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

Effects of somatostatin on the responses of rostrally projecting spinal dorsal horn neurons to noxious stimuli in cats

Somatostatin (SOM) is a widely distributed peptide in the central nervous system and exerts a variety of hormonal and neural actions. Although SOM is assumed to play an important role in spinal nociceptive processing, its exact function remains unclear. In fact, earlier pharmacological studies have provided results that support either a facilitatory or inhibitory role for SOM in nociception. In the current study, the effects of SOM were investigated using anesthetized cats. Specifically, the responses of rostrally projecting spinal dorsal horn neurons (RPSDH neurons) to different kinds of noxious stimuli (i.e., heat, mechanical and cold stimuli) and to the Adelta-and C-fiber activation of the sciatic nerve were studied. Iontophoretically applied SOM suppressed the responses of RPSDH neurons to noxious heat and mechanical stimuli as well as to C-fiber activation. Conversely, it enhanced these responses to noxious cold stimulus and Adelta-fiber activation. In addition, SOM suppressed glutamate-evoked activities of RPSDH neurons. The effects of SOM were blocked by the SOM receptor antagonist cyclo-SOM. These findings suggest that SOM has a dual effect on the activities of RPSDH neurons; that is, facilitation and inhibition, depending on the modality of pain signaled through them and its action site.

Substance P (NK1) and somatostatin (sst2A) receptor immunoreactivity in NTS-projecting rat dorsal horn neurones activated by nociceptive afferent input

Spinal neurones that receive inputs from primary afferent fibres and have axons projecting supraspinally to the medulla oblongata may represent a pathway through which nociceptive and non-nociceptive peripheral stimuli are able to modulate cardiorespiratory reflexes. Expression of the neurokinin-1 (NK1) receptor is believed to be an indicator of lamina I cells that receive nociceptive inputs from substance P releasing afferents, and similarly, sst2A receptor expression may be a marker for neurones receiving somatostatinergic inputs. In this study, immunoreactivity for these two receptors was investigated in rat spinal neurones retrogradely labelled by injections of cholera toxin B or Fluorogold into the nucleus of the solitary tract (NTS). In addition, nociceptive activation of these labelled cells was studied by immunodetection of Fos protein in response to cutaneous and visceral noxious chemical stimuli. NK1 and sst2A receptors in lamina I were localised to mainly separate populations of retrogradely labelled cells with fusiform, flattened and pyramidal morphologies. Examples of projection neurones expressing both receptors were, however observed. With visceral stimulation, many retrogradely labelled cells expressing c-fos were immunoreactive for the NK1 receptor, and a smaller population was sst2A positive. In contrast, with cutaneous stimulation, only NK1 positive retrogradely labelled cells showed c-fos expression. These data provide evidence that lamina I neurones receiving noxious cutaneous and visceral stimuli via NK1 receptor activation project to NTS and so may be involved in coordinating nociceptive and cardiorespiratory responses. Moreover, a subpopulation of projection neurones that respond to visceral stimuli may receive somatostatinergic inputs of peripheral, local or supraspinal origins.

Immunohistochemical distribution of enkephalin, substance P, and somatostatin in the brainstem of the leopard frog, Rana pipiens

