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Sequence dependence of post-tetanic potentiation after sequential heterosynaptic stimulation in the rat auditory cortex

Kenjiro Seki, Masaharu Kudoh and Katsuei Shibuki

1. To investigate the mechanisms for the coding stimulus sequence in the auditory cortex (AC), post-tetanic potentiation (PTP) was recorded after sequentially combined heterosynaptic stimulation was applied in rat AC slices.
2. Brief tetanic stimulation (TS) was applied at two sites on AC slices at intervals of 0.5-10 s. PTP of field potentials was induced by the earlier TS, rather than the later TS. PTP was followed by sequence-dependent long-term potentiation (LTP).
3. Using Ca²⁺ imaging in the slices loaded with rhod-2, a Ca²⁺ indicator, a sequence-dependent distribution of PTP was found in AC slices.
4. The sequence-dependent PTP in excitatory postsynaptic potentials (EPSPs) was observed in supragranular pyramidal neurons.
5. The sequence dependence of PTP was not significantly affected by 1 µM bicuculline, an antagonist of GABA_A receptors, or 100 µM 2-hydroxysaclofen, an antagonist of GABA_B receptors.
6. Depolarization and firing recorded in pyramidal neurons during the later TS were less vigorous than when the slices were incubated in the control medium. However, this suppression of the responses during the later TS was not observed in the presence of 50 µM atropine, an antagonist of muscarinic receptors.
7. PTP was induced by the earlier and later TS in the presence of 50 µM atropine, so that the sequence dependence of PTP was abolished. Pirenzepine (50 µM), an antagonist of muscarinic M1 receptors, but not methoctramine (30 µM), an antagonist of M2 receptors, eliminated the sequence dependence of PTP.
8. These findings suggest that the sequence dependence of PTP in AC might have a role in the temporal processing of auditory information on the scale of seconds.

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