Binding characteristics of the selective alpha 2-adrenoceptor antagonist [3H]idazoxan to rat olfactory cortex membranes

Differential effects of olfactory bulbectomy on GABAA and GABAB receptors in the rat brain

GABAergic mechanisms have been implicated in the bilateral olfactory bulbectomy (OBX) animal model of depression, where GABAB receptor binding sites have been shown to decrease markedly at specific time points after OBX. However, as no detailed time course of events has been determined, the present study investigated the effects of OBX on high-affinity GABAA, GABAB, beta-adrenergic, and benzodiazepine receptor binding parameters in membrane preparations from rat brain regions at weekly intervals (1-4 weeks) after OBX. Persistent significant increases (40-60%) in Bmax values of high affinity GABAA receptors were observed in the frontal cortex throughout the period investigated following OBX. Bmax values in the hippocampus increased significantly after 1 week (53%) but were not statistically significant thereafter. No changes in GABAA binding parameters were observed in the hypothalamus or cerebellum. Conversely, GABAB receptor densities were significantly decreased in the frontal cortex after 1 (-38%) and 2 (-41%) weeks and moderately decreased 3 and 4 weeks (-27 and -23%, respectively) after OBX, while in the cerebellum they were significantly increased after 1 week (96%) and returned to sham-operated levels by 3 weeks. No changes in GABAB receptor binding parameters were observed in the hippocampus or hypothalamus. Binding parameters for benzodiazepine receptor binding sites or beta-adrenoceptors were not modified throughout the time course. GABAergic transmission, reflected by changes in GABAA and GABAB receptor density in the frontal cortex, may be altered in OBX rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Decrease in locus coeruleus [3H]idazoxan binding site density in genetically epilepsy-prone (GEPR) rats

Deficits in norepinephrine synthesis, transmitter level, turnover and reuptake have been reported in the brain of genetically epilepsy-prone (GEPR) rats. We investigated the hypothesis that these alterations may trigger a compensatory downregulation of locus coeruleus alpha 2-adrenergic receptors and an upregulation of postsynaptic alpha 2-adrenergic receptor density in forebrain regions of GEPR rat brain. alpha 2-adrenergic receptor density was measured in the locus coeruleus and 7 forebrain regions of control and GEPR rats by in vitro [3H]idazoxan autoradiography. Specific [3H]idazoxan binding site density was decreased significantly in the locus coeruleus of both GEPR-3 and GEPR-9 rats compared to controls. No significant differences in specific [3H]idazoxan binding were observed in the 7 forebrain regions of GEPR-9 rats compared to control. Reduced locus coeruleus alpha 2-adrenergic receptor density in GEPR rats may produce a net increase in locus coeruleus noradrenergic cell firing, an effect which could, in part, offset the impact of reduced noradrenergic influence in GEPR rat forebrain. Additionally, decreased norepinephrine levels in GEPR rat brain may be a long-term consequence of reduced alpha 2-adrenergic receptor-mediated inhibition of locus coeruleus firing activity.

Autoradiographic distribution of alpha 2 adrenoceptors, NAIBS, and 5-HT1A receptors in human brain using [3H]idazoxan and [3H]rauwolscine

The regional distribution of [3H]idazoxan and [3H]rauwolscine was studied autoradiographically in human brain. [3H]Idazoxan binds with high affinity to alpha 2 adrenoceptors as well as to non-adrenergic sites (NAIBS). [3H]Rauwolscine, besides binding to alpha 2 adrenoceptors, also binds to 5-HT1A receptors. Both radioligands labelled the same population of alpha 2 adrenoceptors, defined as the epinephrine-displaceable binding component. The highest densities of alpha 2 adrenoceptors occur in the leptomeninges, cerebral cortex and claustrum; lower densities were visualised in the basal ganglia, thalamus, pons, substantia nigra, cerebellum and medulla oblongata; no alpha 2 adrenoceptors were detected in amygdala and nucleus ruber. NAIBS were present in all the examined brain areas, with the highest densities found in the basal ganglia and substantia nigra. The finding that certain brain regions, such as the amygdala, contained NAIBS but no detectable alpha 2 adrenoceptors, suggests that the binding sites are independent from each other. The regional distribution of 5-HT1A receptors labelled by [3H]rauwolscine is in agreement with previous studies using [3H]8-OH-DPAT.

