Abstract
Hierarchical processing of sensory information occurs at multiple levels between the peripheral and central pathway. Different extents of convergence and divergence in top down and bottom up projections makes it difficult to separate the various components activated by a sensory input. In particular, hierarchical processing at sub-cortical levels is little understood. Here we have developed a method to isolate extrinsic inputs to the inferior colliculus (IC), a nucleus in the midbrain region of the auditory system, with extensive ascending and descending convergence. By applying a high concentration of divalent cations (HiDi) locally within the IC, we isolate a HiDi-sensitive from a HiDi-insensitive component of responses evoked by afferent input in brain slices and in vivo during a sound stimulus. Our results suggest that the HiDi-sensitive component is a monosynaptic input to the IC, while the HiDi-insensitive component is a local polysynaptic circuit. Monosynaptic inputs have short latencies, rapid rise times, and underlie first spike latencies. Local inputs have variable delays and evoke long-lasting excitation. In vivo, local circuits have variable onset times and temporal profiles. Our results suggest that high concentrations of divalent cations should prove to be a widely useful method of isolating extrinsic monosynaptic inputs from local circuits in vivo.
Highlights
How does the representation of a sensory stimulus in the periphery get transformed into a perceptually reliable code in the cortex? Hierarchical flow of information converts the representation of sensory features extracted at the periphery into central codes
A constant onset latency during stimulus trains would suggest that monosynaptic responses were retained in high concentration of divalent cations (HiDi) and further, that spike invasion of the nerve terminal was not compromised
Our results suggest that the use of HiDi in vivo succeeds in isolating the monosynaptic component of the response to an acoustic stimulus
Summary
How does the representation of a sensory stimulus in the periphery get transformed into a perceptually reliable code in the cortex? Hierarchical flow of information converts the representation of sensory features extracted at the periphery into central codes. The ascending part of the central auditory system involves extensive convergence and divergence of inputs (Oliver et al, 1997; Cant and Benson, 2003), which can be difficult to untangle This gives rise to the need to develop methods for isolating extrinsic inputs from local influences. In the auditory cortex, ascending monosynaptic and local intracortical influences on cortical responses to acoustic input are separated through a cocktail of GABAA agonists and GABAB antagonists, which isolate thalamocortical input from local inhibitory intracortical circuits (Liu et al, 2007) This method would not work in the midbrain, or between different cortical areas, where extrinsic and local pathways consist of a mix of excitatory and inhibitory inputs. We developed a method to separate extrinsic monosynaptic inputs from local circuits in a nucleus that receives extensive ascending and descending projections that include excitatory and inhibitory inputs
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