Inhibition of Purkinje cells in the cerebellum of elasmobranch fishes

GABAergic neuronal circuits in the cerebellum of the dogfish Scyliorhinus canicula (Elasmobranchs): an immunocytochemical study

The presence of GABAergic cells and fibres in the corpus cerebelli and auricles of a dogfish, Scyliorhinus canicula, was studied with immunocytochemistry. In both locations, two types of GABA-immunoreactive (GABA-ir) neurons were observed: stellate cells in the molecular layer and Golgi cells in the granular layer. Stellate cell axons gave rise to numerous GABA-ir boutons distributed throughout the molecular layer and a smaller number of boutons which contacted Purkinje cell perikarya. No GABA-ir baskets around Purkinje cells were observed. Golgi cells of the granular layer gave rise to numerous GABA-ir boutons which were located around cerebellar glomeruli. Purkinje cell perikarya and their axon terminals in the cerebellar nucleus were not GABA-immunoreactive. These findings are discussed in terms of the phylogeny of cerebellar circuits.

Stellate cell inhibition of Purkinje cells in the turtle cerebellum in vitro

1. The stellate cell-mediated inhibition of Purkinje cells was studied by intracellular recordings in an in vitro slice preparation of the turtle cerebellar cortex. A graded inhibitory postsynaptic potential (IPSP) was recorded in Purkinje cells upon stimulation of the parallel fibre-stellate cell pathway. 2. The IPSP was abolished by bicuculline, and had a reversal potential around -75 mV, consistent with a GABAA receptor-operated Cl- conductance dominating the response investigated here. 3. Paired recordings from synaptically coupled stellate cells and Purkinje cells demonstrated that the inhibitory input from a single stellate cell is sufficient to reduce the firing in a Purkinje cell. 4. The extracellular-evoked IPSP interacted with the active postsynaptic membrane properties in the Purkinje cell. Interaction with both the Na+ plateau and the IA prolonged the responses to an IPSP, making the net effect of the inhibitory response dependent on the membrane potential in each postsynaptic neurone. 5. A precisely timed IPSP was particularly efficient in reducing dendritic Ca2+ influx. 6. The voltage-dependent Ca2+ component of a climbing fibre response (CFR) as well as of a parallel fibre (PF) input was reduced by the IPSP. 7. It is suggested that Ca2+ spike-mediated reduction in Purkinje cell excitability may be prevented by the stellate cell IPSP-mediated reduction in Ca2+ influx.

Origin of the parallel fibers in the cerebellar crest overlying the intermediate nucleus of the elasmobranch hindbrain

This study utilizes anterograde transport of horseradish peroxidase (HRP), Golgi staining, and electrophysiological techniques to demonstrate the origin of the parallel fibers which constitute the molecular layer overlying the intermediate nucleus of the dogfish hindbrain. The parallel fibers are shown to arise from granule cells of the auricle. The majority come from the granular area of the lower leaf of the auricles, which is termed the lateral granule cell area of the auricle. A band of parallel fibers at the ventrolateral extent of the molecular layer arises from the ipsilateral and contralateral granule cell layers of the upper leaf of the auricle. The functional continuity of the molecular layer between vestibular and lateral-liner areas may provide an additional basis for interactions between these two sense modalities.

Field potential analysis in elasmobranch cerebellum

Stimulation of cerebellar white matter (WM) in the skate, an elasmobranch fish, evokes a distinctive set of cortical field potentials characterized by 3 negativities and a positivity. The first negativity (N1) has 1 msec latency and is largest in the lateral regions of the corpus cerebelli where Purkinje cells and white matter are most densely congregated. The second negativity (N2) occurs at 2--4 msec latency and is localized to the granular layer. The third negativity (N3) follows with a latency of 4--6 msec and is prominent in the molecular layer. The positivity (P) correlates with N2 in time course and dominates in the midline where granule cell axons ascend en masse to form parallel fibers in the molecular layer. A preceding WM stimulus blocks the N1 potential for 20 msec and the N2 potential for 60 msec and the N2 potential for 20 msec. A conditioning stimulus, applied to the parallel fibers in the dorsal midline (LOC), suppresses the N1 potential for 20 msec and the N2 potential for 40 msec. Intracellular recordings from the Purkinje cell layer reveals short latency action potentials correlating in time with N1. These findings suggest that N1 derives from antidromic Purkinje cell activation and mossy fiber excitation, that N2 represents orthodromic granule cell excitation by white matter fibers, and that N3 stems from parallel fiber activity. Because of its close association with parallel fibers and the similarity of its time course to N2, the positivity is attributed to current sources in parallel fibers generated by granule cell current sinks. Differences between the cerebellar field potentials found in elasmobranchs and mammals can be explained by the unique anatomic arrangement of granule cell axons in the former.