Roberto Frau, Vivien Miczán, Francesco Traccis, Sonia Aroni, Csaba I. Pongor, Pierluigi Saba, Valeria Serra, Claudia Sagheddu, Silvia Fanni, Mauro Congiu, Paola Devoto, Joseph F. Cheer, István Katona and Miriam Melis

The increased legal availability of cannabis has led to a common misconception that it is a safe natural remedy for, among others, pregnancy-related ailments such as morning sickness. Emerging clinical evidence, however, indicates that prenatal cannabis exposure (PCE) predisposes offspring to various neuropsychiatric disorders linked to aberrant dopaminergic func-tion. Yet, our knowledge of how cannabis exposure affects the maturation of this neuromodulatory system remains limited. Here, we show that male, but not female, offspring of Δ9-tetrahydrocannabinol (THC)-exposed dams, a rat PCE model, exhibit extensive molecular and synaptic changes in dopaminergic neurons of the ventral tegmental area, including altered excitatory-to-inhibitory balance and switched polarity of long-term synaptic plasticity. The resulting hyperdopaminergic state leads to increased behavioral sensitivity to acute THC exposure during pre-adolescence. The neurosteroid pregnenolone, a US Food and Drug Administration (FDA) approved drug, rescues synaptic defects and normalizes dopaminergic activity and behavior in PCE offspring, thus suggesting a therapeutic approach for offspring exposed to cannabis during pregnancy.

The use of cannabis among pregnant women is increasing, with a prevalence rate of 3–16% in Western societies (1–4). Together with the boom in cannabis marketing and the increased perception of its safety, cross-sectional analyses indicate that cannabis is often recommended to pregnant women as a treat-ment for morning sickness (5). Although the use of medical cannabis for nausea and vomiting is approved in several states and countries, no legal distinction or warning for its use during pregnancy is men-tioned6. Additionally, doctors or other health-care practitioners seldom advise pregnant women about the risks of taking cannabis during pregnancy (6,7).

The main psychoactive ingredient of cannabis, THC, interferes with the endocannabinoid system, which tightly controls progenitor cell proliferation and neuronal differentiation, axon growth and pathfinding, synapse formation and pruning in the develop-ing brain (3,8–10). Accordingly, four independent longitudinal clinical studies demonstrated that PCE predisposes individuals to a wide array of behavioral and cognitive deficits, including hyperactivity, enhanced impulsivity, loss of sustained attention, increased sensitivity to drugs of abuse (11–13) and susceptibility to psychosis (14). Notably, all these neuropsychiatric impairments are tied to a dysfunction of dopaminergic signaling (15,16). While the effects of acute and chronic cannabis use during adolescence and adulthood have been investi-gated (17–19), the impact of PCE on dopamine neurons within the ven-tral tegmental area (VTA), key players in motivation, reward and cognition (20), remains to be elucidated.

The ‘two-hit’ model of psychiatric disorders posits that genetic background and/or environmental insults act as a first hit, perturbing brain development in a manner that leads to susceptibility to the onset of psychiatric symptoms following a second hit. First hits can also lead to endophenotypes such as neurobehavioral deficits (21,22), and characterizing these may help to elucidate altered trajectories of circuit development that increase susceptibility to subsequent chal-lenges (22,23), which may in turn enable prevention of disease emer-gence. Notably, PCE was recently suggested to act as a first hit by interfering with the known complex developmental functions of endocannabinoid signaling (3,9,23).

Longitudinal studies evaluating the behavioral effects of PCE on offspring have consistently shown that the offspring exhibit increased impulsivity, increased incidence of risk-taking behaviors and vulnerability to psychosis and enhanced sensitivity to drugs of abuse later in life, which can be detected as early as early infancy and throughout child development (11,12,14). Furthermore, it is predicted that the ratio of affected children developing prenatal THC-induced endophenotypes is likely to be substantially higher (24,25), but the com-plexity of uncontrollable genetic, environmental and socioeco-nomic factors in humans makes the determination of causality very difficult. This highlights the advantage of animal models that mimic specific genetic and environmental factors. Here, we tested the hypothesis that PCE triggers molecular and synaptic changes in the VTA, which lead to aberrant dopaminergic activity and behavioral susceptibility to subsequent challenges. In agreement with evidence that the first clinical neuropsychiatric symptoms manifest as early as infancy in PCE offspring (11,14,24), we find that prenatal THC exposure (a model of PCE, hereafter referred to as PCE) engenders ‘silent’ functional abnormalities, such as impaired sensorimotor gating, increased risk-taking and abnormal locomotor responses to THC in juvenile male offspring, that become overt when acutely challenged with THC. Enhanced excitability of VTA dopamine neurons and larger THC-induced dopamine release accompany the PCE-induced endophenotype. Furthermore, we observe altered excitatory–inhib-itory balance of VTA dopamine cells along with switched polarity from long-term depression (LTD) to long-term potentiation (LTP) at afferent excitatory synapses. Postnatal administration of preg-nenolone, a federal-drug-agency (FDA)-approved drug, which is currently under investigation in clinical trials for cannabis use disorder (ClinicalTrials.gov identifier: NCT02811939), schizophrenia (NCT00728728 and NCT00615511), autism (NCT01881737 and NCT02627508), and bipolar disorder (NCT00223197 and NCT01409096), normalized dopamine neuron excitability, restored synaptic properties and abnormal polarity of synaptic plasticity, as well as THC-induced dopamine release and deficits of sensorimotor gating functions.