Functional properties of regenerated optic axons terminating in the primary olfactory cortex

Primary sensory map formations reflect unique needs and molecular cues specific to each sensory system

Interaction with the world around us requires extracting meaningful signals to guide behavior. Each of the six mammalian senses (olfaction, vision, somatosensation, hearing, balance, and taste) has a unique primary map that extracts sense-specific information. Sensory systems in the periphery and their target neurons in the central nervous system develop independently and must develop specific connections for proper sensory processing. In addition, the regulation of sensory map formation is independent of and prior to central target neuronal development in several maps. This review provides an overview of the current level of understanding of primary map formation of the six mammalian senses. Cell cycle exit, combined with incompletely understood molecules and their regulation, provides chemoaffinity-mediated primary maps that are further refined by activity. The interplay between cell cycle exit, molecular guidance, and activity-mediated refinement is the basis of dominance stripes after redundant organ transplantations in the visual and balance system. A more advanced level of understanding of primary map formation could benefit ongoing restoration attempts of impaired senses by guiding proper functional connection formations of restored sensory organs with their central nervous system targets.

Using the Severity of Dependence Scale to screen for DSM-5 khat use disorder

OBJECTIVE

This study aimed to determine the efficacy of the Severity of Dependence Scale (SDS) as a screening tool for Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition-defined khat use disorder.

METHODS

Cross-sectional, purposive sample of past-year khat consumers aged 16 and above were recruited from khat markets and cafes from university and general community in Adama, Ethiopia. Participants self-completed a survey comprising current substance use disorder.

RESULTS

The SDS formed a unifactorial structure, consistent with the dependence construct. Almost three quarters (73%) of the sample were identified as experiencing Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition khat use disorder. The SDS demonstrated excellent discrimination (area under the curve = 0.92) and an optimal cut-off as a score of 3 or greater, with sensitivity of 81% and specificity of 96%. This classification validly identified a group with more frequent and higher dose khat use than participants that did not screen positive.

CONCLUSION

Although khat is a mild stimulant, there is clear evidence that some consumers are both concerned with their use and experience problems associated with their use. Consistent with its application for other drugs, the SDS is a brief and simple screening tool that appears to validly identify individuals experiencing a khat use disorder syndrome and experiencing high rates of adverse consequences in association with use.

Superimposed maps of the monocular visual fields in the caudolateral optic tectum in the frog, Rana pipiens

The superficial layers of the frog optic tectum receive a projection from the contralateral eye that forms a point-to-point map of the visual field. The monocular part of the visual field of the contralateral eye is represented in the caudolateral region of the tectum while the binocular part of the visual field is represented in the rostromedial tectum. Within the representation of the binocular field (rostromedial tectum), the maps of visual space from each eye are aligned. The tectal representation of the binocular visual field of the ipsilateral eye is mediated through a crossed projection from the midbrain nucleus isthmi. This isthmotectal projection also terminates in the caudolateral region of the optic tectum, yet there has been no indication that it forms a functional connection. By extracellular recording in intermediate layer 7 of the caudolateral tectum, we have discovered electrical activity driven by visual stimulation in the monocular visual field of the ipsilateral eye. The units driven from the ipsilateral eye burst upon initial presentation of the stimulus. At individual layer 7 recording sites in the caudolateral tectum, the multiunit receptive field evoked from the ipsilateral eye is located at the mirror image spatial location to the multiunit receptive field driven by the contralateral eye. Thus, as revealed electrophysiologically, there are superimposed topographic maps of the monocular visual fields in the caudolateral tectum. The ipsilateral eye monocular visual field representation can be abolished by electrolytic ablation of contralateral nucleus isthmi.

Transsexual limb transplants in fiddler crabs and expression of novel sensory capabilities

We used transsexual limb transplants in fiddler crabs to examine how peripheral sensory structures interact with the central nervous system (CNS) to produce a sexually dimorphic behavior. Female and male chemosensory feeding claws were transplanted onto male hosts in place of nonfeeding, nonchemosensory claws. Successfully transplanted claws retain donor morphologies and contain chemosensory neurons. Neurons in successfully transplanted female feeding claws express the enhanced sensitivity to chemical cues seen in female, but not male, neurons in claws of normal animals. When chemically stimulated, the transplanted claws evoke feeding behavior not observed in normal males, even though the sensory neurons in the transplanted limb project to the host's sexually dimorphic neuropil not known to receive chemosensory input. Behavioral sensitivity is directly related to the sensitivity of peripheral neurons in the transplanted feeding claw. Thus, the interactions between peripheral neurons and their targets may restructure the CNS so that novel sensory capabilities are expressed, and this can produce sexually dimorphic behaviors.

