Identification of subdivisions in the medial geniculate body of the guinea pig

• Published in: Hearing research 2007-06, Vol.228 (1), p.156-167
• Main Authors: Anderson, L.A., Wallace, M.N., Palmer, A.R.
• Format: Article
• Language: English
• Subjects: Acetylcholinesterase; Acetylcholinesterase - analysis; Acoustic Stimulation; Animals; Auditory Pathways - physiology; Auditory thalamus; Biological and medical sciences; Brain Mapping - methods; Characteristic frequency; Cytochrome oxidase; Drug toxicity and drugs side effects treatment; Ear and associated structures. Auditory pathways and centers. Hearing. Vocal organ. Phonation. Sound production. Echolocation; Electron Transport Complex IV - analysis; Evoked Potentials, Auditory; Female; Frequency response area; Fundamental and applied biological sciences. Psychology; Geniculate Bodies - cytology; Geniculate Bodies - enzymology; Geniculate Bodies - physiology; Guinea Pigs; Image Processing, Computer-Assisted; Immunohistochemistry - methods; Male; Medical sciences; Neural Conduction; Neurons - enzymology; Neurons - physiology; Neuropil - enzymology; Pharmacology. Drug treatments; Reproducibility of Results; Toxicity: respiratory system, ent, stomatology; Vertebrates: nervous system and sense organs
• ISSN: 0378-5955; 1878-5891
• DOI: 10.1016/j.heares.2007.02.005

The accurate and reliable identification of subdivisions within the auditory thalamus is important for future studies of this nucleus. However, in the guinea pig, there has been no agreement on the number or nomenclature of subdivisions within the main nucleus of the auditory thalamus, the medial geniculate body (MGB). Thus, we assessed three staining methods in the guinea pig MGB and concluded that cytochrome oxidase (CYO) histochemistry provides a clear and reliable method for defining MGB subdivisions. By combining CYO with acetylcholinesterase staining and extensive physiological mapping we defined five separate divisions, all of which respond to auditory stimuli. Coronal sections stained for CYO revealed a moderate to darkly-stained oval core. This area (the ventral MGB) contained a high proportion (61%) of V-shaped tuning curves and a tonotopic organisation of characteristic frequencies. It was surrounded by four smaller areas that contained darkly stained somata but had a paler neuropil. These areas, the dorsolateral and suprageniculate (which together form the dorsal MGB), the medial MGB and the shell MGB, did not have any discernable tonotopic frequency gradient and contained a smaller proportion of V-shaped tuning curves. This suggests that CYO permits the identification of core and belt areas within the guinea pig MGB.