ABS 036: Role of Inhibition in Fast Homeostatic Synaptic Plasticity in Xenopus laevis Optic Tectum

Jesus Gonzalez, Carlos Toro, Carlos Aizenman

Van Wickle (2025) Volume 1, ABS 036

Introduction: The stability of neural circuits during development depends on homeostatic plasticity mechanisms that balance excitation and inhibition. While long-term homeostatic adjustments are well-characterized, less is known about how rapidly inhibitory synapses respond to acute changes in excitatory drive. This study investigates if inhibitory synaptic transmission in the Xenopus laevis optic tectum exhibits fast homeostatic plasticity in response to reduced excitation. We pharmacologically suppressed AMPA receptor-mediated excitatory transmission to test this using the non-competitive antagonist GYKI 53655 in an ex vivo preparation. Whole-cell voltage-clamp recordings from tectal neurons were made over a 3–4 hour time frame where we observed changes in spontaneous inhibitory postsynaptic currents (sIPSCs).We focused on the sustained block of excitatory synaptic activity, validating the effectiveness of GYKI. In parallel, inhibitory synaptic activity exhibited a downward trend in amplitude and frequency, though these changes did not reach statistical significance. These results suggest that the decrease in inhibition may reflect a circuit-level reduction in excitatory input onto inhibitory neurons rather than direct, cell-autonomous homeostatic plasticity at inhibitory synapses. Following experiments with tetrodotoxin (TTX) to look at postsynaptic currents resulted in inconclusive findings, something to look into for the future. Our findings underscore the complexity of homeostatic regulation in recurrent circuits and raise key questions about the timing, mechanisms, and cellular specificity of inhibitory plasticity. Understanding how inhibition adapts to acute changes in excitation could provide insights into the pathophysiology of neurological disorders that involve the E/I imbalance, such as epilepsy, autism, and schizophrenia.

Method: Wild-type and albino Xenopus laevis tadpoles were bred and reared to stage 49 in 10% Steinberg’s solution at 18–21°C. All procedures followed Brown University IACUC protocols. To assess homeostatic plasticity, tadpoles were exposed to GYKI 53655, a non-competitive AMPA receptor antagonist, with DMSO vehicle controls. Following anesthesia, brains were dissected and pinned flat in a recording chamber. Periventricular membranes over the optic tectum were removed to expose neurons for whole-cell patch-clamp recordings using glass micropipettes (8–12 MΩ) filled with potassium-based internal solution. Voltage-clamp recordings assessed voltage-gated sodium and potassium currents, as well as spontaneous postsynaptic currents (sEPSCs at –45 mV, sIPSCs at +5 mV). Intrinsic excitability was quantified from peak inward (Na⁺) and steady-state outward (K⁺) currents. Event amplitudes and frequencies were measured from 60s traces using Axograph. All data were acquired using Clampex 11.2 and analyzed using GraphPad Prism 10.

Results: We confirmed that GYKI 53655 effectively suppressed excitatory synaptic transmission in tectal neurons, validating our pharmacological perturbation. Inhibitory activity showed a consistent decrease in spontaneous inhibitory postsynaptic current (sIPSC) amplitude and frequency over a 3–4 hour window. However, these changes did not reach statistical significance, suggesting the decrease may reflect a loss of excitatory drive onto inhibitory neurons rather than cell-autonomous plasticity. Overall, our findings highlight a potential network-level reduction in inhibition rather than direct fast homeostatic adjustment at inhibitory synapses.

Discussion: Our results show that acute suppression of excitatory synaptic activity using GYKI 53655 leads to a consistent, though statistically non-significant, decline in sIPSCs in the Xenopus laevis optic tectum. While this trend might suggest fast inhibitory homeostatic plasticity, it more likely reflects an indirect, network-level effect: reduced excitatory drive onto inhibitory interneurons. AMPAR blockade may raise the activation threshold of these neurons, reducing presynaptic firing rather than synaptic strength. This interpretation aligns with previous findings in hippocampal and cortical circuits, highlighting the need for future experiments to isolate intrinsic versus network-driven mechanisms of inhibitory adaptation.