Effects of acute inhalation exposure to isoamyl nitrite on the hypothalamo-pituitary-adrenal axis in male Sprague-Dawley rats.

Isoamyl nitrite (IAN) is a member of the family of volatile organic nitrites that exert vasodilatory effects and have recently exhibited a considerable potential for inhalation abuse. In an effort to provide mechanistic insight into the neurotoxic effects and abuse potential of these agents, the present study was designed to evaluate the acute effects of IAN on the hypothalamo-pituitary-adrenal (HPA) axis. Attempts were also made to correlate the neuroendocrine effects of IAN with its pharmacokinetic profile. Male Sprague-Dawley rats were exposed to 600 or 1200 ppm IAN by inhalation for 10 or 30 min. Following exposure, adrenocorticotropic hormone (ACTH) and corticosterone in plasma and corticotropin-releasing factor (CRF) in three brain regions (hypothalamus, hippocampus, and frontal cortex) were determined by radioimmunoassay. Levels of IAN in the three brain regions as well as in blood were measured by gas chromatography to determine the target tissue concentrations responsible for neuroendocrine changes. Uptake of IAN into blood and all brain regions was very rapid, as stable concentrations were achieved within 10 min of exposure and maintained for 30 min of continuous inhalation. Plasma corticosterone decreased significantly after 10 min inhalation of both IAN doses, and returned to control levels after 30 min. Moreover, plasma ACTH was significantly increased by 10 and 30 min of exposure to 600 and 1200 ppm IAN, while hypothalamic CRF increased significantly after 30 min of exposure to the 600 ppm dose. These latter findings suggest activation of the hypothalamus and pituitary due to a reduction in negative feedback resulting from the initial decrease in corticosterone. Although plasma ACTH was greatly increased after 30 min, plasma corticosterone levels were unchanged, indicating that IAN primarily acts to inhibit the synthesis or secretion of adrenal steroids and that activation of the HPA axis is not involved in the behavioral manifestations of IAN inhalation. These compensatory effects of HPA axis regulation, and possibly the vasodilatory properties of IAN, also likely precluded the establishment of definitive relationships between observed changes in hormone levels and blood or regional brain concentrations of the inhalant.

Effects of acute inhalation exposure to 1,1,1-trichloroethane on the hypothalamo-pituitary-adrenal axis in male Sprague-Dawley rats.

1,1,1-Trichloroethane (TRI) is a commonly used industrial solvent with a considerable potential for inhalation abuse. Previous studies in our laboratory and elsewhere have shown that this agent exerts a suppressant effect on operant responding, as well as a number of additional neurobehavioral effects that are similar to those of central nervous system (CNS) depressant drugs. In an effort to provide information relevant to potential mechanisms involved in the behavioral effects and abuse potential of TRI, the present study evaluated the acute effects of this agent on the activity of the hypothalamo-pituitary-adrenal (HPA) axis . Male Sprague-Dawley rats were exposed to 3500 or 5000 ppm TRI by inhalation for 10 or 30 min. Following exposure, plasma levels of adrenocorticotropic hormone (ACTH) and corticosterone and levels of ACTH and corticotropin-releasing factor (CRF) in three brain regions--hypothalamus, hippocampus, and frontal cortex--were determined by selective radioimmunoassays. Levels of TRI in the three brain regions as well as blood were measured by headspace gas chromatography to determine the target tissue concentrations responsible for neuroendocrine changes. Uptake of TRI in blood and all brain regions was very rapid, with stable concentrations apparently achieved within 10 min and maintained for 30 min. During this time course, a significant decrease in plasma corticosterone was produced at 30 min but no significant change in plasma ACTH was observed with 3500 ppm TRI. However, after exposure to 5000 ppm, both plasma ACTH and plasma corticosterone were significantly reduced at 10 and 30 min. ACTH levels in the three brain regions were not significantly changed by TRI, while hypothalamic CRF was significantly increased during exposure to 3500 ppm. However, hypothalamic concentrations of CRF declined following 30 min at 3500 ppm and were not significantly changed by 5000 ppm. This complexity of effects on the regulation of HPA axis activity likely precluded the establishment of consistent relationships between changes in hormonal levels and blood or regional brain concentrations of the inhalant. However, these actions of TRI were strikingly similar to those previously reported for the benzodiazepines.

Schedule-controlled operant behavior of rats during 1,1,1-trichloroethane inhalation: relationship to blood and brain solvent concentrations.

