Abstract
Migration is a major risk factor for schizophrenia but the neurochemical processes involved are unknown. One candidate mechanism is through elevations in striatal dopamine synthesis and release. The objective of this research was to determine whether striatal dopamine function is elevated in immigrants compared to nonimmigrants and the relationship with psychosis. Two complementary case–control studies of in vivo dopamine function (stress-induced dopamine release and dopamine synthesis capacity) in immigrants compared to nonimmigrants were performed in Canada and the United Kingdom. The Canadian dopamine release study included 25 immigrant and 31 nonmigrant Canadians. These groups included 23 clinical high risk (CHR) subjects, 9 antipsychotic naïve patients with schizophrenia, and 24 healthy volunteers. The UK dopamine synthesis study included 32 immigrants and 44 nonimmigrant British. These groups included 50 CHR subjects and 26 healthy volunteers. Both striatal stress-induced dopamine release and dopamine synthesis capacity were significantly elevated in immigrants compared to nonimmigrants, independent of clinical status. These data provide the first evidence that the effect of migration on the risk of developing psychosis may be mediated by an elevation in brain dopamine function.
Highlights
Geographic mobility has a large impact on sociological, economic, and health factors within communities
Secondary exploratory ANOVA comparing nonimmigrants to first and second generation immigrants separately found an overall significant effect of immigration on striatal dopamine synthesis capacity (F = 3.12; df = 1, 73; P = .047), with post hoc tests indicating this was primarily driven by elevated DA synthesis capacity in the second generation immigrant compared to nonimmigrant group (P = .04), whereas DA synthesis capacity did not differ significantly between first generation immigrant and nonimmigrant (P = .91) or second generation immigrant groups (P = .23)
The main effects of immigration and clinical group on DA synthesis remained after co-varying for previous use of other illicit substances, or after excluding the 2 clinical high risk (CHR) subjects who were currently taking antipsychotics. These results indicate that striatal DA function is elevated in both immigrants and their children, including those at risk for psychosis or with schizophrenia, confirming its relevance for psychotic disorders
Summary
Geographic mobility has a large impact on sociological, economic, and health factors within communities. Over 247 million people, or 3.5% of the world population, became migrants over the last decade.[1,2] One of the most consistent findings in the epidemiology of schizophrenia is the high incidence of the disorder among immigrant groups (relative risk vs nonimmigrant: 2.9).[3] The risk is increased in immigrant groups who migrate from a country where the population is predominantly black skinned to a country where the population is predominantly white skinned (relative risk vs nonimmigrant: 4.8).[3] This increased risk of schizophrenia has been reported both in immigrants and in their children These findings have been replicated in a number of high income countries: The Netherlands,[4] Denmark,[5] Germany,[6] the United Kingdom,[7] and Canada,[8] clearly establishing that the incidence of schizophrenia is higher among migrant groups as compared to host populations. The UK study sought to confirm the association between immigration (first and second generation) and elevated striatal DA function in a different cohort using the complementary technique of [18F]DOPA PET to estimate DA synthesis capacity.[25]
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