Sandy -

Check this one out, among a few:

A combined effect of dextromethorphan and melatonin on neuropathic pain behavior in rats, Wang S, Zhang L, Lim G, Sung B, Tian Y, Chou CW, Hernstadt H, Rusanescu G, Ma Y, Mao J., Brain Res. 2009 Sep 8;1288:42-9. Epub 2009 Jul 9, FULL ONLINE TEXT@ http://www.ncbi.nlm.nih.gov/pmc/arti...ihms132160.pdf

MGH Center for Translational Pain Research, Department of Anesthesia and Critical Care, WACC 324, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA.

Abstract
Previous study has shown that administration of melatonin into the anterior cingulate cortex contralateral to peripheral nerve injury prevented exacerbation of mechanical allodynia with a concurrent improvement of depression-like behavior in Wistar-Kyoto (WKY) rats, a genetic variation of Wistar rats. In the present study, we examined the effect of the individual versus combined treatment of melatonin and/or dextromethorphan (DM), a clinically available N-methyl-d-aspartate (NMDA) receptor antagonist, on pain behaviors in WKY rats with chronic constriction sciatic nerve injury (CCI). Pain behaviors (thermal hyperalgesia and mechanical allodynia) were established at one week after CCI. WKY rats were then treated intraperitoneally with various doses of melatonin, DM or their combination once daily for the following week. At the end of this one-week treatment, behavioral tests were repeated in these same rats. While DM alone was effective in reducing thermal hyperalgesia at three tested doses (15, 30 or 60 mg/kg), it reduced mechanical allodynia only at high doses (30 or 60 mg/kg). By comparison, administration of melatonin alone was effective in reducing thermal hyperalgesia only at the highest dose (120 mg/kg, but not 30 or 60 mg/kg) tested in this experiment. Melatonin alone failed to reverse allodynia at all three tested doses (30, 60 and 120 mg/kg). However, the combined intraperitoneal administration of melatonin (30 mg/kg) and DM (15 mg/kg) effectively reversed both thermal hyperalgesia and mechanical allodynia although each individual dose alone did not reduce pain behaviors. These results suggest that a combination of melatonin with a clinically available NMDA receptor antagonist might be more effective than either drug alone for the treatment of neuropathic pain.

PMID: 19595681 [PubMed - indexed for MEDLINE]

http://www.ncbi.nlm.nih.gov/pubmed/19595681

And while I only have the abstract, I think you might appreciate this one in particular:

An intravenous ketamine test as a predictive response tool in opioid-exposed patients with persistent pain, Cohen SP, Wang S, Chen L, Kurihara C, McKnight G, Marcuson M, Mao J, J Pain Symptom Manage. 2009 Apr;37(4):698-708. Epub 2008 Sep 11.

Pain Management Division, Department of Anesthesiology & Critical Care Medicine, Johns Hopkins School of Medicine, Baltimore, Maryland 21205, USA. scohen40@jhmi.edu

Abstract
Chronic pain patients who are treated with opioid therapy represent a significant challenge to medical professionals. When pain recurs in the face of a previously effective opioid regimen, treatment options include dose escalation, opioid rotation, drug holidays, and the addition of adjuvants. Some experts advocate the use of N-methyl-D-aspartate receptor (NMDA-R) antagonists to combat tolerance. Recently, the use of an intravenous (i.v.) ketamine infusion to predict the response to a dextromethorphan (DX) treatment trial has been described. In this study, 56 opioid-exposed patients with recurrent pain were treated with a low-dose (0.1mg/kg) i.v. ketamine test followed by a DX treatment course. Using previously designated cutoff values for a positive response to ketamine (67% or more pain relief) and DX (50% or more pain relief), the sensitivity, specificity, positive predictive value, and negative predictive value for an i.v. ketamine infusion to predict subsequent response to DX treatment were 72%, 68%, 52%, and 85%, respectively. The observed agreement between analgesic responses was 78%, indicating a highly significant correlation (r=0.54, P=0.0001). Subgroup classification revealed no significant differences in the response to either ketamine or DX treatment based on pain classification (i.e., nociceptive, neuropathic, or mixed) or placebo response. In contrast, a weaker correlation between ketamine and DX response was found in subjects requiring high-dose rather than low-dose opioid therapy. A significant correlation also was noted between the development of side effects for the two NMDA-R antagonists. Based on these results, we conclude that an i.v. ketamine test may be a valuable tool in predicting subsequent response to DX treatment in opioid-exposed patients. with persistent pain.

PMID: 18789637 [PubMed - indexed for MEDLINE]

http://www.ncbi.nlm.nih.gov/pubmed/18789637

Mike