Tissue content of epinephrine and norepinephrine following adrenal medullectomy

Molecular pathways underlying sympathetic autonomic overshooting leading to fear and traumatic memories: looking for alternative therapeutic options for post-traumatic stress disorder

The sympathoadrenal medullary system and the hypothalamic-pituitary-adrenal axis are both activated upon stressful events. The release of catecholamines, such as dopamine, norepinephrine (NE), and epinephrine (EPI), from sympathetic autonomic nerves participate in the adaptive responses to acute stress. Most theories suggest that activation of peripheral β-adrenoceptors (β-ARs) mediates catecholamines-induced memory enhancement. These include direct activation of β-ARs in the vagus nerve, as well as indirect responses to catecholamine-induced glucose changes in the brain. Excessive sympathetic activity is deeply associated with memories experienced during strong emotional stressful conditions, with catecholamines playing relevant roles in fear and traumatic memories consolidation. Recent findings suggest that EPI is implicated in fear and traumatic contextual memories associated with post-traumatic stress disorder (PTSD) by increasing hippocampal gene transcription (e.g., Nr4a) downstream to cAMP response-element protein activation (CREB). Herein, we reviewed the literature focusing on the molecular mechanisms underlying the pathophysiology of memories associated with fear and traumatic experiences to pave new avenues for the treatment of stress and anxiety conditions, such as PTSD.

Epinephrine increases contextual learning through activation of peripheral β2-adrenoceptors

RATIONALE

Phenylethanolamine-N-methyltransferase knockout (Pnmt-KO) mice are unable to synthesize epinephrine and display reduced contextual fear. However, the precise mechanism responsible for impaired contextual fear learning in these mice is unknown.

OBJECTIVES

Our aim was to study the mechanism of epinephrine-dependent contextual learning.

METHODS

Wild-type (WT) or Pnmt-KO (129x1/SvJ) mice were submitted to a fear conditioning test either in the absence or in the presence of epinephrine, isoprenaline (non-selective β-adrenoceptor agonist), fenoterol (selective β2-adrenoceptor agonist), epinephrine plus sotalol (non-selective β-adrenoceptor antagonist), and dobutamine (selective β1-adrenoceptor agonist). Catecholamines were separated by reverse-phase HPLC and quantified by electrochemical detection. Blood glucose was measured by coulometry.

RESULTS

Re-exposure to shock context induced higher freezing in WT and Pnmt-KO mice treated with epinephrine and fenoterol than in mice treated with vehicle. In addition, freezing response in Pnmt-KO mice was much lower than in WT mice. Freezing induced by epinephrine was blocked by sotalol in Pnmt-KO mice. Epinephrine and fenoterol treatment restored glycemic response in Pnmt-KO mice. Re-exposure to shock context did not induce a significant difference in freezing in Pnmt-KO mice treated with dobutamine and vehicle.

CONCLUSIONS

Aversive memories are best retained if moderately high plasma epinephrine concentrations occur at the same moment as the aversive stimulus. In addition, epinephrine increases context fear learning by acting on peripheral β2-adrenoceptors, which may induce high levels of blood glucose. Since glucose crosses the blood-brain barrier, it may enhance hippocampal-dependent contextual learning.

Attenuated aortic vasodilation and sympathetic prejunctional facilitation in epinephrine-deficient mice: selective impairment of β2-adrenoceptor responses

