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R J Bicknell

Publications and source records attributed to R J Bicknell.

At least 19 recordsLinked to original sources
PubMed1999

Fluctuating estrogen and progesterone receptor expression in brainstem norepinephrine neurons through the rat estrous cycle.

Norepinephrine (NE) neurons within the nucleus tractus solitarii (NTS; A2 neurons) and ventrolateral medulla (A1 neurons) represent gonadal steroid-dependent components of several neural networks regulating reproduction. Previous studies have shown that both A1 and A2 neurons express estrogen receptors (ERs). Using double labeling immunocytochemistry we report here that substantial numbers of NE neurons located within the NTS express progesterone receptor (PR) immunoreactivity, whereas few PRs are found in ventrolateral medulla. The evaluation of ERa and PR immunoreactivity in NE neurons through the estrous cycle revealed a fluctuating pattern of expression for both receptors within the NTS. The percentage of A2 neurons expressing PR immunoreactivity was low on metestrus and diestrus (3-7%), but increased significantly to approximately 24% on proestrous morning and remained at intermediate levels until estrus. The pattern of ERalpha immunoreactivity in A2 neurons was more variable, but a similar increment from 11% to 40% of NE neurons expressing ERa was found from diestrus to proestrus. Experiments in ovariectomized, estrogen-treated and estrogen-plus progesterone-treated rats revealed that PR immunoreactivity in A2 neurons was induced strongly by estrogen treatment, whereas progesterone had no significant effect. The numbers of ERalpha-positive NE neurons were not influenced by steroid treatment. These observations provide direct evidence for PRs in NE neurons of the brainstem and show that cyclical patterns of gonadal steroid receptor expression exist in A2, but not A1, neurons through the rat estrous cycle. The expression of PR in A2 neurons appears to be driven principally by circulating estrogen concentrations. The fluctuating levels of ERalpha and PR expression in these brainstem NE neurons may help generate cyclical patterns of biosynthetic and electrical activity within reproductive neural networks.

Animals↗
PubMed1998

Sex-steroid actions on neurotransmission.

Although it is well recognized that sex-steroids exert both developmental and activational influences on the brain, the cellular and molecular mechanisms underlying their actions are less well understood. Progress is rapid, however, and this paper reviews recent insights gained through manipulation of sex-steroid receptor genes, identification of phenotypes expressing these receptors and how these proteins may also be activated to regulate transcription by ligand-independent pathways. Advances in our understanding of more rapid actions of sex-steroids and the molecular targets involved are also reviewed, as are new studies describing effects on synaptic plasticity and the recent excitement regarding the neuroprotective effects of oestrogen replacement therapy in Alzheimer's disease.

Aged↗
PubMed1997

Up-regulation of nitric oxide synthase messenger RNA in an integrated forebrain circuit involved in oxytocin secretion.

The hypothalamo-neurohypophysial system contains high levels of neuronal nitric oxide synthase and this increases further during times of neurohormone demand, such as that following osmotic stimulation. Using double in situ hybridization, we demonstrate here an increase in the expression of nitric oxide synthase messenger RNA by oxytocin neurons, but not vasopressin neurons, of the supraoptic nucleus at the time of lactation, when oxytocin is in demand due to another neuroendocrine stimulus, the milk-ejection reflex. In addition, using immunocytochemical retrograde tracing, we show that neurons of the subfornical organ, median preoptic nucleus and organum vasculosum of the lamina terminalis, which project to the supraoptic nucleus, contain nitric oxide synthase. These three structures of the lamina terminalis, together with the hypothalamo-neurohypophysial system, make up the forebrain osmoresponsive circuit that controls osmotically-stimulated release of oxytocin in the rat. The expression of nitric oxide synthase messenger RNA in the lamina terminalis was also shown to increase during lactation. The increases in nitric oxide synthase messenger RNA were not apparent during pregnancy. These results provide evidence for an integrated nitric oxide synthase-containing neural network involved in the regulation of the hypothalamo-neurohypophysial axis. The expression of nitric oxide synthase messenger RNA increases in this circuit during lactation and correlates with a reduction in the sensitivity of the circuit to osmotic stimuli also present in lactation but not pregnancy. As nitric oxide is believed to attenuate neurohormone release, it seems that the increased nitric oxide synthase messenger RNA expression detected here during lactation at a time of high oxytocin demand may be involved in reducing the sensitivity of the whole forebrain circuit to osmotic stimuli.

