Michael J. Meldrum

Prejunctional and postjunctional effects of neuropeptide Y at the noradrenergic neuroeffector junction of the perfused mesenteric arterial bed of the rat

Summary: The effect of neuropeptide Y (NPY) on periarterial nerve stimulation-induced release of norepinephrine (NE) and increase in perfusion pressure in the perfused mesenteric arterial bed of the rat was examined. Perfusate effluents were continuously collected and assayed for endogenous NE by high-pressure liquid chromatography (HPLC) coupled to electrochemical detection. Perfusion pressure was continuously monitored by means of a pressure transducer. Periarterial nerve stimulation (8 or 16 Hz, 60 V, 2-ms duration for 30 s) resulted in a readily detectable increase in NE release and perfusion pressure that was attenuated by the prior administration of tetrodotoxin (TTX) (10−5 M) or guanethidine (5 × 10−5 M). NPY exerted both prejunctional and post-junctional effects on noradrenergic neurotransmission in this preparation. The peptide produced a concentration-dependent reduction in the release of NE over a concentration range of 10−10-10−7 M. A similar inhibition effect occurred at 8. 10. and 16 Hz. In contrast, low concentrations (10−10 and 10−9 M) decreased the effect of nerve stimulation on perfusion pressure, whereas higher concentrations (10−7 M) produced a marked potentiation. The α2-adrenoceptor antagonist, yohimbine, did not alter the inhibitory effect of NPY on evoked NE release or the effect on perfusion pressure. Prazosin similarly did not alter the inhibitory effect of NPY on NE release but prevented the increase in perfusion pressure. We conclude that NPY modulates noradrenergic neurotransmission in the mesenteric arterial bed by decreasing the evoked release of NE and producing a concentration-dependent bi-phasic response on vascular smooth muscle. The inhibition of NE produced by low concentrations of NPY results in a decrease in perfusion pressure whereas high concentrations produce a marked increase in perfusion pressure.

Endogenous glutamate release from frontal cortex of adult and aged rats

Glutamate (GLU) is a major excitatory neurotransmitter in the frontal cortex. Alterations in GLU neurotransmission are present in a number of neurodegenerative diseases, however, little is known about the normal aging process of GLU utilizing neurons. GLU release, uptake and content were examined in the frontal cortex of adult (6 months old) and aged (24 months old) male, Fisher 344 rats. These markers were used to assess the functional integrity of intrinsic and extrinsic GLU utilizing pathways innervating the frontal cortex. Basal- and potassium- (56 mM) evoked GLU release from brain slices of aged rats were not significantly different from that of adults. Kainic acid (1.0 mM) failed to significantly augment basal or potassium-stimulated GLU release in the frontal cortex of either aged or adult rats. Uptake of [3H] GLU into brain slices was also unaltered as a function of age. In contrast, GLU content was decreased 17% in the frontal cortex of aged rats when compared to the adults. These results suggest that the functional integrity of GLU utilizing nerve terminals in the frontal cortex is maintained in 24-month-old Fisher 344 rats. The decrease in GLU content may reflect a generalized neuronal loss or a defect in neuronal and/or glial GLU metabolism in the metabolic compartment.

Noradrenergic transmission in the isolated portal vein of the spontaneously hypertensive rat.

The effect of electrical field stimulation (1, 2, 5, 10 Hz for a total of 480 pulses at 15-minute intervals) on the release of 3H-norepinephrine from the superfused portal vein of spontaneously hypertensive rats (SHR) or Wistar-Kyoto rats (WKY) of various ages was studied. The ages of the animals were (in weeks) 5-6 (prehypertensive), 8-10 (young hypertensives), 16-18 (older hypertensives), and 28 (mature hypertensives). There was no difference in the release of 3H-norepinephrine or developed tension of the portal vein to any frequency of field stimulation of SHR or WKY at 5-6 weeks of age. However, there was a significantly greater release of 3H-norepinephrine and developed tension of veins of SHR in response to low (1 or 2 Hz) but not high frequencies (5 or 10 Hz) at 8-10, 16-18, and 28 weeks of age. Vessels from hypertensive animals also developed greater resting tension and spontaneous activity, which was reduced to that of WKY in the presence of an alpha-adrenergic antagonist. The alpha 2 selective adrenergic antagonist yohimbine produced the same degree of enhancement of release of 3H-norepinephrine to field stimulation of veins obtained from both SHR and WKY at 5-6, 8-10 and 16-18 weeks of age. However, the facilitory effect of yohimbine was significantly attenuated in portal veins obtained from SHR at 28 weeks of age compared to age-matched WKY.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin Facilitation of Noradrenergic Neurotransmission in Central Tissues of the Rat: Effects of Sodium Restriction

