The Study Of Inhibition Mechanisms Underlying Vocalization Suppression And Forward Masking In Inferior Collicular Neurons Of Echolocating Bat

The vocalization suppression (V-S) is described as the emitted sound impedes animals perceiving sounds which arrived later for a limited time. For bats, the reflected echoes arriving right after V-S could be apperceived much better. Even human beings, as non-echolocation animals, also need V-S to eliminate the environment noise in sound communication. In the other hand, the inhibition exerts by leading sounds to lagging sounds is also found in forward masking (F-M). In order to understand whether the V-S and F-M underly the same mechanism for inhibition, we mimicked pulse-echo pair and masker-probe pair sound stimulation under free field for V-S and F-M respectively, and intracellularly recorded in vivo neurons’ responses to these two kind of paired sound stimulation in inferior colliculus (IC) of Pratt’s roundleaf bat, Hipposideros pratti. The main new result are as follows:1. A total of 149 (N=149) neurons were tested with pulse-echo pair sound in which 3 pulses were composed of 3 harmonics (H1, H2, H3) respectively in bat’s vocalization and masker-probe pair sound stimulation. Responses to V-S of second harmonic (VH2-S) and F-M were recorded in 22 neurons, while responses to V-S of first harmonic (VH1-S) and F-M were recorded in 52 neurons.2. According to whether hyperpolarization was evoked by H2 and masker,21 neurons could be divided into two classes:38.1%(n= 8/21,38.1%) of neurons burst action potentials (APs) followed by hyperpolarization to both H2 and masker:50% recovery cycles in VH2-S and F-M have no correlation with the duration of hyperpolarizations which induced by H2 and masker respectively, but the duration of hyperpolarizations were significant less than the 50% recovery cycles in VH2-S and F-M, it was hypothesized that postsynaptic inhibition, synaptic depression or presynaptic inhibition are all likely involved in the formation of VH2-S and F-M. The other 61.9% (n= 13/21,61.9%) neurons burst APs without hyperpolarization to both H2 and masker. There was no hyperpolarization related to inhibition were recorded, the inhibition in VH2-S and F-M might be inherited from the nuclear below IC. Although that the 50% recovery cycle in VH2-S was significant long than that in F-M was probably result from the intensity of H2 being higher than masker. In brief, VH2-S and F-M highly share the same mechanisms for inhibition.3.52 neurons could be divided into four classes according to whether hyperpolarization was evoked by H1 and masker. First, H1 and masker could evoke exclusive hyperpolarization or hyperpolarization following APs in 15.4% of neurons (n=8/52). The 50% recovery cycles in VH1-S and F-M were linear correlated with the duration of hyperpolarization in this kind of neurons, which suggested that the integration of inhibition induced by H1 or masker and excitation induced by echo or probe resulted in the formation of VH1-S and F-M. Second,26.9%(n=14/52) of neurons showed no hyperpolarization followed AP to both H1 and masker. And no significant difference between the 50% recovery cycle in VH1-S and F-M also suggested that the inhibition in VH1-S and F-M might be inherited by the nuclear below IC. Third,15.4% of neurons (n=8/52) showed no reaction to H1 but AP followed by hyperpolarization to masker, the very short of 50% recovery cycle in VH1-S and the duration of hyperpolarization correlating with 50% recovery cycle in F-M suggested that the vibration of the basement membrane mainly contributed to the formation of VH1-S, while the integration of inhibition and excitation take part in the formation of F-M. Fourth, 42.3% of neurons (n=22/52) showed no responses to H1 but AP without hyperpolarization to masker. Comparing the very short of 50% recovery cycle in VH1-S to the extensive range of 50% recovery cycle in F-M, it was speculated that the inhibition in VH1-S was mainly induced by the vibration of the basement membrane, while the inhibition in F-M was inherited from the nuclear below IC. In brief, VH1-S and F-M partly share neural mechanisms for inhibition.Above results indicated that V-S and F-M share the inhibition mechanism to a large extent, however the difference between inhibition in VH1-S and F-M suggests VH1-S probably plays a particular role in the echolocation for Pratt’s roundleaf bat.