Adrenaline turnover rates in the medial preoptic area and mediobasal hypothalamus in relation to the release of luteinizing hormone in female rats

The turnover rates of adrenaline in the medial preoptic area and mediobasal hypothalamus, areas which, respectively, include the cell bodies and terminals of luteinizing hormone-releasing hormone neurons, have been measured in female rats on pro-oestrus, the day of the preovulatory surge of luteinizing hormone, and on dioestrus, the preceding day. A rise in the rate of turnover was found in the medial preoptic area coinciding with the surge of luteinizing hormone in the late afternoon of pro-oestrus; the rate of turnover at this time was higher than at the same time on dioestrus. No changes in turnover rate were found in the mediobasal hypothalamus within either of these days.The results indicate that the adrenaline-containing projections to the preoptic area may be actively involved in the production of the spontaneous preovulatory surge of luteinizing hormone in rats.     

Morphologic evidence for intranuclear circuits in the hypothalamic arcuate nucleus

Three weeks after complete deafferentation of the medial basal hypothalamus (MBH) of adult female rats, the hypothalamic arcuate nucleus (ARCN) was examined ultrastructurally. Axodendritic and axosomatic synapses were counted in a field of 18,000 μm² in the middle part of the ARCN in each brain. Intraventricular infusion of 5-hydroxydopamine before autopsy resulted in the differentiation of two types of axon terminals. One axon terminal contained small spherical vesicles (SSVs, about 50 nm in diameter) and the other contained 5-hydroxydopamine-labeled small granular vesicles (SGVs) which were regarded as monoaminergic terminals. In the completely deafferented MBH, mean numbers of SSV and SGV synapses of the ARCN decreased to about one-half and one-fourth, respectively, compared to those of the unoperated rats. However, considerable numbers of intact SSV and SGV synapses were still encountered in the ARCN following deafferentation. There was no significant difference in the number of synapses between the large MBH island (including ARCN and ventromedial nucleus) and the small MBH island (mostly localized in ARCN). These results suggested that numerous converging nonmonoaminergic and monoaminergic fibers terminated in the ARCN and also suggested that nonmonoaminergic and monoaminergic arcuate neurons connected directly with other arcuate neurons.     

Poststimulatory endocytosis, microvesicle repopulation and changes in smooth endoplasmic reticulum in nerve endings of the median eminence superfused in vitro

Mediobasal hypothalami of adult rats were superfused in vitro. A single 5 min pulse of 60 mM KCl-containing medium was infused, followed by either 15, 30, 45, 60 or 75 min superfusions with standard medium. In some experiments, 5 or 10% dextran was added followed by a 15 min recovery. Morphologically, two recovery phases were recognized. The early phase (15-30 min) was characterized by two features: (1) A clear-cut increase in the quantity of large, pleomorphic vacuoles occupying the axoplasm of nerve endings; these vacuoles were observed to be connected to caveolae of the same diameter in the axolemma and they were either coated or uncoated. (2) Progressive increase in the quantity of microvesicles (synaptic vesicles) from an initial depleted state. The vacuoles were found to contain dextran aggregates. Microvesicle-like protrusions bulged from the membrane of vacuoles. The late phase, from 45 min poststimulation onward, was typically identified after the appearance of tubules of smooth endoplasmic reticulum at the most distal segments of the nerve terminals. During this period, large vacuoles tended to decrease in quantity. Granular vesicles remained scant during the entire observation period. Images suggesting formation of microvesicles from tubules of smooth endoplasmic reticulum were observed. These results open the possibility that endocytosis of patches of membranes forming large vacuoles be an important mechanism for retrieving the membranes belonging to microvesicles and granular vesicles. Some of these large vacuoles may contribute to the early regeneration of microvesicles. More microvesicles could later be produced from the smooth endoplasmic reticulum.     

Single-cell responsiveness to sensory stimuli in the mediobasal hypothalamus of rats with partial hypothalamic deafferentations and medial forebrain bundle lesions

Neuroendocrine studies have demonstrated that photic, acoustic, and sciatic nerve stimulation produce adrenocortical discharges and these can be blocked by partial hypothalamic deafferentations and medial forebrain bundle (MFB) lesions. The effects of these sensory stimuli on single-cell activity of mediobasal hypothalamic (MBH) neurons were studied in intact male rats and in animals with anterior, anterolateral, and posterolateral hypothalamic deafferentations and bilateral MFB lesions. The data were analyzed as to changes in responsiveness, facilitation: inhibition ratio, patterns of firing, magnitude of response, frequency distribution, and sensory convergence. Photic projections enter the MBH both anteriorly and posteriorly and via the MFB, acoustic afferent fibers utilize the MFB, and the sciatic impulses arrive posteriorly. The MFB has also an inhibitory modulation on the photic, acoustic, and sciatic responses and AHD has the same effect on the latter. MFB lesions reduced most markedly the sensory convergence of the three modalities. The relation of these electrophysiologic findings to neuroendocrine studies on the neural pathways mediating adrenocortical responses are discussed.     

