1Neuroglial Interactions in Cerebral Physiopathology, Center for Interdisciplinary Research in Biology, CNRS UMR 7241, INSERM U1050, Collège de France, 2Paris Diderot University
Pannasch, U., Sibille, J., Rouach, N. Dual Electrophysiological Recordings of Synaptically-evoked Astroglial and Neuronal Responses in Acute Hippocampal Slices. J. Vis. Exp. (69), e4418, doi:10.3791/4418 (2012).
Astrocytes form together with neurons tripartite synapses, where they integrate and modulate neuronal activity. Indeed, astrocytes sense neuronal inputs through activation of their ion channels and neurotransmitter receptors, and process information in part through activity-dependent release of gliotransmitters. Furthermore, astrocytes constitute the main uptake system for glutamate, contribute to potassium spatial buffering, as well as to GABA clearance. These cells therefore constantly monitor synaptic activity, and are thereby sensitive indicators for alterations in synaptically-released glutamate, GABA and extracellular potassium levels. Additionally, alterations in astroglial uptake activity or buffering capacity can have severe effects on neuronal functions, and might be overlooked when characterizing physiopathological situations or knockout mice. Dual recording of neuronal and astroglial activities is therefore an important method to study alterations in synaptic strength associated to concomitant changes in astroglial uptake and buffering capacities. Here we describe how to prepare hippocampal slices, how to identify stratum radiatum astrocytes, and how to record simultaneously neuronal and astroglial electrophysiological responses. Furthermore, we describe how to isolate pharmacologically the synaptically-evoked astroglial currents.
1. Preparation of Artificial Cerebrospinal Fluid and Intracellular Solution
2. Acute Hippocampal Slice Preparation
3. Dual Recording of Evoked Astroglial Currents and Neuronal Field Potentials
We here describe how to record synaptically-evoked astroglial and neuronal responses, i.e. responses induced by synapse activation through afference stimulation using an extracellular electrode.
A representative simultaneous recording of synaptically-evoked astroglial and neuronal responses (fEPSPs) in the CA1 area of the hippocampus is shown in Figure 2 A-B. The evoked astroglial current is biphasic, i.e. it consists of a transient outward current and a slowly decaying inward current (> 10 sec) (Figure 2B). The outward current reflects the evoked fEPSP, and is blocked after inhibition of ionotropic glutamate receptors by kynurenic acid (dark grey trace, Figure 2B)3. The majority of the slow inward current reflects potassium entry into the astrocyte following postsynaptic depolarization, since it is also abolished by kynurenic acid, which inhibits postsynaptic ionotropic glutamate receptor activity (Figure 2B and Figure 2C1)3-6, known to represent the main source (80 %) of potassium release7. The remaining rapidly rising and decaying inward current is inhibited by the GLT antagonist DL-TBOA (light grey trace, Figure 2B and Figure 2C2). Post-hoc subtraction of the remaining slow current in TBOA (light grey trace) from the current in kynurenic acid (dark grey trace) allows the isolation of the pure astroglial glutamate transporter current (black trace), as illustrated in Figure 2C2. The persistent slowly decaying current in kynurenic acid and TBOA (light grey trace, Figure 2B and Figure 2C2) can be blocked by TTX (data not shown), and reflects most likely the accumulation of extracellular K+ released during presynaptic afferent firing3. Moderate single stimulation of Schaffer collaterals induces a relatively large synaptically-evoked astroglial current compared to the small evoked depolarization recorded in the same cell (Figure 2D). This is due to the low membrane resistance of astrocytes. Recording of the synaptically-evoked astroglial membrane potential dynamics, as illustrated in Figure 2D, is a direct measure of local extracellular potassium levels8. Normalization of the evoked astroglial responses to the underlying neuronal activity allows the direct comparison of different experiments, as recently shown6. Astroglial currents can furthermore monitor very reliably alterations in excitatory transmission, as the total synaptically-evoked astroglial current follows linearly the increase in the fEPSP (Figure 2E). Astroglial currents also reflect short-term synaptic plasticity, since they show, as neurons, paired-pulse facilitation (Figure 2F). Paired whole-cell recording of a CA1 pyramidal cell and an astrocyte reveal very different electrophysiological behavior in both cell types, since the neuron display action potentials in response to a depolarizing pulse, while the neighboring astrocyte is silent (Figure 3A-B). However, moderate stimulation of the Schaffer collaterals can evoke simultaneously a fast excitatory posynaptic potential in the CA1 pyramidal cell and a fast outward and slow inward currents in the adjacent astrocyte (Figure 3B2). Dual recordings of synaptically-evoked neuronal and astroglial responses can also be recorded in the CA3 area of the hippocampus, as shown in Figure 3C. Indeed, single stimulation of CA3 mossy fibers evokes in basal conditions very small neuronal responses, recorded as local fEPSPs, associated to small fast outward and slow inward currents in astrocytes (Figure 3D1). In contrast, 1 Hz stimulation of CA3 mossy fibers for a few seconds strongly potentiates the fEPSP, while it only moderately increases the astroglial response (Figure 3D2).
