IMPORTANCE OF POLYSYNAPTIC INPUTS AND HORIZONTAL CONNECTIVITY IN THE GENERATION OF TETANUS-INDUCED LONG-TERM POTENTIATION IN THE RAT AUDITORY-CORTEX

Supragranular pyramidal neurons in the adult rat auditory cortex (AC) show marked long-term potentiation (LTP) of population spikes after te tanic white matter stimulation (TS). For determination of whether this marked LTP is specific to AC, LTP in rat AC slices was compared with LTP in slices of the visual cortex (VC). The amplitude of TS-induced L TP in AC was twice that in VC. LTP of EPSPs was also studied with perf orated patch or whole-cell recording. Although the amplitude of TS-ind uced LTP of EPSPs in AC was larger that in VC, no cortical difference was found in LTP elicited by low-frequency stimulation paired with cur rent injection. Neocortical LTP is dependent on the activation of NMDA receptors, and induction of LTP requires postsynaptic depolarization for removal of Mg2+ blockade of NMDA receptors. The postsynaptic depol arization elicited by TS in supragranular pyramidal neurons in AC was significantly larger than that in VC. Cutting of supragranular horizon tal connections resulted in a decrease in the depolarization amplitude in AC but an increase in the depolarization amplitude in VC. The cort ical difference in TS-induced LTP was diminished in the slices in whic h horizontal connections in supragranular layers were cut. The estimat ed density of horizontal axon collaterals of supragranular pyramidal n eurons in AC was approximately twice that in VC. These results strongl y suggest that the marked polysynaptic and postsynaptic depolarization during TS and the resulting marked LTP in AC are attributed to well d eveloped horizontal axon collaterals of supragranular pyramidal neuron s in AC.