Lennart Heimer

Two methods for selective silver impregnation of degenerating axons and their synaptic endings in the central nervous system.

Abstract The original, non-suppressive Natua method for impregenation of terminal degeneration has been modified by the introduction of a potassium permanganate-uranyl nitrate sequence. With proper timing, this step enchances the argyrophilia of degenerating fibers while suppresing normal-fiber impregnation. The result is a selective impregnation of degenarated axons inclusive of their synaptic thickenings.

Impairment of mating behavior in male rats following lesions in the preoptic-anterior hypothalamic continuum

Abstract Electrolytic lesions were made in the medial preoptic-anterior hypothalamic continuum, the lateral anterior hypothalamus, the dorsal part of the middle hypothalamus and in the septal nuclei of male rats. Sexual behavior was observed before and after the operations. Large lesions in the medial preoptic-anterior hypothalamic continuum permanently eliminated mating in all subjects. Small lesions within this region never completely abolished sexual behavior although 19 of 42 rats showed a temporary impairment. The sexual impairment was not related to a particular location of the lesion within the continuum. Treatment with testosterone propionate did not restore mating activity in any male with extensive lesions in the medial preoptic-anterior hypothalamic continuum. Animals with small lesions within the continuum, which had become sexually inactive after the operation, resumed mating after the hormone treatment. None of the operated animals showed any abnormalities in the testes or in the accessory organs. The medial preoptic-anterior hypothalamic continuum is situated intermediate between the limbic and olfactory telencephalon, on the one hand, and borders the gonadotropic mechanisms of the tuberal hypothalamic region. This position of the medial preoptic-anterior hypothalamic continuum appears to be quite compatible with the present physiological evidence that it is virtually involved in mating behavior.

In vivo anterograde and retrograde axonal trnasport of the fluoresecent rhodamine-dextran-amine, Fluor-Ruby, within the CNS

A number of fluorescent dextrans were screened for axonal transport properties within the rat CNS. One compound, Fluoro-Ruby (FR), was found to be particularly sensitive for demonstrating retrograde and particularly anterograde axonal transport. The tracer may be either pressure or iontophoretically injected, and the fixed tissue can be examined without histochemical processing. The technique can be combined with a wide variety of other neuroanatomical methods.

The hypothalamic distribution of the stria terminalis in the rat

Abstract The hypothalamic distribution of the stria terminalis was studied by the Fink-Heimer methods and electron microscopy in rats sacrificed 3–6 days after unilateral amygdalectomy or stria terminalis section. Each of the major hypothalamic stria components was found to be distributed to fairly sharply defined regions, as follows: (1) The postcommissural component terminals massively in the rostral half of the anterior hypothalamic nucleus; an apparently small number of its fibers join the medial forebrain bundle. (2) The supracommissural component terminates predominantly in a dense synaptic zone surrounding the ventromedial nucleus and containing a massive plexus dendrites protruding from this nucleus, in smaller quantity also in lateral and ventral parts of the ventromedial nucleus proper and in the ventral premammillary nucleus. The prevalence in the hypothalamus of neurons with long dendrites suggests that the stria terminalis may have synaptic contracts not only with perikarya and dendrites of neurons lying within its synaptic fields, but also with dendrites of neurons whose cell bodies lie well outside these fields.

Mating behavior of male rats after olfactory bulb lesions

Abstract Interruption of the afferent olfactory connections by lesions in the olfactory bulbs caused a marked impairment of the sexual behavior of male rats. The animals showed a prolonged latency in their responses to estrous females, a reduction in the number of ejaculations and an increased tendency to remain totally unresponsive to the sexual stimulation. It is suggested that olfactory impulses exert a strong influence upon the preoptic-anterior hypothalamic region, which is known to be of crucial importance for mating behavior.

The dorsal root distribution to the substantia gelatinosa of the rat with a note on the distribution in the cat.

SummaryThe terminal degeneration in the substantia gelatinosa of the rat was studied with the Fink-Heimer silver technique following dorsal root section. Providing the survival time of the animal was in the range of 1–4 days, a massive degeneration was seen in lamina I, II and III of Rexed. The light microscope findings were corroborated by electronmicroscopic observations of degenerating boutons. Spinal cord material examined with silver methods one week after dorsal root section showed few signs of degeneration in the substantia gelatinosa. Although a significant dorsal root distribution to the substantia gelatinosa was found also in the cat, the terminal degeneration in lamina II showed considerable regional variations in this species.

