= Medial habenula
The \b[medial habenula] is the medial division of the <habenula>, a nuclear complex of the <epithalamus>. It participates in a circuit projecting to the <interpeduncular nucleus>. The <medial habenula> and <lateral habenula> are neighboring anatomical divisions with different circuitry; results from one should not automatically be assigned to the other.<Habenular α5 nicotinic receptor subunit signalling controls nicotine intake by Christie D. Fowler and others>[Originating 2011 article, medial-habenula/interpeduncular circuit and region-specific receptor manipulations.]{ref}
A motivating experiment tested an inhibitory contribution to <nicotine> intake. <Christie D. Fowler> and colleagues found increased <nicotine> intake in mice lacking the α5 <nicotinic acetylcholine receptor> subunit. Re-expression of that subunit in the <medial habenula> rescued the altered intake phenotype; knockdown in the <medial habenula> of rats reproduced increased intake. The work distinguished nicotine's rewarding effects from an inhibitory effect appearing at higher tested doses.<Habenular α5 nicotinic receptor subunit signalling controls nicotine intake by Christie D. Fowler and others>[Abstract: knockout, local rescue, rat knockdown and reward/inhibitory-signal distinction.]{ref}
The rescue is particularly informative. A whole-animal mutation can alter many tissues and developmental processes. Restoring the relevant component in a specified region tests whether that regional circuit can account for an important part of the phenotype. Conversely, the comparison does not show that every <medial habenula> neuron has the same function or that the entire region is merely a nicotine-detector. The measured intake change is a result in specified rodent preparations, not a direct measurement of human smoking behavior or a clinical intervention.
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