Ultrastructure of somatostatin-immunoreactive nerve terminals in laminae I and II of the rat trigeminal subnucleus caudalis

The somatostatin sst2A receptor in the rat trigeminal ganglion

Immunohistochemistry for the somatostatin sst2A receptor was performed on the rat trigeminal ganglion to know its function in the trigeminal nervous system. The immunoreactivity was detected in 9.4% of primary sensory neurons in the ganglion. These neurons were small to medium-sized (range=106.5-1123.2 microm(2); mean+/-S.D.=506.3+/-213.2 microm(2)) and predominantly located in the rostromedial part of the ophthalmo-maxillary division. They were also immunoreactive for calcitonin gene-related peptide and the vanilloid receptor subtype 1. In addition, 13.7% of trigeminal neurons which were retrogradely traced with fluorogold from the nasal mucosa exhibited sst2A receptor-immmunoreactivity. Trigeminal neurons which innervated the facial skin and tooth pulp were devoid of the immunoreactivity. In the brainstem trigeminal sensory nuclear complex, both the neuronal cell body and the neuropil exhibited sst2A receptor-immunoreactivity in the superficial medullary dorsal horn.The present study indicates that sst2A receptor-immunoreactive trigeminal nociceptors innervate the nasal mucosa. They may project to the superficial laminae of the medullary dorsal horn.

Intracisternal octreotide does not ameliorate orthodromic trigeminovascular nociception

Octreotide is a long-acting somatostatin analogue that has been effectively used to treat migraine. Octreotide poorly penetrates the blood-brain barrier, but has potential central target sites in the trigeminal nucleus caudalis, which is the primary central relay station for trigeminal nociceptive information in the brain. We studied the effect of intracisternally applied octreotide in a model of trigeminovascular stimulation in the unrestrained rat using intracisternal capsaicin infusion to stimulate intracranial trigeminal nerves. Fos expression in the outer layers of the trigeminal nucleus caudalis (TNC I-II) and behavioural analysis were used to measure the effects of octreotide on capsaicin-induced trigeminovascular activation. Increases of head grooming and scratching behaviour are an indication of octreotide-induced trigeminal activation. However, octreotide did not alter the average capsaicin-induced Fos expression in the TNC I-II and capsaicin sensitive behaviours were not modified by octreotide pretreatment. This argues against a role for central (TNC I-II) somatostatin receptors in the processing of the nociceptive trigeminovascular signals.

The neuropeptide Y Y1 receptor is a somatic receptor on dorsal root ganglion neurons and a postsynaptic receptor on somatostatin dorsal horn neurons

Using indirect immunofluorescence, neuropeptide Y Y1 receptor (Y1 receptor)-like immunoreactivity (LI) was localized close to the plasmalemma of small neurons in lumbar dorsal root ganglia (DRGs) and neurons in the inner lamina II of the lumbar spinal cord of the rat. Using confocal microscopy, colocalization of Y1 receptor-LI and transferrin receptor-LI, a marker for endosomes and coated vesicles, was observed in dot-like structures along the plasmalemma. Under the electron microscope, Y1 receptor-LI was localized in coated vesicles and endosomes, in the membrane of tubular cisternae, sometimes connected to multivesicular bodies, and in the plasmalemma. These complex distribution patterns may reflect receptor turnover and internalization processes. In the lamina II of the spinal dorsal horn, Y1 receptor-LI was localized in the plasmalemma of neurons without any apparent association with paramembrane structures, as described above for the DRG neurons. Many dendrites were Y1 receptor-positive, and some of them made synaptic contacts with unstained axonal terminals. In general, Y1 receptor-LI was localized in the membrane outside the postsynaptic density. Double-immunofluorescence staining showed that most Y1 receptor-immunoreactive neurons in lamina II contained somatostatin-LI. Both in DRG and dorsal horn neurons, the Y1 receptor thus seems to represent a postjunctional/postsynaptic receptor.

