C1 catecholamine neurons form local circuit synaptic connections within the rostroventrolateral medulla of rat

C1 catecholamine neurons reside within the rostroventrolateral medulla (RVLM), an area that plays an integral role in blood pressure regulation through reticulospinal projections to sympathetic preganglionic neurons in the thoracic spinal cord. In a previous investigation we mapped the efferent projections of C1 neurons, documenting supraspinal projections to cell groups in the preautonomic network that contribute to the control of cardiovascular function. Light microscopic study also revealed putative local circuit connections within RVLM. In this investigation we tested the hypothesis that RVLM C1 neurons elaborate a local circuit synaptic network that permits communication between C1 neurons giving rise to supraspinal and reticulospinal projections. A replication defective lentivirus vector that expresses enhanced green fluorescent protein (EGFP) under the control of a synthetic dopamine beta hydroxylase (DβH) promoter was used to label C1 neurons and their processes. Confocal fluorescence microscopy demonstrated thin varicose axons immunopositive for EGFP and tyrosine hydroxylase that formed close appositions to C1 somata and dendrites throughout the rostrocaudal extent of the C1 area. Dual-labeled electron microscopic analysis revealed axosomatic, axodendritic and axospinous synaptic contacts with C1 and non-C1 neurons with a distribution recapitulating that observed in the light microscopic analysis. Labeled boutons were large, contained light axoplasm, lucent spherical vesicles, and formed asymmetric synaptic contacts. Collectively these data demonstrate that C1 neurons form a synaptic network within the C1 area that may function to coordinate activity among projection-specific subpopulations of neurons. The data also suggest that the boundaries of RVLM should be defined on the basis of function criteria rather than the C1 phenotype of neurons.

c-Fos Expression in Rat Brain and Brainstem Nuclei in Response to Treatments That Alter Food Intake and Gastric Motility

Expression of the proto-oncogene protein c-Fos was evaluated immunocytochemically in individual brain cells as a marker of treatment-related neuronal activation following pharmacological and physiological treatments that are known to alter food intake and gastric motility in rats. c-Fos expression in response to each treatment was analyzed in the brainstem dorsal vagal complex, the limbic system, and the hypothalamus, representing the areas thought to be involved in coordinating the autonomic, behavioral, and neuroendocrine responses that occur during conditions of stimulated or inhibited food intake. Our results indicate that the patterns of brain c-Fos expression associated with treatments that inhibit food intake differ significantly from the patterns produced by treatments that potentiate food intake. Treatments that inhibited food intake (administration of the anorexigenic agents cholecystokinin, LiCl, and hypertonic saline, and food ingestion following fasting or insulin treatment) were associated with widespread stimulation of c-Fos expression in cells in the nucleus tractus solitarius (NTS), and to a more variable degree the area postrema (AP), but without significant activation of neurons in the dorsal motor nucleus of the vagus nerve (DMN). In contrast, treatments that potentiated food intake (food deprivation and insulin-induced hypoglycemia) were associated with marked stimulation of c-Fos expression in DMN neurons, but little or no activation in cells in the NTS or the AP. Pharmacological treatments that inhibited food intake and gastric motility also were associated with pronounced c-Fos expression in several forebrain areas, including the parvocellular and magnocellular subdivisions of the paraventricular nucleus of the hypothalamus (PVN), the central nucleus of the amygdala (CeA), and the bed nucleus of the stria terminalis (BNST). In contrast, more physiological inhibition of food intake resulting from spontaneous food ingestion did not cause significant activation of c-Fos expression in these forebrain regions, nor did treatments that stimulated food intake. Central administration of oxytocin, which also is known to inhibit food intake, was associated with a pattern of c-Fos activation analogous to that produced by spontaneous food ingestion after fasting (c-Fos expression in the NTS and AP, but without significant activation in the DMN or forebrain areas). Finally, acute gastric distension produced complex results, in that it was associated with activation of c-Fos expression in all areas of the brainstem (NTS, AP, and DMN), as well as in multiple forebrain areas (PVN, CeA, and BNST). Our results therefore demonstrate that specific patterns of brain c-Fos expression are associated with treatments that alter food intake in rats, and indicate that assessment of c-Fos immunoreactivity in different brain areas can identify common functional neuroanatomical networks that are activated by diverse treatments which nonetheless produce similar behavioral, autonomic, and neuroendocrine effects in animals.

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).

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.

Cue dependency of nicotine self-administration and smoking

A paradox exists regarding the reinforcing properties of nicotine. The abuse liability associated with smoking equals or exceeds that of other addictive drugs, yet the euphoric, reinforcing and other psychological effects of nicotine, compared to these other drugs, are more subtle, are manifest under more restricted conditions, and do not readily predict the difficulty most smokers experience in achieving abstinence. One possible resolution to this apparent inconsistency is that environmental cues associated with drug delivery become conditioned reinforcers and take on powerful incentive properties that are critically important for sustaining smoking in humans and nicotine self-administration in animals. We tested this hypothesis by using a widely employed self-administration paradigm in which rats press a lever at high rates for 1 h/day to obtain intravenous infusions of nicotine that are paired with two types of visual stimuli: a chamber light that when turned on signals drug availability and a 1-s cue light that signals drug delivery. We show that these visual cues are at least as important as nicotine in sustaining a high rate of responding once self-administration has been established, in the degree to which withdrawing nicotine extinguishes the behavior, and in the reinstatement of lever pressing after extinction. Additional studies demonstrated that the importance of these cues was manifest under both fixed ratio and progressive ratio (PR) schedules of reinforcement. The possibility that nicotine-paired cues are as important as nicotine in smoking behavior should refocus our attention on the psychology and neurobiology of conditioned reinforcers in order to stimulate the development of more effective treatment programs for smoking cessation.

Area postrema lesion alters the effects of peptide YY on 2-DG-stimulated pancreatic exocrine secretion

Previously we demonstrated that circulating peptide YY (PYY), which inhibits pancreatic exocrine secretion, binds to specific receptors in the area postrema (AP); therefore we have tested the hypothesis that the removal of the AP (APX) will alter the effects of PYY on pancreatic secretion in awake rats. One-month after AP lesion or sham lesion, rats were implanted with pancreatic, biliary, duodenal, and intravenous catheters. After recovery from the surgery, unanesthetized rats were infused with vehicle or PYY (30 pmol/kg/hr or 100 pmol/kg/hr) under basal or 2-deoxy-D-glucose (2-DG) stimulated (75 mg/kg, intravenous bolus) conditions. PYY at 30 pmol/kg/hr inhibited basal pancreatic fluid secretion in sham-operated rats, but not APX rats. PYY at 100 pmol/kg/hr stimulated basal pancreatic protein secretion in sham-operated rats, and this effect was also lost in APX rats. PYY at 30 and 100 pmol/kg/hr inhibited peak 2-DG stimulated protein secretion to a greater extent in APX rats as compared to sham-operated rats (P < 0.05). Since PYY inhibition of basal pancreatic secretion is AP dependent and inhibition of 2-DG stimulated pancreatic secretion is AP independent, we conclude that the 2-DG pathway of pancreatic secretion differs from the pathway responsible for basal secretion, and that APX potentiates the inhibitory effect of PYY on the 2-DG pathway.