Figure 1. Hippocampus isolation to prepare transversal slices. To dissect the brain, cut the scull along the midline (a). Make a coronal cut at the level of the olfactory bulb (b) and subsequently at the level of the cerebellum (c). Carefully remove the scull with the help of a forceps (d), separate the two hemispheres with a blade (e), and transfer them on a small spoon into cold oxygenated ACSF (f). After ~ 5 min equilibration, place one hemisphere on dry tissue with the medial surface up (g). With the help of two spoons remove the diencephalon (h-j). The hippocampus is now visible, as illustrated by the dashed lines (k). Dissect the hippocampus with a spoon out, starting from the fimbria, visible as white structure (l-m). Transfer the hippocampus back into the cold ACSF. Prepare a small agarose-block, position the two hippocampi with the alveus side up and the ventral hippocampus facing the edge of the agar block, and soak carefully all liquid away to allow a good attachment to the agar (n). Glue the hippocampus attached to the agar block onto the ventral part (o).
Figure 2. Simultaneous neuronal and astroglial responses evoked-synaptically in the CA1 area of the hippocampus. A) Scheme of the hippocampal slice illustrating the arrangement of the stimulating electrode, to activate the Schaffer collaterals (SC), the patch pipette electrode, to record astrocytic currents, and the extracellular electrode, to record fEPSP, evoked by SC stimulation in the hippocampal CA1 area. B) Representative traces of simultaneous recordings of fEPSPs (upper panel) and astrocytic currents (lower panel) evoked-synaptically by SC stimulation in the presence of pharmacological drugs. The responses are first recorded in the presence of a GABAA receptor blocker (picrotoxin, 100 μM, black traces) to isolate excitatory responses. Subsequent application of an ionotropic glutamate receptor blocker (kynurenic acid, 5 mM, dark grey traces), inhibits the fEPSP and the major part of the long-lasting astroglial current, unmasking a small and fast transient component of the astrocytic current response, which is sensitive to a glutamate transporter blocker (TBOA, 200 μM, light grey traces). Scale bar, fEPSP 0.1 mV, astrocytic current 15 pA, 10 msec. C1). Sample trace of the astroglial potassium current (1-2), which can be isolated from the evoked response, shown in B (lower panel, black trace), by subtracting the current component remaining in kynurenic acid (2) from the total current (1). Scale bar, 20 pA, 1 sec. C2) Sample trace of the glutamate transporter current (2-3), obtained by subtraction of the TBOA insensitive slow component (3) from the current in kynurenic acid (2). Scale bar, 2.5 pA, 25 msec. D) Sample traces of an inward current recorded in voltage-clamp (lower panel) and the corresponding membrane depolarization recorded in current-clamp (upper panel) induced in an astrocyte by SC stimulation. Scale bar, current-clamp 1.5 mV, voltage-clamp 5 pA, 1 sec. E) Input-output curves illustrating the relationship between the presynaptic fiber volleys (input) and the total astroglial current (output) recorded simultaneously in response to SC stimulation (n = 6). The astroglial current increases linearly with the increased fiber volleys, as the neuronal fEPSP. F) Sample traces of the neuronal response (fEPSP) and the astrocytic current are shown for paired-pulse stimulation at a 40 msec interpulse interval. The synaptically-evoked astroglial current exhibits, like neurons, paired-pulse facilitation. Scale bar, 0.1 mV, 5 pA, 20 msec.
