DREADDing the lateral habenula: a review of methodological approaches for studying lateral habenula function

Abstract

The lateral habenula (LHb) is part of the habenular complex in the dorsal diencephalon. The LHb is an important regulator of several neurotransmitter systems in the midbrain; disturbances in this regulation may contribute to mood disorders, abnormalities in cognition, drive, and addiction. Owing to the critical role this nucleus plays in modulating activity of midbrain nuclei, there has been a rapid increase in studies targeting the LHb in the recent years. In this review, we describe studies using traditional approaches to elucidate the function of this brain region, such as lesion, electrical and chemical stimulation, electrophysiology and in vivo microdialysis. We have selected a variety of illustrative studies to discuss each of these methods. Next, we describe studies using methods that are based upon recent advances in molecular biology techniques including recent results from our laboratory using the Designer Receptor Exclusively Activated by Designer Drug (DREADD) technology. Using a Gi/o-coupled DREADD, we found that inhibition of the LHb reduces depression-like behavior in the forced swim test in a manner that suggests enhanced serotonergic activity. The emerging picture reveals that the LHb is likely to be a critical node in the network of subcortical nuclei that regulate aversive learning, motivation, stress responses, etc. We describe how recently developed methods have advanced the study of the LHb and are leading research of this brain region in promising new directions. This article is part of a Special Issue entitled Optogenetics (7th BRES).

Figure 1. Virus-mediated gene transfer

(A) Illustration of the herpes simplex virus-hM4Di green fluorescent protein transgene amplicon (B) Illustration of rat brain coordinates (Paxinos plate 32, −3.6 mm) used for viral vector infusion. The red oval depicts the target zone for the viral vector unilaterally. MHb: Medial habenula, LHb: Lateral habenula (C) Representative histological plate of demonstrating viral vector injection from a coronal section of the LHb four days after viral vector infusion.

Figure 2. Effects of HSV-hM4D mediated LHb inhibition on behavioral responses in the modified forced swim test

Viral vector was injected into the LHb and a modified forced swim test was performed. Rats treated with CNO (1 mg/kg, i.p., n=5) demonstrated an increase in swim time and a decrease in immobility time compared to vehicle treated rats (n=7). Rats treated with CNO, but with missed LHb injections (lateral to the lateral boundary of the LHb; n=3) did not demonstrate reversal of behavioral effects in the forced swim test. p< 0.001, significantly differs from vehicle-treated group.

Figure 3. FLEX switch strategy

(A) Illustration of the FLEX Viral vector design. L-ITR: Left inverted terminal repeat, R-ITR: Right inverted terminal repeat, WPRE: Woodchuck hepatitis post-transcriptional regulatory element (B) Illustration of LHb rat brain coordinates (Paxinos plate 32, −3.6 mm) used for viral vector infusion. The red oval depicts the target zone for the viral vector infusion. MHb: Medial habenula, LHb: Lateral habenula (C) Representative histological plate demonstrating viral vector injection from a coronal section of the LHb twenty four days after viral vector infusion (GFP signal is enhanced using an anti-GFP antibody).