NMDA-RECEPTOR-MEDIATED SYNAPTIC CURRENTS IN GUINEA-PIG LATERODORSAL TEGMENTAL NEURONS IN-VITRO

1. Whole cell voltage-clamp techniques were used to record glutamate-r eceptor-mediated synaptic currents from neurons of the laterodorsal te gmental nucleus (LDT). The principal cells of the LDT contain acetylch oline and nitric oxide synthase, and are believed to be involved in th e control of sleep-waking behavior via widespread projections to the t halamus and brain stem. LDT cells were recorded from slices of mature guinea pig brain stem with patch pipette solutions containing cesium a s the primary cation. 2. Application of N-methyl-D-aspartate (NMDA) el icited currents that were strongly voltage dependent with a mean rever sal potential of +16.3 mV. Peak currents occurred near -15 mV, and a r egion of negative slope conductance was seen at more negative potentia ls. Application of lpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid evoked currents that exhibited a nearly linear current-voltage re lation with a mean reversal potential of -3.4 mV. 3. Electrical stimul ation of local afferents elicited dual-component excitatory postsynapt ic currents (EPSCs) with decays that were well fitted by the sum of tw o exponentials. Mean decay time constants at -60 mV were 8.77 ms for t he 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-dinitroquinoxali ne-2,3-dione, indicating that it was mediated by non-NMDA receptors, w hereas the slower component displayed a voltage dependence similar to that for NMDA-evoked currents and was blocked by 2-amino-5-phosphonope ntanoic acid (AP-5), indicating its mediation by NMDA receptors. 4. Th e fractional contribution of NMDA receptors to the EPSC was estimated from double-exponential curve fits to the decay phases. With this meth od, NMDA receptors were estimated on average to carry 10.1% of the tot al peak EPSC at -60 mV. Blockade of the non-NMDA-receptor-mediated com ponent with CNQX revealed a residual EPSC whose amplitude was 14.4% of the control value, whereas AP-5 alone reduced the control EPSC peak b y 16.1%, both values were comparable with those obtained from curve fi t estimates. 5. Previous work has shown that the presence of a 4-amino pyridine-sensitive, A-like transient current in LDT cells is correlate d with the cholinergic phenotype. The majority of cells in this study exhibited A-like transient currents that were blocked by 4-aminopyridi ne, suggesting that the majority of the data were obtained from the ch olinergic and NOS-containing neurons of the LDT nucleus. 6. These expe riments demonstrate the synaptic activation of functional NMDA and non -NMDA receptors in LDT neurons, and indicate that NMDA receptors contr ibute to fast excitatory transmission in these cells. The results sugg est that afferents releasing excitatory amino acids may play an import ant role in controlling the state-dependent activity of LDT neurons.