Ionophoretically-applied dopamine and noradrenaline
selectively inhibited the nociceptive responses of
multireceptive somatosensory dorsal horn neurones, whilst
non-nociceptive responses, spontaneous activity and
activity evoked by an ionophoretically-applied excitatory
amino acid, DL-homocysteic acid were unaffected. Many
of the neurones tested had long ascending projections,
capable of transmitting nociceptive information to
supraspinal sites; in the rat (spinothalamic tract
neurones) and in the cat (spinocervical tract neurones).
The use of ionophoretically-applied receptor-specific
agonists and antagonists demonstrated that the actions of
noradrenaline and dopamine were pharmacologically
distinct. The selective antinociceptive action produced
by noradrenaline was mediated by an 06,-adrenoreceptor,
whilst the selective antinociceptive effect of dopamine
was mediated by a dopamine receptor.
A glyoxylic-acid histofluorescence study was undertaken
to ascertain the optimal stereotaxic placement of
stimulating electrodes, in the regions of those dopamine
cell groups (A9 and All) that have been considered to
provide a spinal projection.
Focal electrical stimulation in the region of the All
dopamine cell group evoked a selective antinociceptive
effect on multireceptive dorsal horn neurones in the rat.
This stimulus-evoked effect was rapidly and consistently
reversed by ionophoresis of the D₂ dopamine-receptor
antagonist, sulpiride, in the vicinity of the dorsal horn
neurone being tested, whilst an opiate antagonist
(naloxone) and an α₂-antagonist (RX781094) had no
effect. Using the same parameters, focal electrical
stimulation in the region of the A9 dopamine cell group
did not affect the evoked responses of any multireceptive
The results of this study present data supporting
selective antinociceptive roles for dopamine and
noradrenaline at the spinal level. The All dopamine cell
group was demonstrated as a supraspinal source of a
selective antinociceptive effect, mediated by dopamine at
the level of the dorsal horn.