Investigating Long-term Synaptic Plasticity in Interlamellar Hippocampus CA1 by Electrophysiological Field Recording

The hippocampus has been widely used in cognitive studies^1,2,3. The hippocampal lamellar network in the transverse axis forms the tri-synaptic circuitry that is made up of the dentate gyrus, CA3, and CA1 regions. The lamellar network is considered to be a parallel and independent unit^4,5. This lamellar viewpoint has influenced the use of transverse orientation and transverse slices for both in vivo and in vitro electrophysiological studies of the hippocampus. In light of emerging research, the lamellar hypothesis is being reevaluated⁶ and attention is also being given to the interlamellar network of the hippocampus. With regards to the hippocampal interlamellar network, the CA3 region has long been investigated^7,8,9,10, however the longitudinal CA1 hippocampal region has received relatively little attention until recently. With regards to the CA1 interlamellar network, the short-term synaptic properties along the dorsoventral longitudinal hippocampal CA1 axis of rats have been shown to vary¹¹. Also, clusters of hippocampal cells responding to the phase and the place were found to be arranged systematically along the longitudinal axis of the hippocampus in rats, undergoing a short term memory task¹². Also, epileptic seizure activities were found to be synchronized along the whole hippocampus along the longitudinal axis¹³.

Most studies of the longitudinal CA1 hippocampal region however, have utilized input from the CA3 to the CA1 regions^11,14,15. Using a unique protocol to make longitudinal brain slices, our previous work demonstrated the associational connectivity of CA1 pyramidal neurons along the longitudinal axis and implicated its ability to process neuronal signaling effectively¹⁶. However, there is a need to determine whether the CA1 pyramidal neurons along the longitudinal axis without transverse input can support long term synaptic plasticity. This finding can add another angle into investigations of neurological issues pertaining to the hippocampus.

The ability of neurons to adapt the efficacy of information transfer is known as synaptic plasticity. Synaptic plasticity is implicated as the underlying mechanism for cognitive processes such as learning and memory^17,18,19,20. Long-term synaptic plasticity is demonstrated as either long-term potentiation (LTP), which represents the strengthening of neuronal response, or long-term depression (LTD), which represents the weakening of neuronal response. Long-term synaptic plasticity has been studied in the transverse axis of the hippocampus. However, this is the first study to demonstrate long-term synaptic plasticity in the hippocampal longitudinal axis of CA1 pyramidal neurons.

Building from a protocol used by Yang et al.¹⁶, we designed the protocol to demonstrate LTP and LTD in the hippocampal longitudinal axis of CA1 pyramidal neurons. We used C57BL6 male mice with ages ranging between 5-9 weeks old for in vitro experiments and 6-12 weeks old for in vivo experiments. This detailed article shows how longitudinal hippocampal brain slices from mice were obtained for in vitro recordings and how in vivo recordings were recorded in the longitudinal axis. For in vitro recordings, we investigated directional specificity of longitudinal CA1 synaptic plasticity by targeting the septal and temporal end of the hippocampus. We also investigated layer specificity of the longitudinal CA1 synaptic plasticity by recording from the stratum oriens and stratum radiatum of the hippocampus. For in vivo recordings, we investigated the angles that best correspond to the longitudinal direction of the hippocampus.

Using both in vivo and in vitro extracellular field recordings, we observed that the longitudinally connected CA1 pyramidal neurons presented with LTP, not LTD. The transverse orientation involving both CA3 and CA1 neurons, however, supports both LTP and LTD. The distinction in the synaptic capabilities between the transverse and the longitudinal orientation of the hippocampus could speculatively signify differences in their functional connectivity. Further experiments are needed to decipher the differences in their synaptic capabilities.

All animals were treated in accordance with the guidelines and regulations from the Animal Care and Use of Laboratory of National Institute of Health. All methods described here have been approved by the Institutional Animal Care and Use Committee (IACUC) of City University of Hong Kong and Incheon National University.

