Kevin Cox

Parvalbumin-lmmunoreactive Neurons in the Neocortex are Resistant to Degeneration in Alzheimer's Disease

Recent studies have stressed the fact that specific neuronal subtypes may display a differential sensitivity to degeneration in Alzheimers disease. For example, large pyramidal neurons have been shown to be vulnerable, whereas smaller neurons are resistant to pathology. Using a monoclonal antibody against the calcium-binding protein parvalbumin, we investigated the possible changes in a subpopulation of interneurons in two cortical areas known to be strongly damaged in Alzheimers disease. In the prefrontal cortex as well as in the inferior temporal cortex, we observed no differences in parvalbumin-immunoreactive cell counts or cell size in Alzheimers disease brains as compared to control cases. Moreover, the general cellular morphology of these neurons was preserved in the Alzheimers disease cases, in that their perikarya and dendritic arborizations were intact. These results suggest that paravalbumin-immunoreactive cells represent a neuronal subset resistant to degeneration, and further support the hypothesis that the pathological process in Alzheimers disease involves specific neuronal subtypes with particular morphological and molecular characteristics.

Two distinct populations of tectal neurons have unique connections within the retinotectorotundal pathway of the pigeon (Columba livia)

The tectofugal pathway is a massive ascending polysynaptic pathway from the tectum to the thalamus and then to the telencephalon. In birds, the initial component of this pathway is known as the tectorotundal pathway; in mammals, it is known as the tectopulvinar pathway. The avian tectorotundal pathway is highly developed; thus, it provides a particularly appropriate model for exploring the fundamental properties of this system in all amniotes. To further define the connectivity of the tectorotundal projections of the tectofugal pathway, we injected cholera toxin B fragment into various rotundal divisions, the tectobulbar projection, and the ventral supraoptic decussation of the pigeon. We found intense bilateral retrograde labeling of neurons that stratified within layer 13 and, in certain cases, granular staining in layer 5b of the optic tectum. Based on these results, we propose that there are two distinct types of layer 13 neurons that project to the rotundus: 1) type I neurons, which are found in the outer sublamina of layer 13 (closer to layer 12) and which project to the anterior and centralis rotundal divisions, and 2) type II neurons, which are found in the inner sublamina of layer 13 (closer to layer 14) and which project to the posterior and triangularis rotundal divisions. Only the labeling of type I neurons produced the granular dendritic staining in layer 5b. An additional type of tectal neuron was also found that projected to the tectobulbar system. We then injected Phaseolus vulgaris‐leucoagglutinin in the optic tract and found that the retinal axons terminating within tectal layer 5b formed narrow radial arbors (7–10 μm in diameter) that were confined to layer 5b. Based on these results, we propose that these axons are derived from a population of small retinal ganglion cells (4.5–6.0 μm in diameter) that terminate on the distal dendrites of type I neurons.

Bottlebrush dendritic endings and large dendritic fields: Motion‐detecting neurons in the tectofugal pathway

In avian and mammalian brains, visual information from the retina is conveyed to the telencephalon via two separate pathways: the thalamofugal and the tectofugal pathways. Recently, Karten et al. ([1997] J. Comp. Neurol. 387:449–465) examined a portion of the tectofugal pathway, the projection from the optic tectum to the nucleus rotundus thalami, in pigeons. They defined two distinct subpopulations of tectal neurons projecting from the stratum griseum centrale (SGC; tectal layer 13) to specific divisions of the rotundus. The goal of this study in chick was to verify the existence of the type I and type II SGC neurons, as defined by Karten et al., and then examine in greater detail the connectivity and morphology of these SGC neurons. Furthermore, our results suggest how the unique morphological characteristics of SGC neurons contribute to the large receptive fields (20–50°) found in physiological recordings and the SGC neuronal response to extremely small (ca. 0.05°), fast‐moving (100°/second) stimuli.

GABAergic inputs to the nucleus rotundus (pulvinar inferior) of the pigeon (Columba livia)

The avian nucleus rotundus, a nucleus that appears to be homologous to the inferior/caudal pulvinar of mammals, is the major target of an ascending retino‐tecto‐thalamic pathway. Further clarification of the inputs to the rotundus and their functional properties will contribute to our understanding of the fundamental role of the ascending tectal inputs to the telencephalon in all vertebrates, including mammals.

Cholera toxin mapping of retinal projections in pigeons ( Columba livia ), with emphasis on retinohypothalamic connections

Anterograde transport of cholera toxin subunit B (CTb) was used to study the retinal projections in birds, with an emphasis on retinohypothalamic connections. Pigeons (Columbia livia) were deeply anesthetized and received unilateral intraocular injections of CTb. In addition to known contralateral retinorecipient regions, CTb-immunoreactive fibers and presumptive terminals were found in several ipsilateral regions, such as the nucleus of the basal optic root, ventral lateral geniculate nucleus, intergeniculate leaflet, nucleus lateralis anterior, area pretectalis, and nucleus pretectalis diffusus. In the hypothalamus, CTb-immunoreactive fibers were observed in at least two contralateral cell groups, a medial hypothalamic retinorecipient nucleus, and a lateral hypothalamic retinorecipient nucleus. To compare retinorecipient hypothalamic nuclei in pigeons with the mammalian suprachiasmatic nucleus, double-label experiments were conducted to study the existence of neurophysin-like immunoreactivity in the retinorecipient avian hypothalamus. The results showed that only cell bodies in the medial hypothalamic nucleus contained neurophysin-like immunoreactivity. The results demonstrate CTb to be a sensitive anterograde tracer and provide further anatomical information on the avian equivalent of the mammalian suprachiasmatic nucleus.

Depletion and recovery of the calcium-binding proteins calbindin and parvalbumin in the pigeon optic tectum following retinal lesions ☆

Retinal lesions in pigeons produced a marked depletion of somata and neuropil staining for both calbindin-D28k and parvalbumin immunoreactivities in the contralateral optic tectum. Calbindin-like immunoreactivity reappeared in some tectal layers by 6 weeks postlesion, whereas paravalbumin-like immunoreactivity recovered almost completely after 5 weeks. These data indicate that the retinal input may control the expression of calbindin and parvalbumin in the pigeon optic tectum.