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The Interpeduncular nucleus is a unpaired, ovoid cell group at the base of the midbrain tegmentum. It is located in the mesencephalon below the interpeduncular fossa. As the name suggests, the interpeduncular nucleus lies in between the cerebral peduncles.

Composition
The Interpeduncular nucleus is primarialy GABAergic and contains at least two neuron clusters of different morphologies. The region is divided into 4 paired and 3 unpaired subnuclei.

Subdivisions
The presence of non-homologous subdivisions of the Interpeduncular nucleus was first noticed by Cajal over a hundred years ago. The currently recognized standard subdivision notation was mostly established by Hammill and Lenn in 1984 by combining the work and notations of four groups. Although most of their proposed convention stuck, at some point the proposed "rostral lateral" sub-nucleus was renamed "dorsomedial" and became immortalized in brain atlases. This unpaired region was originally thought to be an extension of the Dorsal raphe nucleus and contains serotonergic neurons. The apical sub-nucleus makes up 10% of the IPN. Former names include "cuadal dorsal", "dorsal", and "pars dorsalis magnocellularis." This region is thought to have reciprocal connections with the IPR. This region is notable for its intense acetylcholinesterase(AChE) staining. This sub-nucleus has the weakest innervation from the medial Habenula of any of the other sub-nuclei. This unpaired region makes up aproximately 13% of the IPN. Former names include: "rostral ventral", "caudal central", "posterior inter", and "pars medianus". This paired sub-nucleus makes up 10% of the IPN This paired sub-nucleus makes up approximately 2% of the IPN. Former names include: "rostral lateral"and "interstitial." This paired subnucleus makes up 25% of the IPN's volume. Former names include: "caudal intermediate", "posterior inner" and "pars medianus." This subdivision has the strongest Cytochrome oxidase staining and is thought to develop later than the surrounding regions. The IPI has perhaps the strongest cholinergic medial habenula innervation of any sub-IPN region. This paired subnucleus makes up 10% of the IPN's volume. Former names include: "caudal lateral", "paramedian" and "pars lateralis." This sudivision is notable for its intense acetylcholinesterase(AChE) staining This unpaired subnucleus makes up 30% of the IPN's volume. Former names include: "central" and "pars dorsalis."
 * Apical sub-nucleus (IPA)
 * Central sub-nucleus (IPC)
 * Dorsolateral sub-nucleus (IPDL)
 * Dorsomedial sub-nucleus (IPDM)
 * Intermediate sub-nucleus (IPI)
 * Lateral sub-nucleus (IPL)
 * Rostral sub-nucleus (IPR)

Inputs
The major input to the IPN arrives via the fasciculus retroflex from the medial habenula. This pathway presents the IPN with several excitatory neurotransmitters including ACh, Glutamate, L-Enkephalin and Substance P. Other brain regions that project to the Interpeduncular nucleus include: the Nucleus of diagonal band, the dorsal Tegmentum, the Raphe nuclei, the Central grey, and the Locus coeruleus.

Outputs
The major output pathways are inhibitory and project to the dorsal Tegmental area ie. the Periaqueductal gray and also the Raphe nuclei. Additionally, output is sent to the following: the mediodorsal nucleus of the Thalamus, the lateral Habenula, the Septal nuclei, the anterior Mammillary nuclei, the Nucleus of diagonal band, the Preoptic area, the Ventral tegmental area, and in some species the dorsal and/or lateral Hypothalmus.

