Intracranial injections of 6-OHDA. Comparison of catecholamine-depleting effects of different volumes and concentrations.

Fluorescence histochemistry was used to assess monoamine depletion after injections of 6-OHDA into selected brain areas. Two volumes (2 and 4 mul) and 4 concentrations (1, 2, 4 and 8 mug/mul) of 6-OHDA were injected into the olfactory tubercle, the posterior lateral hypothalamus and the lateral hyopthalamus. Selective destruction of catecholamine-containing neurons resulted from all injections of 6-OHDA with the exception of the 2 lowest doses (2 and 4 mul of 1 mug/mul) and the highest dose (4 mul of 8 mug/mul) which produced nonspecific damage of brain parenchyma. The results indicate that, in addition to the selection of an effective dose, it is also possible to choose a site of injection which will produce a maximal area of specific depletion. In cases where injections into terminal areas caused limited specific depletion the same dose injected into preterminal axons often caused a more widespread loss of fluorescence. With volume, concentration and anatomical location being important variables to consider, caution is needed in the interpretation of behavioural experiments. When using 6-OHDA it is necessary to show that specific depletion of catecholamines has been achieved.