Destruction of noradrenergic neurons in rabbit brainstem elevates plasma vasopressin, causing hypertension.

When A1 noradrenergic neurons in the caudal ventrolateral medulla of rabbits are destroyed electrolytically or by local injection of the neurotoxin kainic acid, the concentration of vasopressin in plasma increases, causing hypertension. The A1 neurons may tonically inhibit the activity of vasopressin-secreting neuroendocrine cells through a direct hypothalamic projection.

The motivation to obtain nicotine-conditioned reinforcers depends on nicotine dose.

Stimuli associated with nicotine (NIC) can acquire new meaning via Pavlovian conditioning. If a stimulus is associated with the primary reinforcing effects of NIC, the new conditional properties of the stimulus should make it a more valuable reinforcer (i.e., increase the motivation to obtain the stimulus), and this value should be based, in part, on the strength or intensity of the primary reinforcer (i.e., NIC dose). The purpose of the present study was to investigate whether NIC-conditioned reinforcement increased motivation to obtain non-NIC stimuli, as reflected by performance on a progressive ratio (PR) reinforcement schedule, and whether this increased motivation was systematically related to NIC dose. Two Paired groups were allowed to nose-poke for NIC (0.03 or 0.09mg/kg/infusion, IV) accompanied by 15-s illumination of a stimulus light (conditional stimulus or CS). Two Unpaired groups (0.03 or 0.09mg/kg/infusion) could also make a nose-poke response for the CS; however their NIC infusions were controlled by the Paired group (i.e., yoked design). A fifth group (CS-Only) was allowed to nose-poke for CS presentations and saline infusions. After 29 conditioning sessions the nose-poke operant was prevented by obscuring the receptacle and the CS (accompanied by saline infusion for all groups) was made contingent upon a novel operant response (lever press). During the acquisition of this novel response, each CS/saline infusion earned increased the number of responses required to earn the next CS/saline infusion. Pairings with the primary reinforcing effects of NIC resulted the acquisition of a novel response for the CS. Motivation to obtain the CS depended on salience (dose) of the primary reinforcement (NIC).

Sodium excretion and renin secretion after continuous versus pulsatile infusion of oxytocin in rats.

Neurohypophyseal oxytocin (OT), secreted continuously under conditions of hyperosmolality, is a potent natriuretic hormone in rats. In contrast, OT secretion during lactation is pulsatile and is not accompanied by increased urinary Na+ excretion. The present experiments compared the effects of continuous and pulsatile infusion of OT on natriuresis in rats. In male rats anesthetized with Inactin, continuous infusion of OT (125 ng/kg x h) increased plasma OT to about 70 pg/ml; renal Na+ excretion increased 10-fold, and urine volume and K+ excretion also were elevated. However, when OT was administered i.v. in the same amount but in pulses given once every 5 or 10 min, to simulate the pattern of OT secretion during lactation, rats did not excrete significantly more urine, Na+, or K+ than did vehicle-treated animals. The plasma renin concentration, measured in these experiments because OT receptors are present in the macula densa, increased 2-fold when OT was infused either continuously or in pulses. These results indicate that the effects of OT administration on urinary Na+ excretion in rats varies depending on whether the infusion is pulsatile or continuous, whereas the effects of OT on renin secretion show no such difference.

Peripheral pressor systems in hypertension caused by nucleus tractus solitarius lesions.

