Changes in gene expression and neuronal phenotype in brain stem pain modulatory circuitry after inflammation

Modulation of visceral hyperalgesia by morphine and cholecystokinin from the rat rostroventral medial medulla

Using a model of visceral nociception, we examined whether cholecystokinin (CCK) acts as an anti-opioid peptide in the rat rostral ventromedial medulla (RVM). Because such interaction may be affected by inflammation, rats with and without inflamed colons were studied. The visceromotor response to noxious colorectal distension (CRD), quantified electromyographically, was recorded before and after intra-RVM administration of CCK, CCK receptor antagonists, and morphine. Either 50% ethanol/saline (vehicle) or 2,4,6-trinitrobenzenesulfonic acid (TNBS), which inflames the colon, was instilled into the colon 5 days before experiments. Intra-RVM morphine dose-dependently attenuated responses to CRD in intracolonic vehicle-treated rats. In TNBS-treated rats with inflamed colons, responses to CRD were significantly increased and 0.3, 3.0 and 6.0 microg doses of intra-RVM morphine reduced responses to control (i.e. were anti-hyperalgesic); the greatest dose tested (30 microg) further reduced responses to 40% control. In intracolonic vehicle-treated rats, intra-RVM pre-treatment with a selective CCK(B) (but not CCK(A)) receptor antagonist dose-dependently and significantly enhanced the effect of a low dose of morphine. Intra-RVM CCK-8 peptide enhanced responses to CRD in intracolonic vehicle-treated, but not TNBS-treated rats. Intra-RVM naloxone was without effect in intracolonic vehicle-or TNBS-treated rats, suggesting an absence of tonic opioid activity in RVM. These results document a CCK-opioid interaction in RVM, suggesting that colon inflammation leads to tonic activity at CCK(B) receptors in RVM.

Inflammation-induced upregulation of AMPA receptor subunit expression in brain stem pain modulatory circuitry

Our previous study demonstrated an increase in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor sensitivity in the rostral ventromedial medulla (RVM) associated with enhanced net descending inhibition after inflammatory hyperalgesia. The present study further studied the time-dependent changes in AMPA-produced inhibition after inflammation and the underlying molecular mechanisms. Inflammation was induced by intraplantar injection of complete Freund's adjuvant (CFA, 0.2ml). There was a significant increase in AMPA-produced inhibition at 5h that was further enhanced at 24h (P<0.05), as compared to that at 3h post-inflammation. The AMPA-produced inhibition returned to the control level at 14 days post-inflammation. We analyzed mRNA and protein levels of the GluR1 and GluR2 AMPA receptor subunits in the RVM at 2h to 14 days post-inflammation. AMPA receptor subunits exist in the two 'flip' and 'flop' isoforms that differentially affect the desensitization properties of the receptor. Reverse transcription-polymerase chain reaction analysis indicated that there was a significant upregulation of mRNAs encoding the GluR1-flip (5-24h), GluR2-flip (24h) and GluR2-flop (24h) isoforms in the RVM after inflammation, whereas the levels of GluR1-flop mRNAs showed no significant change. Western blots demonstrated that the GluR1 protein levels were significantly upregulated at 24h-3 days (P<0.05) post-inflammation, compared to that of naive animals. GluR2 protein levels remained unchanged. Immunohistochemistry further demonstrated an increase in GluR1-like immunoreactivity localized to the RVM at 24h post-inflammation. These findings suggest that AMPA receptors in the RVM undergo selective transcriptional and translational modulation following inflammation and may contribute to activity-dependent plasticity in descending pain modulatory systems after prolonged noxious input.

Repetitive acute pain in infancy increases anxiety but does not alter spatial learning ability in juvenile mice

We assessed the long-term behavioural effects of a single acute or repetitive inflammatory pain experienced during infancy. Groups of male and female CD1 mice were subjected to either an acute single pain, i.e. a tail clip or sham pain at P8, or acute repetitive pain in the form of needle pricks or sham pain from P8 to P14. All of the subjects were tested in the elevated plus maze at P30 and in the Morris Water Maze from P31 to P38. Mice in the acute single pain and sham groups did not differ on measures of anxiety in the plus maze. Mice in the repetitive pain group demonstrated significantly more anxious behaviours than controls in the elevated plus maze as they spent less time in the open arms, made fewer open arm entries, displayed fewer head dips and showed more stretch attend postures. There were no effects of single or repetitive pain treatments in the latency to find the hidden platform in the Morris Water Maze. Overall, these data suggest that acute repetitive pain experienced during infancy may increase anxiety later in life but it does not influence spatial learning as measured in the Morris Water Maze. The origin of the anxiogenic profile shown in the acute repetitive pain mice may be a result of changes in neural circuitry or context dependent learning and is currently under further investigation with this paradigm.