1. In vivo field recording

1. Animal preparation
   1. Inject urethane (0.06 g per 25 g weight) intraperitoneally to anesthetize mouse. Supplement with intramuscular injection of atropine (0.05 mg/kg). Keep the mouse in a dark quiet spot until full anesthesia takes effect.
      CAUTION: Urethane has carcinogenic potential. Handle it with care and wear protective clothing to avoid contact with exposed skin.
      NOTE: Alternatively, isoflurane can be used as anesthesia.
   2. Check for depth of anesthesia intermittently until a full surgical plane of anesthesia takes effect. Check for depth of anesthesia by performing toe pinch, ear pinch, tail pinch and corneal touch tests to observe the response of the mouse to physical stimuli.
      NOTE: A reflex or voluntary movement should not be observed when the mouse is at a full surgical plane of anesthesia.
   3. For the toe pinch test, extend either the hind or foreleg of the mouse and pinch firmly with a pair of blunt forceps or fingers. The mouse is not confirmed for full anesthesia if it withdraws the leg or shakes body, has an observable increased respiratory rate or makes vocal sounds.
   4. For the ear pinch test, pinch the ends of the pinna with a pair of blunt forceps or fingers firmly. Full anesthesia is not confirmed if the mouse moves the whiskers forward, shakes its head, makes vocal sounds or has an observable increased respiratory rate.
   5. For the tail pinch test, hold the tail of the mouse gently and firmly pinch with a blunt forcep or finger. No tail movement, vocal sound or observed increase in respiratory rate should be observed when fully anesthetized for surgical procedures.
   6. For the corneal touch test, touch the cornea of the mouse gently, with a cotton wick. No eyelid movement, whisker movement or observable increase in respiratory rate should be observed when fully anesthetized for surgery.
   7. Shave the hair on the neck and skull of the mouse.
   8. Place the fully anesthetized mouse on a heating pad set to 37 °C and insert the rectal temperature probe in the rectum. This enables the heat produced by the heating pad to adjust in response to changes of the mouse’s body temperature.
   9. Apply eye gel to moisten the eyes of the mouse.
   10. Pull the tongue out to the side of the lips gently using forceps and fix the two front teeth into the second or third teeth hole of the stereotactic instrument. Fix the skull of the mouse firmly using the eye-clamp.
       NOTE: Alternatively, a stereotactic ear clamp can be used.
   11. Observing under a microscope, separate the subcutaneous tissue and muscles at the end of the interparietal and occipital bone of the mouse with a scalpel to expose the cisterna magna. Blot the dura mater dry with a cotton swab.
   12. Gently puncture the cisterna magna by making a shallow cut with a sharp pointed scalpel blade to drain the cerebrospinal fluid (CSF). Attach a cotton swab to keep draining the CSF.
2. Craniotomy
   1. Holding the skin on the scalp with forceps, cut and remove the skin with a pair of surgical scissors. Cut enough skin to expose the bregma and lambda marks on the scalp. Keep the exposed region dry.
   2. Adjust the clamped skull of the mouse to enable the bregma and lambda points to be aligned in a horizontal level of similar height. Avoid tilting of the head in either the anterior-posterior position or the medial lateral position.
   3. Mark the points corresponding to the hippocampal region using the aid of a Vernier caliper. Use the mouse brain in a stereotactic coordinate book as a reference to help determine the exact coordinates.
      NOTE: The location to mark for incision for the hippocampus are 1 mm x 3 mm on the midline referred to as anterior-posterior (AP) using the bregma as reference point, and 3 mm x 3 mm perpendicular to the midline (ML) to connect the points on the midline. The stereotactic coordinates are given as (AP: 1,3 and ML: 3,3).
   4. Make an incision in the skull above the dorsal region of the hippocampus along the marked points using a scalpel or high-speed drill while observing under a microscope.
      NOTE: A rectangular shaped opening with a size of 2 mm x 3 mm should be obtained after incision. Keep the exposed area clean to avoid injury to the brain.
   5. Carefully take out the loose skull with forceps to expose the dura mater and use a syringe or dropper to gently apply physiological saline solution to keep the surface moist.
   6. Remove the dura mater carefully with needle or sharp pointed tip forceps.
      NOTE: Be careful not to cause any damage to the brain tissue during craniotomy. This will lead to the swelling of the brain and will affect the results.
   7. Keep the exposed brain tissue moist by applying physiological saline or inert oil using a dropper or syringe.
3. In vivo recording
   1. Fix and position the stimulation and recording electrode firmly in the stereotactic holder. Adjust the stereotactic instrument according to the position of the corresponding AP and ML coordinates for the stimulation and recording electrodes above the CA1 dorsal hippocampus.
