strengthens previous indications that some populations of neurones might be activated by stimuli applied in each of these regions of the brain. 6. In contrast to group II afferents, group Ia afferents investigated in the same experiments were only exceptionally affected. Of seven fibres tested with stimuli applied in the LC/SC, six with stimuli applied in the raphe nuclei and seven with stimuli applied in the nucleus ruber, only one fibre showed any clear change in threshold and this was a single fibre which was similarly affected by stimuli in all three sites. 7. It is concluded that presynaptic inhibitory mechanisms contribute to the depression of transmission in spinal reflex pathways from group II muscle afferents produced by stimulation in the LC/SC and raphe nuclei.

Evidence for nonsynaptic serotonergic and noradrenergic innervation of the rat dorsal horn and possible involvement of neuron-glia interactions

We investigated the synaptic incidence of the contacts established by serotonergic and noradrenergic descending fibers in the dorsal horn of the rat spinal cord. Serial electron microscopic sections were performed. Synapses were scarce. The majority of serotonergic and noradrenergic varicosities (more than 60%) are characterized by nonsynaptic contacts. Numerous glial profiles, and particularly astrocytic profiles, were observed in apposition with serotonergic and noradrenergic varicosities. The proportion of astroglia was higher around serotonergic and noradrenergic varicosities devoid of synaptic specialization. The length of the contact between immunoreactive nonsynaptic varicosities and astrocytes was twice as long as that between synaptic varicosities and astrocytes. Thus, the modulation of sensitive messages by serotonin and noradrenaline through pauci-synaptic varicosities in the dorsal horn of the spinal cord could be an example of the concept of "volume transmission" [Fuxe and Agnati (1991) Volume Transmission in the Brain: Novel Mechanisms for Neural Transmission, Advances in Neuroscience, Vol. 1, pp. 1-9.] in the central nervous system. Analysis of the microenvironment of serotonergic and noradrenergic varicosities led us to make the hypothesis that glial cells, particularly astrocytes, could play some role in volume transmission.

Ultrastructural analysis of noradrenergic nerve terminals in the cat lumbosacral spinal dorsal horn: a dopamine-beta-hydroxylase immunocytochemical study

Noradrenaline-containing nerve terminals within the cat spinal dorsal horn were studied by immunocytochemical localization of dopamine-beta-hydroxylase. Immunoreactive terminals formed symmetrical (Gray type II) synaptic specializations with dendrites and somata throughout laminae I-IV, but no junctions were formed with other axons. These findings suggest that noradrenaline regulates sensory transmission through the dorsal horn via a postsynaptic action.