Novelty of the test environment contributes to the expression of triethyl lead-induced antinociception

Acute subcutaneous injection of 7.88 mg/kg of triethyl lead chloride (TEL) increased hot-plate latencies 7 days later. This antinociception has been shown to be unrelated to neuromotor impairments and is modulated by environmental factors such as novelty of the test situation. Furthermore, behavioral observations revealed that TEL induced a profile of behavior during hot-plate testing which was unlike that produced by an equianalgesic dose of morphine (10 mg/kg). While morphine-injected rats were hypokinetic and remained in one location the rats injected with TEL vigorously explored the apparatus. It was subsequently found that manipulation of test variables such as familiarity with the apparatus and test environment could modify TEL's antinociceptive effects. For example, repeated exposure to a non-functional hot-plate as well as daily hot-plate testing attenuated the antinociception induced by TEL. These studies suggest that TEL-induced antinociception is related to an enhanced behavioral and/or biological response to stress and/or "novelty." The relationship of this effect to stress-induced analgesia is discussed.