Electrophysiological effects of dridocainide on isolated canine, guinea-pig and human cardiac tissues.

The cellular electrophysiological effects of dridocainide (EGIS-3966), a novel class I antiarrhythmic agent, was studied using conventional microelectrode techniques in canine cardiac Purkinje fibres and papillary muscle preparations obtained from humans and guinea-pigs. In each preparation, dridocainide (0.6-2 mumol/l) decreased the maximum velocity of action potential upstroke (Vmax) in a frequency-dependent manner, although marked differences were observed in its effects in Purkinje fibre and ventricular muscle preparations. In canine Purkinje fibres, action potential duration measured at 50% and 90% of repolarization was decreased, while action potential duration measured at 10% of repolarization was increased by dridocainide. In addition, the plateau of the action potential was depressed by the drug. These changes in action potential configuration were not observed in guinea pig or human papillary muscles. The offset kinetics of the dridocainide-induced Vmax block were different in Purkinje fibres and in ventricular muscle: the slow time constant of recovery of Vmax was estimated to be 2.5 s in dog Purkinje fibre and 5-6 s in human and guinea-pig papillary muscle. In guinea-pig papillary muscle, the rate of onset of the Vmax block was 0.15 and 0.2 per action potential in the presence of 0.6 and 2 mumol/l dridocainide, respectively. Dridocainide also decreased the force of contraction in this preparation. On the basis of the present results, dridocainide appears to possess mixed class I.C and I.A properties, with I.C predominance in human and guinea-pig ventricular muscle. Present results also indicate that results of conventional classification of class I drugs may depend on the parameters chosen, as well as on the preparation selected.

Role of CRH in glucopenia-induced adrenomedullary activation in rats.

Acute glucoprivation profoundly stimulated hypothalamic-pituitary-adrenocortical (HPA) and adrenomedullary outflows. Whether these responses reflect a single central mechanism regulated by corticotropin-releasing hormone (CRH) has been unclear. This study examined the role of endogenous CRH in HPA and adrenomedullary responses to hypoglycemia in Sprague-Dawley rats, by using anti-CRH immune serum or a CRH antagonist (alpha-helical h/r CRH9-41, and in Lewis rats, a strain characterized by deficient hypothalamic CRH responses during stress. In conscious Sprague-Dawley rats with indwelling arterial and venous cannulas, insulin (0.3 U/kg was injected iv, and responses of serum glucose concentrations and plasma levels of corticotropin (ACTH) and catechols (including epinephrine, EPI; norepinephrine, NE; dihydroxyphenylalanine, DOPA; dihydroxyphenylglycol, DHPG; and dihydroxyphenylacetic acid, DOPAC) were assessed, with or without pretreatment with anti-CRH immune serum (0.5 or 1.0 ml iv or 10 microl icv) or alpha-helical h/r CRH9-41 (130 nmol iv or 13 nmol icv). Responses to insulin (1.0 U/kg iv) were also measured in conscious juvenile Lewis and Fischer 344/N rats. Insulin-induced hypoglycemia markedly increased plasma levels of EPI and ACTH in all groups. Pretreatment iv with 1/0 ml of anti-CRH immune serum blocked the ACTH response to insulin but failed to attenuate the EPI response. alpha-helical h/r CRH9-41, whether given iv or icv, failed to alter ACTH or EPI responses to insulin, although the antagonist did block EPI responses to icv CRH. Hypoglycemia elicited similar increments in ACTH levels in Lewis rats and Fischer 344/N control rats; and although Lewis rats had lower baseline EPI and smaller responses of NE, DHPG, DOPA, and DOPAC levels, the groups did not differ in proportionate increments in EPI levels. The results indicate that the ACTH response to hypoglycemia depends on availability of CRH outside the blood-brain barrier--presumably in the pituitary gland. The findings with icv alpha-helical h/r CRH9-41 can be explained by failure of the antagonist to reach effective concentrations at central sites of action of endogenous CRH, or by mechanisms other than CRH release determining the adrenomedullary response to hypoglycemia. Lewis rats seem to have less adrenomedullary secretion at baseline and smaller responses of NE synthesis and release during hypoglycemia than do Fischer 344/N rats. Neurochemical evidence for differential adrenomedullary and sympathoneural responses during hypoglycemia in all three rat strains is inconsistent with Cannon's view of a functionally unitary sympathoadrenal system. Lewis rats have deficient CRH responses to some stressors but not to others, or else pituitary-adrenomedullary responses in this setting depend on mechanisms other than CRH release in the brain. Both explanations are inconsistent with the doctrine of non-specificity, the main tenet of Selye's stress theory.

