1 alpha, 25-Dihydroxyvitamin D3 nuclear receptors in pituitary.

Specific binding of 1 alpha,25-dihydroxyvitamin D(3) was found in nuclear and cytosol fractions of the bovine pituitary. For nuclear binding. the dissociation constant was 0.1 namomole per liter, and maximum binding was 104 femtomoles per milligram of protein. In competition studies, 25-hydroxyvitamin D(3) was 300 times weaker than 1 alpha,25-dihydroxyvitamin D(3). The existence of high-affinity sites supports a physiologic role for 1 alpha,25-dihydroxyvitamin D(3) in the pituitary.

beta-Adrenergic regulation of alpha 2-adrenergic receptors in the central nervous system.

Incubation of rat cerebral cortical slices with the beta-adrenergic agonist isoproterenol causes an increase in alpha 2-adrenergic receptor binding in addition to a decrease in beta-adrenergic receptor binding. The effects are rapid and reversible, show a parallel time course, and are blocked by sotalol, a beta-adrenergic receptor antagonist. The beta-mediated regulation of alpha 2-receptor sensitivity at brain norepinephrine synapses may be a mechanism for the homeostatic control of central noradrenergic activity.

Tricyclic antidepressants: therapeutic properties and affinity for alpha-noradrenergic receptor binding sites in the brain.

Tricyclic antidepressants vary in their capacity to cause psychomotor activation, to relieve agitated depressive states, and to cause sedation and hypotension. We have quantified relative potencies of tricyclic antidepressants in competing for the binding of 3H-labeled WB-4101 to alpha-noradrenergic receptor sites in rat brain membranes. Affinities of tricyclic drugs for alpha-noradrenergic receptor sites in the brain correlate well with the capacity of these agents to relieve psychomotor agitation and to induce sedation and hypotension; these affinities also correlate inversely with tendencies to elicit psychomotor activation.

Strain differences in rat brain epinephrine synthesis: regulation of alpha-adrenergic receptor number by epinephrine.

Inbred tht strains Fischer 344 (F344) and Buffalo (BUF) differ in serveral physiological and behavioral measures. It was found that the activity of adrenomedullary and regional brain phenylethanolamine N-methyltransferase is at least four times higher in F344 rats than in BUF rats; these strain-dependent differences corresponded directly with the epinephrine content of the medulla-pons and hypothalamus. Conversely, alpha-adrenergic receptor density in brain regions containing phenylethanolamine N-methyltransferase is two to three times lower in F344 rats than in BUF rats; alpha-receptors in frontal cortex (a brain region lacking phenylethanolamine N-methyltransferase activity and epinephrine) are similar in both strains. These findings suggest that strain-dependent differences in alpha-receptors are regulated by inherited differences in presynaptic adrenergic neuronal function in different brain regions.

Inheritance of adrenal phenylethanolamine N-methyltransferase activity in the rat.

Phenylethanolamine N-methyltransferase (PNMT) is the enzyme that catalyzes the S-adenosyl-L-methionine-dependent methylation of (-)norepinephrine to (-)epinephrine in the adrenal medulla. Adrenal PNMT activity is markedly different in two highly inbred rat strains; enzyme activity in the F344 strain is more than fivefold greater than that in the Buf strain. Initial characterization of the enzyme in the two inbred strains reveals evidence for catalytic and structural differences, as reflected in dissimilar Km values for the cosubstrate (S-adenosyl-L-methionine) and prominent differences in thermal inactivation curves. To assess adrenal PNMT activity in an F344 X Buf pedigree, we employed a statistical procedure to test for one- and two-locus hypotheses in the presence of within-class correlations due to cage or litter effects. The PNMT data in the pedigree are best accounted for by segregation at a simple major locus superimposed upon a polygenic background; data obtained from the biochemical studies suggest that the major locus is a structural gene locus.

Changes in brain alpha-adrenergic receptors after alpha-methyldopa administration to spontaneously hypertensive rats.

