Elsevier

Physiology & Behavior

Volume 65, Issue 1, August 1998, Pages 133-139
Physiology & Behavior

Original Articles
Prepulse inhibition following lesions of the inferior colliculus: prepulse intensity functions

https://doi.org/10.1016/S0031-9384(98)00143-7Get rights and content

Abstract

LI, L., L. M. KORNGUT, B. J. FROST AND R. J. BENINGER. Prepulse inhibition following lesions of the inferior colliculus: Prepulse intensity functions. PHYSIOL BEHAV 65(1) 133–139, 1998.—The magnitude of the acoustic startle response can be reduced by a relatively weak sound presented immediately before the startle-eliciting sound; this phenomenon has been termed prepulse inhibition (PPI). Previous studies reported that PPI was present in the decerebrate rat, indicating that the primary neural pathways mediating PPI are located in the brainstem. The present study investigated the effects of focal excitotoxic lesions of the inferior colliculus (IC) on acoustic PPI in rats. In the first part, startle magnitudes were measured in six normal rats as the interstimulus interval (ISI) between the prepulse and startle-eliciting sounds varied between 10 and 100 ms. Prepulse-inhibited startle changed in an ISI-dependent manner with the most effective ISI at 50 ms. In the second part, 21 rats were assigned to three groups: normal unoperated, cortical lesion, and IC lesion. With the ISI fixed at 50 ms, as the prepulse sound level increased from 29 to 49 dB SPL, startle responses decreased quickly in both normal and cortical lesion rats. However, rats with unilateral IC lesions made with ibotenic acid had significantly lower PPI but did not display any increase in startle magnitude. These data suggest that the IC is an important structure in the neural circuit mediating acoustic PPI.

Section snippets

Animals

Experiments were conducted on 27 male adult Wistar rats (Rattus norvegicus) (300–450 g) obtained from Charles River Canada, St. Constant, Quebec. Rats were housed individually on a 12-h light:dark cycle (lights on at 0700 hours) with food and water freely available.

Startle apparatus

All testing was carried out in a dark and soundproof chamber. Detailed descriptions of the sound delivery and calibration systems have also been published elsewhere (29). Briefly, broad-band noise bursts (40 Hz–20 kHz; 10 ms in

Anatomical results

Injection of ibotenic acid into the IC or overlying cortex resulted in neural loss, gliosis, and some cavitation in the affected area. Restricted cortical lesions were made in the seven rats, and no neural damage was found in any subcortical structure (Fig. 1 ).

The histologic reconstruction for each rat with IC lesions are shown in Fig. 2. No damage was found beyond the IC that received injection of ibotenic acid.

General post-surgical observations

Rats receiving unilateral injection of ibotenic acid into the IC had brief and

Discussion

The results of the present study indicate that the IC makes an important contribution to acoustic PPI. The startle suppression that is normally augmented with an increase in prepulse sound level is reduced by unilateral restricted lesions of the IC. In contrast, damage to the cortex overlying IC has no effect on PPI. The present data are consistent with previous observations of the effects of radio-frequency lesions of the IC and surrounding areas (27) and, therefore, support the view that the

Acknowledgements

The authors gratefully acknowledge the assistance of Sharon David and Stephen Ferguson. This work was supported by a postdoctoral fellowship to L. L. and research grants to B. J. F. and R. J. B. from the Natural Sciences and Engineering Research Council of Canada.

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