Light microscope autoradiography of peripheral dopamine receptor subtypes

Radioligand binding assay techniques associated with light microscope autoradiography were used for investigating the pharmacological profile and the micro anatomical localization of peripheral dopamine receptor subtypes. In systemic arteries, the predominant dopamine D1-like receptor belongs to the D5 (or D1B) subtype. It is located within smooth muscle of the tunica media. In pulmonary arteries, dopamine D1-like receptors have primarily an endothelial localization and belong to the dopamine D1 (or D1A) receptor subtype. Both systemic and pulmonary arteries express a dopamine D2-like receptor belonging to the D2 receptor subtype. It has a prejunctional localization in the majority of vascular beds investigated. In cerebral, coronary and mesenteric arteries, it has also an endothelial localization. In the heart, a dopamine D4 receptor was identified. It is expressed by atrial tissue and has a widespread distribution overall atrial musculature. The kidney expresses both dopamine D1-like and D2-like receptors. Renal dopamine D1-like receptors have a vascular and tubular localization. The majority of these sites belongs to the D5 receptor subtype. A smaller D1 receptor population has primarily a tubular localization. Renal dopamine D2-like receptors belong to the dopamine D3 subtype and in lesser amounts to the D2 and D4 receptor subtypes. Renal dopamine D3 receptor has to a greater extent a tubular localization, whereas the D4 receptor is located within glomerular arterioles. The above results suggest that radioligand binding assay and autoradiographic techniques, if performed in the presence of compounds displaying specific receptor subtype selectivity, may contribute to characterize, mainly from a quantitative point of view, peripheral dopamine receptors.

Age-dependent changes in the expression of dopamine receptor subtypes in human peripheral blood lymphocytes

The pharmacological profile and the density of dopamine D3 and D5 receptor subtypes expressed by human peripheral blood lymphocytes of subjects of different ages (ranging from 20 to 75 years) were assessed using radioligand binding techniques. Dopamine D3 receptor was assayed with [3H]7-hydroxy-N,N-di-n-propyl-2-aminotetraline ([3H]7-OH-DPAT) as a ligand. Dopamine D5 receptor was assayed using [3HIR]-(+)-(-chloro-2,3,4,5, tetrahydro-5-phenyl-1H-3-benzazepin-al-hemimaleate) ([3H]SCH 23390) as a ligand. The affinity and the pharmacological profile of [3H]7-OH-DPAT and [3H]SCH 23390 at dopamine D3 and D5 receptor, respectively, were similar in subjects of different ages. The density of dopamine D3 receptor binding sites was slightly decreased in subjects of 30-39 years in comparison with younger individuals. A remarkable loss of dopamine D3 receptor was then found between 40 and 49 years of age in comparison with younger subjects. A further slight decrease was noticeable between 50 and 59 years of age. The number of [3H]7-OH-DPAT binding sites was then stabilized after 60 years of age. The density of dopamine D5 receptor binding sites did not show age-dependent changes. The above findings indicate the occurrence of a decline in the density of lymphocyte dopamine D3 but not D5 receptor between adult and mature subjects. The possibility that dopamine D3 receptor assay in peripheral blood lymphocytes may represent a tool for investigating dopamine receptor function in aging and age-related neurological disorders is discussed.

Muscarinic thioligands with cyclopentane nucleus

Some thio- and the benzoyl-derivatives of deoxamuscarine were synthesized and tested as muscarinic agonists using radioligand binding assays and functional tests. In comparison with deoxamuscarine, used as reference compound, no dimension/distance modification is tolerated for correct lipophilic pocket recognition. The substitution of the ammonium group with a sulphonium group significantly decreased muscarinic potency. The so-called 'muscarinic sub-site' accepts relatively bulky functions as long as it is bound to the cyclopentane carrier by an oxygen bridge. Esterification of this moiety increases the M2 subtype selectivity, while etherification heightens that of M3.