The central nervous system is the principal target of 1,1,1-trichloroethane (TRI), and several studies of this volatile solvent have demonstrated effects on learned animal behaviors. There have been few attempts, however, to quantitatively relate such effects to blood or target organ (brain) solvent concentrations. Therefore, Sprague-Dawley rats trained to lever-press for evaporated milk on a variable interval 30-s reinforcement schedule were placed in an operant test cage and exposed to clean air for 20 min, followed by a single concentration of TRI vapor (500-5000 ppm) for 100 min. Additional rats were exposed to equivalent TRI concentrations for 10, 20, 40, 60, 80, or 100 min to determine blood and brain concentration vs. time profiles. Inhalation of 1000 ppm slightly increased operant response rates, whereas 2000, 3500, and 5000 ppm decreased operant response rates in a concentration- and time-dependent manner. Accumulation of TRI in blood and brain was rapid and concentration dependent, with the brain concentration roughly twice that of blood. Plots of blood and brain TRI concentrations against operant performance showed responding in excess of control rates at low concentrations, and decreasing response rates as concentrations increased. Linear regression analyses indicated that blood and brain concentrations, as well as measures of time integrals of internal dose, were strongly correlated with operant performance. Neurobehavioral toxicity in laboratory animals, as measured by changes in operant performance, can therefore be quantitatively related to internal measures of TRI exposure to enhance its predictive value for human risk assessment.

Activity wheel running reduces escape latency and alters brain monoamine levels after footshock.

We examined the effects of chronic activity wheel running on brain monoamines and latency to escape foot shock after prior exposure to uncontrollable, inescapable foot shock. Individually housed young (approximately 50 day) female Sprague-Dawley rats were randomly assigned to standard cages (sedentary) or cages with activity wheels. After 9-12 weeks, animals were matched in pairs on body mass. Activity wheel animals were also matched on running distance. An animal from each matched pair was randomly assigned to controllable or uncontrollable inescapable foot shock followed the next day by a foot shock escape test in a shuttle box. Brain concentrations of norepinephrine (NE), dopamine (DA), dihydroxyphenylacetic acid (DOPAC), 5-hydroxytryptamine (5-HT), and 5-hydroxyindole acetic acid (5-HIAA) were assayed in the locus coeruleus (LC), dorsal raphe (DR), central amygdala (AC), hippocampus (CA1), arcuate nucleus, paraventricular nucleus (PVN), and midbrain central gray. After prior exposure to uncontrollable foot shock, escape latency was reduced by 34% for wheel runners compared with sedentary controls. The shortened escape latency for wheel runners was associated with 61% higher NE concentrations in LC and 44% higher NE concentrations in DR compared with sedentary controls. Sedentary controls, compared with wheel runners, had 31% higher 5-HIAA concentrations in CA1 and 30% higher 5-HIAA concentrations in AC after uncontrollable foot shock and had 28% higher 5-HT and 33% higher 5-HIAA concentrations in AC averaged across both foot shock conditions. There were no group differences in monoamines in the central gray or in plasma prolactin or ACTH concentrations, despite 52% higher DA concentrations in the arcuate nucleus after uncontrollable foot shock and 50% higher DOPAC/DA and 17% higher 5-HIAA/5-HT concentrations in the PVN averaged across both foot shock conditions for sedentary compared with activity wheel animals. The present results extend understanding of the escape-deficit by indicating an attenuating role for circadian physical activity. The altered monoamine levels suggest brain regions for more direct probes of neural activity after wheel running and foot shock.

Schedule-controlled operant behavior of rats following oral administration of perchloroethylene: time course and relationship to blood and brain solvent levels.

Previous studies have indicated that human exposure to perchloroethylene (PCE) produces subtle behavioral changes and other neurological effects at concentration at or below the current occupational exposure limit. Since comparable effects in animals may be reflected by changes in schedule-controlled operant behavior, the ability of orally administered PCE to alter fixed-ratio (FR) responding for a food reward was investigated in male Sprague-Dawley rats. Furthermore, since behavioral effects of solvents are likely to be more closely related to blood or target tissue (i.e, brain) concentrations than administered dose, the relationship between the pharmacokinetic distribution of PCE and its effects on operant responding was also evaluated. Rats trained to lever-press for evaporated milk on an FR-40 reinforcement schedule were gavaged with 160 or 480 mg/kg PCE and immediately placed in an operant test cage for 90 min. Separate animals gavaged with equivalent doses of PCE were used to determine profiles of blood and brain concentrations versus time. Perchloroethylene produced changes in responding that varied not only with dose but also among animals receiving the same dose. Changes in the response rates of rats receiving 160 mg/kg PCE were either not readily apparent, restricted to the first 5 min of the operant session, or attributable to gavage stress and the dosing vehicle. However, 480 mg/kg produced either an immediate suppression of responding for 15-30 min before a rapid recovery to control rates or a complete elimination of lever-pressing for the majority of the operant session. Although the two doses of PCE produced markedly different effects on operant behavior during the first 30 min of exposure, differences in brain concentrations of PCE were minimal. Furthermore, the majority of animals receiving 480 mg/kg PCE fully recovered from response suppression while blood and brain levels of the solvent continued to rise. Thus, relationships between blood and brain PCE levels and performance impairment were not discernible over the monitored time course. Since the rapid onset of response suppression suggests that the precipitating event occurs within the first few minutes of exposure, it is possible that altered responding is related to the rate of increase in blood or brain concentrations rather than the absolute solvent concentrations themselves. The relationship between the pharmacokinetic distribution of solvents and their effects on the central nervous system is obviously complex and may involve acute neuronal adaptation as well as the dynamics of solvent distribution among the various body compartments.

