In Dr. Steidl’s 2022 paper titled “Glutamate inputs from the laterodorsal tegmental nucleus to the ventral tegmental area are essential for the induction of cocaine sensitization in male mice,” the researchers aimed to determine how cocaine sensitization is affected by glutamatergic projections from the laterodorsal tegmental nucleus (LDTg) to the ventral tegmental area (VTA). They used locomotor assessments to deduce whether cocaine sensitization occured after optogenetic manipulation where LDTg glutamatergic cells and their VTA projections were inhibited after blue light was inputted to the brains of halorhodopsin infected mice. The study found that mice in the halorhodopsin experimental group exposed to cocaine had no change in locomotor activity, indicating no increase in cocaine sensitization, as opposed to the control group which did show increases and thus sensitization. This led the researchers to deduce that glutamatergic inputs are crucial for developing cocaine sensitization.
A neuroscience research group at Florida Atlantic University, led by Amanda Catalfio, explored how cocaine-induced sensitization and glutamate plasticity in the nucleus accumbens (NAc) core are affected by biological sex. Similar to Dr. Steidl’s study, who studied the role of upstream glutamate inputs in sensitization, Catalfio’s group aimed to recognize if repeated cocaine exposure leads to neuroplasticity in glutamatergic signaling in the nucleus accumbens core, and if these changes may be different between sexes. In their experiment, rats of both sexes were exposed to repeated cocaine injections, and measures of locomotor sensitization (similar to the test in Dr. Steidl’s study) and spontaneous excitatory postsynaptic currents (sEPSCs) in the nucleus accumbens core were collected to assess behavioral and neural adaptations.
The study found that both male and female rats exhibited psychomotor sensitization — which is increased locomotor activity over repeated cocaine exposure. However, only males showed enhanced sEPSC frequency in the nucleus accumbens core, suggesting a sex-specific strengthening of glutamatergic inputs higher in males. Females, despite showing behavioral sensitization, did not display the same glutamatergic plasticity shown by changing postsynaptic currents. These results indicate that while behavioral sensitization after sustained cocaine exposure occurs in both sexes, the underlying synaptic mechanisms, particularly glutamate signaling in the nucleus accumbens, may differ between males and females.
Based off of Dr. Steidl’s study finding that glutamate signaling into the VTA is necessary for initiating sensitization (as the group with inhibited glutamate inputs to VTA did not show sensitization), and Dr. Catalfio’s study findings that glutamate plasticity within the nucleus accumbens is sex-dependent following cocaine sensitization, perhaps cocaine addiction involves a multi-level glutamatergic cascade: glutamate first acts upstream to drive VTA dopamine neuron changes, and later induces circuit-specific plasticity in the basal ganglia- specifically the nucleus accumbens- that may be influenced differently across biological sexes. However, the discovery that females experienced behavioral sensitization without major neural circuitry changes in nucleus accumbens glutamate activity suggests that females may rely on different or additional neural mechanisms that lead to the behavioral changes exhibited. Further research could explore how upstream VTA inputs and downstream nucleus accumbens adaptations interact differently between sexes, which could have important implications for developing sex-specific addiction treatments. Either way, these studies together highlight the critical and complex role of glutamate pathways in drug-induced plasticity and addiction.
Works Cited
Catalfio AM, Fetterly TL, Nieto AM, Robinson TE, Ferrario CR. Cocaine-induced sensitization and glutamate plasticity in the nucleus accumbens core: effects of sex. Biol Sex Differ. 2023 Jun 24;14(1):41. doi: 10.1186/s13293-023-00525-8. PMID: 37355656; PMCID: PMC10290362.
Puranik A, Buie N, Arizanovska D, Vezina P, Steidl S. Glutamate inputs from the laterodorsal tegmental nucleus to the ventral tegmental area are essential for the induction of cocaine sensitization in male mice. Psychopharmacology (Berl). 2022 Oct;239(10):3263-3276. doi: 10.1007/s00213-022-06209-2. Epub 2022 Aug 25. PMID: 36006414.