Peripheral nerve vascular changes in spontaneously hypertensive rats

Circulation to the brain is affected by hypertension. Hypertension-dependent cerebrovascular changes were documented primarily in brain pial arteries, whereas no information is so far available concerning changes of peripheral nerve vascularization in hypertension. This study was designed to assess the occurrence of structural changes of interfascicular and intrafascicular arteries supplying peripheral nerves (the so called vasa nervorum) in spontaneously hypertensive rats (SHR). The investigation was performed in 8-month-old SHR, by using standard microanatomical techniques associated with quantitative image analysis. In SHR a significant increase of systolic pressure values accompanied by thickening of the arterial wall, narrowing of the lumen and increase of the wall-to-lumen ratio were observed in comparison with age-matched normotensive Wistar-Kyoto rats. Hypertension-related structural changes involved primarily interfascicular arteries and to a lesser extent intrafascicular arteries. These findings indicate that similarly as documented for cerebral arteries, the vascular supply to peripheral nerves is impaired in hypertension. Structural changes of interfascicular and intrafascicular arteries of SHR could lead to ischemia of peripheral nerves. Further work is in progress to evaluate the functional relevance of hypertensive changes to peripheral nerve vasculature.

Synthesis, resolution, and preliminary evaluation of trans-2-amino-6(5)-hydroxy-1-phenyl-2,3-dihydro-1H-indenes and related derivatives as dopamine receptors ligands

The present work reports the synthesis of enantiomeric pairs of the trans-2-amino-6-hydroxy-1-phenyl-2,3-dihydro-1H-indene [(+)-14a, (-)-14a] and trans-2-amino-5-hydroxy-1-phenyl-2,3-dihydro-1 H-indene [(+)-14b, (-)-14b] and their N,N-di-n-propyl [(+)-and (-)-15a,b], N-methyl-N-allyl [(+)-and (-)-16a,b], and N-methyl-N-n-propyl [(+) and (-)-17a,b] derivatives obtained by a combination of stereospecific reactions and optical resolution. The new compounds were evaluated for their affinity at the dopamine D1 and D2 receptors. The amines (+)- and (-)-14a, incorporating the D1 pharmacophore 2-phenyl-2-(3-hydroxyphenyl)ethylamine in a trans extended conformation, and their derivatives displayed D1 and D2 affinity in the nanomolar range. On the other hand, the enantiomers (+)- and (-)-14b, (+)- and (-)-15b displayed high affinity and selectivity for the D1 receptor. In a preliminary behavioral study on rats (+)-14b, and to a greater extent (+)-15b, promoted episodes of intense grooming, thus indicating that they act as central D1 agonists. The trans-2-amino-5-hydroxy-1-phenyl-2,3-dihydro-1H-indenes (+)-14b and (+)-15b represent selective D1 agonists lacking a catechol group, which should meet the prerequisites for a central nervous system penetration.

Vascular and neuronal hypertensive brain damage: protective effect of treatment with nicardipine

AIM

The brain is sensitive to hypertension, which causes a variety of vascular and neuronal cerebral changes. The present study was designed to assess the effect of long-term treatment with the Ca2+ channel blocker nicardipine on intracerebral (intraparenchymal) arteries in spontaneously hypertensive rats (SHR) by using microanatomical techniques associated with image analysis. The effects of hypertension and treatment with nicardipine on nerve cells and glial fibrillary acid protein (GFAP)-immunoreactive glial cells were also evaluated.

EFFECTS OF NICARDIPINE ON BLOOD PRESSURE

In SHR a significant increase in systolic blood pressure in comparison with age-matched normotensive Wistar-Kyoto (WKY) rats was noticeable. Treatment with nicardipine significantly reduced systolic pressure in the SHR. The media: lumen ratio and the thickness of the tunica media were increased in medium (diameter between 150 and 50 microns and small (diameter < 50 microns intracerebral arteries. This phenomenon was accompanied by luminal narrowing. Treatment with nicardipine significantly reduced the thickness of the tunica media, the media: lumen ratio and increased the luminal area, primarily at the level of small pial arteries and of intracerebral arteries.

EFFECTS OF NICARDIPINE IN THE BRAIN

In control SHR, the number of neurones in the frontal and occipital cortex was reduced in comparison with normotensive WKY rats. GFAP-immunoreactive astrocytes were increased in number (hyperplasia) and in size (hypertrophy), both in the frontal cortex and in the occipital cortex of control SHR. In the CA1, field of the hippocampus, the number of neurones and their size were decreased in SHR in comparison with normotensive WKY rats. Hyperplasia of GFAP-immunoreactive astrocytes of white matter and hypertrophy of those of grey matter was also noticeable. No important changes were found in other portions of the hippocampus. Treatment with nicardipine increased the number of neurones in the frontal cortex and in the occipital cortex of SHR and countered hyperplasia and hypertrophy of GFAP-immunoreactive astrocytes. Moreover, it increased the number of neurones in the CA1 field of the hippocampus and decreased the number and the size of astrocytes of the white matter and grey matter, respectively.

