Endorphin hypothesis of schizophrenia

Dopamine and the action of opiates: a reevaluation of the dopamine hypothesis of schizophrenia. With special consideration of the role of endogenous opioids in the pathogenesis of schizophrenia

It is suggested that the antipsychotic efficacy of opioids in patients suffering from schizophrenia may result from an interaction of opioids with the dopaminergic system. The modulatory effect of opioids on dopaminergic functions has already been demonstrated in basic experiments: Anatomical and biochemical data reveal an interaction between opioid receptors and dopamine (DA) actions on dopaminergic nerve terminals, cell bodies, and afferent nerve endings. Endogenous enkephalin levels correlate well with the endogenous dopamine content in various brain areas. Systemic or iontophoretic administration of morphine alters the spontaneous activity of ventral tegmental dopaminergic neurons. Morphine and enkephalin effectively enhance pituitary prolactin release, whereas dopamine inhibits it. Opioid agonists effectively alter DA release, DA reuptake, and DA metabolism in the striatum and substantia nigra. In reverse, chronic neuroleptic treatment enhances the synthesis and release of pituitary beta-endorphin. Opioids affect contralateral rotation elicited by dopamine agonists in animals with unilateral lesions of the nigrostriatal pathway. Phencyclidine, a psychotropic drug that shares certain pharmacological characteristics with the putative sigma-opioid receptor ligand SKF 10,047, indirectly mimics the effects of dopamine agonists on prolactin release, release of acetylcholine, etc. It is suggested that an imbalance of opiate-DA interaction might be involved in the pathogenesis of schizophrenia. Consequently, clinical studies on the effects of opioids on psychotic symptoms should also examine opioid influence on dopaminergic functions in these patients.

Endogenous opiates and schizophrenia: directions in clinical research

In the past decade both basic and clinical research on endogenous opiates has involved many aspects of medicine. This review focuses on the clinical literature that relates physiology and pathophysiology of the endogenous opiates to schizophrenia. Three major investigative approaches are described and their results evaluated. These approaches are described and their results evaluated. These approaches include assays of endogenous opiates in the context of clinical illness and use of exogenous opiates as well as exogenous opiate antagonists in the treatment of clinical illness. Current understanding at both the basic science and clinical research levels does not allow firm conclusions about the role of endogenous opiates in schizophrenia; however, the evolution of the growing understanding is used by the author to provide a speculative hypothesis.