The brainstems of frogs contain many of the neurochemicals that are found in mammals. However, the clustering of nuclei near the ventricles makes it difficult to distinguish individual cell groups. We addressed this problem by combining immunohistochemistry with tract tracing and an analysis of cell morphology to localize neuropeptides within the brainstem of Rana pipiens. We injected a retrograde tracer, Fluoro-Gold, into the spinal cord, and, in the same frog, processed adjacent sections for immunohistochemical location of antibodies to the neuropeptides enkephalin (ENK), substance P (SP), and somatostatin (SOM). SOM+ cells were more widespread than cells containing immunoreactivity (ir) to the other substances. Most reticular nuclei in frog brainstem contained ir to at least one of these chemicals. Cells with SOM ir were found in nucleus (n.) reticularis pontis oralis, n. reticularis magnocellularis, n. reticularis paragigantocellularis, n. reticularis dorsalis, the optic tectum, n. interpeduncularis, and n. solitarius. ENK-containing cell bodies were found in n. reticularis pontis oralis, n. reticularis dorsalis, the nucleus of the solitary tract, and the tectum. The midbrain contained most of the SP+ cells. Six nonreticular nuclei (griseum centrale rhombencephali, n. isthmi, n. profundus mesencephali, n. interpeduncularis, torus semicircularis laminaris, and the tectum) contained ir to one or more of the substances but did not project to the spinal cord. The descending tract of V, and the rubrospinal, reticulospinal, and solitary tracts contained all three peptides as did the n. profundus mesencephali, n. isthmi, and specific tectal layers. Because the distribution of neurochemicals within the frog brainstem is similar to that of amniotes, our results emphasize the large amount of conservation of structure, biochemistry, and possibly function that has occurred in the brainstem, and especially in the phylogenetically old reticular formation.

Anti-somatostatin antisera, but neither a somatostatin agonist (octreotide) nor antagonist (CYCAM), attenuates hyperalgesia in the rat

Somatostatin has been reported to have both nociceptive and antinociceptive roles in sensory transmission in the spinal cord. In this study, antisera against SOM (alpha-SOM), a somatostatin antagonist (CYCAM) and a somatostatin agonist (octreotide), were evaluated for their role in thermal and mechanical hyperalgesia in a model of carrageenan-induced inflammatory pain in the rat. Intrathecal administration of alpha-SOM prior to hindpaw inflammation dose-dependently attenuated thermal and mechanical hyperalgesia and the increase in paw size for up to 4 h following injury. Administration of alpha-SOM 3 h following injury had no effect. Intrathecal administration of octreotide or CYCAM prior to or following injection of carrageenan had no effect on any measure. It is suggested that the lack of effect of octreotide and CYCAM resulted from low affinity for the SOM receptor subtypes in the rat spinal cord. The attenuation of hyperalgesia and paw size produced by alpha-SOM may have resulted from attenuation of somatostatin's role in producing a dorsal root reflex that modulates the increase in paw size following injury.

Transganglionic labelling of primary sensory afferents in the rat lumbar spinal cord: comparison between wheatgerm agglutinin and the I-B4 isolectin from Bandeiraea simplicifolia

We recently reported that the I-B4 isolectin from Bandeiraea simplicifolia could be used as a transganglionic neuronal tracer which appears to be selective for unmyelinated cutaneous afferents (C fibres) and their terminals in the superficial dorsal horn. As terminals in the superficial dorsal horn are also labelled by wheatgerm agglutinin, we sought to compare these two neuronal tracers. Three days after the injection of 1% wheatgerm agglutinin-HRP or 1% BSI-B4-HRP into the sciatic nerve of adult rats the lumbar spinal cord was processed for HRP reactivity. The majority of labelled structures was found in the superficial dorsal horn, with fewer labelled structures seen in the overlying white matter (including Lissauer's tract). In wheatgerm agglutinin-HRP experiments most labelled structures were synaptic terminals (63%) and unmyelinated axons (32%). About 3% of wheatgerm agglutinin-HRP-labelled structures were fine myelinated fibres (which were found only in lamina I and outer lamina II) and about 2% of label was located in neuronal somata. In contrast, label from BSI-B4-HRP experiments was found only in synaptic terminals (37%) and unmyelinated axons (63%). Previous studies have shown that small diameter dorsal root ganglion neurons and their terminals in the superficial dorsal horn express a range of structurally related carbohydrates that contain binding sites for BSI-B4 or wheatgerm agglutinin or both. Comparison of the labelling patterns produced by the two transganglionic tracers in the present study suggests that unmyelinated sciatic afferents express wheatgerm agglutinin and BSI-B4 binding sites, but some thin myelinated afferents, and a distinct form of synaptic terminal in lamina I/II outer, express the wheatgerm agglutinin binding site and not the BSI-B4 binding site.