CONCLUSIONS

These findings show that treatment of SHR with nicardipine significantly reduced systolic blood pressure and induced moderate vasodilation of both extracerebral and intracerebral arteries regulating cerebrovascular resistance. The compound also countered some microanatomical changes occurring in the hypertensive brain. The frontal and occipital (visual) cortex and the CA1 field of the hippocampus were the cerebral areas more sensitive to treatment with nicardipine. This suggests that nicardipine induces moderate cerebrovascular dilation and exerts neuroprotective effects on SHR neurones. The possible relevance of the neuroprotective actions of nicardipine in the hypertensive brain deserves to be evaluated in future studies.

Pharmacological characterization and autoradiographic localization of dopamine receptors in the rat adrenal medulla

The pharmacological profile and the anatomical localization of dopamine D1-like and D2-like receptors were studied in sections of rat adrenal medulla, with radioligand binding and autoradiographic techniques, respectively. [3H]([R]-(+)-chloro-2,3,4,5-tetrahydro-5-phenyl-1 H-3benzazepin-al hemimaleate) (SCH 23390) was used as a ligand for dopamine D1-like receptors and [3H]spiperone was used as a ligand for dopamine D2-like receptors. Radioligand binding and light microscope autoradiography did not show specific [3H]SCH 23390 binding in sections of rat adrenal medulla. This suggests that rat adrenal medulla does not express dopamine D1-like receptors. [3H]Spiperone was specifically bound to sections of rat adrenal medulla. The binding was time-, temperature- and concentration-dependent, with a dissociation constant (Kd) of 1.05 nM and a maximum density of binding sites (Bmax) of 100.2 +/- 3.8 fmol/mg tissue. The pharmacological profile of [3H]spiperone binding to rat adrenal medulla was similar to that displayed by neostriatum, which is known to express dopamine D2 receptors. Light microscope autoradiography showed the accumulation of specifically bound [3H]spiperone as silver grains within sections of adrenal medulla. Silver grains were found primarily over the cellular membrane of chromaffin cells. The above data indicate that chromaffin cells of the rat adrenal medulla express dopamine receptors belonging to the dopamine D2 receptor subtype. These receptors are probably involved in the modulation of catecholamine release from chromaffin cells, as documented by functional studies.

Effect of treatment with L-deprenyl on age-dependent microanatomical changes in the rat kidney

The influence of aging and of treatment with L-deprenyl on the structure of the kidney was investigated in 24-month-cld male Sprague Dawley rats by microanatomical techniques associated with image analysis. L-Deprenyl was administrated orally for 5 months at a dose not inhibiting monoamine oxidase (MAO) B activity (1.25/mg/kg/day) and at a dose inhibiting MAO B activity (5 mg/kg/day). In 24 month-old-rats the number and the volume of glomeruli was reduced in comparison with 12-month-old rats used as reference adult animals. Vascular changes characterised by increased thickness of the tunica media, decreased size of arterial lumen and increased wall-to-lumen ratio were also noticeable in 24-month-old rats. Moreover, an increased MAO B reactivity was noticeable within glomerular tufts and renal tubules. Treatment with the low dose of L deprenyl did not cause changes in MAO B reactivity, or in the number of glomeruli, but increased glomerular volume and reduced the wall-to-lumen ratio in medium-sized renal artery branches. The dose of 5 mg/kg/day of L-deprenyl significantly decreased MAO B reactivity within both glomerular tufts and tubules, increased the number and the volume of glomeruli and countered-age-related vascular changes. The above results suggest that treatment with L-deprenyl counters to some extent microanatomical changes occurring in the kidney of aged rats. The observation that the dose of the compound inactive on MAO B activity reduces in part age dependent renal microanatomical changes, indicates that the renal protective effect of L-deprenyl is only in part related to MAO B inhibition.