Animals↗
PubMed1997

Plasticity in fast synaptic inhibition of adult oxytocin neurons caused by switch in GABA(A) receptor subunit expression.

We found that magnocellular oxytocin neurons in adult female rats exhibit an endogenous GABA(A) receptor subunit switch around parturition: a decrease in alpha1:alpha2 subunit mRNA ratio correlated with a decrease in allopregnanolone potentiation and increase in decay time constant of the GABA(A) receptor-mediated IPSCs in these cells. The causal relationship between changes in alpha1:alpha2 mRNA ratio and the ion channel kinetics was confirmed using in vitro antisense deletion. Further, GABA(A) receptors exhibited a tonic inhibitory influence upon oxytocin release in vivo, and allopregnanolone helped to restrain oxytocin neuron in vitro firing only before parturition, when the alpha1:alpha2 subunit mRNA ratio was still high. Such observations provide evidence for the physiological significance of GABA(A) receptor subunit heterogeneity and plasticity in the adult brain.

Animals↗
PubMed1997

Monoamine transporter gene expression in the central nervous system in diabetes mellitus.

Diabetic animals exhibit altered neurotransmission in brain monoaminergic systems. By means of in situ hybridization, we have investigated the expression of dopamine, noradrenaline, and serotonin transporter (DA-T, NA-T, and 5-HT-T, respectively) mRNAs in the brains of alloxan- and streptozotocin-diabetic rats. The expression of DA-T mRNA is decreased in 1- (-11%) and 4- (-17%) week alloxan-diabetic and 4- (-9%) and 8- (-20%) week streptozotocin-diabetic rats in the ventral medial bundle. The expression of NA-T mRNA is decreased in the locus coeruleus of 8- (-26%) week streptozotocin-diabetic rats, in the noradrenergic A1 cell group of 4- (-27%) week alloxan- and 8- (-25%) week streptozotocin-diabetic rats, and in the noradrenergic A2 cell group of 1- (-21%) and 4- (-28%) week alloxan-diabetic and 4- (-27%) and 8- (-25%) week streptozotocin-diabetic animals. The expression of 5-HT-T mRNA in the dorsal raphe nucleus is increased in 1- (+14%) and 4- (+44%) week alloxan- and 4- (+28%) and 8- (+44%) week streptozotocin-diabetic rats. The expression of each of the three monoamine transporter genes may be differentially regulated in diabetes and dependent on the duration of diabetes. Altered monoamine transporter gene expression may possibly contribute to the observed dysfunctions in brain monoamine transmission in chronic diabetes.

Animals↗
PubMed1997

Kappa-opioid restraint of oxytocin secretion: plasticity through pregnancy.

Oxytocin neurone terminals display a powerful opioid-mediated auto-inhibitory mechanism which restrains secretory activity. In pregnancy, upregulation of this mechanism may underlie the accumulation of large stores of oxytocin in the pituitary. In the present study, conscious pregnant rats were cannulated to allow blood samples to be taken for measurement of oxytocin secretion in response to intravenous administration of cholecystokinin (CCK). In each rat, we measured the secretory response to CCK before and after administration of norbinaltorphine (norBNI), a selective kappa-opioid antagonist, or of naloxone, a relatively mu-selective antagonist. Throughout pregnancy, the stimulatory effect of CCK was enhanced by prior administration of norBNI. In pregnant rats norBNI also consistently increased basal oxytocin secretion, while naloxone had no effect, suggesting that in pregnancy there is active restraint of oxytocin secretion by endogenous opioids acting at kappa-receptors. However, in late pregnancy, between days 17 and 20, there was a significant reduction in the efficacy of norBNI in potentiating the oxytocin release in response to CCK, compared to its efficacy either in non-pregnant rats or in rats between days 8 and 13 of pregnancy. This suggests that in late pregnancy, endogenous kappa-opioid restraint is downregulated. In addition, the present results demonstrate an attenuation in the potency with which CCK evoked oxytocin release in late pregnancy. The attenuation was unrelated to opioid restraint at the level of the pituitary since it coincided with apparent desensitization of this auto-inhibitory mechanism.

