title: Synaptic modulation by dopamine of calcium currents in rat pars intermedia.
abstract: Melanotrophs of the rat pars intermedia are innervated by dopaminergic fibers traveling through the pituitary stalk which inhibit secretion via an action on D-2 receptors. As secretion from the melanotroph has been shown to be calcium (Ca2+) dependent, it is possible that dopamine may have an action to inhibit Ca2+ currents in these cells. This possibility was tested by examining the effects of exogenously applied dopaminergic agonists or synaptically released dopamine upon Ca2+ currents recorded under single electrode voltage clamp in intact rat pars intermedia in vitro. Following blockade of sodium and potassium currents in melanotrophs, Ca2+ spikes were elicited with intracellular injection of depolarizing currents; electrical stimulation of the pituitary stalk caused an inhibition of the Ca2(+)-based action potentials which lasted for several seconds. Using single-electrode voltage-clamp techniques, we recorded inward Ca2+ currents corresponding to the T, N, and L types (see Williams et al., 1990). Stimulation of the pituitary stalk inhibited both the low- and high-threshold peak inward Ca2+ currents elicited from a holding potential of -90 mV. In contrast, when noninactivating Ca2+ currents were elicited from a holding potential of -30 mV, the currents were not altered by stalk stimulation. This pattern of inhibition of the Ca2+ currents was consistent with the preferential inhibition, by stalk stimulation, of the N and T Ca2+ currents, while sparing the L current. We observed that inhibition of Ca2+ currents due to stalk stimulation was completely reversed by bath perfusion of domperidone (1 microM), an antagonist of dopamine at the D-2 receptor.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Neuroscience Research Group, University of Calgary, Alberta, Canada.
authors.forename: P J
authors.initials: PJ
authors.lastname: Williams
authors.forename: B A
authors.initials: BA
authors.lastname: MacVicar
authors.forename: Q J
authors.initials: QJ
authors.lastname: Pittman
mesh_terms: D000818:Animals; D002118:Calcium / Q000502:physiology*; D004294:Domperidone / Q000494:pharmacology; D004298:Dopamine / Q000494:pharmacology*; D004558:Electric Stimulation; D004594:Electrophysiology; D004873:Ergolines / Q000494:pharmacology; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives; D008297:Male; D008839:Microelectrodes; D009490:Neurosecretory Systems / Q000166:cytology / Q000502:physiology; D010902:Pituitary Gland / Q000166:cytology / Q000502:physiology*; D019257:Quinpirole; D051381:Rats; D011919:Rats, Inbred Strains; D013569:Synapses / Q000502:physiology*; D013873:Thionucleotides
chemical_list: D004873:Ergolines; D013873:Thionucleotides; D019257:Quinpirole; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D004294:Domperidone; D006160:Guanosine Triphosphate; D002118:Calcium; D004298:Dopamine