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.

The area postrema lesions alter the inhibitory effects of peripherally infused pancreatic polypeptide on pancreatic secretion

Circulating PP binds to specific receptors in the DVC through the AP, but the mechanism through which these brain receptors affect pancreatic secretion is not clear. We hypothesize that the removal of the AP (APX) will alter the effects of PP on pancreatic secretion. APX or sham procedures were performed in anesthetized male Wistar rats. After a 1-month recovery, one group of rats were infused with either PP (30 or 100 pmol/kg per h) or vehicle under basal or 2-DG-stimulated (75 mg/kg, i.v. bolus) conditions for studying pancreatic exocrine secretion. A second parallel group was sacrificed for examination of PP receptor binding in the brain stem. A third group received an intraperitoneal injection of PP at the dose of 4.15x10(4) pmol/kg (200 microg/kg) and c-fos expression in the brain stem was examined. APX eliminated PP binding sites in the DVC as assessed by autoradiography. PP infusion caused a dose-dependent decrease in basal protein secretion. APX partially reversed PP inhibition of basal protein secretion when infused at 30 pmol/kg per h, and at 100 pmol/kg per h stimulated pancreatic fluid secretion and reversed the inhibition of protein secretion. During 2-DG stimulation the effects of PP and 2-DG on pancreatic fluid and protein secretion were parallel. PP dose-dependently inhibited 2-DG-stimulated secretion in sham rats. APX reduced the pancreatic fluid (54%) and protein (46%) secretory response to 2-DG. However, PP at 30 pmol/kg per h remained a potent inhibitor of 2-DG-stimulated pancreatic secretion in APX rats. This effect was blunted with PP at 100 pmol/kg per h in APX rats, possibly related to the stimulatory effect of high-dose PP in APX rats without 2-DG. Furthermore, i.p. PP induced significantly greater c-fos activation of NTS neurons in APX rats than sham rats, despite the apparent absence of PP binding sites in the DVC. We conclude that in awake rats, PP inhibits basal secretion, in part, through the AP. Furthermore, and unlike PYY, PP inhibits 2-DG-stimulated pancreatic secretion, and it does so through an AP-independent mechanism. The possibility that the mechanism may involve the DVC cannot be excluded since i.p. injection of PP activates c-fos expression in DVC neurons. Thus, PP and PYY may regulate different components of the pancreatic secretory control system through unique pathways.

PYY inhibits CCK-stimulated pancreatic secretion through the area postrema in unanesthetized rats

Peptide YY (PYY) inhibits CCK-8-secretin-stimulated pancreatic secretion in vivo. To investigate whether CCK-8-secretin-stimulated pancreatic secretion is mediated through a vago-vagal pathway and whether PYY inhibits this pathway through the area postrema (AP), chronic pancreatic, biliary, and duodenal catheters were implanted in AP-lesioned (APX) or sham-operated rats. The effects of APX on pancreatic secretion stimulated by bethanechol, pancreatic juice diversion (PJD), or CCK-8-secretin, were tested, with and without background PYY infusion, in unanesthetized rats. APX reduced basal pancreatic secretion by 15-20% (P < 0.01). APX had no effect on bethanechol-stimulated secretion and potentiated protein secretion stimulated by PJD (396 vs. 284%) and exogenous CCK-8-secretin. In sham-operated rats, background PYY potently inhibited CCK-8-secretin-stimulated pancreatic fluid (1.8 vs. 48.2%) and protein secretion (3.7 vs. 45.8%) but potentiated fluid (52.9 vs. 43.1%) and protein (132.9 vs. 68.9%) secretion in APX rats. Our findings demonstrate that PYY inhibits CCK-8-secretin-stimulated pancreatic secretion through an AP-dependent mechanism in sham-operated rats. The AP also contributes to basal pancreatic secretion.

Excitatory inputs to the RVLM in the context of the baroreceptor reflex

The central neural circuit mediating baroreceptor control of sympathetic vasomotor outflow involves an excitatory projection from arterial baroreceptors to nucleus tractus solitarius, an excitatory projection from nucleus tractus solitarius to the caudal ventrolateral medulla, an inhibitory projection from the caudal ventrolateral medulla to the rostral ventrolateral medulla (RVLM), and an excitatory projection from the RVLM to sympathetic preganglionic neurons in the spinal cord. For this circuit to be operational, the relevant neurons in the RVLM must be tonically active. Indeed, numerous studies have demonstrated that RVLM vasomotor neurons are tonically active; however, little is known regarding the nature of the tonic excitatory drive to these neurons. We present a model in which RVLM vasomotor neurons are tonically excited by inputs to the RVLM that can be blocked by the excitatory amino acid receptor antagonist, kynurenic acid, as well as an input from the caudal ventrolateral medulla that is not sensitive to kynurenic acid.

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.

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.

Oxytocin antagonist disrupts hypotension-evoked renin secretion and other responses in conscious rats

Previous experiments have indicated that arterial hypotension increases plasma oxytocin (OT) levels in rats and that OT infused intravenously causes an increase in plasma renin activity (PRA). The goal of the present study was to determine whether systemic administration of an OT receptor antagonist would attenuate the increase in PRA that is normally evoked by arterial hypotension in rats. In conscious male rats, intravenous injection of hydralazine or diazoxide produced sustained hypotension and evoked a significant increase in PRA, as expected. Intravenous infusion of an OT receptor antagonist did not alter the hypotension induced by hydralazine or diazoxide, but it did markedly blunt the induced increase in PRA. The OT receptor antagonist also blunted the hypotension-evoked increase in heart rate and plasma vasopressin levels, suggesting that the antagonist may have generally disrupted afferent signaling of hypotension. Thus hypotension-evoked OT secretion may contribute to cardiovascular homeostasis by enhancing baroreceptor signals that stimulate increases in renin secretion, vasopressin secretion, and heart rate during arterial hypotension in rats.

