Anatomical and physiological properties of the superior paraolivary nucleus in the rat

The superior paraolivary nucleus (SPON) is a group of neurons located within the superior olivary complex, a constellation of brainstem nuclei involved in auditory processing. The major inputs to the SPON arise from the contralateral ear and SPON axons target primarily the ipsilateral inferior colliculus. However, little is known regarding the neurochemical phenotypes present in the SPON and how these neurons respond to auditory stimuli. Understanding the neurochemical and physiological properties of the constituent neurons will provide insight into the functional role of the SPON and will contribute to our understanding of the neural circuitry involved in hearing. Immunocytochemical, stereological, physiological and pharmacological techniques were used to characterize SPON neurons in the rat. The presence of inhibitory neurotransmitters was investigated with immunocytochemistry and provides evidence that the SPON contains a morphologically homogeneous population of GABAergic neurons and further that these neurons receive a robust inhibitory innervation in the form of glycinergic and GABAergic inputs. Stereological estimates of total neuron number in eighteen subcortical auditory nuclei provide evidence that the SPON is a prominent brainstem cell group and a major source of ascending inhibition to the inferior colliculus. Extracellular in vivo recordings provide evidence that nearly all SPON neurons respond to sound played in the contralateral ear with spike activity timed to the stimulus offset and phase lock to amplitude modulations in complex sounds. Pharmacologically blocking glycinergic input abolished the offset response (indicating that offset activity is a rebound from glycinergic inhibition); blockade of glycinergic and GABAergic input simultaneously, resulted in broader receptive fields and reduced phase locking capabilities. Taken together, these data indicate the rat SPON is a prominent auditory cell group that provides GABAergic inhibition to the ipsilateral inferior colliculus indicating the sound offset. GABAergic inhibition has been implicated in numerous aspects of auditory physiology, including sound localization and sensitivity to stimulus duration. Therefore, the SPON plays an important role in auditory processing and offset inhibition may be involved in processing complex sounds and in creating sensitivity to stimulus duration, both important features of animal and human communication.