Neurotransmitter involvement in development and maintenance of the auditory space map in the guinea pig superior colliculus

The mammalian superior colliculus (SC) is a complex area of the midbrain in terms of anatomy, physiology, and neurochemistry. The SC bears representat ions of the major sensory modalites integrated with a motor output system. It is implicated with saccade generation, in behavioral responses to novel sensory stimuli and receives innervation from diverse regions of the brain using many neurotransmitter classes. Ethylene-vinyl acetate copolymer (Elva x-40W polymer) was used here to deliver chronically neurotransmitter recept or antagonists to the SC of the guinea pig to investigate the potential rol e played by the major neurotransmitter systems in the collicular representa tion of auditory space. Slices of polymer containing different drugs were i mplanted onto the SC of guinea pigs before the development of the SC azimut hal auditory space map, at similar to 20 days after birth (DAB). A further group of animals was exposed to aminophosphonopentanoic acid (AP5) at simil ar to 250 DAB. Azimuthal spatial tuning properties of deep layer multiunits of anesthetized guinea pigs were examined similar to 20 days after implant ation of the Elvax polymer. Broadband noise bursts were presented to the an imals under anechoic, free-field conditions. Neuronal responses were used t o construct polar plots representative of the auditory spatial multiunit re ceptive fields (MURFs). Animals exposed to control polymer could develop a map of auditory space in the SC comparable with that seen in unimplanted no rmal animals. Exposure of the SC of young animals to AP5, 6-cyano-7-nitroqu inoxaline-2,3-dione, or atropine, resulted in a reduction in the proportion of spatially tuned responses with an increase in the proportion of broadly tuned responses and a degradation in topographic order. Thus N-methyl-D-as partate (NMDA) and non-NMDA glutamate receptors and muscarinic acetylcholin e receptors appear to play vital roles in the development of the SC auditor y space map. A group of animals exposed to AP5 beginning at similar to 250 DAB produced results very similar to those obtained in the young group expo sed to AP5. Thus NMDA glutamate receptors also seem to be involved in the m aintenance of the SC representation of auditory space in the adult guinea p ig. Exposure of the SC of young guinea pigs to gamma-aminobutyric acid (GAB A) receptor blocking agents produced some but not total disruption of the s patial tuning of auditory MURFs. Receptive fields were large compared with controls, but a significant degree of topographical organization was mainta ined. GABA receptors may play a role in the development of fine tuning and sharpening of auditory spatial responses in the SC but not necessarily in t he generation of topographical order of the these responses.