Activation of σ–receptors by an agonist ligand may induce hallucinogenic effects and also may be responsible for the paradoxical convulsions sometimes seen in opiate overdose. Drugs known to be σ–agonists include cocaine, morphine/diacetylmorphine, opipramol, PCP, fluoxetine, methamphetamine, dextromethorphan, and the herbal antidepressant berberine. Physiologic effects when the σ–receptor is activated include hypertonia, tachycardia, tachypnea, antitussive effects, and mydriasis. However the exact role of σ–receptors is difficult to establish as many σ–agonists also bind to other targets such as the κ-opioid receptor and the NMDA glutamate receptor. In animal experiments, σ–antagonists such as rimcazole were able to block convulsions from cocaine overdose. σ–antagonists are also under investigation for use as antipsychotic medications.