Function
The Interpeduncular nucleus is thought to have a broad inhibitory effects on many other brain regions. IPN activity is linked with decreased dopamine release and utilization from dopamine producing regions. The Interpeduncular nucleus is implicated with a role in the regulation of Rapid eye movement sleep The IPN appears to inhibit the Septal nuclei which is the functional equivalent of turning off euphoria. The projection to the Mammilary bodies suggests input in the processing of recognition memory. Projection to the Preoptic area suggest the IPN modulates REM sleep onset, thermal regulation(probably increases breathing and speeds of metabolism or vis versa), and inhibits sexual drive in males.

nAChR combinations
There are 241 possible combinations of functional nAChRs in the IPN: α2α2β2β2β3, α2α2β2β2α5, α2α2β2β4β3, α2α2β2β4α5, α2α2β4β4β3, α2α2β4β4α5, α2α3β2β2β3,α2α3β2β2α5, α2α3β2β4β3, α2α3β2β4α5, α2α3β4β4β3, α2α3β4β4α5, α2α4β2β2β3, α2α4β2β2α5, α2α4β2β4β3, α2α4β2β4α5, α2α4β4β4β3, α2α4β4β4α5, α2α6β2β2β3, α2α6β2β2α5, α2α6β2β4β3, α2α6β2β4α5, α2α6β4β4β3, α2α6β4β4α5, α3α3β2β2β3,α3α3β2β2α5, α3α3β2β4β3, α3α3β2β4α5, α3α3β4β4β3, α3α3β4β4α5, α3α4β2β2β3, α3α4β2β2α5, α3α4β2β4β3, α3α4β2β4α5, α3α4β4β4β3, α3α4β4β4α5, α3α6β2β2β3, α3α6β2β2α5, α3α6β2β4β3, α3α6β2β4α5, α3α6β4β4β3, α3α6β4β4α5, α4α4β2β2β3, α4α4β2β2α5, α4α4β2β4β3, α4α4β2β4α5, α4α4β4β4β3, α4α4β4β4α5, α4α6β2β2β3, α4α6β2β2α5, α4α6β2β4β3, α4α6β2β4α5, α4α6β4β4β3, α4α6β4β4α5, α6α6β2β2β3, α6α6β2β2α5, α6α6β2β4β3, α6α6β2β4α5, α6α6β4β4β3, α6α6β4β4α5, α2α2β2β2β2, α2α2β2β2β4, α2α2β2β4β2, α2α2β2β4β4, α2α2β4β4β2, α2α2β4β4β4, α2α3β2β2β2, α2α3β2β2β4, α2α3β2β4β2, α2α3β2β4β4, α2α3β4β4β2, α2α3β4β4β4, α2α4β2β2β2, α2α4β2β2β4, α2α4β2β4β2, α2α4β2β4β4, α2α4β4β4β2, α2α4β4β4β4, α2α6β2β2β2, α2α6β2β2β4, α2α6β2β4β2, α2α6β2β4β4, α2α6β4β4β2, α2α6β4β4β4, α3α3β2β2β2, α3α3β2β2β4, α3α3β2β4β2, α3α3β2β4β4, α3α3β4β4β2, α3α3β4β4β4, α3α4β2β2β2, α3α4β2β2β4, α3α4β2β4β2, α3α4β2β4β4, α3α4β4β4β2, α3α4β4β4β4, α3α6β2β2β2, α3α6β2β2β4, α3α6β2β4β2, α3α6β2β4β4, α3α6β4β4β2, α3α6β4β4β4, α4α4β2β2β2, α4α4β2β2β4, α4α4β2β4β2, α4α4β2β4β4, α4α4β4β4β2, α4α4β4β4β4, α4α6β2β2β2, α4α6β2β2β4, α4α6β2β4β2, α4α6β2β4β4, α4α6β4β4β2, α4α6β4β4β4, α6α6β2β2β2, α6α6β2β2β4, α6α6β2β4β2, α6α6β2β4β4, α6α6β4β4β2, α6α6β4β4β4, α2α2β2β2α2, α2α2β2β2α3, α2α2β2β4α2, α2α2β2β4α3, α2α2β4β4α2, α2α2β4β4α3, α2α3β2β2α2, α2α3β2β2α3, α2α3β2β4α2, α2α3β2β4α3, α2α3β4β4α2, α2α3β4β4α3, α2α4β2β2α2, α2α4β2β2α3, α2α4β2β4α2, α2α4β2β4α3, α2α4β4β4α2, α2α4β4β4α3, α2α6β2β2α2, α2α6β2β2α3, α2α6β2β4α2