Ultrastructural study of electrophysiologically identified neurones in the paraventricular nucleus of the rat

The ultrastructural characterization of electrophysiologically identified neurones of the rat paraventricular hypothalamic nucleus was performed with extracellular labelling technique. The extracellularly recorded neurons are labelled with an electrophoretic deposit of alcian blue contained in the recording micropipette. The neurone thus labelled takes on a dark and shrunken appearance which enables its detection among neighbouring cells without, however, concealing its main morphological characteristics. 1) Spontaneously firing neurones, invaded by an antidromic action potential elicited by electrical stimulation of the neurohypophysis, were identified as magnocellular cells containing dense-cored vesicles of 200-250 nm in diameter. Dense-cored vesicles were not found in the antidromically activated neurones devoid of spontaneous activity. 2) Trans-synaptically activated neurones in the PVN or in its dorso-lateral edge were small cells devoid of dense secretory vesicles. 3) PV neurones in which neurohypophysial stimulation evoked no response, contained small, dense vesicles (100 nm in diameter) comparable with those found in parvocellular peptidergic neurones.

[The Harderian gland--target for cyclic nonapeptide neurohypophyseal hormones. Autoradiographic studies on the rat (author's transl)]

Among autoradiograms of different tissues prepared after intravenous injection of 3H-tyrosine oxytocin into male white rats, only those of the kidney and the Harderian gland, an exocrine gland in the orbit of most terrestrial vertebrates, displayed a saturable labelling, i.e., pretreatment of the rats with an excess of unlabelled oxytocin led to a significantly lowered tissue content of radioactivity. Liquid scintillation analysis confirmed this result. Moreover, besides the kidney only the Harderian gland proved to be labelled in autoradiograms prepared after intracarotid injection of 125I-lysine vasopressin into male white rats. The intensity of this label is enhanced in Brattleboro rats. The results strongly suggest that the Harderian gland represents a hitherto not recognized target for posterior pituitary cyclic nonapeptide hormones. Concerning the physiological role these hormones may play in the gland, the speculation is put forward that oxytocin like its role in the mammary gland might be involved in the regulation of Harderian gland secretory processes.

A study of psychological and endocrine variables on 14 patients treated by chronical haemodialysis

In 13 patients suffering from renal dysfunction and treated by chronic haemodialysis, the mnemic functions appeared within the limits of normality and there was no positive correlation between blood levels of neurophysines and free cortisol. On the other hand, a negative correlation was found between levels of neurophysine I and items 3 and 5 of the memory test PRM. This negative result indicates that in haemodialyzed patients a discrepancy may exist between blood levels of neurophysines and vasopressin release, or that there is no direct relationship between plasma and CSF concentrations of such hormones.

The effects of neural stalk stimulation upon firing patterns in rat supraoptic neurones

Antidromically identified neurones were recorded from the supraoptic nucleus of urethane-anaesthetized rats. In test periods of about 10 min, each recorded action potential was followed 1 ms later by a shock to the neural stalk. In phasically firing cells, this stimulation consistently modified the observed firing pattern: burst length was shortened but intraburst activity was unchanged. The stimulation invaded most supraoptic neurones antidromically, but antidromic spikes evoked in the recorded neurone were extinguished by collision. Thus, the observed changes in firing pattern were probably the consequence of interaction between the recorded unit and other neurones projecting to the neural stalk--most probably other supraoptic neurones. The behaviour of continuously firing supraoptic neurones was studied following single shocks and following trains of shocks. The results suggest that the widely reported evidence for recurrent inhibition in the rat supraoptic nucleus has been misinterpreted, and that inhibition cannot be demonstrated in most supraoptic neurones following single shocks to the neural stalk. However, the experiments using trains of shocks produced evidence consistent with a complex interaction between magnocellular neurosecretory cells involving both excitation and inhibition.

