Clonidine injections into the lateral nucleus of the amygdala block acquisition and expression of fear-potentiated startle.
Numerous studies of aversive learning with different animal models have shown that the noradrenergic system has an important role in the acquisition, consolidation and expression of aversive learning. We used intracerebral clonidine injections to investigate the role of the noradrenergic amygdaloid system in the fear-potentiated startle paradigm. Clonidine is a noradrenergic alpha2-receptor agonist which can decrease noradrenergic transmission by stimulating presynaptic alpha2-receptors. Rats received injections of 0, 2.5, 5 and 10 nmol clonidine into the lateral amygdala (i) before fear-conditioning, (ii) immediately after fear-conditioning, (iii) before testing and (iv) before both fear-conditioning and the testing of conditioned fear. Clonidine injections blocked the acquisition and expression of conditioned fear. The effect on acquisition was not caused by state-dependency or possible side-effects of clonidine on consolidation. Given that clonidine decreases amygdaloid noradrenaline release, these results show a crucial role of noradrenergic transmission within the amygdala in classical fear-conditioning. Surprisingly, both the acquisition and the expression of conditioned fear were blocked after amygdaloid injections of clonidine, suggesting that amygdaloid noradrenaline is necessary to induce both unconditioned and conditioned fear.
Schulz
Brigitte
B
Department of Animal Physiology, University of Tübingen, Auf der Morgenstelle 28, D-72076 Tübingen, Germany.
Fendt
Markus
M
Schnitzler
Hans-Ulrich
HU
D000161:Acoustic Stimulation; D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage / Q000494:pharmacology*; D000679:Amygdala / Q000033:anatomy & histology / Q000502:physiology*; D000818:Animals; D003000:Clonidine / Q000008:administration & dosage / Q000494:pharmacology*; D005239:Fear / Q000187:drug effects*; D008845:Microinjections; D051381:Rats; D017207:Rats, Sprague-Dawley; D013216:Reflex, Startle / Q000187:drug effects*; D013565:Sympatholytics / Q000008:administration & dosage / Q000494:pharmacology*
D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists; D013565:Sympatholytics; D003000:Clonidine