The hypotensive action of methyldopa has been linked to production of the metabolites methyldopamine and methylnorepinephrine in brain. We have studied the effect of long-term (72 hour) intravenous infusions of methyldopa to awake restrained spontaneously hypertensive rats and normotensive Wistar-Kyoto control animals to look for differences in hypotensive effect, differences in concentrations of natural and alpha-methylated catecholamines, and differences in alpha 1 and alpha 2-adrenergic receptor populations. Results described here indicate that hypertensive rats have a greater reduction in blood pressure and a larger increase in hypothalamic and brain stem methylnorepinephrine concentrations than do the normotensive animals. The methylnorepinephrine concentration reached a plateau value in hypothalamus in both strains while pons and medulla showed progressive, dose-related increases in concentration. These regional and strain differences in the metabolism of alpha-methyldopa suggest that the production of methylnorepinephrine in brain stem nuclei is most correlated with the hypotensive action of methyldopa. alpha 2 Agonist binding (p-amino-clonidine) declined in both hypothalamus and brain stem, and the fall was greater in hypertensive than in normotensive rats. alpha 1-Adrenergic receptor binding (prazosin) was increased, again more in hypertensive than in normotensive rats. The down regulation of alpha 2-adrenergic receptors and the up regulation of alpha 1-adrenergic receptors are compatible with increased alpha 2-adrenergic agonist presynaptic inhibition of catecholamine release with resultant postsynaptic alpha 1-adrenergic receptor supersensitivity. Spontaneously hypertensive rats showed greater methylnorepinephrine production, larger up regulation of alpha 1-adrenergic receptors, and greater down regulation of alpha 2-adrenergic receptors than did the normotensive animals; these changes may be physiological markers for the greater antisympathetic action of methyldopa in hypertensive animals.

Clozapine and haloperidol have differential effects on glutamic acid decarboxylase mRNA in the pallidal nuclei of the rat.

The striatum, and one of its targets, the pallidum (globus pallidus and entopeduncular nucleus) are based ganglia nuclei involved in extrapyramidal movement control. Gamma-aminobutyric acid (GABA)ergic neurons of the pallidum may be important for the expression of the effects of agents which alter striatal neurotransmission. In this study, rats were treated once daily for 28 days with either haloperidol or clozapine, two drugs which respectively, do and do not, induce extrapyramidal movement disorders. In situ hybridization histochemistry was used to quantify the levels of labeling for the messenger ribonucleic acid encoding glutamic acid decarboxylase, the main synthesizing enzyme for GABA in neurons of the striatum, globus pallidus, and entopeduncular nucleus. Neither drug treatment altered levels of labeling in the striatum. Haloperidol treatment increased the level of labeling in the entopeduncular nucleus and clozapine treatment increased labeling in the globus pallidus suggesting that these drugs exert different regulatory effects on pallidal neurons.

The N- and C-terminal boundaries of myelin basic protein determinants required for encephalitogenic and proliferative responses of Lewis rat T cells.

Highly purified synthetic peptides representing portions of the 68-86 sequence of guinea pig (GP) myelin basic protein (GPMBP) were used to define the N- and C-termini of encephalitogenic determinants that cause experimental autoimmune encephalomyelitis (EAE) in Lewis rats. Each peptide was tested for: (a) induction of EAE, (b)in vitro potentiation of EAE transfer activity by GPMBP-sensitized lymph node cells (LNC), (c) in vitro proliferation of GPMBP-sensitized LNC, and (d) in vitro proliferation of a GPMBP-reactive line of EAE-inducing T cells. In these bioassays, the general rank order of potency was: GPMBP greater than or equal to GP68-86 greater than or equal to GP72-86 greater than [G84]GP68-86 greater than or equal to GP68-84 much greater than GP75-85 greater than or equal to GP75-84 = virtually no activity. These results demonstrate that the encephalitogenic region is bounded by the 72-74 and 84-86 sequences. Further evidence presented herein indicates that the 75-84 sequence contains the primary antigenic features required for specific T cell recognition of the encephalitogenic region.

Persistence of immunoreactive TRH and GnRH in long-term primary anterior pituitary cultures.

