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Journal of Neuroscience, Vol 10, 3627-3634, Copyright © 1990 by Society for Neuroscience
Membrane properties and response to opioids of identified dopamine neurons in the guinea pig hypothalamus
MD Loose, OK Ronnekleiv and MJ Kelly
Department of Physiology, Oregon Health Sciences University, Portland 97201-3098.
The electrophysiological properties and opioid responsiveness of the
dopamine-containing neurons in the arcuate nucleus of the guinea pig
hypothalamus were examined. Dopamine-containing neurons, identified
immunocytochemically by the presence of tyrosine hydroxylase, had a mean
length-to-width profile of 14.9 +/- 4.4 x 11.5 +/- 3.1 microns (N = 14).
The Na+ action potential of these neurons was of short duration, and
induction of repetitive firing (20-50 Hz) caused an afterhyperpolarization
of 6-9 mV in amplitude, with a decay half-time of approximately 1.5 sec.
Dopamine-containing cells exhibited a low threshold spike, which induced
1-4 Na+ action potentials. This potential had a threshold close to -65 mV,
could not be induced without prior hyperpolarization and was not sensitive
to TTX. Dopamine- containing neurons also exhibited a time- and
voltage-dependent inward current at potentials negative to -70 mV, and Cs+
blocked this conductance. The mu-opioid agonist Tyr-D-Ala-Gly-mePhe-Gly-ol
hyperpolarized (14 +/- 3 mV) dopamine neurons via induction of an outward
current (93 +/- 44 pA near the resting membrane potential) which had a
reversal potential similar to that expected for a selective potassium
conductance. TTX (1 microM) did not block the opioid effects. These results
show that dopamine neurons of the arcuate nucleus differ in their intrinsic
conductances and their responsiveness to opioids from other CNS
dopaminergic neurons. Furthermore, opioid activation of a potassium
conductance resulted in a direct hyperpolarization of dopamine neurons of
the arcuate nucleus, and we suggest that this mechanism may underlie the
effects of opioids on dopamine-mediated prolactin release.
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