Acetylcholine Facilitates a Depolarization-Induced Enhancement of Inhibition in Rat CA1 Pyramidal Neurons

Adolescent administration of Δ9-THC decreases the expression and function of muscarinic-1 receptors in prelimbic prefrontal cortical neurons of adult male mice

Long-term cannabis use during adolescence has deleterious effects in brain that are largely ascribed to the activation of cannabinoid-1 receptors (CB1Rs) by delta-9-tetrahydrocannabinol (∆9-THC), the primary psychoactive compound in marijuana. Systemic administration of ∆9-THC inhibits acetylcholine release in the prelimbic-prefrontal cortex (PL-PFC). In turn, PL-PFC acetylcholine plays a role in executive activities regulated by CB1R-targeting endocannabinoids, which are generated by cholinergic stimulation of muscarinic-1 receptors (M1Rs). However, the long-term effects of chronic administration of increasing doses of ∆9-THC in adolescent males on the distribution and function of M1 and/or CB1 receptors in the PL-PFC remains unresolved. We used C57BL\6J male mice pre-treated with vehicle or escalating daily doses of ∆9-THC to begin filling this gap. Electron microscopic immunolabeling showed M1R-immunogold particles on plasma membranes and in association with cytoplasmic membranes in varying sized dendrites and dendritic spines. These dendritic profiles received synaptic inputs from unlabeled, CB1R- and/or M1R-labeled axon terminals in the PL-PFC of both treatment groups. However, there was a size-dependent decrease in total (plasmalemmal and cytoplasmic) M1R gold particles in small dendrites within the PL-PFC of mice receiving ∆9-THC. Whole cell current-clamp recording in PL-PFC slice preparations further revealed that adolescent pretreatment with ∆9-THC attenuates the hyperpolarization and increases the firing rate produced by local muscarinic stimulation. Repeated administration of ∆9-THC during adolescence also reduced spontaneous alternations in a Y-maze paradigm designed for measures of PFC-dependent memory function in adult mice. Our results provide new information implicating M1Rs in cortical dysfunctions resulting from adolescent abuse of marijuana.

Muscarinic Long-Term Enhancement of Tonic and Phasic GABAA Inhibition in Rat CA1 Pyramidal Neurons

Acetylcholine (ACh) regulates network operation in the hippocampus by controlling excitation and inhibition in rat CA1 pyramidal neurons (PCs), the latter through gamma-aminobutyric acid type-A receptors (GABA_ARs). Although, the enhancing effects of ACh on GABA_ARs have been reported (Dominguez et al., 2014, 2015), its role in regulating tonic GABA_A inhibition has not been explored in depth. Therefore, we aimed at determining the effects of the activation of ACh receptors on responses mediated by synaptic and extrasynaptic GABA_ARs. Here, we show that under blockade of ionotropic glutamate receptors ACh, acting through muscarinic type 1 receptors, paired with post-synaptic depolarization induced a long-term enhancement of tonic GABA_A currents (tGABA_A) and puff-evoked GABA_A currents (pGABA_A). ACh combined with depolarization also potentiated IPSCs (i.e., phasic inhibition) in the same PCs, without signs of interactions of synaptic responses with pGABA_A and tGABA_A, suggesting the contribution of two different GABA_A receptor pools. The long-term enhancement of GABA_A currents and IPSCs reduced the excitability of PCs, possibly regulating plasticity and learning in behaving animals.

Neuromodulation of the Feedforward Dentate Gyrus-CA3 Microcircuit

The feedforward dentate gyrus-CA3 microcircuit in the hippocampus is thought to activate ensembles of CA3 pyramidal cells and interneurons to encode and retrieve episodic memories. The creation of these CA3 ensembles depends on neuromodulatory input and synaptic plasticity within this microcircuit. Here we review the mechanisms by which the neuromodulators aceylcholine, noradrenaline, dopamine, and serotonin reconfigure this microcircuit and thereby infer the net effect of these modulators on the processes of episodic memory encoding and retrieval.