Vertebrate photoreceptors have a modified cilium composed of a basal body, axoneme and outer segment. The outer segment includes stacked membrane discs, containing opsin and the signal transduction apparatus mediating phototransduction. In photoreceptors, two distinct classes of vesicles are trafficked. Synaptic vesicles are transported down the axon to the synapse, whereas opsin-containing vesicles are transported to the outer segment. The continuous replacement of the outer segments imposes a significant biosynthetic and trafficking burden on the photoreceptors. Here, we show that Ahi1, a gene that when mutated results in the neurodevelopmental disorder, Joubert syndrome (JBTS), is required for photoreceptor sensory cilia formation and the development of photoreceptor outer segments. In mice with a targeted deletion of Ahi1, photoreceptors undergo early degeneration. Whereas synaptic proteins are correctly trafficked, photoreceptor outer segment proteins fail to be transported appropriately or are significantly reduced in their expression levels (i.e., transducin and Rom1) in Ahi1 Ϫ / Ϫ mice. We show that vesicular targeting defects in Ahi1 Ϫ/Ϫ mice are cilium specific, and our evidence suggests that the defects are caused by a decrease in expression of the small GTPase Rab8a, a protein required for accurate polarized vesicular trafficking. Thus, our results suggest that Ahi1 plays a role in stabilizing the outer segment proteins, transducin and Rom1, and that Ahi1 is an important component of Rab8a-mediated vesicular trafficking in photoreceptors. The retinal degeneration observed in Ahi1 Ϫ/Ϫ mice recapitulates aspects of the retinal phenotype observed in patients with JBTS and suggests the importance of Ahi1 in photoreceptor function.

Central regulation of cardiac output via the sympathetic and parasympathetic branches of the autonomic nervous system allows the organism to respond to environmental changes. Sudden onset stimuli, startle stimuli, are useful probes to study central regulatory responses to the environment. In mammals, startle stimuli induce a transient bradycardia that habituates with repeated stimulation. Repeated presentation of the stimulus results in tachycardia. In this study, we investigate the behavioral regulation of heart rate in response to sudden stimuli in the zebrafish. Larval zebrafish show a stereotyped heart rate response to mild electrical shock. Naïve fish show a significant increase in interbeat interval that resolves in the 2 s following stimulation. This transient bradycardia decreases on repeated exposure to the stimulus. Following repeated stimulation, the fish become tachycardic within 1 min of stimulation. Both the transient bradycardia and following tachycardia responses are blocked with administration of the ganglionic blocker hexamethonium, demonstrating that these responses are mediated centrally. The transient bradycardia is blocked by the muscarinic antagonist atropine, suggesting that this response is mediated by the parasympathetic system, while the following tachycardia is specifically blocked by the beta-adrenergic antagonist propranolol, suggesting that this response is mediated by the sympathetic nervous system. Together, these results demonstrate that at the larval stage, zebrafish actively regulate cardiac output to changes in their environment using both the parasympathetic and sympathetic branches of the autonomic nervous system, a behavioral response that is markedly similar to that observed in mammals to similar sudden onset stimuli. cardiac regulation; zebrafish AUTONOMIC DYSREGULATION APPEARS to play a key role in the development of cardiovascular disease (14), and genetic factors are hypothesized to contribute to the variable progression of the disease (30, 34). The startle response can be used as a probe for autonomic regulation of cardiac activity (4, 7). Both parasympathetic and sympathetic responses to the startle stimulus can be observed, which allows for both components of the autonomic nervous system to be probed with a single behavioral manipulation. This study seeks to establish the zebrafish larvae as a model organism to investigate autonomic regulation of cardiac activity by developing a rapid behavioral assay to probe for autonomic regulation and dysregulation of the cardiac output.In mammals, sudden onset stimuli induce a rapid change in behavior that consists of both locomotor activity and coincident autonomic behavioral changes. The locomotor activity, the startle response, may be characterized by a complex motor response involving contraction of the muscles of the eyelid, neck, and limbs [reviewed in (15)]. In rats, for example, an air-puff stimulus results in a stereotyped motor response and transient bradycardia (7). Repeated exposure to the stimulus results in decre...

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