These electrophysiological investigations of spinal cord neu¬
rones responding to noxious and non-noxious cutaneous stimulation
were conducted to evaluate (1) their ascending projection into the
spinothalamic tract, (2) the involvement of endogenous opioids in
mediating the tonic descending and segmental inhibition and (3) the
role of the nucleus locus coeruleus (LC) in modulating spinal cord
A total of 252 neurones were antidromically activated from the
ventrolateral quadrant but only 14 of these were found to project
into the spinothalamic tract. The spinothalamic tract (STT) neurones
responded to noxious and non-noxious, or to noxious stimulation alone.
All the units were located in lamina VII and most of them displayed
wide receptive fields. None of the 39 specific nociceptor-driven
neurones from lamina I was found to project into the spinothalamic
Naloxone (0.3-2.0 mgm/kg) did not alter the response of multireceptive neurones to heat, to tonic descending inhibition or to the
inhibition generated by stimulation of contralateral plantar nerve
or dorsal columns.
Stimulation in the nucleus locus coeruleus produced a predominantly inhibitory effect on nociceptive transmission in the spinal cord.
The descending inhibition produced from LC was antagonised by the
administration of a-noradrenergic receptor antagonists but was not
changed by 3-receptor antagonists. Methysergide did not alter the
inhibition from LC or NRM whereas the GABA antagonist, bicuculline,
abolished the inhibition. Naloxone partially reduced the inhibition
from LC. The ipsilateral ventral quadrant lesion (VLF and VF)
abolished the actions produced from LC but a bilateral DLF lesion
was required for abolishing the NRM actions. Electrolytic lesions
made in the midline raphe complex did not block the actions produced
from LC. The stimuli in LC and NKM that evoked inhibition of multireceptive neurones also produced DRPs.
These data support the conclusion that the spinothalamic tract
in the cat plays a role in the transmission of nociceptive and
tactile messages to the brain. The endogenous opioids do not appear
to be involved in mediating the types of inhibition examined (Section III). The
descending actions produced from LC are not mediated through NRM
and are most probably mediated through the direct coeruleospinal
projection. Catecholaminergic, opioid and Gabaergic transmission
is involved in mediating these actions which may involve both the
pre- and postsynaptic mechanisms.