Locus Coeruleus Optogenetic Light Activation Induces Long-Term Potentiation of Perforant Path Population Spike Amplitude in Rat Dentate Gyrus

Abstract

Norepinephrine (NE) in dentate gyrus (DG) produces NE-dependent long-term potentiation (NE-LTP) of the perforant path-evoked potential population spike both in vitro and in vivo. Chemical activators infused near locus coeruleus (LC), the source of DG NE, produce a NE-LTP that is associative, i.e., requires concurrent pairing with perforant path (PP) input. Here, we ask if LC optogenetic stimulation that allows us to activate only LC neurons can induce NE-LTP in DG. We use an adeno-associated viral vector containing a depolarizing channel (AAV8-Ef1a-DIO-eChR2(h134r)-EYFP-WPRE) infused stereotaxically into the LC of TH:Cre rats to produce light-sensitive LC neurons. A co-localization of ~62% in LC neurons was observed for these channels. Under urethane anesthesia, we demonstrated that 5–10 s 10 Hz trains of 30 ms light pulses in LC reliably activated neurons near an LC optoprobe. Ten minutes of the same train paired with 0.1 Hz PP electrical stimulation produced a delayed NE-LTP of population spike amplitude, but not EPSP slope. A leftward shift in the population spike input/output curve at the end of the experiment was also consistent with long-term population spike potentiation. LC neuron activity during the 10 min light train was unexpectedly transient. Increased LC neuronal firing was seen only for the first 2 min of the light train. NE-LTP was more delayed and less robust than reported with LC chemo-activation. Previous estimates of LC axonal conduction times suggest acute release of NE occurs 40–70 ms after an LC neuron action potential. We used single LC light pulses to examine acute effects of NE release and found potentiated population spike amplitude when a light pulse in LC occurred 40–50 ms, but not 20–30 ms, prior to a PP pulse, consistent with conduction estimates. These effects of LC optogenetic activation reinforce evidence for a continuum of NE potentiation effects in DG. The single pulse effects mirror an earlier report using LC electrical stimulation. These acute effects support an attentional role of LC activation. The LTP of PP responses induced by optogenetic LC activation is consistent with the role of LC in long-term learning and memory.

A Feasibility Study to Investigate Chemogenetic Modulation of the Locus Coeruleus by Means of Single Unit Activity

“…We found that LC unit recording is feasible following manipulations to induce DREADD expression although the yield is relatively low as we were only able to perform successful electrophysiological recordings in 35% of the animals. This is similar to other research groups targeting locus coeruleus with success rates around 45% and is possibly due to the small size and location of the nucleus in the pontine brainstem (Quinlan et al, 2019). In the current experimental setting, we were not able to draw conclusions on the feasibility to chemogenetically increase LC neuronal activity with clozapine.…”

“…An alternative for increasing transduction efficiency and selectivity is the use of a Cre-Lox system for hM3Dq expression in LC (Witten et al, 2011). The group of Harley and Walling et al used a TH:Cre rat line to induce expression of an opsin in LC by injecting a Cre-dependent AAV with the promoter and gene of interest between loxP sites, assuring selective expression in LC cells that contain Cre recombinase (Quinlan et al, 2019). Because of these findings we believe that for further optimization of the experimental approach studied in this work, transduction efficiency studies with different viral vector serotypes is required when targeting a new brain structure such as a brainstem nucleus, even when a similar type of neurons i.e., noradrenergic neurons are the primary content of the nucleus.…”

The eyes reflect an internal cognitive state hidden in the population activity of cortical neurons

“…Although the LC is limited in size (~3mm caudorostrally) and buried deep within the brainstem [78,79], several studies have successfully recorded from single neurons in LC of the macaque [26,27,[80][81][82][83][84]. In addition, LC can be activated using optogenetics [85][86][87][88], electrical microstimulation [14,16,58], and pharmacological manipulations [17,89,90]. Thus, it should be possible to alter the course of slow drift in the cortex using some, if not all, of these methods.…”

Locus Coeruleus Activation Patterns Differentially Modulate Odor Discrimination Learning and Odor Valence in Rats

“…The surgical procedure was carried out following appropriate sterilization. A hole was drilled in the skull (12.3-mm posterior and 1.3-mm left lateral to bregma) and an optrode, assembled just before the experiment (400-μm glass optical fiber; Thorlabs Inc), bundled with a 200/280-μm tungsten electrode; FHC), was lowered down at a 20° angle to 6.1–6.9-mm ventral to brain surface until LC neurons were identified by slow spontaneous spiking and burst response to toe pinch (audio-monitor and oscilloscope response; Quinlan et al. 2018 ).…”