Autoradiographic evaluation of forskolin and D1 dopamine receptor binding in a rat model of focal cerebral ischaemia

Different Doses of Dopamine Have Heterogeneous Effects on Cerebral Hemodynamics and Dopamine Receptors in Young Rabbits as Measured with Near Infrared Spectroscopy

BACKGROUND

Fluctuations in cerebral blood volume and cerebral oxygenation may be important in the pathogenesis of intraventricular hemorrhage and hypoxic-ischemic brain injury in the neonate. The cerebral hemodynamic response to dopamine infusion in premature infants is not well established. The newborn rabbit, a rather immature species at birth, is a suitable model for monitoring the physiological changes of the cerebral circulation.

METHODS

The effect of dopamine upon cerebral hemodynamics and basal ganglia dopaminergic receptors were studied using four different dopamine doses.

RESULTS

No significant changes in near infrared spectroscopy (NIRS) parameters were observed in the animals that received 0.5 (n = 5) and 1 microg/kg/min (n = 4) of dopamine intravenously. In contrast, in those animals that received dopamine at 5 microg/kg/min (n = 7) and 50 microg/kg/min (n = 7), there was a significant decrease in oxygenated hemoglobin. Moreover, this was accompanied by a significant increase in deoxygenated hemoglobin soon after drug infusion. Cerebral blood volume was increased in the group that received 5 microg/kg/min, but significantly decreased in the group that received 50 microg/kg/min. In both groups NIRS parameters returned to baseline values soon after stopping dopamine infusion.

CONCLUSION

Despite evidence of a physiological response, we found no difference in the distribution of dopamine receptors between experimental and control animals. We therefore speculate that dopamine has an effect on the cerebrovasculature that could be mediated by factors other than changes in the basal ganglia dopamine receptors.

Discrepancy between cell injury and benzodiazepine receptor binding after transient middle cerebral artery occlusion in rats

We investigated postischemic alterations in benzodiazepine receptor, D1 dopamine receptor, and muscarinic acetylcholine receptor binding after transient middle cerebral artery (MCA) occlusion in rats using [3H]-flumazenil, [3H]-SCH23390, and [3H]-N-methyl-4-piperidyl benzilate ([3H]-NMPB), respectively, as radioligand. These ligand bindings were determined at 3 and 24 h and at 3 and 7 days after ischemia/reperfusion of MCA by using autoradiographic methods. Ischemic cell injury was clearly detected from 3 h after ischemia/reperfusion and progressively increased from 3-24 h after ischemia/reperfusion of MCA. The area of cell injury reached maximum at 24 h after ischemia/reperfusion of MCA. [3H]-SCH23390 binding was reduced to 47% of the contralateral side at 3 days after ischemia/reperfusion of MCA. After 7 days, [3H]-SCH23390 binding was further reduced by 20% in the striatum. [3H]-NMPB binding was slightly decreased in both the striatum and cerebral cortex at 3 days after ischemia/reperfusion of MCA, and [3H]-NMPB binding in the striatum and cerebral cortex were reduced to 42 and 62% of the contralateral side at 7 days after ischemia/reperfusion of MCA. [3H]-NMPB was also decreased at 24 h. In contrast, [3H]-flumazenil binding was not decreased in the striatum and cerebral cortex within 7 days after ischemia/reperfusion of MCA. These results suggest that [3H]-SCH23390 and [3H]-NMPB binding do not correlate with cell injury by ischemia/reperfusion, although vulnerability to ischemia/reperfusion was observed with these receptors. In addition, central benzodiazepine receptor imaging might be essentially stable to neuronal cell injury induced by transient focal cerebral ischemia in rats, in contrast to the results of PET studies.

Alteration of second messengers during acute cerebral ischemia - adenylate cyclase, cyclic AMP-dependent protein kinase, and cyclic AMP response element binding protein