Rostral ventromedial medulla neurons that project to the spinal cord express multiple opioid receptor phenotypes

The rostral ventromedial medulla (RVM) forms part of a descending pathway that modulates nociceptive neurotransmission at the level of the spinal cord dorsal horn. However, the involvement of descending RVM systems in opioid analgesia are a matter of some debate. In the present study, patch-clamp recordings of RVM neurons were made from rats that had received retrograde tracer injections into the spinal cord. More than 90% of identified spinally projecting RVM neurons responded to opioid agonists. Of these neurons, 53% responded only to the mu-opioid agonist D-Ala2, N-Me-Phe4, Gly-ol5 enkephalin, 14% responded only to the kappa-opioid agonist U-69593, and another group responded to both mu and kappa opioids (23%). In unidentified RVM neurons, a larger proportion of neurons responded only to mu opioids (75%), with smaller proportions of kappa- (4%) and mu/kappa-opioid (13%) responders. These RVM slices were then immunostained for tryptophan hydroxylase (TPH), a marker of serotonergic neurons. Forty-percent of spinally projecting neurons and 11% of unidentified neurons were TPH positive. Of the TPH-positive spinally projecting neurons, there were similar proportions of mu- (33%), kappa- (25%), and mu/kappa-opioid (33%) responders. Most of the TPH-negative spinally projecting neurons were mu-opioid responders (67%). These findings indicate that functional opioid receptor subtypes exist on spinally projecting serotonergic and nonserotonergic RVM neurons. The proportions of mu- and kappa-opioid receptors expressed differ between serotonergic and nonserotonergic neurons and between retrogradely labeled and unlabeled RVM neurons. We conclude that important roles exist for both serotonergic and nonserotonergic RVM neurons in the mediation of opioid effects.

The N-methyl-D-aspartate receptor subunit NR2B: localization, functional properties, regulation, and clinical implications

The N-methyl-D-aspartate (NMDA) receptor is an example of a heteromeric ligand-gated ion channel that interacts with multiple intracellular proteins by way of different subunits. NMDA receptors are composed of seven known subunits (NR1, NR2A-D, NR3A-B). The present review focuses on the NR2B subunit of the receptor. Over the last several years, an increasing number of reports have demonstrated the importance of the NR2B subunit in a variety of synaptic signaling events and protein-protein interactions. The NR2B subunit has been implicated in modulating functions such as learning, memory processing, pain perception, and feeding behaviors, as well as being involved in a number of human disorders. The following review provides a summary of recent findings regarding the structural features, localization, functional properties, and regulation of the NR2B subunit. The review concludes with a section discussing the role of NR2B in human diseases.

The roles of NMDA receptor activation and nucleus reticularis gigantocellularis in the time-dependent changes in descending inhibition after inflammation

Previous studies indicate that descending modulation of nociception is progressively increased following persistent inflammation. The present study was designed to further examine the role of supraspinal neurons in descending modulation following persistent inflammation. Constant levels of paw withdrawal (PW) and tail flick (TF) latencies to noxious heat stimuli were achieved in lightly anesthetized rats (pentobarbital sodium 3-10 mg/kg/h, i.v.). Electrical stimulation (ES, 0.1 ms, 100 Hz, 20-200 A) was delivered to the rostral ventromedial medulla (RVM), mainly the nucleus raphe magnus (NRM). ES produced intensity-dependent inhibition of PW and TF. Following a unilateral hindpaw inflammation produced by injection of complete Freund's adjuvant (CFA), ES-produced inhibition underwent time-dependent changes. There was an initial decrease at 3 h after inflammation and a subsequent increase after inflammation in the excitability of RVM neurons and the inhibition of nocifensive responses. These changes were most robust after stimulation of the inflamed paw although similar findings were seen on the non-inflamed paw and tail. The inflammation-induced dynamic changes in descending modulation appeared to be correlated with changes in the activation of the N-methyl--aspartate (NMDA) excitatory amino acid receptor. Microinjection of an NMDA receptor antagonist, AP5 (1 pmol), resulted in an increase in the current intensity required for inhibition of the PW and TF. The effect of AP5 was less at 3 h after inflammation and significantly greater at 11-24 h after inflammation. In a subsequent experiment, ES-produced inhibition of nocifensive responses after inflammation was examined following selective chemical lesions of the nuclei reticularis gigantocellularis (NGC). Compared to vehicle-injected animals, microinjection of a soma-selective excitotoxin, ibotenic acid, enhanced ES-produced inhibition at 3 h but not at 24 h after inflammation. We propose that these time course changes reflect dynamic alterations in concomitant descending facilitation and inhibition. At early time points, NMDA receptor and NGC activation enhance descending facilitation; as time progresses, the dose-response curve of NMDA shifts to the left and descending inhibition dominates and masks any descending facilitation.