Activation of LH GABAergic inputs counteracts fasting-induced changes in tVTA/RMTG neurons

Functional properties of corticothalamic circuits targeting paraventricular thalamic neurons

Corticothalamic projections to sensorimotor thalamic nuclei show modest firing rates and serve to modulate the activity of thalamic relay neurons. By contrast, here we find that high-order corticothalamic projections from the prelimbic (PL) cortex to the anterior paraventricular thalamic nucleus (aPVT) maintain high-frequency activity and evoke strong synaptic excitation of aPVT neurons in rats. In a significant fraction of aPVT cells, such high-frequency excitation of PL-aPVT projections leads to a rapid decay of action potential amplitudes, followed by a depolarization block (DB) that strongly limits aPVT maximum firing rates, thereby regulating both defensive and appetitive behaviors in a frequency-dependent manner. Strong inhibitory inputs from the anteroventral portion of the thalamic reticular nucleus (avTRN) inhibit the firing rate of aPVT neurons during periods of high-spike fidelity but restore it during prominent DB, suggesting that avTRN activity can modulate the effects of PL inputs on aPVT firing rates to ultimately control motivated behaviors.

The rostromedial tegmental nucleus gates fat overconsumption through ventral tegmental area output in male rats

Excessive consumption of palatable foods that are rich in fats and sugars has contributed to the increasing prevalence of obesity worldwide. Similar to addictive drugs, such foods activate the brain's reward circuit, involving mesolimbic dopaminergic projections from the ventral tegmental area (VTA) to the nucleus accumbens (NAc) and the prefrontal cortex. Neuroadaptations occurring in this circuit are hypothesized to contribute to uncontrolled consumption of such foods, a common feature of most of eating disorders and obesity. The rostromedial tegmental nucleus (RMTg), also named tail of the VTA (tVTA), is an inhibitory structure projecting to the VTA and the lateral hypothalamus (LH), two key brain regions in food intake regulation. Prior research has demonstrated that the RMTg responds to addictive drugs and influences their impact on mesolimbic activity and reward-related behaviors. However, the role of the RMTg in food intake regulation remains largely unexplored. The present study aimed to investigate the role of the RMTg and its projections to the VTA and the LH in regulating food intake in rats. To do so, we examined eating patterns of rats with either bilateral excitotoxic lesions of the RMTg or specific lesions of RMTg-VTA and RMTg-LH pathways. Rats were exposed to a 6-week 'free choice high-fat and high-sugar' diet, followed by a 4-week palatable food forced abstinence and a 24 h re-access period. Our results indicate that an RMTg-VTA pathway lesion increases fat consumption following 6 weeks of diet and at time of re-access. The RMTg-LH pathway lesion produces a milder effect with a decrease in global calorie intake. These findings suggest that the RMTg influences palatable food consumption and relapse through its projections to the VTA.

Relationship between the role of muscarinic M3 receptors in morphine-induced conditioned place preference and the mesolimbic dopamine system

Opioid use disorder mainly results from functional defects in the brain reward loop, which includs the ventral tegmental area (VTA) and nucleus accumbens (NAc; consisting of shell and core, NAcS and NAcC). Reward effects contribute to opioid use disorder. RMTg M₃ receptors play a role in opioid reward by regulating the γ-aminobutyric acid (GABA) neuron activity. Dopamine D₁ receptors expressed on GABA neurons regulate opioid reward by mediating the dopamine neuron activity in the VTA. Therefore, we investigated the effect of activating M₃ receptors by microinjecting pilocarpine into the RMTg along with activating D₁ receptors by microinjecting SKF38393 into the VTA on morphine-induced reward effect, using the conditioned place preference (CPP) paradigm (locomotion was also recorded). We also investigated whether the activation of M₃ receptors in the RMTg influenced dopamine release in the NAcS. The results showed that the inhibitory role of RMTg pilocarpine (60 μg/rat) infusions in morphine-induced CPP was reversed by VTA SKF38393 (4 μg/rat) infusions. Moreover, morphine (5 mg/kg, i.p.) increased dopamine release in the NAcS, which was blunted by microinjecting pilocarpine (60 μg/rat) into the RMTg. These results indicate that RMTg M₃ receptors mediate morphine-induced reward effect, which is probably related to the dopamine activity within the VTA and NAcS. The relationship between RMTg M₃ receptors and the mesolimbic dopamine system could be a potential direction for the treatment of opioid use disorder, but further verification through more comprehensive techniques is needed.