Effects of 6-hydroxydopamine lesions of locus coeruleus on startle in rats

To examine the possible involvement of the norepinephrine (NE) containing neurons of the locus coeruleus in the modulation of behavioral reactivity to sensory stimulation, bilateral chemical lesions of the locus coeruleus were made by local injection of the catecholamine neurotoxin 6-hydroxydopamine. Both histochemical and biochemical analyses confirmed the effectiveness of the lesions in specifically eliminating the NE containing cell bodies of the locus coeruleus and reducing the NE content of the hippocampus and substantia nigra by 45% and 69% respectively. Rats were tested both 5 and 36 days after lesioning for their startle response to a repetitive series of tactile stimuli. On both days, locus coeruleus lesioned rats exhibited consistently reduced startle responses throughout the stimulus series. Additionally, lesioned rats showed a far more rapid rate of response habituation, particularly in the first test. The results are discussed in terms of a possible influence of the locus coeruleus on the process of sensitization to sensory stimuli.