Many forms of learning require temporally ordered stimuli. CS-evoked spiking. This bidirectional modulation of Purkinje cell activity offers a basis for the requirement of a minimum CSCUS interval for conditioning, but we argue that it cannot be fully explained by LTD, even when previous studies of stimulus-timing-dependent LTD are taken into account. studies suggest that LTD is strongest for PFCCF intervals of 100 ms (Chen and Thompson, 1995; Wang et al., 2000; Safo and Regehr, 2008), but these studies also find LTD with simultaneous PFCCF input. Thus, if LTD is the synaptic mechanism underlying conditioning, training at short ISIs would be expected to lead to acquisition of CRs. Thus, either no Purkinje cell CRs develop with short ISIs, or they do occur but are not expressed as overt behavior (Wetmore et al., 2008). To address this paradox, we recorded Purkinje cells in decerebrate ferrets during conditioning with various ISIs. Materials and Methods Subjects for Purkinje cell recording experiments were 48 male ferrets (4C12 months, 1C2 kg) decerebrated under propofol anesthesia (10 mg/ml Diprivan; AstraZeneca). Subsequent Purkinje cell recordings occurred without anesthesia. The experimental setup is illustrated in Figure 1? 1, where = (? < 0 s, and = 0 is defined as CS onset. To quantify changes in CS-evoked Purkinje cell firing after training, we computed = [(? is the baseline spike rate as defined above, is the spike rate during the CS defined as for < 0.2 s, and the final CS-alone session recorded is designated by the subscript ? values are from two-sample, two-tailed tests with no assumption of equal population variances. Results Eyeblink conditioning in decerebrate ferrets In rabbits, acquisition of a CR requires a minimum ISI of 100 ms (Fig. 1test with no assumption of equal population variances, < 0.05), but not after training with a short ISI of 45 ms (> 0.05; Fig. 1< 0.01; Fig. INNO-206 (Aldoxorubicin) 4< 0.01). Previous training with a different ISI did not influence the direction of Purkinje modulation in any recorded cell (data not shown), confirming that individual cells can acquire both increases and decreases in CS-evoked firing. Discussion Relationship between behavioral and Purkinje cell conditioning The motivation behind this study was the apparent conflict between the fact that learning of overt conditioned responses does not occur with short ISIs (Schneiderman and Gormezano, 1964; Smith, 1968; Smith et al., 1969; Salafia et al., 1980) and the hypothesis that the synaptic learning mechanism in conditioning is LTD at the PF-to-Purkinje cell synapse, a process that would be expected to occur at short ISIs. Our main finding is that the acquisition of Purkinje cell CRs satisfies the same temporal requirements as the overt behavior. Consistent with previous behavioral studies and with our behavioral data from decerebrate INNO-206 (Aldoxorubicin) ferrets (Fig. 1and the effects observed in the present work is problematic, and not only because of the difference between the preparations. In the standard conditioning protocol we used, the mossy fiber CS, and consequently the PF inputs to neurons in the cerebellar cortex, continues throughout and often far beyond the ISI (Jirenhed and Hesslow, 2011a,b). Therefore, the first PF input precedes the CF input by the full ISI, but during the remainder of the CS, PF inputs precede the CF inputs at gradually decreasing intervals until they become concurrent. When the CS outlasts the ISI, as in experiments reported here, CF input will precede the later parts of the PF input. Thus, many PFCCF delays, both positive and negative, will actually be represented in the same trials in a standard conditioning protocol. Yet the direction and amplitude of HDAC5 learned changes in Purkinje cell spiking is determined by the ISI, suggesting that the cerebellar circuits recognize the onset of INNO-206 (Aldoxorubicin) these stimulus trains for the purpose of inducing plasticity and acquiring Purkinje CRs. Plasticity studies are generally not designed to measure this temporal profile of activity, thus limiting the conclusions that can be drawn from the work discussed here. In short, our findings suggest that LTD of the parallel fiberCPurkinje cell synapses is unlikely to be INNO-206 (Aldoxorubicin) the primary mechanism underlying conditioning, a conclusion that is also supported by recent evidence from genetically modified mice (Schonewille et al., 2011). An alternative to LTD that we are currently addressing in our laboratory is long-term potentiation of the inhibitory action of molecular layer interneurons, a mechanism that has been demonstrated previously (J?rntell and Ekerot, 2002; Jirenhed et al., 2013). The temporal properties of this form of plasticity are not well studied, but.