Mice were anesthetized by halothane and then decapitated; brains were rapidly removed and placed into ice-cold physiological saline containing (mM): NaCl 130, KCl 3, CaCl₂ 0.5, MgCl₂ 2.5, NaH₂PO₄ 1, NaHCO₃ 25, glucose 15, pH of 7.4 gassed with 95% O₂ - 5% CO₂. Transverse slices (280 μm) were cut at 4^°C and then placed in physiological saline containing (mM): NaCl 130, KCl 3, CaCl₂ 2.5, MgCl₂ 1, NaH₂PO₄ 1, NaHCO₃ 22, glucose 15, pH of 7.4 gassed with 95% O₂ - 5% CO₂ and kept for 1 - 5 h prior to cell isolation and recording. The same extracellular saline was used for the recordings with addition of 1 μM TTX except experiments that required the evoking synaptic activity, namely: registration of astrocytic Ca²⁺-signaling elicited by stimulation of neuronal afferents and long-term plasticity experiments (detailed below).

Whole-cell voltage-clamp recordings from the CA1 and neocortical neurones were performed using glass patch-pipettes (4 - 5 MΩ) filled with intracellular solution (in mM): 110 CsCl, 10 NaCl, 10 HEPES, 5 MgATP, 1 D-Serine, 0.1 EGTA, pH 7.35; Transmembrane currents were monitored using an MultiClamp 700B patch-clamp amplifier and Digidata1440A data acquisition board (Axon Instruments, USA), filtered at 2 kHz and digitized at 4 kHz. Experiments were controlled by WinWCP software; recordings were analyzed using self-designed software. Liquid junction potentials were compensated with the patch-clamp amplifier. The series and input resistances were respectively 5–7 MΩ and 600–1100 MΩ; both series and input resistance varied by less than 20% in the cells accepted for analysis.

In the synaptic plasticity experiments, the same slice preparations were used. Field excitatory postsynaptic potentials (fEPSPs) were measured via a glass micropipette filled with extracellular solution (0.5 - 1 MΩ resistance) placed in neocortical layer 2/3 or in the stratum radiatum of the CA1 area. The fEPSPs were evoked correspondingly by the stimulation of neuronal afferents descending from layers IV-V or stimulation of CA3-CA1 Schaffer collaterals with a bipolar coaxial electrode (WPI, Stevenage, UK); stimulus duration was 300 μs, the stimulus strength was set to provide average the fEPSP amplitude about 30–40% of maximal level (typically 1.5–3 μA in neocortex and 1.2–2.5 μA in hippocampus).

To induce the long-term potentiation (LTP), several of trains of high-frequency theta-burst stimulation (HFS-trains) have been delivered; each HSF-train consisted of 10 pulses stimulated at 100 Hz (100 ms total length), trains were delivered with 200 ms interval and every 10 trains were separated by 10 s-long intervals. For the weak and strong stimulation of CA3-CA1 synapses correspondingly 5 and 10 trains were used. In the L2/3 area, the weak and strong stimulation consisted correspondingly of 20 and 50 trains.

To evaluate the intracellular concentration of free Ca²⁺ ([Ca²⁺]_i) in CA1 and layer 2/3 astrocytes in situ, brain slices were incubated with 1 μM of Rhod-2AM for 30 min at 33^°C. Usage of relatively short incubation time ensured preferential loading of astrocytes (as compared to neurons) with Ca²⁺-dye, as reported previously (Perea and Araque, 2005; Rasooli-Nejad et al., 2014) Astrocytes were identified initially by their morphology under DIC observation and EGFP fluorescence (astrocytes from dnSNARE mice and GFAP-EGFP mice). After the recording identification of astrocytes was confirmed by their electrophysiological properties as described previously (Lalo et al., 2014; Rasooli-Nejad et al., 2014; Pankratov and Lalo, 2015).

