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Journal of Neuroscience, Vol 10, 757-763, Copyright © 1990 by Society for Neuroscience
Synaptic modulation by dopamine of calcium currents in rat pars intermedia
PJ Williams, BA MacVicar and QJ Pittman
Neuroscience Research Group, University of Calgary, Alberta, Canada.
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)
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