Ultrastructural studies on peptides in the dorsal horn of the rat spinal cord--II. Co-existence of galanin with other peptides in local neurons

Using light microscopic immunoperoxidase and immunofluorescence histochemistry, double-staining methodology, and electron microscopic pre-embedding and post-embedding immunocytochemistry, we studied galanin-immunoreactive neurons in the superficial dorsal horn of the rat spinal cord. Co-existence of galanin with other neuropeptides was also analysed. The lumbar 4 and 5 segments of normal rats and after rhizotomy or spinal cord transection were studied. Galanin-positive local neurons in lamina II were often islet cells and could be classified as type A, which had abundant electron-dense cytoplasm containing many large dense-core vesicles, and type B, which had electron-lucent cytoplasm with only a few large dense-core vesicles. Galanin-positive and -negative peripheral afferent terminals made synaptic contact mostly with galanin-negative dendrites and cell bodies, but also with type B galanin cell bodies and with galanin-positive dendrites of unidentified type. Galanin-immunoreactive terminals from local neurons could also be classified into two types. Type alpha terminals were most common; they contained densely packed synaptic vesicles and many large dense-core vesicles, were strongly immunostained and most frequently made synaptic contact with galanin-negative dendrites. Type beta terminals contained loosely packed synaptic vesicles and a few large dense-core vesicles, and were weakly immunostained. Axosomatic synaptic contact were sometimes found between type beta terminals and type B galanin-positive cell bodies, but were most often associated with galanin-negative dendrites. Double immunostaining showed that galanin-like immunoreactivity co-localized mainly with enkephalin-like, but sometimes also with neuropeptide Y-like immunoreactivity in some local neurons in lamina II. Galanin-like and substance P-like immunoreactivities were identified in the same neurons in deeper layers of the dorsal horn. Coexistence of these neuropeptides and neurotensin with galanin was demonstrated not only in terminals in lamina II but also in large dense-core vesicles, as revealed by post-embedding immunocytochemistry. These results show that galanin-immunoreactive neurons in lamina II receive inputs directly from primary afferents and frequently make synaptic contacts with other intrinsic neurons. Galanin in the superficial dorsal horn may be released both from primary afferents and local neurons to modulate sensory processing in many different ways, including interacting with enkephalin, neuropeptide Y, neurotensin and substance P released from the same and/or other local neurons.

Descending projections from the medullary dorsal reticular nucleus make synaptic contacts with spinal cord lamina I cells projecting to that nucleus: an electron microscopic tracer study in the rat

An ultrastructural study is made of the synaptic contacts occurring between structures labelled anterogradely and retrogradely in the superficial dorsal horn following injections of cholera toxin subunit B or horseradish peroxidase in the dorsal reticular nucleus of the medulla oblongata of the rat. Both tracers revealed labelled axonal boutons in lamina I with round synaptic vesicles and a few large granular vesicles making asymmetrical synaptic contacts upon labelled somata and dendrites. After injections of Phaseolus vulgaris leucoagglutinin in the dorsal reticular nucleus, labelled boutons identical to those revealed by the two other tracers were presynaptic to unlabelled somata and dendrites. In addition, dorsoreticular neurons were labelled retrogradely following injections of cholera toxin subunit B into the superficial dorsal horn of the cervical enlargement. These observations show the occurrence of a reciprocal connection between dorsal reticular and lamina I neurons. Considering the putative excitatory nature of the axodendritic contacts in lamina I, a positive feedback circuit is suggested, whereby the nociceptive signals transmitted to the dorsal medullary reticular formation by marginal neurons are intensified.