Intrinsic and extrinsic factors that shape neocortical specification

Increasing evidence points to the importance of intrinsic molecular cues in specifying the regional identity of mammalian neocortex. Few such cues, however, have been found to be restricted to individual functionally defined cortical areas before the arrival of afferent information. In contrast, thalamocortical axons are specifically targeted to individual cortical areas, raising the possibility that they can instruct some aspects of cortical areal identity. Cortical structure and function can be altered by modifying the source or pattern of activity in thalamocortical afferents. In particular, studies of cross-modal plasticity have shown that in many respects, one sensory cortical area can substitute for another after a switch of input modality during development. Afferent inputs might therefore direct the formation of their own processing circuitry, a possibility that has important implications for brain development, plasticity and evolution.

A compartment-based, asymmetric representation of the retina in an induced projection to the olfactory cortex

Displacing the optic nerve into the telencephalon in adult Rana pipiens induces a projection to olfactory cortex. We have examined the topographic organization of this projection anatomically by injecting a mixture of biotin dextran (BDA) with 3H-amino acids into the affected eye immediately after making cuts across defined sectors of the nerve fiber layer to trace the complementary patterns of anterograde migration of BDA and 3H label in the cut and intact retinal axons, respectively. Fibers from the temporal side of the optic disc terminated in an oblique band along the posterior two-thirds or more of the ectopic projection field. In contrast, fibers arising in the nasal retina terminated in a parallel strip occupying the anterior one-third or less of the field. Varying the location of the cuts within each hemiretina did not reveal any further organization along the nasotemporal or dorsoventral axes of the retina. The retinal location of the cells involved in this projection was further studied with injections of wheat germ agglutinin conjugated to horseradish peroxidase into the olfactory cortex. Ganglion cells labeled by retrograde transport were found throughout the retina, but they were much more numerous on the temporal side, having a mean spatial density 3.7-7.4 times greater in the temporal hemiretina, whereas the overall ganglion cell density (labeled plus unlabeled) was roughly the same in the two halves of the retina. These data provide an example of a permanent projection in which the overall representation of the retina, though nontopological, is polarized in one axis (nasotemporal) and, therefore, compartmentally organized.

Concentration and storage of biotin in the amphibian brain

Prominent displays of endogenous biotin reactivity can be observed at specific locations in histochemical preparations of the forebrain and midbrain in the northern leopard frog (Rana pipiens) and common American toad (Bufo americanus). At the light microscopic level, the biotin reactivity appears in clusters of darkly stained puncta of either spherical or rodlike shape in the olfactory cortex, nucleus isthmi, and hypothalamus. With the electron microscope, the biotin reactive spheres are identified as neuronal varicosities and synaptic boutons and the rods as short segments of axons. Appropriate controls demonstrate that the punctate biotin-reactive structures are sites of concentration of biotin or a biotin analog in the processes of certain neurons. These data represent the first observation on the selective concentration of a vitamin in vertebrate neurons and suggest that biotin may have specialized functions in anatomically delimited areas of the central nervous system. Localization of the densest concentration of the biotin-reactive puncta in the dorsolateral prominence of the olfactory cortex may have relevance to the functional organization of the olfactory system. The distributions of biotin-reactive puncta were observed in laboratory-housed frogs and in wild toads captured in the summer months but were sparse or absent in batches of commercially obtained frogs examined immediately upon arrival in the laboratory. Systemic administration of biotin or biocytin hydrochloride did not alter the appearance or numbers of the biotin-reactive structures either in newly received or laboratory-housed frogs. These findings suggest that the capacity of the biotin-storage mechanism in the amphibian brain may be set by environmental factors and may be readily saturable from natural dietary or enteric sources.