Evidence for opiate receptors on pituicytes

A hypothalamo-neurohypophyseal enkephalinergic pathway has been described and the pars nervosa of the rat pituitary contains enkephalin-like material which may coexist in vasopressin and oxytocin terminals. At the level of the pars nervosa itself, stereospecific opiate receptors with properties very similar to those of brain receptors have been described, and opiates have been shown to inhibit the release of both vasopressin and oxytocin. The location of the opiate receptors involved has been presumed to be pre-terminal on the neurosecretory fibres. Using an autoradiographic technique to visualize opiate receptors, however, we now report that destruction of the neurosecretory fibres following pituitary stalk section does not result in a significant change in the neural lobe opiate receptor population. This suggests that the opiate receptors within the neural lobe may be present on pituicytes rather than on neurosecretory fibres.

Some well-kept hypothalamic secrets disclosed

The magnocellular neuropeptidergic cells of the supraoptic and paraventricular nuclei comprise much of what is known as the hypothalamoneurohypophyseal system and is involved in several functions, including body fluid balance, parturition, and lactation. In vivo experiments have not produced a clear understanding of some of the crucial features associated with the functioning of this system. In particular, questions relating to the osmosensitivity of magnocellular neurons and the mechanisms(s) by which their characteristic firing patterns are generated have not been answered by using the older approaches. Electrophysiological studies with brain slices present direct evidence for osmosensitivity, and perhaps even osmoreceptivity, of magnocellular neurons. Other evidence is reviewed indicating that the phasic bursting patterns of activity associated with vasopressin-releasing neurons: 1) occur in the absence of patterned chemical synaptic input, 2) are probably influenced by localized changes in extracellular K+ concentrations, 3) may be modulated by electrotonic conduction across gap junctions connecting magnocellular neurons, and 4) are likely to be generated by endogenous membrane currents.

Central cardiovascular effects of mammalian neurohypophyseal peptides in conscious rats

To confirm and extend the results of previous studies which demonstrated central cardiovascular effects of vasopressin in anesthetized rats, we determined blood pressure and heart rate changes for 30 minutes after intracerebroventricular injections of arginine vasopressin, arginine vasotocin and oxytocin in conscious rats. As compared to sham injections, significantly greater increases in either systolic or diastolic blood pressure were noted over the 30 minutes which followed the injection of 0.15, 1.0 or 10.0 nM of either vasopressin or vasotocin. In animals given vasopressin, plasma levels of the peptide were determined. There was a substantial increase in plasma vasopressin only after the highest dose. Overall blood pressure responses to doses of oxytocin as high as 100 nM were not significantly different than sham injections. Heart rate following both vasopressin and vasotocin was increased at 0.15 nM, was initially decreased then increased at 1.0 nM and was substantially decreased after the 10.0 nM dose. There was a significant increase in heart rate at the 10.0 nM and 100 nM doses of oxytocin. Dose response curves for systolic blood pressure and heart rate 20 minutes after injection were similar for vasopressin and vasotocin. We conclude that arginine vasopressin has significant central pressor and tachycardic effects in conscious rats, and it is related, at least in part, to the tail structure of the peptide, which is shared with arginine vasotocin.

Analysis of the neurohypophyseal components accumulating in the supraoptic nucleus of the rat after injection of colchicine

[35S]Cysteine has been injected into the supraoptic nuclei of normal rats and of animals given 7 micrograms colchicine into the cerebrospinal fluid to inhibit transport of neurosecretory granules. Analysis of extracts of the supraoptic nuclei 20 min or 6 h after isotope injections showed that (i) colchicine does not affect biosynthesis, i.e., incorporation of tracer into the common precursors of neurohypophyseal hormones and their associated neurophysins, and (ii) processing of precursors inside the arrested granules proceeds normally.

Calcium-containing mitochondrial granules in neurohypophysial axon terminals disappear following vasopressin treatment of Brattleboro rats

The presence of intramitochondrial calcium-containing electron-dense granules was demonstrated in axon terminals of chronically hyperactive neurosecretory neurons of untreated homozygous Brattleboro rats. Following vasopressin treatment for 30 days, which has been shown to attenuate this neuronal hyperactivity, calcium-containing deposits could not be detected in mitochondria. It is concluded that the presence of intramitochondrial calcium-containing dense deposits is connected with the functional state of neurosecretory neurons.

[Age-related reactions of the rat hypothalamo-hypophyseal neurosecretory system to physical load]

In two age groups of rats (5 to 6 months--young animals and 10 to 12 months--aging rats) the response of peptidergic Gomori-positive hypothalamic-hypophyseal neurosecretoryoryory (NS) system to moderate exercises (swimming during 30 minutes) was studied. The age differences in histology of NS centers and neurogemnic organs were found immediately after swimming and within 4 subsequent days. More rapid but less significant devastation of neurogemnic regions, combined with sharp intensification of the neurosecret synthesis in NS centers, was seen in the young animals. In the aging rats the response was slower, the neurosecret synthesis and its release from NS centers were not so activated. The anterocommissural group of the NS centers studied (especially in the young animals) and supraoptic nucleus were most closely associated with the response to exercises. The paraventricular nucleus was activated later, that appears to indicate the secondary inclusion of this NS center into elimination of the aftereffects of an upset balance.