Pertussis toxin treatment counteracts intramembrane interactions between neuropeptide Y receptors and alpha 2-adrenoceptors

The effect of intracerebroventricular injections of pertussis toxin were investigated on the neuropeptide Y-induced modulation of alpha 2-adrenoceptor binding in membranes from the dorsomedial medulla oblongata of the rat. Concentration-response experiments showed that neuropeptide Y reduced the binding affinity of the alpha 2-agonist, p-[3H]aminoclonidine, with a maximal effect of 30% at 3-30 nM. Pertussis toxin treatment (10 micrograms, 24 h) counteracted this modulation, without reducing the binding of neuropeptide Y to its own receptor. The results indicate that pertussis toxin-sensitive G-proteins are essential for the mediation of the intramembrane interaction between neuropeptide Y receptors and alpha 2-adrenoceptors.

Reciprocal interactions between alpha 2-adrenoceptor agonist and neuropeptide Y binding sites in the nucleus tractus solitarius of the rat. A biochemic and autoradiographic analysis

Interactions between a alpha 2-adrenoceptor agonist and neuropeptide Y (NPY) binding sites have been studied in the rat medulla oblongata (MO) using biochemical binding techniques as well as quantitative autoradiography. Tritiated para-amino clonidine (3H-PAC; alpha 2-adrenoceptor agonist), idazoxan (3H-IDA; alpha 2-adrenoceptor antagonist) and iodinated neuropeptide Y (125I-NPY) were used as radioligands. (1) Neuropeptide Y (NPY; 10(-8) M) but not bovine pancreatic polypeptide (BPP) nor peptide YY (PYY 10 nM) increased the KD value of 3H-PAC binding sites. However, intraventricular administration of a high dose of NPY (1.25 nmol) did not change the 3H-PAC binding characteristics in MO membrane preparations of these animals. (2) GTP 10(-4) lowered the affinity of 3H-PAC binding. NPY (10 nM) had no additional effect, nor did NPY influence the GTP induced shift in potency of clonidine to displace 3H-IDA from its binding sites. (3) In the autoradiographical experiments NPY (10 nM) significantly reduced 3H-PAC binding (2 nM) in the nucleus tractus solitarius (NTS) area by 35%. (4) When clonidine, either given centrally in vivo (3.75 nmol) or in vitro (10 nM) the binding of 125I-NPY was reduced (34 and 24%, respectively) in the NTS. When the monoamine receptors were irreversibly blocked in vivo by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 10 micrograms i.c. 24 h) 125I-NPY (0.5 nM) binding was increased by 137% in the NTS. This effect of EEDQ was prevented by pretreatment with the alpha 2-adrenoceptor antagonist idazoxan. These results provide support for a direct intramembrane interaction between the alpha 2-receptor and the NPY receptor within the NTS and may be of importance in central cardiovascular regulation.

Transient elevation of locus coeruleus alpha 2-adrenergic receptor binding during the early stages of amygdala kindling

Enhancement of noradrenergic neurotransmission retards, but does not prevent, the development of kindling. The firing activity of noradrenergic locus coeruleus (LC) neurons is partially regulated by axon collateral recurrent inhibition mediated via alpha 2-adrenergic receptors. We tested the hypothesis that LC autoinhibitory alpha 2-adrenergic receptors may change during the kindling process thereby altering LC excitability. Specific binding of the alpha 2-adrenergic receptor antagonist [3H]RX781094 (idazoxan) was measured in the LC of rats at 3 different stages of kindling development using in vitro neurotransmitter receptor autoradiography techniques. Specific [3H]RX781094 binding was elevated significantly in rats kindled to two Class 1 kindled motor seizures. No differences in binding were observed in animals kindled to Class 3 or Class 5 kindled motor seizures. Saturation of binding experiments indicated that the increase in binding following two Class 1 kindled motor seizures was due to an increase in the total number of alpha 2-receptors without a change in the affinity of the binding site for [3H]RX781094. The transient increase in number of LC alpha 2-adrenergic receptors is consistent with the idea that noradrenergic neurotransmission inhibits the early progress of kindling development, but then subsequently becomes ineffective in maintaining the inhibition during later stages of kindling development.