A variety of neurotransmitters and other chemical substances are released into the extracellular space in the brain in response to acute ischemic stress, and the biological actions of these substances are exclusively mediated by receptor-linked second messenger systems. One of the well-known second messenger systems is adenylate cyclase, which catalyzes the generation of cyclic AMP, triggering the activation of cyclic AMP-dependent protein kinase (PKA). PKA controls a number of cellular functions by phosphorylating many substrates, including an important DNA-binding transcription factor, cyclic AMP response element binding protein (CREB). CREB has recently been shown to play an important role in many physiological and pathological conditions, including synaptic plasticity and neuroprotection against various insults, and to constitute a convergence point for many signaling cascades. The autoradiographic method developed in our laboratory enables us to simultaneously quantify alterations of the second messenger system and local cerebral blood flow (lCBF). Adenylate cyclase is diffusely activated in the initial phase of acute ischemia (< or = 30 min), and its activity gradually decreases in the late phase of ischemia (2-6 h). The areas of reduced adenylate cyclase activity strictly coincide with infarct areas, which later become visible. The binding activity of PKA to cyclic AMP, which reflects the functional integrity of the enzyme, is rapidly suppressed during the initial phase of ischemia in the ischemic core, especially in vulnerable regions, such as the CA1 of the hippocampus, and it continues to decline. By contrast, PKA binding activity remains enhanced in the peri-ischemia area. These changes occur in a clearly lCBF-dependent manner. CREB phosphorylation at a serine residue, Ser(133), which suggests the activation of CREB-mediated transcription of genes containing a CRE motif in the nuclei, remains enhanced in the peri-ischemia area, which is spared of infarct damage. On the other hand, CREB phosphorylation at Ser133 rapidly diminishes in the ischemic core before the histological damage becomes manifest. The Ca2+ influx during membrane depolarization contributes to CREB phosphorylation in the initial phase of post-ischemic recirculation, while PKA activation and other signaling elements seem to be responsible in the later phase. These findings suggest that derangement of cyclic AMP-related intracellular signal transduction closely parallels ischemic neuronal damage and that persistent enhancement of this signaling pathway is important for neuronal survival in acute cerebral ischemia.

Inhibition of cyclic AMP-dependent protein kinase in the acute phase of focal cerebral ischemia in the rat

Binding of cyclic AMP to the regulatory subunit of cyclic AMP-dependent protein kinase is an essential step in cyclic AMP-mediated intracellular signal transduction. In the present study, the binding capacity of cyclic AMP-dependent protein kinase for cyclic AMP was examined by autoradiography with local cerebral blood flow in focal cerebral ischemia in the rat, which was induced by occlusion of the middle cerebral artery using the intraluminal suture method. The binding capacity of cyclic AMP-dependent protein kinase and local cerebral blood flow were assessed by the in vitro [3H]cyclic AMP binding and the [14C]iodoantipyrine methods, respectively. At 3 h of occlusion, a significant reduction in the binding of cyclic AMP-dependent protein kinase to cyclic AMP was already noted in the lateral region of the caudate-putamen and the parietal cortex. Between three and five hours of occlusion, the area with reduced cyclic AMP binding was significantly expanded to the peri-ischemic regions including the frontal cortex and the medial region of the caudate-putamen. The threshold in local cerebral blood flow for reduced cyclic AMP binding was clearly noted at 5 h of ischemia, and was 45 ml/100 g per min in the cerebral cortices, and 38 ml/100 g per min in the caudate-putamen, respectively. No threshold was noted at 3 h of ischemia, since cyclic AMP binding showed a large variation ranging from reduced to normal values even when local cerebral blood flow was below 20 ml/100 g per min. Recirculation for 3.5 h following 1.5 h of ischemia restored the normal cyclic AMP binding in the cerebral cortices, but failed to normalize cyclic AMP binding in the caudate-putamen despite good recovery of local cerebral blood flow. Western blot analysis suggested that this reduction in cyclic AMP binding was not due to loss or degradation of the subunit protein of cyclic AMP-dependent protein kinase, and may therefore have resulted from conformational changes in the protein. A significant increase in cyclic AMP binding was noted after recirculation in the non-ischemic regions such as the frontal and the cingulate cortices on the occluded side and in the contralateral cortices. These data indicate that cyclic AMP-mediated signal transduction in the brain tissue may be very susceptible to ischemic stress, and the region of disrupted signal transduction may expand progressively from the ischemic core to peri-ischemic regions in the acute phase of ischemia. Such impairment of signal transduction may not be restored in the caudate-putamen even when cerebral circulation is fully recovered after short-term ischemia, suggesting that a regional vulnerability to ischemic stress may also exist in cyclic AMP-mediated signal transduction. A significant increase in cyclic AMP binding after recirculation in regions outside of ischemic area may be closely related with the protective mechanisms of brain tissue, since cyclic AMP has been reported to exert various neuroprotective actions.

