Regulation of Hippocampal 5-HT Release by P2X7 Receptors in Response to Optogenetic Stimulation of Median Raphe Terminals of Mice

Abstract

Serotonergic and glutamatergic neurons of median raphe region (MRR) play a pivotal role in the modulation of affective and cognitive functions. These neurons synapse both onto themselves and remote cortical areas. P2X7 receptors (P2rx7) are ligand gated ion channels expressed by central presynaptic excitatory nerve terminals and involved in the regulation of neurotransmitter release. P2rx7s are implicated in various neuropsychiatric conditions such as schizophrenia and depression. Here we investigated whether 5-HT release released from the hippocampal terminals of MRR is subject to modulation by P2rx7s. To achieve this goal, an optogenetic approach was used to selectively activate subpopulation of serotonergic terminals derived from the MRR locally, and one of its target area, the hippocampus. Optogenetic activation of neurons in the MRR with 20 Hz was correlated with freezing and enhanced locomotor activity of freely moving mice and elevated extracellular levels of 5-HT, glutamate but not GABA in vivo. Similar optical stimulation (OS) significantly increased [3H]5-HT and [3H]glutamate release in acute MRR and hippocampal slices. We examined spatial and temporal patterns of [3H]5-HT release and the interaction between the serotonin and glutamate systems. Whilst [3H]5-HT release from MRR neurons was [Ca2+]o-dependent and sensitive to TTX, CNQX and DL-AP-5, release from hippocampal terminals was not affected by the latter drugs. Hippocampal [3H]5-HT released by electrical but not OS was subject to modulation by 5- HT1B/D receptors agonist sumatriptan (1 μM), whereas the selective 5-HT1A agonist buspirone (0.1 μM) was without effect. [3H]5-HT released by electrical and optical stimulation was decreased in mice genetically deficient in P2rx7s, and after perfusion with selective P2rx7 antagonists, JNJ-47965567 (0.1 μM), and AZ-10606120 (0.1 μM). Optical and electrical stimulation elevated the extracellular level of ATP. Our results demonstrate for the first time the modulation of 5-HT release from hippocampal MRR terminals by the endogenous activation of P2rx7s. P2rx7 mediated modulation of 5-HT release could contribute to various physiological and pathophysiological phenomena, related to hippocampal serotonergic transmission.

P2X7 Receptor Signaling in Stress and Depression

“…However, the specific purinergic receptor subtype involved in this effect was not characterized, due to the lack of selective drugs. However, a role for P2X7R in regulating 5-HT release has been confirmed in more recent studies with genetic and pharmacological approaches, where it was reported that median raphe nuclei stimulation decreased the 5-HT release in the hippocampus of genetically deficient in P2X7R mice, and after perfusion with selective antagonists (JNJ-47965567 or AZ-10606120) [19]. In another study, however, P2X7R KO mice presents with increased hippocampal 5-HT levels, increased 5-HT transporter binding sites and 5-HT uptake, along with behavioral adaptation to stress [20].…”

“…The P2X7 receptor (P2X7R) is an ion channel activated by high concentrations of ATP, which for example can be observed following stress exposure [18]. The P2X7R has been involved in several process observed in MDD, such as impaired monoaminergic neurotransmission [19,20,21], increased glutamatergic neurotransmission [22], stimulation of a neuroinflammatory response [23] as well as reduced neuroplasticity [20,24]. In this scenario, the P2X7R becomes an interesting target for the understanding of depression neurobiology and treatment.…”

Salient brain entities labelled in P2rx7-EGFP reporter mouse embryos include the septum, roof plate glial specializations and circumventricular ependymal organs

“…The DR, the developing Cb and the Ecu stand out for their known relationship with P2X7R activity. P2X7R has been shown to mediate the release of serotonin in the Hi through their activity in the raphe nucleus (Goloncser et al 2017 ). Regarding the Cb, we observed a strong P2rx7 -EGFP signal located in the EGL, known to produce granule neurons destined to reside in the internal granular layer (Alvarez Otero et al 1993 ; Hallonet et al 1990 ).…”

Differential Roles of the Two Raphe Nuclei in Amiable Social Behavior and Aggression – An Optogenetic Study

“…After 2 h equilibration period we collected 9 samples, one in every 30 min. Perfusion rate was 2 μl/min ( Figure 5 ) ( Goloncser et al, 2017 ). The first three samples served as baseline.…”

“…Method ( Goloncser et al, 2017 ). The extraction solution (PCA) was 0.1 M perchloric acid that contained theophylline (as an internal standard) at 10 mM concentration.…”