The morphology of retinal ganglion cells projecting to the superior colliculus (SC) of the thirteen-lined ground squirrel (Spermophilus tridecemlineatus) was studied after retrogradely labeling the cells with cholera toxin subunit B. On the basis of previous reports, labeled cells were classified as small (6-10 microm in soma diameter), medium (11-14 microm), or large (>14 microm). A total of 3,427 cells were studied. Small cells constituted 78% of the population, 21% were medium cells, and only 1% were classified as large. The morphology of medium-sized cells was studied in more detail because large cells were few in number and the staining of the dendritic tree of small cells was not optimal. The best labeled medium-sized cells were classified on the basis of the shape and size of their dendritic tree and the pattern of dendritic ramification. Four types were identified among the medium-sized ganglion cells. Two types were classified as symmetric delta-like and asymmetric delta-like cells considering the relative symmetric or asymmetric distribution of their dendritic branches and their similarities with the delta type of the cat. Approximately 52% of all the medium-sized cells studied were symmetrical delta-like, and 19% were classified as asymmetrical delta-like. These cells were also very similar to the symmetrical and asymmetrical directionally selective ganglion cells described in rabbit retina. Other cells were termed beta-like. They had the smallest dendritic tree diameter, and their tree size seemed to be related to retinal eccentricity. Medium beta-like cells comprised approximately 21% of all cells projecting to the SC. The fourth type was termed "acute angle" because most of their dendritic branches were relatively straight and formed acute angles (10-45 degrees) at their branching points. These cells were few in number (approximately 8% of all medium-sized cells studied) and did not resemble any reported previously in cats. Thus, a variety of morphological types of retinal ganglion cells projected to the SC. Of these, the symmetrical and asymmetrical delta-like cells appeared to correspond to the directionally selective type described in the ground squirrel (Michael, C.R. [1968] J. Neurophysiol. 31:257-267) and reported in the rabbit retina.

The intergeniculate leaflet (IGL) and its neuropeptide Y (NPY) projection to the main circadian clock, the suprachiasmatic nucleus (SCN), have been the focus of extensive research conducted, for the most part, on nocturnal rodent species. However, a variety of anatomical and physiological differences between the circadian system of diurnal and nocturnal species have been reported. These differences led us to question whether the role of NPY in the circadian system of the diurnal ground squirrel differs from that in nocturnal rodents. We used semi-quantitative immunohistochemistry to analyze NPY content in SCN terminals of squirrels sacrificed at specific times of the day and compared the data to previous published results from the rat. Additionally, NPY mRNA was quantified using real-time PCR to determine if varying NPY-immunoreactivity (-ir) levels could be the result of changes in peptide transcription. Our results demonstrate that NPY-ir levels in the ground squirrel SCN peak during the middle of the night unlike what is observed in the rat. Cell counts of NPY-ir neurons in the IGL revealed a pattern of variation 6 hr out of phase compared to what was observed in the SCN. NPY mRNA levels showed only one sharp increase in the middle of the night, coinciding with increases in NPY-ir levels observed in the SCN. Differences in the pattern of fluctuation of NPY in the SCN between the rat and squirrel suggest that this peptide may serve distinct roles in the circadian system of diurnal and nocturnal species. Our data provide the first evidence of the relationship between transcript and peptide levels in the circadian system of a diurnal species.

Immunocytochemical techniques were employed to locate somatostatin (SS)-containing cells in the retina of the 13-lined ground squirrel (Spermophilus tridecemlineatus). In normal retinas immunostain was limited to neuronal processes, yet distinctly labeled somata were detected in retinas of animals pretreated with colchicine. Labeled cell bodies were located in the outermost and innermost portions of the inner nuclear layer (INL) and in the ganglion cell layer (GCL). The largest population of SS-like immunoreactive neurons was found in the innermost INL. These cells were identified as small and medium sized amacrine cells whose soma diameters ranged from 4 to 14 microns. A smaller population of immunoreactive cells was observed in the outermost region of the INL. These cells, presumptive horizontal cells, were found mainly in peripheral regions of the retina. Immunoreactive cells in the GCL were of two types: displaced amacrines, and retinal ganglion cells. SS-positive axons in the optic fiber layer suggest that some of the immunoreactive GCL neurons were ganglion cells, and it is our opinion that these cells belong to a class of associational ganglion cells previously identified in other species.

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Mucociliary activity is an important clearance mechanism in the respiratory system of air breathing vertebrates. Substance P (SP) and acetylcholine play a key role in the stimulation of the mucociliary transport in the frog palate. In this study, retrograde neuronal tracing was combined with immunocytochemistry for SP and choline acetyl transferase (ChAT) in the trigeminal ganglion and for neurokinin-1 receptor (NK1R) in the palate of Rana pipiens. The cells of origin of the palatine nerve were identified in the trigeminal ganglion using the retrograde tracer Fluorogold (FG). Optimal labeling of FG cells in the trigeminal ganglion was obtained at 96 h of exposure. Immunoflorescent shows that SP and acetylcholine are co-localized in 92% of the cells labeled with FG in the trigeminal ganglion. NK1 receptors were found in the membrane of epithelial and goblet cells of the palate. Ultrastructural study of the palate showed axonal-like endings with vesicles in connection with epithelial and goblet cells. These results further support the concerted action of both neurotransmitters in the regulation of mucociliary activity in the frog palate.