Evidence for the presence of neurokinin-1 receptors on dorsal horn spinocerebellar tract cells in the rat

Abstract

Dorsal horn spinocerebellar tract cells of adult rats were labelled by retrograde axonal transport with the B subunit of cholera toxin. Sections were prepared from lumbar and thoracic spinal segments and incubated with antisera which specifically recognise neurokinin-1 receptor protein and substance P. Labelled cells and immunoreactivity for the receptor and substance P were identified by using three different fluorophores and the relationships between them were assessed in single optical sections with three-colour confocal laser scanning microscopy. Forty-eight cells were examined and 23 of them displayed immunoreactivity for the receptor. Many substance P-immunoreactive profiles were present in lamina V and some formed contacts with spinocerebellar tract cells possessing neurokinin-1 receptor immunoreactivity. The evidence suggests that substance P may influence the activity of a subpopulation of dorsal horn spinocerebellar tract cells by acting through neurokinin-1 receptors.

Introduction

Substance P (SP) is a peptide neuromodulator which is present throughout the grey matter of the spinal cord. It is particularly concentrated in a population of primary afferent fibres in laminae I and II of the dorsal horn, but concentrations of SP-containing axons have been noted in other regions of the grey matter, including lamina V 6, 15, 24, 31which also contains neurons which form the dorsal horn component of the dorsal spinocerebellar tract (dhDSCT) 16, 27. It is generally agreed that SP acts preferentially at the neurokinin-1 (NK-1) tachykinin receptor 14, 20. The availability of antisera directed against parts of the amino acid sequence of this receptor [35]has enabled the locations of cells which express NK-1 receptor (NK-1R) immunoreactivity in the rat spinal cord to be mapped 4, 5, 22, 23, 30. These studies show that neuronal cell bodies and dendrites in laminae I and III–VI express the receptor. Immunoreactive cells correspond to several morphological types 5, 22, but some in lamina V are similar in appearance to dhDSCT cells 16, 27. The cells of origin of the dhDSCT pathway are located in caudal thoracic and rostral lumbar segments in rats 16, 27. In the cat, they are powerfully excited by group II muscle and cutaneous afferents but, in contrast to the cells in Clarke's column which form the major component of the dorsal spinocerebellar tract, they are not activated by group I muscle afferents [10]. Group II afferents provide information about muscle length from muscle spindle secondary fibres and it has been suggested that this pathway is concerned with limb position sense [10]. Furthermore, while all DSCT neurons project to the cerebellum, collateral axons of some of these neurons also project to nucleus Z in the medulla 2, 25, which is a relay nucleus conveying information to the contralateral thalamus and consequently to the cortex. It is probable, therefore, that the pathway has a role in the conscious appreciation of limb position in addition to supplying the cerebellum with information. Knowledge of the transmitters which modify the activity of these cells is thus likely to be of importance in understanding proprioceptive mechanisms.

It is often assumed that SP in the spinal cord is principally involved in nociceptive transmission; however, SP also regulates the activity of non-nocireceptive neurons, such as motor neurons and preganglionic sympathetic neurons 3, 18, 33. The aim of this study was to determine if SP is likely to influence transmission through a proprioceptive pathway. We examined dhDSCT cells for NK-1 receptor immunoreactivity and investigated the relationship between these cells and SP-immunoreactive fibres with a confocal laser scanning microscope.