This study addressed the role of neurokinin] (NKi) receptors in nociceptive transmission and
their participation in a series of events involving glycine and NMDA receptor-mediated
effects on spinal neurones. Using an in vivo electrophysiology protocol utilising ionophoresis
and extracellular recording from laminae III-V dorsal horn neurones of anaesthetised rats,
the mechanisms of these interactions were assessed.
The functions of the inflammatory cytokine leukaemia inhibitory factor (LIF) were also
considered. Injury-induced alterations in the spinal expression pattern of this factor and the
consequences of these changes to neuropeptide and excitatory amino acid expression were
measured using in situ hybridisation.
1. The involvement of NK] receptors in spinal pain transmission may be dependent upon
the duration and intensity of the nociceptive stimulus. Since activation of spinal NK]
receptors leads to increases in the concentration of glycine in the dorsal horn, the role of the
inhibitory glycine receptor as a regulator of NKj receptor function was investigated.
Ionophoretic application of GR82334, a selective NK] receptor antagonist did not alter
activity evoked by cutaneous applications of mustard oil. Flowever in the presence of the
glycine antagonists strychnine or phenylbenzene-co-phosphono- a-amino acid (PMBA),
GR82334 displayed inhibitory properties. Therefore inhibitory glycine receptors may mask
the contribution made by NK] receptors to nociceptive processing. This is discussed with
reference to the role of NK] receptors during brief nociceptive transmission.
2. NK] receptors can contribute to the processing of sustained nociceptive stimuli by
modulating excitatory amino acid-mediated transmission, particularly through potentiation of
NMDA receptor activity. Experiments were carried out to investigate the possible role of the
glycine site of the NMDA receptor (GlyNMDA) and of protein kinase C (PKC) activation to
the NK]/NMDA interaction.
Ionophoresis of the NMDA receptor agonist 1 -aminocyclobutane-m-1,3-dicarboxylic acid
(ACBD) produced a sustained increase in the firing rate of dorsal horn neurones that was
facilitated by the NK] agonist acetyl-[Arg6,Sar9,Met(02)11]-SP6-ii (Sar9-SP). GlyNMDA site
antagonists 2-carboxy-4,6-dichloro-(lH)-indole-3-propanoic acid (MDL 29951), 7-chloro-3-
(cyclopropylcarbonyl)-4-hydroxy-2(lH)-quinoline (L701,252), 5,7-dinitroquinaxoline-2,3-
dione (MNQX) and 7-chlorothiokynurenic acid (7-CTK) or PKC inhibitors, chelerythrine
and GF109203X significantly reduced this facilitation whilst having no effect on activity
driven by ACBD alone. Like the NK] receptor agonist, a selective GlyNMDA site agonist 1-
aminocyclopropanecarboxylic acid (ACPC) also caused facilitation of ACBD-evoked
activity and this was inhibited by the GlyNMDA site antagonist L701,252 in a similar manner.
These data suggest that NKi receptors facilitate NMDA receptor by potentiating the positive
influence of the GlyNMDA site. This may be bought about by the NKi receptor-induced
glycine release and/or by a PKC mediated increase in the sensitivity of the GlyNMDA site.
This demonstrates that co-transmitters released from fine primary afferent fibres may interact
postsynaptically to alter central hyperexcitability, particularly that associated with more
prolonged noxious events.
3. LIF is a neuroactive cytokine that is associated with peripheral nerve injury. Using in situ
hybridisation, the present study has examined the distribution of LIF mRNA in the spinal
cord, normally or following peripheral inflammation or nerve injury and determined the
consequences of intrathecally applied LIF on spinal expression of NK, receptor and the high
affinity glutamate transporter, EAAT2.
In control animals, dorsal horn expression of LIF was high in superficial laminae I-II with
only light expression in the deeper laminae III-V and in the ventral horn. Both peripheral
inflammation and neuropathy significantly increased levels of LIF mRNA in the dorsal horn
and this was most evident in the lateral parts of laminae I and II.
NKi and EAAT2 expression was normally associated with cells in laminae I-II, IV-V and
ventral horn motorneurones. Intrathecal LIF administration significantly increased this
expression for all three mRNAs and resulted in dense hybridisation throughout the dorsal
These results show that LIF is normally expressed in the spinal cord in a specific laminar
pattern that increases dramatically following peripheral inflammation or nerve damage.
Furthermore, LIF upregulates the expression of NKi receptors and EAAT2 transporters
indicating that it may be a critical regulator of the central changes that occur following
Interactions within the spinal cord may underlie the plasticity of the dorsal horn in sensory
processing. This has been discussed with reference to the regulation of short-term co¬
operation between NIG and NMDA receptors by glycine and to long-term modifications of
peptide and excitatory amino acid neurotransmission by altered LIF gene expression.