Preparation and Maintenance of Primary Embryonic Rat Hippocampal Neuron Cultures (E18) on Poly-D-Lysine for Long-Term Synaptic Studies

Each preparation yields neurons from pooled hippocampi of one E18 litter (8-12 embryos). Plate technical replicates of ≥3 coverslips per condition and biological replicates from ≥3 independent dissections (different litters) for any downstream comparison. Plating densities: 50,000 cells/cm² (low, for morphology/imaging) and 100,000 cells/cm² (high, for biochemistry). Viability assessed at plating (trypan blue) and culture health monitored at DIV1, 4, 7, 14. Half-medium changes every 3-4 days.

BSL-1; embryonic rodent tissue under IACUC approval. Hazards: papain/trypsin are sensitizers (gloves, avoid aerosols); AraC is a cytotoxic antimetabolite (chemotherapeutic) — handle with nitrile gloves in a BSC, collect AraC-contaminated waste as cytotoxic waste. Borate buffer is an irritant. Work in a Class II biosafety cabinet to maintain sterility and personal protection. Sharps (scalpels, glass pipettes) into sharps containers. Animal waste per institutional biohazard policy.

Purity control: stain a coverslip for MAP2 (neurons) and GFAP (astrocytes) at DIV14; expect >90% MAP2+. Viability control: trypan blue exclusion at plating should show >85% viable cells. Coating control: a non-PDL-coated coverslip should show poor adhesion/no neurite extension, confirming PDL necessity. Density control: parallel plates at low and high density confirm the effect of density on survival and network formation. Medium control: a no-B-27 arm demonstrates the survival dependence on B-27 supplement.

Healthy cultures show phase-bright somata with smooth, extending neurites by DIV1, an elaborated dendritic arbor by DIV7, and a dense synaptic network by DIV14. Viability at plating >85%; purity >90% MAP2+ at DIV14. Spontaneous calcium transients/network bursts appear by DIV10-14. Synapsin/PSD-95 puncta density of ~0.2-0.5 puncta/µm of dendrite at DIV14-21. Minimal glial carpet if AraC used; mild glial layer supports survival if AraC omitted.

• Timed-pregnant Sprague-Dawley rat, E18
• HBSS without Ca²⁺/Mg²⁺ (dissection buffer), ice-cold, +10 mM HEPES
• 0.25% trypsin-EDTA or papain (20 U/mL) for dissociation
• DNase I (0.1 mg/mL) to reduce clumping
• Plating medium: Neurobasal + 2% B-27 + 0.5 mM GlutaMAX + 1% penicillin/streptomycin (+ optional 5% heat-inactivated FBS for first 4 h)
• Maintenance medium: Neurobasal + 2% B-27 + 0.5 mM GlutaMAX + 1% pen/strep
• Poly-D-lysine (0.1 mg/mL in borate buffer) for coverslip coating
• Laminin (5 µg/mL, optional co-coating)
• Cytosine arabinoside (AraC, 1-5 µM, optional anti-mitotic at DIV3-4)
• 18 mm round glass coverslips (#1.5) or PDL-coated plates
• 0.4% trypan blue

To dissect hippocampi from embryonic day 18 (E18) rat embryos, dissociate them enzymatically and mechanically into a single-cell suspension, and plate neurons at defined density on poly-D-lysine-coated coverslips in serum-free Neurobasal/B-27 medium. The goal is a culture that reaches >90% neuronal purity (MAP2+), forms dense synaptic networks by DIV14, and remains viable through DIV21 for downstream functional and morphological assays.

1. The day before: coat coverslips with 0.1 mg/mL poly-D-lysine overnight at 37 °C; wash 3× with sterile water; air-dry in hood.
2. Euthanize the pregnant rat per IACUC; remove the uterine horns into ice-cold HBSS-HEPES.
3. Extract embryos; decapitate; remove brains into fresh ice-cold HBSS under a dissecting scope.
4. Dissect out both hippocampi; carefully strip the meninges (critical to limit fibroblasts).
5. Pool hippocampi; incubate in 0.25% trypsin + DNase I at 37 °C for 15 min.
6. Wash 3× in HBSS to remove trypsin; add plating medium.
7. Triturate 8-10× with a fire-polished glass pipette (avoid bubbles) until a homogeneous suspension forms.
8. Filter through a 70 µm strainer; count viable cells with trypan blue on a hemocytometer.
9. Plate at 50,000-100,000 cells/cm² onto PDL-coated coverslips; incubate 37 °C, 5% CO2.
10. After 2-4 h, exchange to serum-free maintenance medium (full medium change).
11. Optionally add 2 µM AraC at DIV3-4 to limit glial overgrowth.
12. Perform half-medium changes every 3-4 days; use neurons DIV7-21.

Independent variables: plating density, coating substrate (PDL ± laminin), AraC treatment, embryonic age. Dependent variables: neuronal viability (% live), neuronal purity (% MAP2+), synapse density (synapsin/PSD-95 puncta at DIV14), and neurite outgrowth. Controlled variables: medium formulation (Neurobasal/B-27/GlutaMAX), CO2 (5%), temperature (37 °C), feeding schedule, coverslip lot, dissociation enzyme and time.

We hypothesize that E18 hippocampal neurons plated at 50,000-75,000 cells/cm² on poly-D-lysine in Neurobasal/B-27 will, without serum, mature into a synaptically connected network exhibiting spontaneous activity by DIV10-14, and that glial proliferation will be suppressed sufficiently (with optional AraC) to maintain a neuron-dominated culture without compromising neuronal survival.

Cell counts and viability computed from hemocytometer (live/total). Purity quantified by counting MAP2+ vs DAPI+ nuclei across ≥5 fields per coverslip in Fiji/ImageJ. Synapse density measured by thresholded puncta counts (synapsin/PSD-95) normalized to MAP2+ dendrite length using the Fiji Puncta Analyzer or SynQuant. Neurite length measured with the Simple Neurite Tracer (SNT) plugin. Report per-coverslip and per-dissection means.

1. Poor adhesion / cells lift off: ensure PDL coating concentration/time was adequate, rinse PDL thoroughly (residual PDL is toxic), and verify coverslips are #1.5 borosilicate, not plastic. 2. Excessive cell death by DIV3: over-trituration or trypsin over-digestion — reduce trituration passes and limit enzyme to 15 min; verify B-27 lot and osmolarity. 3. Glial overgrowth: strip meninges more carefully and add 2-5 µM AraC at DIV3-4. 4. Clumpy suspension / low yield: add DNase I and triturate more gently with a fire-polished pipette; filter through 70 µm strainer. 5. Contamination: confirm aseptic technique, fresh pen/strep, and sterile water rinses; pre-test medium for endotoxin if recurrent.

Compare conditions (e.g., density, AraC) by one-way ANOVA with Tukey post-hoc (alpha = 0.05), treating independent dissections as the biological unit (n ≥ 3 dissections). For purity/viability proportions use a generalized linear model or arcsine-transformed ANOVA. Power: detecting a 15% difference in survival at 80% power (SD ~10%) requires n ≈ 4 dissections. Report n at both coverslip and dissection levels and avoid pseudoreplication.