Perry B. Molinoff

Dopamine receptor changes following destruction of the nigrostriatal pathway: Lack of a relationship to rotational behavior

Phenomena consistent with postsynaptic supersensitivity developed in the rat neostriatum following the destruction of dopaminergic afferent neurons. A gradual increase in the density of binding sites for [3H]spiperone occurred over a 2-3 week period. This increase was apparent only after the almost complete loss of dopamine-containing nerve terminals as measured by the depletion of endogenous dopamine. The properties of the receptor labeled by [3H]spiperone were not altered by denervation. Elimination of dopamine-containing nerve endings in the neostriatum was accompanied by the gradual development of an increase in dopamine-sensitive adenylate cyclase activity in homogenates of the caudate ipsilateral to the lesion as compared to the contralateral side. The administration of apomorphine led to pronounced circling behavior. This effect occurred rapidly and was maximal within 3 days following destruction of dopaminergic neurons. The increase in the density of dopamine receptors and in a receptor-mediated function may partially account for the development of enhanced electrophysiological responses to dopamine agonists in the neostriatum. However, the results do not explain the drug-induced rotational behavior which develops after destruction of the dopamine-containing nigrostriatal pathway. This behavioral phenomenon clearly preceded the appearance of receptor alterations in the corpus striatum.

Ontogeny of β-adrenergic receptors in rat cerebral cortex

Abstract The ontogeny of β-adrenergic receptors in rat cerebral cortex has been studied using [125I]iodohydroxyzylpindolol as a ligand in an in vitro binding assay. The concentration of β-adrenergic receptors was very low during the first week after birth. Between days 7 and 14 there was a rapid increase in the density of receptors. Adult levels were reached by the end of the second week. The affinities of 1-isoproterenol and iodohydroxybenzylpindolol for β-adrenergic receptors did not vary with the age of the animal. Fluoride stimulated adenylate cyclase activity in the cerebral cortex was 40% of the adult level at birth and gradually increased to maximal levels over the next two weeks. On the other hand, catecholamine stimulated cyclic-3′,5′-adenosine monophosphate accumulation was barely detectable during the first week after birth, but increased rapidly to adult levels bwetween days 7 and 14. The results suggest that is the development of β-adrenorgic receptors that permits the expression of catecholamine sensitive adenylate cyclase activity. Norepinephrine stores in the cerebral cortex developed slowly reaching adult levels approximately two months after birth. There is therefore little correlation between the ontogeny of presynaptic adrenergic nerve terminals and the postsynaptic development of β-adrenergic receptors.

Ontogeny of β1- and β2-adrenergic receptors in rat cerebellum and cerebral cortex

The development of β1- and β2-adrenergic receptors was studied in rat cerebral cortex and cerebellum. In the cerebral cortex, which contains mostly β1-adnergic receptors, total β-adrenergic receptor density increased sharply between postnatal days 10 and 21. The density of receptors remained fairly constant through 6 weeks of age and then subsequently declined. The proportion of β1 and β2 receptors was relatively constant throughout the development of the cerebral cortex. The development of the two receptor subtypes thus paralleled the development of total β-adrenergic receptors in the cerebral cortex. The ontogeny of β-adrenergic receptors in the cerebellum, which contains mainly β2 receptors, was strikingly different from that observed in the cortex. Total cerebellar β receptor density exhibited a slow but steady increase from postnatal day 5 through day 42. The density of receptors then plateaued and remained constant until the animals were approximately 6 months of age. Unlike the results obtained in the cortex, the relative proportions of β1 and β2 receptors in the cerebellum changed markedly during development. Between postnatal days 8 and 13 approximately 18% of the receptors were of the β1 subtype. This proportion steadily decreased with age, and in 3- and 6-month-old animals only approximately 2% of the receptors were of the β1 subtype. The results demonstrate that two subtypes of β-adrenergic receptors can have different development patterns in the same brain area, and that a single receptor subtype can follow different development patterns in different brain regions. Possible correlations between the ontogeny of β1 and β2 receptors and various developmental events are discussed.

Effect of Central Catecholamine Depletion on the Osmotic and Nonosmotic Stimulation of Vasopressin (Antidiuretic Hormone) in the Rat

The central nervous system (CNS) mechanism(s) for the release of antidiuretic hormone (ADH) by various stimuli is unknown. In this study, the role of CNS catecholamines in effecting ADH release was examined in conscious rats 10-14 d after the cerebroventricular injection of 6-hydroxydopamine (6-OHDA). This dose of 6-OHDA caused a 67% depletion of brain tissue norepinephrine and only 3% depletion of heart norepinephrine, as compared with controls, which were injected with the vehicle buffer alone. Either intravenous 3% saline (osmotic stimulus) or intraperitoneal hyperoncotic dextran (nonosmotic stimulus) was administered to water-diuresing rats through indwelling catheters. Neither of these maneuvers changed arterial pressure, pulse, or inulin clearance in control or 6-OHDA rats. The 3% saline caused similar increases in plasma osmolality (15 mosmol/kg H(2)O) in control and 6-OHDA rats. The control rats, however, increased urinary osmolality (Uosm) to 586 mosmol/kg H(2)O, whereas 6-OHDA rats increased Uosm only to 335 mosmol/kg H(2)O (P < 0.005). These changes in Uosm were accompanied by an increase in plasma ADH to 7.6 muIU/ml in control animals vs. 2.9 muIU/ml in 6-OHDA rats (P < 0.005). All waterdiuresing animals had undetectable plasma ADH levels. Dextran-induced hypovolemia caused similar decrements (- 10%) in blood volume in both control and 6-OHDA animals, neither of which had significant changes in plasma osmolality. This nonosmotic hypovolemic stimulus caused an increase in Uosm to 753 mosmol/kg H(2)O in control rats, whereas Uosm in 6-OHDA rats increased to only 358 mosmol/kg H(2)O (P < 0.001). At the same time, ADH levels also were significantly greater in Cont rats (2.4 muIU/ml) than in the 6-OHDA animals (0.69 muIU/ml; P < 0.05). These results therefore suggest that CNS catecholamines may play an important role in mediating ADH release in response to both osmotic and nonosmotic (hypovolemic) stimuli.

