Attenuation of 1-methyl-4-phenylpyridinium (MPP⁺) neurotoxicity by deprenyl in organotypic canine substantia nigra cultures

Summary

Systemic administration of MPTP to experimental animals induces neurodegeneration of dopaminergic neurons in the central nervous system. MPTP crosses the blood-brain barrier where it is taken up by astrocytes and converted to MPP⁺ by monamine oxidase-B (MAO-B). Subsequently, MPP⁺ is selectively taken up by dopaminergic neurons upon which it exerts intracellular neurotoxic effects. Systemic administration of the selective MAO-B inhibitor deprenyl prevents the conversion of MPTP to MPP⁺ and by this mechanism is able to protect against MPTP neurotoxicity. Deprenyl has also been reported to exert neuroprotective effects that are independent of its MAO-B inhibitory properties, but since MPP⁺ itself does not cross the blood-brain barrier it is difficult to directly study the MAO-B independentin vivo effects of MPP⁺ itself. One approach is to use organotypic tissue cultures of the canine substantia nigra (CSN) which permit administration of precise concentrations of pharmacological agents directly to mature, well-developed and metabolically active dopaminergic neurons. These neurons as well as other components of the cultures exhibit morphological and biochemical characteristics identical to theirin vivo counterparts. This study was undertaken to evaluate the neuroprotective effects of deprenyl in MPP⁺-treated cultures by measuring changes in the levels of HVA as an indicator of dopamine release and metabolism by dopaminergic neurons and to correlate this indication of dopaminergic function with morphological evidence of survival or loss of dopaminergic neurons in mature CSN cultures. Mature CSN cultures, at 44 days in vitro (DIV), were exposed to either MPP⁺ alone, deprenyl alone or simultaneously to both deprenyl and MPP⁺ or to MPP⁺ following 4 day pretreatment with deprenyl. Exposure to MPP⁺ alone caused significant reduction in HVA levels, evidence of widespread injury and ultimate disappearance of large neurons in the cultures. These effects were attenuated by simultaneous exposure to MPP⁺ and deprenyl and the destructive effects of MPP⁺ appeared to be prevented by pretreatment with deprenyl. Thus the neuroprotective effects of deprenyl on MPP⁺-induced reduction of HVA levels in living cultures appears similar to the effects of deprenyl on dopamine levels and tyrosine hydroxylase activity reported by others in cultures previously exposed to deprenyl and MPP⁺. These studies also confirm that the neuroprotective effects of deprenyl against MPP⁺ in dopaminergic neurons are, at least in part, independent of deprenyl's inhibition of MAO-B.