Electrophysiological effects of kainic acid on vasopressin-enhanced green fluorescent protein and oxytocin-monomeric red fluorescent protein 1 neurones isolated from the supraoptic nucleus in transgenic rats

The supraoptic nucleus (SON) contains two types of magnocellular neurosecretory cells: arginine vasopressin (AVP)-producing and oxytocin (OXT)-producing cells. We recently generated and characterised two transgenic rat lines: one expressing an AVP-enhanced green fluorescent protein (eGFP) and the other expressing an OXT-monomeric red fluorescent protein 1 (mRFP1). These transgenic rats enable the visualisation of AVP or OXT neurones in the SON. In the present study, we compared the electrophysiological responses of AVP-eGFP and OXT-mRFP1 neurones to glutamic acid in SON primary cultures. Glutamate mediates fast synaptic transmission through three classes of ionotrophic receptors: the NMDA, AMPA and kainate receptors. We investigated the contributions of the three classes of ionotrophic receptors in glutamate-induced currents. Three different antagonists were used, each predominantly selective for one of the classes of ionotrophic receptor. Next, we focused on the kainate receptors (KARs). We examined the electrophysiological effects of kainic acid (KA) on AVP-eGFP and OXT-mRFP1 neurones. In current clamp mode, KA induced depolarisation and increased firing rates. These KA-induced responses were inhibited by the non-NMDA ionotrophic receptor antagonist 6-cyano-7-nitroquinoxaline-2,3(1H4H)-dione in both AVP-eGFP and OXT-mRFP1 neurones. In voltage clamp mode, the application of KA evoked inward currents in a dose-dependent manner. The KA-induced currents were significantly larger in OXT-mRFP1 neurones than in AVP-eGFP neurones. This significant difference in KA-induced currents was abolished by the GluK1-containing KAR antagonist UBP302. At high concentrations (250-500 μm), the specific GluK1-containing KAR agonist (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA) induced significantly larger currents in OXT-mRFP1 neurones than in AVP-eGFP neurones. Furthermore, the difference between the AVP-eGFP and OXT-mRFP1 neurones in the ATPA currents was approximately equal to the difference in the KA currents. These findings suggest that the GluK1-containing KARs may be more highly expressed in OXT neurones than in AVP neurones. These results may provide new insight into the physiology and synaptic plasticity of SON neurones.

Computational study of synthetic agonist ligands of ionotropic glutamate receptors

Neurological glutamate receptors are among the most important and intensely studied protein ligand binding systems in humans. They are crucial for the functioning of the central nervous system and involved in a variety of pathologies. Apart from the neurotransmitter glutamate, several artificial, agonistic and antagonistic ligands are known. Of particular interest here are novel photoswitchable agonists that would open the field of optogenetics to glutamate receptors. The receptor proteins are complex, membrane-bound multidomain oligomers that undergo large scale functional conformational changes, making detailed studies of their atomic structure challenging. Therefore, a thorough understanding of the microscopic details of ligand binding and receptor activation remains elusive in many cases. This topic has been successfully addressed by theoretical studies in the past and in this paper, we present extensive molecular dynamics simulation and free energy calculation results on the binding of AMPA and an AMPA derivative, which is the basis for designing light-sensitive ligands. We provide a two-step model for ligand binding domain activation and predict binding free energies for novel compounds in good agreement to experimental observations.

[Activation of presynaptic ionotropic glutamate receptors stimulates GABA release from hippocampal and cortical nerve terminals in rats]

One of the pathways implicated in a fine-tuning control of neurosecretory process is the activation of presynaptic receptors. The present study was focused on the role of presynaptic glutamate receptor activation in the regulation of inhibitory synaptic transmission in the rat hippocampus and cortex. We aimed to clarify what types of ionotropic glutamate receptors are involved in the modulation of GABA secretion, and what mechanism underlies this modulation. We have revealed that specific agonists of kainate and NMDA receptors, kainate and NMDA, like glutamate, induced the release of [3H]GABA from hippocampal and cortical nerve terminals suggesting the involvement of both types in the regulation of GABAergic transmission. Our results indicate preferential involvement of vesicular, but not cytosolic, pool in response to glutamate receptor activation. This is based on the finding that NO-711 (a specific inhibitor of plasma membrane GABA transporters), fails to attenuate [3H]GABA release. We have concluded that presynaptic glutamate receptor-induced modulation of the strength of synaptic response is due to increasing the release probability of synaptic vesicles.

Role of ionotropic glutamate receptors in the regulation of hippocampal norepinephrine output in vivo

In vitro evidence indicates that norepinephrine release in the mammalian hippocampus is modulated by glutamate receptors. With the use of microdialysis, we have now evaluated the role of ionotropic glutamate receptors in the regulation of hippocampal norepinephrine output in vivo. Stimulation of N-methyl-D-aspartate (NMDA)-sensitive glutamate receptors by local administration of NMDA (1-100μM) resulted in a concentration-dependent decrease in the extracellular concentration of norepinephrine in the hippocampus of freely moving rats, whereas the blockade of these receptors with MK801 (1-100μM) induced a concentration-dependent increase in norepinephrine output. Activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-sensitive glutamate receptors with AMPA (1-100μM) resulted in a biphasic effect on the extracellular norepinephrine concentration, with a decrease in this parameter apparent at 10μM and an increase at 100μM. The AMPA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione had no effect on norepinephrine output. The GABA(A) receptor antagonist bicuculline (10μM) prevented the decrease in hippocampal norepinephrine output induced by either NMDA or 10μM AMPA. Our results thus implicate ionotropic glutamate receptors as key regulators of norepinephrine release in the hippocampus and may therefore provide a basis for the development of new drugs for stress-related disorders.