Jillian M. Pavia

Neuropeptide Y and [Leu31,Pro34]neuropeptide Y potentiate potassium-induced noradrenaline release in the paraventricular nucleus of the aged rat

This microdialysis study investigated the effects of NPY and the Y1 selective agonist [Leu31, Pro34]NPY on basal and potassium-stimulated noradrenaline release in the PVN of 18-month-old anaesthetised male Sprague-Dawley rats. Microdialysate noradrenaline, DOPAC and HVA concentrations were measured by HPLC after i.c.v. administration of 2 nmol NPY, [Leu31, Pro34]NPY or vehicle. [Leu31, Pro34]NPY produced a significant 40% reduction in basal noradrenaline concentration (P < 0.05). Aged rats had blunted noradrenaline response to potassium stimulation, however stimulated noradrenaline release was similar in 18-month-old NPY-treated animals and 3-month-old saline treated age controls (2.8 and 3.2 times resting, respectively). [Leu31, Pro34]NPY induced a significantly greater release of noradrenaline in response to KC1 (5.0 times resting, P < 0.05). Thus, in 18-month-old animals with reduced endogenous hypothalamic NPY content, administration of NPY or [Leu31, Pro34]NPY increased potassium-induced noradrenaline release to levels seen in 3-month-old rats. This effect may be mediated by an NPY Y1 receptor.

Catecholamine release in the rat hypothalamic paraventricular nucleus in response to haemorrhage, desipramine and potassium

In vivo microdialysis and HPLC were used to measure catecholamine release in the rat hypothalamic paraventricular nucleus (PVN) during haemorrhage. The effects of noradrenaline uptake blockade with 1 microM desipramine (DMI) and a depolarising concentration of potassium (100 mM) through the probe were also examined. Dialysis probes implanted in the PVN of urethane anesthetised rats were perfused with modified Ringer solution at 1.1 microliter/min. Thirty minute collections were analysed for DOPA, noradrenaline, DOPAC, HVA and 5-HIAA. Basal concentrations, in the absence of DMI, were: DOPA 203.6 +/- 44.0 pg/ml, noradrenaline 128.0 +/- 20.4 pg/ml; DOPAC 5.6 +/- 0.7, HVA 5.1 +/- 2.2 and 5-HIAA 87.2 +/- 17.8 ng/ml. Basal noradrenaline was doubled in the presence of DMI while basal and stimulated DOPA, DOPAC, HVA and 5-HIAA were not affected by DMI. Haemorrhage resulted in a significant noradrenaline release (48% over resting levels) in the presence of DMI (n = 10, P < 0.05); in the absence of DMI, a smaller and non-significant increase (30% over basal levels) was observed. Potassium-induced depolarisation caused a significant two- and four-fold increase in noradrenaline release (P < 0.001), with decreases in the dopamine metabolites DOPAC (31%, 44%) and HVA (35%, 28%), and the serotonin metabolite, 5-HIAA (41%, 33%), in the presence and absence of DMI, respectively. The catecholamine precursor DOPA did not vary throughout either experiment. The results indicate that haemorrhage induces a 48% increase in noradrenaline release in the rat PVN which provides evidence for a role of noradrenergic projections to the PVN in cardiovascular control.

Centtral Interactions Between Noradrenaline and Neuropeptide Y in the Rat: Implications for Blood Pressure Control

Neuropeptide Y (NPY) and noradrenaline are co-localised in central neurones and both transmitters exert cardiovascular effects. Using microdialysis and push-pull techniques to measure transmitter release in vivo, and microinjection studies, we examined the role(s) of central noradrenaline and NPY in blood pressure regulation in the hypothalamus and nucleus tractus solitarius (NTS) of the rat. Hypothalamic noradrenaline release was increased following haemorrhage and reduced after phenylephrine infusion. Ageing is associated with markedly reduced NPY concentrations in the hypothalamus. 18-month old animals showed a reduced ability to release both NPY and noradrenaline to a potassium depolarisation stimulus. NTS administration of NPY induced dose-dependent decreases in blood pressure and heart rate. The depressor but not the bradycardic response was attenuated by prior administration of yohimbine. NTS microinjection of 23 pmol NPY induced similar cardiovascular effects in spontaneously hypertensive and Wistar Kyoto rats. NPY and noradrenaline appear to interact at several sites in the brain known to be important for blood pressure control.

Neuropeptide Y potentiation of potassium-induced noradrenaline release in the hypothalamic paraventricular nucleus of the rat in vivo

NPY is co-localised with catecholamines in the brain and periphery. Noradrenaline and NPY are present in high concentrations in the PVN of the hypothalamus, an area implicated in autonomic regulation. This microdialysis study examined whether NPY can modulate rat PVN noradrenaline release in vivo, as has been shown in vitro. Basal and K(+)-stimulated noradrenaline release was measured after i.c.v. administration of 2 nmol NPY or vehicle. No effect of NPY was observed on basal release, however a significant doubling of K(+)-induced release was observed, both 60 and 150 min following i.c.v. NPY. This raises the possibility that NPY may potentiate rather than inhibit brain noradrenaline release in vivo.

TRANSITORY REDUCTION IN ANGIOTENSIN AT2 RECEPTOR EXPRESSION LEVELS IN POSTINFARCT REMODELLING IN RAT MYOCARDIUM

1. Myocardial infarction (MI) poses a significant risk for sudden cardiac death. The effectiveness of angiotensin‐converting enzyme (ACE) inhibitors and AT1 receptor blockade in attenuating unfavourable post‐MI outcomes indicates an important role for angiotensin (Ang) II signalling in the post‐MI remodelling process.

