Synaptic plasticity of local connections in rat motor cortex.

This paper reviews studies that investigated mechanisms of the induction of long-term synaptic efficacy increase in local horizontal connections in slices of adult rat motor cortex. Long-term potentiation (LTP) could be induced by electrical stimulation of afferents using theta burst stimulation (TBS) conditionally, when synaptic inhibition was transiently blocked by focal application of GABA(A) receptor antagonist. Robust, long-lasting enhancement of synaptic transmission in horizontal connections was induced by brief application of the potassium channel blocker, tetraethylammonium (TEA, 25 mM), to the incubation medium. This TEA-LTP could be blocked by nifedipine, a voltage-dependent calcium channel blocker. A transient exposure of slices to elevated extracellular calcium (5 mM) resulted in a long-lasting enhancement of responses, termed Ca-LTP, which could be blocked by the antagonist of NMDA receptors, APV. The induction of both TEA-LTP and Ca-LTP, could be prevented by inhibitors of the extracellular signal regulated kinase (ERK) cascade U0126 and PD98059. A transient activation of the ERK, 15 min after application of TEA or elevated [Ca2+], was demonstrated using immunofluorescence. Both forms of plasticity could also be prevented by the inhibitor of cAMP-dependent protein kinases (PKA), Rp-cAMPS. These studies indicate the involvement of the ERK and PKA signaling mechanisms in synaptic plasticity of the motor cortex in vitro. Since LTP in horizontal connections of the motor cortex has previously been shown to be related to the acquisition of a motor skill, it is suggested that the ERK and PKA signaling pathways may be involved in motor learning.