The effects of estrogen on the response of hypothalamic arcuate neurons to mu-opioid and GABAB agonists were investigated. Intracellular recordings were made from arcuate neurons in slices prepared from ovariectomized guinea pigs that were pretreated with estrogen or vehicle. Estrogen shifted the dose-response curve to the mu-opioid agonist DAMGO (Tyr-D-Ala-Gly-MePhe-Gly-ol) by 3.4-fold; the EC50 for DAMGO was 240 +/- 25 nM in estrogen-treated females versus 70 +/- 12 nM in the controls. The maximal hyperpolarization induced by DAMGO was equivalent in neurons from both groups. The Ke for the naloxone antagonism of the DAMGO response was similar in both groups, which would indicate that the affinity of the mu-receptor was unchanged. To explore where in the receptor/G-protein/K+ channel cascade estrogen may be acting to attenuate the mu-opioid-mediated hyperpolarization, the response to the GABAB agonist baclofen was also tested. Estrogen treatment also shifted the dose-response curve for the baclofen-induced hyperpolarization by 3.3-fold without altering the maximum hyperpolarization; the EC50 shifted from 11.0 +/- 4.0 microM to 36.0 +/- 5.0 microM. All of the neurons were identified after linking the intracellular biocytin with streptavidin-FITC, and a subpopulation of cells in both groups were immunoreactive for beta-endorphin. We conclude that estrogen decreases the functional coupling of the mu- opioid and GABAB receptors to the inwardly rectifying K+ channel possibly through an action on the G-protein.

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.

The behavioral and neuroendocrine phenotypes displayed by F 2 (129, C57) D2 dopamine receptor-deficient mice (D2R −/− ) result from a complex interaction between the absence of the receptor and additional polymorphic genetic loci contributed by the parental mouse strains.

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