Novel synaptic potentials in cerebellar Purkinje cells: probable mediation by metabotropic glutamate receptors

Glutamate receptors of both the ionotropic (ion channel-linked) and metabotropic (enzyme-linked) categories are abundantly expressed by Purkinje cells in the cerebellum but the functional significance of the latter receptors is unknown. We have tested the possibility that they are activated by the parallel fibre input by recording from Purkinje cells within a biplanar cerebellar slice preparation using the grease-gap technique. Under conditions where ionotropic (NMDA and non-NMDA) glutamate and GABA receptors were blocked pharmacologically, electrical stimulation of parallel fibres gave rise to two very slow potentials. The first peaked about 400 msec from the start of stimulation and was depolarising. It was not evident with single stimuli but reached maximum amplitude after 6 shocks delivered at 50 Hz. The wave was abolished when the slices were perfused with Ca(2+)-free solution or with drugs that inhibit synaptic transmission, but it was resistant to blockade of GABAB receptors, acetylcholine receptors and adrenergic receptors. Next came a slow hyperpolarising potential that peaked about 30 sec after stimulation and which was also Ca(2+)-dependent. The sequence of potentials was replicated by perfusion of an exogenous agonist acting selectively on metabotropic glutamate receptors. We conclude that parallel fibre-to-Purkinje cell synaptic transmission involves not only fast signals generated through ionotropic non-NMDA receptors but also much slower potentials that are likely to be mediated by metabotropic glutamate receptors. These potentials are likely to be significant both for shorter-term (seconds to minutes) Purkinje cell excitability as well as for the induction of longer-term synaptic plasticity.