EVOLUTION OF GABAERGIC CIRCUITRY IN THE MAMMALIAN MEDIAL GENICULATE-BODY

Many features in the mammalian sensory thalamus, such as the types of neurons, their connections, or their neurotransmitters, are conserved in evolution, We found a wide range in the proportion of gamma-aminobu tyric acidergic (GABAergic) neurons in the-medial geniculate body, fro m <1% (bat and rat) to 25% or more (cat and monkey), In the bat, some medial geniculate body subdivisions have no GABAergic cells. Species-s pecific variation also occurs in the somesthetic ventrobasal complex, In contrast, the lateral geniculate body of the visual system has abou t the same proportion of GABAergic cells in many species, In the centr al auditory pathway, only the medial geniculate body shows this arrang ement; the relative number of GABAergic cells in the inferior collicul us and auditory cortex is similar in each species, The range in the pr oportion of GABAergic neurons suggests that there are comparative diff erences in the neural circuitry for thalamic inhibition, We conclude t hat the number of GABAergic neurons in thalamic sensory nuclei map hav e evolved independently or divergently in phylogeny, Perhaps these ada ptations reflect neurobehavioral requirements for more complex, less s tereotyped processing, as in speech-like communication.