Thomas C. Westfall

Preferential stimulation of ventral tegmental area dopaminergic neurons by nicotine

The effect of intravenous (i.v.) nicotine on the single unit activity of midbrain dopamine (DA) neurons was studied in rats under either local or general anesthesia. Nicotine (50-500 micrograms/kg) produced a dose-related increase in the firing rate of nigral pars compacta DA cells (A9), up to 25% above baseline, irrespective of the preparation. The same range of doses was more than three times as effective on ventral tegmental area DA cells (A10) in rats paralyzed and given a local anesthetic. By contrast, the majority of these cells were temporarily depressed in deeply anesthetized animals. All of the above effects were reversed and prevented by i.v. mecamylamine suggesting the involvement of nicotine cholinergic receptors. Moreover, after nicotine-induced stimulation, low doses of i.v. apomorphine inhibited the firing rate similar to controls indicating that dopamine receptors are not directly involved in the nicotinic action. The results suggest that acute nicotine shares with other drugs of abuse the characteristic of being more effective in stimulating A10 than A9 neurons.

Inhibition of periarterial nerve stimulation-induced vasodilation of the mesenteric arterial bed by CGRP (8-37) and CGRP receptor desensitization.

We provide the first functional evidence that calcitonin gene-related peptide (8-37) induces a direct vasoconstriction and reversibly antagonizes vasodilation of the mesenteric arterial bed induced by calcitonin gene-related peptide (CGRP) suggesting that CGRP (8-37) is a competitive antagonist of vascular CGRP receptors. Vasodilation induced by periarterial nerve stimulation was inhibited both by CGRP (8-37) and by desensitization of CGRP receptors. These results further support the evidence that the periarterial nerve stimulation-induced nonadrenergic noncholinergic vasodilation of the mesenteric vasculature is mediated by endogenous CGRP and its receptors.

Cannabinoids augment the release of neuropeptide Y in the rat hypothalamus

Little is known about the mechanism of action behind the orexigenic activity of cannabinoids. Neuropeptide Y (NPY) is one of the most potent orexigenic factors and is a key mediator in the hypothalamic control of food intake. We examined the effect of cannabinoids on NPY release using a rat hypothalamic explant model. The cannabinoid agonists anandamide (AEA) and CP55,940 both significantly augmented resting and KCl-evoked NPY release. AM251, a cannabinoid receptor antagonist, blocked the augmentation of NPY release elicited by AEA and CP55,940. Additionally, AM251 administered alone, in the absence of exogenous cannabinoid agonists, inhibited NPY release demonstrating the role of endogenous cannabinoids in NPY release. Combined, these findings demonstrate that cannabinoids augment NPY release in the hypothalamus and that this may be a potential mechanism behind the orexigenic activity of cannabinoids.

Release of dopamine and 5-hydroxytryptamine from rat striatal slices following activation of nicotinic cholinergic receptors

1. The administration of the nicotinic cholinergic agonists dimethylphenylpiperazinium iodide (DMPP) or nicotine caused a concentration dependent release of [3H] dopamine, [3H]5-hydroxytryptamine as well as endogenous dopamine and 5-hydroxytryptamine from superfused slices of rat striatum. 2. Release of both labelled and non-labelled transmitter was antagonized by the nicotinic antagonist, hexamethonium but not the muscarinic antagonist, atropine. 3. The present study provides additional evidence that nicotinic-cholinergic receptors are present in the mammalian central nervous system. 4. Activation of these nicotinic-cholinergic receptors in the striatum results in the release of both dopamine and 5-hydroxytryptamine.

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.

Autoreceptor-induced inhibition of neuropeptide Y release from PC-12 cells is mediated by Y2 receptors

Pheochromocytoma (PC)-12 cells express Y1, Y2, and Y3 neuropeptide Y (NPY) receptors when differentiated with nerve growth factor (NGF). The present work evaluated NGF-differentiated PC-12 cells as a model system to study modulation of NPY release by NPY autoreceptors. We demonstrated that both K+ and nicotine stimulated concomitant release of NPY and dopamine from differentiated PC-12 cells. We also showed in this study that NPY release from PC-12 cells was attenuated in a concentration-dependent manner by peptide YY (PYY)-(13-36), a selective agonist for the Y2 type of NPY receptors. This result demonstrated that NPY release could be modulated by NPY autoreceptors of the Y2 subtype. The inhibitory action of PYY-(13-36) may be mediated at least in part by inhibition of N-type Ca2+channels, because PYY-(13-36) could not produce further inhibitory effects in the presence of a maximum effective concentration of ω-conotoxin, an N-type Ca2+-channel blocker. The inhibition by PYY-(13-36) could be blocked by pretreatment of cells with pertussis toxin, suggesting that an inhibitory GTP-binding protein was involved. Furthermore, the function of NPY autoreceptors could be modulated by other receptors such as β-adrenergic and ATP receptors. The evoked release of NPY was also attenuated by ATP and adenosine, which have been shown to be colocalized and coreleased with NPY from sympathetic nerve terminals. These results suggest that PC-12 cells differentiated with NGF may be an ideal model to study regulatory mechanisms of NPY release and that autoreceptor-mediated regulation of NPY release appears to act through the Y2 subtype of the NPY receptor.