Animals↗
PubMed1997

Dissociated adult rat subfornical organ neurons maintain membrane properties and angiotensin responsiveness for up to 6 days.

We have utilised standard dissociation techniques to obtain a preparation of subfornical organ (SFO) cells that have been maintained in tissue culture for up to 1 week. Stable (> 15 min) whole cell recordings were obtained from 80 cells displaying rapid (<2 ms) voltage-dependent sodium currents (blocked by tetrodotoxin in 10 of 10 cells tested), and current evoked action potentials, which were thus classified as SFO neurons. These neurons had a resting membrane potential of-63.8 +/- 1.3 mV (mean +/- SEM), spike amplitude of 86.8 +/- 2.5 mV, and input resistance of 1.2 +/- 0.1 G omega, characteristics which did not change significantly in recordings obtained for up to 6 days after dissociation. Current clamp recording showed that of 65 cells tested with bath application of angiotensin (ANG; 1,000-10nM), 41 responded to this peptide with decreases in input resistance (control 1.4 +/- 0.16 G omega, after ANG 0.78 +/- 0.1 G omega, p < 0.0001), and depolarisations (mean 18.3 +/- 2.0 mV, p < 0.0001). Similar recordings were obtained from viable cells up to 6 days after initial cell dissociation. These studies provide the first description of the basic membrane properties of dissociated SFO neurons. The responsiveness of these cells to ANG supports the conclusion that their properties are similar to those in vivo. These data suggest that use of this technique will permit systematic analysis of the membrane events underlying the actions of multiple ligands on this uniquely specialised group of CNS neurons.

Angiotensin II↗
PubMed1996

Estrogen uncouples noradrenergic activation of Fos expression in the female rat preoptic area.

The preoptic area of the rat brain is a site at which gonadal steroids act to regulate sexual behaviour and gonadotrophin secretion. The expression of the immediate-early gene product, Fos, in the preoptic area was investigated in conscious ovariectomised, vehicle and estrogen-treated animals which had received an intracerebroventricular (i.c.v.) infusion of noradrenaline, and also in anaesthetised proestrous and ovariectomised rats following electrical stimulation of the brainstem A1 or A2 noradrenergic cell groups. In ovariectomised oil-treated rats, a third ventricular infusion of noradrenaline (45 micrograms) resulted in a significant (P < 0.05) increase in the numbers of Fos-immunoreactive cell nuclei throughout the preoptic area, compared to vehicle controls. In contrast, Fos expression in animals which had received estrogen replacement showed no change in response to i.c.v. noradrenaline compared with saline-treated controls. In anaesthetised, ovariectomised animals electrical stimulation of the A1 cell group resulted in a significant increase (P < 0.05) in Fos-like immunoreactivity compared with sham controls, specifically within the ventral preoptic area whilst stimulation of the A2 cell group had no significant effect. In anaesthetised, proestrous rats receiving electrical stimulation no significant changes in Fos-like immunoreactivity were detected within the preoptic area after either A1 or A2 stimulation compared with paired controls. These results show that noradrenaline-induced Fos expression in the preoptic area is dependent on estrogen status and suggest that the estrogenic regulation of reproductive functions may thus involve altered responses to noradrenaline in sub-populations of preoptic neurones.

Animals↗
PubMed1996

A stress-sensitive chemokinergic neuronal pathway in the hypothalamo-pituitary system.

