Nicotine exposure refines visual map topography through an NMDA receptor-mediated pathway

Smoking, nicotine and visual plasticity: does what you know, tell you what you can see?

Nicotine exposure alters activity-dependent synaptic plasticity processes. Effects on learning and memory outcomes, and the synaptic changes that underlie them, are well-documented. Parallels in hippocampal and visual system pharmacology suggest that nicotine has the potential to alter activity-dependent structural organization in visual areas. Such alterations may contribute to deficits in visual performance reported in smoking exposed individuals.

Spatial and intracellular relationships between the alpha7 nicotinic acetylcholine receptor and the vesicular acetylcholine transporter in the prefrontal cortex of rat and mouse

The alpha 7 subunit of the nicotinic acetylcholine receptor (alpha7nAChR) is expressed in the prefrontal cortex (PFC), a brain region where these receptors are implicated in cognitive function and in the pathophysiology of schizophrenia. Activation of this receptor is dependent on release of acetylcholine (ACh) from axon terminals that contain the vesicular acetylcholine transporter (VAChT). Since rat and mouse models are widely used for studies of specific abnormalities in schizophrenia, we sought to determine the subcellular location of the alpha7nAChR with respect to VAChT storage vesicles in axon terminals in the PFC in both species. For this, we used dual electron microscopic immunogold and immunoperoxidase labeling of antisera raised against the alpha7nAChR and VAChT. In both species, the alpha7nAChR-immunoreactivity ((-)ir) was principally identified within dendrites and dendritic spines, receptive to axon terminals forming asymmetric excitatory-type synapses, but lacking detectable alpha7nAChR or VAChT-ir. Quantitative analysis of the rat PFC revealed that of alpha7nAChR-labeled neuronal profiles, 65% (299/463) were postsynaptic structures (dendrites and dendritic spine) and only 22% (104/463) were axon terminals or small unmyelinated axons. In contrast, VAChT was principally localized to varicose vesicle-filled axonal profiles, without recognized synaptic specializations (n=240). Of the alpha7nAChR-labeled axons, 47% (37/79) also contained VAChT, suggesting that ACh release is autoregulated through the presynaptic alpha7nAChR. The VAChT-labeled terminals rarely formed synapses, but frequently apposed alpha7nAChR-containing neuronal profiles. These results suggest that in rodent PFC, the alpha7nAChR plays a major role in modulation of the postsynaptic excitation in spiny dendrites in contact with VAChT containing axons.

Isthmotectal axons maintain normal arbor size but fail to support normal branch numbers in dark-reared Xenopus laevis

Developing binocular projections to the Xenopus tectum require visual input in order to establish matching topographic maps. In dark-reared Xenopus, the ipsilateral eye's map, relayed via the retino-tecto-isthmotectal pathway, fails initially to acquire normal rostrocaudal order. Moreover, with extended time in the dark, the ipsilateral map becomes progressively less well organized. This phenomenon showed that without binocular cues, the isthmotectal axons are unable to locate proper sites for their terminal zones but left open the issue of whether the axons are able to establish arbors of normal dimensions and/or to sustain normal numbers of branches. In order to test whether dark-rearing modifies isthmotectal axon branching, we have used horseradish peroxidase to examine axons of Xenopus after dark-rearing for periods from 3 to 298 weeks. The results demonstrate that these axons never acquire more than about half the normal numbers of terminals. Surprisingly, however, the dark-reared axons' terminal zones are normal in mediolateral and rostrocaudal extent despite the lack of binocular cues that normally could constrain arbor size by inducing pruning of branches in regions with mismatched visual inputs. The effects of dark-rearing are reversible. After a return to normal lighting conditions, the recovery process begins quickly, with a significant increase in branch numbers within 4 weeks. The terminal zone remains of normal dimensions. These results support the hypothesis that correlated binocular visual input is essential for the maintenance of normal numbers of isthmotectal branches but that normal termination zone size can be established in the absence of visual cues.

The instructive role of binocular vision in the Xenopus tectum

This review presents the fascinating neurobiology underlying the development of the frog optic tectum, the brain structure where the two separate inputs from the two eye are combined into a single, integrated map. In the species Xenopus laevis, binocular visual information has a dramatic impact on axon growth and connectivity, and the formation of binocular connections in this system provides a rich basis for both theoretical and experimental investigations.