      NOTE: Use the mouse brain in stereotactic coordinates as a guide in locating the coordinates for the dorsal longitudinal CA1 hippocampal region. For example, a stereotactic coordinate for the CA1 hippocampal region will be (AP 1.5, ML 1.0) for the recording electrode and (AP 1.7, ML 1.5) for the stimulation electrode.
   2. Locate the stimulating electrode lateral to the recording electrode in a longitudinal direction.
      NOTE: Ensure that both stimulation and recording electrodes are clean before usage. This prevents the introduction of noise when recording.
   3. For recordings, use multichannel electrodes. First, locate the stereotactic coordinates for the CA1 hippocampal region using the first channel. Position the remaining electrodes such that the angle of stimulation and recording electrodes are in the range of 30° to 60° in relation to the midline from bregma point.
      NOTE: This angle corresponds to the longitudinal orientation of the CA1 hippocampal region.
   4. As a control, locate the recording electrode above the CA1 region and the stimulation electrode above the CA3 region of the dorsal hippocampus. For example, using the mouse brain in stereotactic coordinates as a guide, a stereotactic coordinate for CA1-CA3 hippocampal region will be (AP 1.8, ML 1.0) for the recording electrode and (AP 1.5, ML 1.5) for the stimulation electrode.
      NOTE: This step is an alternative control and should be done in a separate experiment.
   5. Place the reference electrode at a distal part of the exposed brain region or under the skin of the mouse.
   6. Turn on the recording system.
   7. Open the software for recording and data acquisition.
      NOTE: Different laboratories have their preferred software for recording and data acquisition.
   8. Observing under a microscope, lower the recording and stimulation electrodes slowly using the micromanipulator until it just touches the surface of the brain. Mark the point as the zero point to start calculating the accurate depth to the hippocampal region.
      NOTE: The micromanipulator can be used to monitor the depth at which the electrode is inserted at any given time. Observe an increase in the impedance just when the electrode touches the brain surface.
   9. Slowly lower the electrodes to the approximate depth corresponding to the chosen stereotactic coordinates for CA1 hippocampus.
      NOTE: Use the mouse brain in stereotactic coordinates as a guide to obtain the approximate depth corresponding to the chosen stereotactic coordinates.
   10. Give stimulation (100 μs duration, repeated at 30 s intervals) and adjust the electrode depth in steps of 50 μm or less until a stable evoked field excitatory postsynaptic potential (fEPSP) is observed.
       NOTE: A stimulus of 20 μA is usually enough to evoke an observable response.
   11. Ensure that a stable fEPSP has been evoked by varying the stimulus intensity. A notable increase in the slope or amplitude of the fEPSP should be observed with each increased stimulus intensity. This is termed as the input-output curve. Create an input-output (I-O) curve to detect maximum stimulus intensity at which there is no more increase in the slope of the evoked fEPSP (Figure 6).
       NOTE: Discard data and change electrode position if the fiber volley disappears during experiment.
   12. Use the input-output curve to set the baseline intensity to 40 - 50% of the maximum. Use the corresponding stimulus intensity for baseline recording.
   13. Record the local field potential as a baseline for 20 - 30 min.
   14. Use the same input-output curve to set the stimulus intensity for evoking high frequency stimulation (HFS) or tetanus to 75% of the maximum intensity. Alternatively, when inducing long-term depression, maintain the same stimulus intensity used for recording the baseline when evoking low frequency stimulation (LFS).
   15. Apply a tetanic stimulation of 100 Hz pulses 4 times with a 10 s interval to induce LTP.
       NOTE: Use this protocol only when working on a long-term potentiation experiment.
   16. Apply low frequency stimulation of 5 Hz (900 stimuli during 3 min), 1 Hz LFS (900 stimuli during 15 min), or 1 Hz paired-pulse (50 ms paired-pulse interval, 900 pairs of stimuli during 15 min) to induce LTD according to already established protocols.
       NOTE: Use these protocols only when working on a long-term depression experiment.
   17. Record the local field potential for 1 h after HFS or LFS, alternatively.
   18. Export data and analyze using the software.
   19. Verify the position of the recording and stimulation electrode by giving a stimulation of 10 μA current for 30 s to lesion the recorded areas. Transcardially perfuse the mouse with 4% paraformaldehyde and harvest the brain for slicing and staining with Cresyl Violet according.
   20. Euthanize mouse by cervical dislocation or injection of lethal anesthetic dosage after experiment.

