Comparison of Effects of L-Dopa, Amphetamine and Apomorphine on Firing Rate of Rat Dopaminergic Neurones

Abstract

WE have reported that systemically administered d-amphet-amine selectively depresses the firing rate of dopamine-containing cells in the substantia nigra zona compacta and adjacent ventral tegmental area of the rat midbrain¹. d-Amphetamine is believed to act by releasing dopamine and blocking its re-uptake, leading to stimulation of dopamine receptors by increasing the availability of dopamine at postsynaptic receptor sites^2–4. The increased receptor stimulation is then thought to initiate neuronal feedback inhibition of dopamine units⁵. Two other drugs are believed to stimulate dopamine receptors: L-dopa is converted to dopamine by an aromatic amino acid decarboxylase, increasing the amount of dopamine potentially available at receptor sites^6–8, while apomorphine seems to stimulate the receptors directly^9–11. Because of this alleged ability to stimulate dopamine receptors, it has been predicted that these drugs also slow dopamine cells¹⁰.