Eric A. Stone

Brain Adrenergic receptors and resistance to stress

The relationship between brain beta-adrenergic receptors and adaptation to stress was studied in rats subjected to repeated restraint stress. The stress was found to produce a reduction in the density of these receptors in the hypothalamus, cerebral cortex and brain stem. This change first appeared after 4-7 days and persisted for the duration of the two-week stress. Adaptation, as judged by resistance to the anorexic and gastric lesion-inducing effects of the stress, occurred progressively over the full two-week period. The loss of beta-receptors was found to correlate positively with the degree of adaptation. The relationship was strongest for the hypothalamus but was also apparent in the cortex and brain stem. These findings support the hypothesis that a reduction in the number of brain adrenergic receptors is one of the biochemical factors underlying adaptation to stress.

Are glial cells targets of the central noradrenergic system? A review of the evidence

It has been suggested by a number of investigators that glial cells as well as neurons are targets of the central noradrenergic system. This important hypothesis, however, has not been presented previously in a systematic and unified manner. The present review was therefore undertaken to accomplish this. The evidence supporting noradrenergic action on glia consists primarily of findings that beta-adrenoceptors, norepinephrine (NE)-stimulated cyclic AMP (cAMP) responses and glycogen are localized preferentially in glial cells and that beta-receptor density and glycogen hydrolysis are under the control of neuronally released NE. While there is some disagreement as to the extent to which beta-receptors are preferentially localized in glia, there is a consensus that most glycogen in the forebrain is localized in this cellular compartment. The presumed function of the noradrenergic action on glia appears to be the release of glucose for production of energy, the synthesis of neurotrophic factors such as nerve growth factor, and the release of substances which may affect local neurotransmission including taurine, cAMP and its metabolites. These glial responses may be intimately related to the electrophysiological actions of NE on neurons.

Reduction by stress of norepinephrine-stimulated accumulation of cyclic AMP in rat cerebral cortex

I N A RECENT stud? we used chronic electric footshock to determine if severe emotional stress can affect the responsiveness of the norepinephrine (NE)-sensitive C A M P generating system in the rat brain (STONE. 1978). I t was reasoned that footshock. which increases the release of NE in the rat brain (STONE. 1975). should reduce responsibeness, as d o other procedures which increase the content or release of this brain amine (VETULANI et ul.. 19760. h). Chronic footshock however was found to produce only a weak and variable decrease in the C A M P response to NE of slices of the rat hypothalamus and cerebral cortex. Interpretation of these findings was hampered by the fact that the brain slice technique used to measure NE-stimulated adenylate cyclase activity (KRELGER rt id.. 1976) had several drawbacks as used in our laboratory. First. the method frequently gave widely divergent C A M P values for duplicate samples of slices exposed to the same N E concentration. Second. the assay employed the phosphodiesterase inhibitor. isobutylmethylxanthine (IBMX) which may have obscured possible stress-induced changes in the latter enzyme. IBMX is also a potent adenosine antagonist (MAH & DALY. 1976) and may have blocked a population of adrenergic receptors that arc functionally dependent on adenosine receptor actbat ion (SATTIN er a/.% 1975). For these reasons we hace reexamined the effects of footshock using a modified brain slice technique which produces more reliable results and omits the use of IBMX. We now report that chronic footshock stress significantly reduces the C A M P response to NE in the rat cerebral cortex.

Chronic restraint stress elicits a positive antidepressant response on the forced swim test

The present study investigated behavioral parallels between adaptation to stress and antidepressant treatment using the forced swim test. Restraint stress given repeatedly for 11 days significantly reduced immobility on this test. A single application of stress had no effect. The reduction in immobility produced by repeated restraint was quantitatively similar to that produced by repeated administration of desmethylimipramine. These results confirm previous findings of similarities in the behavioral and neurochemical response to chronic stress and chronic antidepressant treatment.

ACCUMULATION AND METABOLISM OF NOREPINE-PHRINE IN RAT HYPOTHALAMUS AFTER EXHAUSTIVE STRESS1

—Exhaustive stress in rats is followed by a temporary reduction of hypothalamic norepinephrine (NE) together with a persistent increase in turnover during recovery. To test for persistent alterations of NE storage and metabolism produced by stress, rats were subjected to 3 h of forced running and were then injected intraventricularly with [3H]NE or [3H]dopamine (DA). The hypothalamus was assayed for [3H]NE and its metabolites at various intervals after injection. The effects of stress were compared with those of reserpine (7·5 mg/kg) or α‐methyltyrosine (AMT, 300 mg/kg) pretreatment. It was found that the stress‐induced reduction of endogenous NE was not accompanied by a change in the accumulation of exogenous [3H]NE either 10 or 30 min after injection, whereas the NE depletions produced by reserpine or AMT were associated with decreased or increased accumulation, respectively. However, stress did produce an increased accumulation of [3H]NE endogenously synthesized from [3H]DA. These results indicate that exhaustive stress does not adversely affect the storage of NE. They also suggest that stores of NE depleted by stress are replenished chiefly with newly synthesized NE and not through an increased uptake and binding or decreased metabolism of extraneuronal NE. The latter factors may play a role in the maintenance of brain NE stores when biosynthesis is low, i.e. after AMT. The major metabolites of exogenous [3H]NE, at 30 min after injection, were identified as conjugates of 3,4‐dihydroxyphenylglycol (DOPEG) and 3‐methoxy‐4‐hydroxyphenylglycol (MOPEG) in approximately equal amounts. The finding of high levels of conjugated DOPEG confirms a recent report (Slgden and Eccleston, 1971) that this compound is a major metabolite of brain NE. Reserpine produced marked elevations of both conjugates; AMT slightly reduced each. Prior stress increased only conjugated MOPEG, an observation suggesting that CNS levels of this metabolite may reflect NE released by nervous activity.

