In animal experimentation, welfare and severity assessments are necessary to meet legal and ethical requirements, as well as public interests. Injection of 6-hydroxydopamine (6-OHDA) into the nigrostriatal system of rats is used as a model for Parkinson's disease (PD) to investigate the pathophysiological background and treatment. The classical method of unilateral intranigral infusion of 6-OHDA leads to a massive destruction of nigrostriatal dopaminergic neurons and concomitant motor disturbances. After daily injection of Levodopa (L-Dopa), these rats develop dyskinesias, a devastating side effect after long-term treatment in PD patients. In the bilateral model, 6-OHDA infused into the striatum leads to slow and incomplete retrograde degeneration of dopaminergic neurons. This model can be used to study the development of PD or cognitive function. For severity assessment, we here quantitatively measured weight, heart rate and activity in the unilateral and bilateral 6-OHDA model, as well as during 21 days of L-Dopa injection.

In male Sprague Dawley (SD) rats (n=16) a telemetric device was subcutaneously implanted under general anesthesia and perioperative pain management. After recovery, rats received either unilateral stereotaxic injection of 6-OHDA into the substantia nigra (n=8) or bilateral injection into the striatum (n=8). Four weeks after unilateral 6-OHDA injection, rats were subcutaneously injected with L-Dopa (10 mg/kg) for 21 days. Perioperatively and during the L-Dopa injection we measured weight, heart rate and activity during the first two hours after light on and light off, as well as directly before and after L-Dopa injection.

Bilateral injection of 6-OHDA led to weight loss for the first four postoperative days (two rats nearly 20% of their preoperative weight; p<0.05), while unilateral 6-OHDA injection had no effect. Heart rate was enhanced for the first postoperative three days in both models, while activity measures were not affected after surgery. L-Dopa injection had no effect on weight and heart rate, but enhanced activity.

The results indicate that perioperatively rats` wellbeing is more affected by bilateral injection of 6-OHDA with incomplete loss of nigral dopamine, although heart rate measures also indicate disturbed wellbeing after unilateral injection of 6-OHDA. The development of dyskinesias, however, has only mild and transient effects on rat`s wellbeing.