It has been suggested that there is a link between epinephrine synthesis and the development of β2-adrenoceptor-mediated effects, but it remains to be determined whether this development is triggered by epinephrine. The aim of this study was to characterize β-adrenoceptor-mediated relaxation and facilitation of norepinephrine release in the aorta of phenylethanolamine-N-methyltransferase-knockout (Pnmt-KO) mice. Catecholamines were quantified by reverse-phase high-performance liquid chromatography-electrochemical detection. Aortic rings were mounted in a myograph to determine concentration-response curves to selective β1- or β2-adrenoceptor agonists in the absence or presence of selective β1- or β2-adrenoceptor antagonists. Aortic rings were also preincubated with [(3)H]norepinephrine to measure tritium overflow elicited by electrical stimulation in the presence of increasing concentrations of nonselective β- or selective β2-adrenoceptor agonists. β2-Adrenoceptor protein density was evaluated by Western blotting and β2-adrenoceptor localization by immunohistochemistry. Epinephrine is absent in Pnmt-KO mice. The potency and the maximal effect of the β2-adrenoceptor agonist terbutaline were lower in Pnmt-KO than in wild-type (WT) mice. The selective β2-adrenoceptor antagonist ICI 118,551 [(±)-erythro-(S*,S*)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride] antagonized the relaxation caused by terbutaline in WT but not in Pnmt-KO mice. Isoproterenol and terbutaline induced concentration-dependent increases in tritium overflow in WT mice only. β2-Adrenoceptor protein density was decreased in membrane aorta homogenates of Pnmt-KO mice, and this finding was supported by immunofluorescence confocal microscopy. In conclusion, epinephrine is crucial for β2-adrenoceptor-mediated vasodilation and facilitation of norepinephrine release. In the absence of epinephrine, β2-adrenoceptor protein density was decreased in aorta cell membranes, thus potentially hindering its functional activity.

Urinary excretion of catecholamines in the night monkey (Aotus trivirgatus) (Primates, Cebidae)

A seasonal variation in the urinary catecholamines output has been demonstrated in two simians kept under constant ambient conditions : the nocturnal Aotus and the diurnal Saïmiri sciureus. In Aotus, catecholamines output (NA + A), in spring, is higher than in other Primates including man and even more so in winter. Cold exposure increases the NA + A excretion in Aotus as it does in squirrel monkey and rat but the A output is particularly prominent in Saïmiri. Fasting does not alter significantly the catecholamines excretion. Associated fasting and cold exposure do not modify the adrenosympathetic response observed in Aotus in cold conditions alone, but depresses the sympathetic activity and greatly enhance the adrenomedullary excretion in squirrel monkey, as it is the case in rat. Associated fasting and cold represents a highly stressful situation for squirrel monkey but not for night monkey. Catecholamines metabolites (MN, NMN, DOPAC, HVA, VMA and MHPG) are found in urine of both species, DOPAC and VMA being predominant in Aotus but DOPAC and MHPG in Saïmiri. The proportions of conjugated forms vary according to the metabolite : DOPAC and VMA are mainly under free form but NMN, MN and MHPG are mostly conjugated in both species. The daily output of pooled adrenergic metabolites (expressed as ng/mg creatinine) is higher in Aotus than in Saïmiri and man. Both monkey species display a high adrenosympathetic activity which does not correlate with their resting metabolic rate.

Role of endogenous catecholamines in the anti-inflammatory activity of alpha-adrenoceptor blocking agents

1 Drugs which release or modify the response to catecholamines were examined for their effect on the permeability of the mouse peritoneal vascular bed to circulating plasma albumin, labelled with Evans blue.2 Phenoxybenzamine, phentolamine, piperoxane, yohimbine or cocaine reduced the extravasation of Evans blue into the peritoneum, an effect which was antagonized by beta-adrenoceptor blocking drugs. The inhibitory effect of desipramine on the extravasation of Evans blue was less completely antagonized by beta-adrenoceptor blockade.3 Inhibition of catecholamine biosynthesis, ganglion blockade or adrenergic neurone blockade antagonized the reduction in dye extravasation by alpha-adrenoceptor blocking agents and cocaine, but had no significant effect on the response to desipramine. The inhibitory effects of alpha-adrenoceptor blocking agents on dye extravasation were not prevented by bilateral adrenalectomy.4 Mice subjected to the procedure for estimation of vascular permeability excreted increased amounts of adrenaline and noradrenaline. Pretreatment with phenoxybenzamine, piperoxane or cocaine further increased catecholamine excretion, but desipramine caused only a small increase in catecholamine excretion which did not correlate with its effect on dye extravasation.5 It is suggested that phenoxybenzamine, phentolamine, piperoxane and cocaine reduce vascular permeability in the mouse peritoneum by releasing and/or potentiating the effects of endogenous catecholamines on beta-adrenoceptors. Endogenous catecholamines do not appear to be involved in the anti-inflammatory activity of desipramine.

Current evaluation and management of pheochromocytoma

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