Increased endogenous noradrenaline and neuropeptide Y release from the hypothalamus of streptozotocin diabetic rats

Noradrenaline and neuropeptide Y (NPY) in the hypothalamus regulate a number of important endocrine and autonomic functions. Alterations in brain neurotransmitter content have been described in type 1 diabetes but there is little understanding of whether these changes affect neurotransmitter release. This study examined for the first time, region-specific co-release of NPY and noradrenaline from the hypothalamus of male Sprague-Dawley rats treated intravenously with 48 mg/kg streptozotocin (STZ) or vehicle. Five weeks later, the release of endogenous noradrenaline and NPY was monitored by in vitro superfusion of ventral and dorsal hypothalamus slices under basal and potassium-stimulated conditions. STZ-diabetes induced significant increases in basal noradrenaline and NPY overflow from the ventral hypothalamus (P<0.05); only NPY overflow was increased in the dorsal hypothalamus (P<0.05). Noradrenaline overflow increased similarly to potassium depolarisation in vehicle and STZ-diabetic rats, whereas diabetic rats showed a significantly increased NPY overflow response to potassium depolarisation compared to vehicle rats. These region-specific increases in endogenous noradrenaline and NPY overflow from the hypothalamus in diabetes suggest increased neuronal activity at rest and enhanced responses under some conditions. Increased hypothalamic NPY and noradrenaline overflow most likely contributes to diabetic hyperphagia.

AGE-RELATED CHANGES IN NEUROPEPTIDE Y CONTENT IN BRAIN AND PERIPHERAL TISSUES OF SPONTANEOUSLY HYPERTENSIVE RATS

1. This study examined neuropeptide Y (NPY) concentrations in brain regions and peripheral tissues of young (3–4 months) and old (17–18 months) normotensive Wistar‐Kyoto (WKY) rats and spontaneously hypertensive rats (SHR).

Stimulation of Neuropeptide Y Overflow in the Rat Paraventricular Nucleus by Corticotropin‐Releasing Factor

Abstract: Neuropeptide Y (NPY) and corticotropin‐releasing factor (CRF) are present at high concentrations in the hypothalamus where they mediate important endocrine and autonomic functions. Morphological and physiological studies have suggested an interaction between these peptides, and opposing actions of CRF and NPY have been reported on feeding and other behaviors. This study investigated the effect of CRF on NPY release in vivo, measured by push‐pull techniques, in the anesthetized rat. Push‐pull probes implanted into the paraventricular nucleus of the hypothalamus (PVN) were perfused with modified Ringer solution containing bovine serum albumin at 15 µl/min, and the perfusate was lyophilized prior to NPY radioimmunoassay. NPY overflow from the rat PVN was increased threefold by perfusion of a depolarizing concentration of potassium (50 mmol/L KCI). When CRF was administered into the PVN via the push‐pull cannula at 1 or 5 µg/ml, dose‐dependent increases in NPY overflow of two‐ and fivefold were observed (p < 0.05). These increases were abolished by prior intracerebroventricular (i.c.v.) administration of the CRF antagonist [d‐Phe12,Nle21,38,CαMeLeu32]CRF (12–41) at 1 or 5 µg/µl, respectively. NPY overflow returned promptly to resting levels following CRF administration. In contrast, when CRF was administered by i.c.v. bolus at a similar total dose (2 µg), no significant effect on NPY overflow was observed. These data provide in vivo evidence for an interaction between CRF and NPY at the level of the PVN.

Altered in vivo catecholamine release in the hypothalamic paraventricular nucleus of the aged rat

The effect of age on basal and stimulated noradrenaline release in the hypothalamic paraventricular nucleus (PVN) of the rat was examined by in vivo microdialysis. Microdialysis probes were inserted into the PVN of 3 and 18 month old anaesthetised Sprague Dawley rats and perfused with a modified Ringer solution. Following four basal 30-min collections, transmitter release was stimulated by perfusion with 100 mM potassium for one collection. After re-equilibration, blood pressure was raised 60 mmHg for 30 min by phenylephrine infusion (1–1.3 mg/kg) then a 2-h recovery period followed. Dialysate collections were injected directly onto a reverse phase HPLC-ECD (HPLC with electrochemical detection).Basal extracellular noradrenaline concentrations were found to be similar in adult and old animals. Basal dihydroxyphenylacetic acid (DOPAC) concentrations were significantly greater in old compared to adult rats (P < 0.05). Potassium depolarisation induced a significant increase in noradrenaline concentrations in both age groups (P < 0.001), however the noradrenaline response to potassium stimulation was significantly reduced in the aged rats (P < 0.05). Potassium-induced decreases in DOPAC and homovanillic acid (HVA) concentrations were seen in both age groups. Following phenylephrine infusion, a modest delayed reduction in noradrenaline levels, which failed to reach statistical significance, was seen. Phenylephrine-induced hypertension was associated with decreased DOPAC and HVA concentrations in adult (P < 0.05) and old (P < 0.05) rats, respectively.These results indicate that ageing is associated with changes in dopaminergic and noradrenergic activity in the PVN of the rat. A reduction in noradrenaline response to maximal stimulation induced by potassium depolarisation was observed with ageing. The alteration in the activity of the catecholaminergic pathways to the PVN induced by phenylephrine infusion appears to be age dependent.

Lack of effect of age on the cardiovascular response to neuropeptide Y injection in the rat nucleus tractus solitarius.

1. Neuropeptide Y (NPY) is colocalized with catecholamines in central regions involved in blood pressure regulation and exerts depressor responses in the nucleus tractus solitarius (NTS). Ageing is accompanied by a decline in baroreflex function and a reduction in NPY concentrations in some brain areas. The present study investigated whether the cardiovascular response to NPY microinjection into the NTS and medullary NPY concentrations were conserved in aged rats.