The ultrastructure of the central nucleus of the inferior colliculus of the Sprague-Dawley rat.

Previous studies from this laboratory have indicated that increased numbers of GABAergic neurons, as well as total neurons, occur in the central nucleus of the inferior colliculus (IC) of genetically epilepsy-prone rats (GEPRs) as compared to non-seizuring Sprague-Dawley rats. Since electron microscopic studies of the IC have not been reported for rats, we wanted to determine the ultrastructure of neurons and their processes in this brain region to serve as a basis for future studies on neuronal circuitry in the GEPRs. Both disc-shaped and stellate types were found for each of three size categories: large, medium and small. Thus, six types of neuron were distinguished by differences in somatic size, shape, organelles and dendritic orientation. Large neurons (longest diameter greater than 25 micron), which are the least frequent cell type, contained vast perikaryal cytoplasm, eccentrically located nuclei and abundant granular endoplasmic reticulum (GER) adjacent to the nucleus as well as clustered in the cytoplasm; many axosomatic, symmetric synapses were present. Medium-sized neuronal somata (15-25 micron in diameter) had smooth as well as infolded nuclear membranes and clusters of GER in their cytoplasm but no GER adjacent to the nucleus; synapses were sparse along the surface of their somata. Small neurons (10-15 micron in diameter), which are the most frequent cell type, had scant perikaryal cytoplasm, usually infolded nuclei, frequently two nucleoli, and few or no stacked cisternae of GER in the perikaryal cytoplasm; only infrequent axosomatic synapses were found. Based on previous retrograde and immunocytochemical studies, most large disc-shaped and stellate cells project to the medial geniculate body and are probably excitatory, but some large stellate neurons have been shown to be GABAergic and it is doubtful that such neurons participate in this projection. A dense plexus of terminals that form symmetric synapses covers the soma and proximal dendrites of large neurons, and may provide a strong GABAergic inhibition of this type of projection neuron. Small and medium-sized disc-shaped cells also project to the thalamus but they lack this dense axosomatic plexus. The stellate cells from these same two size categories probably do not project to the thalamus and may be GABAergic local circuit neurons. Other ultrastructural features of IC neurons that were analysed include dendrites, dendritic spines, axon hillocks, initial segments and terminals, as well as the laminae of myelinated axons. Dendrites were either beaded or smooth and few spines were observed.(ABSTRACT TRUNCATED AT 400 WORDS

An electron microscopic study of GABAergic neurons and terminals in the central nucleus of the inferior colliculus of the rat.

Neurons and terminals in the ventral lateral portion of the central nucleus of the inferior colliculus (ICCN) of the rat were labelled immunocytochemically with antisera to GABA or to its synthesizing enzyme, glutamic acid decarboxylase. Four types of GABAergic neuron are described: small, medium-sized and large multipolar neurons, as well as medium-sized bipolar neurons. All sizes of GABAergic multipolar neurons are characterized by highly infolded nuclei, many mitochondria and both asymmetric and symmetric axosomatic synapses. A dense plexus of terminals occurs on the proximal dendrites of GABAergic neurons, and most of these terminals form asymmetric axodendritic contacts. Small GABAergic neurons (diameter less than 15 microns) are multipolar, and have a large nucleus to cytoplasm ratio, prominent nucleoli and usually two to five axosomatic synapses per thin section, with the majority of these contacts being symmetric. Medium-sized GABAergic neurons (15-25 microns in diameter) display both multipolar and fusiform shaped somata, have a more abundant cytoplasm than the small neurons and show about ten axosomatic contacts per thin section. Large GABAergic neurons (diameters greater than 25 microns) have eccentrically located, highly infolded nuclei, abundant cytoplasm and a denser plexus of terminals that form axosomatic synapses than the other cell types. These results indicate that four of the six major cell types in the ICCN are probably GABAergic inhibitory neurons. The axon initial segments of GABAergic neurons in the ICCN all have similar features in that they are contacted by only one or two terminals that form symmetric synapses on their proximal portions and are invested by a glial sheath from 3 to 20 microns from the cell body. Many immunoreactive myelinated axons (approximately 0.5 micron in diameter) are observed and some terminals that arise from these axons form synapses with small neuronal somata. Both these and other labelled terminals are shown to form symmetric synapses. These data suggest a complex circuitry for the GABAergic neurons within the ICCN

The ultrastructure of the central nucleus of the inferior colliculus of the genetically epilepsy-prone rat.

