Intracranial self-stimulation in rats as a function of various stimulus parameters

The effects of different subcutaneous doses (0.08, 0.16, 0.31, 0.63 and 1.25 mg/kg) of apomorphine on self-stimulation in rats, with monopolar nichrome electrodes, implanted in the medial forebrain bundle at the level of the lateral hypothalamus were studied. Six different selected stimulus parameter combinations inducing different predictable response rates were used. Apomorphine was found to produce a dose-related response stimulation and a dose-related response depression. The highest stimulation was obtained at 0.63 mg/kg, the highest depression at 1.25 mg/kg. The response stimulation with apomorphine was 1. inversely related to the control response rate, i.e. the higher the control response rate, the lower the response stimulation after apomorphine and vice versa, 2. directly related to the control response rate of the individual rats, i.e. the highest response stimulation was obtained with the most sensitive rat and vice versa. The response inhibition with apomorphine was not related to the control response rates but was more pronounced during the first 1/2 h of the session. It is postulated that

1. increased self-stimulation with apomorphine could be the result of an increased motor response output;
2. decreased self-stimulation with apomorphine could be due to non-adaptive behaviour as a result of non-physiological overexcitation with interruption of integrated behaviour;
3. a complex behavioural pattern like intracranial self-stimulation depends on different interacting systems, mediated by different transmitters.