CCKergic Tufted Cells Differentially Drive Two Anatomically Segregated Inhibitory Circuits in the Mouse Olfactory Bulb.

Delineation of functional synaptic connections is fundamental to understanding sensory processing. Olfactory signals are synaptically processed initially in the olfactory bulb (OB) where neural circuits are formed among inhibitory interneurons and the output neurons mitral cells (MCs) and tufted cells (TCs). TCs function in parallel with but differently from MCs and are further classified into multiple subpopulations based on their anatomic and functional heterogeneities. Here, we combined optogenetics with electrophysiology to characterize the synaptic transmission from a subpopulation of TCs, which exclusively express the neuropeptide cholecystokinin (CCK), to two groups of spatially segregated GABAergic interneurons, granule cells (GCs) and glomerular interneurons in mice of both sexes with four major findings. First, CCKergic TCs receive direct input from the olfactory sensory neurons (OSNs). This monosynaptic transmission exhibits high fidelity in response to repetitive OSN input. Second, CCKergic TCs drive GCs through two functionally distinct types of monosynaptic connections: (1) dendrodendritic synapses onto GC distal dendrites via their lateral dendrites in the superficial external plexiform layer (EPL); (2) axodendritic synapses onto GC proximal dendrites via their axon collaterals or terminals in the internal plexiform layer (IPL) on both sides of each bulb. Third, CCKergic TCs monosynaptically excite two subpopulations of inhibitory glomerular interneurons via dendrodendritic synapses. Finally, sniff-like patterned activation of CCKergic TCs induces robust frequency-dependent depression of the dendrodendritic synapses but facilitation of the axodendritic synapses. These results demonstrated important roles of the CCKergic TCs in olfactory processing by orchestrating OB inhibitory activities.SIGNIFICANCE STATEMENT Neuronal morphology and organization in the olfactory bulb (OB) have been extensively studied, however, the functional operation of neuronal interactions is not fully understood. We combined optogenetic and electrophysiological approaches to investigate the functional operation of synaptic connections between a specific population of excitatory output neuron and inhibitory interneurons in the OB. We found that these output neurons formed distinct types of synapses with two populations of spatially segregated interneurons. The functional characteristics of these synapses vary significantly depending on the presynaptic compartments so that these output neurons can dynamically rebalance inhibitory feedback or feedforward to other neurons types in the OB in response to dynamic rhythmic inputs.

Dose-dependent effects of adenosine antagonists on tacrine-induced tremulous jaw movements.

The present study examines the effect of three adenosine receptor antagonists on tremulous jaw movements (TJMs), an animal model of tremor. Forty-five rats were pre-treated with one adenosine antagonist: caffeine (0.0, 5.0, or 10.0 mg/kg; non-selective adenosine receptor antagonist), 8-cyclopentyltheophylline (CPT; 0.0, 5.0, or 10.0 mg/kg; selective adenosine A1 receptor antagonist), or SCH 58261 (0.0 or 8.0 mg/kg; selective adenosine A2A receptor antagonist) followed by TJM induction with tacrine (0.0, 0.75, or 2.5 mg/kg; acetylcholinesterase inhibitor). CPT and SCH 58261 both significantly reduced TJMs while caffeine did not. Unexpectedly, both SCH 58261 and CPT reduced TJMs even in the absence of tacrine. Also, CPT showed a robust reduction of TJMs, achieved at both (5.0 mg/kg) and (10.0 mg/kg) doses and regardless of tacrine dose. In conclusion, this study shows adenosine receptor antagonism to generally suppress low-dose tacrine-induced TJMs. In concert with two prior studies, these results are suggestive of behavioral evidence for a biphasic effect of adenosine A2A receptor antagonists (caffeine and SCH 58261) that is modulated by tacrine, and a model of this effect is proposed.

Pituitary adenylate cyclase activating polypeptide induces long-term, transcription-dependent plasticity and remodeling at autonomic synapses.