Two-photon images of neurons and astrocytes were acquired at 5Hz frame-rate using a Zeiss LSM-7MP multi-photon microscope coupled to a SpectraPhysics MaiTai pulsing laser; experiments were controlled by ZEN LSM software (Carl Zeiss, Jena, Germany). Images were analyzed offline with aid of ZEN LSM (Carl Zeiss) and ImageJ 1.52 (NIH) software. The [Ca²⁺]_i levels were expressed as ΔF/F ratio averaged over a region of interest (ROI). To analyze spontaneous Ca²⁺–signaling in astrocytes, several ROIs located over peripheral astrocytic processes (functional microdomains) and 1 ROI located over the soma were chosen. The initial identification of peripheral ROIs as microdomains was performed with the aid of standard particle analysis routine of ImageJ. After then, identification of microdomains was verified by quantification of relative fluorescence time course. Only regions, exhibiting average fluorescence 3 times larger than standard deviation of noise for the period of 1 s at least once over 30 min, were selected as active microdomains for further analysis. The average frequency and amplitude of spontaneous Ca²⁺-transients were calculated for all ROIs detected in each cell (typically 10–15 ROIs per cell). To test the response of astrocyte to synaptic stimulation, a single HFS train (as describe above) was used. The net Ca²⁺ -response to agonists or synaptic stimulation was quantified using an ROI covering the whole cell image.

The DHPG-induced changes in the ATP concentration in the neocortical and hippocampal tissue were evaluated with the aid of microelectrode biosensors produced by the Sarissa Biomedical (Coventry, UK). A detailed description of the biosensors and recording procedure has been published previously in Frenguelli et al. (2007). Briefly, biosensors consisted of the thin (25 μM) Pt/Ir wire coated with permselective polymer matrix with immobilized enzymes metabolizing ATP in a cascade of redox reactions, so the current output of the sensor was proportional to the ambient ATP concentration. To calculate the ATP concentration, the auxiliary null-sensor, coated with the matrix but containing no enzymes was inserted in the vicinity of main the ATP-sensor and its signal was subtracted from the signal of main sensor. Usage of null-sensor enabled to minimize a putative interference from any unspecific electro-active endogenous substances (Frenguelli et al., 2007). To further reduce an interference from unspecific signals, only transient elevation of the ATP-signal from baseline level was assessed. To calibrate the measurements and compensate for any reduction in sensitivity during experiment, biosensor response to the application of known concentrations of ATP (10 μM) was recorded twice in each experiment, before the slice was present in the perfusion chamber and after the slice had been removed.

All data are presented as mean ± standard deviation (SD) and the statistical significance of differences between data groups was tested by two-tailed unpaired t-test, unless indicated otherwise. For all cases of statistical significance reported, the statistical power of the test was 0.8–0.9. Each brain slice was used only for one experiment (e.g., fluorescent recordings in single astrocyte or single LTP experiment either in CA1 or layer 2/3). The number of experiments/cells reported is therefore equal to the number of slices used. The experimental protocols were allocated randomly so the data in any group were drawn from at least from 3 animals, typically from 4 to 10 mice. The average ratio of experimental unit per animal was 1.5 for the LTP experiments and 1.4 whole-cell recordings and fluorescent Ca²⁺-measurements.

The spontaneous transmembrane currents recorded in neurons were analyzed off-line using methods described previously (Lalo et al., 2014, 2016). The amplitude distributions of spontaneous and evoked currents were analyzed with the aid of probability density functions and likelihood maximization techniques; all histograms shown were calculated as probability density functions.

Receptor antagonists and agonists (NMDA, D-AP5, NBQX, ryanodine, TFLLR, DPCPX, SCH58261) were from the Tocris (Bristol, UK). Other salts and chemicals (Rhod-2, Rhod-2AM, noradrenaline, caffeine, TTX) were from Sigma (Dorset, UK) unless specifically indicated.

We investigated the action of RyRs activator caffeine and selective inhibitor ryanodine (Zucchi and Ronca-Testoni, 1997) on the Ca²⁺-signaling in astrocytes of CA1 hippocampal area and neocortical layer 2/3 in situ (Figures 1, 2). We used the dnSNARE mice and their wild type littermates of two age groups, adult (2.5 – 4 months old) and old (14 – 18 months old).