Figure 3. Dual recordings of synaptically-induced neuronal and astroglial responses in the CA1 and CA3 areas of the hippocampus. A) Reconstruction of a CA1 pyramidal cell injected with sulforhodamine-B (red, 0.1 %) and an astrocyte filled with fluorescein dextran (green, 0.1 %), using the whole-cell patch-clamp technique. Scale bar, 10 μm. B1) Representative traces of simultaneous whole-cell recordings of membrane potentials recorded in current-clamp from a CA1 pyramidal cell and an adjacent astrocyte. Neuronal action potential firing (black trace), evoked by injection of a 20 pA depolarizing current pulse, evokes no response in the adjacent astrocyte (grey trace). Scale bar, 20 mV, 10 pA, 100 msec. B2) Sample traces of dual whole-cell recordings of a CA1 pyramidal cell in current-clamp and a neighboring astrocyte in voltage-clamp after SC stimulation in the presence of picrotoxin (100 μM). SC stimulation evokes an excitatory postsynaptic potential (EPSP, black trace), associated to a small and long-lasting astrocytic current (grey trace). Scale bar, 5 mV, 10 pA, 100 msec. C) Scheme of the hippocampal slice depicting in the dentate gyrus (DG) and CA3 areas the arrangement of the stimulating electrode, to activate the mossy fibers, the patch pipette electrode (grey), to record astrocytic currents, and the extracellular electrode, to record fEPSP, evoked by CA3 mossy fibers stimulation. D, E) Representative traces of paired recordings of CA3 fEPSPs (upper panels, black traces in D1 and D2) and astrocytic whole-cell responses (lower panels, grey traces in D1 and D2) to single stimulation of CA3 mossy fibers at 0.02 Hz (zoom in the inset) (D1) or at 1 Hz stimulation frequency (D2). Scale bar for D1 and D2, 0.2 mV, 15 pA, time: D1 1 sec; D2 100 msec. Inset scale bar, 0.1 mV, 100 msec.
Dual recording of synaptically-induced neuronal and glial responses is a useful method to study online alterations in pre- and postsynaptic activities associated to changes in astroglial properties. The synaptically-evoked glial membrane depolarization is a direct measure of the extracellular potassium rise8, due in part to presynaptic action potential firing, but mostly to postsynaptic depolarization 7. Therefore recordings of glial membrane potential dynamics can be used to investigate modifications in presynaptic excitability, postsynaptic activity, extracellular space volume and potassium buffering capacities6, 8. The astroglial glutamate transporter current is a sensitive measure of presynaptic glutamate release, able to monitor short-term changes in release probability3, 5, 9. It can furthermore be used to characterize the functional synapse-glia interactions at different synapses or at different developmental stages10. It should be highlighted that GLTs are highly temperature sensitive11 and are driven by the electrochemical gradient of Na+, K+ and H+12. Thus the amplitude and kinetics of the GLT current highly depend on the chosen experimental conditions. Furthermore, the actual time course of astroglial glutamate clearance derived from the recorded GLT current is known to be partially obscured. This is due to the filtering of GLT currents by factors such as the electrotonic properties of astrocytes or the asynchronous transmitter release, which distort their kinetics13. Methods extracting the temporal features of the filtering mechanisms have been developed and can be used to derive the actual glutamate clearance time course in physiological or pathological situations, as recenly performed6,13,14. Additionally, the simultaneous recording of the astroglial membrane depolarization, in current clamp, can provide insights into possible alterations of extracellular potassium transients. Single astrocytes contact up to 100,000 synapses of ~ 100 different neurons, and do therefore integrate and modulate the activity of local neuronal networks.