Osmoregulation in water-deprived rats drinking hypertonic saline: effect of area postrema lesions

Rats drank rapidly when 0.3 M NaCl was the only drinking fluid available after overnight water deprivation, consuming approximately 200 ml/24 h. Although such large intakes of this hypertonic solution initially elevated plasma osmolality, excretion of comparable volumes of urine more concentrated than 300 meq Na(+)/l ultimately appears to restore plasma osmolality to normal levels. Rats drank approximately 100 ml of 0.5 M NaCl after overnight water deprivation, but urine Na(+) concentration (U(Na)) did not increase sufficiently to achieve osmoregulation. When an injected salt load exacerbated the initial dehydration caused by water deprivation, rats increased U(Na) to void the injected load and did not significantly alter 24-h intake of 0.3 or 0.5 M NaCl. Rats with lesions of area postrema had much higher saline intakes and lower U(Na) than did intact control rats; nonetheless, they appeared to osmoregulate well while drinking 0.3 M NaCl but not while drinking 0.5 M NaCl. Detailed analyses of drinking behavior by intact rats suggest that individual bouts were terminated by some rapid postabsorptive consequence of the ingested NaCl load that inhibited further NaCl intake, not by a fixed intake volume or number of licks that temporarily satiated thirst.

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.

Tonic excitatory input to the rostral ventrolateral medulla in Dahl salt-sensitive rats

The goal of the present study was to test the hypothesis that the balance of tonic excitation and inhibition of vasomotor neurons in the rostral ventrolateral medulla (RVLM) driven by excitatory amino acid (EAA)-mediated inputs to the RVLM is shifted toward excitation in Dahl salt-sensitive (DS) rats compared with Dahl salt-resistant (DR) rats. Glutamate and the EAA antagonist kynurenic acid were microinjected into the RVLM of chloralose-anesthetized DS and DR rats maintained on diets containing either 0.3% NaCl or 8.0% NaCl. DS rats had a higher arterial pressure than DR rats, and this difference was greatly exaggerated by high dietary salt intake. Bilateral injection of kynurenic acid (2.7 nmol) into the RVLM decreased mean arterial pressure by 16+/-2 mm Hg in DS rats fed a diet containing 0.3% NaCl, and this effect was significantly larger in DS rats fed the high-salt diet (40+/-2 mm Hg). In contrast, injections of kynurenic acid into the RVLM did not significantly decrease arterial pressure in DR rats fed either diet. In DR rats, the pressor response elicited by the injection of glutamate into the RVLM was potentiated in rats fed the high-salt diet. The glutamate-evoked pressor response was greater in DS rats compared with DR rats, and the response in DS rats was not influenced by the salt content of the diet. These data suggest that tonically active EAA inputs to the RVLM may contribute to salt-sensitive hypertension in the Dahl model.

Tonic excitatory input to the rostral ventrolateral medulla in Dahl salt-sensitive rats

The goal of the present study was to test the hypothesis that the balance of tonic excitation and inhibition of vasomotor neurons in the rostral ventrolateral medulla (RVLM) driven by excitatory amino acid (EAA)-mediated inputs to the RVLM is shifted toward excitation in Dahl salt-sensitive (DS) rats compared with Dahl salt-resistant (DR) rats. Glutamate and the EAA antagonist kynurenic acid were microinjected into the RVLM of chloralose-anesthetized DS and DR rats maintained on diets containing either 0.3% NaCl or 8.0% NaCl. DS rats had a higher arterial pressure than DR rats, and this difference was greatly exaggerated by high dietary salt intake. Bilateral injection of kynurenic acid (2.7 nmol) into the RVLM decreased mean arterial pressure by 16+/-2 mm Hg in DS rats fed a diet containing 0.3% NaCl, and this effect was significantly larger in DS rats fed the high-salt diet (40+/-2 mm Hg). In contrast, injections of kynurenic acid into the RVLM did not significantly decrease arterial pressure in DR rats fed either diet. In DR rats, the pressor response elicited by the injection of glutamate into the RVLM was potentiated in rats fed the high-salt diet. The glutamate-evoked pressor response was greater in DS rats compared with DR rats, and the response in DS rats was not influenced by the salt content of the diet. These data suggest that tonically active EAA inputs to the RVLM may contribute to salt-sensitive hypertension in the Dahl model.

Neuroanatomical specificity of the circuits controlling sympathetic outflow to different targets

1. Despite the emerging framework that central neural pathways controlling the activity of the sympathetic nervous system are capable of producing highly selective responses, the specific neural pathways governing different sympathetic outflows are poorly understood. 2. Anatomical studies suggest that five brain areas, namely the rostral ventrolateral medulla, the rostral ventromedial medulla, the caudal raphe nuclei, the region containing the A5 noradrenergic neurons and the paraventricular hypothalamic nucleus, provide dominant supraspinal innervation of sympathetic preganglionic neurons. 3. The anatomical parcellation of different functions within and among these cell groups is uncertain. However, recent studies using transynaptic retrograde labelling of neural pathways connected to various sympathetic targets suggest that the circuits controlling these different targets may be partially distinct. Similarly, anatomical studies relying on stimulus-evoked expression of immediate early genes, such as c-fos, suggest that different sympathetic responses may be controlled by distinct, neural circuits. 4. Thus, although many similarities exist in the anatomical circuits innervating different sympathetic targets, possibly supporting the orchestration of global sympathetic responses, differences are also discernible.

Acute hypertension inhibits thirst stimulated by ANG II, hyperosmolality, or hypovolemia in rats

The present study sought to determine whether increases in arterial blood pressure inhibited drinking behavior evoked by ANG II, hyperosmolality, or hypovolemia in rats. Cumulative water intakes in 60- or 90-min tests and latency to the first lick were recorded as indexes of thirst. During intravenous infusions of 100 ng. kg(-1). min(-1) ANG II, attenuation of the induced increases in arterial pressure with the arteriolar vasodilator diazoxide resulted in greater water intakes and shorter latencies to drink. Drinking behavior stimulated by intravenous infusion of hypertonic saline was significantly inhibited by increases in arterial pressure caused by intravenous infusion of phenylephrine or endothelin-1, and this inhibition of drinking was proportional to the induced increase in pressure. Upon termination of the phenylephrine infusion, mean arterial pressure returned to basal values, and drinking was restored. Phenylephrine-induced increases in arterial pressure also inhibited drinking behavior in response to hypovolemia that could not be explained by differences in plasma renin activity, plasma protein concentration, or plasma osmolality. Thus increases in arterial pressure inhibit water drinking behavior in response to each of these three thirst stimuli in rats.

Pharmacological profile of depressor response elicited by sarthran in rat ventrolateral medulla

Injection of sarthran, an angiotensin receptor antagonist, bilaterally into the rostral ventrolateral medulla (RVLM) of alpha-chloralose-anesthetized rats decreases arterial pressure (AP) to the same extent as total autonomic blockade. This response is not reproduced by selective AT(1) antagonists. To examine the pharmacological profile of the response elicited by [Sar(1), Thr(8)]ANG II (sarthran), the ability of angiotensin analogs to inhibit the effect of sarthran injected into the RVLM was tested. Coinjection of angiotensin II (ANG II) prevented the sarthran-evoked decrease in AP, but this action of ANG II was markedly attenuated by pretreatment of the RVLM with the aminopeptidase inhibitor amastatin. Coinjection of ANG(3-8) or a selective agonist of AT(4) receptors prevented the effect of sarthran injected into the RVLM. ANG(1-7) was also able to prevent the effect of sarthran. None of the angiotensin fragments tested substantially altered blood pressure when injected alone into the RVLM. These results suggest that the depressor action of sarthran injected into the RVLM is not dependent on ANG II receptors, though the nature of the site or sites of action of sarthran within the RVLM remains uncertain.

