Lateral habenula and the rostromedial tegmental nucleus innervate neurochemically distinct subdivisions of the dorsal raphe nucleus in the rat

Abstract

The lateral habenula (LHb) is an epithalamic structure differentiated in a medial (LHbM) and a lateral division (LHbL). Together with the rostromedial tegmental nucleus (RMTg), the LHb has been implicated in the processing of aversive stimuli and inhibitory control of monoamine nuclei. The inhibitory LHb influence on midbrain dopamine neurons has been shown to be mainly mediated by the RMTg, a mostly GABAergic nucleus that receives a dominant input from the LHbL. Interestingly, the RMTg also projects to the dorsal raphe nucleus (DR), which also receives direct LHb projections. To compare the organization and transmitter phenotype of LHb projections to the DR, direct and indirect via the RMTg, we first placed injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin into the LHb or the RMTg. We then confirmed our findings by retrograde tracing and investigated a possible GABAergic phenotype of DR-projecting RMTg neurons by combining retrograde tracing with in situ hybridization for GAD67. We found only moderate direct LHb projections to the DR, which mainly emerged from the LHbM and were predominantly directed to the serotonin-rich caudal DR. In contrast, RMTg projections to the DR were more robust, emerged from RMTg neurons enriched in GAD67 mRNA, and were focally directed to a distinctive DR subdivision immunohistochemically characterized as poor in serotonin and enriched in presumptive glutamatergic neurons. Thus, besides its well-acknowledged role as a GABAergic control center for the ventral tegmental area (VTA)-nigra complex, our findings indicate that the RMTg is also a major GABAergic relay between the LHb and the DR.

Review of the cytology and connections of the lateral habenula, an avatar of adaptive behaving

“…They also noted, based on a report by Behzadi et al (1990), that LHb projections to the median raphe that take up radiolabeled D-aspartate arise in their LHbMS, which contains neurons that express Fos after persistent noxious stimulation (Nagao et al, 1993). They further cited work indicating that major DA innervation of the LHb is relatively limited to their LHbMPc (Skagerberg et al, 1984), the small-celled site of striking peptidergic enrichments mentioned above, where also neurons in which Fos-immunoreactivity is elicited by daytime stress are concentrated (Chastrette et al, 1991), and which was early reported to be the LHb origin of projections to the dorsal raphe nucleus (Aghajanian and Wang, 1977; Pasquier et al, 1977; Sego et al, 2014). …”

“…In any event, the ‘feigned’ DA (but actually GABAergic and glutamatergic) mesohabenular pathway thus may disproportionately affect LHb outputs to the DR, which mainly arise from LHbMC and LHbMPc (Aghajanian and Wang, 1977; Pasquier et al, 1977; Sego et al, 2014). In line with this connectivity, Luo et al (2015) hypothesized that loss of DA neurons in Parkinson’s disease (PD) may produce a disinhibition of LHb neurons (due to loss of GABA transmission), which itself is dysphoric and may, in turn, act to decrease serotonin levels – another deficit likely to be associated with behavioral depression in PD patients.…”

“…For example, LHb stimulation produces mainly profound inhibition of midbrain DA neurons (Christoph et al, 1986; Ji and Shepard, 2007; Matsumoto and Hikosaka, 2007; 2008; 2009; but see Brown and Shepard, 2016), consistent with an LHb role as a final arbiter of negative modulation of affect in response to aversive stimuli and reward omission (Brady and Nauta, 1955; Caldecott-Hazard et al, 1988; Chastrette et al, 1991; Nagao et al, 1993; Wirtshafter et al, 1994; Vale-Martinez et al, 1997; Sica et al, 2000; Amat et al, 2001; Timofeeva and Richard, 2001; Villarreal et al, 2002; Huang and Cheng, 1998; Yakabi et al, 2002; Shumake et al, 2003; Ullsperger and Cramon, 2003; Kazi et al, 2004; Wang et al, 2004; Pobbe and Zangrossi, 2008; Smith and Lonstein, 2008; Yang et al, 2008; Shumake et al, 2010; Hong et al, 2011; Winter et al, 2011; Jhou et al, 2013; Hennigan et al, 2015; Li et al, 2016a). This major influence of LHb on the activity of DA neurons, in combination with its interactions with other brainstem ascending modulatory projection systems (e.g., Aghajanian and Wang, 1978; Pasquier et al, 1978; Reisine et al, 1982; Gottesfeld, 1983; Goto et al, 2001; Olucha-Bordinau et al, 2003; Sego et al, 2014; Ma and Gundlach, 2015; Zhou et al, 2017), fits with the kaleidoscopic breadth of reported LHb functional involvements in, e.g., olfactory-guided behavior, addiction, ingestion, endocrine control, brain stimulation reward, stress response, pain, maternal behavior, nociception, circadian rhythms, learning and memory, attention, sleep, motor activity, anxiety and depression (reviewed, together with then current information on LHb connections, in Sutherland, 1982; Risold and Swanson, 1997; Andres et al, 1999; Klemm, 2004; Lecourtier and Kelly, 2007; Bianco and Wilson, 2008; Geisler and Trimble, 2008; Hikosaka et al, 2008; Meye et al, 2013; Proulx et al, 2014). A reasonable expectation that the remarkable diversity of LHb function should be accompanied by a complex organization both in terms of its cytology and axonal connections (Andres et al, 1999) will be addressed in the following sections.…”

Gata2 and Gata3 regulate the differentiation of serotonergic and glutamatergic neuron subtypes of the dorsal raphe

“…In addition to the Vglut3 + serotonergic neurons, a specific subgroup of non-serotonergic Vglut3 + cells is located in the central dorsal raphe. Furthermore, recent studies demonstrated the importance of glutamatergic neurotransmission from the dorsal raphe to ventral tegmental area (VTA) in the regulation of motivation and reward processing (Hioki et al, 2010;Liu et al, 2014;McDevitt et al, 2014;Qi et al, 2014;Sego et al, 2014). Developmental mechanisms underlying the neuronal heterogeneity in the dorsal raphe are still poorly understood.…”

Differentiation and molecular heterogeneity of inhibitory and excitatory neurons associated with midbrain dopaminergic nuclei

“…Collectively, these GABAergic neurons are referred to as dopaminergic neuron-associated GABAergic (D-GABA) neurons . It has been suggested that the RMTg integrates signals from different parts of the brain, including the lateral habenula (LHb), converting them into inhibitory inputs to both DA and 5-HT networks (Lavezzi and Zahm, 2011;Barrot et al, 2012;Bourdy and Barrot, 2012;Proulx et al, 2014;Sego et al, 2014).…”