Plasma catecholamine, renin activity, and ACTH responses to the serotonin receptor agonist DOI in juvenile spontaneously hypertensive rats.

Mean arterial blood pressure (MAP), heart rate (HR) and arterial plasma levels of corticotropin (ACTH), renin activity (PRA) and catechols [norepinephrine (NE), epinephrine (EPI), and the intraneuronal NE metabolite dihydroxyphenylglycol (DHPG)] at baseline and in response to the serotonin-1C/2 (5-HT1C/5-HT2) agonist 1-(2,5-dimethoxy-4-iodophenyl)2-aminopropane (DOI, 1.0 mg/kg i.a.) in conscious, freely-moving, juvenile (4 week old) spontaneously hypertensive rats (SHR's) and age-matched Wistar-Kyoto (WKY) normotensive control rats were measured simultaneously. Baseline levels of MAP, NE, DHPG, and EPI all were significantly higher in the SHR's. There was a similar trend for PRA, but ACTH did not differ between the two strains. DOI produced marked increases in levels of MAP, ACTH, EPI, and also PRA but did not affect NE or DHPG concentrations. HR decreased only in the WKY group after administration of DOI. The magnitudes of the EPI and ACTH responses did not differ between the rat strains. Responses of MAP and PRA were significantly larger in SHR's. These results suggest that there is a selective hyperresponsiveness of PRA and blood pressure to 5-HT2 receptor stimulation parallel to a deficient baroreceptor reflex in juvenile SHR's.

Effect of 5-HT1C and 5-HT2 receptor stimulation on excessive grooming, penile erection and plasma oxytocin concentrations.

Serotonin (5-HT) receptor agonist-induced excessive grooming, penile erection and oxytocin secretion were studied in chronically cannulated freely moving rats. The 5-HT1C receptor agonist, m-chlorophenylpiperazine (m-CPP), which also binds to other 5-HT receptors, produced dose-dependent excessive grooming, penile erection and increases in circulating oxytocin concentrations. Maximal responses for excessive grooming and penile erection occurred at 0.3-0.6 mg/kg i.v. m-CPP. Higher doses (0.9-2.5 mg/kg i.v.) caused further increases in oxytocin concentrations, but attenuated both behavioral responses. All three responses to m-CPP (0.6 mg/kg) were attenuated by antagonists with high affinity for the 5-HT1C receptor site (mianserin, LY-53857 and metergoline), but not by the 5-HT2 receptor antagonist ketanserin. The 5-HT2/5-HT1C agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), increased plasma oxytocin concentrations only. After ketanserin pretreatment, DOI caused penile erection and diminished the oxytocin response. All responses to DOI were blocked completely by pretreatment with LY-53857 plus ketanserin. Excessive grooming and penile erection showed significant bimodal correlations with the oxytocin response. These data suggest that stimulation of 5-HT1C receptors induces excessive grooming, penile erection and increased oxytocin secretion. Stimulation of 5-HT2 receptors causes a further increase in plasma oxytocin concentration, but inhibits both behavioral responses.

Effects of handling or immobilization on plasma levels of 3,4-dihydroxyphenylalanine, catecholamines, and metabolites in rats.

In conscious animals, handling and immobilization increase plasma levels of the catecholamines norepinephrine (NE) and epinephrine (EPI). This study examined plasma concentrations of endogenous compounds related to catecholamine synthesis and metabolism during and after exposure to these stressors in conscious rats. Plasma levels of 3,4-dihydroxyphenylalanine (DOPA), NE, EPI, and dopamine (DA), the deaminated catechol metabolites 3,4-dihydroxyphenylglycol (DHPG), and 3,4-dihydroxyphenylacetic acid (DOPAC), and their O-methylated derivatives methoxyhydroxyphenylglycol (MHPG) and homovanillic acid (HVA) were measured using liquid chromatography with electrochemical detection at 1, 3, 5, 20, 60, and 120 min of immobilization. By 1 min of immobilization, plasma NE and EPI levels had already reached peak values, and plasma levels of DOPA, DHPG, DOPAC, and MHPG were increased significantly from baseline, whereas plasma DA and HVA levels were unchanged. During the remainder of the immobilization period, the increased levels of DOPA, NE, and EPI were maintained, whereas levels of the metabolites progressively increased. In animals immobilized briefly (5 min), elevated concentrations of the metabolites persisted after release from the restraint, whereas DOPA and catecholamine levels returned to baseline. Gentle handling for 1 min also significantly increased plasma levels of DOPA, NE, EPI, and the NE metabolites DHPG and MHPG, without increasing levels of DA or HVA. The results show that in conscious rats, immobilization or even gentle handling rapidly increases plasma levels of catecholamines, the catecholamine precursor DOPA, and metabolites of NE and DA, indicating rapid increases in the synthesis, release, reuptake, and metabolism of catecholamines.

