Displaced amacrine cells in the ganglion cell layer of the ground squirrel retina.

In some mammals a large portion of the retinal neurons of the ganglion cell layer are not ganglion cells. These neurons, lacking axons which pass to the brain via the optic nerve, are termed displaced amacrine cells. The present study assessed the number of displaced amacrine cells in the thirteen-lined ground squirrel (Spermophilus tridecemlineatus). We compared the number of labeled cells in the ganglion cell layer after HRP injection of optic tracts and target nuclei with the total number of neurons in the ganglion cell layer. We conclude that approximately one half of the neurons in the ganglion cell layer are displaced amacrine cells, the other one half are ganglion cells. The displaced amacrine cells are on the average smaller than the ganglion cells. Our results provide a rationale for renewed study of relation of ganglion cell morphology and physiological functional type in this species.

Morphology and distribution of dopaminergic neurons in the ground squirrel retina.

Tyrosine hydroxylase (TH), the rate limiting enzyme in the conversion of tyrosine to DOPA, is a reliable marker for catecholaminergic (dopaminergic) neurons. To investigate the distribution of dopamine in the retina of the thirteen-lined ground squirrel (Spermophilus tridecemlineatus), retinal sections and wholemounts were incubated with an antiserum directed against TH and then processed using the avidin-biotin immunohistochemical method. TH-like immunoreactivity was exhibited by amacrine and interplexiform-like cells in the innermost portion of the inner nuclear layer (INL) and by cells we presume to be displaced amacrines in the ganglion cell layer (GCL). Their somata were 12 to 20 microns in diameter, with the majority measuring approximately 18 microns. In transverse sections the processes of the three types of neurons were seen to extend into lamina 1 of the inner plexiform layer (IPL). In horizontal sections 2-3 primary dendrites were seen to ramify and the branches extended for considerable distances, with overlap between the dendritic fields of neighboring TH cells. A distance to the nearest neighbor analysis suggests the TH-neurons in the INL are distributed in a non-random fashion.

Dopaminergic neurons in the cone-dominated ground squirrel retina: a light and electron microscopy study.

Tyrosine hydroxylase (TH), the rate limiting enzyme in the conversion of tyrosine to DOPA, is a reliable marker for catecholaminergic (dopaminergic) neurons. To investigate the distribution of dopamine in the cone-dominated retina of the thirteen-lined ground squirrel (Spermophilus tridecemlineatus), retinal sections and wholemounts were incubated with an antiserum directed against TH and then processed using immunofluorescence and the avidin-biotin immunohistochemical method. TH-like immunoreactivity was exhibited by amacrine and interplexiform-like cells in the innermost portion of the inner nuclear layer (INL) and by cells we presume to be displaced amacrines in the ganglion cell layer (GCL). Their somata were 12 to 28 microns in diameter, with the majority measuring approximately 18 microns. In transverse sections the processes of the three types of neurons were seen to extend into lamina 1 of the inner plexiform layer (IPL). In horizontal sections 2-3 primary dendrites were seen to ramify and the branches extended for considerable distances, with overlap between the dendritic fields of neighboring TH-cells. A distance to the nearest neighbor analysis suggested the TH-neurons in the INL are distributed in a non-random fashion. The mean overall density of labeled amacrines is 15 cells/mm2, low when compared to the mean density reported in other species. EM results indicate that TH-labeled amacrines make synaptic contacts with unlabeled amacrines, although the possibility that they may contact ganglion cells can not be ruled out.

Localization of GAD- and GABA-like immunoreactivity in ground squirrel retina: retrograde labeling demonstrates GAD-positive ganglion cells.

Glutamic acid decarboxylase (GAD)- and gamma-aminobutyric acid (GABA)-like immunoreactivity was examined in the retina of the 13-lined ground squirrel (Spermophilus tridecemlineatus). Labeling was observed in the inner nuclear layer (INL), inner plexiform layer (IPL) and ganglion cell layer (GCL). The immunoreactive cell bodies in the inner third of the INL were 6-13 microns in diameter and, because of their size and location it was considered that these were amacrine cells. Labeling in the IPL was concentrated in 5 bands corresponding to laminae 1a, 1c, 2, 4 and 5. In the GCL a heterogeneous population of neurons exhibited GAD- and GABA-like immunoreactivity. The soma diameters of the GCL cells ranged from 5 to 17 microns. These may represent displaced amacrines and/or ganglion cells. To determine if any of the immunoreactive cells in the GCL were ganglion cells, double labeling experiments were performed using rhodamine latex microspheres ('beads') as retrograde neuronal tracers. Rhodamine beads were injected into the superior colliculus, and retinas with retrogradely labeled ganglion cells were subsequently incubated with the anti-GAD antiserum. These experiments revealed a small population of GAD-positive ganglion cells, setting a lower limit for the total number of GABAergic ganglion cells.