, α2α6β2β4α3, α2α6β4β4α2, α2α6β4β4α3, α3α3β2β2α2, α3α3β2β2α3, α3α3β2β4α2, α3α3β2β4α3, α3α3β4β4α2, α3α3β4β4α3, α3α4β2β2α2, α3α4β2β2α3, α3α4β2β4α2, α3α4β2β4α3, α3α4β4β4α2, α3α4β4β4α3, α3α6β2β2α2, α3α6β2β2α3, α3α6β2β4α2, α3α6β2β4α3, α3α6β4β4α2, α3α6β4β4α3, α4α4β2β2α2, α4α4β2β2α3, α4α4β2β4α2, α4α4β2β4α3, α4α4β4β4α2, α4α4β4β4α3, α4α6β2β2α2, α4α6β2β2α3, α4α6β2β4α2, α4α6β2β4α3, α4α6β4β4α2, α4α6β4β4α3, α6α6β2β2α2, α6α6β2β2α3, α6α6β2β4α2, α6α6β2β4α3, α6α6β4β4α2, α6α6β4β4α3, α2α2β2β2α4, α2α2β2β2α6, α2α2β2β4α4, α2α2β2β4α6, α2α2β4β4α4, α2α2β4β4α6, α2α3β2β2α4, α2α3β2β2α6, α2α3β2β4α4, α2α3β2β4α6, α2α3β4β4α4, α2α3β4β4α6, α2α4β2β2α4, α2α4β2β2α6, α2α4β2β4α4, α2α4β2β4α6, α2α4β4β4α4, α2α4β4β4α6, α2α6β2β2α4, α2α6β2β2α6, α2α6β2β4α4, α2α6β2β4α6, α2α6β4β4α4, α2α6β4β4α6, α3α3β2β2α4, α3α3β2β2α6, α3α3β2β4α4, α3α3β2β4α6, α3α3β4β4α4, α3α3β4β4α6, α3α4β2β2α4, α3α4β2β2α6, α3α4β2β4α4, α3α4β2β4α6, α3α4β4β4α4, α3α4β4β4α6, α3α6β2β2α4, α3α6β2β2α6, α3α6β2β4α4, α3α6β2β4α6, α3α6β4β4α4, α3α6β4β4α6, α4α4β2β2α4, α4α4β2β2α6, α4α4β2β4α4, α4α4β2β4α6, α4α4β4β4α4, α4α4β4β4α6, α4α6β2β2α4, α4α6β2β2α6, α4α6β2β4α4, α4α6β2β4α6, α4α6β4β4α4, α4α6β4β4α6, α6α6β2β2α4, α6α6β2β2α6, α6α6β2β4α4, α6α6β2β4α6, α6α6β4β4α4, α6α6β4β4α6, α7α7α7α7α7

Repetitive Garbage
Locus Coeruleus= Sends Norepinephrin to the IPC and IPI---norepinephrine will stimulate the secretion of corticotropin-releasing factor IPN synthesizes Galanin and probably has galanin receptors Galanin effects locus coeruleus and VTA Raphe Nuclei sends serotonin substance P? NE? Stimulation of this region partially removes morphine withdrawal opioid receptors exist here and in the IPN reduced activity in the raphe nuclei produces relaxation of muscles Nucleus of diagonal band - regulates hippocamal formation Dorsal Tegmental Area- aka-Periaquaductal gray - Projects ekephalin to raphe and IPN associated with freezing and defensive behavior increased blood pressure and muscle tone inhibition= increased locomor activity also associated with lordosis lession=unconcious IPN probably increases locomotor activity The IPN appears to inhibit the Septal nuclei which is the functional equivalent of turning off euphoria. The projection to the Mammilary bodies suggests input in the processing of recognition memory. Projection to the Preoptic area suggest the IPN modulates REM sleep onset, thermal regulation(probably increases breathing and speeds of metabolism or vis versa), and inhibits sexual drive in males. Septum plays a role in the generation of theta waves