2. In vitro field recording

1. Preparing oxygenated slicing and artificial cerebrospinal fluid (ACSF) solutions
   1. For 2 L of slicing solution, add approximately 1 L of double distilled water in a volumetric flask and stir vigorously on a stirrer plate.
   2. Add the following slicing solution components (in mM): 87.0 NaCl, 2.5 KCl, 1.3 NaH₂PO₄, 25.0 NaHCO₃, 25.0 glucose, 75.0 sucrose, 7.0 MgCl₂.6H₂O and 0.5 CaCl₂∙2H₂O (Table 1).
      NOTE: Alternatively, MgCl₂∙6H₂O and CaCl₂∙2H₂O can be excluded in this stage and added later in a ready to use volume.
   3. Top up to 2 L with double distilled water while stirring vigorously.
   4. For 2 L of ACSF, add approximately 1 L of double distilled water in a volumetric flask and stir vigorously on a stirrer plate.
   5. Add the following ACSF solution components (in mM): 125.0 NaCl, 2.5 KCl, 1.3 NaH₂PO₄, 25.0 NaHCO₃, 25 glucose, 1.0 MgCl₂∙6H₂O and 2.0 CaCl₂∙2H₂O (Table 1).
      NOTE: Alternatively, MgCl₂.6H₂O and CaCl₂∙2H₂O can be excluded in this stage and added later in a ready to use volume.
   6. Top up to 2 L with double distilled water while stirring vigorously.
2. Setup and brain slicing
   1. Pour 400 mL of already prepared slicing solution into a separate flask and oxygenate (95% O₂/5% CO₂) for approximately 20 min.
   2. Store the rest of the 1.6 L solution in a 4 °C refrigerator. Keep solution up to 1 week after which it must be discarded if not used to avoid fungal growth.
   3. Pour 200 mL of the oxygenated slicing solution in the flask, cover with parafilm and transfer to a -80 °C freezer for approximately 20 min to make a slush.
   4. Pour the remaining 200 mL of slicing solution in a brain slice holding chamber and keep in a 32 °C water bath with continuous bubbling.
   5. Prepare the bench for slicing. Place paper towels down and surgical tools on top of bench. Arrange the surgical tools in order of use to facilitate a fast and efficient process (Figure 7).
   6. Take out the chilled slicing solution from the freezer and pour approximately 50 mL in a beaker. Pour approximately 10 mL in a Petri dish containing filter paper to moisten it. Place them on ice by the dissection area.
   7. Pour the rest of the slushed slicing solution into the slicing chamber and fix it on the vibratome.
   8. Anesthetize the mouse with isoflurane using guidelines and regulations on Animal Care and use of laboratory of National Institute of Health, and the approved methods of Institutional Animal Use and Care Committee of City University of Hong Kong and Incheon National University.
   9. Decapitate the anesthetized mouse with a pair of scissors and place the head on tissue paper. Cut the skin covering the skull of the mouse and cut through the cutaneous muscles with scissors. Cut the skull plates along the midline to the occipital bone using surgical scissors.
   10. Open the skull with blunt forceps to expose the brain. Gently scoop out the brain with a spatula and place in the chilled slicing solution in the beaker. Wait about 30 s.
   11. Take out the brain using the spoon and carefully place it on the previously moistened filter paper in the Petri dish.
   12. Separate the two brain hemispheres along the midline with a scalpel blade. Isolate the hippocampus by gently detaching it from the cortex with a spatula and place it carefully on the moistened filter paper. Cut out the septal and temporal end of the isolated hippocampus using the scalpel.
   13. Pick up the isolated hippocampus using a blunt spatula and brush. Gently dab the spatula on a tissue paper to remove excess water.
   14. Apply a small amount of glue to the slicing plate of the vibratome.
   15. For a longitudinal CA1 hippocampal slice, attach the CA3 region of the hippocampus to the slicing plate with the glue. Quickly but carefully place it in the slicing chamber of the vibratome containing the chilled oxygenated slicing solution.
   16. For the transverse slice that will serve as a control, attach the ventral end of the hippocampus to the slicing plate of the vibratome. Quickly but carefully place it in the slicing chamber containing the chilled oxygenated dissection solution.
       NOTE: This step is an alternative to the step above and should be performed separately.
   17. Position the blade angle to 90°. Set the vibratome parameters to a speed of 0.05 mm/s, an amplitude of 1.20 mm and a slice thickness of 400 µm.
   18. Slice the attached hippocampus with the vibratome.
