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The majority of neurons in the mammalian brain reside within the cerebellum (Cb). Yet, the evolutionary origins of the Cb are not well understood. There are several sensory nuclei present across vertebrate phylogeny collectively termed cerebellum‐like structures (CbLS) due to a shared anatomy and physiology with the Cb. Despite the similarities, the CbLS are clearly not phylogenetically homologous with the Cb. Common structure and function may arise due to a shared genetic and developmental toolkit. To examine this possibility, we used sequence analysis, western blotting, immunohistochemistry and RT‐qPCR to test for the expression of three genes that are critical for mammalian cerebellar development in the Cb and CbLS of an elasmobranch fish, Leucoraja erinacea. In the mammalian Cb, Pax6 is necessary for parallel fiber development, while Cbln1 and Grid2 code for proteins necessary for parallel fiber‐principal cell synaptogenesis. Pax6 and Cbln1 are expressed by granule cells in the Cb and CbLS of the adult skate and stage 31 embryo. Grid2 is expressed by principal cells in the Cb and CbLS of the adult and stage 31 embryo. RT‐qPCR showed this expression is spatially and temporally restricted to the Cb and CbLS. If Pax6, Cbln1 and Grid2 perform the same functions in the skate Cb and CbLS as they do in the mammalian Cb, then these structures may develop using a shared genetic toolkit and be considered generatively homologous. It is possible that the evolutionary genesis of the Cb was the result of duplication or expansion of the cerebellum‐like developmental toolkit. The role of generative homology in the evolution of novel neural structures is explored. The cerebellum and cerebellum‐like structures of a basal vertebrate, the elasmobranch fish Leucoraja erinacea, share a restricted genetic toolkit that utilizes genes necessary for mammalian cerebellar development. We also present evidence for conservation of cerebellar development. [ABSTRACT FROM AUTHOR]

The cerebellum is considered a motor control structure, yet it shares strong anatomical similarities with the cerebellar-like nuclei of the fish hindbrain, which are clearly sensory and process electrosensory and [...]

The dorsal nucleus of the little skate is a cerebellum-like sensory structure that adaptively filters out predictable electrosensory inputs. The filter's plasticity is mediated by anti-Hebbian associative depression at the synapses between parallel fibers and ascending efferent neurons (AEN5). Changes in synaptic strength are indicated by the formation of a cancellation signal which is initiated by co-activation of parallel fibers and AEN5, and can be reversed by parallel fiber activity in the absence of AEN activation. In other cerebellum-like sensory structures, the formation of the cancellation signal requires activation of postsynaptic NMDA receptors on the principal neurons. We demonstrate here by immunohistochemistry that the somas and the initial portion of both apical and basal dendrites of the AEN5 are labeled with antibodies raised against the NR1 subunit of NMDA receptors from a South American electric fish. In in vivo physiological experiments, we show that the formation of the cancellation signal induced by coupling an electrosensory stimulus to ventilatory movements or direct parallel fiber stimulation is blocked when either of the NMDA receptor antagonists 2-amino-5-phosphonovaleric acid (APV) or MK8O1 is injected into the molecular layer above the recorded AEN. Blocking NMDA receptors prevented formation of a cancellation signal in 79% (15/19; APV) and 60% (3/5; MK8O1) of the AEN5. This blockage was reversible in 40% (6/15) of the AENs after APV removal. Thus, in the dorsal nucleus, the activity- dependent, long-lasting but reversible change in synaptic strength of the parallel fiber-AEN synapses appears to be an NMDA receptor-dependent process. [ABSTRACT FROM AUTHOR]

Detection of relevant sensory signals requires the filtering out of irrelevant noise, including noise created by the animal's own movements (reafference). This is accomplished in the electrosense of little skates (Raja erinacea) by an adaptive filter in the cerebellar-like electrosensory nucleus (dorsal nucleus) in the medulla. We have shown that electrosensory inputs reliably coupled to the regularly recurring movements of breathing over time are eliminated selectively in the principal neurons (ascending efferent neurons, AEN5) by a cancellation signal that is a negative of the reafference and is supplied by a parallel fiber system. Similarly, electrosensory inputs repeatedly linked to passive fin movements are eliminated suggesting that the filter also functions in relation to other behaviors besides breathing. To determine whether this adaptive filter can eliminate reafference created by brief and infrequent episodic behaviors like swimming in skates, we initiated a series of coupling tests in which an external electrosensory stimulus was coupled to short bouts of either parallel fiber stimulation or passive fin movements, and then measured the ability of AENs to generate a cancellation signal. Following five brief coupling periods (30-60s) separated by long rest periods (1 -9 mm), 38.5% of the AENs developed a cancellation signal when the coupling was to parallel fiber stimulation, and 73% when the coupling was to passive fin movement. We demonstrate that the cancellation signals can be developed incrementally, persist for at least a 3h rest period without reinforcement, and are extinguished within minutes when the association of sensory stimulus and fin movement or parallel fiber stimulation no longer exists. The results indicate that the adaptive filter has the properties necessary to cancel reafference associated with even brief and infrequent behaviors. [ABSTRACT FROM AUTHOR]

Studies the electrosensory system of electric fish elasmobranch. Extraction of weak signals from noise in elasmobranchs; Computer modeling of the synaptic plasticity mechanisms; Intracellular current stimuli.

Compares the signal processing of the primary afferent fibres and secondary projection neurones (crest cells) of the dwarf Scorpionfish `Scopeana papillosus.' How crestcells were identified in the fish; Factors that influenced the differences between the two responses; Frequency sensitivity of both responses.