This study investigated the ability of angiotensin II (Ang II) to facilitate the stimulation-induced release of [3H]norepinephrine [( 3H]NE) from two cardiovascular regulatory areas in normal and sodium-restricted rats. Ang II (10(-7) M) facilitated the field-stimulation-induced release of [3H]NE from the A2 area of the nucleus tractus solitarius but not from the anterior hypothalamus of Sprague-Dawley and Wistar rats. Placement of rats on a sodium-restricted diet abolished the facilitation of [3H]NE release due to Ang II. Captopril given during sodium restriction partially restored the facilitory effects of Ang II. In an effort to determine the interaction of Ang II and sodium reduction, the effects of chronic Ang II were studied. Seven-day intravenous Ang II infusions blocked the facilitory effect of Ang II on [3H]NE release in a manner similar to that seen with sodium restriction. These results suggest that low sodium diets may alter the facilitation of [3H]NE release by Ang II by interactions with the renin-angiotensin system.

The effect of opioid drugs on the release of dopamine and 5-hydroxytryptamine from rat striatum following activation of nicotinic-cholinergic receptors

The effect of (Met5)enkephalin, (D-Ala2,D-Met5)enkephalin, (Leu5)enkephalin, (D-Ala2,D-Met5)enkephalin and morphine on the release of [3H]dopamine, endogenous dopamine and [3H]5-hydroxytryptamine produced by the nicotinic-cholinergic agonist, dimethylphenyl piperazinium iodide (DMPP), was examined in rat striatal slices. The DMPP-induced release of [3H]dopamine and endogenous dopamine was reduced by the presence of (Met5)enkephalin, (D-Ala2,D-Met5)enkephalin (1-10 microM) or morphine (10 microM) but not by (Leu5)enkephalin or (D-Ala2,D-Leu5)enkephalin. The DMPP-induced release of [3H]5-hydroxytryptamine was reduced by (Leu5)enkephalin, (D-Ala2,D-Leu5)enkephalin, (Met5)enkephalin, (D-Ala2,D-Leu5)enkephalin (1-10 microM), and morphine (10 microM). All three opioids failed to alter the release of [3H]dopamine induced by field stimulation or potassium depolarization (30 microM). The inhibitory effects of opioid peptides and morphine demonstrated in the present study appear to be due to an initial interaction with nicotinic-cholinergic receptors in the striatum.

Norepinephrine Content in Cardiovascular Tissues from the Aged Fischer 344 Rat

The major aim of the present study was to examine cardiovascular tissues and to determine if norepinephrine (NE) stores declined in aged (22-30 months old) male Fischer 344 (F344) rats. NE content was measured by HPLC in the whole heart, ventricle, atria, caudal artery, renal arteries, portal vein and kidney. The release of [3H]NE from the caudal artery of adult and aged rats was assessed as a functional index of sympathetic innervation. Renal alpha 2-adrenergic binding sites were also determined using 2.0 nM [3H]rauwolscine. In general, total NE content per organ was not altered in aged rats. The exception was the ventricle of aged rats which exhibited a 25% decrease in NE content. In contrast, NE content per gram of tissue weight was significantly decreased in the whole heart, atria, ventricle, kidney and caudal artery of aged rats when compared to that of adults (6-8 months old). Neither basal nor potassium-stimulated NE release was altered in the caudal artery of aged rats. Presynaptic regulation of NE release by taurine and alpha 2-adrenoceptors was also not affected by age. Renal alpha 2-adrenergic binding sites were decreased 23% in aged rats. Tissue growth that occurs as a function of aging does not appear to receive a concomitant increase in sympathetic growth as indexed by NE content. The findings of the present study would lead to the conclusion that with the exception of the ventricle, sympathetic nerve terminals of the aged male F344 rat are relatively intact.

