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Habenular projections in the monitor lizard (Varanus benegalensis)

Summary

The efferent connections of the medial (MHb) and the lateral (LHb) habenular nuclei in the monitor lizard were studied using experimental degeneration techniques. The MHb was found to project to the interpeduncular nucleus and the parvocellular nucleus of the superior raphe via the core portion of the habenulo-peduncular tract (HPT). The LHb fibers form the mantle portion of the HPT and curve laterally to collect again in the ventral tegmentum. From here, they follow either (1) the medial forebrain bundle to terminate in hypothalamus, ventromedial thalamus, preoptic area, and septum, or (2) they continue caudally to terminate in the superior raphe and the paramedian reticular formation, or (3) they decussate and follow in smaller numbers the ascending and descending pathways on the other side. Some fibers enter the midline and reach the periventricular zone of the midbrain. Short range projections exist to the dorsomedial thalamic nucleus and the paramedian central gray and pretectum. The habenular projections are bilateral, however, much smaller on the contralateral side. Although distinct terminal fields were not found in the substahtia nigra and the central gray of the isthmic region, the overall pattern of habenular pathways is strikingly similar to those found in mammals which confirms a long presumed phylogenetic stability of habenular connections.

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Abbreviations

AC:

anterior commissure

DLA:

nucleus dorsolateralis anterior thalami

DM:

nucleus dorsomedialis thalami

EP:

entopeduncular nucleus

GLd:

nucleus geniculatus lateralis, pars dorsalis

GLv:

nucleus geniculatus lateralis, pars ventralis

GPT:

nucleus geniculatus praetectalis

Hb:

habenula

HbC:

habenular commissure

HPT:

habenulo-peduncular tract

Hy:

hypothalamus

IP:

interpeduncular nucleus

LFB:

lateral forebrain bundle

LHb:

lateral habenular nucleus

LHy:

nucleus lateralis hypothalami

MFB:

medial forebrain bundle

MHb:

medial habenular nucleus

N:

neostriatum

NIII:

oculomotor nucleus

nIII:

oculomotor nerve

nIV:

trochlear nerve

Pa:

paleostriatum

PC:

posterior commissure

PD:

nucleus posterodorsalis

PHy:

nucleus paraventricularis hypothalami

Ra:

raphe nuclei

Re:

nucleus reuniens

Rt:

nucleus rotundus

Ru:

nucleus ruber

RMS:

nucleus magnocellularis raphe superior

RPS:

nucleus parvocellularis raphe superior

SAP:

stratum album periventriculare

SGP:

stratum griseum periventriculare

Se:

septum

SMe:

stria medullaris

SRF:

superior reticular formation

TO:

nucleus opticus tegmenti

VHy:

nucleus ventralis hypothalami

VL:

nucleus ventrolateralis thalami

VM:

nucleus ventromedialis thalami

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Author information

Correspondence to Dr. H. Distel.

Additional information

Supported by a postdoctoral fellowship to H. Distel from the Alfred Sloan Foundation and by the Alexander von Humboldt Stiftung

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Distel, H., Ebbesson, S.O.E. Habenular projections in the monitor lizard (Varanus benegalensis). Exp Brain Res 43, 324–329 (1981). https://doi.org/10.1007/BF00238374

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Key words

  • Reptiles
  • Habenular nuclei
  • Efferent connections
  • Fasciculus retroflexus
  • Medial forebrain bundle