Somatostatin modulation of adenosine receptor coupled G-protein subunits in the caudate nucleus of the rat

Elements toward novel therapeutic targeting of the adrenergic system

Adrenergic receptors belong to the family of the G protein coupled receptors that represent important targets in the modern pharmacotherapies. Studies on different physiological and pathophysiological properties of the adrenergic system have led to novel evidences and theories that suggest novel possible targeting of such system in a variety of pathologies and disorders, even beyond the classical known therapeutic possibilities. Herein, those advances have been illustrated with selected concepts and different examples. Furthermore, we illustrated the applications and the therapeutic implications that such findings and advances might have in the contexts of experimental pharmacology, therapeutics and clinic. We hope that the content of this work will guide researches devoted to the adrenergic aspects that combine neurosciences with pharmacology.

Nikolai Konstantinovich Kulchitsky (1856-1925)

Nikolai Kulchitsky is best remembered for his identification of the Kulchitsky (enterochromaffin) cell. His life spanned a teaching and scientific career at Kharkov University, employment as the Imperial Minister of Education for all Russia, work in a soap factory and flight from the Russian Revolution to London, and a position at the University College with Elliot Smith. His subsequent contributions to the anatomic delineation of dual nerve-endings in the muscle were highly regarded, although his identification of the enterochromaffin cell (1897) remains his enduring scientific legacy. The observation of a cardinal neuroendocrine cell of the gut formed the basis for the subsequent delineation of the diffuse neuroendocrine system and provided the cellular framework on which the discipline of gut neuroendocrinology would be established. Kulchitsky's mysterious demise in a bizarre lift-shaft accident at UCL on his 69th birthday tragically terminated a life of service to science.

Modification of tyrosine hydroxylase activity by chloral derived beta-carbolines in vitro

Beta-carbolines have been suggested to be involved in the pathogenesis of Parkinson's disease as a result of their structural similarity to the neurotoxin N -methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The chloral-derived beta-carboline derivative 1-trichloromethyl-1,2,3,4-tetrahydro-beta-carboline (TaClo) causes cell loss in neuronal and glial cell cultures and induces a slowly developing neurodegenerative process in rats. In our experiments, effects of TaClo and its derivatives 2-methyl-TaClo (2-Me-TaClo), and 1-dichloromethylene-1,2,3,4-tetrahydro-beta-carboline (1-CCl(2) -THbetaC) on tyrosine hydroxylase (TH) activity were investigated in TH assays using homogenate preparations of the rat nucleus accumbens and recombinant human TH (hTH1). TH activity was determined in vitro by measuring l-DOPA production with HPLC-ECD. Using homogenate preparations, TaClo, 2-Me-TaClo, and 1-CCl(2) -THbetaC inhibited TH in concentrations of 0.1 mm, while 1-CCl(2) -THbetaC in low concentrations enhanced TH activity. When TH was activated by PACAP-27, TaClo, 2-Me-TaClo, or 1-CCl(2) -THbetaC also inhibited activated enzyme activity in high concentrations. However, in the case of 2-Me-TaClo and 1-CCl(2) -THbetaC a biphasic effect was observed with a marked increase of TH activity in the nanomolar range. In our experiments using recombinant hTH1, TaClo, 2-Me-TaClo, or 1-CCl(2) -THbetaC did not modify enzyme activity. After activation of hTH1 by PKA all the tetrahydro-beta-carbolines investigated in this study decreased l-DOPA formation. We suggest that these beta-carbolines modulate dopamine synthesis by interacting with a protein kinase TH-activating system.

