Opposite effects of naloxone on substance P-induced changes in brain dopa synthesis and in locomotor activity in rats

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Abstract
Intracerebroventricular injection of substance P (SP) enhanced the synthesis of dopamine and noradrenaline (measured as increase in DOPA formation after inhibition of the aromatic L-amino-acid decarboxylase) in rat brain. These biochemical effects were blocked in most brain regions by pretreatment with naloxone. SP also induced vasodilatation, salivation and increased locomotor activity. These effects were not antagonized, but, in the case of locomotor activity, potentiated by naloxone. The data suggest the existence of specific SP-containing neuronal pathways for behaviour, which pathways are not related to those regulating the synthesis of brain catecholamines.