We evaluated the spontaneous and evoked Ca²⁺-transients in the somata and functional microdomains (Shigetomi et al., 2013; Savtchouk and Volterra, 2018), located on the astrocytic branches (Figures 1A, 2A, also see Materials and methods). Under baseline conditions, neocortical and hippocampal astrocytes exhibited prominent spontaneous Ca²⁺-transients, whose amplitude and frequency showed notable decrease at the old age. In the layer 2/3 astrocytes, the average baseline frequency of spontaneous Ca²⁺-transients (pooled for the whole cell image) was 0.34 ± 0.15 min^–1 in the young WT mice and 0.19 ± 0.08 min^–1 in the old WT mice; the average relative amplitudes of Ca²⁺-transients were correspondingly 0.32 ± 0.13 and 0.21 ± 0.09 (Figures 1A, B). The spontaneous signaling in the layer 2/3 astrocytes of dnSNARE mice was not significantly different from their wild-type littermates (Figure 1B). The similar pattern of age-related changes and lack of difference between WT and dnSNARE mice was exhibited by the spontaneous Ca²⁺-signaling in the CA1 astrocytes (Figure 1C).

Bath application of 20 μM caffeine in the presence of A1 and A2 receptors antagonists (Bastia et al., 2002) DPCPX (3 μM) and SCH58261 (1 μM), caused a slow and sustainable elevation of the cytosolic Ca²⁺-level in cortical astrocytes of adult mice (Figure 1A) accompanied by the notable increase in the number of active microdomains and the amplitude and frequency of microdomain Ca²⁺-transients (Figures 1A, B). Whereas the integral Ca²⁺-response of cortical astrocytes to caffeine underwent a sharp decline with aging, the caffeine-induced augmentation of microdomain signaling was retained in the layer 2/3 astrocytes of old mice, both WT and dnSNARE (Figure 1B). The CA1 astrocytes showed similar behavior in the respect of caffeine-induced effects (Figure 1C). These data suggest that RyR-mediated release from ER can play a prominent role in the augmentation of astroglial Ca²⁺-signaling, both in adult and old age.

To verify the importance of RyR-mediated CICR for the astroglial signaling activated by release of various neurotransmitters, we evaluated the impact of ryanodine (10 μM) on Ca²⁺-transients elicited in astrocytes by the stimulation of synaptic pathways (Figure 2). We used short burst of high-frequency stimulation (see Methods) to mimic the conditions of elevated neuronal activity which can occur either in the physiologic context or during induction of long-term synaptic potentiation. Ca²⁺-signaling was triggered in the CA1 astrocytes by the activation of Schaffer collaterals and in the layer 2/3 astrocytes by the activation of intracortical afferents. We have shown previously that major components of synaptically-induced astrocytic Ca²⁺-activity are mediated by NMDA, mGluR, P2X, P2Y and endocannabinoid receptors (Palygin et al., 2010; Rasooli-Nejad et al., 2014; Pankratov and Lalo, 2015; Lalo et al., 2019; Lalo and Pankratov, 2022).

We also evoked Ca²⁺ -signaling with agonists of PAR1, adrenergic α1 and NMDA receptors, which were previously reported to bring significant contribution into astrocytic signaling and activate release of gliotransmitters (Palygin et al., 2010; Lalo et al., 2014; Pankratov and Lalo, 2015). In terms of Ca²⁺-signaling, the agonists of PAR1 and α1-adrenoreceptors, correspondingly TFLLR and NA, have been shown to act specifically in the astrocytes so they are widely used as a tool to activate astrocyte-driven modulation of synaptic plasticity (Woo et al., 2012; Lalo et al., 2014; Pankratov and Lalo, 2015; Won et al., 2021; Koh et al., 2022).

In line with our previous reports, the HFS activated the robust Ca²⁺-response in the wild-type and dnSNARE mice (Figures 2A, B) and significantly increased the amplitude and frequency of spontaneous Ca²⁺-transients in the astrocytic microdomains (Figures 2A, C). Application of ryanodine did not cause notable effect on the baseline Ca²⁺-level and spontaneous signaling in astrocytes but strongly suppressed the HFS-induced response and facilitatory action of HFS on Ca²⁺-transients in the microdomains (Figures 2A–C). Similar to HFS, the 30 s-long bath application of agonist of astrocytic receptors caused a notable Ca²⁺-elevation accompanied by the increase in the spontaneous Ca²⁺-transients. Again, similar to the effects of HFS, the effects of TFLLR, NA and NMDA on astrocytic signaling were efficiently blocked by the ryanodine.