The roles of vasopressin, the sympathoadrenal system, and the renin-angiotensin system in the production of hypertension after bilateral destruction of the nucleus tractus solitarius (NTS) were examined in chloralose-anesthetized rats. Since the activity of the renin-angiotensin system is high in anesthetized rats, additional studies were performed in unanesthetized, freely moving rats to evaluate the role of the renin-angiotensin system in hypertension caused by NTS lesions. Hypertension produced by bilateral electrolytic NTS lesions in rats was accompanied by elevated plasma levels of vasopressin (approximately 7-fold), norepinephrine (greater than 10-fold), and epinephrine (greater than 10-fold), but not of plasma renin activity. These results suggest that this form of hypertension is due to increased sympathoadrenal activity and increased vasopressin release into plasma and that the renin-angiotensin system is not involved. In rats with NTS lesions, blockade of vasopressin or the sympathoadrenal system, but not the renin-angiotensin system, elicited an acute decrease in arterial pressure. However, blockade of either vasopressin or the autonomic nervous system before production of the lesions had no effect on the resulting hypertension, indicating that in the absence of either one of these systems bilateral NTS lesions still result in severe hypertension. Although the renin-angiotensin system does not normally contribute to this hypertension, it does appear to contribute to the elevation in blood pressure once the actions of vasopressin have been blocked. In rats pretreated with a vasopressin antagonist, plasma renin activity increased following NTS lesions and the angiotensin converting enzyme inhibitor captopril decreased blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of gamma-aminobutyric acid B receptors in baroreceptor reflexes in hypertensive rats.

Previous studies demonstrated that stimulation of gamma-aminobutyric acid B (GABA(B)) receptors in the nucleus tractus solitarius of spontaneously hypertensive rats (SHR) elicited a larger increase in arterial pressure compared with control Wistar-Kyoto rats. Since stimulation of GABA(B) receptors in the nucleus tractus solitarius attenuates cardiovascular responses evoked by electrical stimulation of the aortic depressor nerve in normotensive rats and there is evidence of a central neural attenuation of aortic depressor nerve-evoked responses in SHR, we conducted studies to test the hypothesis that enhanced stimulation of GABA(B) receptors in the nucleus tractus solitarius in SHR is responsible for the attenuation of the aortic depressor nerve-evoked responses. Electrical stimulation of the left aortic depressor nerve resulted in frequency-dependent decreases in arterial pressure, heart rate, and splanchnic sympathetic nerve activity in urethane-anesthetized control rats. These responses were not significantly altered by injection of the GABA(B) receptor antagonist CGP 35348 into the ipsilateral nucleus tractus solitarius. The responses evoked by aortic depressor nerve stimulation were attenuated in SHR. This attenuation was particularly apparent with more prolonged periods (>15 seconds) of high-frequency (25-Hz) stimulation, with the depressor and sympathetic nerve responses diminishing during the course of stimulation. This time- and frequency-dependent attenuation of baroreceptor-evoked depressor responses was reversed by injection of CGP 35348 into the ipsilateral nucleus tractus solitarius. Rats made hypertensive by treatment with deoxycorticosterone plus salt did not have attenuated aortic depressor nerve-evoked responses. These results suggest that alterations in GABA b-mediated neural transmission in the nucleus tractus solitarius contribute to the attenuation of the baroreceptor reflex observed in SHR.

Enhanced gamma-aminobutyric acid-mediated responses in nucleus tractus solitarius of hypertensive rats.

Previous studies demonstrated that stimulation of type B gamma-aminobutyric acid (GABAB) receptors but not type A (GABAA) receptors in the nucleus tractus solitarius of spontaneously hypertensive rats elicited a larger increase in arterial pressure compared with control rats of the Wistar-Kyoto strain. The present studies extended that observation by examining the cardiovascular response to injection into the nucleus tractus solitarius of a selective GABAB receptor antagonist, CGP 35348, in these strains as well as examining the cardiovascular responses to stimulation or blockade of GABAB receptors in the nucleus tractus solitarius in another model of hypertension, the rat treated with deoxycorticosterone acetate and salt. In both groups of hypertensive rats the pressor response to injection into the nucleus tractus solitarius of the GABA uptake blocking drug nipecotic acid was significantly greater compared with control rats (P < .01 in each model). Similarly, in both models of hypertension, the depressor response elicited by blockade of GABAB receptors in the nucleus tractus solitarius by injection of CGP 35348 was approximately 75% greater compared with control rats (P < .05 in each model). These results suggest that alterations in GABAB-mediated neural transmission in the nucleus tractus solitarius may contribute to the elevated arterial pressure observed in these models of hypertension.

