Apomorphine effects on emotional modulation of the startle reflex in rats.
BACKGROUND
Emotional modulation of the startle reflex in the rat may be used to assess whether activation of dopamine receptors specifically increases hedonia, incentive, fear or arousal.


OBJECTIVE
The objective of the study is to determine the effects of apomorphine (0.8 mg/kg s.c.) on the startle reflex of rats (72 male Sprague-Dawley rats) exposed to one of three affective conditions. These conditions were negative affective stimulus (exposure to cat smell), positive affective stimulus (availability of a 20% sucrose solution), neutral stimulus (no additional affective stimulus) and one of two appetitive "drive" states (food deprived or non-food deprived).


METHODS
The startle response (whole-body flinch response) was measured after presentation of a range of intensities of acoustic stimuli (65-120 dB, 40-ms duration white noise). The resulting sigmoidal stimulus intensity-response magnitude (SIRM) curves were fitted using a logistic regression procedure, and features of these functions were abstracted for analysis.


RESULTS
Maximal startle amplitudes were increased by the negative affect (fear) stimulus in non-food-deprived rats and decreased by the positive affect stimulus in food-deprived rats. Apomorphine mimicked the effects of food deprivation under both affect conditions, but also produced an effect in food-deprived rats similar to that of the positive affect condition.


CONCLUSIONS
The results are consistent with both a positive incentive effect and a direct hedonic action of apomorphine, but inconsistent with a role in general arousal. In addition, a method of analysing SIRM functions with logistic regressions is introduced as a useful means of standardising startle reflex measurements.
Martin-Iverson
Mathew T
MT
School of Medicine and Pharmacology, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. matt@ccrn.uwa.edu.au
Stevenson
Kirsten N
KN
D000161:Acoustic Stimulation / Q000379:methods; D000704:Analysis of Variance; D000818:Animals; D001058:Apomorphine / Q000494:pharmacology*; D001835:Body Weight / Q000187:drug effects / Q000502:physiology; D018491:Dopamine Agonists / Q000494:pharmacology; D019260:Expressed Emotion / Q000187:drug effects / Q000502:physiology; D005508:Food Deprivation / Q000502:physiology; D008144:Loudness Perception / Q000187:drug effects / Q000502:physiology; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D013216:Reflex, Startle / Q000187:drug effects* / Q000502:physiology; D013395:Sucrose / Q000008:administration & dosage / Q000494:pharmacology
D018491:Dopamine Agonists; D013395:Sucrose; D001058:Apomorphine