Protein kinase C (PCK) is a family of isoforms that are implicated in subcellular signal transduction. The authors investigated the distribution of several PKC isoforms (PKC-alpha, PKC-beta, PKC-gamma, PKC-delta, and PKC-epsilon) within major cerebellar cell types as well as cerebellar projection target neurons, including Purkinje neurons, cerebellar nuclear neurons, and secondary vestibular neurons. PKC-alpha, PKC-beta, PKC-gamma, PKC-delta, and PKC-epsilon are found within the cerebellum. Of these isoforms, PKC-gamma and PKC-delta are highly expressed in Purkinje cells. PKC-gamma is expressed in all Purkinje cells, whereas the expression of PKC-delta is restricted to sagittal bands of Purkinje cells in the posterior cerebellar cortex. In the lower folia of the uvula and nodulus, Purkinje cell expression of PKC-delta is uniformly high, and the sagittal banding for PKC-delta expression is absent. Within the cerebellar nuclei, PKC-delta-immunolabeled axons terminate within the medial aspect of the caudal half of the ipsilateral interpositus nucleus. PKC delta-immunolabeled axons also terminated within the caudal medial and descending vestibular nuclei (MVN and DVN, respectively), the parasolitary nucleus (Psol), and the nucleus prepositus hypoglossi (NPH). PKC-gamma-immunolabeled axons terminated in all of the cerebellar nuclei as well as in the lateral and superior vestibular nuclei and the MVN, DVN, Psol, and NPH. The projection patterns of PKC-immunolabeled Purkinje cells were confirmed by lesion-depletion studies in which unilateral uvula-nodular lesions caused depletion of PKC-immunolabeled terminals ipsilateral to the lesion in the vestibular complex. These data identify circuitry that is unique to cerebellar-vestibular interactions.

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Horizontal optokinetic stimulation of rabbit retina in vivo evokes increased expression of acyl coenzyme A‐binding protein (ACBP), also known as ‘diazepam binding inhibitor,’ from retinal Müller cells. If the expressed ACBP were also secreted by Müller cells, then stimulus‐evoked secretion of ACBP could influence the activity of GABAA receptor‐expressing retinal neurons. In this study, we examine in vitro whether ACBP is secreted by Müller glial cells and Müller‐like QNR/K2 cells following stimulation with elevated levels of KCl and phorbol myristic acetate (PMA). KCl and PMA stimulation evoked secretion of threonine‐phosphorylated ACBP. A sequence analysis of ACBP shows that it has five potential phosphorylation sites: Two threonine sites fit a protein kinase C phosphorylation pattern. Two threonine sites fit a casein kinase II (CK2) pattern. One serine site fits a CK2 pattern. As CK2 is not expressed in QNR/K2 cells, it is probable that protein kinase C accounts for the phosphorylation of ACBP in these cells and for the PMA‐evoked secretion of ACBP. Serine phosphorylation was constitutive. Horizontal optokinetic stimulation increased threonine‐phosphorylated ACBP in rabbit retina. Phosphorylation of ACBP may influence its target affinity. We used a proteolytic fragment of ACBP, octadecaneuropeptide (ODN), to investigate how threonine phosphorylation influences its affinity for GABAA receptors. Threonine‐phosphorylated ODN had a stronger affinity for GABAA receptors than did unphosphorylated ODN or unphosphorylated ACBP. We conclude that stimulus‐induced Müller cell secretion of phosphorylated ACBP could influence the GABAergic transmission in neighboring retinal neurons.

Long-term horizontal optokinetic stimulation (HOKS) decreases the gain of the horizontal optokinetic reflex and evokes the second phase of optokinetic afternystagmus (OKAN-II). We investigated the possible molecular constituents of this adaptation. We used a differential display reverse transcriptase-PCR screen for mRNAs isolated from retinas of rabbits that received HOKS. In each rabbit, we compared mRNAs from the retina stimulated in the posterior3anterior (preferred) direction with mRNAs from the retina stimulated in the anterior3posterior (null) direction. Acyl-CoA-binding protein (ACBP) mRNA was one of four mRNAs selected by this screen, the proteins of which interact with GABA receptors. HOKS in the preferred direction increased ACBP mRNA transcription and ACBP protein expression. ACBP was localized to Muller glial cells by hybridization histochemistry and by immunohistochemistry. ACBP interacts with the ␣ 1 -subunit of the GABA A receptor, as determined by a yeast two-hybrid technique. This interaction was confirmed by coimmunoprecipitation of ACBP and the ␣ 1 -subunit of the GABA A receptor using an antibody to GABA A ␣ 1 . The interaction was also confirmed by a "pull-down" assay in which histidine-tagged ACBP was used to pull down the GABA A ␣ 1 . ACBP does not cross the blood-brain barrier. However, smaller truncated proteolytic fragments of ACBP do, increasing the excitability of central cortical neurons.Muller cells may secrete ACBP in the inner plexiform layer, thereby decreasing the sensitivity of GABA A receptors expressed on the surface of ganglion cell dendrites. Because retinal directional sensitivity is linked to GABAergic transmission, HOKS-induced expression of ACBP could provide a molecular basis for adaptation to HOKS and for the genesis of OKAN-II.