The temporal and spatial changes in intracellular calcium levels during separate activation of parallel fiber (PF) and climbing fiber (CF) inputs to cerebellar Purkinje cells were studied. PF stimulation (1 Hz), at relatively high stimulus strengths, led to accumulations of calcium that were similar in peak levels to those following CF stimulation but that remained spatially localized. Such stimuli consistently induced a durable depression of PF synaptic transmission that partially occluded further depression by conventional conjunctive stimuli and that was independent of nitric oxide. This depression was accompanied by a reduction of synaptic efficacy in spatially isolated PF inputs to the same cell that was independent of postsynaptic calcium but that was mediated by nitric oxide. These data indicate that LTD comprises at least two separate processes and that parameters of PF stimulation that are capable of raising calcium levels in Purkinje cell dendrites are also able to induce long-term changes in synaptic efficacy.

An understanding of the patterns of mossy fiber transmission to Purkinje cells, via granule cell axons, is fundamental to models of cerebellar cortical signaling and processing. Early theories assumed that mossy fiber input is widely disseminated across the cerebellar cortex along beams of parallel fibers, which spread for several millimeters across the cerebellar cortex. Direct evidence for this has, however, proved controversial, leading to the development of an alternative hypothesis that mossy fiber inputs to the cerebral cortex are in fact vertically organized such that the ascending segment of the granule axon carries a greater synaptic weight than the parallel fiber segment. Here, we report that ascending axon synapses are selectively resistant to cerebellar long-term depression and that they release transmitter with higher mean release probabilities and mean quantal amplitudes than parallel fiber synapses. This novel specialization of synapses formed by different segments of the same axon not only explains the reported patterns of granule cell3 Purkinje cell transmission across the cerebellar cortex but also reveals an additional level of functionality and complexity of cerebellar processing. Consequently, ascending axon synapses represent a new element of cortical signal processing that should be distinguished from parallel fiber synapses in future experimental and theoretical studies of cerebellar function.

In the cerebellar cortex, brief, 8 Hz activation of parallel fibres (PFs) induces a cyclic adenosine 3′5’‐monophosphate (cAMP) and protein kinase A (PKA)‐dependent form of long‐term potentiation between PFs and Purkinje cells. With 10 mm BAPTA in the recording pipette, potentiation evoked by raised frequency stimulation (RFS) to one of two, synaptically independent PF inputs to the same Purkinje cell did not remain input specific but consistently spread to synapses that did not receive RFS, up to the maximum distance tested of 168 μm. LTP at activated and non‐activated sites was accompanied by a decrease in paired pulse facilitation (PPF). The PKA inhibitor H‐89 blocked both of these effects. Inhibition of nitric oxide synthase (NOS), either by 7‐nitro‐indazole (7‐NI) or NG‐nitro‐l‐arginine methyl ester (l‐NAME), completely prevented heterosynaptic potentiation and associated reduction in PPF. LTP at distant synapses was selectively prevented by the nitric oxide scavenger 2‐(4‐carboxyphenyl)‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide (cPTIO). Inhibition of soluble guanylate cyclase or protein kinase G had no effect on either pathway. Synaptic potentiation at PF‐PC synapses, induced by the adenylate cyclase activator forskolin, was also prevented by inhibition of NOS. Forskolin‐induced increases in mEPSC frequency were similarly prevented by NOS inhibition and mimicked by the NO donor spermine NONOate. These results are consistent with the notion that heterosynaptic potentiation is of pre‐synaptic origin and dependent upon activation of cAMP/PKA and NO. Moreover, they suggest that cAMP/PKA activation stimulates NO production and this diffusible messenger facilitates pre‐synaptic transmitter release at synapses within a radius of upwards of 150 μm, through a mechanism that does not involve cGMP.

No abstract

Granule cell axons, via their parallel fibers, form synapses with Purkinje cells across large areas of the cerebellar cortex. Evidence for uniform transmission along parallel fibers to Purkinje cells is controversial, however, leading to speculation that the ascending axonal segment plays a dominant role in cerebellar processing. We have compared the relative susceptibilities of ascending axon and parallel fiber synaptic inputs to several forms of synaptic plasticity. We demonstrate that ascending axon synapses have a limited capability to undergo forms of long-term depression and potentiation compared with parallel fiber synapses. These results demonstrate that these two segments of the same axon play fundamentally different roles in cerebellar signaling, and, as such, the synapses formed between granule cells and Purkinje cells should not be treated as a homogenous population.