Heteromerization of Dopamine D2 and Oxytocin Receptor in Adult Striatal Astrocytes

The ability of oxytocin (OT) to interact with the dopaminergic system through facilitatory D2-OT receptor (OTR) receptor-receptor interaction in the limbic system is increasingly considered to play roles in social or emotional behavior, and suggested to serve as a potential therapeutic target. Although roles of astrocytes in the modulatory effects of OT and dopamine in the central nervous system are well recognized, the possibility of D2-OTR receptor-receptor interaction in astrocytes has been neglected. In purified astrocyte processes from adult rat striatum, we assessed OTR and dopamine D2 receptor expression by confocal analysis. The effects of activation of these receptors were evaluated in the processes through a neurochemical study of glutamate release evoked by 4-aminopyridine; D2-OTR heteromerization was assessed by co-immunoprecipitation and proximity ligation assay (PLA). The structure of the possible D2-OTR heterodimer was estimated by a bioinformatic approach. We found that both D2 and OTR were expressed on the same astrocyte processes and controlled the release of glutamate, showing a facilitatory receptor-receptor interaction in the D2-OTR heteromers. Biochemical and biophysical evidence confirmed D2-OTR heterodimers on striatal astrocytes. The residues in the transmembrane domains four and five of both receptors are predicted to be mainly involved in the heteromerization. In conclusion, roles for astrocytic D2-OTR in the control of glutamatergic synapse functioning through modulation of astrocytic glutamate release should be taken into consideration when considering interactions between oxytocinergic and dopaminergic systems in striatum.

Heterodimer of A2A and Oxytocin Receptors Regulating Glutamate Release in Adult Striatal Astrocytes

BACKGROUND

Roles of astrocytes in the modulatory effects of oxytocin (OT) in central nervous system are increasingly considered. Nevertheless, OT effects on gliotransmitter release have been neglected.

METHODS

In purified astrocyte processes from adult rat striatum, we assessed OT receptor (OTR) and adenosine A2A receptor expression by confocal analysis. The effects of receptors activation on glutamate release from the processes were evaluated; A2A-OTR heteromerization was assessed by co-immunoprecipitation and PLA. Structure of the possible heterodimer of A2A and OT receptors was estimated by a bioinformatic approach.

RESULTS

Both A2A and OT receptors were expressed on the same astrocyte processes. Evidence for A2A-OTR receptor-receptor interaction was obtained by measuring the release of glutamate: OT inhibited the evoked glutamate release, while activation of A2A receptors, per se ineffective, abolished the OT effect. Biochemical and biophysical evidence for A2A-OTR heterodimers on striatal astrocytes was also obtained. The residues in the transmembrane domains 4 and 5 of both receptors are predicted to be mainly involved in the heteromerization.

CONCLUSIONS

When considering effects of OT in striatum, modulation of glutamate release from the astrocyte processes and of glutamatergic synapse functioning, and the interaction with A2A receptors on the astrocyte processes should be taken into consideration.

Reactive Astrocytosis in a Mouse Model of Chronic Polyamine Catabolism Activation

BACKGROUND

In the brain, polyamines are mainly synthesized in neurons, but preferentially accumulated in astrocytes, and are proposed to be involved in neurodegenerative/neuroinflammatory disorders and neuron injury. A transgenic mouse overexpressing spermine oxidase (SMOX, which specifically oxidizes spermine) in the neocortex neurons (Dach-SMOX mouse) was proved to be a model of increased susceptibility to excitotoxic injury.

METHODS

To investigate possible alterations in synapse functioning in Dach-SMOX mouse, both cerebrocortical nerve terminals (synaptosomes) and astrocytic processes (gliosomes) were analysed by assessing polyamine levels, ezrin and vimentin content, glutamate AMPA receptor activation, calcium influx, and catalase activity.

RESULTS

The main findings are as follows: (i) the presence of functional calcium-permeable AMPA receptors in synaptosomes from both control and Dach-SMOX mice, and in gliosomes from Dach-SMOX mice only; (ii) reduced content of spermine in gliosomes from Dach-SMOX mice; and (iii) down-regulation and up-regulation of catalase activity in synaptosomes and gliosomes, respectively, from Dach-SMOX mice.

CONCLUSIONS

Chronic activation of SMOX in neurons leads to major changes in the astrocyte processes including reduced spermine levels, increased calcium influx through calcium-permeable AMPA receptors, and stimulation of catalase activity. Astrocytosis and the astrocyte process alterations, depending on chronic activation of polyamine catabolism, result in synapse dysregulation and neuronal suffering.

Exosomes From Astrocyte Processes: Signaling to Neurons

It is widely recognized that extracellular vesicles subserve non-classical signal transmission in the central nervous system. Here we assess if the astrocyte processes, that are recognized to play crucial roles in intercellular communication at the synapses and in neuron-astrocyte networks, could convey messages through extracellular vesicles. Our findings indicate, for the first time that freshly isolated astrocyte processes prepared from adult rat cerebral cortex, can indeed participate to signal transmission in central nervous system by releasing exosomes that by volume transmission might target near or long-distance sites. It is noteworthy that the exosomes released from the astrocyte processes proved ability to selectively target neurons. The astrocyte-derived exosomes were proven positive for neuroglobin, a protein functioning as neuroprotectant against cell insult; the possibility that exosomes might transfer neuroglobin to neurons would add a mechanism to the potential astrocytic neuroprotectant activity. Notably, the exosomes released from the processes of astrocytes maintained markers, which prove their parental astrocytic origin. This potentially allows the assessment of the cellular origin of exosomes that might be recovered from body fluids.

Near-infrared laser photons induce glutamate release from cerebrocortical nerve terminals

Although photons have been repeatedly shown to affect the functioning of the nervous system, their effects on neurotransmitter release have never been investigated. We exploited in vitro models that allow effects involving neuron-astrocyte network functioning to be detected (mouse cerebrocortical slices) and dissected these effects at cerebrocortical nerve endings and astrocyte processes. Infrared light proved able to induce glutamate release by stimulating glutamatergic nerve endings.

A2A-D2 receptor-receptor interaction modulates gliotransmitter release from striatal astrocyte processes

Evidence for striatal A2A-D2 heterodimers has led to a new perspective on molecular mechanisms involved in schizophrenia and Parkinson's disease. Despite the increasing recognition of astrocytes' participation in neuropsychiatric disease vulnerability, involvement of striatal astrocytes in A2A and D2 receptor signal transmission has never been explored. Here, we investigated the presence of D2 and A2A receptors in isolated astrocyte processes prepared from adult rat striatum by confocal imaging; the effects of receptor activation were measured on the 4-aminopyridine-evoked release of glutamate from the processes. Confocal analysis showed that A2A and D2 receptors were co-expressed on the same astrocyte processes. Evidence for A2A-D2 receptor-receptor interactions was obtained by measuring the release of the gliotransmitter glutamate: D2 receptors inhibited the glutamate release, while activation of A2A receptors, per se ineffective, abolished the effect of D2 receptor activation. The synthetic D2 peptide VLRRRRKRVN corresponding to the receptor region involved in electrostatic interaction underlying A2A-D2 heteromerization abolished the ability of the A2A receptor to antagonize the D2 receptor-mediated effect. Together, the findings are consistent with heteromerization of native striatal astrocytic A2A-D2 receptors that via allosteric receptor-receptor interactions could play a role in the control of striatal glutamatergic transmission. These new findings suggest possible new pathogenic mechanisms and/or therapeutic approaches to neuropsychiatric disorders.