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Nucleus incertus
The nucleus incertus is a brainstem region of the pontine brainstem, just ventral to the 4th ventricle.[1] The term was coined by George Streeter (Latin for "uncertain nucleus") based on its unknown function at the time to name a group of cells he observed near the midline of the floor of the 4th ventricle.[2] It sometimes called the 'nucleus O'.[3] The nucleus incertus is a bilateral structure which sits near the brainstem, in front of the nucleus prepositus hypoglossi.[4] It consists of mostly ascending GABAergic projection neurons and glutamatergic neurons[5] which innervate a broad range of forebrain regions involved in behavioural activation. It is part of the theta network acting as a relay from the reticularis pontis oralis nucleus to the septo-hippocampal system.[6] The stimulation of the nucleus incertus activates the hippocampal theta rhythm and either its lesion or inhibition suppress the theta oscillation induced by brainstem stimulation.[7] The nucleus incertus itself presents theta oscillations coupled to the hippocampal theta rhythm.[8] In addition to hippocampal theta rhythms, the nucleus incertus is involved in the control of locomotor speed and arousal,[9] response to stress[3] and integrating the vestibulo-ocular reflex and gaze holding with hippocampal navigation.[6] Neuroanatomy and NeurochemistryThe NI consists of GABAergic and glutamatergic neurons that project widely to other regions of the brain, including the septum, hippocampus, hypothalamus, amygdala, interpeduncular nucleus and prefrontal cortex.[1] One of the defining neurochemical characteristics of NI GABAergic neurons is their expression of relaxin-3, a neuropeptide that acts via the G-protein-coupled receptor, known as RXFP3 in various brain regions, but can also activate RXFP1. The primary effect of RXFP3 receptor activation is the suppression of neuronal activity, which occurs mainly through the opening of M-channels, allowing an outward flow of potassium ions.[4] The relaxin-3/RXFP3 system has been extensively studied since its discovery in 2002 due to its involvement in stress and arousal-related functions.[5] This peptidergic system is preserved throughout vertebrate evolution and is present in zebrafish and several other species, including human.[6][7] Relaxin-3 (RLN3) is detected in at least two neuronal clusters in both teleosts and mammals, in the periaqueductal grey (PAG) and the NI. However, while in the teleosts the PAG/RLN3 projections target extensive areas of the forebrain and optic tectum, the NI/RLN3 projection is concentrated in the interpeduncular nucleus. By contrast, in mammals, PAG/RLN3 projections are restricted to the brainstem and diencephalon, while NI/RLN3 projections display a wide pattern of ascending projections to areas ranging from the nearby interpeduncular nucleus to the more distant hippocampus and prefrontal cortex. In both teleosts and mammals, the RLN3 signaling system plays a central role in arousal control.[9][10] In addition to relaxin-3, NI GABAergic neurons express other neuromodulators such as cholecystokinin (CCK) and neuromedin-B (NMB). These neurons also express receptors for corticotropin-releasing factor (CRF), orexins (hypocretins), melanin- concentrating hormone (MCH), serotonin (5-HT) and glutamate; and this diverse receptor expression profile suggests that the NI integrates signals from multiple neurotransmitter systems.[11] References
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