The astrocytes of the old WT mice exhibited the decrease in the basal amplitude and frequency of spontaneous Ca²⁺-transients but retained responsiveness to stimulation (Figures 2D, F). Similar to the adult mice, the HFS, TFLLR, NA and NMDA produced notable effect on the Ca²⁺-signaling in the astrocyte of old mice, which was dramatically reduced by ryanodine. In both age groups, there was no significant difference in the effects of stimulation and ryanodine in the astrocytes of dnSNARE mice (n = 7 for adult and 6 for old mice) and their wild-type counterparts (the data are not shown).

One should note that application of exogenous ryanodine could also affect neuronal RyRs which might affect the release of neurotransmitters and, theoretically, alter neuron-to-astrocyte communication. However, the main action of neuronal RyRs would be augmentation of Ca²⁺-influx via voltage-gate Ca²⁺-channels and NMDA receptors which would require significant depolarization and thereby could manifest at high levels of synaptic activity. Therefore, putative inhibition of neuronal RyRs could not strongly affect the above responses of astrocytes to TFLLR, NA and NMDA since they were recorded in the presence of TTX (see Methods). We have also shown that primary effect of application of these compounds in the presence of TTX is activation of astrocytic Ca²⁺-signaling. However, one could not a priori rule out putative influence of neuronal RyRs on the astrocytic signaling evoked by HFS. To dissect the specific contribution of astrocytic RyRs, we evaluated the responses of astrocytes to HFS under conditions of intracellular perfusion with 10 μM ryanodine (Figure 3). In addition to the dnSNARE mice and their wild-type littermates, we also used the transgenic mice expressing enhanced green fluorescent protein (EGFP) under the control of the glial fibrillary acidic protein (GFAP) promoter; usage GFAP-EGFP mice facilitated identification of astrocytes (Palygin et al., 2010; Lalo et al., 2011). In the adult mice of all three genotypes, the net Ca²⁺-response of CA1 astrocytes perfused with ryanodine and Rhod-2 was significantly reduced in the comparison to their counterparts perfused only with Rhod-2 (Figures 3B, C). Intracellular ryanodine also significantly decreased the amplitude of the spontaneous Ca²⁺-transients in the hippocampal astrocytes (Figures 3C, D). These results closely agree with the data obtained using exogenous application of ryanodine (Figure 2) and allow to rule out the impact of neuronal RyRs on the astrocytic Ca²⁺-signaling, at least in our experimental setting.

Thus, our data show that RyR-mediated Ca²⁺-induced Ca²⁺-release from ER brings substantial contribution into signaling in the hippocampal and neocortical astrocytes and, potentially, can play instrumental role in the release of gliotransmitters. This notion was tested in the following experiments.

To verify the participation of the RyRs in the gliotransmission, we evaluated the caffeine-induced release of ATP in the CA1 area of brain slices of wild-type and dnSNARE mice. The extracellular ATP was detected with the aid of microelectrode biosensors, as described previously (Frenguelli et al., 2007; Gourine et al., 2010; Lalo et al., 2014; Rasooli-Nejad et al., 2014). Both in the adult and old mice, activation of CA1 astrocytes by 20 μM caffeine (in the presence of A1 and A2 receptors antagonists) caused the transient elevation in the extracellular ATP (Figure 4A) which was strongly suppressed in the dnSNARE mice. In comparison to their wild-type counterparts (Figures 4A, B), the caffeine-induced ATP-transients were reduced in the adult and old dnSNARE-expressing mice correspondingly by 72 ± 18% and 79 ± 16% (Figure 4B). Also, the caffeine-evoked ATP-transients in the CA1 area of WT mice were dramatically inhibited after 20 min-long incubation of brain slices with glial metabolic poison fluoroacetate (FAC, 2mM) (Figure 4B). These observations strongly suggest that caffeine-induced ATP release originated from astroglial exocytosis.

Then, we verified that caffeine-induced exocytosis of ATP from astrocytes can activate neuronal purinoreceptors. The hippocampal and neocortical pyramidal neurons, by virtue of expression of P2X receptors ( Pankratov et al., 1998, 2007), can act as a natural in situ ATP-sensors. We have previously shown that release of ATP from astrocytes can activate transient transmembrane currents in the CA1 and layer 2/3 neurons which can be distinguished from the purinergic currents of synaptic origin by their slower kinetics and smaller amplitude (Pankratov et al., 1998; Lalo et al., 2014, 2016).