Excitatory amino acids in the rostral ventrolateral medulla support blood pressure in spontaneously hypertensive rats.

Injection of the excitatory amino acid (EAA) antagonist kynurenic acid (KYN) into the rostral ventrolateral medulla (RVLM) of anesthetized rats has no effect on arterial pressure. However, we recently reported that after inhibition of the caudal ventrolateral medulla, injection of KYN into the RVLM decreased arterial pressure to the same level as produced by complete inhibition of the RVLM. We have suggested that these results reflect tonically active EAA-mediated inputs to the RVLM producing both direct excitation of RVLM vasomotor neurons and indirect inhibition of these neurons. On the basis of this model, we hypothesize that the balance between these EAA-driven direct excitatory and indirect inhibitory influences on the RVLM may be altered in models of experimental hypertension. To begin to test this hypothesis, the effects of injecting KYN into the RVLM of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) were compared. In chloralose-anesthetized WKY, bilateral injection of KYN into the RVLM did not alter arterial pressure, whereas similar injections in SHR reduced mean arterial pressure by approximately 40 mm Hg. After inhibition of the caudal ventrolateral medulla, which similarly increased arterial pressure in both strains, injection of KYN into the RVLM reduced mean arterial pressure to the same level as produced by autonomic blockade. These results suggest that the balance of excitatory and inhibitory influences on RVLM vasomotor neurons driven by tonically active EAA-mediated inputs to the RVLM is disrupted in SHR and may contribute to the hypertension in SHR.

Vasopressin contributes to hypertension caused by nucleus tractus solitarius lesions.

Lesions of the nucleus tractus solitarius (NTS) were studied to determine whether they elevate plasma vasopressin levels and, if so, whether these elevated levels of vasopressin contribute to the hypertension caused by NTS lesions. Bilateral electrolytic lesions of the NTS caused acute, severe hypertension in rats anesthetized with chloralose and in conscious, freely moving rats. After placement of the NTS lesions there was a greater than tenfold elevation in plasma vasopressin levels. Administration of an antagonist of the vasoconstrictor action of vasopressin markedly diminished the hypertension in both conscious and anesthetized rats. Following ganglionic blockade with chlorisondamine, NTS lesions still elicited hypertension, and the magnitude of the hypertension was not different from that observed in rats not treated with chlorisondamine. The hypertension produced by lesions of the NTS in ganglionic-blocked rats was completely abolished by administration of a vasopressin antagonist. These results indicate that (1) NTS lesions elevate plasma vasopressin levels and (2) elevated plasma vasopressin contributes to the hypertension produced by such lesions.

Inverse relationship between fenfluramine-induced prolactin release and blood pressure in humans.

Although substantial evidence from experimental animals suggests that augmentation and reduction in serotonergic neurotransmission both affect arterial blood pressure (BP), it is unknown whether "tonic" central serotonergic activity is related to resting BP variability in humans. We tested this hypothesis in a community sample by evaluating the relationship between resting BP and a neuropharmacologic index of brain serotonergic activity (the fenfluramine challenge test). Subjects were 270 generally healthy men and women aged 25 to 60 years who were not receiving prescribed antihypertensive or psychotropic medications. The sample included 216 non-Hispanic whites and 47 blacks. Resting systolic BP ranged from 85 to 161 mm Hg and diastolic from 58 to 98 mm Hg. Each subject received 0.55 to 0.65 mg/kg D,L-fenfluramine hydrochloride, and the plasma prolactin concentration was measured over 3.5 hours. Analyses revealed a linear, inverse relationship between the maximum fenfluramine-induced prolactin rise and systolic and diastolic BP in whites: r=-0.36 and r=-0.29, respectively (P<0.001 for both). These relationships were not observed in the black participants. In whites, the prolactin response to fenfluramine remained a significant predictor of systolic and diastolic BPs in multivariate models including age, gender, body mass index, physical activity, smoking, and alcohol consumption (P135/85 mm Hg. These data reveal that in white but not black adults, fenfluramine-induced prolactin release correlates inversely with BP and may indicate a role of central serotonergic activity in the pathogenesis of hypertension.