Brain norepinephrine and metabolites after treadmill training and wheel running in rats.

Regional changes in concentrations of brain norepinephrine [NE] and its metabolites after chronic exercise have not been described for exercise protocols not confounded by other stressors. We examined levels of [NE], 3-methoxy-4-hydroxyphenylglycol [MHPG], and 3,4-dihydroxyphenylglycol [DHPG] in the frontal cortex, hippocampus, pons-medulla, and spinal cord after 8 wk of exercise. Male Sprague-Dawley rats (N = 36) were randomly assigned to three conditions: 1) 24-h access to activity wheel running (WR), 2) treadmill running (TR) at 0 degrees incline for 1 h.d-1 at 25-30 m.min-1, or 3) a sedentary control group (C). Levels (nmol.g-1) of [NE], [MHPG], and [DHPG] were assayed by high performance liquid chromatography with electrochemical detection. Planned contrasts (P < 0.05) indicated that exercise training increased succinate dehydrogenase activity (mmol cytochrome C reduced.min-1.g-1 wet weight) in soleus muscle for TR compared with WR or C. [NE] was higher in the pons-medulla and spinal cord for both TR and WR compared with C. [DHPG] was higher in the pons-medulla for TR compared with C, and [MHPG] was higher in the frontal cortex and in the hippocampus for TR compared with C. Our results suggest that treadmill exercise training is accompanied by brain noradrenergic adaptations consistent with increased metabolism of NE in areas containing NE cell bodies and ascending terminals, whereas treadmill running and wheel running are accompanied by increases in levels of NE in the areas of NE cell bodies and the spinal cord, independently of an exercise training effect.

The involvement of endogenous opiate systems in learned helplessness and stress-induced analgesia.

The participation of endogenous opiate systems in the induction and expression of learned helplessness (LH) and stress-induced analgesia (SIA) was investigated in rats exposed to escapable and inescapable footshock. Following an initial footshock, analgesia was observed only in those animals that could not control their stress exposure and this SIA was prevented by the administration of naloxone. Analgesia was no longer evident in this inescapable group after 48 h. However, exposure to a shuttlebox escape task at this time reinstated the SIA but did not produce SIA in animals previously exposed to escapable footshock. Shuttlebox escape deficits indicative of LH were also exhibited by animals that received an inescapable footshock stress 48 h prior to testing. The analgesia and LH observed in the inescapable group were not affected by the administration of naloxone (3 mg/kg, IP) 10 min before shuttlebox exposure but were prevented when the same dose of naloxone was given before the initial stress. Thus, endogenous opiates clearly participate in the initial induction of LH and SIA and, although the degree of endogenous opiate involvement in the subsequent expression of these behaviors is unclear, it seems evident that their expression can occur in the presence of opiate antagonism and may therefore require the participation of additional transmitter systems.

Increased brain norepinephrine metabolism correlated with analgesia produced by the periaqueductal gray injection of opiates.

Dose-dependent increases in 3-methoxy-4-hydroxyphenylethyleneglycol sulfate (MOPEG-SO4), a major metabolite of norepinephrine, were produced in the limbic forebrain and cerebral cortex 30 min after the bilateral injection of morphine into the periaqueductal gray (PAG). These effects were also elicited by similar injections of levorphanol. Highly significant correlations were obtained between the concentrations of MOPEG-SO4 and the analgesic effect of the opiates and opiate actions were antagonized by systemic naloxone. These results indicate that activation of opiate receptors in the PAG may elicit the involvement of noradrenergic systems in distant brain regions in the mediation of analgesia.

Analgesia and increases in limbic and cortical MOPEG-SO4 produced by periaqueductal gray injections of morphine.