Induction and habituation of c-fos and zif/268 by acute and repeated stressors

Acute restraint and shaking stress each induced the mRNA for the immediate early gene (IEG), c-fos, within 1 h in locus coeruleus (LC), midbrain raphe nuclei and central gray. Both acute restraint stress and acute shaking stress activated c-fos and zif/268 mRNA in paraventricular nuclei within 1 h. Midbrain A9 and A10 dopaminergic neurons did not show any c-fos or zif/268 mRNA induction by either acute stressor. Repeated restraint stress (14d, 1h/d) produced habituation to acute restraint stress, but not to acute shaking stress, in PVN, raphe and central gray. In LC, repeated restraint stress caused some habituation to both acute stressors.

Noradrenergic activation of immediate early genes in rat cerebral cortex

Previous studies have shown that stimulation of adrenergic receptors in the brain increases the expression of the immediate early gene (IEG), c-fos, in vivo (Mol. Brain Res., 6(1989) 39-45). The present study was undertaken to determine whether this also holds for other IEGs which have been shown to be activated in brain cell culture by adrenergic agonists. Both yohimbine injection and stressful stimulation, two treatments causing brain norepinephrine (NE) release, were found to cause a parallel, transient activation of at least 5 IEGs (c-fos, nur77, tis-7, zif-268 and tis-21) in the rat cortex. Genes that are not immediate early (beta-actin, NGF and HSP70) were found not to be affected in the interval used (6 h). The responses were mediated predominantly by beta-adrenoceptors with some contribution from alpha 1 receptors. The parallel activation of multiple genes by noradrenergic receptors may enable the coding of different biochemical responses to the activation of different receptors.

Adaptation to stress: Tyrosine hydroxylase activity and catecholamine release

The effects of adaptation to stress and of genetic differences on levels of in vitro tyrosine hydroxylase (TH) activity and in vivo catecholamine (CA) release are reviewed. It is shown that adaptation of animals to a wide variety of stressors including immobilization, electroconvulsive shock, footshock, hemorrhage, exercise and cold exposure results in a reduced CA response in the plasma, brainstem and heart to subsequent exposure to the same stress. Adaptation to many of the latter stressors also produces increased in vitro levels of TH activity. A similar inverse relation between in vitro TH activity and in vivo CA release is described for two inbred rat strains which differ in emotionality (Brown-Norway and Wistar Kyoto). The inverse relationship between TH activity and CA release may reflect different processes of biochemical adaptation utilized either for acclimation to stress, for preparation for emergency reactions or for changes in the metabolic costs of transmitter release. The similarity between environmental and genetic effects on these variables suggests that the above changes have a common adaptive function.

Greater behavioral effects of stress in immature as compared to mature male mice

The effect of sexual maturity on behavioral effects of stress was examined in male mice. Immature (4-week-old) or mature (8-week-old) animals were subjected to either social stress (exposure to an isolated adult male) or restraint stress for 5 days and examined for body weight, food intake, or plus-maze behavior. Social stress reduced food intake, body weight, and open-arm entries in 4-week-old but not 8-week-old mice. Restraint reduced body weight in 4-week-old but not 8-week-old mice. It is concluded that immature male mice show greater behavioral disturbances after stress than their mature counterparts. The findings are in agreement with much anecdotal evidence that children are more vulnerable to stress than adults.

Locus coeruleus lesions potentiate neurotoxic effects of MPTP in dopaminergic neurons of the substantia nigra

The observation that Parkinsons disease (PD) is associated with locus coeruleus (LC) noradrenergic neuronal degeneration suggests that the LC noradrenergic system may be involved in the pathogenesis and natural progression of the destruction of the substantia nigra (SN) dopaminergic neurons in Parkinsons disease. The relationship of these two systems was examined by injection of subtoxic doses of MPTP into unilateral LC 6-hydroxydopamine (6-OHDA) lesioned mice. A significant loss of dopaminergic cells was only found in the SN on the side of the LC lesions. These results suggest that the LC may have protective effects on SN dopaminergic neurons.