NMDA-receptor-mediated synaptic currents in guinea pig laterodorsal tegmental neurons in vitro

1. Whole cell voltage-clamp techniques were used to record glutamate-receptor-mediated synaptic currents from neurons of the laterodorsal tegmental nucleus (LDT). The principal cells of the LDT contain acetylcholine and nitric oxide synthase, and are believed to be involved in the control of sleep-waking behavior via widespread projections to the thalamus and brain stem. LDT cells were recorded from slices of mature guinea pig brain stem with patch pipette solutions containing cesium as the primary cation. 2. Application of N-methyl-D-aspartate (NMDA) elicited currents that were strongly voltage dependent with a mean reversal potential of +16.3 mV. Peak currents occurred near -15 mV, and a region of negative slope conductance was seen at more negative potentials. Application of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid evoked currents that exhibited a nearly linear current-voltage relation with a mean reversal potential of -3.4 mV. 3. Electrical stimulation of local afferents elicited dual-component excitatory postsynaptic currents (EPSCs) with decays that were well fitted by the sum of two exponentials. Mean decay time constants at -60 mV were 8.77 ms for the faster component and 129.4 ms for the slower component. The faster component displayed a linear current-voltage relation and was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 6,7-dinitroquinoxaline-2,3-dione, indicating that it was mediated by non-NMDA receptors, whereas the slower component displayed a voltage dependence similar to that for NMDA-evoked currents and was blocked by 2-amino-5-phosphonopentanoic acid (AP-5), indicating its mediation by NMDA receptors. 4. The fractional contribution of NMDA receptors to the EPSC was estimated from double-exponential curve fits to the decay phases. With this method, NMDA receptors were estimated on average to carry 10.1% of the total peak EPSC at -60 mV. Blockade of the non-NMDA-receptor-mediated component with CNQX revealed a residual EPSC whose amplitude was 14.4% of the control value, whereas AP-5 alone reduced the control EPSC peak by 16.1%, both values were comparable with those obtained from curve fit estimates. 5. Previous work has shown that the presence of 4-aminopyridine-sensitive, A-like transient current in LDT cells is correlated with the cholinergic phenotype. The majority of cells in this study exhibited A-like transient currents that were blocked by 4-amino-pyridine, suggesting that the majority of the data were obtained from the cholinergic and NOS-containing neurons of the LDT nucleus. 6. These experiments demonstrate the synaptic activation of functional NMDA and non-NMDA receptors in LDT neurons, and indicate that NMDA receptors contribute to fast excitatory transmission in these cells. The results suggest that afferents releasing excitatory amino acids may play an important role in controlling the state-dependent activity of LDT neurons.