Membrane properties and response to opioids of identified dopamine neurons in the guinea pig hypothalamus

Abstract

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.

Intracellular Membrane and Synaptic Properties in Medial Preoptic Slices Containing the Sexually Dimorphic Nucleus of the Rat

“…Unlike the paraventricular, ventromedial, and arcuate nuclei Loose et al 1990;Minami et al 1986 a and b; , where only subsets of neurons display LTS potentials, all anatomically identified medial preoptic neurons recorded in the present investigation showed these potentials. This was also true for all but one unstained neuron.…”

Heterogeneity in the Basic Membrane Properties of Postnatal Gonadotropin-Releasing Hormone Neurons in the Mouse

“…Developmentally, the major differences between juvenile and adult GnRH neurons were those of reduced GnRH cell type heterogeneity and a more apparent role for specific calcium channels in juvenile GnRH neurons. Other neuroendocrine phenotypes such as the oxytocin and vasopressin neurons do not exhibit substantial heterogeneity in their membrane properties (Stern and Armstrong, 1995), whereas the tuberoinfundibular dopaminergic neurons exhibit only small degrees of variability (Loose et al, 1990). The apparent heterogeneity in levels of channel expression within the GnRH neuronal populations may well result from their scattered distribution and likely individual microenvironments.…”

Membrane-initiated estrogen signaling in hypothalamic neurons

“…Both μ-opioid receptor (e.g., by β-endorphin or by selective μ-opioid agonists) or GABA B receptor activation of these GIRK channels directly hyperpolarize and thereby inhibit hypothalamic neurons (Loose et al, 1990, Kelly et al, 1992, Lagrange et al, 1994, Lagrange et al, 1995, Lagrange et al, 1996. Brief (<20 min) application of E2 or BSA-E2 in vitro causes a four-fold decrease in the potency of μ-opioid receptor agonists and GABA B receptor agonists to inhibit POMC neurons (Lagrange et al, 1994, Lagrange et al, 1996, indicating that a membrane ER (mER) is involved.…”