Cocaine withdrawal reduces GABA_BR transmission at entopeduncular nucleus – lateral habenula synapses

Abstract

Lateral habenula (LHb) hyperactivity plays a pivotal role in the emergence of negative emotional states, including those occurring during withdrawal from addictive drugs. We have previously implicated cocaine-driven adaptations at synapses from the entopeduncular nucleus (EPN) to the LHb in this process. Specifically, ionotropic GABA receptor (R)-mediated neurotransmission at EPN-to-LHb synapses is reduced during cocaine withdrawal, due to impaired vesicle filling. Recent studies have shown that metabotropic GABA R signaling also controls LHb activity, although its role at EPN-to-LHb synapses during drug withdrawal is unknown. Here, we predicted that cocaine treatment would reduce GABA R-mediated neurotransmission at EPN-to-LHb synapses. We chronically treated mice with saline or cocaine, prepared brain slices after two days of withdrawal and performed voltage-clamp recordings from LHb neurons whilst optogenetically stimulating EPN terminals. Compared with controls, mice in cocaine withdrawal exhibited reduced GABA R-mediated input to LHb neurons, and a reduced occurrence of GABA R-signaling at EPN-to-LHb synapses. We then assessed the underlying mechanism of this decrease. Application of GABA R agonist baclofen evoked similar postsynaptic responses in EPN-innervated LHb neurons in saline- and cocaine-treated mice. Release probability at EPN-to-LHb GABAergic synapses was also comparable between groups. However, incubating brain slices in glutamine to facilitate GABA vesicle filling, normalized GABA R-currents at EPN-to-LHb synapses in cocaine-treated mice. Overall, we show that during cocaine withdrawal, together with reduced GABA R transmission, also GABA R-mediated inhibitory signaling is diminished at EPN-to-LHb synapses, likely via the same presynaptic deficit. In concert, these alterations are predicted to contribute to the emergence of drug withdrawal symptoms, facilitating drug relapse.

Entopeduncular Nucleus Projections to the Lateral Habenula Contribute to Cocaine Avoidance

“…Notably, other studies have also implicated the rEPN-LHb pathway in cocaine's aversive effects, specifically the anxiogentic symptoms of cocaine withdrawal (Meye et al, 2016;Tan et al, 2019). Our findings corroborate those, while showing a somewhat different influence on conditioned responses to the drug itself, as rEPN inactivation in a runway operant task enhanced cocaine-seeking behaviors, while inhibiting the rEPN-LHb pathway blocked cocaine-induced CPA.…”

Opposite responses to aversive stimuli in lateral habenula neurons

“…On the other hand, Fs‐mediated phasic inhibition of LHb neuronal firing requires solely GABA A receptor activation without involving the slow component mediated by GABA B receptors (Tan, Nuno‐Perez, Mameli, & Meye, ; Tchenio, Lecca, Valentinova, & Mameli, ). Altogether, these data provide a cellular logic underlying phasic Fs responses in the LHb.…”

Studying Synaptic Connectivity and Strength with Optogenetics and Patch-Clamp Electrophysiology

“…Aside from detecting fast ionotropic signaling, CRACM approaches can be used to assess current responses generated by postsynaptic metabotropic receptor activation. Compared to ionotropic current responses, metabotropic receptor-mediated currents tend to depend on downstream effector channels (e.g., G protein-coupled inwardly rectifying potassium channels, GIRKs), have slower kinetics and often require trains of light stimulation pulses delivered at relatively high frequencies (e.g., >20 Hz) to be revealed [ 24 , 25 , 26 ] ( Figure 1 B).…”