Chronic cold exposure potentiates CRH-evoked increases in electrophysiologic activity of locus coeruleus neurons.

BACKGROUND: Chronic stress exposure can produce sensitization of norepinephrine release in the forebrain in response to subsequent stressors. Furthermore, the increase in norepinephrine release in response to the stress-related peptide corticotropin-releasing hormone (CRH) is potentiated by prior chronic stress exposure. To explore possible mechanisms underlying these alterations in norepinephrine release, we examined the effect of chronic stress on the electrophysiologic activity of locus coeruleus (LC) neurons in response to centrally applied CRH. METHODS: Single-unit recordings of LC neurons in halothane-anesthetized rats were used to compare the effect of intraventricular administration of CRH (0.3-3.0 microg) in control and previously cold-exposed (2 weeks at 5 degrees C) rats. RESULTS: The CRH-evoked increase in LC neuron activity was enhanced following chronic cold exposure, without alteration in basal activity of LC neurons. The enhanced CRH-evoked activation was apparent at higher doses of CRH but not at lower ones, resulting in an increased slope of the dose-response curve for CRH in previously cold-exposed rats. CONCLUSIONS: These data, in combination with previous data, suggest that the sensitivity of LC neurons to excitatory inputs is increased following chronic cold exposure. The altered functional capacity of LC neurons in rats after continuous cold exposure may represent an experimental model to examine the role of central noradrenergic neurons in anxiety and mood disorders.

Characterization of the central nervous system innervation of the rat spleen using viral transneuronal tracing.

Splenic immune function is modulated by sympathetic innervation, which in turn is controlled by inputs from supraspinal regions. In the present study, the characterization of central circuits involved in the control of splenic function was accomplished by injecting pseudorabies virus (PRV), a retrograde transynaptic tracer, into the spleen and conducting a temporal analysis of the progression of the infection from 60 hours to 110 hours postinoculation. In addition, central noradrenergic cell groups involved in splenic innervation were characterized by dual immunohistochemical detection of dopamine-beta-hydroxylase and PRV. Infection in the CNS first appeared in the spinal cord. Splenic sympathetic preganglionic neurons, identified in rats injected with Fluoro-Gold i.p. prior to PRV inoculation of the spleen, were located in T(3)-T(12) bilaterally; numerous infected interneurons were also found in the thoracic spinal cord (T(1)-T(13)). Infected neurons in the brain were first observed in the A5 region, ventromedial medulla, rostral ventrolateral medulla, paraventricular hypothalamic nucleus, Barrington's nucleus, and caudal raphe. At intermediate survival times, the number of infected cells increased in previously infected areas, and infected neurons also appeared in lateral hypothalamus, A7 region, locus coeruleus, subcoeruleus region, nucleus of the solitary tract, and C3 cell group. At longer postinoculation intervals, infected neurons were found in additional hypothalamic areas, Edinger-Westphal nucleus, periaqueductal gray, pedunculopontine tegmental nucleus, caudal ventrolateral medulla, and area postrema. These results demonstrate that the sympathetic outflow to the spleen is controlled by a complex multisynaptic pathway that involves several brainstem and forebrain nuclei.

Release of antidiuretic hormone in quadriplegic subjects in response to head-up tilt.

Plasma antidiuretic hormone (ADH) levels during head-up tilt were measured in quadriplegic patients and normal healthy subjects. In normal subjects, neither rapid tilt (from horizontal to 70 degrees head-up in 20 seconds and remaining head-up for 5 minutes) nor gradual tilt (from horizontal to 70 degrees head-up over 15 minutes) significantly changed the plasma ADH level. In contrast, rapid tilt elevated plasma ADH levels approximately fourfold in quadriplegic subjects. A less dramatic elevation in plasma ADH levels followed gradual tilt in quadriplegic subjects. These data, along with the possibility that the pressor potency of ADH is enhanced in quadriplegic subjects, suggest that ADH is involved in the maintenance of blood pressure in quadriplegics in a head-up position.

