Dopamine release and synthesis in the neurointermediate lobe of the rat hypophysis in vitro after electrical stimulation of the pituitary stalk

The isolated neurointermediate lobe of the rat hypophysis was incubated in Krebs solution and the stalk was electrically stimulated. The endogenous dopamine (DA) released into the medium was estimated by HPLC with electrochemical detection. Stimulation with biphasic pulses (1 ms, 10 Hz) in the presence of pargyline elicited a mean DA output of 200 fg X pulse-1. This release was calcium-dependent but was only partially inhibited by tetrodotoxin (TTX) (1 microM), effects typical for direct electrical depolarization of the nerve endings. Reducing the duration of the electric pulses to 0.2 ms (15 Hz) caused a reduction in DA output to about 40 fg X pulse-1 which was completely blocked by TTX indicating that it was evoked by propagated action potentials. DA overflow was enhanced when the action potential was prolonged with tetraethylammonium (TEA) or when DA uptake was inhibited with GBR 12921. Evidence for a calcium-dependent increase in DA synthesis in electrically stimulated NILs has been obtained when monoamine oxidase was inhibited and TEA or GBR 12921 was present in the Krebs solution. The present results complete the requirements for the DA in the NIL to be classified as a neurotransmitter substance. The NIL-pituitary stalk preparation is a useful model for studying regulatory mechanisms in dopaminergic nerve terminals.