Muscarinic cholinergic receptor subtypes in human peripheral blood lymphocytes

The subtypes of muscarinic cholinergic receptors were studied in human peripheral blood lymphocytes with radioligand binding techniques and the non-selective muscarinic cholinergic receptor antagonist [3H]quinuclidinyl benzylate (QNB) as a ligand. [3H]QNB was bound to human peripheral lymphocytes in a manner consistent with the labelling of muscarinic cholinergic receptors. The dissociation constant (Kd) value was 0.60 +/- 0.08 nM and the maximum density of binding sites (Bmax) was 2.33 +/- 0.03 fmol/2.2 x 10(6) cells. The binding was time-, temperature- and concentration-dependent, belonging to a single class of high affinity sites. Analysis of the pharmacological profile of [3H]QNB binding in the presence of compounds specific for the different muscarinic receptor subtypes suggests that human peripheral blood lymphocytes express mainly muscarinic cholinergic M2 and M3 receptor subtypes and to a lesser extent muscarinic M4 receptors. The characterization of the subtypes of muscarinic cholinergic recognition sites expressed by human peripheral blood lymphocytes may represent a tool for investigating the possible relationships between immune and cholinergic systems in normal and pathologic conditions.

Pharmacological characterisation of Ca2+ channels of the L-type in human peripheral blood lymphocytes

Ca2+ channels of the L-type were characterised in intact human peripheral blood lymphocytes using a radioligand binding technique and the dihydropyridine-type Ca2+ channel antagonist [3H](+)-PN 200-110 (isopropyl-4-(2,1,3-benzoxadiazol-4-yl)1,4-dihydro-5-methoxycarbon yl-2, 6-dimethyl-3-pyridine carboxylate) as a ligand. [3H](+)-PN 200-110 binding to human peripheral blood lymphocytes was time-, temperature-, concentration-dependent and of high affinity. The dissociation constant (Kd) value was 0.4 +/- 0.02 nM and the maximum binding capacity (Bmax) was 33.5 +/- 1.6 fmol/10(6) cells. Pharmacological analysis of [3H](+)-PN 200-110 binding to human peripheral blood lymphocytes was consistent with the labelling of a Ca2+ channel of the L-type. In fact, dihydropyridine derivatives were the most potent competitors of [3H](+)-PN 200-110 binding, whereas phenylalkylamine and benzothiazepine compounds or non-selective Ca2+ channel modulators were weak or ineffective displacers. These findings are the first observation that human peripheral blood lymphocytes express Ca2+ channels of the L-type. The possibility that Ca2+ channel antagonists may interfere with immune system function is discussed.

Increased density of dopamine D5 receptor in peripheral blood lymphocytes of migraineurs: a marker for migraine?

The expression of dopamine D5 receptor was investigated in peripheral blood lymphocytes of 11 migraine patients and of ten healthy control subjects using a radioligand binding technique with [3H]SCH 23390 as a ligand. [3H]SCH 23390 is a benzazepine derivative with potent antagonist properties at the dopamine D1-like receptors. [3H]SCH 23390 was specifically bound to peripheral blood lymphocytes of migraineurs and control subjects in a manner consistent with the labelling of a dopamine D5 receptor. In migraineurs a statistically significant higher density of lymphocyte dopamine D5 receptor compared with controls was noticeable, whereas the affinity of the radioligand was unchanged. The increased density of dopamine D5 receptor in peripheral blood lymphocytes may reflect the dopaminergic hypersensitivity displayed by migraineurs and may represent a relatively simple and reliable peripheral marker of altered dopaminergic function.

Density and pattern of dopamine D2-like receptors in the cerebellar cortex of aged rats

The pharmacological properties and the anatomical localization of dopamine (DA) D2-like receptors were studied in the cerebellum of 3 months, 12 months, and 24 months male Wistar rats using combined radioligand binding and autoradiographic techniques with [3H]-spiroperidol as a ligand. The binding was consistent with the labelling of the DA D3 receptor subtype. The affinity for DA D2-like receptors was similar in the cerebellar cortex of the three animal groups investigated, whereas the density of binding sites (Bmax value) assessed using conventional radioligand binding techniques was reduced as a function of aging. Light microscope autoradiography revealed the localization of binding sites primarily in the molecular layer and to a lesser amount in Purkinje neurons layer. A loss of binding sites was noticeable with aging in the grey matter of the cerebellar cortex. It affected primarily the molecular layer. Analysis of radioligand binding data and light microscope autoradiography suggests that age-related changes of DA D2-like receptors depend in part by structural alterations of cerebellar cortex and in part by modifications in receptor expression.

