Autocrine Neuromodulation and Network Activity Patterns in the Locus Coeruleus of Newborn Rat Slices

Mediation of Sinusoidal Network Oscillations in the Locus Coeruleus of Newborn Rat Slices by Pharmacologically Distinct AMPA and KA Receptors

Brain control by locus coeruleus (LC) neurons involves afferent glutamate (Glu) inputs. In newborns, LC Glu receptors and responses may be sparse due to immaturity of the brain circuits providing such input. However, we reported, using newborn rat brain slices, that Glu and its ionotropic receptor (iGluR) agonist NMDA transform spontaneous local field potential (LFP) rhythm. Here, we studied whether α-amino-3-hydroxy-5-methyl-4-isoxazole propionic-acid (AMPA) and kainate (KA) iGluR subtypes also transform the LFP pattern. AMPA (0.25–0.5 µM) and KA (0.5–2.5 µM) merged ~0.2 s-lasting bell-shaped LFP events occurring at ~1 Hz into ~40% shorter and ~4-fold faster spindle-shaped and more regular sinusoidal oscillations. The AMPA/KA effects were associated with a 3.1/4.3-fold accelerated phase-locked single neuron spiking due to 4.0/4.2 mV depolarization while spike jitter decreased to 64/42% of the control, respectively. Raising extracellular K⁺ from 3 to 9 mM increased the LFP rate 1.4-fold or elicited slower multipeak events. A blockade of Cl⁻-mediated inhibition with gabazine (5 μM) plus strychnine (10 μM) affected neither the control rhythm nor AMPA/KA oscillations. GYKI-53655 (25 μM) blocked AMPA (but not KA) oscillations whereas UBP-302 (25 μM) blocked KA (but not AMPA) oscillations. Our findings revealed that AMPA and KA evoke a similar novel neural network discharge pattern transformation type by acting on pharmacologically distinct AMPAR and KA receptors. This shows that already the neonatal LC can generate oscillatory network behaviors that may be important, for example, for responses to opioids.

NMDA Enhances and Glutamate Attenuates Synchrony of Spontaneous Phase-Locked Locus Coeruleus Network Rhythm in Newborn Rat Brain Slices

Locus coeruleus (LC) neurons are controlled by glutamatergic inputs. Here, we studied in brain slices of neonatal rats NMDA and glutamate effects on phase-locked LC neuron spiking at ~1 Hz summating to ~0.2 s-lasting bell-shaped local field potential (LFP). NMDA: 10 μM accelerated LFP 1.7-fold, whereas 25 and 50 μM, respectively, increased its rate 3.2- and 4.6-fold while merging discrete events into 43 and 56% shorter oscillations. After 4–6 min, LFP oscillations stopped every 6 s for 1 s, resulting in ‘oscillation trains’. A dose of 32 μM depolarized neurons by 8.4 mV to cause 7.2-fold accelerated spiking at reduced jitter and enhanced synchrony with the LFP, as evident from cross-correlation. Glutamate: 25–50 μM made rhythm more irregular and the LFP pattern could transform into 2.7-fold longer-lasting multipeak discharge. In 100 μM, LFP amplitude and duration declined. In 25–50 μM, neurons depolarized by 5 mV to cause 3.7-fold acceleration of spiking that was less synchronized with LFP. Both agents: evoked ‘post-agonist depression’ of LFP that correlated with the amplitude and kinetics of V_m hyperpolarization. The findings show that accelerated spiking during NMDA and glutamate is associated with enhanced or attenuated LC synchrony, respectively, causing distinct LFP pattern transformations. Shaping of LC population discharge dynamics by ionotropic glutamate receptors potentially fine-tunes its influence on brain functions.