Intact anterior pituitary tissue and primary anterior pituitary cultures were stained with 1:30,000 anti-TRH and 1:10,000 anti-GnRH using the peroxidase antiperoxidase immunocytochemical technique. Stains applied to serial ultrathin sections of intact pituitaries showed that TRH immunoreactivity could be localized in secretory granules of thyrotropes, gonadotropes and corticotropes whereas GnRH immunoreactivity was found only in gonadotropes and corticotropes. Long-term primary pituitary cultures were studied to remove the anterior pituitary cells from hypothalamic influences. In these cell populations both TRH and GnRH immunoreactivity persisted. In addition, quantification of the stained cells at the light microscopic level demonstrated that the volume fraction of TRH and GnRH immunoreactive cells remained constant up to 3 weeks of culture. Studies of serial ultrathin sections through cells from these cultures showed TRH or GnRH localized in secretory granules of cells that contained LH and ACTH, but not TSH. Both liquid and solid phase immunoabsorption specificity controls were used to validate the immunocytochemical stains. These studies suggest that the pituitary TRH and GnRH immunoreactivities may not be completely of hypothalamic origin, but may also be endogenous to a subpopulation of unique multihormonal pituitary cells.

Modulation of rat brain alpha- and beta-adrenergic receptor populations by lesion of the dorsal noradrenergic bundle.

Bilateral lesion of the ascending noradrenergic fibers in the dorsal bundle of adult Wistar rats with 4 micrograms 6-hydroxydopamine caused extensive depletion of norepinephrine in all forebrain areas, but led to a 54% increase in norepinephrine levels in the cerebellum. beta-Adrenergic receptor binding of [3H]dihydroalprenolol was significantly increased in all forebrain areas depleted of norepinephrine except hypothalamus. The increase in [3H]dihydroalprenolol binding was due to 62% and 34% increases in the number of beta-receptor sites in the frontal cerebral cortex and hippocampus respectively. Binding of [3H]WB-4101 to alpha 1-adrenergic receptors after dorsal bundle lesion was augmented generally to a lesser extent than beta-receptor binding, with significantly increased numbers of sites only in the frontal cortex (74%), thalamus (20%) and septum. Both alpha 1- and beta-receptor binding sites were reduced in number by 25-28% in the cerebellum of dorsal bundle-lesioned rats, whereas intraventricular administration of 6-hydroxydopamine to adult rats, which depletes norepinephrine in the cerebellum by 96%, increased cerebellar alpha 1- and beta-receptor binding by 33-40%. Binding of [3H]clonidine to forebrain alpha 2-adrenergic receptors was significantly elevated in the frontal cortex, but reduced in the amygdala and septum, after dorsal bundle lesion.

Brain epinephrine systems: detailed comparison of adrenergic and noradrenergic metabolism, receptor number and in vitro regulation, in two inbred rat strains.

Epinephrine content and PNMT activity in medulla pons and hypothalamus of F344 inbred rats is from 3- to 8-fold higher than that of Buf inbred rats. These strain-dependent differences in brain adrenergic neurons are reciprocally related to altered alpha 1- adn alpha 2-adrenergic receptor density in PNMT-containing brain regions. Radioligand binding indices related to alpha 2-receptor function reveal that receptors may be 'desensitized' as well as 'down-regulated' in the strain with high PNMT activity (F344), and may be 'supersensitive' as well as 'up-regulated' in the strain with low PNMT activity (Buf). The association between epinephrine-containing neurons and alpha-adrenergic receptor regulation appears specific, since alpha-adrenergic receptor density and regulation in brain regions devoid of PNMT and epinephrine is similar in F344 and Buf rats. While noradrenergic metabolism in F344 rats is greater than that in Buf rats, this difference is generalized throughout the brain and, thus, bears no apparent relationship to the localized alterations in alpha-adrenergic receptor density. Moreover, beta-adrenergic receptor density in the 2 strains is similar in all brain regions. These data suggest that a significant proportion of alpha-adrenergic receptors in medulla-pons and hypothalamus are intimately related to and regulated by epinephrine-containing nerve endings.

Modulation of agonist and antagonist interactions at kidney alpha 1-adrenoceptors by nucleotides and metal ions.

