G. Jean Kant

Habituation to repeated stress is stressor specific

Rats were exposed to 15 min of restraint or footshock or forced running in an activity wheel once a day for 10 days. Control groups were handled only. On the 11th day, rats from each stressor group and controls were exposed to 15 min of one stressor in a crossed design such that all combinations of one chronic stressor and one acute stressor were performed. Rats were sacrificed immediately following removal from their home cage or after 15 min stressor exposure on the 11th day and plasma corticosterone and prolactin and pituitary cyclic AMP levels were determined. There were no measured differences in these stress indices among groups of rats sacrificed immediately upon removal from their home cage on day 11 regardless of previous history on days 1 through 10. Plasma corticosterone and plasma prolactin and pituitary cyclic AMP levels were elevated in all rats exposed to any of the three stressors immediately prior to sacrifice as compared to all rats not exposed to stress immediately before sacrifice. However, plasma prolactin and pituitary cyclic AMP responses to each of the 3 stressors were attenuated in rats which had previous exposure to that specific stressor as compared to rats which had previous experience with a different or no stressor. We conclude that habituation results from behavioral experience with a particular stressor rather than biochemical adaptation resulting from repeated challenge to hormonal and neurochemical systems responsive to stress.

Behavioral and neurochemical effects of intraventricular AF64A administration in rats

Ethylcholine aziridinium ion solution (AF64A), a putative specific cholinergic neurotoxin, was injected bilaterally into the lateral ventricles of rats. Following administration of 3 or 6 nmoles of AF64A, drinking and eating were depressed but returned to normal levels after several days; increased activity in the 6 nmole group persisted throughout the 21 days of observations. Performance on complex place and cue tasks indicated that injected animals were impaired in reference memory only on the place task, but working memory was impaired on both tasks. Neurochemical measurements in a separate group of animals one week after AF64A injections found large depletions of acetylcholine in hippocampus and corpus striatum, but not depletions of norepinephrine (hippocampus) or dopamine (striatum). Histological examination of the injection site revealed extensive damage to the fimbria-fornix similar to that seen after electrolytic lesions. Since the behavorial and neurochemical changes are similar to those previously found following fimbria-fornix lesions, it is concluded that the present results are possibly due to non-specific lesion effects of the neurotoxin rather than a specific effect on cholinergic systems.

Graded footshock stress elevates pituitary cyclic AMP and plasma β-endorphin, β-LPH, corticosterone and prolactin

Abstract Male rats were subjected to 15 min of various intensities of footshock current (0.0, 0.2, 0.4, 0.8, 1.6, 2.4, 3.2mA) on a variable interval schedule with an average intershock interval of 30 sec (30 shocks/15 min session). Each shock lasted 5 sec. Animals were sacrificed immediately after being removed from the shock box. Two similar studies were conducted. In the first experiment, rats were sacrificed by microwave irradiation and pituitary cyclic AMP levels were determined. In the second study, rats were decapitated and plasma hormones (prolactin, corticosterone, β-endorphin, β-LPH) were measured by radioimmunoassay. Although all biochemical indices of stress measured increased as shock intensity increased, some differences among the substances measured were observed with respect to threshold intensity, range of proportional response and maximal response.

In vivo effects of pentobarbital and halothane anesthesia on levels of adenosine 3', 5'-monophosphate and guanosine 3',5'-monophosphate in rat brain regions and pituitary

Abstract The effects of two general anesthetics, pentobarbital and halothane, on in vivo levels of cyclic AMP and cyclic GMP were examined in seventeen brain regions and the pituitary in the rat. Ventilation was controlled to produce normal values of arterial pH, pCO 2 and pO 2 , to eliminate changes in cerebral perfusion and oxygen delivery which occur as a result of the respiratory depressant effect of these drugs. Arterial pressure was monitored and colonie temperature was maintained within normal limits. Pentobarbital was given as a single i.p. injection of 80mg/kg. Control animals received an equivalent volume of vehicle solution. Induction of halothane anesthesia was accomplished by placing the animals in a jar flushed with 3% halothane in air. After 3 min the animals received 2% halothane in air via a nose cone. Control animals for this experiment were placed in an air-filled jar. Experimental and control animals were killed by microwave irradiation 1 hr after the start of anesthesia. Both drugs decreased levels of cyclic GMP in virtually all regions. The largest changes occurred in the cerebellum, where cyclic GMP dropped to 7.4 per cent of control with pentobarbital and to 9.8 per cent of control following halothane. Levels of cyclic AMP significantly increased in the cerebellum, brainstem and hypothalamus after halothane, by 58, 49 and 65 per cent, respectively. Both pentobarbital and halothane markedly increased cyclic AMP levels in the pituitary (to 784 and 270 per cent of control values, respectively). These results show that halothane and pentobarbital, which modify synaptic transmission, selectively alter cyclic AMP and cyclic GMP levels in specific brain regions and the pituitary.

Effects of lesions of the ventral medial tegmentum on locomotor activity, biogenic amines and response to amphetamine in rats ☆

Rats subjected to electrolytic lesions of the ventral medial tegmentum (VMT) showed long-lasting increased locomotor activity in the open field compared to sham-operated controls. In addition to severe depletion of mesolimbic dopamine, the lesions also caused significant depletions of striatal dopamine, mesolimbic and striatal norepinephrine and striatal and hippocampal serotonin. Administration of d-amphetamine sulfate produced similar dose-response functions for locomotor activity in both VMT-lesioned and sham-operated rats despite the extensive depletion of dopamine in the VMT-lesioned rats. These results suggest that the mesolimbic dopamine pathway is not the sole substrate for amphetamine-stimulated locomotor activity. Electrolytic lesions of the VMT interrupt several neurotransmitter pathways which may produce complex and antagonistic effects on behavior.