Ultrastructural morphology, synaptic relationships, and CGRP immunoreactivity of physiologically identified C-fiber terminals in the monkey spinal cord

The spinal cord terminations of two electrophysiologically identified single C-fibers (one identified as a C-nociceptor) were intra-axonally labeled with horseradish peroxidase and analyzed with both light and electron microscopy. Serial section ultrastructural analysis and postembedding immunocytochemical techniques for calcitonin gene-related peptide (CGRP), substance P (SP), and GABA were used to study the synaptology, and neuropeptide content. All C-terminal synapses were in laminae I and II. The terminals sampled (n = 73) from these two C-fibers rarely established glomerular synaptic complexes, but rather, simple terminals, usually measuring 1-4 microns in length and 1-3 microns in diameter. They most often established 1 or 2 (range 1 to 5) quite large asymmetric axodendritic synaptic contacts. Postsynaptic structures included dendritic spines and shafts with and without vesicles. C-terminals were filled with small round synaptic vesicles (45-60 nm) and also contained variable numbers of large dense-core vesicles (LDCVs, 80-110 nm). LDCVs inside identified C-terminals frequently displayed CGRP immunoreactivity. We were unable to detect SP immunoreactivity inside our sample of C-fiber LDCVs. C-terminals were never found postsynaptic to other profiles. Thus, the C-fiber terminals sampled in this study have simple synaptology, do not receive presynaptic control and contain CGRP immunoreactivity. They differ greatly from the terminals of A delta nociceptors studied previously by our group that had glomerular endings, often received presynaptic input and did not contain CGRP immunoreactivity. This suggests the existence of different processing mechanisms, at the level of the first synapse, for nociceptive inputs arriving to lamina I and II through different types of primary afferents.

Immunocytochemical study of somatostatin, neurotensin, GABA, and glycine in rat spinal dorsal horn

In order to determine whether somatostatin coexists with GABA or glycine in neurones in rat spinal dorsal horn, a combined pre- and post-embedding immunocytochemical study was carried out. One hundred six somatostatin-immunoreactive neurones located in lamina II and the dorsal half of lamina III were tested with antiserum or monoclonal antibody to GABA and none of these cells showed GABA-like immunoreactivity. However, 8 out of 13 somatostatin-immunoreactive neurones located deeper in the dorsal horn (ventral lamina III and lamina IV) showed glycine-like immunoreactivity, and 6 of these were also GABA-immunoreactive. We have previously shown that neurotensin-immunoreactive neurones in laminae II and III are also not immunoreactive when tested with GABA antiserum (Todd et al.: Neuroscience 47:685-691, 1992), and a double-labelling fluorescence method was therefore used to compare the distribution of somatostatin and neurotensin within the superficial dorsal horn. The two types of peptide-immunoreactivity were never found in the same profile. These results suggest that somatostatin and neurotensin are present in different populations of non-GABAergic neurones in rat superficial dorsal horn, but that some somatostatin-containing neurones in the deeper part of the dorsal horn contain glycine, with or without GABA.

Parvalbumin and calbindin immunocytochemistry reveal functionally distinct cell groups and vibrissa-related patterns in the trigeminal brainstem complex of the adult rat

Immunocytochemistry for calbindin (CA) and parvalbumin (PA) was combined with retrograde tracing from the thalamus, superior colliculus (SC), and cerebellum to define the ascending projections of neurons in the rat's trigeminal (V) brainstem complex that express immunoreactivity for these calcium binding proteins. Many PA-immunoreactive neurons were observed in trigeminal nucleus principalis (PrV). Many of these cells projected to thalamus and a few sent axons to SC. In ventral PrV, PA-immunoreactive neurons were arranged in a vibrissa-related pattern. A very small number of large CA-immunoreactive neurons were observed in dorsomedial PrV. None of these cells were labeled by our tracer deposits. Small neurons in V subnucleus oralis (SpO) were also immunoreactive for PA, but none were retrogradely labeled. A small percentage of the large neurons in SpO were CA-immunoreactive; many of these were retrogradely labeled by tracer injections in the thalamus and/or SC. In V subnucleus interpolaris (SpI), many small to medium sized cells were PA-positive and they were arrayed in a vibrissae-like pattern. None of these neurons were retrogradely labeled from any of the above-listed targets, but many were retrogradely labeled by tracer injections into ipsilateral PrV. SpI also contained many large CA-immunoreactive cells. Many of these projected to the thalamus and/or SC and some were also retrogradely labeled by tracer injections into ipsilateral PrV. In V subnucleus caudalis (SpC), very dark PA-immunoreactive neurons were located in the inner part of lamina II and less often in laminae I. Lightly labeled cells were located in the magnocellular laminae and formed vibrissa-related aggregates. None of these neurons were retrogradely labeled by our tracer injections. CA-immunoreactive cells were located throughout the depth of lamina II in SpC and smaller numbers were also visible in lamina I and layers III-V. A small percentage of the CA-positive cells in lamina I and in the magnocellular layers were retrogradely labeled from the thalamus. These data indicate that PA and CA antisera identify two cell populations in whisker-related regions of the V brainstem complex and that PA cells are somatotopically patterned in PrV, SpI, and SpC. These markers also distinguish two cell groups in superficial laminae of the medullary dorsal horn.