Cardiovascular responses elicited by gamma-aminobutyric acid in the nucleus tractus solitarius: evidence for action at the GABAB receptor.

Previous studies in this laboratory have shown that GABAergic neurotransmission in the nucleus tractus solitarius contributes to the maintenance of blood pressure. Increasing the synaptic action of GABA in the nucleus tractus solitarius by injection of the blocker of the uptake of GABA, nipecotic acid elevated blood pressure. The current studies examined the subtype of GABA receptor involved in mediating this response. Bilateral injections of (-)baclofen into the nucleus tractus solitarius (1-100 pmol in 100 nl) of chloralose-anesthetized, paralyzed, ventilated Sprague-Dawley rats increased blood pressure without significantly altering heart rate. This pressor response was neither attenuated nor blocked by the selective GABAA receptor antagonist bicuculline. Similarly, the pressor response elicited by the injection of nipecotic acid into the nucleus tractus solitarius was totally unaffected by the GABAA receptor blockade with bicuculline. Since nipecotic acid acts by potentiating the action of synaptic GABA, and the GABA present presumably can interact with both GABAA and GABAB receptors, the observation that bicuculline did not block or attenuate the pressor response to nipecotic acid suggests that the pressor response to the enhanced synaptic action of GABA in the nucleus tractus solitarius can be produced independently of the involvement of GABAA receptors, and presumably is mediated through an action on GABAB receptors.

Contrasting effects of the GABA transaminase inhibitors gamma-vinylGABA and aminooxyacetic acid on arterial pressure following injection into nucleus tractus solitarius.

The effect of injection of inhibitors of GABA-transaminase into the nucleus tractus solitarius (NTS) on arterial pressure was examined in anesthetized rats. Two typical GABA-transaminase inhibitors, gamma-vinylGABA (GVG) and aminooxyacetic acid (AOAA), increased arterial pressure when injected into the NTS, a response similar to that elicited by direct stimulation of GABA receptors in the NTS. However, with neither drug was the response consistent with the action of inhibiting GABA-T. The response elicited by GVG, despite GVG acting as an irreversible inhibitor of GABA-T, could be reproduced with subsequent injections. Still, the pressor response to GVG could be reversed by blockade of GABAB-receptors with CGP-35348, suggesting that GVG injection did increase GABA-mediated neural transmission. In contrast, the pressor response to AOAA could not be antagonized by blockade of either GABAA or GABAB receptors. These results suggest that acute inhibition of GABA-T does not necessarily result in an increase in GABA-mediated neural transmission.

Contributions of arginine vasopressin and the sympathetic nervous system to fulminating hypertension after destruction of neurons of caudal ventrolateral medulla in the rat.

We sought to determine the role in control of arterial pressure (AP) of the caudal ventrolateral medulla (CVL), a region containing noradrenergic neurons of the A1 group. Electrical stimulation of the A1 area in anaesthetized rats elicits a fall of AP and heart rate at low, but not high, stimulus frequencies. Electrolytic lesions of the A1 area produce fulminating hypertension and a 13-fold increase in plasma arginine vasopressin (AVP). A1-hypertension is attenuated by treatment with an AVP antagonist and is also diminished in rats of the Brattleboro strain. After AVP blockade, the residual hypertension is abolished by treatment with ganglionic or alpha adrenergic blockers, but not by adrenalectomy. We conclude that the region of the CVL containing the A1 noradrenergic cells tonically inhibits the discharge of sympathetic nerve activity as well as the release of AVP from the pituitary.

Injection of corticotropin-releasing hormone into the locus coeruleus or foot shock increases neuronal Fos expression.

