Recurrent axon collaterals underlie facilitating synapses between cerebellar Purkinje cells

Abstract

Morphological studies have provided ample evidence for synaptic connections between cerebellar Purkinje cells (PCs), but the functional properties of these synapses remain elusive. We report on direct recordings of synaptically connected PCs in mice cerebellar slices. In PCs filled with a fluorescent dye to aid axon visualization and postsynaptic target identification, presynaptic action potentials elicited unitary inhibitory postsynaptic currents in neighboring PCs in 10% of potential connections tested. In 11 pairs, postsynaptic currents had a delay onset of 1.62 ؎ 0.16 ms with respect to the presynaptic spike, a 10 -90% rise time of 2.20 ؎ 0.33 ms, and a monoexponential decay with a time constant of 13.3 ؎ 1.7 ms. Average values for peak current and variance-to-mean ratio were 55 ؎ 14 and 30 ؎ 3 pA, respectively. In contrast to the depressing nature of the synapse between PCs and deep cerebellar nuclei neurons, PC-PC synapses exhibited strong facilitation operating within a time window of a few milliseconds; paired-pulse ratios for 3-and 20-ms intervals were 1.79 ؎ 0.18 and 1.01 ؎ 0.14, respectively (n ‫؍‬ 6). The facilitation is of presynaptic nature because it is accompanied by a decrease in failure rate. Trains of action potentials evoked in presynaptic varicosities volume-averaged calcium transients whose peak increased 1.7-fold as the frequency increased from 50 to 166 Hz. We suggest that PC-PC synapses are tuned for high fidelity of transmission during bursts of PC activity and that their operation in the cerebellar circuit modulates synchronized PC firing.presynaptic calcium ͉ cerebellum ͉ synaptic plasticity ͉ GABA ͉ calcium binding protein M ore than a century has elapsed since classical anatomical studies by Golgi revealed a surprising feature of Purkinje cells (PCs), namely the formation by their axon collaterals of profuse ramifications along the parasagittal plane extending from the granule cell layer into the PC layer of the cerebellar cortex (reviewed in ref. 1). The nature of this cortical plexus has since been the subject of many studies. Ramon y Cajal (2), based on light microscopy of mammalian and bird cerebellum, suggested that PC axon collaterals terminate in PCs and in other neurons of the molecular layer. In rodents, more recent work has described the development of the plexus (3-5) and provided ultrastructural evidence for the synaptic nature of the contacts made by PC collaterals (refs. 6 and 7 and see review in ref. 1). For PC-PC synapses some uncertainty remains as to the cellular compartment targeted by the collaterals. In mice, synaptic boutons were found on PC dendrites but not over PC somata (6), whereas in rat both PC regions receive PC synapses and clusters of boutons occur over PC somata (7). The boutons are large, up to 3 m in diameter in adult mice, but quantification on the number of release sites per bouton is not available. A distinctive feature of PC-PC contacts is their wide synaptic cleft, 2-fold larger than the usual value of 125 Å (7).Despite the precocity and abund...

Cerebellum and Cerebellar Connections

“…Purkinje recurrent collaterals terminate within in the vicinity of their parent cell body and contact Purkinje cells and molecular layer interneurons (O'Donoghue et al, 1989). Purkinje cell contacts to neighboring Purkinje cells have been suggested to play a role in synchronization between cells (Orduz and Llano, 2007). The projections of the Purkinje cells to the cerebellar nuclei display a strict longitudinal pattern, which will be discussed at length later.…”

Cytosolic Ca2+ Buffers

“…This indicates that oscillations emerge via a mechanism that synchronizes assemblies of Purkinje cells (mediated by parallel fiber excitation) and the network of chemically-coupled MLI. In addition, recurrent Purkinje cell collaterals (Orduz and Llano 2007) may be implicated in these oscillations (de Solages et al 2008).…”

Principal Cell Spiking, Postsynaptic Excitation, and Oxygen Consumption in the Rat Cerebellar Cortex

“…The feedforward organization of the neuronal circuitry in the cerebellar cortex, including spatial segregation of PC input and output, no recurrent PC excitation (Orduz and Llano 2007), and the inability of PC spikes to back-propagate into the dendrites (Llinás and Sugimori 1980), together with the pacemaker property of PCs (Hausser and Clark 1997;Raman and Bean 1999), form a good model for relating different aspects of neurosignaling to cortical oxygen consumption. In this setting, ongoing PC spiking was modified pharmacologically using bicuculline, a GABA A receptor blocker.…”

“…Because PC axon collaterals are not excitatory, recurrent neuronal activation cannot occur (Orduz and Llano 2007).…”