Golgi impregnated somatostatin immunoreactive neurons in lamina II of the rat spinal cord

The morphology of somatostatin immunoreactive (SOM-I) neurons in lamina (L) II of the rat spinal cord was determined using a combination of Golgi impregnation and immunohistochemistry. Golgi-impregnated SOM-I neurons that resembled islet, stalked and other cells were observed. Islet cells are considered to be inhibitory interneurons while stalked cells are excitatory and are thought to relay information from primary afferent neurons to L I projection cells. The heterogeneous morphology of SOM-I neurons suggest they have diverse functions.

Evidence for dorsal root projection to somatostatin-immunoreactive structures in laminae I–II of the spinal dorsal horn

In order to determine if somatostatin (SOM)-immunoreactive (I) cell bodies and processes in lamina (L) II of the rat spinal cord receive dorsal root input, the latter were anterogradely labeled by wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). SOM-I structures were demonstrated by immunohistochemistry. Cell bodies labeled transscellularly or transsynaptically by WGA-HRP and immunohistochemically for SOM were present in L II. In addition, a L I cell was double labeled. These results suggest that some dorsal root axons innervate SOM-I neurons in L I-II of the spinal cord. In addition to confirming immunohistochemical observations in published reports, we have revealed SOM-I central terminals in the type II glomerulus. Further, a SOM-I CI-terminal, presumed to be of primary afferent origin, contacted a SOM-I dendrite in L II. Since SOM has been implicated in nociceptive function in the dorsal horn, it is possible that some of the SOM-I structures identified are involved in nociceptive processing.

Anatomy of somatostatin-immunoreactive fibres and cell bodies in the rat trigeminal subnucleus caudalis

The distribution of somatostatin-immunoreactive fibres and cells has been analysed in the rat spinal trigeminal subnucleus caudalis. Immunoreactive fibres are most concentrated in lamina II outer but fibres and terminals occur also in lamina I, lamina II inner, and scattered in the magnocellular region and neighbouring lateral reticular area. Immunoreactive cells occur in laminae I and II and in the magnocellular region of the nucleus but are most abundant in lamina II inner. The lamina II immunoreactive cells are morphologically heterogeneous and include types which are similar to cells described in Golgi studies such as stalked and islet cells. In order to distinguish somatostatin-immunoreactive primary afferents from intrinsic sources of somatostatin such as the lamina II neurons, we have used a monoclonal antibody (LD2) which is specific for primary afferents. Using dual-colour immunofluorescence we have shown that all somatostatin-immunoreactive cells in the trigeminal ganglia express LD2 immunoreactivity. Quantitative immunostaining density profiles indicate that LD2- and somatostatin-immunoreactive fibres overlap mainly in lamina II outer and dual-colour immunofluorescence confirms that this region contains somatostatin and LD2 double-labelled fibres. In contrast, lamina I contains more somatostatin- than LD2-immunoreactive fibres and fewer double-labelled fibres. The presence of double-labelled fibres in outer lamina II indicates that somatostatin-immunoreactive primary afferents terminate largely in this sublamina. However, the small number of double-labelled fibres found suggests that somatostatin-immunoreactive fibres in laminae I and II are derived mainly from intrinsic sources such as the various types of lamina II neurons.