Thirst

The homeostasis of body fluid traditionally is viewed as involving the regulation of its osmolality and of blood volume. However, the control of thirst is more complex than can be described in a two-factor model, and consideration of plasma sodium concentration and of arterial blood pressure also must be included in the discussion. This review is organized around those four variables and focuses on the seven distinct signals that appear to influence water intake in rats. These signals include four that are excitatory for thirst: increased plasma osmolality detected by cerebral osmoreceptors, decreased blood volume presumably detected by cardiac stretch receptors, increased circulating levels of angiotensin II detected by angiotensin II receptors in the subfornical organ, and increased gastric sodium load apparently detected by putative sodium receptors in the abdominal viscera. There also appear to be three signals that inhibit thirst: decreased plasma osmolality detected by cerebral osmoreceptors, increased arterial blood pressure detected by arterial baroreceptors, and increased gastric water load apparently detected by putative sodium receptors in the abdominal viscera.

Water ingestion provides an early signal inhibiting osmotically stimulated vasopressin secretion in rats

Dehydrated dogs are known to inhibit secretion of vasopressin (VP) within minutes after drinking water, before plasma osmolality (P(osmol)) diminishes. The present studies determined whether water ingestion causes a similar rapid inhibition of neurohypophyseal hormone secretion in rats. Adult rats were infused with 1 M NaCl (2 ml/h iv) for 240 min to stimulate VP and oxytocin (OT) secretion. After 220 min of infusion, rats were given water to drink for 5 min, and blood samples were taken 5 and 15 min later for RIA. Plasma VP (pVP) was much lower when rats ingested water than when they drank nothing even though P(osmol) was not significantly altered. Plasma OT (pOT) was affected similarly. In contrast, no effects on pVP or pOT occurred when rats drank isotonic NaCl solution for 5 min in amounts comparable to the water intakes (approximately 5.5 ml). These results suggest that neurohypophyseal secretion of VP and OT in rats is inhibited rapidly by water drinking, and that this inhibition is mediated by a visceral signal of osmotic dilution rather than by the act of drinking per se.

Differential effects of response-contingent and response-independent nicotine in rats

Passive administration of nicotine activates the hypothalamic-pituitary-adrenocortical axis and sympathetic nervous system. However, little is known about the effects of self-administered nicotine. Drug-naive rats were trained to respond for food reinforcement and then tested in one, 1-h session in which they received response-contingent i.v. nicotine or response-independent i.v. nicotine or saline. Blood draws were taken immediately prior to the session, 15 min after the first infusion and immediately after the session. Both response-contingent and response-independent nicotine (RI/N) increased corticosterone within 15 min, however, corticosterone levels returned to baseline in animals receiving response-contingent nicotine (RC/N) by the end of the session while remaining elevated in those receiving RI/N. Furthermore, only RI/N increased plasma epinephrine and norepinephrine levels; RC/N produced no effect. These differences indicate that nicotine's acute effects are powerfully modified by the presence of a contingency relationship between drug administration and the animal's behavior and that this relationship develops very rapidly.

Role of renin-angiotensin system in hypotension-evoked thirst: studies with hydralazine

Injection of rats either with diazoxide (25 mg/kg iv), isoproterenol (0.33 mg/kg sc), or hydralazine (HDZ) (10 mg/kg ip) decreased arterial blood pressure from approximately 120 to 70-80 mmHg and stimulated renin secretion. However, diazoxide and isoproterenol treatments each stimulated water ingestion, whereas HDZ treatment did not. HDZ treatment did not reduce water intake evoked by systemic injection of hypertonic saline or 20% polyethylene glycol solution or by 24-h water deprivation, suggesting that HDZ treatment did not interfere with drinking behavior. In contrast, HDZ treatment markedly reduced water intake evoked by injection of diazoxide or isoproterenol or by intravenous infusion of renin. Furthermore, a highly significant correlation was observed when plasma ANG II levels were plotted as a function of plasma renin activity after intravenous infusion of renin and after diazoxide and isoproterenol treatments. However, values obtained after HDZ treatment alone or in combination with intravenous infusion of renin did not fall near the 99% confidence interval of the regression line, suggesting that HDZ treatment blocks ANG II production and/or promotes its clearance. Thus rats apparently do not increase water intake after HDZ treatment, because this drug interferes with the renin-angiotensin system. These results provide further evidence that arterial hypotension evokes thirst in rats predominantly by activation of the renin-angiotensin system.

Nucleus of the solitary tract lesions enhance drinking, but not vasopressin release, induced by angiotensin

Rats with chronic nucleus of the solitary tract lesions (NTS-X) drink water and release vasopressin (VP) in response to reduced blood volume despite an absence of neural signals from cardiac and arterial baroreceptors. The present study determined whether rats with NTS-X have a greater sensitivity to circulating ANG II, which may contribute to the drinking and VP responses to hypovolemia. In conscious control rats and rats with NTS-X, ANG II was infused intravenously for 1 h at 10, 100, or 250 ng. kg(-1). min(-1). At the two higher doses, ANG II stimulated more water intake with a shorter latency to drink in rats with NTS-X than in control rats. In contrast, infusion of ANG II produced comparable increases in plasma VP in the two groups. At the two higher doses, ANG II produced an enhanced increase in arterial pressure (AP) in rats with NTS-X, and the bradycardia seen in control rats was reversed to a tachycardia. Infusion of hypertonic saline, which did not alter AP or heart rate, produced comparable drinking and VP release in the two groups. These results demonstrate that chronic NTS-X increases the dipsogenic response of rats to systemic ANG II but has no effect on ANG II-induced VP release or the osmotic stimulation of these responses.

Mecamylamine prevents tolerance but enhances whole brain [3H]epibatidine binding in response to repeated nicotine administration in rats

RATIONALE

Chronic administration of nicotine in rats results in upregulation of neuronal nicotinic receptors. Upregulation has been proposed to reflect receptor desensitization, which may underlie functional tolerance to nicotine's effects. However, evidence indicates that tolerance and upregulation do not always parallel each other, suggesting that either upregulation does not always reflect desensitization, or mechanisms other than receptor desensitization account for tolerance to nicotine.

