Action of beta-endorphin antiserum of different antibody titres on thermal or tail-shock pain in rats.

Intravenous injection of beta-endorphin antiserum (AS) with antibody titres of 1:1,600, 1:3,200, 1:12,800, and 1:20,000 decreased the voltage threshold in the tail-shock test (electrocutaneous nociceptive stimulus) in rats for 2 days, after which time control levels were regained. In the case of AS of titres 1:12,800 and 1:20,000 this threshold then increased to above control levels for about 40 and 100 days, respectively. This effect was abolished by naloxone. In the same rats, an injection of AS of titre 1:1,600 did not alter the latency of the tail-flick test (thermal nociceptive stimulus) but AS of titres 1:3,200, 1:12,800, and 1:20,000 decreased it for 1-4 h, the actual length of time being titre-dependent. After control levels had been regained, there was no further change in latency in the tail-flick test in the subsequent 42 days. Injection with preimmune serum did not change either voltage threshold or latency in the two tail-stimulation tests in control rats. Thus the hyperalgesic short-term effect seen in response to treatment with beta-endorphin AS was more pronounced in response to an electrocutaneous stimulus than to a thermal one, and the long-term selective analgesic effect was present in response to rats given electrocutaneous stimulation but not to thermal stimulation. It is proposed that beta-endorphin AS has a two-stage selective effect: a reduction in beta-endorphin release followed by a 'rebound' increase.