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Expression of calcium-binding proteins in cerebellar- and inferior olivary-projecting neurons in the nucleus lentiformis mesencephali of pigeons

Abstract In the avian brain, the optokinetic response is controlled by two retinal-recipient nuclei: the nucleus of the basal optic root (nBOR) of the accessory optic system and the pretectal nucleus lentiformis mesencephali (LM). Although considered sister nuclei because of their similar response properties and function, there are both similarities and differences with respect to efferent projections and neurochemistry. Both nBOR and LM project to the cerebellum (Cb) directly as mossy fibers but also indirectly via the inferior olive (IO). In a previous report, we showed that the cerebellar- and inferior olivary-projecting neurons in nBOR of pigeons differentially express the calcium-binding proteins calretinin (CR) and parvalbumin (PV). Both CR and PV are expressed in the somata of LM neurons, although the latter is not as prevalent, and whether expression of CR and PV reflects cerebellar and IO projections is not known. In this report, by combining retrograde neuronal tracing from the Cb and IO with fluorescent immunohistochemistry, we examined the expression of these calcium-binding proteins in the pigeon LM. Half (52%) of the cerebellar-projecting neurons were CR+ve, but only 15% were PV+ve. Almost all (>95%) these PV+ve cells also expressed CR. In contrast, few of the IO-projecting neurons expressed CR or PV (≤5%). This is strikingly similar to what we observed in nBOR and reveals that calcium-binding protein expression is concordant with projection patterns in two nuclei that share similar functions.