Pituitary adenylate cyclase activating polypeptide (PACAP) is a multifunctional neuropeptide, widely expressed in the nervous system (Vaudry et al., 2009; Starr and Margiotta, 2016). At neuronal synapses where transmission is mediated by nicotinic acetylcholine receptors (nAChRs) transient PACAP exposure increases the frequency and amplitude (FS and AS) of spontaneous excitatory postsynaptic currents (sEPSCs) within minutes. This short-term (ST) plasticity requires high-affinity PACAP receptor (PAC1R) signaling via adenylate cyclase (AC), cyclic AMP (cAMP), Protein kinase A (PKA) and obligatory nAChR-dependent stimulation of nitric oxide (NO) synthesis to retrogradely increase presynaptic ACh release (Pugh et al., 2010; Jayakar et al., 2014). Remarkably, synaptic changes persist 48h after transient PACAP exposure, featuring a similar increase in FS and an even larger increase in AS. Pharmacological studies reveal that this long-term (LT) plasticity requires PACAP/PAC1R signaling via AC and cAMP, but unlike ST plasticity, Phospholipase-C and new gene transcription are also necessary, whereas PKA, nAChR, impulse and NO synthase (NOS1) activities are dispensable. In accord with the increases in FS and AS characterizing LT plasticity, miniature EPSC (mEPSC) frequency, ACh release (quantal content), and mEPSC amplitude (quantal size) all increased in parallel. Consistent with these functional changes, imaging studies reveal that LT, but not ST, PACAP-induced plasticity is accompanied by increases in presynaptic terminal size, postsynaptic nAChR cluster size and density, and the size and density of co-localized pre- and post-synpatic sites. Thus PACAP/PAC1R signaling induces mechanistically distinct forms of synaptic plasticity, with a ST form arising from acute, membrane-delimited processes, and a LT form arising from transcription-dependent alterations in the function and structural arrangement of pre- and post-synaptic components.

PACAP induces plasticity at autonomic synapses by nAChR-dependent NOS1 activation and AKAP-mediated PKA targeting.

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic neuropeptide found at synapses throughout the central and autonomic nervous system. We previously found that PACAP engages a selective G-protein coupled receptor (PAC1R) on ciliary ganglion neurons to rapidly enhance quantal acetylcholine (ACh) release from presynaptic terminals via neuronal nitric oxide synthase (NOS1) and cyclic AMP/protein kinase A (PKA) dependent processes. Here, we examined how PACAP stimulates NO production and targets resultant outcomes to synapses. Scavenging extracellular NO blocked PACAP-induced plasticity supporting a retrograde (post- to presynaptic) NO action on ACh release. Live-cell imaging revealed that PACAP stimulates NO production by mechanisms requiring NOS1, PKA and Ca(2+) influx. Ca(2+)-permeable nicotinic ACh receptors composed of α7 subunits (α7-nAChRs) are potentiated by PKA-dependent PACAP/PAC1R signaling and were required for PACAP-induced NO production and synaptic plasticity since both outcomes were drastically reduced following their selective inhibition. Co-precipitation experiments showed that NOS1 associates with α7-nAChRs, many of which are perisynaptic, as well as with heteromeric α3*-nAChRs that generate the bulk of synaptic activity. NOS1-nAChR physical association could facilitate NO production at perisynaptic and adjacent postsynaptic sites to enhance focal ACh release from juxtaposed presynaptic terminals. The synaptic outcomes of PACAP/PAC1R signaling are localized by PKA anchoring proteins (AKAPs). PKA regulatory-subunit overlay assays identified five AKAPs in ganglion lysates, including a prominent neuronal subtype. Moreover, PACAP-induced synaptic plasticity was selectively blocked when PKA regulatory-subunit binding to AKAPs was inhibited. Taken together, our findings indicate that PACAP/PAC1R signaling coordinates nAChR, NOS1 and AKAP activities to induce targeted, retrograde plasticity at autonomic synapses. Such coordination has broad relevance for understanding the control of autonomic synapses and consequent visceral functions.