Role of Calcium in Neurotensin-Evoked Enhancement in Firing in Mesencephalic Dopamine Neurons

Abstract

Neurotensin (NT) increases neurotransmission within the mesolimbic dopamine system by enhancing the firing rate of dopaminergic (DAergic) neurons and by acting at the nerve terminal level. The signal transduction pathways involved in these effects have not been characterized, but NT receptors are coupled to the phospholipase C pathway and Ca²⁺ mobilization. However, an enhancement of intracellular Ca²⁺ concentration ([Ca²⁺]_i) evoked by NT in DAergic neurons has yet to be demonstrated. Furthermore, the hypothesis that the excitatory effects of NT in DAergic neurons are Ca²⁺ dependant is currently untested. In whole-cell recording experiments, DAergic neurons in culture were identified by their selective ability to express a cell-specific green fluorescent protein reporter construct. These experiments confirmed that NT increases firing rate in cultured DAergic neurons. This effect was Ca²⁺ dependent because it was blocked by intracellular dialysis with BAPTA. Using Ca²⁺ imaging, we showed that NT caused a rapid increase in [Ca²⁺]_i in DAergic neurons. Most of the Ca²⁺ originated from the extracellular medium. NT-induced excitation and Ca²⁺ influx were blocked by SR48692, an antagonist of the type 1 NT receptor. Blocking IP₃ receptors using heparin prevented the excitatory effect of NT. Moreover, Zn²⁺ and SKF96365 both blocked the excitatory effect of NT, suggesting that nonselective cationic conductances are involved. Finally, although NT can also induce a rise in [Ca²⁺]_i in astrocytes, we find that NT-evoked excitation of DAergic neurons can occur independently of astrocyte activation.