Recently we found that cytokine-induced neutrophil chemoattractant influenced anterior pituitary hormone release in vitro. These observations prompted us to investigate the possibility of the existence of cytokine-induced neutrophil chemoattractant in the hypothalamus. Immunohistochemistry showed that cytokine-induced neutrophil chemoattractant-like immunoreactivity existed in the paraventricular hypothalamic nucleus, the supraoptic nucleus, both the internal and the external layers of the median eminence and the posterior pituitary. Since the paraventricular hypothalamic nucleus plays a pivotal role in response to stressful stimuli, we examined the effect of a single episode of immobilization stress on cytokine-induced neutrophil chemoattractant messenger RNA expression in the paraventricular hypothalamic nucleus. Immobilization stress induced strong hybridization signals of cytokine-induced neutrophil chemoattractant messenger RNA in the parvocellular and magnocellular subdivision of the paraventricular hypothalamic nucleus within 15 min, and cytokine-induced neutrophil chemoattractant-like immunostaining intensity in the posterior pituitary started to increase around the periphery of the posterior lobe at 30 min after stress and extended to the whole lobe at 1 h after stress. The increase in the serum cytokine-induced neutrophil chemoattractant in response to stress showed a kinetically biphasic pattern. A first phase occurred within 15 min which may be due to an immediate release of stored cytokine-induced neutrophil chemoattractant in the neurohypophysis, since hypophysectomy completely blocked this phase. A second phase may reflect the release of newly synthesized cytokine-induced neutrophil chemoattractant in the paraventricular hypothalamic nucleus and/or peripheral cytokine-induced neutrophil chemoattractant, since hypophysectomy could not reduce this phase. These data suggest that cytokine-induced neutrophil chemoattractant in the paraventricular hypothalamic nucleus was immediately synthesized in response to stress, and then released into the peripheral blood via the hypothalamo-neurohypophysial system, revealing the presence of a stress-sensitive chemokinergic neuronal pathway in the hypothalamo-pituitary system.

Animals↗
PubMed1995

Differential cellular localization of oestrogen receptor immunoreactivity and oxytocin mRNA and immunoreactivity in the rat preoptic area.

Oestrogen influences both oxytocin mRNA and peptide immunoreactivity in the preoptic area and the rat oxytocin gene contains functional oestrogen response elements. However, using combinations of immunocytochemistry and in situ hybridization for oxytocin and oestrogen receptor, we found that preoptic oxytocin neurons do not possess oestrogen receptors. This finding implies oestrogen actions on oxytocin synthesis in preoptic neurones are unlikely to be mediated directly.

Animals↗
PubMed1995

Changing patterns of Fos expression in brainstem catecholaminergic neurons during the rat oestrous cycle.

Brainstem catecholaminergic neurons are believed to play an important role in the activation of luteinising hormone-releasing hormone (LHRH) neurons on the afternoon of proestrus which results in the luteinising hormone (LH) surge. To examine the respective roles of brainstem A1 and A2 neurons and the adjoining C1 and C2 adrenergic cells at this time, we have examined the patterns of Fos-immunoreactivity within tyrosine hydroxylase (TH) and phenylethanolamine-N-methyl transferase (PNMT) neurons during diestrus and proestrus. Initial studies demonstrated that the LH surge commenced at approximately 15:00 h in proestrous animals and that peak plasma levels of LH were observed between 16:00 and 17:00 h. Groups of cycling female rats (n = 6) were then perfused between 09:00 and 11:00 (diestrus early) and 18:00 to 19:30 h (diestrus late) on diestrus and at the same times on proestrus (proestrus early and proestrus late). Double-labelling immunocytochemistry revealed little Fos expression by adrenergic neurons of the C1 or C2 cell groups and this did not change significantly between any of the experimental groups. Analysis of the A2 region was divided into rostral, middle and caudal divisions and all regions showed a significant (P < 0.01) increase in the number of Fos-expressing TH neurons (up to 35% of TH cells) in proestrus early animals compared with diestrus and proestrus late rats. In the A1 region, a significant increase in the number of TH neurons expressing Fos (approximately 33%) was detected in both proestrus early (P < 0.05) and diestrus early (P < 0.01) rats compared with animals perfused in the late afternoon (approximately 12%).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

Oestrogenic activity of an environmentally persistent alkylphenol in the reproductive tract but not the brain of rodents.

