Singhal, S. M. | Search bioRxiv

bioRxiv2024

Mu-opioid receptor activation potentiates excitatory transmission at the habenulo-peduncular synapse

The continuing opioid epidemic poses a huge burden on public health. Identifying the neurocircuitry involved and how opioids modulate their signaling is essential for developing new therapeutic strategies. The medial habenula (MHb) is a small epithalamic structure that projects predominantly to the interpeduncular nucleus (IPN) and represents a mu-opioid receptor (MOR) hotspot. This habenulo-peduncular (HP) circuit can regulate nicotine and opioid withdrawal; however, little is known about the physiological impact of MOR on its function. Using MOR-reporter mice, we observed that MORs are expressed in a subset of MHb and IPN cells. Patch-clamp recordings revealed that MOR activation inhibited action potential firing in MOR+ MHb neurons and induced an inhibitory outward current in IPN neurons, consistent with canonical inhibitory effects of MOR. We next used optogenetics to stimulate MOR+ MHb axons to investigate the effects of MOR activation on excitatory transmission at the HP synapse. In contrast to its inhibitory effects elsewhere, MOR activation significantly potentiated evoked glutamatergic transmission to IPN. The facilitatory effects of MOR activation on glutamate co-release was also observed from cholinergic-defined HP synapses. The potentiation of excitatory transmission mediated by MOR activation persisted in the presence of blockers of GABA receptors or voltage-gated sodium channels, suggesting a monosynaptic mechanism. Finally, disruption of MOR in the MHb abolished the faciliatory action of DAMGO, indicating that this non-canonical effect of MOR activation on excitatory neurotransmission at the HP synapse is dependent on pre-synaptic MOR expression. Our study demonstrates canonical inhibitory effects of MOR activation in somatodendritic compartments, but non-canonical faciliatory effects on evoked glutamate transmission at the HP synapse, establishing a new mode by which MOR can modulate neuronal function.

neuroscience↗

bioRxiv2024

Ventral tegmental area interneurons revisited: GABA and glutamate projection neurons make local synapses

The ventral tegmental area (VTA) contains projection neurons that release the neurotransmitters dopamine, GABA, and/or glutamate from distal synapses. VTA also contains GABA neurons that synapse locally on to dopamine neurons, synapses widely credited to a population of so-called VTA interneurons. Interneurons in cortex, striatum, and elsewhere have well-defined morphological features, physiological properties, and molecular markers, but such features have not been clearly described in VTA. Indeed, there is scant evidence that local and distal synapses originate from separate populations of VTA GABA neurons. In this study we tested whether several markers expressed in non-dopamine VTA neurons are selective markers of interneurons, defined as neurons that synapse locally but not distally. Challenging previous assumptions, we found that VTA neurons genetically defined by expression of parvalbumin, somatostatin, neurotensin, or mu-opioid receptor project to known VTA targets including nucleus accumbens, ventral pallidum, lateral habenula, and prefrontal cortex. Moreover, we provide evidence that VTA GABA and glutamate projection neurons make functional inhibitory or excitatory synapses locally within VTA. These findings suggest that local collaterals of VTA projection neurons could mediate functions prior attributed to VTA interneurons. This study underscores the need for a refined understanding of VTA connectivity to explain how heterogeneous VTA circuits mediate diverse functions related to reward, motivation, or addiction. Significance statementGABA neurons in VTA are key regulators of VTA dopamine neurons and considered central to the mechanisms by which opioids and other drugs of abuse can induce addiction. Conventionally, these VTA GABA neurons are considered interneurons, but GABA projection neurons are also abundant in VTA, and it is unclear if these represent separate populations. We found that several markers enriched in non-dopamine neurons of VTA, including Mu-opioid receptor, are also expressed in projection neurons, and thus are not selective interneuron markers. Moreover, we found that VTA GABA and glutamate projection neurons collateralize within VTA where they make local synapses. These data challenge the notion of a VTA interneuron that synapses only within VTA and suggest that inhibitory projection neurons can serve functions previously attributed to VTA interneurons.

neuroscience↗