OBJECTIVES

The present studies examined tolerance to nicotine-induced antinociception and changes in receptor binding after two regimens of intermittent nicotine injections in rats. The role of receptor activation in upregulation and tolerance was also examined by co-administering nicotine with the non-competitive antagonist, mecamylamine.

METHODS

Sprague-Dawley rats were administered a short (once-daily, s.c. for 6 days (0.35 mg/kg)) or long (twice-daily for 11 days (0.66 mg/kg)) series of injections and tolerance to nicotine-induced antinociception and [3H]epibatidine binding in whole brain were measured.

RESULTS

The short series of injections resulted in tolerance to nicotine-induced antinociception, but failed to increase [3H]epibatidine binding. In contrast, the long series of injections resulted in both tolerance and increased receptor binding. Once-daily pairings of mecamylamine (1 mg/kg, s.c.) with nicotine (0.35 mg/kg) for 6 days blocked the development of tolerance, indicating receptor activation is necessary for tolerance to occur. Pairing mecamylamine with nicotine (0.66 mg/kg) twice daily for 11 days blocked tolerance but produced a greater increase in [3H]epibatidine binding than nicotine alone.

CONCLUSIONS

A dissociation of tolerance from receptor upregulation was observed in the present study. The finding that receptor activation may be necessary for tolerance but not upregulation is discussed within the context of possible mechanisms controlling tolerance to nicotine.

Enhanced serotonin-mediated responses in the nucleus tractus solitarius of spontaneously hypertensive rats

Previous studies have demonstrated that injection of serotonin into the nucleus tractus solitarius (NTS) elicits hypotension and bradycardia in rats. The present study sought to further characterize this response and to examine the role of serotonergic mechanisms in the NTS in cardiovascular regulation in spontaneously hypertensive (SHR) rats. Injections of picomole amounts of serotonin into the NTS of chloralose-anesthetized normotensive Sprague-Dawley (S-D) or Wistar-Kyoto (WKY) rats produced hypotension and bradycardia that were eliminated by prior injection into the NTS of the selective 5HT(2) antagonist sarpogrelate. Bilateral injection of sarpogrelate did not alter blood pressure or reflex changes in heart rate in response to phenylephrine-induced increases in blood pressure or nitroprusside-induced decreases in blood pressure. In SHR rats, the depressor response produced by injection of serotonin into the NTS was markedly larger than in WKY rats, and was larger than depressor responses previously reported for other excitatory substances injected into the NTS. In SHR rats bilateral injection of sarpogrelate produced an increase in blood pressure, although it did not alter baroreceptor-evoked changes in heart rate. These results provide further support for the hypothesis that stimulation of 5HT(2) receptors in the NTS contributes to cardiovascular regulation independent of the baroreceptor reflex. Furthermore, this serotonergic system is altered in SHR rats, apparently acting tonically to reduce blood pressure.

Connections of Barrington's nucleus to the sympathetic nervous system in rats

Barrington's nucleus (BN) has been considered a pontine center related exclusively to the control of pelvic parasympathetic activity. The present study demonstrates an anatomical linkage between BN and autonomic outflow to visceral targets innervated exclusively by the sympathetic division of the autonomic nervous system. Temporal analysis of infection after injection of pseudorabies virus (PRV), a retrograde transynaptic tracer, into two sympathetically innervated organs, the spleen and the kidney, revealed the presence of infected neurons in BN at early post-inoculation survival intervals. Immunohistochemical localization of PRV after spleen injections showed that a small subpopulation of BN neurons became labeled in a time frame coincident with the appearance of infected neurons in other brain regions known to project to sympathetic preganglionic neurons (SPNs) in the thoracic spinal cord; a larger number of infected neurons appeared in BN at intermediate intervals after PRV injections into the spleen or kidney. Coinjection of the retrograde tracer Fluoro-Gold i.p. and PRV into the spleen demonstrated that parasympathetic preganglionic neurons in the caudal medulla or lumbo-sacral spinal cord were not infected, indicating that infected BN neurons were not infected via a parasympathetic route. Thus, BN neurons become infected after PRV injections into the spleen or kidney either directly through BN projections to SPNs, or secondarily via BN projections to infected pre-preganglionic neurons. These results demonstrate an anatomical linkage, either direct or indirect, between BN and sympathetic activity. Because BN receives numerous inputs from diverse brain regions, the relation of BN with both branches of the autonomic nervous system suggests that this nucleus might play a role in the integration of supraspinal inputs relevant to the central coordination of sympathetic and parasympathetic activity.

Vasopressin and oxytocin release evoked by NaCl loads are selectively blunted by area postrema lesions

The present study investigated the effect of area postrema lesions (APX) on stimulated neurohypophysial secretion of vasopressin (VP) and oxytocin (OT) in conscious rats. Blunted increases in plasma levels of both pituitary hormones were observed when rats with APX were infused intravenously with 1 M NaCl solution (2 ml/h for 6 h). In contrast, plasma VP and OT increased normally in rats with APX when equivalent increases in plasma osmolality (but not plasma Na(+)) resulted from intravenous infusion of an equiosmotic solution of 1 M mannitol and 0.5 M NaCl. Furthermore, APX did not affect increases in plasma VP and OT stimulated by plasma volume deficits, nor did APX disrupt OT secretion stimulated by intravenous injection of cholecystokinin. These findings suggest that the area postrema plays an important role in mediating secretion of VP and OT in response to an NaCl load, but not in response to an equiosmotic load that does not cause substantial hypernatremia, and not in response to other stimuli of neurohypophysial hormone secretion. Together with previous reports, these results suggest that APX impairs Na(+) regulation in rats.

Baroreflex dependent and independent roles of the caudal ventrolateral medulla in cardiovascular regulation

The caudal ventrolateral medulla (CVLM) plays a critical role in cardiovascular regulation. Convincing data now support the hypothesis that inhibition of sympathoexcitatory neurons in the rostral ventrolateral medulla (RVLM) by CVLM neurons constitutes the necessary inhibitory link in baroreceptor reflex mediated control of sympathetic vasomotor outflow. Inhibition or destruction of the CVLM produces severe acute hypertension, consistent with blockade of baroreceptor reflexes and withdrawal of inhibition of RVLM sympathoexcitatory neurons. However, other data indicate that the CVLM also tonically inhibits RVLM sympathoexcitatory neurons in a manner not driven by baroreceptor input. In some studies, inhibition of the CVLM results in an increase in arterial pressure (AP) without inhibiting baroreceptor reflexes, possibly reflecting baroreceptor-independent and baroreceptor-dependent sub-regions of the CVLM. Furthermore, in baroreceptor-denervated rats, inhibition of the CVLM still leads to large increases in AP. In addition, in spontaneously hypertensive rats (SHR) central processing of baroreceptor reflexes appears normal but CVLM-mediated inhibition of the RVLM seems to be attenuated, suggesting that it is specifically a baroreceptor-independent mechanism of cardiovascular regulation in SHR that is altered. Taken together, these findings support an important, tonic, baroreceptor-independent inhibition of RVLM sympathoexcitatory neurons exerted by the CVLM.