Compounds with oestrogenic actions present in the environment as a result of human activity may represent a threat to health and reproductive efficiency in human and wildlife populations. We show here that parenteral administration of octylphenol, a recently described environmental oestrogen derived from one group of non-ionic surfactants, is active in stimulating oestrogen-dependent uterine growth in prepubertal rats, but has no influence on perinatal sexual differentiation of the rat brain. These results extend previous in vitro findings to show that alkylphenols exert weak oestrogenic activity in vivo in mammals.

Animals↗
PubMed1995

Opioid tolerance and dependence in the magnocellular oxytocin system: a physiological mechanism?

At the neurosecretory terminals in the neural lobe, oxytocin secretion is restrained by co-secreted endogenous opioids, which act via kappa-receptors. The co-secreted opioids include products of pro-dynorphin (released by both vasopressin and oxytocin terminals) and proenkephalin (released by oxytocin terminals). In morphine-tolerant rats this opioid mechanism is more effective, but in late pregnancy it is less effective. Opioids also act directly on oxytocin cell bodies, via separate mu- and kappa-receptors, inhibiting excitation by all stimuli tested, and also exert presynaptic and more distal actions on afferent systems. During chronic morphine exposure, tolerance and dependence develop in oxytocin neurones; the former involves reduction in mu-opioid receptor density, while the latter may involve compensatory upregulation of mechanisms regulating Ca2+ influx. In mid-pregnancy, the effectiveness of opioid mechanisms in the neural lobe increases, assisting the accumulation of oxytocin stores in advance of parturition, but by the end of pregnancy the effectiveness of these mechanisms is reduced. At this time, a separate endogenous opioid system, acting via mu-receptors, actively restrains the electrical activity of oxytocin neurones. Release of this endogenous opioid inhibition may contribute to the increase in activity during parturition analogous to that occurring during morphine withdrawal excitation. Central opioid mechanisms retain the ability to control oxytocin neurones during parturition, and can interrupt established parturition by inhibiting oxytocin neurone firing rate in disadvantageous environmental circumstances.

Animals↗
PubMed1995

Induction of uterine activity with oxytocin in late pregnant rats replicates the expression of c-fos in neuroendocrine and brain stem neurons as seen during parturition.

We investigated whether delivery could be induced with pulsatile oxytocin and whether such treatment activated neurons in the supraoptic nucleus (SON) and putative afferent neurons in the nucleus tractus solitarii and ventrolateral medulla, as seen during spontaneous parturition. Rats were implanted with a jugular venous cannula 1 day before the expected term and on the next morning were given a pulse of oxytocin or saline every 10 min for 4 h. Pulses of 10 and 20 mU oxytocin induced delivery in 77% (14 of 18) of rats, whereas none of the control animals (0 of 9) gave birth during the treatment. Lower doses of oxytocin were not effective at this time in inducing delivery. Animals were killed either before (prepartum groups) or during (parturient groups) delivery, and the brains were processed for immunocytochemistry. Oxytocin treatment induced Fos expression in SON and brain stem neurons in both parturient rats and rats in which parturition was not induced. Fos expression in all sites was significantly higher than that in control prepartum rats, but was similar in extent and distribution to that in spontaneous parturient rats. In the brain stem, a substantial proportion of Fos-immunoreactive cells contained tyrosine hydroxylase-like immunoreactivity, and the number of these cells was increased in response to oxytocin treatment. As only very few Fos-immunoreactive nuclei in either the SON or the nucleus tractus solitarii were observed in virgin rats injected with oxytocin, we suggest that intermittent oxytocin injections in late pregnant rats induce strong uterine activity, which can stimulate magnocellular and putative afferent neurons even before the expulsion of pups.

Animals↗
PubMed1995

Oxytocin antagonists delay the initiation of parturition and prolong its active phase in rats.