To detect the putative astrocyte-derived purinergic activity, excitatory whole-cell currents were recorded in the CA1 pyramidal neurons at a membrane potential of −80 mV in the presence of TTX (1 μM) and inhibitors of A1, A2 receptors. For pharmacological isolation of purinergic currents, NBQX (30 μM), D-APV (30 μM) and picrotoxin (100 μM) have also been applied to eliminate the synaptic signaling via AMPA, NMDA and GABAA receptors. Similar to our previous reports (Pankratov et al., 1998, 2007; Lalo et al., 2016), we detected the non-glutamatergic excitatory spontaneous currents which were fully inhibited by application of P2X receptor antagonists PPADS (10 μM) and 5-BDBD (5 μM) in all 15 neurons tested (Figure 4C).

Under baseline conditions, the purinergic spontaneous currents recorded in the CA1 neurons of adult WT mice had the mean amplitude of 8.9 ± 3.2 pA and decay time of 8.9 ± 2.9 ms (n = 8). After bath application of caffeine, the frequency of the purinergic spontaneous currents underwent a dramatic increase in the neurons of wild-type mice (Figures 4C, E) which was paralleled by the reduction of their mean amplitude to 6.7 ± 1.7 pA and slowing down of their decay kinetics.

Such changes in the average amplitude and decay time of purinergic currents were caused by occurrence of large number of spontaneous events of smaller amplitude and slower kinetics, as it was evidenced by appearance of two distinct peaks in their amplitude and decay time distributions (Figure 4D). This result is consistent with our previous experiments where bimodal distributions of amplitude and decay time of spontaneous purinergic currents were observed in the neocortical (Lalo et al., 2014; Rasooli-Nejad et al., 2014; Pankratov and Lalo, 2015) and hippocampal (Lalo and Pankratov, 2022) neurons. Our previous experiments have proved that purinergic currents with smaller amplitude and large decay time originate from the astrocytic exocytosis of ATP (Lalo et al., 2014).

Application of caffein caused the significant increase in the number of these smaller and slower currents (Figures 4C, D). The caffein-evoked burst of purinergic EPSCs (pEPSCs) in the CA1 neurons was dramatically inhibited in the dn-SNARE mice, strongly supporting their origin from astroglial exocytosis (Figures 4D, E).

The putative mechanisms of Ca²⁺-dependent astroglial release of glutamatergic gliotransmitters, Glutamate and D-Serine, are still under intensive study and debate (Lalo et al., 2021). Recent studies provided strong evidence supporting both fast exocytosis (Won et al., 2021; de Ceglia et al., 2023) and slow channel-mediated release (Woo et al., 2012; Koh et al., 2022). The most plausible target of glutamate released by astrocytes is neuronal NMDA receptors which have high affinity to glutamate and are abundantly expressed both at synaptic densities and extrasynaptic sites (Paoletti et al., 2013; Sanz-Clemente et al., 2013; Papouin and Oliet, 2014). One of the prominent effects of glia-derived glutamatergic transmitters on neuronal NMDA receptors is their tonic activation, whose origin from Ca²⁺-dependent release of glutamate and D-Serine has been recently verified by Koh et al. (2022). Thus, we measured the tonic NMDA current in the neocortical pyramidal neuron as a readout of glutamatergic gliotransmission (Figure 5).

To assess the neuronal response to glia-derived glutamate we recorded the whole-cell transmembrane NMDAR-mediated currents in neocortical pyramidal neurons at membrane potential of + 40 mV and physiological Mg²⁺concentration (Figure 5). Activity of AMPA, GABA and P2X receptors was blocked by DNQX, picrotoxin and PPADS plus 5-BDBD. The amplitude of NMDAR-mediated tonic current was elevated by the downward shift in the holding current caused by application of NMDAR antagonist D-AP5 (3 μM). The selective activation of Ca²⁺-signaling in astrocytes (Woo et al., 2012; Lalo et al., 2014; Pankratov and Lalo, 2015) was achieved with a 30 sec-long application of 3 μM noradrenaline or 20 μM caffein (as in Figure 1).

To verify that astrocytes regulate the neuronal NMDAR tone, the tonic NMDAR-current was measured in the pyramidal neurons while Ca²⁺ signaling in nearby astrocytes was inhibited either by Ca²⁺-clamping or intracellular ryanodine. For attenuation of Ca²⁺-signaling, astrocytes were perfused via the patch-pipette with intracellular solution containing fluorescent dye AlexaFluor488 and 3 mM BAPTA or 10 μM ryanodine; as a control condition, loading of astrocytes with AlexaFluor488 only was used (Figures 5A, C).

