In studies of the effect on nociception of chronic administration of
antidepressants, the stress of the
injections may influence the results. In this experiment,
desipramine or
zimelidine were administered in the
drinking water of rats, in a concentration yielding a dose of approximately 8 mg/kg/24 hr.
Desipramine, given both for a short time (24 hr) and chronically (14 days), induced antinociception in the increasing temperature hot-plate test;
zimelidine did not significantly influence the results of this test. In the tail-flick test, neither short-term nor chronic administration of these
antidepressants had any effect on nociception, when correction was made for the changes in the temperature of the tail skin. In the
formalin test, nine behavioural categories were scored for 1 hr and the data were treated statistically, using a multivariate analysis. Chronic administration of
desipramine increased nociceptive behaviour during the first 10 min of the test.
Desipramine and, to a lesser extent,
zimelidine, changed the response in the late phase (10-60 min), showing less focussed
pain-related behaviour (jerks and shaking, licking and biting of the injected paw) and more non-focussed
pain-related behaviour (activity states with elevation or protection of the injected paw). It was concluded that
desipramine is antinociceptive in the increasing temperature hot-plate test.
Desipramine and
zimelidine, administered chronically, modify the late phase of the
formalin test towards less focussed
pain-related behaviour, suggesting an antinociceptive effect. Multivariate analysis of the data of the
formalin test seemed to be of value for the interpretation of the data.