Opposite changes in synaptic activity of organotypic rat spinal cord cultures after chronic block of AMPA/kainate or glycine and GABAA receptors

1. The well-developed cytoarchitecture of rat organotypic spinal cord culture makes it a suitable model to explore how persistent suppression of certain synaptic inputs might be compensated by increased synaptic efficacy (homeostatic plasticity). 2. Spontaneous or electrically evoked synaptic transmission of patch-clamped ventral horn interneurons was studied in control solution after blocking, for the second week in culture, AMPA/kainate receptors with CNQX or glycine and GABA, receptors with strychnine and bicuculline, or indiscriminately removing inputs with tetrodotoxin (TTX). 3. In untreated cells, spontaneous postsynaptic currents (PSCs) had fast (tau < 5 ms) or slow (tau > 10 ms) decay. A similar separation was observed when recording miniature currents (mPSCs). Slow decay PSCs were suppressed by strychnine plus bicuculline while fast decay events were eliminated by CNQX. 4. After chronic CNQX treatment the frequency of spontaneous, fast PSCs (of larger amplitude) or mPSCs was almost doubled with respect, to control. These events were blocked by acutely applied CNQX, which unmasked slow PSCs. 5. After chronic TTX treatment neither the frequency nor the amplitude of spontaneous events was changed. 6. After chronic strychnine and bicuculline treatment the frequency and amplitude of all PSCs was decreased in most cells. mPSCs were also decreased in frequency. Spontaneous or electrically evoked currents acquired a larger component mediated by NMDA receptor activity. 7. The developing spinal network thus operated distinct homeostatic processes which led to strong enhancement in glutamatergic transmission after CNQX block or to broad down-regulation of synaptic activity following chronic exposure to strychnine and bicuculline.