ABSTRACT

 The information on relative acuity of visual and auditory sense in neonate and juvenile African giant rats (AGR) (Cricetomys gambianus) is lacking. This research was designed to evaluate the postnatal development of visual and auditory brain centres in AGR. Thirty-six captive AGR consisting of 12 neonates, juveniles and adults were obtained alive from Uturu villages in Isuikwuato Local Government Area, Abia state, Nigeria. The live body weights were obtained. Each animal was perfused with 4% paraformaldehyde fixative, decapitated at atlanto-axial joint, and head fixed in 10% formalin for 3 days. The intact brain was extracted and weighed. Gross anatomical studies of rostral and caudal colliculi as well as medulla oblongata were done. Cytoarchitecture of visual neural substrates: CN III, CN IV, CN VI nuclei and auditory neural substrates: medial geniculate, superior olivary complex, medial trapezoid, dorsal trapezoid nuclei were also studied. The mean values for live body weight were 58.71±2.29, 375.00±23.43 and 855.83±42.91; nose-rump length 13.55±0.75, 24.95±0.76 and 31.01±0.57; Tail length 12.52±1.36, 29.07±1.65 and 37.67±1.07; Rostral colliculus height 0.55±0.06, 0.36±0.33 and 0.41±0.03; Caudal colliculus height 0.72±0.06, 0.52±0.04 and 0.47±0.03; Rostral colliculus width 0.71±0.02, 0.70±0.04 and 0.72±0.05; Caudal colliculus width 0.70±0.01; 0.76±0.02 and 0.83±0.02; Midbrain weight 0.32±0.02, 0.57±0.05 and 0.54±0.04; Medulla oblongata weight 0.16±0.01, 0.40±0.03 and 0.41±0.02; Rostral colliculus weight 0.07±0.00, 0.12±0.01 and 0.14±0.01; Caudal colliculus weight 0.15±0.05, 0.15±0.01 and 0.17±0.01 for neonates, juveniles and adults respectively. There was a positive linear relationship between nose–rump length and caudal colliculus height in neonates (R2 = 0.60; p < .05) with a regression formula deduced as y = 1.492 – 0.057x; positive linear relationship between nose–rump length and rostral colliculus height in neonates (R2 = 0.60; p < .05) with a regression formula deduced as y = 1.461 – 0.067x; width of caudal and rostral colliculi of neonates were not statistically significant (P > 0.05). In juveniles, width of caudal colliculus was statistically wider than rostral colliculus width just as in adults (P < 0.05); height of caudal colliculus was statistically higher than rostral colliculus height in each of neonates, juveniles and adults; caudal colliculus was statistically heavier than rostral colliculus in each of neonates, juveniles and adults. There were poor, improved and dominant positions of acoustic structures in neonates, juveniles and adults of AGR respectively, with their visual structures being poorly developed, though with those of adults being better than the other two age groups. Bushy cells with neuronal axons were absent in neonates, present but with short axons in juveniles and well developed in adults. The inferior olivary nucleus was marked with radial glial cells in neonates, formed neurones in juveniles and well delineated in adults. The results of this study suggest that auditory sense is more acute than visual sense in adult AGR. Furthermore, it has established that adults have better acuity of the senses than juveniles and neonates, thus, should be preferred for scientific research of vision and audition.