Dopamine constricts porcine pial veins

Dopamine has been shown to induce pial arterial relaxation and constriction in several species. Its mode of action on pial veins, however, remains unclarified. The vasomotor effect of dopamine on porcine pial veins was, therefore, examined using an in vitro tissue bath technique. The results indicated that dopamine constricted exclusively isolated ring segments of pial veins in the presence or absence of active muscle tone. The constriction induced by dopamine was not affected by N(omega)-nitro-L-arginine (L-NNA, 2 x 10(-5) M) or indomethacin (10(-5) M). Only in few preparations was the constriction induced by maximum concentration of dopamine potentiated by L-NNA, suggesting that dopamine at high concentrations may release NO or a NO-related substance. In the presence of L-NNA (2 x 10(-5) M), dopamine-induced constriction was inhibited by phentolamine and yohimbine (but not prazosin) in a concentration-dependent manner with maximum inhibition at 10(-6) M. SKF38393 and 6-bromo-APB (selective dopamine D1 receptor agonists) and LY171555 (a selective dopamine D2 receptor agonist) also induced pial venous constriction exclusively in the presence of L-NNA. The constriction was not affected by phentolamine (10(-6) M). The order of potency for these agonists in the presence of phentolamine, propranolol, guanethidine and L-NNA was: 6-bromo-APB > SKF38393 > dopamine > LY171555. The dopamine-induced constriction in the presence of phentolamine was further inhibited by both SCH23390 (a selective dopamine D1 receptor antagonist) and sulpiride (a selective dopamine D2 receptor antagonist), but was not affected by dopamine D3 or D4 receptor antagonists. These results indicate that dopamine at low and high concentrations induces exclusively constriction of isolated porcine pial veins. The constriction is mediated by postsynaptic alpha2-adrenoceptors, and dopamine D1 and D2 receptors.

The temporal evolution of striatal dopamine receptor binding and mRNA expression following hypoxia-ischemia in the neonatal rat

Neonatal hypoxic-ischemic (HI) brain injury in the rat alters dopamine receptors. To determine whether such changes are permanent, dopamine receptors and corresponding mRNA were examined at various time points after neonatal HI using receptor autoradiography and in situ hybridization. Rat pups underwent ligation of the left common carotid artery followed by hypoxic exposure (8.5% O2 for 3 h). Controls underwent sham surgery alone. Animals surviving for 2-80 days following HI were studied. Striatal D1 receptors (labeled by [3H]SCH23390) were reduced as early as 2 days following HI, remained depressed for 21 days, but recovered to control levels by young adulthood (3 months of age). D2 receptors (labeled by [125I] iodosulpride) did not decline until 10 days after HI, and remained uniformly depressed throughout the caudate-putamen thereafter. Changes in D1 receptor mRNA transcripts closely paralleled alterations in receptors: early reductions in D1 mRNA signal recovered by young adulthood. D2 mRNA exhibited a unique temporal profile with an early decrease (2 days following HI), and prompt, persistent recovery. Dopamine receptors and transcripts are differentially affected by HI injury early in development. Whereas D1 receptor expression recovers from neonatal HI injury, D2 receptors remain permanently affected despite the presence of normal levels of D2 receptor transcripts. A persistent, post-transcriptional effect of HI on D2 receptor expression is suggested.

Consequences of transient focal cerebral ischaemia for second messenger and neurotransmitter binding in the rat: quantitative autoradiographic analysis of forskolin, dopamine D1 receptor binding and cerebral blood flow changes

In order to study the consequences of reperfusion for ischaemic brain injury, quantitative ligand binding autoradiography was carried out in a model of reversible focal cerebral ischaemia. Endothelin-1 applied to the abluminal surface of the middle cerebral artery in anaesthetized Sprague-Dawley rats induced severe focal ischaemia and subsequent reperfusion (assessed by blood flow tracers [99mTc]HMPAO and [14C]iodoantipyrine respectively) by 2 h after insult. Ligand binding autoradiography on consecutive sections demonstrated these blood flow changes to be associated with a significant reduction in forskolin binding throughout the middle cerebral artery territory (e.g. 25% in parietal cortex, 11% in dorsolateral caudate nucleus). The most marked losses in forskolin binding were in areas where ischaemia was severe and reperfusion was poor. However, the same changes in cerebral blood flow had no significant effect on D1 dopamine receptor binding (e.g. < 2% reduction in the caudate nucleus). These data demonstrate that ligand binding characteristics are significantly affected as early as 2 h after insult, with evidence of differential sensitivity for forskolin and D1 dopamine binding. With regard to the consequences of reperfusion, comparison with our previous study of 2 h maintained ischaemia demonstrates reperfusion-related salvage of dopamine and forskolin binding in the caudate nucleus but possible exacerbation of forskolin binding loss in the cortex.

Is TISCH a suitable tracer for in vivo study of modifications of the dopamine D-1 receptor?

