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Projections to the median eminence and the arcuate nucleus with special reference to monoamine systems: Effects of lesions

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The external layer of the median eminence (ELME) and the arcuate nucleus of male rats were studied with the Falck-Hillarp technique and electron microscopy of aldehyde-OsO4 or KMnO4 fixed material after various types of hypothalamic deafferentation experiments with the Halász knife. Special reference was paid to the monoamine systems and the results can be summarized as follows.

  1. 1.

    The main monoaminergic input to the ELME comes from the arcuate nucleus-periventricular area via a dorsal approach. A horizontal transection through the arcuate nucleus decreases the percentage of monoamine boutons i.e. boutons with small granular vesicles, from 31.6% in the controls to 4.4% in the lesion group, whereas only a small effect is seen after anterior (or complete) deafferentations.

  2. 2.

    A major input to the ELME enters the basal hypothalamus at the anterior-lateral aspects (see Réthelyi and Halász, 1970). The fibers cut after anterior deafferentations in all probability mainly come from cell bodies localized in the anterior hypothalamus or even further rostrally but some may represent NA axons ascending from the lower brain stem.

  3. 3.

    The degeneration course of nerve endings in the ELME both after anterior deafferentations as well as after lesions in the arcuate nucleus is rapid (within 2–3 days) and morphologically characterized by an initial aggregation of large dense cored vesicles seemingly to electron dense bodies within the boutons and probably also to a closer spacing of the small electron lucent synaptic vesicles (see Raisman, 1972). This type of degeneration seems to take place both in monoamine and non-monoamine neurons.

  4. 4.

    Degenerating boutons are found in the arcuate nucleus after anterior and complete deafferentations. Thus, the anterior hypothalamus may exert an “indirect” control of the pituitary gland via synapses on arcuate neurons although quantitatively the “direct” influence through the projection to the ELME is of more importance.

  5. 5.

    After anterior deafferentations enlarged axons containing large amounts of large dense cored vesicles and other organelles are found caudally of the cut indicating the existence of rostral projections from the medial hypothalamus.

These results have been reported in part at the 47th Annual Meeting of the Japan Endocrinological Society, April 8–10, 1974 in Tokyo.

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Correspondence to Dr. Katsuya Ajika.

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Dr. K. Ajika was supported by a personal Grant from the Swedish Medical Research Council (Visiting Scientist Programme; B72-14V-3664-01). This work was supported by Grants from the Swedish Medical Research Council (K72-14X-3664-1K; B73-04X-2887-04C; B73-14P-3262-03C; B71-14X-2887-02AK) and by grants from Karolinska Institutets Forskningsfonder, Ollie och Elof Ericssons Stiftelse and from Magn. Bergvalls Stiftelse.

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Ajika, K., Hökfelt, T. Projections to the median eminence and the arcuate nucleus with special reference to monoamine systems: Effects of lesions. Cell Tissue Res. 158, 15–35 (1975). https://doi.org/10.1007/BF00219949

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Key words

  • Median eminence
  • Arcuate nucleus (Rat)
  • Afferent projections
  • Monoamine neurons
  • Deafferentations
  • Electron and fluorescence microscopy