Pharmacological characterization and autoradiographic localization of dihydropyridine-type calcium channels in the kidney of spontaneously hypertensive rats

1. The pharmacological profile and the microanatomical localization of Ca2+ channels of the L-type were analysed in sections of the kidney of Wistar-Kyoto (WKY) rats and of spontaneously hypertensive rats (SHR) of different ages. 2. [3H]-Nicardipine was used as a ligand. It was bound to sections of rat kidney in a manner consistent with the labelling of Ca2+ channels of the L-type. The density of [3H]-nicardipine binding sites was similar in WKY rats of different ages and in SHR of 2 and 4 months, but was significantly increased in SHR of 6 months. 3. Light microscope autoradiography revealed the highest density of binding sites in the tubular portion of the nephron and to a lesser extent within smooth muscle of renal arteries and renal corpuscles. In SHR of 4 and 6 months the density of [3H]-nicardipine binding sites was increased within the epithelium of proximal tubules and of the loop of Henle and decreased in renal corpuscles in comparison with WKY rats or 2 month old SHR. 4. These results show that the density of Ca2+ channels of the L-type increases with the worsening of hypertension in SHR. The observation of a different sensitivity to hypertension of Ca2+ channels located in the various portions of the nephron indicates the usefulness of light microscope autoradiography for assessing hypertension-related changes of Ca2+ channels in the kidney.

Protective effect of nicardipine treatment on cerebrovascular microanatomical changes in spontaneously hypertensive rats

1. The effect of long-term treatment with the dihydropyridine Ca2+ antagonist, nicardipine, on the morphology of different sized pial arteries was assessed in spontaneously hypertensive rats (SHR) using histological techniques associated with image analysis. 2. In control 20 week old SHR blood pressure values, the thickness of the tunica media, the media-to-lumen ratio and connective tissue content were significantly increased in comparison with reference normotensive Wistar-Kyoto (WKY) rats. 3. Treatment for 8 weeks with a daily dose of 3 mg/kg of nicardipine decreased blood pressure values in SHR and significantly reduced the area occupied by the tunica media and the media-to-lumen ratio. This effect was observed primarily in small sized pial arteries and to a lesser extent in medium sized pial arteries. Nicardipine administration was without effect on connective tissue content in the wall of cerebral arteries. 4. These results indicate that treatment with nicardipine reduces blood pressure elevation in SHR and exerts a protective effect on arteries controlling cerebrovascular resistance. The activity of the compound primarily on small sized pial arteries may protect the brain from generalized vasodilation which could cause cerebral hypoperfusion.

Influence of long-term treatment with the dihydropyridine-type calcium antagonist nicardipine on renal microanatomical changes in spontaneously hypertensive rats

1. The influence of hypertension and treatment with the dihydropyridine-type Ca2+ antagonist, nicardipine, on the structure of the kidney was assessed in spontaneously hypertensive rats (SHR) of 12 weeks of age. Treatment went for 8 weeks with a daily oral dose of 1 mg/kg of nicardipine. 2. Control SHR exhibited hypertension and microanatomical vascular and glomerular changes. Vascular changes consisted of a thickening of the tunica media and decreased luminal area of medium- and small-sized intrarenal artery branches. Glomerular changes included glomerulosclerosis and atrophy of varying degrees. 3. Administration of nicardipine significantly reduced blood pressure. The drug also decreased the thickening of tunica media and luminal narrowing of renal artery branches as well as the degree of glomerular injury in SHR. 4. These data indicate that nicardipine treatment is able to control elevated blood pressure in SHR, and to counter hypertension-dependent microanatomical impairment of the kidney. This suggests that the compound exerts a protective effect on hypertensive kidney.

Dopamine D5 receptor expression is unchanged in peripheral blood lymphocytes in essential hypertension

The present study was designed to investigate possible changes in the expression of lymphocyte dopamine receptor in essential hypertension. The expression of dopamine D5 receptor was evaluated by radioligand binding techniques using [3H]-SCH 23390 as ligand. Plasma catecholamines, aldosterone levels and plasma renin activity were also measured. Eleven borderline hypertensive patients, 15 patient with the mild essential hypertension, 7 patients with moderate essential hypertension and 5 patients with severe essential hypertension were examined. Plasma catecholamine levels were assayed by high pressure liquid chromatography with electrochemical detection. Dopamine D5 receptor was measured by radioligand binding techniques. Plasma aldosterone levels and renin activity were determined by radio immunoassay. [3H]-SCH 23390 was specifically bound to human peripheral blood lymphocytes. The binding was time-, temperature- and concentration-dependent with a dissociation constant (Kd) value of 0.59 nM and a maximum density of binding sites (Bmax) of 223 pmol/10(6) cells. Dopamine competed with [3H]-SCH 23390 binding in the submicromolar range suggesting the labelling of a dopamine D5 receptor. No changes in the density of [3H]-SCH 23390 binding sites were observed in human peripheral blood lymphocytes between essential hypertensive patients and normotensive subjects. Also catecholamines, plasma renin activity and aldosterone levels were unchanged. In spite of the availability of a sensitive technique for measuring dopamine receptors in human peripheral lymphocytes, no change in their expression was noticeable in essential hypertension. This suggests that dopamine receptor analysis in essential hypertension is not a useful marker for investigating hypertension-dependent changes of the peripheral dopaminergic system.

