Internalized GABA-receptor subunits are transferred to an intracellular pool associated with the postsynaptic density

Endocytosis represents an important mechanism regulating cell-surface expression of neurotransmitter receptors, including GABAA receptors, in neurons. Little is known, however, about trafficking of internalized receptors. Here, we used antibody tagging in living rat hippocampal neurons in culture to monitor GABAA receptor internalization. We show that cell-surface receptors have a homogeneous distribution reflecting their mobility in the membrane. Unexpectedly, internalized GABAA receptors were detected mainly in a subsynaptic pool associated with gephyrin at postsynaptic sites, whereas AMPA-type glutamate receptors were accumulated in the soma. This process was time-dependent and could be prevented by blocking clathrin-coated vesicle endocytosis. In control experiments, the existence of an intracellular pool of GABAA receptors associated with gephyrin was confirmed independently of internalization of surface receptors, and constitutive endocytosis, unrelated to antibody-tagging, could be demonstrated for both AMPA and GABAA receptors using a biotinylation assay. These results suggest that cycling of GABAA receptors between the cell surface and the subsynaptic pool provides a mechanism for the short-term regulation of GABAergic neurotransmission. Furthermore, the close association of gephyrin with internalized GABAA receptors suggests a role in intracellular receptor trafficking.

Requirement of alpha5-GABAA receptors for the development of tolerance to the sedative action of diazepam in mice

Despite its pharmacological relevance, the mechanism of the development of tolerance to the action of benzodiazepines is essentially unknown. The acute sedative action of diazepam is mediated via alpha1-GABA(A) receptors. Therefore, we tested whether chronic activation of these receptors by diazepam is sufficient to induce tolerance to its sedative action. Knock-in mice, in which thealpha1-,alpha2-,alpha3-, oralpha(5)-GABA(A) receptors had been rendered insensitive to diazepam by histidine-arginine point mutation, were chronically treated with diazepam (8 d; 15 mg x kg(-1) x d(-1)) and tested for motor activity. Wild-type, alpha2(H101R), and alpha3(H126R) mice showed a robust diminution of the motor-depressant drug action. In contrast, alpha5(H105R) mice failed to display any sedative tolerance. alpha1(H101R) mice showed no alteration of motor activity with chronic diazepam treatment. Autoradiography with [3H]flumazenil revealed no change in benzodiazepine binding sites. However, a decrease in alpha5-subunit radioligand binding was detected selectively in the dentate gyrus with specific ligands. This alteration was observed only in diazepam-tolerant animals, indicating that the manifestation of tolerance to the sedative action of diazepam is associated with a downregulation of alpha5-GABA(A) receptors in the dentate gyrus. Thus, the chronic activation of alpha(5)-GABA(A) receptors is crucial for the normal development of sedative tolerance to diazepam, which manifests itself in conjunction with alpha1-GABA(A) receptors.

The role of sensory nerve endings in nerve growth factor-induced airway hyperresponsiveness to histamine in guinea-pigs

1. Nerve growth factor induces an airway hyperresponsiveness in vivo in guinea-pigs, as we have shown previously. Since antagonizing the neurokinin-1 (NK(1)) receptor can prevent this NGF-induced airway hyperresponsiveness and since sensory nerves release tachykinins, we investigated the role of sensory nerves in the NGF-induced airway hyperresponsiveness. 2. We used isolated tracheal rings from guinea-pigs to measure tracheal contractility. In these rings sensory nerve endings are present, but these endings lack any contact with their cell bodies. 3. In this in vitro system, NGF dose-dependently induced a tracheal hyperresponsiveness to histamine. The NK(1) receptor antagonist SR140333 could block the induction of tracheal hyperresponsiveness. 4. To further investigate the involvement of sensory nerve endings we used the cannabinoid receptor 1 (CB(1)) agonist R-methanandamide to inhibit excitatory events at the nerve terminal. The CB(1) receptor agonist was capable of blocking the tracheal hyperresponsiveness to NGF in the isolated system, as well as the airway hyperresponsiveness to NGF in vivo. 5. This indicates that NGF can induce an increase in airway responsiveness in the absence of sensory nerve cell bodies. NGF may act by increasing substance P release from sensory nerve endings, without upregulation of substance P in the neurons. Substance P in its turn is responsible for the induction of the NGF-induced airway hyperresponsiveness.