The physiological importance of oxytocin for the initiation and maintenance of labour and delivery is controversial. We investigated the effects of two novel peptide oxytocin antagonists on the onset and the progress of delivery in rats implanted with a jugular vein cannula one day before term. During delivery rats were given either an oxytocin antagonist (OVT16, n = 10, or F382, n = 7, 30 micrograms/kg) or vehicle (n = 10, 9) after the birth of the second pup and the time to deliver five more pups was recorded. Other rats were given an injection of F382 (30 micrograms/kg, n = 7) or vehicle (n = 9) after the birth of the fourth pup and the time to deliver three more pups was recorded. In another experiment rats were given repeated injections of F382 (30 or 60 micrograms/kg, n = 13, 11) or vehicle (n = 32) prepartum on the day of expected term and the time of onset and the progress of delivery was recorded. Rats given an antagonist after the second pup delivered the next five pups in 100 +/- 8 min (F382) and 83 +/- 12 min (OVT16), significantly slower than the respective controls (51 +/- 6 and 49 +/- 6 min, U-test, P < 0.05). Four of the 7 rats given F382 after the fourth pup showed no prolongation of delivery (time between pups 4-7: 24.7 +/- 2.9 vs 27.5 +/- 3.1 min in controls), while in the other three rats delivery was prolonged (time between pups 4-7: 86 +/- 4.3 min).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

Pathways to parturition.

Oxytocin is secreted during parturition to stimulate myometrial contractions and birth. Prior to the start of labour, oxytocin neurones undergo changes to prepare for optimal secretion during labour. Thus, during late pregnancy oxytocin secretion is limited by endogenous opioid inhibition. This does not appear to act at the oxytocin nerve terminals in the neural lobe since they in fact become desensitised to opioid inhibition, responding less to either the general opioid antagonist, naloxone, or to the specific kappa-opioid agonist U50,488, and kappa-receptor binding decreases. However, removal of opioid inhibition on oxytocin neurones by naloxone activates oxytocin cell bodies and there is an increase in the number of cells expressing Fos protein in the supraoptic nucleus. This action is mediated via mu- and not kappa-opioid receptors since norBinaltorphimine (kappa-antagonist) is ineffective. Endogenous opioids are likely to act pre-synaptically on inputs to oxytocin neurones, especially those from the brainstem since naloxone potentiates the firing rate response of oxytocin neurones to intravenous cholecystokinin administration which acts via noradrenergic neurones. The endogenous opioid, beta-endorphin may be responsible for inhibition of oxytocin neurones as both the peptide content and its precursor proopiomelanocortin mRNA content increase in the arcuate nucleus during pregnancy, whereas expression of the co-localized opioids, prodynorphin or proenkephalin A, in magnocellular neurones does not alter.

Animals↗
PubMed1994

Stimulus-induced depletion of pro-enkephalins, oxytocin and vasopressin and pro-enkephalin interaction with posterior pituitary hormone release in vitro.

The secretion of oxytocin (OXT) from the neurohypophysis is modulated by the actions of opioids acting via kappa-receptors. The vasopressin (AVP)-containing nerve terminals in the neurohypophysis contain the kappa-opioid agonist dynorphin, but endogenous opioid restraint of OXT secretion is observed even when AVP release is not activated, suggesting that another source of opioids is responsible for modulating OXT secretion. We now report that acute stimulation of the rat neural lobe in vivo results in depletion of the neural lobe content of OXT, AVP, dynorphin A1-17, dynorphin A1-8 and metenkephalin (Met-Enk). The dynorphin content is depleted to a similar extent as that of OXT and AVP; a correlation analysis suggests that while most dynorphin is co-secreted with AVP, a significant portion is co-secreted with OXT, consistent with a co-localisation of dynorphin with OXT. Met-Enk was depleted to a lesser extent than either hormone, consistent with a partial localisation in non-releasable pools. However, depletion of Met-Enk was also observed following naloxone-precipitated opioid withdrawal accompanying selective hypersecretion of OXT, suggesting co-secretion of OXT and Met-Enk. Met-Enk is a mu-opioid receptor agonist, but extended forms of Met-Enk, as we now report, are active at neurohypophysial kappa-receptors.

Animals↗