Identification of age-related changes of dopamine D1-like receptors in the rat cerebellar cortex

The present study was designed to characterize the pharmacological profile of dopamine D1-like receptors in the rat cerebellar cortex and to assess if these receptor sites undergo age-related changes. Cerebella of young (3 months), adult (12 months), and old (27 months) male Wistar rats were examined by using radioligand binding techniques and light microscope autoradiography. The non-selective dopamine D1-like radioligand [3H]SCH 23390 was specifically bound to sections of rat cerebellum. The findings that dopamine displaced [3H]SCH 23390 binding in the submicromolar range suggest that labelling of a dopamine D5 (or D1B) receptor subtype. The affinity of [3H]SCH 23390 for dopamine D1-like receptors was similar in the cerebellar cortex of the three animal groups investigated, whereas radioligand binding techniques revealed a gradual age-related reduction of the density of binding sites. Light microscope autoradiography showed the localization of [3H]SCH 23390 binding sites primarily in the molecular layer and to a lesser extent in the Purkinje neuron layer of the cerebellar cortex. Aging was accompanied by a loss of [3H]SCH 23390 binding sites affecting mainly the molecular layer. The age-dependent loss of dopamine D1-like receptors is more pronounced if detected with radioligand binding techniques than with light microscope autoradiography. This suggests that the decrease of dopamine D1-like receptors observed in aging rat cerebellar cortex may depend in part on changes in the receptor expression and in part on cortico-cerebellar structural changes.

Localization of calcium channels of the L-type in human epicardial arteries: a light microscope autoradiographic study

The anatomical localization of Ca2+ channels of the L-type was analyzed in sections of the human right and anterior interventricular coronary arteries by using in vitro light microscope autoradiography associated with radioligand binding techniques. [3H]Nicardipine was utilised as a ligand. Binding of the radioligand to sections of the two coronary arteries was time-, temperature- and concentration-dependent. Analysis of binding isotherms revealed a dissociation constant value of about 0.5 nM in the two arteries and maximum binding capacities of 139 +/- 6.4 fmol/mg tissue for the right coronary artery and of 173 +/- 9.5 for the anterior interventricular branch. The pharmacological profile of [3H]nicardipine binding to sections of human coronary arteries was consistent with the labelling of Ca2+ channels of the L-type. Dihydropyridine derivatives were the most powerful competitors of [3H]nicardipine binding, whereas phenylalkylamines, benzothiazepine or non-selective channel modulators were weak competitors or ineffective. Light microscope autoradiography revealed the highest density of [3H]nicardipine binding sites in the tunica media of the coronary arteries. In this layer Ca2+ channels of the L-type are located within smooth muscle cells. A lower accumulation of the radioligand occurred in the tunica adventitia, whereas no specific binding was found in the tunica intima. Study of the localization of Ca2+ channels in sections of human coronary arteries may contribute to a better understanding of the mechanism of the marked coronary dilatory activity elicited by Ca2+ antagonists demonstrable in both in vitro preparations and in vivo.

Nicardipine and treatment of cerebrovascular diseases with particular reference to hypertension-related disorders

Nicardipine is a second generation dihydropyridine-type Ca2+ antagonist with high vascular selectivity and strong cerebral and coronary vasodilatory activity. The compound is used in the treatment of hypertension, primarily in the elderly. In this review the main evidence of the cerebrovascular activity of nicardipine in preclinical studies using in vitro and in vivo models is detailed. A particular physico-chemical property of nicardipine is the almost complete protonation in acid environment. This allows its accumulation in ischemic brain regions and makes it a candidate for the treatment of cerebrovascular disorders characterised by impaired brain perfusion. The main clinical data on the use of nicardipine in cerebral ischemia and related disorders, subarachnoid haemorrhage and stroke, are also reviewed. These studies included 5940 patients affected by chronic cerebrovascular insufficiency (cerebral ischemia, cerebral atherosclerosis mainly associated with hypertension, transient ischemic attacks, sequelae of cerebral infarction, thrombosis or embolia, hypertensive encephalopathy), 1540 patients affected by sequelae of subarachnoid haemorrhage and 206 patients affected by stroke. Both preclinical studies and clinical trials have shown that nicardipine is a safe Ca2+ antagonist with powerful cerebrovascular activity. This suggests its possible use in cerebrovascular disorders in which blockade of Ca2+ channels of the L-type and/or selective cerebral vasodilatation is desirable. Further studies are necessary to establish if modulation of neuronal Ca2+ channels of the L-type by nicardipine may have a neuroprotective effect independent by the cerebrovascular activity of the compound.

