The relationship between the olfactory system and emotional processing is an area of growing interest in schizophrenia research. Both the orbitofrontal cortex and amygdala are involved in the processing of olfactory information, and olfactory deficits may be also influenced by endogenous opioids and calcitonin gene-related peptide (CGRP), which is probably involved in dopaminergic transmission. However, the relationship between endorphins and dopaminergic transmission has not been fully explored.

Odor identification performance and valence interaction was evaluated among 50 schizophrenic patients and 50 controls. Schizophrenia symptoms were assessed using the Positive and Negative Syndrome Scale (PANSS). All study participants were subjected to the University of Pennsylvania Smell Identification Test (UPSIT), blood β-endorphin (BE) and CGRP measurement.

Insignificantly higher BE concentrations were observed in the patient group, while significantly higher UPSIT scores were seen in controls (mean UPSIT 32.48 vs 26.82). The patients demonstrated significantly more identification errors for pleasant (P=0.000) and neutral (P=0.055) odors than for unpleasant odors. Patients with higher BE concentrations made more identification errors concerning pleasant (R_s=−0.292; P=0.04) and neutral odors (R_s=−0.331; P=0.019). Although the concentration of CGRP was significantly higher in the patient sample (P<0.001), no relationship was observed between concentration and UPSIT performance. A strong negative correlation was observed between PANSS N score and UPSIT total score (R_s=−0.646; P=0.000), between PANSS N score and identification by valence for pleasant and neutral odors (UPSIT n/16: R_s=−0.450, P=0.001; UPSIT n/15: R_s=−0.586, P=0.000), and a weak negative correlation between PANSS N score and identification of unpleasant odors (UPSIT n/9: R_s=−0.325, P=0.021).

Schizophrenic patients present a unique pattern of smell identification characterized by aberrant hedonic ratings for pleasant odors but not unpleasant ones. Individuals with predominant negative symptoms and higher BE concentrations are most able to identify negative odors.