The inferior colliculus of the genetically epilepsy-prone rat (GEPR) was examined at the ultrastructural level to determine if any abnormalities exist in the inferior colliculus of the GEPR as compared to the non-epileptic Sprague-Dawley rat. Both routine electron microscopic preparations and glutamate decarboxylase (GAD) and GABA immunocytochemical preparations were examined in the GEPR and compared to previous studies from this laboratory that described the normal ultrastructure of the Sprague-Dawley rat. Cell counts from 2 micron semi-thin sections confirmed our previous observations that showed a large, significant increase in the number of neurons in the inferior colliculus of the GEPR as compared to the Sprague-Dawley rat. Many of the small neurons in the inferior colliculus of the GEPR were found to be smaller than those in the inferior colliculus of the Sprague-Dawley rat. Moreover, the small neurons in the GEPR were frequently clumped in clusters of 3-5. Several ultrastructural abnormalities present in the inferior colliculus of the GEPR have been observed at epileptic foci or in brain regions along the pathway of seizure spread in other experimental models of epilepsy. These changes included the presence of dendrites which are almost completely devoid of organelles, hypertrophy of glial processes, and terminals that contain either swollen vesicles or very few vesicles. Other features that were frequently observed in the GEPR but were rarely found in preparations of Sprague-Dawley rats included an abundance of extra membranes, whorl bodies and multivesicular bodies within somata, dendrites and axons.(ABSTRACT TRUNCATED AT 250 WORDS

GABAergic nerve terminals decrease in the substantia nigra following hemitransections of the striatonigral and pallidonigral pathways.

Glutamic acid decarboxylase (GAD), the enzyme that synthesizes the neurotransmitter, GABA, was immunocytochemically localized in axon terminals as well as in small and medium-sized neurons of the rat substantia nigra. The pattern formed by GAD-containing axon terminals with the dendrites and somata of neurons in the substantia nigra was altered following ipsilateral hemitransections of the striatonigral and pallidonigral pathways. A marked reduction of GAD-positive terminals occurred throughout this brain region, but the ventral fifth of the pars reticulata showed a nearly normal pattern of GAD-positive axon terminals. The results of this investigation are consistent with results from biochemical studies which have indicated that the striatonigral and/or pallidonigral pathways are GABAergic. In addition, these results suggest that the residual GABAergic terminals remaining after hemitransection are derived from intrinsic neurons of the substantia nigra

Increased numbers of neurons occur in the inferior colliculus of the young genetically epilepsy-prone rat

To determine if the increase in the number of neurons observed in the inferior colliculus (IC) of the adult genetically epilepsy-prone rat (GEPR) as compared to the Sprague-Dawley rat was present in the young GEPRs prior to the time at which seizure activity commences, brains from both types of rats 4-10 days of age were studied. A statistically significant increase in the numbers of small neurons occurred in the IC of the young GEPR. At 4 days of age, a 55% increase in the number of small neurons was found in the GEPR as compared to the Sprague-Dawley rat and at 10 days of age this increase was 105%. The numbers of the medium and large neurons were similar in the older group of rats. These data suggest that the increase in cell number observed in the adult GEPR is not compensatory to the seizure activity, but is genetically programmed. © 1985

Increased numbers of GABAergic neurons occur in the inferior colliculus of an audiogenic model of genetic epilepsy.

