Selective developmental increase in the climbing fiber input to the cerebellar interpositus nucleus in rats

Abstract

Previous studies have demonstrated that learning-related cerebellar plasticity and stimulus-elicited neuronal activity emerge ontogenetically in parallel with delay eyeblink conditioning in rats. The present study examined cerebellar interpositus field potentials and multiunit neuronal activity evoked by microstimulation of the inferior olive in Postnatal Day 17 and 24 rats. The slope and amplitude of the excitatory postsynaptic potential and the number of evoked multiunit spikes increased with age, whereas the inhibitory postsynaptic potential caused by Purkinje cell input remained stable. These results are consistent with the notion that the postsynaptic depolarization of cerebellar interpositus neurons caused by cerebellar afferents (e.g., the climbing fibers of the inferior olive) is a critical factor contributing to the ontogeny of delay eyeblink conditioning in rats.

Figure 1

The neurophysiology of the olivocerebellar trisynaptic circuit. a: Diagram of the olivocerebellar circuit showing that climbing fibers (cf) from the dorsal accessory olive (dao) excite cerebellar interpositus neurons (ipn), and that complex spikes in Purkinje cells (pkj) inhibit cerebellar interpositus neurons milliseconds later. b: Four overlaid Purkinje cell complex spikes elicited by the climbing fiber inputs that also activate cerebellar interpositus neurons. c: Four overlaid multiunit sweeps from the interpositus nucleus. d–f: The field potential in the cerebellar interpositus nucleus evoked by DAO microstimulation under control (Ctl) conditions (d) and after picrotoxin (Ptx; e) and picrotoxin plus kynurenic acid (Ptx + Kyn; f) infusion into the cerebellar interpositus nucleus. Note that the field potential after picrotoxin infusion does not have an inhibitory postsynaptic potential (IPSP) and that the field potential after picrotoxin plus kynurenic acid infusion has neither an IPSP nor an excitatory postsynaptic potential (EPSP). Arrow in b indicates stimulus onset in b–f. g: Diagram of the experimental design, with a recording electrode in the cerebellar interpositus nucleus and a stimulating electrode in the DAO. h: Two representative brain sections showing marking lesions in the cerebellar interpositus nucleus and DAO (arrows). i: Evoked interpositus field potentials (upper traces; gray line = subthreshold, black line = threshold +100 μA) and multiunit activity (lower traces; three trials overlaid) from a Postnatal Day (P) 17 rat after microstimulation in the DAO. j: Evoked interpositus field potentials (upper traces) and multiunit activity (lower traces) from a P24 rat after microstimulation in the DAO. Arrows in i and j indicate stimulus onset. k: Illustration of EPSPs and IPSPs for each rat superimposed on each other (black line = P17 rat; gray line = P24 rat). Scale bar in f = 2 ms/600 μV. Scale bar in k = 1 ms/500 μV.

Figure 2

Selective developmental increase in the climbing fiber excitatory postsynaptic potential (EPSP). a: Amplitude of the EPSP and inhibitory postsynaptic potential (IPSP) for each of four current levels in Postnatal Day (P) 17 (solid) and P24 (open) rats. b: Slope of the EPSP and IPSP for each of four different current levels. c: Peak latencies for the EPSP (circles) and IPSP (triangles). Plotted along the x-axis is the time between each peak at each of the four current levels for P17 (solid) and P24 (open) rats. d: Total number of multiunit spikes evoked in the six trials at each current level for three 4-ms time windows after microstimulation of the dorsal accessory olive in P17 and P24 rats. All data are means (± SEM). T = thresholds. Asterisks indicate statistically significant differences (p < .05).