To detect quantitative modification of dopamine D-1 receptors in vivo, we used [125I]-TISCH in an animal modelin which the striatum was unilaterally lesioned with quinolinic acid. [125I]-TISCH was injected into rats 5 days after the lesion, and the changes in receptor density obtained in vivo were compared to in vitro quantification of dopamine D-1 receptors by binding with either [125I]-TISCH or [3H]-SCH 23390 as a reference ligand. In vivo and in vitro, we obtained the same decrease (-70%) in binding of these ligands in the lesioned striatum. Using an injection of [99mTc]-DTPA to lesioned rats, we also showed the disruption of the blood-brain barrier (BBB) in the lesioned area. Thus, the equivalent decrease observed in vitro and in vivo with [125I]-TISCH confirmed that this molecule would be a valuable tool for exploration of dopamine D-1 receptors by SPECT imaging. Moreover, the fact that the breakdown of the BBB did not interfere with the receptor binding obtained in vivo may be important for future investigations in pathologies with BBB disruption, such as ischemia.

Effects of post-mortem delay on high affinity forskolin binding sites and adenylate cyclase activity in rat and human striatum and cerebral cortex

High affinity [3H]forskolin binding was measured using quantitative autoradiography in the striatum and frontal cortex of rat and human brain. Forskolin binding in rat striatum (310.8 +/- 26.0 pmol/g mean +/- S.E.M.) was approximately 4 times that in the frontal cortex (75.5 +/- 8.4 pmol/g), whereas in post-mortem human brain each region exhibited similar levels of forskolin binding (striatum 51.3 +/- 1.2 and cortex 53.2 +/- 2.1 pmol/g). Basal adenylate cyclase activity was assayed in membranes prepared from striatum and frontal cortex of rat and human; enzyme activity in the rat striatum was approximately 2-fold that in rat frontal cortex whereas enzyme activity in the human striatum was similar to that in the human frontal cortex. To investigate the effect of the interval between death and freezing of the brain, rats were killed by decapitation, then maintained at 37 degrees C for up to 4 hours before freezing and subsequent assay of forskolin binding and adenylate cyclase activity. Striatal forskolin binding declined markedly post-mortem such that 4 h post-mortem it was only 13% of the level in control animals while levels of cortical forskolin binding declined minimally during the immediate post-mortem period. Striatal and cortical adenylate cyclase activity (basal) was minimally influenced by post-mortem delay although in both regions there was a rapid loss of the ability of fluoroaluminate to stimulate adenylate cyclase. The data suggest that the striatum contains a population of high affinity forskolin binding sites which is extremely sensitive to post-mortem delay.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction in second-messenger ligand binding sites after brain ischemia--autoradiographic Bmax and Kd determinations using digital image analysis

Changes in forskolin (FK) and phorbol 12,13-dibutyrate (PDBu) binding were evaluated in relation to local cerebral blood flow (CBF) after 6-h unilateral carotid artery occlusion in the gerbil striatum employing a quantitative autoradiographic method, which permitted these three parameters to be measured in the same brain. CBF was measured by the [14C]iodoantipyrine method at the end of the experiment. [3H]FK and [3H]PDBu were utilized as specific ligands to assess the activity of adenylate cyclase (AC) and protein kinase C (PKC), respectively. A saturation study was undertaken to measure the Kd (dissociation constant) and Bmax (maximal binding capacity) of each ligand by digital image processing of sequential autoradiograms employing pixel-by-pixel Scatchard analysis. The Bmax values of FK and PDBu were significantly decreased on the ischemic side, but the reduction in Bmax of FK was greater than that of PDBu. The K4 of each ligand remained unchanged. The FK binding underwent a progressive decline as CBF fell below 30 ml/100 g/min. The PDBu binding showed only a gradual decline in parallel with the CBF reduction. These findings suggest that a reduction in CBF below 30 ml/100 g/min for 6 h may induce a remarkable suppression of the AC system with less significant inhibition of the PKC system in the striatum.

Early changes in second messenger but not receptor binding sites after acute subdural hematoma: an in vitro autoradiographic study

Neurotransmitter receptor-coupled mechanisms have been recently recognized as important determinants of cell damage after central nervous system (CNS) trauma and ischemia. Many of these receptors exert their intracellular effects via second messenger systems. This study used in vitro autoradiographic radioligand binding to measure beta-adrenergic and muscarinic cholinergic receptors and adenylate cyclase and protein kinase C (PKC) binding sites two h after acute subdural hematoma in rats. Both beta-adrenergic and cholinergic receptor binding sites were unchanged in comparison to controls, while adenylate cyclase binding significantly decreased in the ischemic cortex under the hematoma. These changes may constitute a major limiting factor on receptor-linked therapeutic strategies in trauma and ischemia. Protein kinase C activation significantly increased in the ischemic area under the hematoma in these studies. This appears to be a response to calcium flux, which may be in part glutamate mediated.