Response Properties of Units in the Dorsal Cochlear Nucleus of Unanesthetized Decerebrate Gerbil. Davis, Kevin A., Jiang Ding, Thane E. Benson. Department of Biomedical Engineering, Boston University, Boston MA 02215- 2407 and Departments of Biomedical Engineering and Otolaryngology, Boston University, Boston MA 02215-2407.
APStracts 2:0319N, 1995.
SUMMARY AND CONCLUSIONS
1. The electrophysiological responses of single units in the dorsal cochlear nucleus (DCN) of unanesthetized decerebrate Mongolian gerbil ( Meriones unguiculatus ) were recorded. Units were classified according to the response map scheme of Evans and Nelson (1973) as modified by Young and Brownell (1976), Young and Voigt (1982), and Shofner and Young (1985). Type II units have a V-shaped excitatory response map similar to typical auditory nerve tuning curves but little or no spontaneous activity (SpAc<2.5 spikes/s) and little or no response to noise. Type I/III units also have a V-shaped excitatory map and SpAc<2.5 spikes/s, but have an excitatory response to noise. Type III units have a V-shaped excitatory map with inhibitory sidebands, SpAc>2.5 spikes/s, and an excitatory response to noise. Type IV-T units typically also have a V-shaped excitatory map with inhibitory sidebands, but have a highly non-monotonic rate versus level response to best frequency (BF) tones like type IV units, SpAc>2.5 spikes/s, and an excitatory response to noise. Type IV units have a predominantly inhibitory response map above an island of excitation at BF, SpAc>2.5 spikes/s, and an excitatory response to noise. We present results for 133 units recorded with glass micropipette electrodes. The purpose of this study was to establish a normative response map data base in this species for ongoing structure/function and correlation studies. 2. The major types of units (type II, type I/III, type III, type IV-T and type IV) found in decerebrate cat (Shofner and Young 1985) are found in decerebrate gerbil. However, the percentages of type II (7.5%) and type IV (11.3%) units encountered are smaller and the percentage of type III (62.4%) units is larger in decerebrate gerbil than in decerebrate cat. In comparison, Shofner and Young (1985) found 18.5% type II units, 30.6% type IV units and 23.1% type III units using metal electrodes. 3. Two new unit subtypes are described in gerbil: type III-i and type IV-i units. Type III-i units are similar to type III units except that type III-i units are inhibited by low levels of noise and excited by high levels of noise while type III units have strictly excitatory responses to noise. Type IV-i units are similar to type IV units except that type IV-i units are excited by low levels of noise and become inhibited by high levels of noise while type IV units have strictly excitatory responses to noise. Type III-i units are approximately 30% of the type III population and type IV-i units are approximately 50% of the type IV population. 4. On the basis of the paucity of classic type II units and the reciprocal responses to broadband noise of type III-i and type IV-i units, we postulate that some gerbil type III-i units are the same cell type and have similar synaptic connections as cat type II units. 5. Type II and type I/III units are distinguished from one another on the basis of both their relative noise response, r , and the normalized slope of the best frequency tone rate versus level functions beyond the first maximum. Previously, type II units were defined to be those non-spontaneously active units with r values less than 0.3 (Shofner and Young 1985) where r is defined as the ratio of the maximum noise response minus spontaneous rate to the maximum best frequency tone response minus spontaneous rate. In the gerbil, the average r value for type II units is 0.25, although a few values are greater than 0.3, and the rate-level curves are consistently non-monotonic with normalized slopes steeper than than Ä0.007/dB. The average r value for type I/III units is 0.54, although a few values are less than 0.3, and the rate-level curves tend to saturate with slopes shallower than Ä0.006/dB. In general, the response properties of type II units recorded in gerbil are similar to those recorded in decerebrate cat (Young and Voigt 1982; Shofner and Young 1985). 6. In comparison to decerebrate cat, the lower percentage of type IV units recorded in decerebrate gerbil may be due to a species difference (a reduced number of type II units in gerbil) or an electrode bias. As the difference between a type IV and a type III unit is thought to be the balance between their inhibitory and excitatory inputs, the paucity of classic type II units could reflect a decreased inhibitory input to "nominal" type IV units resulting in these units exhibiting type III or type IV-T unit properties. 

Received 22 June 1995; accepted in final form 23 October 1995.
APS Manuscript Number J372-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 November 95