       NOTE: A good longitudinal CA1 hippocampal brain slice will have one layer of CA1 and 2 layers of dentate gyrus. A maximum of 2 good hippocampal slices can be obtained. Alternatively, a transverse hippocampal brain slice will have the dentate gyrus, CA3 and CA1 regions intact.
   19. Transfer the longitudinal hippocampal brain slices from the vibratome with a pipette and incubate it in the brain slice holding chamber in the water bath.
   20. Incubate slices in the water bath for 20 min at a temperature of 32 °C and bring out to room temperature for 30 min of recovery.
       NOTE: Alternatively, transfer the brain slice when out of the water bath and gently place it in the recording chamber with flowing oxygenated ACSF at a temperature of 32 °C for 30 min of recovery.
   21. While incubating in the water bath, pour 400 mL of ACSF solution into a separate flask and oxygenate (95% O₂/5% CO₂) for approximately 20 - 30 min before transferring the brain slice into the recording chamber. Continuously superfuse the ACSF into the recording chamber at a speed of 2 mL/min.
   22. Turn on the temperature controller to heat the flowing ACSF to 32 °C.
   23. Store the rest of the 1.6 L solution in a 4 °C refrigerator. Keep the solution up to one week after which it must be discarded if not used to avoid fungal growth.
3. In vitro recording
   1. Set up a recording rig by turning on all needed hardware.
   2. Transfer the brain slice from the slice holding chamber into the recording chamber. Adjust the position of the brain slice with blunt forceps and hold it in place with a harp. Allow the ACSF to run for at least 20 min. This enables the brain slice to be stable before recording.
   3. Fill the recording pipette with ACSF as an internal solution.
   4. Check the recording pipette resistance with the designated software. The recording pipette resistance should be within the range of 3-5 MΩ.
   5. Turn on the software for data acquisition. This software should have similar features that enable data acquisition in a similar way as the in vivo recordings shown earlier.
   6. Fix the recording and the stimulation electrodes firmly in the stereotactic instrument holder. Place the reference electrode in the ACSF in the recording chamber.
   7. For longitudinal slices, position the stimulation and recording electrode in the stratum oriens (S.O.). Keep the distance between electrodes to about 300 to 500 µm. Place the stimulation electrode on either the septal or temporal side of the brain slice and record from the same layer (Figure 8).
   8. Alternatively, position the stimulation and recording electrode in the stratum radiatum (S.R.). Place the stimulation electrode on either the septal or temporal side of the brain slice.
   9. For transverse slices as a control, position the stimulating electrode on the CA3 (Schaffer collateral pathway) region, and the recording electrode on the CA1 region (Figure 9).
      NOTE: This is an alternative control step and should be performed in a separate experiment.
   10. Turn on the isolated stimulus generator and give stimulation (100 μs duration, repeated at 30 s intervals). Adjust the recording electrode depth and/or position till a stable evoked excitatory postsynaptic field potential is observed.
   11. Ensure that a stable fEPSP has been evoked by varying the stimulus intensity. A notable change in the slope of the fEPSP should be observed with each change of stimulus intensity. This is termed as the input-output curve. Create an input-output (I-O) curve to detect maximum stimulus intensity at which there is no more increase in the slope of the FEPSP (Figure 6).
       NOTE: Discard data and change electrode position if the fiber volley disappears during experiment.
   12. Use the input-output curve to set the stimulus intensity for baseline recording and for evoking high frequency stimulation (HFS) or tetanus to 30-40% of the maximum evoked fEPSP.
   13. Alternatively, for experiments regarding LTD, use the input-output curve to set the baseline intensity for baseline recording and for evoking low frequency stimulation (LFS) to 70% of the maximum evoked fEPSP.
   14. Record the local field potential as the baseline for 20 - 30 min.
   15. Apply HFS of 100 Hz pulses twice with a 30 s interval to induce LTP.
   16. Alternatively, for LTD experiments, apply low frequency stimulation of 5 Hz (900 stimuli during 3 min) or 1 Hz LFS (900 stimuli during 15 min) or 1 Hz paired-pulse (50 ms paired-pulse interval, 900 pairs of stimuli during 15 min) to induce LTD according to already established protocol.
   17. Record the local field potential for 1 h after HFS or LFS.
   18. Export data and analyze.

