A detailed parcellation of the entire cochlear nucleus of the cat was prepared with the Nissl and Protargol methods in the form of a cytoarchitectonic atlas. Neuronal cell types were characterized in rapid Golgi impregnations. Golgi impregnations were used to define the specific features of the mature neuronal types. Neurons from the Golgi preparations were systematically mapped according to type and location in serial sections. These neurons were then plotted in corresponding positions in the cytoarchitectonic atlas. This combined GolgiNissl approach provides a scheme in which neuronal types, defined in terms of a number of functionally significant features, can be precisely localized within the cochlear nucleus.The cochlear nucleus provides the first opportunities for recoding of signals from the auditory nerve. So the neuronal architecture of the cochlear nucleus must play a key role in defining the functional organization of the ascending auditory system. The structural organization of the cochlear nucleus is complicated by the fact that it is not a single nucleus but a complex of distinct neuronal populations. Previous investigators, using several histological techniques, have attempted to identify and describe types of neurons and to subdivide the nucleus. Most schemes of parcellation have been based on cell body stains, especially the Nissl method, whereas neuronal cell types have been characterized with a variety of techniques, including Nissl, reduced silver, and Golgi methods (e.g., in the cut: Ram6n y Cajal,

ABSTRACT

Young adult cyclic female rats were each injected with 2 mg estradiol valerate (EV) in sesame oil. Controls received an equivalent volume of sesame oil. Within 2 months after injection, most of the EV-treated animals showed persistent vaginal estrus and small polyfollicular ovaries as well as pathological changes in the hypothalamic arcuate nucleus. This pathological process was gradually progressive such that by 6 months after EV injection, the basal lateral region of the nucleus contained numerous reactive microglia, astrocytes, and degenerating elements of the neuropil. The experimental rats had elevated plasma PRL and GH concentrations which gradually diminished. Plasma estradiol concentration remained elevated 2 months after injection, while plasma LH and FSH concentrations stayed within the high and low normal range, respectively. The pituitary glands of injected animals weighed significantly more than those of controls 5.5 months after injection, but the enlarged glands did not cause hypothalamic compression. As mechanical anterior deafferentation of the medial basal hypothalamus has previously been shown to produce similar endocrine and reproductive alterations, it may be that estradiol treatment results in a functional-anatomical disconnection of the arcuate nucleus from the more anterior hypothalamic areas that regulate cyclicity. Whether this type of functional-anatomical phenomenon underlies other varieties of induced or secondary acyclicity in females remains to be determined.

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Rapid Golgi impregnations of the ascending branches of the auditory nerve fibers and of the types of neurons in the anteroventral cochlear nucleus (AVCN) were studied. Entire ascending branches could be observed, some of these branches project to each subdivision, others do not. There are two main typesof large neurons: the bushy and stellate cells. Criteria were established for identifying unimpregnated bushy and stellate perikarya by means of Nomarski optics, and these criteria were checked by Momarski observations on neurons which had either impregnated dendrites and unimpregnated cell bodies or impregnated portions of perikarya. In this way, the relations of unimpregnated cell bodies to auditory nerve endings were observed. Furthermore, with Nomarski optics, the cytoarchitectonic subdivisions of AVCN could be determined. Differences in the end-bulbs and collateral endings formed by the auditory nerve fibers were distinguished in three of the cytoarchitectonic subdivisions of the AVCN. End-bulbs in the anterior division were much larger than those in the dorsal and ventral parts of the posterior division. The large end-bulbs of Held in the anterior division of the AVCN were consistently associated with the perikarya of bushy cells and not with those of stellate cells. The large end-bulbs are not observed in the posterior division. Thus, bushy cells in the posterior division, although morphologically similar to those in the anterior division, must have a different synaptic organization. This difference may correspond to electrophysiological distinctions in the time-patterns of response recorded in these regions following acoustic stimulation.

Specific binding sites for blood-borne insulin were determined to be selectively localized on axons and axon terminals in the external median eminence and the hypothalamic arcuate nucleus by means of quantitative fine structural radioautography. This localization suggests that discrete populations of hypothalamic nerve terminals are potential targets for the direct effects of insulin and that insulin may act through synaptic mechanisms to influence hypothalamic circuits regulating energy balance and hypophyseal function.

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The effects of estradiol valerate and constant light exposure on the histological appearance of the arcuate nucleus were assessed in female rats. Both of these treatments caused significant increases in the numbers of reactive microglial cells and astrocytic granules in the nucleus. Ovariectomy before either treatment prevented the glial reaction, indicating that the experimental manipulations triggered the secretion of an ovarian product which appears to be selectively toxic to the arcuate nucleus. The fact that monthly injections of estradiol valerate in male rats produced the same profile of degeneration in the arcuate nucleus suggests that the neuropathological agent may itself be estradiol. Ovariectomy also significantly reduced arcuate microglial reactivity associated with normal aging, which suggests that cyclic surges of endogenous estradiol may be capable of gradually producing an arcuate lesion. This phenomenon may accout for the hypothalamic reproductive failure associated with normal aging in the rat.