Homogeneity of intracellular electrophysiological properties in different neuronal subtypes in medial preoptic slices containing the sexually dimorphic nucleus of the rat

Abstract

The sexually dimorphic nucleus of the preoptic area (SDN-POA) is larger in male than in female rats, the male phenotype requiring the presence of circulating androgens perinatally. These experiments investigated the intracellular electrophysiology and morphology of SDN-POA neurons and compared these properties with those of other medial preoptic area (MPOA) neurons. Biocytin-injected cells in the SDN-POA either had one or two primary dendrites, or they had multipolar dendritic arrays; dendrites were aspiny or sparsely spiny and displayed limited branching. Neurons in other parts of the MPOA were similar morphologically. Regardless of morphology, neurons situated in either the SDN-POA or surrounding MPOA had low-threshold potentials and linear or nearly linear current-voltage relations. In most (73%) cells, stimulation of the dorsal preoptic region evoked a fast excitatory postsynaptic potential followed by a fast inhibitory postsynaptic potential (IPSP). Bicuculline blocked the fast IPSPs, which reversed near the Cl2 equilibrium potential (-71 +/- 5 mV), indicating their mediation by gamma-aminobutyric acid (GABA)A receptors. Neurons in the SDN-POA have electrophysiological properties similar to those of other medial preoptic cells. When compared with the hypothalamic paraventricular nucleus, the MPOA appears relatively homogeneous electrophysiologically. This is despite the morphological variability within this population of neurons and heterogeneities that are also apparent at other levels of analysis. Finally, GABA-mediated, inhibitory synaptic contacts are widespread among medial preoptic neurons, consistent with indications from earlier reports that GABA provides a link in the feedback actions of gonadal steroids on the release of gonadotropic hormones.

Allopregnanolone enhancement of GABAergic transmission in rat medial preoptic area neurons

“…Large numbers of MPOA neurons are GABAergic (5) and form local circuits within this region (10). Progesterone is rapidly metabolized into neurosteroids in the brain (1).…”

“…MPOA neurons form local circuits in the preoptic area (10). Large numbers of MPOA neurons stain positive for glutamate decarboxylase, a GABA-synthesizing enzyme (5), and GABA is detectable at relatively high concentrations in the MPOA (22).…”

Estrogen Upregulates T-Type Calcium Channels in the Hypothalamus and Pituitary

“…Low-threshold calcium spikes evoke several fast Na ϩ action potentials and therefore are involved in signal amplification in CNS neurons. The occurrence of LTS is prevalent in rat and guinea pig hypothalamic neurons (Erickson et al, 1993b;Hoffman et al, 1994;Fisher and Bourque, 1995;Niespodziany et al, 1999;Wagner et al, 2000), and we found that the number of arcuate neurons expressing lowthreshold spikes is increased with estrogen treatment (Kelly and Rønnekleiv, 1994). Voltage-clamp studies have revealed that a relative fast and large inward current, which exhibits activation/ inactivation characteristics analogous to the T-type calcium channels, underlies the LTS.…”

Functional significance of the rapid regulation of brain estrogen action: Where do the estrogens come from?

“…Electrophysiological studies indicated that preoptic aromatase-containing cells are sensitive to glutamate, dopamine and norepinephrine [63,66] and showed the existence in quail and rat preoptic area of a tonic electrical activity that can be blocked by glutamate and GABA antagonists [63,66,113]. A recent study also showed a modulation of estrogen synthesis within 30 min in hippocampal slices of male rats incubated in the presence of Mg 2+ and NMDA.…”