We explored long-term synaptic plasticity of longitudinal CA1 pyramidal neurons of the hippocampus using extracellular field recordings both in vivo and in vitro. LTP and LTD are facets of long-term synaptic plasticity that have been demonstrated in the transverse axis of the hippocampus to be unidirectional.

We showed here that using longitudinal hippocampal brain slices, there is LTP in the CA1 longitudinal axis of the hippocampus. We prepared longitudinal slices of the hippocampus along the septotemporal axis, which is perpendicular to the transverse slices (Figure 1). Using recordings from the CA1 region of the hippocampus, we showed the presence of LTP that was not direction specific. There were no statistically significant differences in the recordings from the septal or temporal (Figure 2) side of the longitudinal hippocampal brain slice. We also showed the presence of LTP that was not layer specific; thus, recordings from both stratum radiatum and stratum oriens (Figure 2) showed successfully induced LTP in the longitudinal brain slice. We used D-AP5, an NMDAR antagonist to demonstrate that the LTP induced was dependent on NMDA receptors (Figure 3). What happens in vitro does not necessarily reflect in vivo conditions, so we investigated LTP in vivo. Figure 4a shows a schematic diagram of the stimulation and recording electrode positioned in the dorsal hippocampus along the longitudinal axis of CA1 region in vivo. The position of the electrodes used for the recording and stimulation was verified by lesion marks and crystal violet staining (Figure 4a). We demonstrated the presence of LTP in vivo in the longitudinal CA1 region (Figure 4b).

Using already established protocols for inducing LTD, we failed to successfully induce LTD both in vivo and in vitro (Figure 5).

Figure 1. A schematic drawing of transverse and longitudinal hippocampal brain slices. This figure is adapted and modified from Sun et al. 2018²¹. Please click here to view a larger version of this figure.

Figure 2. LTP in longitudinal slices. Synaptic responses at S.R. (a) or S.O. (b) in longitudinal slices are potentiated right after tetanus stimulation with both temporal and septal inputs (S.R./temporal (n = 12, c), S.R./septal (n = 12, c), S.O./temporal (n = 10, d), S.O./septal (n = 9, d).
The n stands for the number of slices. Error bars represent SE. This figure is adapted and modified from Sun et al. 2018²¹. Please click here to view a larger version of this figure.

Figure 3. NMDAR-dependent LTP in longitudinal slices. (a,b) LTP induction in temporal and septal direction is blocked by 50 μM D-AP5 (temporal, n = 6, a) (septal, n = 5, b). (c,d) LTP induction in temporal and septal direction is also blocked by D-AP5. The n stands for slices. Error bars represent SE. This figure is adapted and modified from Sun et al. 2018²¹. Please click here to view a larger version of this figure.

Figure 4. In vivo LTP in the interlamellar network. (a) A schematic drawing of recording and stimulation electrodes in anesthetized animals. The loci of recording (on the septal side of CA1) and stimulating electrodes (on the temporal side of CA1) were identified by lesion marks. (b) LTP is induced in the interlamellar connection by 100 Hz high frequency stimulation (HFS) (n = 10 mice). Color traces: before (black) and after (red) HFS. Error bars represent SE. This figure is adapted and modified from Sun et al. 2018²¹. Please click here to view a larger version of this figure.