Analysis of tyrosine hydroxylase gene transcription using an intron specific probe

The nuclear run-on assay is the most commonly used technique to determine transcription rates of specific genes such as tyrosine hydroxylase. Its application to studies in the nervous system is problematic, however, as a result of limitations in sensitivity and the loss of anatomical integrity. We observed that the relative levels of tyrosine hydroxylase intron 2-containing RNA using a ribonuclease protection assay in the adrenal medulla changed in response to pharmacological treatments consistently with changes shown by the nuclear run-on assay. Our results indicate that measures of tyrosine hydroxylase primary transcript levels offer an alternative to the nuclear run-on assay and validate the application of intron-specific in situ hybridization as a means of assessing the relative transcriptional activity of the tyrosine hydroxylase gene. Similar quantitative results were obtained using intron-specific in situ hybridization with oligonucleotide probes specific for rat tyrosine hydroxylase intron 2. Furthermore, we observed that intron-specific in situ hybridization could be used to measure tyrosine hydroxylase transcription rates in the locus coeruleus, providing resolution at the level of single neurons. Thus, measuring the levels of tyrosine hydroxylase intron 2 provides a sensitive measure of tyrosine hydroxylase transcription rate that can be applied to the study of brain catecholaminergic neurons.

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.

Oxytocin-induced renin secretion in conscious rats

Arterial hypotension and hypovolemia are known to stimulate neurohypophysial secretion of oxytocin (OT) in rats, although the physiological function of OT under these circumstances is uncertain. We now report that OT infused intravenously into conscious rats at 125 ng x kg(-1) x h(-1), a dose selected to mimic plasma OT levels during hypotension or hypovolemia, increased plasma renin concentration and plasma renin activity by twofold. This effect was prevented by systemic pretreatment with an OT receptor antagonist [[1-(3-mercaptopropionic acid)-2-O-ethyl-D-Tyr-Thr(4)-Orn(8)]-OT]. The OT antagonist did not block renin secretion induced by systemic injection of the beta-adrenergic receptor agonist isoproterenol, indicating that the OT antagonist does not interfere nonselectively with renin release. Pretreatment of rats with the beta-adrenergic receptor antagonist nadolol also prevented OT-induced renin secretion. Similarly, nadolol injected during infusion of OT markedly reduced the elevated plasma renin levels. These observations raise the possibility that pituitary OT secretion during hypotension or hypovolemia in rats may serve to support blood pressure by enhancing activation of the renin-angiotensin system via a beta-adrenergic receptor-dependent mechanism.

Lesions of the C1 catecholaminergic neurons of the ventrolateral medulla in rats using anti-DbetaH-saporin

Phenylethanolamine-N-methyltransferase (PNMT)-containing neurons in the rostral ventrolateral medulla (RVLM) are believed to play a role in cardiovascular regulation. To determine whether injection of anti-dopamine beta-hydroxylase (DbetaH)-saporin directly into the RVLM in rats could selectively destroy these cells and thereby provide an approach for evaluating their role in cardiovascular regulation, we studied rats 2 wk after unilateral injection of 21 ng anti-DbetaH-saporin into the RVLM. There was an approximately 90% reduction in the number of PNMT-positive neurons in the RVLM, although the number of non-C1, spinally projecting barosensitive neurons of this area was not altered. The A5 cell group was the only other population of DbetaH-containing cells that was significantly depleted. The depressor response evoked by injection of tyramine into the RVLM was abolished by prior injection of toxin. The pressor response evoked by injection of glutamate into the RVLM was attenuated ipsilateral to the toxin injection but was potentiated contralateral to the toxin injection. Thus anti-DbetaH-saporin can be used to make selective lesions of PNMT-containing cells, allowing for the evaluation of their role in cardiovascular regulation.

Impaired osmoregulatory responses in rats with area postrema lesions

Area postrema lesions (APX) in adult male rats produced a robust spontaneous intake of 0.5 M NaCl, as reported previously. The largest NaCl intakes (up to 108 ml/day) were observed when there was little incidental damage in the medial subnucleus of the nucleus of the solitary tract adjacent to the caudal and middle portions of the area postrema. Rats with discrete APX also drank substantial amounts of 0.5 M NaCl when access to saline was restricted to 7 h/day (up to 30 ml in 1 h, 48 ml in 7 h). Such large NaCl intakes stimulated considerable water ingestion and renal sodium excretion, but together these responses usually were insufficient for osmoregulation during the 7-h test period. After systemic administration of hypertonic NaCl solution, rats with APX excreted less Na(+) in urine and secreted less vasopressin and oxytocin than control rats did. The prominent salt appetite, insufficient thirst and natriuresis in response to an ingested NaCl load, and blunted natriuresis and neurohypophysial hormone secretion in response to an injected NaCl load, all indicate that osmoregulatory responses are impaired in rats after APX.

Sensitization of norepinephrine release in medial prefrontal cortex: effect of different chronic stress protocols

Previously, we demonstrated that continuous exposure of rats to cold (5 degrees C) for 2-3 weeks potentiates the increase in extracellular norepinephrine in the medial prefrontal cortex produced by acute tail shock. In the present study, we used in vivo microdialysis to examine whether this sensitization of evoked norepinephrine release also occurs in the medial prefrontal cortex following exposure to other chronic stress protocols. Rats exposed to 30 min of intermittent foot shock (0.6 mA) each day for 14 days, did not exhibit a greater increase in extracellular norepinephrine in response to acute tail shock. To determine whether this discrepancy between cold exposure and foot shock might be related to differences in the nature or the pattern of exposure to the chronic stressor, we also examined the effect of intermittent exposure to cold or continuous exposure to a foot shock protocol on tail shock-evoked norepinephrine release. Sensitized norepinephrine release did not develop following either intermittent exposure to cold (5 degrees C; 4 h/day for 14 days) or continuous exposure to a foot shock protocol (0.6 mA trains at random intervals 24 h/day for 14 days), suggesting that both the nature of the stressor as well as the pattern of exposure to the chronic stressor play a role in the development of sensitized norepinephrine release.