Pituitary adenylate cyclase-activating polypeptide (PACAP-27) enhances tyrosine hydroxylase activity in the nucleus accumbens of the rat

Pituitary adenylate cyclase-activating polypeptide (PACAP-27) was incubated in a tyrosine hydroxylase (TyrOH) assay with a homogenate preparation of the nucleus accumbens of the rat. TyrOH activity was determined in vitro by measuring the production of L-dopa with HPLC-ECD. Only in the presence of adenosine nucleotides (ATP, App(NH)p) PACAP-27 increased TyrOH activity with a EC(50)of 100 nM. Since the PACAP-27 effect on TyrOH was abolished when homogenate or pellet of the nucleus accumbens were coincubated with CHAPS, the peptide effect appears to be receptor mediated. TyrOH activation produced by PACAP-27 increased in the presence of the phosphodiesterase inhibitor papaverine indicating the involvement of cAMP. The marked effect of the non-hydrolysable adenosine nucleotide App(NH)p also supports a cAMP-dependent TyrOH activation not related to ADP or an ADP-dependent mechanism. This report's data suggest that PACAP-27 activates TyrOH in the rat nucleus accumbens through receptor-mediated cAMP formation. The exact receptor type present in the nucleus accumbens has yet not been specified.

alpha-MSH changes cyclic AMP levels in rat brain slices by an interaction with the D1 dopamine receptor

The exposure of rat brain slices containing caudate putamen and accumbens nuclei to alpha-MSH or dopamine (DA) results in an increase in cyclic AMP (cAMP) levels. When tissues are compared with those containing both alpha-MSH and DA, a reduction in the cyclic nucleotide is observable. This study was carried out to determine whether variations in tissular cAMP levels induced by alpha-MSH might be explained by an interaction between the peptide and some dopaminergic receptors. Therefore, we measured cAMP in tissues and medium in response to alpha-MSH in the presence of haloperidol, the selective D1 (SCH 23390) or D2 (sulpiride) antagonists, or the selective D1 (SKF 38393) or D2 (bromocriptine) agonists. Haloperidol by itself induced no changes either in the cAMP content or in the cAMP efflux to the medium. When slices were exposed to alpha-MSH and haloperidol, the latter blocked the alpha-MSH effect of inducing an increase in the content of cAMP. None of the specific antagonists (at the administered doses) induced changes in the content of cAMP when compared with the control group. The presence of SCH 23390 in the incubation medium together with alpha-MSH yielded a reduction in cAMP levels compared with those incubated with alpha-MSH. A slight stimulatory effect on cAMP formation was observed when the dopaminergic agonists (SKF 38393 10 microM) were used. We conclude that alpha-MSH interacts with the D1 dopamine receptor, changing the cAMP levels in striatum and accumbens nuclei.

Interactions between inhibitory and excitatory modulatory signals in single submucosal neurons

Intracellular recordings were made in submucosal neurons from the guinea pig ileum to study the actions of norepinephrine and somatostatin on slow depolarizations induced by 2-chloroadenosine (CADO) and substance P. Local application (by pressure) of CADO and substance P induced a slow depolarization that occurred concomitantly with an increase in input membrane resistance. Norepinephrine, UK-14304 (alpha 2-adrenoceptor agonist), and somatostatin blocked the excitatory responses induced by CADO in a concentration-dependent manner. The alpha 2-adrenoceptor antagonists idazoxan and yohimbine antagonized these inhibitory effects of UK-14304 and norepinephrine. UK-14304 also decreased depolarizations induced by forskolin, but not those induced by the adenosine 3',5'-cyclic monophosphate analogue 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate. Slow depolarizations induced by substance P were blocked neither by UK-14304 nor by somatostatin. It was previously shown that staurosporine (an inhibitor of various protein kinases) and KT-5720 (an inhibitor of protein kinase A) inhibited slow depolarizations induced by CADO. Here, substance P depolarizations were inhibited by staurosporine and calphostin C (a blocker of protein kinase C) but not by KT-5720. In conclusion, activation of alpha 2-adrenoceptors and somatostatin receptors selectively blocks excitatory responses induced by CADO, most likely by inhibition of adenylyl cyclase and via pertussis toxin-sensitive G proteins. Slow depolarizations induced by substance P are independent of adenylyl cyclase activation and involve activation of protein kinase C.