Cyclic AMP and cyclic GMP response to stress in brain and pituitary: Stress elevates pituitary cyclic AMP

Male rats were exposed to six stressors (saline injection, cold, forced running, Formalin injection, immobilization, electric footshock) for 15, 30, or 60 min. Following sacrifice by microwave irradiation, cyclic AMP and cyclic GMP levels were measured in pituitary, pineal and 8 regions of rat brain. All stressors except saline increased plasma corticosterone, plasma prolactin and pituitary cyclic AMP levels compared to control animals. The magnitude of the pituitary cyclic AMP response was highly correlated with the intensity of the stress as determined by the levels of plasma prolactin. Electric footshock increased pituitary cyclic AMP levels over 10 fold and plasma prolactin over 60 fold. Cyclic AMP levels in other brain regions were not altered. Cerebellar cyclic GMP was increased only by stressors that involved increased motor activity.

Biochemical indices of reactivity and habituation in rats with hippocampal lesions.

The response of rats with hippocampal lesions to acute and repeated footshock stress was assessed by measurement of pituitary cyclic AMP, plasma corticosterone and plasma prolactin. Levels of pituitary cyclic AMP and plasma prolactin and corticosterone were similar in never-shocked sham controls, and never-shocked hippocampal and neocortical lesion groups. Acute first time shock markedly elevated all measured stress indices with no statistically significant differences observed among surgical groups. In rats subjected to repeated stress (one 15 min footshock session per day for 10 days) and sacrificed 24 hours after the last shock session, levels of pituitary cyclic AMP and plasma hormones were similar to levels in never-shocked shams with the exception of the hippocampal animals. The rats with hippocampal lesions had higher levels of pituitary cyclic AMP, plasma corticosterone and plasma prolactin compared to never-shocked animals. We suggest that these data reflect a hyperreactive response of the hippocampal animals to a situation previously associated with shock. Finally, rats in all surgical groups subjected to repeated stress and sacrificed immediately after the last shock session showed a diminished cyclic AMP response to the stressor as compared to first footshock session response, demonstrating a habituation to the stressor as measured by this index. No differences in habituation were observed among hippocampal, neocortical and sham groups. Plasma hormone responses did not habituate in any group. These data support the behavioral observations of hyperreactivity in hippocampal animals and indicate that hippocampal animals are able to habituate to repeated stressful stimuli.

Regional sensitivity of cyclic AMP and cyclic GMP in rat brain to central cholinergic stimulation

Abstract We determined cyclic AMP and cyclic GMP levels in 18 regions of rat brain following administration of two different centrally active cholinergic agonists. Administration of oxotremorine (2 mg/Kg IP), a muscarinic agonist, 10 minutes prior to sacrifice by exposure to high power microwave irradiation resulted in significant increases in cyclic GMP in cerebellum, brainstem, hippocampus, midbrain, thalamus and septal region. Cyclic AMP levels were significantly elevated in substantia nigra, nucleus interpeduncularis, hypothalamus, brainstem, midbrain and in the pituitary where a greater than tenfold increase was observed. Levels of plasma prolactin and corticosterone did not differ in any of the groups examined, but growth hormone was significantly lower in animals exposed to oxotremorine. Physostigmine (0.5 mg/Kg) a cholinesterase inhibitor, administered IP also produced elevations in cyclic AMP and cyclic GMP in several of the brain regions examined. These results indicate that multiple regions of the brain are responsive to central cholinergic activation of not only cyclic GMP, but also cyclic AMP system.

In vivo effects of apomorphine and 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (RO 20–1724) on cyclic nucleotides in rat brain and pituitary

Abstract The effect of apomorphine or of 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (RO 20–1724), a potent phosphodiesterase inhibitor, on levels of cyclic AMP and cyclic GMP in vivo was examined in the pituitary, cerebellum, corpus striatum and nucleus accumbens-olfactory tubercle. RO 20–1724 was also tested in combination with apomorphine to determine whether this drug could potentiate the effect of apomorphine. Rats were injected with vehicle or RO 20–1724 (30 mg/kg) 30 min prior to an injection of saline or apomorphine hydrochloride (1 or 10 mg/kg). The animals were killed by microwave irradiation 7 min after the second injection. RO 20–1724 increased levels of cyclic AMP in all four regions, especially in the pituitary. RO 20–1724 increased levels of cyclic GMP in the cerebellum, but not in the pituitary. Apomorphine increased cyclic AMP in the pituitary, and cyclic GMP in all four regions. RO 20-1724 did not produce supra-additive effects with apomorphine. The system most responsive to either drug was cyclic AMP in the pituitary, where cyclic AMP increased approximately 10-fold after either apomorphine or RO 20–1724.

Brain and Pituitary Cyclic Nucleotide Response to Stress

Publisher Summary Endocrine responses to stress have been studied, and several brain neurochemical systems have been shown to be responsive to stress. Stress activates central noradrenergic, dopaminergic, and serotonergic neurons. Noradrenergic stimulation is a potent elevator of brain cyclic adenosine 3′5′ monophosphate (cyclic AMP) in vitro. Cyclic nucleotides in brain function as second messengers, mediating cellular responses to neurotransmitters by activating enzymes, which phosphorylate specific membrane proteins. Brain cyclic AMP increases following stress. Cyclic AMP mediates pituitary responses to humoral stimulation in vitro. Cyclic guanosine monophosphate (GMP) levels in the cerebellum and in a number of other brain regions are not influenced by stress, but by the amount of locomotor activity elicited by the experimental situation. This chapter presents a useful model for assessment of in vivo biochemical responses to stress at the pituitary level. This model is used to compare the responses of pituitary to a variety of acute and chronic stressors and to elucidate the mechanisms of neuronal and neurotransmitter regulation of pituitary response.