LONG-TERM POTENTIATION OF GLYCINERGIC INHIBITORY SYNAPTIC TRANSMISSION

1. Tetanizing protocols were used to test whether glycinergic inhibition undergoes long-term plasticity in vivo. For this purpose we studied the inhibition evoked disynaptically in the teleost Mauthner (M) cell by stimulation of the posterior branch of the contralateral VIIIth nerve. The advantage of this experimental design is that the inhibition, which is mediated by identified second-order commissural interneurons, is not contaminated by parallel excitation. 2. The VIIIth-nerve-evoked inhibitory postsynaptic potentials (IPSPs), which are generated at the level of the soma, are depolarizing in Cl--loaded M cells. After VIIIth nerve tetanization, these IPSPs exhibited potentiation lasting >30 min in 23 of 31 cells. The maximum enhancement measured 5-10 min after the onset of the tetanization averaged 100 +/- 19% (mean +/- SE). In contrast, the non-''tetanized'' collateral IPSP induced by antidromic stimulation of the M axon did not increase significantly, suggesting synaptic specificity of the potentiation. 3. Single-electrode voltage-clamp studies of Cl--loaded M cells indicated that this plasticity is due to an increased synaptic conductance that occurs without obvious modifications of the kinetics or voltage dependence of the inhibitory postsynaptic currents. 4. The synaptic conductance and its changes during potentiation were quantified by measuring the inhibitory shunt of the antidromic spike while recording with potassium-acetate-filled electrodes. For this purpose the ratio, r', of the inhibitory to resting membrane conductances, was calculated using the expression (V/V') - 1, where V and V' are the amplitudes of the control and the test antidromic spikes, respectively. This ratio was called fractional conductance. Measured at the peak of the expected VIIIth-nerve-evoked IPSP, r' increased by 114 +/- 18% (n = 46). Again the collateral inhibitory conductance was not modified. 5. Because there are two synapses in the inhibitory pathway, it became important to determine whether modifications of the second-order inhibitory junctions contribute to the overall potentiation. Several experimental procedures were used for this purpose. 6. The input-output relationship at the inhibitory synapses was determined by comparing the size of the presynaptic volley and r'. The former was recorded intra- or extracellularly as a monophasic positive potential, the so-called extrinsic hyperpolarizing potential, which increases in parallel with the strength of VIIIth nerve stimulation. In 12 experiments where the presynaptic volley was unaffected by the tetanization, suggesting lack of involvement of the first relay, r' nevertheless increased in amplitude by 79 +/- 14%. 7. In another eight experiments, the volley was increased by the conditioning protocol, indicating potentiation at the first-order excitatory relay. When the input-output function of the inhibitory synapses was examined over a wide range of test stimulus intensities, its slope was 50 +/- 20% steeper at 30 min posttetanization. This enhancement could occur when only a few commissural interneurons were tetanized by low-intensity trains. 8. Direct evidence for plasticity of the inhibitory synapses was obtained with paired recordings from identified commissural interneurons and Cl--loaded M cells. Stability of Cl- loading was monitored using the amplitude of the collateral IPSP as a control. Eight of 23 pairs exhibited potentiation, with an average IPSP increase of 70 +/- 20%, the longest recording session lasting 24 min after tetanization. 9. Taken together, these results demonstrate that inhibitory synapses undergo a form of plasticty comparable with that termed long-term potentiation (LTP) at excitatory junctions. 10. Evidence suggests that a minimum level of M cell cytoplasmic Ca2+ is necessary for the induction of the inhibitory LTP: depolarizations applied to the M cell in conjunction with repeated test stimulations of the contralateral VIIIth nerve resulted in a persistent potentiation. This observation is consistent with an earlier finding that postsynaptic injection of a Ca2+ chelator blocks induction of this potentiation. 11. Although quantal analysis was not systematically performed, results obtained using the coefficient of variation and constraint deconvolution methods were in close agreement and suggested that the inhibitory LTP is mainly expressed presynaptically. 12. The functional meaning of this form of plasticity is discussed in the context of its role in the control and adaptability of reflex activities rather than within the more conventional scheme of learning and memory. Consideration of network design, properties of the M cell's afferent inputs, and factors involved in acoustic localization lead to the suggestions that inhibitory LTP contributes to the direction and orientation of the sound-evoked startle response triggered by the M cell.