Pharmacological characterisation and autoradiographic localisation of dopamine receptor subtypes in the cardiovascular system and in the kidney

Combined radioligand binding and light microscope autoradiography techniques were used for investigating the pharmacological profile and the microanatomical localisation of dopamine receptor subtypes in the cardiovascular system and in the kidney. In superior mesenteric and renal arteries the predominant dopamine D1-like receptor belongs to the D5 (or D1b) subtype. This site is located within smooth muscle of the tunica media. The same receptor subtype predominates in the kidney, where it has a vascular and tubular localisation. The dopamine D2-like receptor subtype expressed by systemic arteries belongs to the D2 receptor subtype. It has a prejunctional and endothelial localisation. In the kidney the predominating dopamine D2-like receptor belongs to the dopamine D3 subtype. Atria but not ventricles express dopamine D2-like receptors belonging to the D4 receptor subtype. The above results suggest that in spite of the emerging complexity of the dopamine receptor profile demonstrated by molecular biology techniques, radioligand binding and autoradiographic techniques, if performed with appropriate radioligands and/or in the presence of compounds active on specific receptor subtypes, may represent a useful tool for better understanding the biological significance of peripheral dopamine receptors.

Localisation of dopamine D1-like and D2-like receptors in the pulmonary vasculature

The pharmacological profile and the anatomical localisation of dopamine receptor subtypes were investigated in the rabbit pulmonary vascular bed using combined radioligand binding and light microscope autoradiography techniques. Dopamine D1-like receptor sites, which probably belong to the dopamine D1 receptor subtype, were characterized in sections of lung using [3H]-SCH 23390 as a ligand. These sites were located within the tunica intima and the tunica media of large sized intrapulmonary artery branches and in the tunica media of medium sized intrapulmonary artery branches. Dopamine D2-like receptor sites, which probably belong to the dopamine D2 receptor subtype, were characterized using [3H]-spiroperidol as a ligand. These sites were located within the tunica adventitia of both extra- and intrapulmonary artery branches. Dopamine D2-like receptor sites were also found in the tunica adventitia of the human pulmonary artery, but not of the rat pulmonary artery. The different anatomical localization of dopamine D1 and D2 receptor subtypes in the pulmonary vasculature suggests that these sites are involved in the modulation of pulmonary vascular tone by interacting with different receptors unevenly distributed throughout the pulmonary vascular bed.

Localisation of dopamine D3 receptor in the rat cerebellar cortex: a light microscope autoradiographic study

The pharmacological properties and the anatomical localisation of dopamine D3 receptor were assessed in the rat cerebellar cortex using radioligand binding techniques associated with light microscope autoradiography and 7-[3H]hydroxy-N,N-di-n-propyl-2-aminotetralin (7-[3H]OH-DPAT) as a ligand. 7-[3H]OH-DPAT was specifically bound to sections of rat cerebellar cortex with a dissociation constant (Kd) of 0.5 nM and a maximum density of binding sites (Bmax) of 97 +/- 4 fmol/mg tissue. The rank order of potency of competitors of 7-[3H]OH-DPAT binding and the observation that guanosine triphosphate did not affect radioligand binding suggest the labelling of a dopamine D3 receptor. 7-[3H]OH-DPAT binding sites are located mainly in the molecular layer and in lesser amounts in the Purkinje neuron layer, primarily within the cell body of Purkinje neurons. No specific accumulation of silver grains was observed in the granule neuron layer or in the white matter of the cerebellar cortex. The localisation of a putative dopamine D3 receptor within Purkinje neurons suggests that this site may have functional relevance in the cerebellar cortex.