Morphology of ganglion cells which project to the dorsal lateral geniculate and superior colliculus in the ground squirrel.

We wished to determine whether retinal ganglion cells that have axons terminating in the dorsal lateral geniculate and/or the superior colliculus have specific sizes of somata, comprising only part of the entire size range of ganglion cell somata. If so, then perhaps the specific functional types described by Michael might be associated with morphological types based on soma size. HRP was injected into either the superior colliculus (SC) or dorsal lateral geniculate nucleus (LGd) of thirteen-lined ground squirrels. Soma diameter of labeled ganglion cells was measured and the relation between cell size and frequency determined. After SC injections HRP-filled cells were mostly small and medium-sized. They ranged in diameter from 3 to 14 microns and the mean diameter of labeled neurons was 7.35 microns. Cells labeled after SC injections were often distributed as doublets or triplets in the retina. After LGD injections the majority of labeled cells were medium and large-sized. They ranged from 4 to 18 microns in diameter with a mean of 9.1 microns and were more regularly spaced within the retinal region of labeled cells. Thus, the present results provide reason to believe that functional classes of ganglion cells in ground squirrels may be correlated with particular morphological types.

Thalamic connections of the ground squirrel superior colliculus and their topographic relations.

The connections of the superior colliculus (SC) of the ground squirrel Spermophilus tridecemlineatus were studied with the horseradish peroxidase (HRP) method. Multiple pressure injections of HRP served to define the total pattern of SC projections while iontophoretic injections allowed differentiation of connections of the deep and superficial layers and determination of topographic relations of SC with its associated nuclei. The deep laminae were mainly connected with auditory, somatosensory and reticular regions of the brain, including the inferior colliculus, zona incerta, substantia nigra, mesencephalic central grey, pontine nuclei, spinal trigeminal nucleus, nucleus of the posterior commissure, thalamic reticular nucleus, raphe nuclei, lateral vestibular nucleus, the lateral superficial reticular formation of the medulla, the mesencephalic reticular formation, nucleus gracilis and the cervical spinal cord. The superficial laminae were connected with visual system structures. They were reciprocally connected with the dorsal and ventral lateral geniculate nuclei, the pretectum, nucleus lateralis posterior (LP), the parabigeminal nucleus and the contralateral SC. Connections between the SC and the dorsal lateral geniculate were topologic. LP was found to consist of three divisions: rostrolateral, rostromedial and caudal. SC was interconnected with the rostrolateral and caudal divisions. The connections between the SC and the rostrolateral division were topologic; those with the caudal division were not. The connections of the deep collicular layers in ground squirrels were similar to those which have been reported for cats and monkeys. The connections of the superficial laminae were more extensive than has been reported in other species. These elaborate interconnections indicate extensive interaction between primary retinal projection nuclei in the processing of visual information.

Superior colliculus efferents to five subcortical visual system structures in the ground squirrel.

We compared the laminar location and morphology of superior colliculus cells projecting to the dorsal and ventral lateral geniculate nuclei (LGd, LGv), the pretectum (PT), the parabigeminal nucleus (Pb), and nucleus lateralis posterior (LP) in the ground squirrel Spermophilus tridecemlineatus. Horseradish peroxidase was iontophoretically injected into LGd, LGv, PT, Pb and each of the 3 subdivisions of the LP. After survival periods of 24-72 h the animals were perfused intracardially and brain sections processed histochemically. A Zeiss ZIDAS image analysis system was used to determine the soma size of labeled neurons and to prepare histograms showing the relation between cell size and frequency. After injections in the LGd, LGv, Pb and PT, labeled neurons were present throughout the stratum griseum superficiale and the upper portion of the stratum opticum. They were mainly fusiform neurons whose long axes ranged from 12 to 44 microns. There were also some multipolar cells 9-22 microns in diameter with the highest frequencies found in the 12-14 and 16-17 microns ranges. Differences were found in the exact location and/or soma size of the neurons projecting to the 4 nuclei. After injections in rostrolateral and caudal LP the labeled cells were always large multipolar neurons specifically located in the lower half of the stratum griseum superficiale. Their somata measured 9-22 microns in diameter but the highest frequencies were found in the 16-17 and 19-20 microns ranges. Our findings suggest that there are different populations of superior colliculus cells projecting to different visual system structures.