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Apomorphine effects on emotional modulation of the startle reflex in rats

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Abstract

Rationale

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.

Objectives

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.

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Acknowledgements

We would like to thank Peter (Mouse) Meyer for his excellent technical support and all those cat owners who offered up their cat collars for our use. We greatly appreciate the financial support of the Australian Research Council and the Western Australian Institute for Medical Research for the financial support that made this research possible.

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Authors and Affiliations

  1. Pharmacology Unit, School of Medicine and Pharmacology, and Centre for Clinical Research in Neuropsychiatry, University of Western Australia, 35 Stirling Highway, Crawley, WA, 6009, Australia

    Mathew T. Martin-Iverson & Kirsten N. Stevenson

  2. Graylands Hospital, CCRN, Private Bag #1, Claremont, WA, 6910, Australia

    Mathew T. Martin-Iverson

Authors
  1. Mathew T. Martin-Iverson
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  2. Kirsten N. Stevenson
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Correspondence to Mathew T. Martin-Iverson.

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Martin-Iverson, M.T., Stevenson, K.N. Apomorphine effects on emotional modulation of the startle reflex in rats. Psychopharmacology 181, 60–70 (2005). https://doi.org/10.1007/s00213-005-2217-3

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  • DOI: https://doi.org/10.1007/s00213-005-2217-3

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