Dietary salt intake alters cardiovascular responses evoked from the rostral ventrolateral medulla

The present experiments examined whether in rats consuming diets with either high NaCl content (8%) or low Na+ content (0.01%) for 2 wk excitatory inputs to the rostral ventrolateral medulla (RVLM) would be altered. In chloralose-anesthetized rats, injection of glutamate into the RVLM elicited a pressor response that, compared with rats fed a control diet, was 50% larger in rats fed a diet containing 8% NaCl and was 25% smaller in rats fed a diet containing 0.01% Na+. Pressor responses produced by electrical stimulation of sciatic nerve afferents, as well as by microinjections into the RVLM of L-dihydroxyphenylalanine or carbachol, were all potentiated by high dietary salt intake and reduced by low dietary salt intake. Dietary salt intake had no effect on pressor responses produced by intravenous injection of phenylephrine, indicating that salt-related alterations in cardiovascular responses produced by central activation could not be accounted for by changes in peripheral vascular reactivity. The decrease in arterial pressure produced by injection of glutamate into the nucleus of the solitary tract was also potentiated by the high salt diet, suggesting that the sensitivity of central baroreceptor reflex pathways may be altered by dietary NaCl. These results indicate that the amount of NaCl consumed in the diet can change the sensitivity of RVLM sympathoexcitatory neurons, and this change in sensitivity is not restricted to any particular class of cell surface receptors.

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.

GABA-mediated inhibition of raphe pallidus neurons regulates sympathetic outflow to brown adipose tissue

Sympathetic nerve activity to brown adipose tissue (BAT) regulates adipocyte metabolism of its stored lipid fuel and thus the thermogenesis in BAT. To determine if the discharge of neurons in the rostral raphe pallidus (RPa) can influence BAT thermogenesis, changes in sympathetic nerve activity to BAT were recorded after microinjection (60 nl) of the GABAA receptor antagonist bicuculline (500 microM) into the RPa in chloralose-urethan-anesthetized, ventilated rats. Bicuculline caused a large, rapid rise in the sympathetic nerve activity to BAT (which had also increased during acute hypothermia) from very low, normothermic control levels to maximum values (mean: 1,949 +/- 604% control; n = 13) after 4-6 min. The sympathetic nerve discharge to BAT had a mean burst frequency (3. 5 +/- 0.3 Hz) that was significantly less than the heart rate (7.3 +/- 0.2 beats/min), and it was not inhibited during baroreceptor reflex activation. Bicuculline-stimulated increases in the sympathetic nerve activity to BAT and cold-evoked increases in neuronal fos expression were localized to the RPa at the level of the caudal half of the facial nucleus. This dramatic increase in sympathetic nerve activity to BAT after disinhibition of neurons in rostral RPa is consistent with a major role for RPa neurons, perhaps as sympathetic premotoneurons for BAT, in medullary control of BAT thermogenesis.

Role of excitatory amino acids in the regulation of dopamine synthesis and release in the neostriatum

We have explored the role of excitatory amino acids in the increased dopamine (DA) release that occurs in the neostriatum during stress-induced behavioral activation. Studies were performed in awake, freely moving rats, using in vivo microdialysis. Extracellular DA was used as a measure of DA release; extracellular 3,4-dihydroxyphenylalanine (DOPA) after inhibition of DOPA decarboxylase provided a measure of apparent DA synthesis. Mild stress increased the synthesis and release of DA in striatum. DA synthesis and release also were enhanced by the intra-striatal infusion of N-methyl-D-aspartate (NMDA), an agonist at NMDA receptors, and kainic acid, an agonist at the DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate (AMPA)/kainate site. Stress-induced increase in DA synthesis was attenuated by co-infusion of 2-amino-5-phosphonovalerate (APV) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), antagonists of NMDA and AMPA/kainate receptors, respectively. In contrast, intrastriatal APV, CNQX, or kynurenic acid (a non-selective ionotropic glutamate receptor antagonist) did not block the stress-induced increase in DA release. Stress-induced increase in DA release was, however, blocked by administration of tetrodotoxin along the nigrostriatal DA projection. It also was attenuated when APV was infused into substantia nigra. Thus, glutamate may act via ionotropic receptors within striatum to regulate DA synthesis, whereas glutamate may influence DA release via an action on receptors in substantia nigra. However, our method for monitoring DA synthesis lowers extracellular DA and this may permit the appearance of an intra-striatal glutamatergic influence by reducing a local inhibitory influence of DA. If so, under conditions of low extracellular DA glutamate may influence DA release, as well as DA synthesis, by an intrastriatal action. Such conditions might occur during prolonged severe stress and/or DA neuron degeneration. These results may have implications for the impact of glutamate antagonists on the ability of patients with Parkinson's disease to tolerate stress.

Mecamylamine blocks the development of tolerance to nicotine in rats: implications for the mechanisms of tolerance

Chronic injections of nicotine in rats produce upregulation of nicotinic cholinergic receptors. It has been proposed that this upregulation is a reflection of receptor desensitization and is the basis of functional tolerance. Mecamylamine, a non-competitive antagonist that blocks activation of nicotinic receptors, does not prevent upregulation produced by nicotine injections. This suggests that receptor activation is not a prerequisite for nicotine-induced receptor upregulation. Therefore, the present experiments tested whether mecamylamine would also fail to prevent the development of tolerance to nicotine. Six daily pairings of mecamylamine (1 mg/kg SC) with nicotine did block the development of tolerance to nicotine-induced antinociception (0.35 mg/kg) and to the ability of nicotine to suppress milk intake (0.66 mg/kg). In another experiment, six daily injections of mecamylamine, when given alone, did not alter the effects of a subsequent, acute injection of nicotine (0.35 mg/kg) in inducing antinociception in rats. There was no evidence that after six pairings of mecamylamine with nicotine, the cues associated with mecamylamine delivery took on conditioned antagonistic properties. These findings suggest that, unlike the receptor upregulation that results from either continuous or repeated nicotine administration, the tolerance following a short series of intermittent nicotine injections is dependent on receptor activation.

Thirst and salt appetite elicited by hypovolemia in rats with chronic lesions of the nucleus of the solitary tract

Cardiac vagal afferents terminating in the nucleus of the solitary tract (NTS) are believed to participate in stimulating neurohypophysial secretion of vasopressin as well as increased ingestion of water and NaCl solution in response to decreased blood volume. However, we recently reported that chronic lesions of NTS, which eliminate neural input from cardiac and arterial baroreceptors, do not impair hypovolemia-induced vasopressin secretion in rats. In the present investigation we sought to determine whether those sensory signals were necessary for hypovolemia-induced thirst and salt appetite. Rats with chronic lesions of the NTS increased consumption of water and NaCl solution normally when plasma volume was reduced isosmotically by subcutaneous injection of polyethylene glycol solution. These results were obtained whether rats were allowed to drink water or 0.15 M NaCl in one-bottle tests or water and 0.5 M NaCl in two-bottle tests. The induction of thirst and salt appetite by hypovolemia despite the apparent loss of neural input to the brain from cardiac volume-sensitive receptors indicates that other signals generated by plasma volume deficits can stimulate these behavioral responses in rats.