The numbers of GABAergic neurons as determined by GAD immunocytochemistry and total neurons as determined from Nissl preparations were counted and classified at the light microscopic level in the inferior colliculus (IC) of the genetically epilepsy-prone rat (GEPR) and the non-epileptic Sprague-Dawley (SD) strain of rat. GAD-positive neurons are abundant in the IC in all 3 subdivisions. Several sizes of multipolar neurons as well as medium-sized bipolar or fusiform neurons are GAD-positive. GAD-positive punctate structures that were interpreted to be axon terminals and transversely-sectioned dendrites and preterminal axons are abundant in the IC of both the GEPR and SD. A dramatic increase in the number of GAD-positive neurons occurs in the GEPR as compared to the SD. This increase is most evident in the middle of the rostrocaudal extent of the IC. Although the increase is statistically significant in all subdivisions of the IC, it is most pronounced in the central nucleus, especially the ventral lateral portion. Within the central nucleus, the increase in the number of GAD-positive neurons is due to a selective increase in the small (200%) and medium (90%) cell body size populations (10-15 micron and 15-25 micron in diameter, respectively). Concomitant with this increase in the number of GAD-positive neurons, an increase in total numbers of neurons occurs as determined from Nissl preparations. A 100% increase in the number of small neurons and a 30% increase in the number of medium-sized neurons occur in the GEPR as compared to the SD rat. The proportion of GAD-positive neurons to total neurons is also increased in the GEPR. Approximately 25% of the neurons in the IC in SD rat are GAD-positive, while about 35% of the neurons in the GEPR are GAD-positive. These data demonstrate an anatomical difference in the IC of the GEPR as compared to the SD which appears to be preferential for the GABAergic system

Anatomical and behavioral analyses of the inheritance of audiogenic seizures in the progeny of genetically epilepsy-prone and Sprague-Dawley rats.

Our previous studies have shown an increase in the number of GABAergic and total neurons in the inferior colliculus (IC) of the genetically epilepsy-prone rat (GEPR-9) as compared to the non-seizing Sprague-Dawley (SD) rat. To determine whether an increase in neuron number in the IC is genetically associated with seizure behavior, seizing and non-seizing offspring of GEPR-9 and SD progenitor strains were studied as well as offspring from backcrosses made with F1 and either GEPR-9 or SD rats. In addition, the ontogeny of seizure behavior was studied in seizing rats from these same backgrounds. The development of seizure behavior in GEPR-9s was shown to be dependent on age and the number of exposures to sound stimulus up until the age of 9 weeks. The F1 and F2 generations displayed different audiogenic seizure profiles than those of the two progenitor strains. In the F1 generation, the ratio of seizing to non-seizing rats was always greater than 3:1, and the distribution of seizure scores was similar for males and females. In addition, the off-spring from backcrosses made with F1 rats (high or low seizing) and GEPR-9s displayed maximal audiogenic response scores (ARS) of 9, a characteristic of the GEPR-9s used in this study. The results of these genetic studies indicate a polygenetic inheritance of this autosomal dominant trait of audiogenic seizure susceptibility. For the quantitative study of neuronal density in the IC, neurons were counted from cresyl violet-stained preparations from seizing and non-seizing F1 and F2 rats, backcrosses from different categories and age-matched SD rats. Statistically significant increases in the number of both small (70% increase) and medium-sized (14% increase) neurons occurred in the high seizing animals (ARS = 7-9) as compared to either the non-seizing F2 or SD rats. In addition, a significant increase in the number of small neurons (77% increase) occurred in the high seizing offspring of the F1 X GEPR-9 backcross as compared to that of the non-seizing offspring of the F1 X SD backcross. The data from 25 rats generated a 0.9 coefficient of linear correlation between ARS and the number of small neurons. The results from the anatomical studies suggest that the inheritance of audiogenic seizures appears to be closely linked to the increase in cell number. Therefore, the increase in cell number in the IC may be an important determinant of seizure behavior for GEPR-9s