Effect of electrical stimulation of the neurohypophysis on ACTH release in rats with hypothalamic lesions

Electrical stimulation of the neural lobe (NL) of the pituitary induces a rise in plasma corticosterone indicating the release of adrenocorticotropin (ACTH) in rats pretreated with dexamethasone, morphine and pentobarbitone. 7-8 days after placing an anterolateral cut around the medial basal hypothalamus (MBH), the rats failed to respond with ACTH release to electrical stimulation of the NL; the number of nerve fibers and terminals in the NL decreased to less than 5% of the normal; and ACTH releasing activity of acid extracts of the NL was undetectable using both in vivo and in vitro tests, which are insensitive to vasopressin. After lesions of the paraventricular nuclei the stimulation of the NL elecited a rise of plasma corticosterone that was significantly less than that in the controls. These results suggest that the NL of the pituitary contains electrically excitable fibers capable of releasing corticotropin-releasing factor (CRF; distinct from vasopressin) and that these fibers probably originate from outside the MBH, with a portion of them coming from or through the paraventricular nuclei or their immediate vincinity.

Oxytocin in microdissected hypothalamic nuclei. Significant differences between prepubertal and sexually mature female rats

We measured the concentration of oxytocin and vasopressin in microdissected areas of the hypothalamus and posterior pituitary in weanling and adult female rats. Oxytocin concentration was significantly less in adult rats as compared to weanling rats in the supraoptic nucleus, median eminence, and retrochiasmatic area with no significant differences in vasopressin content. This may indicate differences in hypothalamic-neurohypophyseal pathways in adult and weanling rats. There was a significantly greater concentration of oxytocin in the adult rat in the paraventricular nucleus, suprachiasmatic nucleus, anterior hypothalamic nucleus, and posterior pituitary, again with no significant difference in vasopressin concentration. Our results are consistent with a role for sexual maturation in increasing synthesis of oxytocin in specific hypothalamic nuclei resulting in increased storage of oxytocin in the posterior pituitary.

Role of endogenous opioids in neurohypophysial function of man

Studies were carried out in normal human subjects to determine the effect of two narcotic antagonists, oxilorphan and butorphanol, on antidiuretic hormone (ADH) release. Oxilorphan given to eight subjects on ad libitum fluid intake resulted in a transient but significant increase in 24-h free water clearance and a decrease in urine osmolality. These changes were not accompanied by an increase in creatinine or solute excretion, and urinary ADH levels did not rise even though plasma osmolality rose significantly from 289.4 +/- 0.9 to 292.9 +/- 0.8 mosmol/kg. Treatment with oxilorphan did not interfere with the response to ADH infusion in water-loaded subjects. Both oxilorphan and butorphanol significantly elevated the plasma osmolality at which ADH release became evident after the infusion of hypertonic saline in water-loaded subjects. Oxilorphan suppressed urinary ADH excretion at the time of the osmotic threshold for ADH release in spite of the greater plasma osmolality. The data indicate that the narcotic antagonists are capable of inhibiting ADH release in man during ad libitum fluid intake and in response to an osmotic stimulus by elevating the osmotic threshold for ADH release. It is concluded that narcotic antagonist inhibition of endogenous opioid action provides further evidence supporting a role for the brain opiates in the normal regulation of neurohypophysial hormone release in man.

Neural lobe of pituitary modulates corticotropin release in the rat

The hypothesis was tested that the neural lobe of the pituitary may modulate the release of anterior pituitary hormones. The neural lobe of anesthetized lactating rats was electrically stimulated at 30 Hz (5 sec on and 5 sec off) for 3 min while taking blood samples for RIA of ACTH. Plasma ACTH increased within 3 min by 22 +/- 9% (average +/- SEM; P less than 0.025) in intact rats and by 38 +/- 17% (P less than 0.025) in rats where the nerve supply to the median eminence and neural lobe was interrupted. Electrical stimulation of the anterior pituitary was ineffective. No significant changes in plasma ACTH were observed in rats with coagulated hypophysial portal vessels or in Brattleboro rats with congenital diabetes insipidus. Apparently, neither peripheral plasma vasopressin (estimated at 150 microU/ml maximum) nor intermediate lobe ACTH could account for the observed rise in ACTH. Results suggest a vasopressin dependent modulation of ACTH release by the neural lobe, mediated either by axon collaterals to the median eminence or by a vascular interconnection between posterior and anterior pituitaries.