In order to characterize putative high- and low-affinity states of the renal alpha 1-adrenoceptor, binding sites for the selective antagonist radioligand [3H]prazosin were examined in washed membranes prepared from rat renal cortex and medulla. Norepinephrine competition curves at [3H]prazosin sites were biphasic and were best fit by a two-site model. Na+ and GTP selectively decreased the proportion of sites exhibiting a high affinity for norepinephrine. In contrast, Mg2+ facilitated high-affinity interactions of norepinephrine at the renal alpha 1-receptor. Guanine nucleotides and Na+ increased the affinity of some antagonists [( 3H]prazosin, WB-4101), but not others (phentolamine). Mg2+ again had opposite effects. The effects of ions and nucleotides on both agonist and antagonist interactions were concentration-dependent. The order of potencies for monovalent cations (Na+ greater than Li+ much greater than K+), divalent cations (Mn2+ greater than Mg2+) and nucleotides (Gpp (NH)p, GTP much greater than GMP, ATP) were similar to those reported for cyclase-coupled receptor systems. However, unlike other divalent cations Ca2+ decreased both agonist and antagonist binding, possibly due to a Ca2+-sensitive proteinase. Receptor binding properties were similar in renal cortex and medulla. Renal alpha 1-receptor sites appear to display high- and low-affinity states with respect to agonists, and the equilibrium between these states may be modulated by guanine nucleotides and mono- and divalent metal ions. Some antagonists appear to bind preferentially to sites with low agonist affinity, and this effect is probably independent of retained endogenous catecholamines.

[3H]dihydroalprenolol binding in the rat brainstem.

The binding of [3H]dihydroalprenolol to beta-adrenergic receptors in the rat brainstem was studied. Propranolol inhibited dihydroalprenolol binding to a greater extent than did isoproterenol or epinephrine. Scatchard plots obtained with propranolol were biphasic whereas those obtained with isoproterenol and epinephrine were monophasic. The results indicate that in the brainstem propranolol displaces dihydroalprenolol from sites other than beta-adrenergic receptors. Nonspecific binding of dihydroalprenolol in this tissue therefore cannot be assayed using propranolol but should be measured with isoproterenol or epinephrine.

[3H]rauwolscine (alpha-yohimbine): a specific antagonist radioligand for brain alpha 2-adrenergic receptors.

[3H]Rauwolscine, a specific and potent alpha 2-antagonist radioligand, was used to characterize alpha 2-receptor binding in bovine cerebral cortex. [3H]Rauwolscine binding was reversible, stereospecific, and saturable. Association, dissociation, and saturation studies revealed one site interactions (k -1/k+1 = 1.2 nM, KD = 2.5 nM, Bmax = 160 fmol/mg protein) and competition studies indicated that [3H]rauwolscine labeled the alpha 2-receptor. Agonists inhibited [3H]rauwolscine binding in a shallow, GTP-sensitive manner. These results suggest that [3H]rauwolscine specifically labels both the high and low affinity states of the alpha 2-receptor in brain membranes.

In vitro modulation of CNS beta-receptor number by antidepressants and beta-agonists.

In vitro incubation of rat cerebral cortex slices with antidepressant drugs reduced beta-adrenergic receptor binding of 3H-dihydroalprenolol by 30%. The decrease was maximum after 60 min, and was reversible after 120 min. (-)-Isoproterenol incubation caused a rapid and reversible loss (60%) of beta-receptor binding. Both effects were due to a decrease in beta-receptor sites. In vitro beta-receptor subsensitivity due to desipramine and isoproterenol was non-additive. The observed reversible loss of beta-receptor sites may be an initial phase of the beta-receptor down-regulation by antidepressants seen in vivo.

3H-Catecholamine binding to alpha-receptors in rat brain: enhancement by reserpine.

