Reduction of [3H]nicotine binding in hypothalamic and cortical membranes by dopamine D1 receptors

Molecular mechanisms and therapeutical implications of intramembrane receptor/receptor interactions among heptahelical receptors with examples from the striatopallidal GABA neurons

The molecular basis for the known intramembrane receptor/receptor interactions among G protein-coupled receptors was postulated to be heteromerization based on receptor subtype-specific interactions between different types of receptor homomers. The discovery of GABAB heterodimers started this field rapidly followed by the discovery of heteromerization among isoreceptors of several G protein-coupled receptors such as delta/kappa opioid receptors. Heteromerization was also discovered among distinct types of G protein-coupled receptors with the initial demonstration of somatostatin SSTR5/dopamine D2 and adenosine A1/dopamine D1 heteromeric receptor complexes. The functional meaning of these heteromeric complexes is to achieve direct or indirect (via adapter proteins) intramembrane receptor/receptor interactions in the complex. G protein-coupled receptors also form heteromeric complexes involving direct interactions with ion channel receptors, the best example being the GABAA/dopamine D5 receptor heteromerization, as well as with receptor tyrosine kinases and with receptor activity modulating proteins. As an example, adenosine, dopamine, and glutamate metabotropic receptor/receptor interactions in the striatopallidal GABA neurons are discussed as well as their relevance for Parkinson's disease, schizophrenia, and drug dependence. The heterodimer is only one type of heteromeric complex, and the evidence is equally compatible with the existence of higher order heteromeric complexes, where also adapter proteins such as homer proteins and scaffolding proteins can exist. These complexes may assist in the process of linking G protein-coupled receptors and ion channel receptors together in a receptor mosaic that may have special integrative value and may constitute the molecular basis for some forms of learning and memory.

Acute and subchronic corticosterone treatment differentially modulates subcortical limbic and neostriatal nicotinic cholinergic receptors

The effects of acute and subchronic corticosterone treatment were analyzed on the competition by (-)nicotine ((-)nicotine hydrogen(+)-tartrate) on N-[3H]methylcarbamyl choline iodide ([ 3H]MCC) binding sites in membranes from the subcortical limbic forebrain and the neostriatum. Acute treatment with corticosterone (5 mg/kg, i.p., 2 h) increased the IC50 values of (-)nicotine by 230% in the subcortical limbic areas but not in the neostriatum. Subchronic corticosterone treatment (5 mg/kg, twice a day, 7 days) increased the IC50 values of (-)nicotine by 50% and slightly decreased the specific binding of [3H]MCC (5 nM) in the subcortical limbic area. In the neostriatum, subchronic treatment with corticosterone instead decreased by 50% the IC50 values of (-)nicotine and slightly increased the specific binding of [3H]MCC. The results indicate that corticosterone treatment selectively reduces the affinity of nicotinic cholinergic receptors within the subcortical limbic forebrain.