Afferent regulation of inhibitory synaptic transmission in the developing auditory midbrain

To determine whether afferent innervation regulates the strength of inhibit ory connections in the gerbil auditory midbrain, both cochleas were surgica lly removed in postnatal day 7 animals, before sound-driven activity is fir st observed. Inhibitory synaptic currents were measured in a brain slice pr eparation 1-7 d after the ablations. Whole-cell and gramicidin-perforated p atch recordings were obtained from inferior colliculus neurons, and IPSCs w ere evoked by stimulation of the commissure of the inferior colliculus (CIC ) or the ipsilateral lateral lemniscus (LL) in the presence of kynurenic ac id. Deafferentation led to a 24 mV depolarizing shift in the IPSC equilibri um potential within 1 d of deafferentation. As a consequence, there was a l arge reduction of IPSC amplitude at a holding potential of -20 mV in neuron s from bilaterally ablated animals. Furthermore, both afferent pathways dis played a 50% reduction of the inhibitory synaptic conductance after deaffer entation, indicating that driving force was not solely responsible for the decline in IPSC amplitude. When paired pulses were delivered to the LL or C IC pathway in control neurons, the evoked IPSCs exhibited facilitation. How ever, paired pulse facilitation was nearly eliminated after deafferentation . Thus, normal innervation affects inhibitory synaptic strength by regulati ng postsynaptic chloride homeostasis and presynaptic transmitter release pr operties.