(+/-)-3H-Epinephrine and (-)-3H-norepinephrine bind to rat cortex membranes in a saturable manner with dissociation constants of 16.7 and 27 nM respectively. The maximum number of 3H-catecholamine binding sites, 10--12 pmoles/g tissue, and the pharmacological characteristics of (+/-)-3H-epinephrine binding, indicate that the catecholamines label the same alpha-noradrenergic receptor in the rat as does 3H-clonidine. At 25 degrees, (+/-)-3H-epinephrine binding associates rapidly to equilibrium, and dissociates in a biphasic manner. The affinities of alpha-agonists at the 3H-catecholamine binding site are 2--4 fold weaker in the rat than in the calf cortex under the same experimental conditions. Ergot alkaloids and phenoxybenzamine have similar affinities in the two tissues, whereas phentolamine and WB-4101 are 8--10 times weaker in the rat. Reserpine (0.25 mg/kg s.c. per day for 3 weeks) causes 25 and 46% increases in the numbers of (+/-)-3H-epinephrine and 3H-WB-4101 alpha-receptor binding sites respectively, and a 51% increase in the number of 3H-dihydroalprenolol beta-receptor sites, in rat forebrain. Reserpine pretreatment does not alter the affinities of either alpha- or beta-receptor 3H-ligands.

Interactions of neuroleptic compounds at alpha 2-adrenergic receptor affinity states in bovine caudate nucleus.

Several recent studies have suggested a connection between neuroleptics and/or dopamine systems, and alpha 2-adrenergic receptors. Competition of a variety of neuroleptics at specific alpha 2-adrenergic binding sites (labeled with [3H]rauwolscine or [3H]p-aminoclonidine) in bovine caudate and cortex was examined. Few of the compounds were potent competitors, with (+)-butaclamol, alpha-flupenthixol, clozapine and pimozide the most potent (IC50 range 100-600 nM), while triflupromazine, mesoridazine, and haloperidol were the least potent (IC50 range 5 000-10 000 nM). These findings indicate that few, if any, of the examined neuroleptics have specific, high-affinity interactions with the multiple affinity states of the alpha 2-adrenergic receptor.

Differential effects of antidepressant treatment on brain monoaminergic receptors.

Chronic (21 days) antidepressant administration to rats results in a decrease in both serotonin and beta-adrenergic, but not cholinergic muscarinic, receptor binding in selected brain regions, with the frontal cortex appearing to be somewhat more sensitive to this effect. Neither nisoxetine nor fluoxetine, potent and specific inhibitors of norepinephrine and serotonin uptake respectively, caused receptor binding changes after chronic administration, suggesting that inhibition of transmitter uptake, in itself, is insufficient to cause receptor subsensitivity. In vitro experiments indicated that antidepressants are relatively weak alpha 2-receptor blocking agents, but some are potent on the alpha 1-receptor system indicating that the norepinephrine releasing potency of some antidepressants may not be mediated by blockade of presynaptic autoreceptors.

Tricyclic antidepressant radioreceptor assay.

A radioreceptor assay for tricyclic antidepressants described here is based on the ability of these drugs to compete with [3H]-3-quinuclidinyl benzilate (3H-QNB) for binding to muscarinic cholinergic receptors in rat brain membranes. The assay is sensitive, in that in can detect, for example, 2 ng/ml nortriptyline in plasma. Seven plasma samples from depressed patients treated with nortriptyline were assayed with the radioreceptor and gas liquid chromatographic methods, and the results from these two methods were almost identical. This assay should be used cautiously, if at all, in patients treated with other drugs that have potent anticholinergic effects.

Para-azidoclonidine: a novel photoaffinity ligand for the alpha 2-receptor.

Reversible and irreversible interactions of the photoreactive clonidine analogue p-azidoclonidine (PAZC) with brain alpha 2-adrenergic receptors were examined. In the absence of light, PAZC displayed selective, high affinity, competitive interactions with sites labeled by the alpha 2-agonist 3H-p-aminoclonidine (3H-PAC). Reversible binding characteristics resembled those of other alpha 2-agonists. Preincubation of bovine frontal cortex membranes with 100 nM PAZC followed by ultraviolet irradiation and thorough washing decreased alpha 2-receptor density 42% relative to controls receiving irradiation alone. The loss of receptors could be prevented by inclusion of 500 nM phentolamine in the preincubation medium. Alpha 1- and beta-adrenergic receptors were relatively unaffected. PAZC is a potential photoaffinity ligand for the alpha 2-adrenergic receptor.