[3H]L-657,743 (MK-912): a new, high affinity, selective radioligand for brain alpha 2-adrenoceptors

L-657,743 (MK-912), a highly potent and selective alpha 2-adrenoceptor antagonist was tritiated to a high specific activity and its binding characteristics to brain tissue were determined. The specific binding of [3H]L-657,743 to rat cerebrocortex was saturable, reversible, and dependent on tissue concentration. In saturation studies, [3H]L-657,743 binding was resolved into two high affinity components exhibiting Kd values of 86 pM and 830 pM with densities of 82 fmol/mg protein and 660 fmol/mg protein, respectively. Based on the binding potencies of a variety of compounds with differing receptor selectivities, the sites labeled by [3H]L-657,743 were characteristic of alpha 2-adrenoceptors. In contrast to alpha 2-antagonists, alpha 2-agonists displayed shallow competition curves. In the presence of 100 microM GTP, Gpp(NH)p or 150 mM NaCl, the competition curve for epinephrine was shifted to the right, whereas that for yohimbine was unaffected. In studies utilizing human cerebrocortical tissue, [3H]L-657,743 also bound with high affinity to sites characteristic of alpha 2-adrenoceptors.

Electrophysiological characterization of adrenoceptors in the rat dorsal hippocampus. I. Receptors mediating the effect of microiontophoretically applied norepinephrine

The rat hippocampus receives a dense noradrenergic innervation originating exclusively from the locus coeruleus. The present electrophysiological study was undertaken to characterize the adrenoceptor mediating the suppressant effect of microiontophoretically applied norepinephrine (NE) on CA1 and CA3 dorsal hippocampus pyramidal neurons of the rat. The rank order of potency of microiontophoretically applied agonists, in suppressing the firing rate of hippocampus pyramidal neurons was: oxymetazoline greater than NE greater than phenylephrine greater than isoproterenol greater than clonidine. In the hippocampus, oxymetazoline was more potent than NE, whereas it was ineffective in the lateral geniculate nucleus where the effect of NE is mediated by an alpha 1-adrenoceptor. Low currents of clonidine antagonized the effect of NE suggesting that clonidine may exert a partial agonistic effect. The rank order of potency of i.v. administered adrenergic antagonists in blocking the suppressant effect of microiontophoretically applied NE was: idazoxan much greater than prazosin much greater than propranolol. Idazoxan also blocked the effect of oxymetazoline, phenylephrine, and isoproterenol but did not modify the effect of microiontophoretically applied gamma-aminobutyric acid (GABA). In addition, idazoxan, applied by microiontophoresis, readily blocked the suppressant effect of NE without affecting that of GABA. These results suggest that the suppressant effect of microiontophoretically applied NE on rat dorsal hippocampus pyramidal neurons is primarily mediated by alpha 2-adrenoceptors.

Identification and characterization of (3H)-rauwolscine binding to alpha 2-adrenoceptors in the canine saphenous vein

The biochemical exploration of the alpha 2-adrenergic receptors was investigated in the canine saphenous vein using the highly selective alpha 2-adrenergic antagonist rauwolscine as a tritiated ligand. Following an enzymatic digestive pretreatment, we isolated a purified smooth muscle cell membrane fraction from saphenous veins in quantity sufficient to permit us to study the venous alpha 2-adrenoceptor content. The binding of tritiated rauwolscine was rapid, specific, saturable and reversible. The presence of high affinity binding sites (Kd = 1.53 +/- 0.71 nM) with a density of binding Bmax of 125.2 +/- 43.1 fmol/mg protein was demonstrated on a unique class of non interacting sites (nHill = 1.001 +/- 0.06). The kinetically derived Kd was 1.28 nM, in good agreement with the value obtained from saturation isotherms. The pharmacological profile of these sites was assessed by the comparison of the potency of alpha-adrenergic agonists and antagonists to inhibit 1 nM (3H)-rauwolscine. Their efficacy was respectively: rauwolscine greater than phentolamine greater than RX 781094 greater than clonidine much greater than prazosin greater than (-)-phenylephrine greater than (-)-noradrenaline. The results showed that (3H)-rauwolscine bound specifically to sites in our membranal preparation, which had the pharmacological characteristics of the alpha 2-adrenoceptors. The correlation between biochemical and pharmacological data revealed the usefulness of binding methods in the further study of adrenergic mechanisms in the canine saphenous vein.