When using the technique presented here, i.e. recording electrophysiological whole-cell responses from astrocytes to gain insights into basal synaptic activity, one should keep in mind that in astrocytes, patch-clamp recordings at the soma level allow detecting currents mostly originating from the cell soma or proximal processes. Indeed, currents detected at the soma only partially originate from fine distal processes when a strong activation of receptors and channels occuring in multiple fine processes can generate currents propagating to the cell soma. Thus basal receptor and channel activity in individual small astroglial processes covering synaptic compartments is hardly detectable. This is due in part to the limited spatial and temporal control of membrane currents and voltages by whole-cell patch-clamp recordings from astrocytes in situ. However, it should be noted that the surface of the abundant tiny astrocytic processes exceeds by far the membrane area of the soma and main processes. In addition, these perisynaptic astroglial microdomains contain the functionally relevant receptors and channels, which likely play an important role in neuroglial communication and synaptic regulation. The technique we presented here is therefore mostly useful to study the astrocytic integration of synchronous activity from neuronal ensembles, occurring in particular during afference stimulation. It should not be used to study the dialogue between individual synapses and adjacent fine astrocytic processes occurring during basal spontaneous activity. An alternative method to study local astroglial responses induced by basal synaptic activity would be to perform patch-clamp recordings from fine processes, as done in dendrites15. Although patching these fine astroglial processes is likely challenging due to their small size, it is probably an avenue to pursue to unravel more intimate dialog between astroglial microdomains and individual synapses. However, the likely small electrophysiological astroglial responses resulting from individual fine astroglial processes may be below threshold detection, since electrical noise reaches in average 3-5 pA in patch-clamp recordings. Another method to study astroglial responses to synaptic activity is calcium imaging, since activation of astrocytic membrane receptors or transporters by neuroactive substances can trigger intracellular calcium transients. However, bulk loading of astrocytes with calcium indicators may also mainly reflect somatic activity16. The combination of electrophysiology and calcium imaging also enables detecting small calcium signals from fine astroglial processes, either occurring spontaneously or triggered by minimal synaptic stimulation17, 18. However, one should keep in mind that high-affinity calcium indicators might act like calcium buffers, inhibiting important calcium signaling pathways, whereas low-affinity indicators might work below detection level. Finally, an elegant and non-invasive technique to study calcium events in fine astrocytic processes, which also circumvents the washout of intracellular signaling molecules during whole-cell patch-clamp, consists in using a membrane targeted calcium sensor, which can be expressed in astrocytes in situ, as well as in vivo19. However, calcium imaging can only provide information about one signaling molecule, which is involved in many, but not all cellular activities, whereas whole-cell patch-clamp provides quantitative information about all the different ionic currents triggered upon channel and receptor activation. Therefore simultaneous electrophysiological recordings from neurons and astrocytes are a unique and powerful method to unravel online the dynamics of neuroglial ionic signalling and its role brain information processing.
No conflicts of interest declared.
The authors would like to thank Dana Kamalidenova, Morgan Autexier, and Roch Chopier, who made the video and animations, as well as Florian Beck for the photographs and the post-production of the video voice-over. This work was sponsored by Olympus and supported by grants from the HFSPO (Career Development Award), ANR (Programme Jeunes chercheurs and Programme Blanc Neurosciences), FRC (Fédération pour la Recherche sur le Cerveau), INSERM and La Pitié Salpêtrière hospital (Translational research contract) to N.R., from French Research Ministry and Deutsche Forschungsgemeinschaft postdoc fellowships to U.P., and from the doctoral school "Frontiers in Life Science", Paris Diderot University, Bettencourt Schuller foundation, and FRM (Fondation pour la Recherche Médicale) doctoral fellowship to J.S.
|Picrotoxin||Sigma||P1675||dissolve in DMSO|
|Kynurenic Acid||Tocris||0223||dissolve at 34 °C stirring or sonication|
|DL-TBOA||Tocris||1223||DCG IV Tocris 0975|