Receptors, second messengers and protein kinases required for heterosynaptic cerebellar long-term depression

Raising the frequency and intensity of stimulation to one of two sets of parallel fibre synaptic inputs to cerebellar Purkinje cells results in a localised calcium influx and a long-term depression (LTD) of parallel fibre-Purkinje cell responses. Although the calcium influx remains spatially constrained, depression spreads heterosynaptically to distant sites. Inhibition of the synthetic enzyme for cGMP, guanylate cyclase, did not significantly affect the overall level of calcium-dependent synaptic depression observed at the site of raised stimulation (test site), but it entirely prevented synaptic depression at the distant (control) site. Inhibition of protein kinase G produced identical results. In contrast, protein kinase A inhibition had no effect. Selective inhibition of either metabotropic glutamate receptors (mGluRs), protein kinase C (PKC) or tyrosine protein kinase (PTK) blocked depression at both sites equally effectively. These data reveal that two, inter-dependent cellular pathways capable of inducing cerebellar LTD exist. The levels of PF stimulation required to induce heterosynaptic depression were similar to those used routinely in more widely accepted models of LTD. The data predict that cerebellar long-term depression will not be input specific at the single cell level under those conditions of PF-activation that give rise to NO/cGMP production.