The NMDAR tonic current recorded in the WT mice under basal conditions (without astrocytes activation) reached 13.2 ± 4.1 pA (n = 6) and 9.4 ± 3.5 pA (n = 5) at adult and old age correspondingly (Figure 5C). Activation of astrocytes with NA or caffein substantially increased the tonic current in both age groups with NA producing larger effect (Figure 4A). The NA- and caffein-induced elevation in the tonic NMDAR-current was significantly inhibited by perfusion of astrocyte both with BAPTA and ryanodine.

Taken together, the above results indicate that RyR-mediated CICR is instrumental for Ca²⁺- dependent release of ATP and glutamate from astrocytes. Both exocytosis of ATP and glial modulation of NMDAR-tone have been implicated in astroglial control of synaptic plasticity and memory (Lalo et al., 2016; Koh et al., 2022; Lalo and Pankratov, 2023). So, one might expect an involvement of astrocytic RyRs in regulation of neuronal plasticity in neocortex and hippocampus.

We assessed the long-term potentiation (LTP) of the field EPSPs in the CA1 hippocampal area and layer II/III of somatosensory cortex of the wild-type mice of two age groups (Figures 6, 7). In the first series of experiments, we used intracellular perfusion of astrocytes with ryanodine and Alexa488 in the vicinity of fEPSP-recording electrode similarly to the experiments described in the Figure 5. As a control condition, a perfusion of astrocytes with Alexa488 alone was used.

As we showed previously, a modulatory role for astrocytes manifested prominently in facilitation of LTP induction by sub-threshold high-frequency stimulation (Pankratov and Lalo, 2015; Lalo et al., 2018). So, we used short and long trains of theta-burst high-frequency stimulation further referred for simplicity as “weak TBS” and “strong TBS”; details of induction protocols for CA1 and L2/3 areas are given in the Methods. In the CA1 area of adult mice, the weak TBS induced notable increase in the fEPSP slope only in 3 of 6 trials, with the average potentiation at 60^th min reaching only 44 ± 26% (above the baseline) whereas the strong TBS induced robust the LTP amounting to 105 ± 26% (Figures 6A–C). In the old mice, the weak TBS was not able to induce any notable LTP (n = 5, data not shown) and the magnitude of LTP induced by the strong TBS was significantly reduced as compared to the adult mice (Figures 6C, D). The activation of astrocytes with TFLLR during the induction enabled the weak TBS to induce the substantial LTP in both age groups. The facilitatory effect of TFLLR on LTP induced by the strong stimulus was statistically significant only in the old mice. When fEPSPs were recorded in the vicinity of CA1 astrocytes perfused with ryanodine, the weak TBS did not cause any marked potentiation and the magnitude of the LTP induced by strong TBS decreased considerably (Figures 6A, D). Importantly, the inhibition of astrocytic RyRs strongly attenuated the facilitatory effect of astrocytes activation (Figure 6D) in both age groups.

In the second series of experiments, we explored the effects of caffein on the LTP in the neocortex of wild-type and dnSNARE mice (Figure 7). In line with our previous results, the LTP registered in the layer 2/3 of somatosensory cortex of wild-type mice under control conditions exhibited sharp stimulus- and age-dependence. In particular, the weak TBS induced a moderate long-term depression of fEPSPs and the magnitude of LTP was substantially lower in the old mice (Figures 7A, B) as compared to their younger counterparts. In the wild type mice, application of caffein during the LTP induction led to dramatic changes in the effect of weak TBS, switching it to the long-term potentiation in all trials (125 ± 22%, n = 5). Caffein also increased the magnitude of LTP induced by the strong TBS with a prominent effect in the old mice (Figure 7B). The facilitatory effect of caffein was abolished in the dnSNARE mice (Figures 7B, C), strongly supporting an involvement of gliotransmission in this effect.

Combined, these data imply that astroglial ryanodine receptors contribute to Ca²⁺-dependent release of gliotransmitters and, thereby, into astrocyte-driven modulation of synaptic plasticity in the hippocampus and neocortex.