Apomorphine enhances harmaline-induced tremor in rats

BackgroundHarmaline-induced tremor is a well-known model of essential tremor in humans. The aim of the present study was to examine the influence of apomorphine, a non-selective dopamine receptor agonist, on the tremor induced by harmaline in rats. Propranolol (a first-line drug in essential tremor) was used as a reference compound.MethodsTremor, locomotor activity and focused stereotypy were measured objectively using force plate actimeters. Tremor was analyzed using a Fourier transform to generate power spectra for rhythmic behavior.ResultsThe tremor induced by harmaline administered at a dose of 15 mg/kg ip was associated with an increase in power in the 9–15 Hz band (AP2) and in the tremor index, calculated as a difference between AP2 and power in the 0–8 Hz band (AP1). Propranolol injected at a dose of 20 mg/kg ip reversed both of these effects of harmaline. Apomorphine administered at the doses of 0.5 and 1 mg/kg sc further enhanced AP2 and at the lower dose also the tremor index elevated by harmaline. This increase in AP2 was stronger than enhancement of locomotor activity induced by apomorphine in the harmaline-treated animals.ConclusionsThe present study suggests that the dopamine agonist apomorphine enhances the tremor induced by harmaline, and this effect is at least partly independent of hyperactivity.