Direct evidence for the role of neuropeptide Y in sympathetic nerve stimulation-induced vasoconstriction

Neuropeptide Y (NPY) is a vasoconstrictor peptide and a cotransmitter with norepinephrine (NE) in sympathetic nerve terminals and is thought to be involved in sympathetic nerve stimulation (SNS)-induced vasoconstriction. Using BIBP-3226, a Y1 receptor selective antagonist, we examined this hypothesis in the isolated and perfused mesenteric vascular bed. SNS produced a frequency-dependent increase in perfusion pressure and concomitant overflow of NPY immunoreactivity in the perfusate. [Leu31,Pro34]NPY potentiated NE-induced and ATP-induced vasoconstriction, indicating the presence and biological action of Y1 receptors in this vascular bed. The potentiation effect of [Leu31,Pro34]NPY of the increase in perfusion pressure by NE, ATP, or SNS was prevented by BIBP-3226. In addition, SNS-induced vasoconstriction at both high and low frequencies was significantly attenuated by BIBP-3226 at a concentration that completely blocked the [Leu31,Pro34]NPY-induced potentiation of the NE- or ATP-induced vasoconstrictor effect. These results suggest that approximately 30% of vasoconstriction produced by SNS depends on NPY in the mesenteric vascular bed.Neuropeptide Y (NPY) is a vasoconstrictor peptide and a cotransmitter with norepinephrine (NE) in sympathetic nerve terminals and is thought to be involved in sympathetic nerve stimulation (SNS)-induced vasoconstriction. Using BIBP-3226, a Y1 receptor selective antagonist, we examined this hypothesis in the isolated and perfused mesenteric vascular bed. SNS produced a frequency-dependent increase in perfusion pressure and concomitant overflow of NPY immunoreactivity in the perfusate. [Leu31,Pro34]NPY potentiated NE-induced and ATP-induced vasoconstriction, indicating the presence and biological action of Y1 receptors in this vascular bed. The potentiation effect of [Leu31,Pro34]NPY of the increase in perfusion pressure by NE, ATP, or SNS was prevented by BIBP-3226. In addition, SNS-induced vasoconstriction at both high and low frequencies was significantly attenuated by BIBP-3226 at a concentration that completely blocked the [Leu31,Pro34]NPY-induced potentiation of the NE- or ATP-induced vasoconstrictor effect. These results suggest that ∼30% of vasoconstriction produced by SNS depends on NPY in the mesenteric vascular bed.

Inhibition of the electrically induced release of [3H]dopamine by serotonin from superfused rat striatal slices

The effect of serotonin (5-hydroxytryptamine) on the electrically induced release of [3H]dopamine from superfused slices of the rat striatum has been studied. It was observed that serotonin produced a concentration dependent decrease in the field stimulation-induced release of [3H]dopamine with the threshold concentration being 10(-6) M or lower. Methysergide, in a concentration which did not alter the evoked release, antagonized the inhibitory effect of serotonin. The present results suggest that serotonin should be added to the list of endogenous substances that can influence dopaminergic transmission in the striatum.

Effects of endothelin on regional hemodynamics in conscious rats.

Endothelin is a potent vasoactive peptide in anesthetized rats and isolated vascular smooth muscle. This study was performed to describe the hemodynamic effects of endothelin in conscious, freely moving rats. Endothelin (0.067-2 nmol/kg i.v.) produced long-lasting, dose-dependent increases in arterial pressure, mesenteric and, to a lesser degree, hindquarters vascular resistances and decreases in heart rate. We suggest that endothelin may play an important role in regulation of arterial pressure by modulating peripheral vasomotor tone.

Allosteric modulation by leucine-enkephalin of [3H]naloxone binding in rat brain

The interaction of morphine and leucine-enkephalin with the binding site labeled by [3H]naloxone in the presence of sodium was compared. The effect of fixed concentrations of morphine and leucine enkephalin on the saturation binding of [3H]naloxone demonstrated that whereas morphine was a competitive inhibitor, leucine enkephalin caused a dose-dependent masking of binding sites. From these data we conclude that the enkephalin receptor is allosterically coupled to the morphine receptor.