Microanatomical and electrophysiological changes of the rat dentate gyrus caused by lesions of the nucleus basalis magnocellularis

The effect of unilateral or bilateral lesions of the nucleus basalis magnocellularis (NBM) on the dentate gyrus of the hippocampus were assessed using microanatomical and electrophysiological techniques. NBM is the main cholinergic basal forebrain nucleus that supplies the fronto-parietal cortex. Lesions were induced using the neurotoxin ibotenic acid or a radio-frequency system and did not affect glutamic acid decarboxylase activity both in the frontal cortex and in the hippocampus. At 4 weeks after lesioning, a loss of choline acetyltransferase (ChAT) activity and of ChAT-immunoreactive fibres was observed in the frontal cortex but not in the hippocampus and no changes in the density of granule neurons of the dentate gyrus or in the hippocampal long-term potentiation (LTP) were noticeable. At 8 weeks after lesioning the loss of both ChAT activity and of ChAT-immunoreactive fibres persisted in the frontal cortex of NBM-lesioned rats. Moreover, at this time a significant decrease in the density of granule neurons in the dentate gyrus accompanied by a reduced probability of dentate LTP induction were observed in both ibotenic acid- and radio-frequency-lesioned rats. These findings have shown that although NBM does not send direct cholinergic projections to the hippocampus, lesions of this cholinergic nucleus are accompanied by delayed neurodegenerative changes involving the dentate gyrus. This suggests the occurrence of indirect connections between NBM and the hippocampus, the functional relevance of which should be explored.

Localization of dopamine receptors in the rabbit lung vasculature

The localization of dopamine receptor subtypes in the rabbit pulmonary circulation was investigated by using combined radioligand binding and light microscope autoradiography techniques. Dopamine D1-like receptors, probably belonging to the dopamine D1 receptor subtype, were demonstrated in sections of rabbit lung using [3H]-SCH 23390 as a ligand. This site is located within the tunica intima and the tunica media of large intrapulmonary artery branches (diameter > 300 microns) as well as within the tunica media of medium-sized pulmonary artery branches (diameter between 100 and 300 microns). No [3H]-SCH 23390 binding sites were found in small-sized pulmonary artery branches (diameter < 100 microns) or in pulmonary veins. Dopamine D2-like receptors, probably belonging to the dopamine D2 receptor subtype, were demonstrated with radioligand binding techniques in sections of extrapulmonary arteries but not of lung using [3H]-spiroperidol as a ligand. Light microscope autoradiography revealed the localization of this site within the tunica adventitia of extrapulmonary arteries as well as of large- and medium-sized intrapulmonary artery branches. The different anatomical localization of dopamine D1- and D2-like receptors in the rabbit pulmonary vasculature suggests that these sites are involved in the modulation of pulmonary vascular tone by interacting with different receptors having endothelial, muscular and probably prejunctional localization. The functional significance of the uneven distribution of dopamine receptor subtypes throughout the pulmonary vascular bed should be clarified in future studies.

Autoradiographic localization of dopamine D2-like receptors in the rat adrenal gland

The pharmacological profile and the anatomical localization of dopamine D2-like receptors were studied in sections of the rat adrenal gland using combined radioligand binding and autoradiographic techniques with [3H]-spiroperidol as a ligand. [3H]-Spiroperidol was bound to sections of the rat adrenal gland in a manner consistent with the labelling of dopamine D2-like receptor sites. The binding was time-, temperature- and concentration-dependent and of high affinity with a dissociation constant (Kd) value of 1.6 +/- 0.04 nM and a maximum density of binding sites (Bmax) of 60 +/- 3.6 fmol/mg tissue. Experiments on the pharmacological specificity of [3H]-spiroperidol binding to sections of the rat adrenal gland suggest the labelling of dopamine D3 and/or D4 receptors. The presence of dopamine D3 and D4 receptors in the rat adrenal gland was confirmed by the demonstration of a specific binding for the D3 radioligand [3H]-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (DPAT) and for the D4 radioligand [3H]-clozapine. Light microscope autoradiography showed the highest accumulation of silver grains which correspond to [3H]-spiroperidol binding sites in the rat adrenal medulla. In the adrenal cortex, where density of silver grains is about 40% lower than in the medulla, the radioligand is accumulated primarily in the zona glomerulosa and to a lesser extent in the zona reticularis. These findings suggest that dopamine D2-like receptor sites in the rat adrenal gland cortex are primarily involved in the modulation of catecholamine secretion from the medulla and of aldosterone secretion from the cortex. The possible relevance of the occurrence of dopamine D3 and D4 receptor subtypes in the adrenal gland is discussed.