Glutamate inputs from the laterodorsal tegmental nucleus to the ventral tegmental area are essential for the induction of cocaine sensitization in male mice.

RATIONALE: Drug-induced potentiation of ventral tegmental area (VTA) glutamate signaling contributes critically to the induction of sensitization - an enhancement in responding to a drug following exposure which is thought to reflect neural changes underlying drug addiction. The laterodorsal tegmental nucleus (LDTg) provides one of several sources of glutamate input to the VTA.

OBJECTIVE: We used optogenetic techniques to test either the role of LDTg glutamate cells or their VTA afferents in the development of cocaine sensitization in male VGluT2::Cre mice. These were inhibited using halorhodopsin during each of five daily cocaine exposure injections. The expression of locomotor sensitization was assessed following a cocaine challenge injection 1-week later.

RESULTS: The locomotor sensitization seen in control mice was absent in male mice subjected to inhibition of LDTg-VTA glutamatergic circuitry during cocaine exposure. As sensitization of nucleus accumbens (NAcc) dopamine (DA) overflow is also induced by this drug exposure regimen, we used microdialysis to measure NAcc DA overflow on the test for sensitization. Consistent with the locomotor sensitization results, inhibition of LDTg glutamate afferents to the VTA during cocaine exposure prevented the sensitization of NAcc DA overflow observed in control mice.

CONCLUSIONS: These data identify the LDTg as the source of VTA glutamate critical for the development of cocaine sensitization in male mice. Accordingly, the LDTg may give rise to the synapses in the VTA at which glutamatergic plasticity, known to contribute to the enhancement of addictive behaviors, occurs.