Previous research suggests that corticotropin-releasing hormone can act in the locus coeruleus to increase the firing of locus coeruleus neurons and elicit physiological responses resembling those associated with stress. The present study used immunocytochemical detection of Fos as a measure of neuronal activation to identify brain areas that were activated by bilateral injections of corticotropin-releasing hormone into the locus coeruleus of rats. Injection of corticotropin-releasing hormone into the locus coeruleus increased the expression of Fos in the locus coeruleus as compared with injection of vehicle into the locus coeruleus or injection of corticotropin-releasing hormone into neighbouring pontine sites. The pattern of Fos expression throughout the brain after injections of corticotropin-releasing hormone into the locus coeruleus was generally consistent with the anatomical organization of efferent projections arising from the locus coeruleus; increased Fos expression was observed in many brain areas including the ventral lateral septum, septohypothalamic nucleus, bed nucleus of the stria terminalis, the central amygdaloid nucleus, the dorsomedial nuclei of the hypothalamus, and the thalamic paraventricular and rhomboid nuclei. Foot shock also increased Fos expression in the locus coeruleus and the other brain regions that expressed Fos after corticotropin-releasing hormone injections into the locus coeruleus. A few brain regions, most notably the hypothalamic paraventricular nucleus, expressed Fos in response to foot shock but not corticotropin-releasing hormone. These results indicate that local injection of corticotropin-releasing hormone into the locus coeruleus stimulates the activity of the locus coeruleus neurons. However, the pattern of Fos expression throughout the brain evoked by injection of corticotropin-releasing hormone into the locus coeruleus does not fully replicate the effects of foot shock.

Corticotropin-releasing hormone receptor blockade fails to alter stress-evoked catecholamine release in prefrontal cortex of control or chronically stressed rats.

Although it is well documented that stress can increase the activity of central dopamine and norepinephrine neurons, little is known about the role of other neurotransmitters in modulating this response. Previous studies have implicated corticotropin-releasing hormone in modulating stress-evoked changes in the activity of locus coeruleus neurons. The present study examines whether corticotropin-releasing hormone contributes to stress-evoked increases in extracellular norepinephrine and dopamine in rat medial prefrontal cortex, as monitored by in vivo microdialysis. As noted previously, 30 min of tail-shock increased extracellular levels of norepinephrine and dopamine in the medial prefrontal cortex of naïve rats, and this was enhanced in rats previously exposed to chronic cold ( approximately 5 degrees C for 2-3 weeks). Previous intraventricular administration of a corticotropin-releasing hormone antagonist (D-Phe-corticotropin-releasing hormone; 3 and 9 microg) did not alter the tail-shock evoked in increase in extracellular levels of norepinephrine and dopamine in either naïve or chronically cold-exposed rats. Intraventricular administration of 3 microg of D-Phe-corticotropin-releasing hormone attenuated the increase in extracellular norepinephrine induced by co-administration of 3 microg of corticotropin-releasing hormone, confirming the efficacy of this compound. Results of the present study suggest that endogenous corticotropin-releasing hormone does not play a role in modulating the release of norepinephrine and dopamine occurring in response to acute tail-shock or the expression of a potentiated response to tail-shock in rats exposed chronically to cold.

Increased transcription of the tyrosine hydroxylase gene in individual locus coeruleus neurons following footshock stress.

Footshock-evoked change in transcriptional activity of tyrosine hydroxylase in neurons of the locus coeruleus was examined using an intron-specific in situ hybridization histochemical technique. A significant increase in the cellular concentration of tyrosine hydroxylase primary transcripts was found in locus coeruleus neurons 3h following 30 min of intermittent footshock. However, the footshock-induced increase in tyrosine hydroxylase transcription was not homogeneously expressed in locus coeruleus neurons. Similarly, administration of the alpha(2)-adrenergic receptor antagonist idazoxan produced a significant increase in the cellular concentration of tyrosine hydroxylase primary transcripts that was heterogeneously distributed among locus coeruleus neurons. Both footshock and idazoxan significantly increased the regional levels of tyrosine hydroxylase messenger RNA in the locus coeruleus. The time-course of changes in tyrosine hydroxylase transcription rate and messenger RNA levels in the locus coeruleus was examined after a 15 min exposure to footshock. A robust increase in tyrosine hydroxylase transcription rate was found at the end of 15 min of footshock, which remained elevated for 6h and was back to the control levels by 24h. In contrast, in response to a 15 min period of footshock tyrosine hydroxylase messenger RNA concentrations in the locus coeruleus did not increase until 6h and remained elevated at 24h. These findings demonstrate that transcription of the tyrosine hydroxylase gene in locus coeruleus neurons in response to footshock stress occurs rapidly, is sustained for many hours and is heterogeneously distributed. These data also suggest that the increase in tyrosine hydroxylase messenger RNA following footshock is mediated, at least in part, by an increase in tyrosine hydroxylase gene transcription.

