Gene expression and neurochemical characterization of the rostromedial tegmental nucleus (RMTg) in rats and mice

Abstract

The rostromedial tegmental nucleus (RMTg), also known as the tail of the ventral tegmental area (tVTA), is a GABAergic structure identified in 2009 that receives strong inputs from the lateral habenula and other sources, sends dense inhibitory projections to midbrain dopamine (DA) neurons, and plays increasingly recognized roles in aversive learning, addiction, and other motivated behaviors. In general, little is known about the genetic identity of these neurons. However, recent work identified the transcription factor FoxP1 as enhanced in the mouse RMTg (Lahti et al. 2016). Hence, in the current study, we used RNA sequencing to identify genes significantly enhanced in the rat RMTg as compared to adjacent VTA, and then examined the detailed distribution of two genes in particular, prepronociceptin (Pnoc) and FoxP1. In rats and mice, both Pnoc and FoxP1 were expressed at high levels in the RMTg and colocalized strongly with previously established RMTg markers. FoxP1 was particularly selective for RMTg neurons, as it was absent in most adjacent brain regions. We used these gene expression patterns to refine the anatomic characterization of RMTg in rats, extend this characterization to mice, and show that optogenetic manipulation of RMTg in mice bidirectionally modulates real-time place preference. Hence, RMTg neurons in both rats and mice exhibit distinct genetic profiles that correlate with their distinct connectivity and function.

Generality and opponency of rostromedial tegmental (RMTg) roles in valence processing

“…Out of 151 recorded neurons, 59 were located in the RMTg, as defined by immunostaining for FOXP1 (Lahti et al, 2016; Smith et al, 2018) (Figure 2—figure supplement 1A,B). Consistent with previous studies that RMTg neurons encode motivational valence (Hong et al, 2011; Jhou et al, 2009), we found that RMTg neurons on average showed significant inhibition to reward cues during a time window 200–400 ms post-stimulus, and rapid excitations to all other phasic stimuli 0–100 ms post-stimulus (p < 0.0001 for all stimuli) (Figure 2B; Figure 2C).…”

“…Thus, we used endoscopic calcium imaging and selectively recorded RMTg neurons that project to either the VTA or the dorsal raphe nucleus (DRN), both of which are implicated in encoding of motivational stimuli (Cohen et al, 2012; Li et al, 2016). Because of the need to keep GRIN lenses short, this experiment was performed in mice, rather than rats, but the anatomy of the RMTg is very similar between species (Smith et al, 2018). We injected into wild type mice a retrogradely transported canine adenovirus expressing Cre recombinase (CAV2-Cre) into either the VTA or (in separate mice) the DRN, along with a second virus into the RMTg expressing a Cre-dependent fluorescent calcium indicator (gCaMP6f) (Figure 4A–C, Figure 4—figure supplement 1A,B).…”

“…Notably, we observed a delay of several hundred milliseconds from the bulk of the RMTg activation to the bulk of the DA inhibition, a delay that may seem rather long for a monosynaptic connection, and certainly longer than expected for ionic conductance. However, the RMTg also expresses a number of peptides (nociceptin and somatostatin) that could mediate slower responses (Jhou et al, 2012; Smith et al, 2018).…”

Valence-encoding in the lateral habenula arises from the entopeduncular region

“…The lateral habenula (LHb) has been implicated in processing aversive events across many species (Jhou et al, 2013; Matsumoto and Hikosaka, 2008; Salas et al, 2010; Stopper and Floresco, 2014; Wang et al, 2017). Neurons in the LHb are activated by negative motivational stimuli, which in turn suppress dopamine (DA) firing via activation of GABAergic neurons in the rostromedial tegmental nucleus (RMTg) (Brown et al, 2017; Hong et al, 2011; Jhou et al, 2009a; Jhou et al, 2009b; Ji and Shepard, 2007; Smith et al, 2019; Vento et al, 2017). Dysregulation of this pathway can lead to cognitive disorders associated with abnormal processing of motivational stimuli including depression (Baker et al, 2016; Elmer et al, 2019; Lawson et al, 2017; Shumake and Gonzalez-Lima, 2003).…”

Inhibition of the rostromedial tegmental nucleus reverses alcohol withdrawal-induced anxiety-like behavior

“…One such potential target is a small GABAergic region known as the rostromedial tegmental nucleus (RMTg), also known as the tail of the VTA (tVTA). First identified in 2009, the RMTg receives glutamatergic input from the lateral habenula (LHb), sends a dense projection to the VTA thereby exerting inhibitory control over midbrain dopamine neurons, and is delineated by expression of the transcription factor FoxP1 [25][26][27]. This neural circuit (LHb-RMTg-VTA), which exhibits only modest direct connectivity with the amygdala [28], has been characterized for its involvement in negative reward prediction error [29].…”