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 (P</=0.001). When compared with subjects in the highest quartile of prolactin response, individuals whose prolactin responses to fenfluramine comprised the lowest quartile were 2.6 times more likely to have a resting systolic/diastolic BP of >135/85 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.

Pseudorabies virus-induced leukocyte trafficking into the rat central nervous system

When the swine alphaherpesvirus pseudorabies virus (PRV) infects the rat retina, it replicates in retinal ganglion cells and invades the central nervous system (CNS) via anterograde transynaptic spread through axons in the optic nerve. Virus can also spread to the CNS via retrograde transport through the oculomotor nucleus that innervates extraocular muscles of the eye. Since retrograde infection of the CNS precedes anterograde transynaptic infection, the temporal sequence of infection of the CNS depends on the route of invasion. Thus, motor neurons are infected first (retrograde infection), followed by CNS neurons innervated by the optic nerve (anterograde transynaptic infection). This temporal separation in the appearance of virus in separate groups of neurons enabled us to compare the immune responses to different stages of CNS infection in the same animal. The data revealed focal trafficking of peripheral immune cells into areas of the CNS infected by retrograde or anterograde transport after PRV Becker was injected into the vitreous body of the eye. Cells expressing the leukocyte common antigen, CD45(+), entered the area of infection from local capillaries prior to any overt expression of neuropathology, and quantitative analysis demonstrated that the number of cells increased in proportion to the number of infected neurons within a given region. Recruitment of cells of monocyte/macrophage lineage began prior to the appearance of CD8(+) cytotoxic lymphocytes, which were, in turn, followed by CD4(+) lymphocytes. These data demonstrate that PRV replication in CNS neurons stimulates the focal infiltration of specific classes of CD45(+) cells in a time-dependent, temporally organized fashion that is correlated directly with the number of infected neurons and the time that a given region has been infected.

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.

Acquisition of nicotine self-administration in rats: the effects of dose, feeding schedule, and drug contingency

The studies presented here were designed to further clarify the nature of nicotine self-administration (SA) based on a limited access model in which rats are food restricted, receive operant training using food reinforcement, and are then tested in daily 1-h drug sessions. We examined the effects of dose, feeding schedule, and contingency of drug delivery on acquisition of nicotine SA. Two doses of nicotine bitartrate, 0.03 and 0.06 mg/kg per infusion (free base), supported the transition from food-reinforced to drug-reinforced responding, although the pattern of behavior differed between these doses. In contrast, 0.01 mg/kg per infusion failed to maintain nicotine SA. In a second study, animals were divided into three groups according to feeding schedule. Rats that were both weight restricted and food deprived showed the highest level of SA behavior, although neither food deprivation nor weight restriction was necessary to establish SA. In the third experiment, rats that were switched from food to nicotine as the response-dependent reinforcer maintained higher response rates throughout a 9-day period than animals switched to response-independent (i.e., yoked) nicotine which showed minimal responding after day 1. Furthermore, the differences between self-administering and yoked animals emerged during the first session, suggesting that nicotine may serve as a reinforcer during the first drug exposure in naive animals. These results indicate that acquisition of nicotine SA can be influenced by both dose of nicotine and feeding schedule and that, in animals previously trained on a food-reinforced operant, active lever pressing is maintained only when nicotine delivery is contingent upon responding.

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.

Influence of GABA in the nucleus of the solitary tract on blood pressure in baroreceptor-denervated rats

We have previously reported that inhibition of the nucleus of the solitary tract (NTS) in chronically sinoaortic baroreceptor-denervated (SAD) rats has no effect on blood pressure in contrast to the marked increase in blood pressure it elicits in baroreceptor-intact rats. This could result either from a lack of tonic excitatory input to this region or from overriding inhibition of NTS neurons involved in the control of blood pressure. The present study aimed to distinguish between these two possibilities by examining the changes in blood pressure elicited by injection of bicuculline (Bic), a gamma-aminobutyric acid (GABA) antagonist, into the NTS of SAD and control rats. In chloralose-anesthetized baroreceptor-intact rats or acutely SAD rats, injection of 10 pmol Bic into the NTS elicited minimal changes in blood pressure. In contrast, in chronic SAD rats injection of Bic into the NTS elicited a large decrease in blood pressure. The maximal decrease in blood pressure elicited by Bic in chronic SAD rats was equivalent to the maximal decrease in blood pressure that could be evoked by direct excitation of the NTS with L-glutamate. These results suggest that the lack of a tonic role of the NTS in the regulation of blood pressure in chronic SAD rats is a result of maximal GABA-mediated inhibition of relevant NTS neurons.

Adrenal epinephrine and norepinephrine release to hypoglycemia measured by microdialysis in conscious rats

Experiments were conducted in conscious male rats to determine whether hypoglycemia induced by insulin administration preferentially stimulated epinephrine (Epi) or norepinephrine (NE) adrenal medullary chromaffin cells. The release of Epi and NE from the adrenal medulla was continuously monitored using a microdialysis probe of novel design that had been inserted in the adrenal medulla approximately 16 h before the administration of insulin. Following insulin, 3 U/kg i.v., blood glucose declined and dialysate Epi levels rose. No measurable increment in dialysate NE was obtained. Similarly, plasma Epi increased with no detectable change in NE. Patterns of dialysate and plasma catecholamine changes were similar in two groups of animals that had been fed or fasted overnight before insulin treatment. However, the magnitude of the Epi increase was greater in the fasted animals. After recovery of the blood glucose concentration to preinsulin levels, dialysate and plasma catecholamine concentrations returned to control values. These experiments clearly demonstrate that adrenal medullary chromaffin cells that produce Epi are preferentially stimulated in response to insulin-induced hypoglycemia.

Decreases in arterial pressure activate oxytocin neurons in conscious rats

Hemorrhage and nonhypotensive hypovolemia are known to increase plasma levels of oxytocin (OT) and vasopressin (VP) in rats. The present experiments demonstrated that secretion of OT and VP also are stimulated by acute drug-induced hypotension. Injection of hydralazine abruptly decreased arterial blood pressure in conscious rats and induced Fos expression, a marker of neuronal activation, within OT and VP neurons in the hypothalamus. Hydralazine also elicited substantial increases in plasma levels of both OT and VP. Injection of chlorisondamine similarly elicited acute hypotension and increased plasma levels of OT and VP. Furthermore, when the hypotensive effect of chlorisondamine was blunted by coinfusion of phenylephrine, the induced increases in OT and VP were markedly attenuated. Across all treatments, arterial blood pressure was inversely related to plasma levels of OT and VP. Plasma osmolality was not increased by hydralazine, nor was there evidence of gastric malaise, two known stimuli for OT secretion in rats. These results suggest that arterial hypotension increases neurohypophysial release of OT and VP in conscious rats.