Analgesia and changes in limbic and cortical concentrations of the major brain noradrenaline metabolite, 3-methoxy-4-hydroxy-phenylethylene glycol sulphate (MOPEG-SO4), were investigated in rats following the bilateral injection of morphine into the periaqueductal gray (PAG). Morphine, at a dose of 5 micrograms per bilateral site, produced a significant antinociceptive effect within 15 min of injection. This effect, as measured by the tail flick analgesic test, remained constant at a level of approximately 75% of the maximum for 60 min. Significant increases in limbic and cortical MOPEG-SO4 were also observed 15, 30 and 60 min after the 5 micrograms bilateral PAG injection of morphine. However, MOPEG-SO4 concentrations exhibited a sharp peak in both brain areas at 30 min. Analgesia and the regional increases in MOPEG-SO4 were antagonized by the prior systemic injection of naloxone (1 mg kg-1, i.p.). Thus, analgesia and increases in noradrenaline metabolism in two brain regions appear to be mediated by the specific activation of opiate receptors in the PAG. Although these findings indicate that brain noradrenergic systems may be involved in the mediation of morphine analgesia, the lack of a strict temporal relationship between antinociceptive action and increases in MOPEG-SO4, suggests that analgesia cannot be totally attributed to changes in brain noradrenergic transmission.

Behavioral and neurochemical interactions of dextroamphetamine and methylphenidate in rats.

The effects of dextroamphetamine and methylphenidate on locomotor activity and brain levels of norepinephrine and dopamine were compared in male Sprague-Dawley rats. Both drugs produced a dose-related increase in locomotor activity during the hour immediately following intraperitoneal administration. However, combined administration of drugs elicited only the effect of dextroamphetamine. Brain levels of norepinephrine and dopamine also increased 1 hr after dextroamphetamine dosing. Methylphenidate did not exhibit these effects and antagonized the neurochemical changes produced by dextroamphetamine. Although both drugs are considered to exert their effects by indirect activation of brain catecholamine systems, differences in their mechanism of action appear to result in a lack of additive or antagonistic effects when dextroamphetamine and methylphenidate are coadministered. These findings may have clinical significance with respect to the use of such agents in minimal brain dysfunction.

The effects of tranylcypromine isomers on norepinephrine-H3 metabolism in rat brain.

The effects of d- and l-tranylcypromine on the disposition and metabolism of intracisternally administered l-norepinephrine-H3 were studied in rat brain. Both isomers inhibited the deamination of norepinephrine-H3. However, d-tranylcypromine was considerably more potent than the l-isomer in this respect. In addition, the l-isomer of tranylcypromine was found to enhance the disappearance of endogenous and tritiated norepinephrine from brain. Although this action appeared to result from an increase in catecholamine release, the possibility of uptake inhibition could not be eliminated. The l-isomer of tranylcypromine enhanced the disappearance of norepinephrine-H3 from brain when administered both 20 and 90 min following intracisternal injection of the label. Comparable doses of d-tranylcypromine did not exhibit this effect. Larger increases in brain levels of normetanephrine-H3 were produced by d,l-tranylcypromine than by either the d- or l-isomer alone, indicating that the racemic mixture may produce the greatest increase in the interaction of norepinephrine with its postsynaptic receptors.

The effects of several narcotic analgesics on brain levels of 3-methoxy-4-hydroxyphenylethylene glycol sulfate in the rat.

The acute administration of levorphanol, morphine, anileridine, methadone, cyclazocine and pentazocine was found to increase brain levels of 3-methoxy-4-hydroxyphenylethylene glycol sulfate (MOPEG-SO4) in rats. Drug-induced increases in brain levels of this norepinephrine metabolite were dose-dependent and peak drug effects generally occurred 1 hr after intraperitoneal injection. Six to 8 hr after opiate agonist or partial agonist administration, MOEG-SO4 levels returned to control values or below. The narcotic effect appeared to be stereospecific, since high doses of d-methadone and dextrorphan produced either no change or only minimal increases in brain MOPEG-SO4 levels when compared with saline-treated controls. The relative potency of the analgesics tested in producing increases in brain MOPEG-SO4 levels appeared to correlate reasonably well with the ability of these drugs to produce other characteristic pharmacological effects. The drug-induced increases in brain MOPEG-SO4 levels were antagonized by naloxone. The rate of disappearance of MOPEG-SO4 from the brains of animals treated with pargyline was not decreased by the opiate agonists or partial agonists tested indicating that these drugs did not inhibit the acid transport process. These results indicate that the production of an increase in brain norepinephrine turnover is a specific component of the pharmacological actions of narcotic analgesics.

Effects of short-and long-term administration of tricyclic antidepressants and lithium on norepinephrine turnover in brain.

The findings summarized in this paper show that norepinephrine turnover in brain is decreased after acute administration of imipramine or desmethylimipramine but tends to increase during chronic administration of these tricyclic antidepressants. Similarly, it appears that there also may be important differences between the effects of acute and chronic administration of lithium salts on norepinephrine turnover in the central nervous system. Such changes in norepinephrine turnover that develop gradually over the course of long-term drug administration may help to explain the need for chronic administration of tricyclic antidepressants or lithium salts in the treatment of patients with affective disorders.