Effect of central serotonin depletion on adrenocortical responses to neural stimuli

In view of the possible role of serotonin in adrenocortical regulation, basal plasma corticosterone concentrations and the response to ether stress, photic, acoustic, or sciatic nerve stimulation, were studied in rats with 5,7-dihydroxytryptamine or vehicle injected into the raphe nuclei. The neurotoxin inhibited the response to photic stimulation without affecting the other modalities. This may suggest that the depletion of brain serotonin has a differential effect on the transmission of neural impulses which activate the adrenal cortex.     

The hypothalamus exhibits electrophysiologic evidence for morphine tolerance and dependence

The electrophysiologic effects of morphine (10 mg/kg) were studied in naive rats and after chronic morphine treatment for 5 days. Spontaneous multiunit activity was recorded from the ventromedial hypothalamus of unanesthetized, freely behaving rats implanted previously with permanent electrodes. In the naive condition, firing rates were altered by morphine in 60 of 73 multiunits. After development of tolerance, 28 of multiunits responded to the morphine challenge dose, in 22 of which the direction of change was opposite that observed in the naive condition. The other six were not affected by morphine on day 1 of the experiment and became sensitive to the morphine challenge dose only after the animal had become physically dependent on morphine. Predrug baseline activity in morphine physically dependent rats was altered in 23 recordings compared to the predrug activity obtained in morphine naive animals. Based on effects of morphine and naloxone, responses could be grouped into six electrophysiologic patterns which provide for separation of tolerance from the dependence phenomena.     

Response of medial septal neurons to the iontophoretic application of glucocorticoids

In view of the influence of glucocorticoids on adrenocortical activity, the iontophoretic effects of cortisol and corticosterone on the electrical activity of medial septal neurons were investigated. About one-half of these cells were steroid sensitive. The hormonal effect was short in the excitatory neurons; however, it persisted as long as 2 min in inhibitory units after cessation of iontophoretic current. This was contrary to the finding in the mediobasal hypothalamic neurons in which both in excitatory and inhibitory cells the effect lasted as long as 30 s. Autocorrelation analysis of the firing of septal neurons revealed that most cells showed a regular nonrhythmic, nonbursting spike activity. Only 10 of the 48 units studied changed their pattern of firing during hormone iontophoresis. These results are significant in relation to the role played by the septum in the neuroendocrine control of adrenocortical secretion.     

Structural changes in nerve endings of rat median eminence superfused with media rich in potassium ions

In vitro fragments of male rat mediobasal hypothalami were superfused with Krebs--Ringer solution in the presence or absence of CaCl2. Infusions containing up to 60 mM potassium chloride were applied, at the end of which tissues were fixed in osmium tetroxide and prepared for transmission electron microscopy. Control superfusions were run in parallel. Quantitative measurements performed on electron micrographs of the outermost palisade region showed significant (20-30%) increase in caliber of axon endings after intensive potassium ion stimulation. Ultrastructurally, widespread depletion of granular vesicles and microvesicles was found. Vesicle shift to the outer zone of the terminals, formation of membrane-bound tubules of the same diameter as microvesicles, and images of attachment and collapse of vesicles into the axolemma were found, particularly after 1 min stimulation. These findings were interpreted as consistent with exocytosis. Longer stimulations were followed by the appearance of large pleomorphic vacuoles that are probably the result of post-exocytotic membrane retrieval. Axon enlargement and vesicle depletion were absent in specimens superfused with calcium-free medium containing high potassium. The functional significance of these ultrastructural changes are interpreted as supporting the hypothesis that exocytosis of calcium-loaded microvesicles can contribute to extrude this ion from median eminence nerve endings during secretion.     

Steroid sensitive identified and unidentified neurons in the medial preoptic area of the rat

In view of the role played by the preoptic area and the glucocorticoids in adrenocortical regulation, the iontophoretic effects of cortisol on non-identified and identified preoptic neurons, which send their axons to the mediobasal hypothalamus, were investigated. More than half of the cells changed their rate or pattern of firing, as studied by autocorrelation, but few cells changed their pattern only. The iontophoretic effect persisted only few seconds beyond cessation of current. Twenty three percent of the studied cells were antidromically identified and most of them responded to the iontophoretic application of cortisol. The possible relation of these findings to the neuroendocrine regulation of adrenocortical secretion is discussed.     

Recovery of sexual receptivity in female rats with lesions of the ventromedial hypothalamus

Bilateral lesions of the hypothalamic ventromedial nucleus (VMN) effectively eliminated the induction by estrogen-progesterone of lordosis behaviors 4 to 6 weeks after the brain surgery. However, recovery of the lordosis response was clearly evident in the rats with effective VMN lesions 2 weeks after the first test. This suggests that the VMN may not be essential to the expression of hormonally induced lordosis behavior, but may be involved in a supplemental lordosis mediating mechanism.