Agonist--antagonist properties of phenoxybenzamine in antinociception and opiate dependence tests

Evaluation of phenoxybenzamine in the CFA model of pain following gene expression studies and connectivity mapping

Background

We have previously used the rat 4 day Complete Freund's Adjuvant (CFA) model to screen compounds with potential to reduce osteoarthritic pain. The aim of this study was to identify genes altered in this model of osteoarthritic pain and use this information to infer analgesic potential of compounds based on their own gene expression profiles using the Connectivity Map approach.

Results

Using microarrays, we identified differentially expressed genes in L4 and L5 dorsal root ganglia (DRG) from rats that had received intraplantar CFA for 4 days compared to matched, untreated control animals. Analysis of these data indicated that the two groups were distinguishable by differences in genes important in immune responses, nerve growth and regeneration. This list of differentially expressed genes defined a "CFA signature". We used the Connectivity Map approach to identify pharmacologic agents in the Broad Institute Build02 database that had gene expression signatures that were inversely related ('negatively connected') with our CFA signature. To test the predictive nature of the Connectivity Map methodology, we tested phenoxybenzamine (an alpha adrenergic receptor antagonist) - one of the most negatively connected compounds identified in this database - for analgesic activity in the CFA model. Our results indicate that at 10 mg/kg, phenoxybenzamine demonstrated analgesia comparable to that of Naproxen in this model.

Conclusion

Evaluation of phenoxybenzamine-induced analgesia in the current study lends support to the utility of the Connectivity Map approach for identifying compounds with analgesic properties in the CFA model.

Yohimbine both increases and decreases nociceptive thresholds in rats: evaluation of the dose-response relationship

The effect of yohimbine on a response to nociceptive stimulation mediated by supraspinal structures was studied after subcutaneous administration in doses from 0.25 mg/kg up to 25 mg/kg. The results showed that to describe adequately the dose-response relationship of yohimbine in pain modulation, characteristics of opposing effects had to be considered: low doses with an ED50-value of 0.56 mg/kg were found to dose-dependently facilitate nociception whereas high doses instead induced antinociception with an ED50 of 4.74 mg/kg. The possible capacity of alpha 2-adrenoceptors to modulate responses to nociception is discussed.

Opposing effects of apomorphine on pain in rats. Evaluation of the dose-response curve

The effect of apomorphine on a supraspinally mediated response to pain was studied after subcutaneous administration of 10 different doses (25 micrograms/kg up to 10 mg/kg). Depending on the dose given, apomorphine was found to induce opposing effects on pain, so that low doses, 25-100 micrograms/kg, dose-dependently increased the sensitivity to pain. This effect then gradually declined in potency with increasing doses and high doses induced antinociception. The data therefore suggest that the net effect recorded involves the sum of responses from at least two functional systems. Using the Hill equation and the digital computer program NONLIN, we have dissociated the observed effect into two components, each having its particular dose-response characteristics: low doses having an ED50 value of 36 micrograms/kg produced hyperreactivity to pain, and high doses having an ED50 of 465 micrograms/kg (in the absence of hyperalgesia) induced antinociception.