The normal occurrence of octopamine in the central nervous system of the rat

An enzymatic assay for octopamine capable of detecting 50 pg of amine was developed and used to study the distribution of octopamine in regions of the rat central nervous system. The presence of octopamine in the rat pineal organ was confirmed by mass spectrometry; Administration of a monoamine oxidase inhibitor and of tyramine led to increases in CNS octopamine levels while the administration of reserpine intraperitoneally or 6-hydroxydopamine intraventricularly led to decreases in octopamine levels. The results suggest that in the mammalian CNS octopamine is present in neural structures where it may be involved in synaptic function.

Endogenous inhibitors of dopamine-β-hydroxylase in rat organs

Abstract Endogenous inhibitors of dopamine-β-hydroxylase (DBH) are present in organ homogenates from several mammalian species. An assay for these inhibitors which utilizes DBH partially purified from the bovine adrenal medulla has been developed. A rat spleen homogenate, diluted 1:8000 in Tris buffer, inhibited bovine DBH activity by more than 50 per cent. Similar amounts of inhibitory activity were present in other organs. The inhibitory activity in homogenates of rat heart was reduced by treatment at 95° while most of that in the spleen was resistant to heat denaturation. Mixing experiments showed that there were constituents in heart homogenates which could cause the inhibitory activity in spleen to become heat labile. Divalent cations and the heme containing protein cytochrome c also conferred heat lability on the inhibitory activity in rat spleen homogenates. N-Ethylmaleimide had no effect on the inhibitory activity in rat heart, spleen or adrenal gland but Cu2+ and p-chloromercuribenzoate completely reversed the effects of the endogenous inhibitors in these organs. Sulfhydryl reagents can inhibit DBH but there was no correlation between inhibitory activity and either total tissue sulfhydryl concentration or TCA soluble sulfhydryl concentration.

The regulation of the noradrenergic neuron

Publisher Summary This chapter discusses the regulation of the noradrenergic neuron. A number of regulatory processes exist that influence the biosynthesis of catecholamines in adrenergic neurons. The large number of events, occurring in a highly synchronized fashion, implies a wide variety of possible regulatory sites. Norepinephrine (NE) that is released into the synaptic cleft can act on presynaptic α-receptors to inhibit the release of additional NE. A further possible mechanism for regulating the release of NE from adrenergic nerves involves prostaglandins of the E type. These compounds are present in most mammalian tissues and an increased formation and release of prostaglandins has been observed in conjunction with increased nerve activity. Nerve growth factor is another substance that appears to be involved in regulating the synthesis of enzymes in adrenergic neurons. When it is administered to newborn rats, for example, it can cause a selective induction of TH and DBH. The increase in the activities of these enzymes is far greater than is that of other enzymes.

REGULATION OF β-ADRENERGIC RECEPTORS IN RAT CEREBRAL CORTEX

ABSTRACT The intraventricular administration of 6-hydroxydopamine (6-OHDA) to adult rats or its subcutaneous administration to newborns leads to an increase in the density of β-adrenergic receptors with no change in their properties. The administration of inhibitors of norepinephrine (NE) uptake or metabolism or the incubation of slices of rat cerebral cortex with isoproterenol (ISO) leads to an apparent loss of receptors. The pharmacological specificity of β-adrenergic receptors in rat cerebral cortex suggests that both β-1 and β-2-adrenergic receptors are present. The chronic administration of 6-OHDA or desemethylimipramine (DMI) cause changes in the density of β-1 receptors without affecting the density of β-2 receptors.

THE REGULATION OF THE NORADRENERGIC NEURON

Publisher Summary This chapter discusses the regulation of the noradrenergic neuron. A number of regulatory processes exist that influence the biosynthesis of catecholamines in adrenergic neurons. The large number of events, occurring in a highly synchronized fashion, implies a wide variety of possible regulatory sites. Norepinephrine (NE) that is released into the synaptic cleft can act on presynaptic α-receptors to inhibit the release of additional NE. A further possible mechanism for regulating the release of NE from adrenergic nerves involves prostaglandins of the E type. These compounds are present in most mammalian tissues and an increased formation and release of prostaglandins has been observed in conjunction with increased nerve activity. Nerve growth factor is another substance that appears to be involved in regulating the synthesis of enzymes in adrenergic neurons. When it is administered to newborn rats, for example, it can cause a selective induction of TH and DBH. The increase in the activities of these enzymes is far greater than is that of other enzymes.