Effects of systemic naloxone upon ventrobasal thalamus neuronal responses in arthritic rats

This study deals with the effect of various doses of systemic naloxone (10 microgram, 300 microgram, 1 mg/kg) upon activities of 21 ventrobasa thalamus neurons recorded in 20 rats rendered arthritic by injection of Freund's adjuvant into the tail. These neurons presented reproducible responses to movement and/or mild lateral pressure on a joint and were recorded for at least 30 min after naloxone administration. Several neurons (5) were tested with two doses. After intravenous injection of naloxone at the dose of 10 microgram/kg (10 cases) there was a rapid decrease of the responses. The maximum effect occurred at 15 min when the mean value expressed as a percentage of the control was 46.20 +/- 8.51% (n = 10, P less than 0.001). Recovery could be considered as complete at 30 min. At the dose of 300 microgram/kg (9 cases), the decrease in the responses was less important, variable from one neuron to another but significant between 5 and 20 min (mean = 67.43 +/- 9.00% at 20 min, n = 7, P less than 0.01). At the dose of 1 mg/kg (7 cases), there was no significant modification of the response. Spontaneous firing rate of the neurons was slightly but significantly increased after injection of the two highest doses and unmodified after the lowest. The relationship between the depressive effect produced by low doses of naloxone upon the neuronal responses, and the 'bi-directional' analgesic-hyperalgesic action of the drug, demonstrated in these suffering rats, is discussed.