Research reportLateral asymmetries in the trigeminal ganglion of the male rat
Introduction
It is well established that certain functions in humans are asymmetrically disposed in the brain, but the structural substrate of these asymmetries is poorly known. While macroscopic morphological asymmetries related to language and handedness have been repeatedly found in some cortical gyri and the corpus callosum [9], [15], [34], the existence of asymmetries in other regions or at other levels of analysis remains controversial [21], [33]. Less attention has been paid to functional asymmetries in non-primate mammals, perhaps with the exception of sensorimotor behaviors in rodents (see Ref. [6] for review). Moreover, the structural correlates of these asymmetries were practically ignored until recently, after a variety of neuroanatomical techniques introduced in the last two decades allowed the study of a whole new range of anatomical features in brain structures.
Such a multifactorial anatomical approach has been particularly rewarding in the trigeminal system. This system offers very attractive characteristics because of its importance in a wide variety of activities, including exploratory behavior and texture discrimination or learning, and its wide and easily discernible cortical representation. Recently, cortical whisker barrels were shown to be larger in the left hemisphere, but only in males, while the opposite was true in females [29]. Similar findings were reported using 2-deoxyglucose [7], although size differences did not reach statistical significance in this study. While such differences at the cortical level could underlie a better capacity of rats to learn a somesthetically cued foraging task using their right vibrissae, the contribution of subcortical structures to such behavioral asymmetries cannot be disregarded [19].
A logical first step in the search for side differences in the anatomy of subcortical sensory structures would be to analyze the trigeminal ganglion. This ganglion contains the first neurons of the somatosensory pathways arising from receptors and free nerve endings in the facial and oral tissues and part of the dura mater. Although not yet analyzed as extensively as the dorsal root ganglia (DRG), neurons in the trigeminal ganglia share many morphological features with DRG neurons. Both can be morphologically classified into two major subtypes (A and B) on the basis of body size and clumping pattern of Nissl substance [20], but further subdivisions are prompted by the heterogeneous expression of a variety of peptides, enzymes or transmitters [14], [23]. There have been a great number of morphometric studies on rats’ DRG, but only a few quantitative studies are available that report neuron numbers in the trigeminal ganglion, and these differ widely [1], [4], [8]. In the present study, we aimed at investigating both the neuron number and the cell body volume of neurons in the trigeminal ganglion of rats, searching for side and inter-animal variability, with the help of unbiased and efficient stereological techniques [10], [16], [31].
Section snippets
Animals and fixation
Eight 3-month-old, male Sprague–Dawley rats born from different dams, weighing 210–350 g, were used in this study. Under deep sodium pentobarbital anesthesia (30 mg/kg), the animals were perfused through the ascending aorta using a peristaltic pump. After a brief wash with saline at room temperature, 1000 ml of cold 4% paraformaldehyde in 0.1 M Na phosphate buffer (pH 7.4) were passed during 30 min. Trigeminal ganglia from both sides were exposed after removing the cranium and brain. Both
Cell number
The number of neurons in each of the trigeminal ganglia is shown in Table 1 and Fig. 2. The mean total number of neurons per ganglion was 35,300, with a moderate variability among ganglia (range: 25,800–43,000, CV=0.16). Variability was lower when both ganglia of each subject were pooled (mean number per rat: 70,600, range: 54,100–80,500, CV=0.14). On average, 66% were A-type and 34% were B-type. Although for individual cases there could be notable side differences in the number of neurons of
Discussion
The high specialization and wide representation of the rodent trigeminal system are two characteristics that have favored its use for research in different fields such as development, sensory systems, neural plasticity or learning. For the most part, however, the possibility that lateral asymmetries existed in this system has been neglected. Until recently it was accepted that, for any given strain, no significant side or inter-individual differences existed in the number and size of cortical
Acknowledgements
Thanks are due to Rosa Sánchez-Lozano for her help in preparing the celloidin-embedded material. This work was supported by Grant 08.5/0033/98 from the C.A.M.
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