Sleep: a prerequisite for reflex milk ejection in the rat

Electroencephalographic activity (EEG) was recorded from the frontal cortex of unanaesthetized and urethane-anaesthetized lactating rats and analysed in relation to the pattern of milk ejection evoked by the nursing pups. The EEG of the anaesthetized rat fluctuated without experimental intervention between three distinctive patterns defined as synchronized, desychronized, and stage III activity, whilst reflex milk ejection recurred at intervals of about 6 min (range 2- greater than 20 min) throughout the 1-4 h period the pups were left attached to the nipples. For greater than 10 s before and for up to 60 s after each milk ejection, as judged from recordings of intramammary pressure and pup behaviour, the EEG was invariably synchronized throughout. Conversely, milk ejection (n greater than 300) was never observed during long periods of desynchronized, or stage III EEG activity. The vigorous increase in the sucking of the pups at milk ejection failed to produce a desynchronization (arousal) of the EEG as observed with other forms of sensory stimulation. Indeed, the sucking of the pups appeared to produce a soporific change i, the maternal EEG for spontaneous periods of desynchronization were not observed in the 30-60 min following the initial attachment of the pups to the nipples. Similar EEG patterns were seen in the unanaesthetized rat, though arousal from the synchronized state was more easily produced, e.g., by weak auditory signals. Milk ejection, as judged from the behaviour of the pups, recurred at intervals of 2 min or more during each 20-80 min period of nursing. The rat appeared somnolent for most of the nursing period and the EEG was always synchronized for greater than 10 s before each milk ejection (n greater than 200), though her eyes usually remained open. Arousal and desynchronization of the EEG was invariably observed in association with the increased pup behaviour at milk ejection. From these observations and the knowledge that oxytocin release from the neurohypophysis occurs about 10 s before milk ejection, we conclude that a synchronized EEG pattite for the expression of the milk-ejection reflex in the rat.

The antidromic compound action potential of the hypothalamo-neurohypophysial tract, a tool for assessing posterior pituitary activity in vivo

A sensitive technique was developed for measuring continuously the summated neural activity of the rat hypothalamo-neurohypophysial tract. Large compound potentials of 4-9 mV peak amplitude were recorded from the fibre tract in response to electrical stimulation of the neurohypophysis. It was estimated that all fibres in the tract were electrically activated and contributed to the compound potential. It was shown that a decrease of peak amplitude could be used as a measure of changes in endogenous neural activity. The decrease in peak amplitude during an increase of endogenous neural activity was due to fatigue and temporal spread of the compound potential, and also to collision of antidromic and orthodromic action potentials. Decrease of the compound potentials was related to the intensity of vaginal distension, haemorrhage and plasma hyperosmolality, and it correlated with neurohypophysial hormone release. This method may usefully complement hormone assays and single cell analysis.

Neurohypophyseal principles and memory

The neurohypophyseal hormones vasopressin and oxytocin modulate memory processes. Vasopressin facilitates, while oxytocin attenuates memory consolidation and retrieval. These influences are located in different regions of the molecules. Thus, the neurohypophyseal hormones act as precursor molecules for neuropeptides involved in memory processes. The covalent ring structures of both vasopressin and oxytocin mainly affect consolidation; the linear parts, retrieval processes; while nearly the whole oxytocin or vasotocin molecule is needed for attenuation of consolidation and retrieval. Regional studies, utilizing microdissection techniques in combination with a sensitive radioenzymatic catecholamine assay, revealed a distinct pattern of effects on cerebral alpha-methyl-p-tyrosine methylester-induced catecholamine disappearance following intraventricular vasopressin administration in limbic midbrain structures. In situations in which the amount of bioavailable vasopressin in the brain is absent, as is the case in the Brattleboro rat with hereditary diabetes insipidus, or neutralized in normal Wistar rats following the intraventricular administration of antivasopressin serum, regional catecholamine disappearance in most cases is altered in a direction opposite to that observed after intracerebroventricular vasopressin administration. These results indicate that vasopressin modulates memory processes by modulation of neurotransmission in distinct catecholamine systems. Recent experiments suggest that the influence of vasopressin on memory consolidation is mediated by the dorsal noradrenergic bundle via terminal regions of this bundle.