Facilitation of long-term potentiation in layer II/III horizontal connections of rat motor cortex following layer I stimulation: route of effect and cholinergic contributions

The ability of layer I activation to facilitate the induction of long-term potentiation (LTP) in layer II/III horizontal connections of motor cortex ( MI) was examined in rat brain slice preparations. Field potentials evoked i n layer I and layer II/III horizontal pathways were recorded from radially aligned MI sites. While theta burst stimulation (TBS) of layer II/III pathw ays alone failed to induce LTP, simultaneous TBS of layer I and layer II/II I inputs on alternate sides of the recording electrodes induced LTP in the layer II/III input in 8 out of 13 slices (mean change +20+/-6%; N=13). In t he same cases, the layer I connections showed mixed effects: LTP in three c ases, LTD in five cases, and no modification in five slices. Despite the fa cilitatory effect of layer I activation on layer II/III LTP induction, we f ound that the critical circuitry for this effect was outside layer I. Cutti ng the layer I fibers selectively in the slice did not prevent layer II/III LTP induction, while cuts preserving only layer I blocked layer II/III LTP after conjoint I+II/III TBS. Cholinergic fibers were evaluated as candidat es for the facilitatory effect because they branch widely in both layers an d they are thought to participate in synaptic modification. The cholinergic contribution to layer II/III LTP facilitation was investigated using bath application of muscarinic antagonists. Muscarinic blockade prevented facili tation of layer II/III LTP by layer I coactivation. Instead, conjoint stimu lation in 10 mu M atropine produced long-term depression (LTD) of layer II/ III (-18+/-9%; N=11) as well as of layer I(-21+/-6%; N=11) horizontal respo nses. These results indicate that connections formed within layer I are ine ffective in promoting LTP in the deeper-lying horizontal connections; the c ritical route by which layer I stimulation influenced LTP induction require d the circuitry in the deeper layers, particularly the cholinergic system. Thus, it appears that diffuse cholinergic afferents provide an additional r oute to regulate activity-dependent synaptic modificaton in horizontal cort ical connections.