Presence of vasopressin, oxytocin and neurophysin in the retina of mammals, effect of light and darkness, comparison with the neuropeptide content of the neurohypophysis and the pineal gland

Arginine vasopressin (AVP) and oxytocin (OT) as well as their CNS carrier neurophysins (Np) have been found in the pineal gland. In view of the analogy between the pineal gland and the retina, the contents of these neuropeptides in rat, human and bovine retinae were determined. AVP, OT and Np were detected by specific radioimmunoassay (RIA) and their presence confirmed by RIA measurements (1) in rat and human retinae on HPLC fractions and (2) by the detection of the C-terminal portion of the precursor to AVP and its associated Np = propressophysin (CPP). The AVP and OT content in the retina of the rat was modified by light: AVP and OT content was smaller at 2 a.m. than at 2 p.m., but was increased by a 7 day constant exposure to darkness. In contrast, pituitary content was decreased after 7 days of constant darkness. If one optic nerve was cut we observed a decrease in retinal AVP content compared to the contralateral side and a decrease in pituitary AVP content. Our data clearly demonstrated the presence of AVP, OT and Np in the retina and their variation induced by light. It is probable that these peptides are of central origin.

Electrical properties of paraventricular neurosecretory neurons with and without recurrent inhibition

Twelve out of 32 neurosecretory neurons in the paraventricular nucleus of rats showed a silent phase following subthreshold stimulation to the posterior pituitary gland. After suprathreshold stimulation, the duration of the silent phase was significantly longer than that of the remaining 20 neurons, which did not show the silent phase at subthreshold stimulation. The latency and threshold in the former neurons were significantly longer and higher than those of the latter neurons. These data indicate a relationship between the recurrent inhibitory system and other electrical properties in the paraventricular neurons.

Voltage-sensitive Na+ channels in the neurohypophysis of the rat as demonstrated by 125I-labelled scorpion toxin

Fresh rat neural lobe slices were incubated in the presence of [125I] alpha-scorpion toxin (ScTX), a specific marker of Na+ channels. Quantitative electron microscope autoradiography revealed preferential, irregularly spaced labeling of the axolemma of neurosecretory axons, with a significantly higher crude specific activity than any other neuronal or non-neuronal compartment. The number of specific binding sites at the neural lobe surface was calculated to be about 23 per microns2 of axolemma.

Neuroendocrine effects of endogenous opioid peptides in human subjects: a review

In the decade since the discovery of specific opioid receptors in the brain, there have been rapid advances in our understanding of the physiological and pathological roles of the endogenous opioid systems in humans. Endogenous opioid peptides have been demonstrated to play a role as modulators of a number of hormonal functions in humans. In particular they appear to inhibit luteinizing hormone and ACTH release, and the response of arginine vasopressin to osmotic stimuli. They appear to participate in the modulation of carbohydrate homeostasis. In pathophysiological states, they appear to play a role in the decreased pulsatile luteinizing hormone release seen in patients with prolactinomas. Circulating beta-endorphin appears to be an important regulator of immune function. Preliminary studies in humans have suggested a role for endogenous opioid peptides in appetite regulation. In the last few years, a few case reports have suggested the possibility of a series of syndromes due to endogenous opioid excess. Within the next decade, we can expect to see the routine use of opioid antagonists in a variety of pathophysiological states.

Effects and site of action of morphine on gonadotropin secretion in the female rhesus monkey

The effects of morphine on gonadotropin secretion, and the site of its action, were tested in female rhesus monkeys. In Exp 1, morphine sulfate (3, 6, or 9 mg iv) was injected into ovariectomized monkeys, and its effects on tonic (pulsatile) LH and FSH secretion were examined. Administration of morphine (9 mg) resulted in a significant decrease in circulating LH and FSH, which lasted for 4-5 h. Exp 2 was performed to evaluate the site of action of morphine, whether hypophyseal or suprahypophyseal. The effects of morphine (6, 9, or 12 mg) on the LH response to GnRH pulses were evaluated in stalk-sectioned monkeys, in which gonadotropin secretion had been restored by long term pulsatile infusion of GnRH. LH responses to GnRH were not significantly altered by morphine. Exp 3 was performed to determine the effects of morphine on the estrogen-induced LH surge. Estradiol benzoate (330 micrograms in oil) was administered on days 2-5 of the menstrual cycle to nine animals. Four of these also were injected with 9 mg morphine at 5-h intervals for 40 h. Four of the five control and three of the four morphine-treated monkeys showed similar LH surges. The results demonstrate that, in the monkey, opiates inhibit tonic (pulsatile) gonadotropin secretion, most probably by acting at a suprahypophyseal site. In contrast, morphine does not alter the estradiol-induced LH surge, a result that differs from that seen in lower species and that may be related to differences in estradiol positive feedback characteristics.

Structural requirements for neurohypophyseal peptide maintenance of ethanol tolerance

Tolerance to the hypnotic effect of ethanol in mice is prolonged by daily subcutaneous administration of arginine vasopressin and certain analogs of this hormone. The major structural requirement for maintenance of ethanol tolerance by these peptides appears to be the N-terminal "ring" structure of vasopressin containing two amino acid residues with aromatic side chains. Peptides structurally related to the C-terminal portion of the neurohypophyseal hormones are less active in maintaining tolerance than the intact hormones. The structure-activity pattern observed for the effects of peptides on ethanol tolerance is similar to that described for neurohypophyseal peptide inhibition of extinction of an active avoidance response, an action thought to reflect peptide effects on memory consolidation. The results are in line with our hypothesis that similar CNS recognition sites may mediate neurohypophyseal peptide effects on ethanol tolerance and certain memory processes. The neurohypophyseal hormones and analogs did not affect the hypnotic or hypothermic response to an acute injection of ethanol, indicating that the determination of tolerance was not influenced by a direct peptide-ethanol interaction. The hormones themselves, however, did cause a drop in body temperature in the mice, which could be a result of either central or peripheral hormonal actions.