Role of locus coeruleus in foot shock-evoked Fos expression in rat brain.

The robust activation of locus coeruleus neurons in response to a variety of stressors, in conjunction with the widespread outputs of the locus coeruleus, suggest that the locus coeruleus may be important in mediating responses to stress. Previous studies in rats have demonstrated that exposure to foot shock elicits Fos expression, a marker of neuronal activation, in the locus coeruleus and other brain sites. In order to evaluate the involvement of the locus coeruleus in foot shock-induced activation of other brain sites, shock-induced Fos expression was examined in the locus coeruleus and other brain areas known to be activated by foot shock, following direct inhibition of the locus coeruleus by local infusion of muscimol, a GABA agonist, prior to foot shock. Control rats received infusions of artificial cerebrospinal fluid into the locus coeruleus or muscimol into areas outside of locus coeruleus. Rats infused with artificial cerebrospinal fluid and then exposed to foot shock had significant increases in Fos expression in several brain areas, including locus coeruleus, nucleus O, several subdivisions of the hypothalamus, subnuclei of amygdala, bed nucleus of the stria terminalis and cingulate cortex. Inhibition of the locus coeruleus prior to foot shock significantly inhibited Fos expression in the locus coeruleus, nucleus O, some subdivisions of the hypothalamus including the magnocellular and medial parvicellular paraventricular hypothalamic nucleus, subnuclei of amygdala, and cingulate cortex. In contrast, inhibition of the locus coeruleus did not affect shock-induced Fos expression in other areas, including certain subdivisions of the hypothalamus and bed nucleus of the stria terminalis. We suggest that foot shock may activate multiple pathways, with activation of certain discrete nuclei requiring input from the locus coeruleus and activation of others occurring independently of locus coeruleus input.

The role of corticosteroids in nicotine's physiological and behavioral effects.

This paper reviews evidence indicating that adrenal corticosteroids modulate the responsiveness of mice and rats to nicotine. Adrenalectomy increases, and both acute and chronic corticosteroid administration decrease, some of the physiological and behavioral effects of nicotine. One function of adrenal steroids may be to regulate stress-induced changes in nicotine sensitivity. Another is to mediate the development of chronic tolerance when nicotine is given intermittently, and when the resulting tolerance has a learned component. A role of glucocorticoids in the development of tolerance to nicotine is suggested by the findings that a conditioned elevation of plasma corticosterone, which anticipates nicotine delivery, accompanies the development of chronic tolerance and that environmental cues evoke a conditioned corticosterone response, but only after they have become associated with nicotine delivery. The mechanisms by which adrenal steroids modulate nicotine sensitivity are not known, although recent in vitro evidence suggests that steroids can rapidly and reversibly reduce nicotinic receptor function. While most of the data are consistent with the hypothesis that corticosteroids reduce nicotine responsiveness, and thus promote a learned form of tolerance, there are new findings that corticosteroids increase the development of sensitization to the locomotor-activating effects of nicotine. These data suggest that formulations postulating a unidirectional effect of corticosteroids on nicotine's actions (e.g. decreased sensitivity) must be revised to take into account interacting variables such as the specific nicotine effect being studied and whether that effect normally exhibits tolerance or sensitization. Finally, research is presented which indicates that the corticosterone-elevating effects of nicotine, previously reported for experimenter-administered drug, are also produced when nicotine administration is contingent on an operant response, and at a dose which sustains the development of nicotine self-administration in rats. These findings highlight the feasibility of using self-administration models in future explorations of the relationship between adrenal steroids and nicotine function.