The Effect of Taurine on Blood Pressure, and Urinary Sodium, Potassium and Calcium Excretion

Taurine is a sulfur-containing amino acid found in large quantities in muscle, brain, heart and blood. Its numerous physiological and biological properties are still not fully understood. Studies have examined the physiological effects of taurine in several different areas. Proposed functions include: major inhibitory neurotransmitter; inhibitory neuromodulator; regulator of Ca2+ movements in muscle (cardiac, smooth and skeletal); transmitter within the retina; growth factor in brain development; and major osmoregulater within cells in the periphery and the central nervous system (CNS) (2,6–8,10,14,15).

[3H]d-Aspartic acid release in brain slices of adult and aged fischer 344 rats

Alterations in glutamate content and uptake have been reported to occur in aged animals. The present studies used [3H]d-Aspartic acid ([3H]-D-ASP) release as a marker for glutamate neurotransmission. Frequency dependent [3H]-D-ASP release was measured in adult (8 month) and aged (28–30 month) Fischer 344 rats. Relatively high stimulation frequencies (>10 Hz) were required to induce [3H]-D-ASP release in both adult and aged F344 rats in temporal cortex and hippocampus. In both brain areas aged animals showed significantly more [3H]-D-ASP release than adult animals Kainic acid 1 mM failed to induce the release of [3H]-D-ASP in either temporal cortex or hippocampus. Omega conotoxin GVIA (5×10−9M) a N and L type voltage sensitive calcium channel antagonist failed to inhibit [3H]-D-ASP stimulated release. These results demonstrate an increase in [3H]-D-ASP release in aged compared to adult F344 rats. The data also suggest a novel calcium channel may be involved in [3H]-D-ASP release.

Comparison of Norepinephrine Release in Hypertensive Rats: I Hypothalamic and Brainstem Tissues

Stimulation induced 3H-norepinephrine release was measured in hypothalamus and brainstem of spontaneously hypertensive (SHR) and normotensive (WKY) rats. Age dependent changes in 3H-norepinephrine release were shown to occur in the anterior and posterior hypothalamus and the A2 region of the nucleus tractus solitarius (NTS). In an attempt to determine whether these changes in 3H-transmitter release were causal or merely secondary to the increase in blood pressure, similar release studies were carried out in DOCA-salt and one kidney-one clip hypertensive animals with similar levels of systolic blood pressure. The changes in stimulus-induced 3H-norepinephrine release seen in the SHR were not observed in the other two models of hypertension, suggesting that one: they were not secondary to an increase in systolic blood pressure; and two that the changes observed in the SHR may possibly play a role in the development and/or maintenance of the hypertension.

A long-acting silicone delivery system for morphine and naloxone administration

Abstract A silicone (dimethylpolysiloxane) slow-release pellet containing morphine sulfate or naloxone HCI was developed for tissue implantation. The morphine-silicone pellet (300 mg morphine sulfate per pellet) induced a high level of tolerance and physical dependence in mice within 24 hr of implantation and the effects were maintained for 96 hr. Morphine was released immediately upon subcutaneous implant of the pellet and peak blood levels of morphine were observed within 12 hr of implantation. The pellet continued to release morphine over a 144-hr period and the blood levels remained elevated. Naloxone HCI-silicone slow-release pellets (25 mg naloxone HCI) blocked the analgesic action of morphine sulfate (10 mg kg ) over 144 hr of implantation. The present study indicates that a silicone pellet containing naloxone can effectively antagonize the acute action of morphine sulfate over a prolonged period of time.