Figure 5. Absence of in vivo and in vitro LTD in Interlamellar CA1 network. (a) 1 Hz-pp LFS does not induce in vivo LTD. (b) 1 Hz pp-LTP, (c) 5 Hz LFS, and (d) 1 Hz LFS do not produce LTD on either the temporal or septal sides of longitudinal brain slice. while LTD is induced by 1 Hz pp-LFS in transverse slices: temporal (n = 8), septal (n = 11) and transverse (n = 6) with 1 Hz pp-LFS; temporal (n = 3) and septal (n = 3) with 5 Hz LFS; temporal (n = 3) and septal (n = 3) with 1 Hz LFS. The n stands for slices. Error bars represent SE. This figure is adapted and modified from Sun et al. 2018²¹. Please click here to view a larger version of this figure.

Figure 6. Input-output curve presenting fEPSP slope in response to increasing stimulus input in hippocampal brain slice. Please click here to view a larger version of this figure.

Figure 7. Surgical tools used for hippocampal isolation during in vitro brain slicing. Please click here to view a larger version of this figure.

Figure 8. A longitudinal brain slice ready for recording. Stimulation electrode and recording pipette are inserted in the stratum radiatum. Please click here to view a larger version of this figure.

Figure 9. A transverse hippocampal brain slice ready for recording. Stimulation electrode is inserted at Schaffer collateral CA3 region and recording pippette is inserted at CA1 region. Please click here to view a larger version of this figure.

Table 1: Concentrations of compounds in brain slice and artificial cerebrospinal fluid solutions.

The protocol demonstrates the method to induce long-term synaptic plasticity in vivo as well as from brain slices in the longitudinal CA1-CA1 axis of the hippocampus in vitro. The steps outlined give enough details for an experimenter to investigate LTP and LTD in a longitudinal hippocampal CA1-CA1 connection. Practice is needed to hone the skills required to successfully record field excitatory potentials.

In addition to needing practice, there are several critical steps that are essential to obtaining good results. First, it was shown previously that the angle to which the brain slices were made could either truncate or preserve the longitudinal projections of the pyramidal neurons in the CA1-CA1 region of the hippocampus¹⁶. The longitudinal pyramidal neurons project from the transverse neurons at an angle that is nearly perpendicular. As the CA1 neurons propagate in diverse angles within the hippocampus, the longitudinal connection between them lays out along the dorsoventral axis of the hippocampus. Thus, for in vitro recording, the experimenter must keep this in mind to accurately target the CA1-CA1 hippocampal neurons along the longitudinal axis by cutting the isolated hippocampus tissue along the dorsal-ventral axis. Also, for in vivo recordings, the angle at which the stimulation and the recording electrodes are positioned determines whether the results obtained are representative of the longitudinal axis or a mixture of both the transverse and longitudinal axis. Further investigations utilizing CRISPR-Cas9 can be done to confirm whether the evoked response is solely from the CA1 region since it could be a mixture of responses from both the CA1 and the CA3 regions.

Secondly, for in vitro experiments, the experimenter must ensure that the brain slicing solution, ACSF, work bench and all equipment or instruments that come in contact with the brain slice are free from contaminants. Any form of contamination will lead to the deterioration of the integrity or death of the brain slice. Maintaining a clean electrode surface will ensure good and stable recordings for both in vitro and in vivo experiments.

We have shown that the longitudinal hippocampal CA1 network exhibits NMDAR-dependent LTPs, but not LTDs. The trisynaptic circuit, however, presents with both LTP and LTD^22,23. This implies that the longitudinal CA1 network and the tri-synaptic circuitry have unique features. Our protocol makes use of only electrophysiological recordings and therefore is limited in finding the difference between these two networks.

The search for a cure for brain diseases such as schizophrenia continues. Decline or deformity of CA1 hippocampal subregions have been linked with some schizophrenic symptoms^24,25. The application of our protocol, though basic, has brought to light a unique synaptic capability of the CA1 longitudinal hippocampal subregion. This knowledge is useful in designing experiments that can further investigate this debilitating brain disease along the longitudinal CA1 axis of the hippocampus.