Neurotensin Enhances Endogenous Extracellular Glutamate Levels in Primary Cultures of Rat Cortical Neurons: Involvement of Neurotensin Receptor in NMDA Induced Excitotoxicity

Primary cultures of cortical neurons were employed to investigate the modulatory effects of neurotensin on glutamate excitotoxicity and the possible neuroprotective actions of the neurotensin receptor antagonist SR48692. NT(1–13) and its biologically active fragment NT(8–13) at 10 nM (30 min) increased endogenous glutamate levels. The inactive fragment NT(1–7) (10–100 nM; 30 min) was ineffective. SR48692, applied 20 min before NT and maintained in contact with cells during NT exposure as well as a low calcium medium (from the onset of the experiment) prevented the NT(1–13)-induced increase in extracellular glutamate levels. The addition of NMDA (0.01–10 µM; 10 min) to the medium concentration-dependently increased extracellular glutamate levels. When 0.1 nM NT(1–13) was added in combination with 0.01 µM NMDA, in concentrations by themselves ineffective, a significant increase in glutamate levels was observed. SR48692 at 100 nM counteracted the increase in glutamate levels induced by 0.1 nM NT(1–13) plus 0.01 µM NMDA. The inhibitor of the protein kinase C (PKC) calphostin C (0.1 µM; 10 min before NT) prevented the increase in glutamate levels induced by the combined treatments. The morphological analysis indicated that 10 nM NT(1–13) enhanced the glutamate (10 min)-induced apoptosis. The peptide was added 30 min prior to glutamate and maintained in contact with cells during the glutamate exposure. The presence of 100 nM SR48692 (20 min before NT) antagonized this effect of NT(1–13). These findings support the view of a pathophysiological role for NT in the cerebral cortex. Thus, under pathological conditions NT by enhancing glutamate outflow and by amplifying the NMDA-mediated glutamate signaling may be involved in increasing the degeneration of cortical neurons.