An electron microscope study of a silver stain for degenerating boutons.

Summary Hand-cut sections of glutaraldehyde-formalin-fixed experimental rat brains were impregnated with silver following the Fink-Heimer method 7 reported to impregnate degenerating boutons terminaux. Impregnated sections were examined wet , and small parts of regions exhibiting evidence of massive terminal degeneration were excised, post-fixed, and embedded in an Epon-Araldite mixture for ultrathin sectioning. Although the electron micrographs demonstrate a scattering of silver particles throughout the tissue, the silver granules are both more numerous and larger within degenerating axons. Such relatively large silver particles were also observed in all of the synaptic endstructures that exhibit characteristic degenerative changes. It can therefore be concluded that the Fink-Heimer silver impregnation technique is preferential in its impregnation of degenerating axons and axon terminals.

Projections of the olfactory tract fibers in the nurse shark (Ginglymostoma cirratum)

Summary Five nurse sharks were killed between 6 and 43 days after the olfactory tract had been cut unilaterally. Whereas the axon degeneration was demonstrated best with the Nauta-Laidlaw technique after 3–4 weeks survival time, the terminal degeneration, as identified in Fink-Heimer preparations, was best seen in a 13-day specimen. The distribution of olfactory tract fibers is entirely ipsilateral to the lesion, the majority terminating near the attachment of the olfactory peduncle to the hemisphere. Terminal degeneration is restricted caudally to a tongue-like extension of the lateral olfactory area on the ventral surface of the hemisphere with a spill-over into the area superficialis basalis of Johnston. The results indicate that the distribution of olfactory bulb fibers in the shark is much more restricted than had been suggested by most earlier descriptions based on studies of normal material.

THE SECONDARY OLFACTORY CONNECTIONS IN MAMMALS, REPTILES AND SHARKS*

The common observation that our knowledge of the nervous system is a reflection of our methods of investigation is dramatically illustrated in the everchanging concept of the rather ambiguous term rhinencephalon, commonly a p plied to those regions of the brain that are completely or predominantly related to the olfactory system. Macroscopic and microscopic studies of normal fiber anatomy have often led to the conclusion that fibers from the olfactory bulb are distributed, not only to the primitive cortex of the piriform lobe, but also to more remote brain regions such as the hippocampus and the septum. In more primitive vertebrates, the olfactory bulb fibers have, until quite recently, been assumed to reach practically the whole cerebral hemisphere, which, accordingly, has been referred to as the “smell-brain.” However, modern variations of the reduced silver-impregnation technique for the impregnation of degenerating axons and their synaptic endstructures, combined with the electron microscopic identification of degenerating boutons now allow a more accurate mapping of the projection fields of primary olfactory bulb fibers in different vertebrates than was previously possible. Recent experimental studies by such methods have shown that the olfactory bulb fibers terminate i n circumscript regions of the telencephalon in mammals, reptiles, amphibians, and fishes. This review, which summarizes some recent progress in the experimental neuroanatomy of the olfactory system, is in part based on collaborative work-in large part still in progress-in this laboratory (with Dr. Harvey Karten and Dr. Walle J. H. Nauta) and in the Laboratory of Perinatal Physiology in San Juan, Puerto Rico (with Dr. Sven 0. Ebbesson).

A Note on the Termination of Commissural Fibers in the Neocortex

The general arrangement of fibers in the corpus callosum shows a strongly symmetrical (homotopic) pattern characteristic of a true commissure. Connections between non-symmetrical (heterotopic) areas also have been demonstrated for many regions, both by anatomical studies using the Marchi technique13,14,16,24,26,29 and by electrophysiological methods6,18. The contribution of recently developed silver techniques, notably the suppressive Nauta-Gygax method23, has been to emphasize the extent of non-homotopic connections3,8,9,19,21. Some clearly delineated areas of neocortex appear not to receive any commissural fibers1,2,6,8,9,17,18,21,28; outstanding examples of such areas are ‘visual cortex’ or area 17, and the hand and foot portions of somatosensory cortex. It is generally presumed that each cortical area contributes fibers to the corpus callosum, and the sparse histological evidence available indicates that the cells of origin reside in cortical layers III through VI5,12,25.