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

Abstract

Already in newborns, the locus coeruleus (LC) controls multiple brain functions and may have a complex organization as in adults. Our findings in newborn rat brain slices indicate that LC neurons (i) generate at ~1 Hz a ~0.3 s-lasting local field potential (LFP) comprising summated phase-locked single spike discharge, (ii) express intrinsic ‘pacemaker’ or ‘burster’ properties and (iii) receive solely excitatory or initially excitatory–secondary inhibitory inputs. μ-opioid or ɑ2 noradrenaline receptor agonists block LFP rhythm at 100–250 nM whereas slightly lower doses transform its bell-shaped pattern into slower crescendo-shaped multipeak bursts. GABAA and glycine receptors hyperpolarize LC neurons to abolish rhythm which remains though unaffected by blocking them. Rhythm persists also during ionotropic glutamate receptor (iGluR) inhibition whereas <10 mV depolarization during iGluR agonists accelerates spiking to cause subtype-specific fast (spindle-shaped) LFP oscillations. Similar modest neuronal depolarization causing a cytosolic Ca2+ rise occurs (without effect on neighboring astrocytes) during LFP acceleration by CNQX activating a TARP-AMPA-type iGluR complex. In contrast, noradrenaline lowers neuronal Ca2+ baseline via ɑ2 receptors, but evokes an ɑ1 receptor-mediated ‘concentric’ astrocytic Ca2+ wave. In summary, the neonatal LC has a complex (possibly modular) organization to enable discharge pattern transformations that might facilitate discrete actions on target circuits.

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

“…At this section level, the neonatal rat LC is not surrounded by, or intermingled with, another obvious brain nucleus. It can thus be identified as a dense cluster of large neuron somata with a diameter of ≥20 µm as illustrated in our previous reports [ 27 , 30 , 31 , 32 ].…”

“…So far, it has not been studied which activity patterns the neonatal LC can generate in vivo under physiological conditions, how the tone of LC neuron spiking is modulated by (sensory) inputs from remote afferents or (neighboring) interneurons and intrinsic neuromodulators. Regarding intrinsic neuromodulators, we noted recently [ 31 ] that the LFP rhythm in newborn rat slices is not depressed by blockers of μ-opioid and (auto) α 1 , α 2 , or β NA receptors. This indicates that, in addition to Glu acting on iGluR, GABA acting GABA A R, or the activation of glycine receptors, neither NA or endogenous opioids are involved in generating the pattern of this spontaneous activity.…”

“…This indicates that, in addition to Glu acting on iGluR, GABA acting GABA A R, or the activation of glycine receptors, neither NA or endogenous opioids are involved in generating the pattern of this spontaneous activity. Here, we can only hypothesize that, in the intact LC, specific Glu input to neurons of one of the presumptive modules (for references, see [ 31 ] would evoke the oscillations mainly in this subnetwork.…”

“…The access resistance typically ranged between 10–50 MΩ and was stable in >90% of neurons even during recordings lasting >1 h. For determining the neuronal input resistance ranging from 120–370 MΩ, hyperpolarizing current pulses (50–100 pA) were injected, mostly at an interval of 10–15 s. Only cells were analyzed in which the spike amplitude was >70 mV while the V m was stable for a 5 min control period. Due to ongoing subthreshold oscillations (STOs) [ 22 , 31 ], LC neurons do not have a ‘resting’ V m which was thus defined as the value at 50% of the interval between spikes at the oscillation peak.…”

“…It was the aim of the present study to use the same in vitro approaches as in our latter report [ 1 ] to investigate whether AMPARs and KARs are both functional, have different pharmacological properties and may thus have distinct effects on the neonatal LC network. Such information will enhance the understanding of mechanistic neonatal LC network organization to study in the future, e.g., whether (one of) these iGluR sub-types contributes to the in vivo-like opioid-evoked slow bursting that we found in newborn rat slices [ 30 , 31 ].…”