Pharmacological characterization of serotonin receptor subtypes involved in vasopressin and plasma renin activity responses to serotonin agonists.

Serotonergic drugs with 5-HT2 receptor agonist properties have been suggested to increase plasma vasopressin concentration, blood pressure (BP) and plasma renin activity (PRA). To study whether these actions are mediated by the same or different receptors, we used three potent 5-HT agonists with different structures and receptor binding profiles. All drugs were administered i.v. to conscious, unrestrained rats. The selective agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), which has high affinity for 5-HT2 receptors, caused marked increases in BP and PRA but no change in plasma vasopressin concentrations. The 5-HT1C agonist, m-chlorophenylpiperazine (m-CPP), which also binds to other 5-HT receptors, caused moderate increases in BP and PRA and significantly elevated plasma vasopressin concentrations. The 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), did not increase any of these parameters. BP and PRA elevations paralleled each other after all drugs, while vasopressin responses were clearly different. Vasopressin responses to m-CPP were entirely antagonised by the 5-HT1/5-HT2 antagonist metergoline, partially by the 5-HT2/5-HT1C antagonists ritanserin and LY 53857, but not by the 5-HT2 antagonist ketanserin. Ritanserin, LY53857 and ketanserin all very effectively blocked BP responses to m-CPP. These findings suggest that BP and PRA but not vasopressin responses are mediated by 5-HT2 receptors. Vasopressin secretion is mediated by 5-HT1 receptors, most likely by 5-HT1C receptors.

Sympathoneural and skeletal muscle contributions to plasma dopa responses in pithed rats.

Dihydroxyphenylalanine (DOPA) in plasma has been thought to originate from sympathetic nerve endings and to reflect catecholamine biosynthesis, because changes in DOPA levels follow pharmacologically- or environmentally-induced manipulations that alter turnover of the sympathetic neurotransmitter, norepinephrine (NE). Skeletal muscle may be an additional, non-neural source of circulating DOPA. In the present study we examined sympathoneural and skeletal muscle contributions to DOPA in arterial plasma in pithed rats. Electrical stimulation of the spinal cord causes discharges of sympathetic post-ganglionic neurons, with attendant release of NE into the bloodstream, and discharges of spinal motoneurons, which causes diffuse contraction of skeletal muscle. Stimulation of the spinal cord rapidly elevated arterial plasma concentrations of NE, dihydroxyphenylglycol (DHPG), and DOPA. Pre-treatment with curare, a skeletal muscle relaxant, did not affect the NE and DHPG responses but attenuated the DOPA responses by about 50%. Administration of chlorisondamine, a ganglionic blocker, abolished NE and DHPG responses to cord stimulation, and DOPA responses were decreased by about 90%. Adrenal-demedullation did not affect the stimulation-induced DOPA responses. The results demonstrate that in pithed rats undergoing spinal cord stimulation, DOPA is released into the bloodstream. Since this response is markedly inhibited after ganglionic blockade and also attenuated after skeletal muscle paralysis, the results provide indirect evidence that DOPA formed in sympathetic neurons can be stored in a non-neuronal pool and released during skeletal muscle contraction.

Simultaneous measurement of plasma and brain extracellular fluid concentrations of catechols after yohimbine administration in rats.

The present study examined whether systemic injection of the alpha 2 adrenoceptor blocker, yohimbine, affects concentrations of norepinephrine (NE) and its metabolites in extracellular fluid in the brain and in blood. Microdialysis probes were inserted into the posterior hypothalamus, medulla, and caudate/putamen in rats. Microdialysate and arterial blood were sampled after intravenous administration of yohimbine. In the hypothalamus yohimbine produced significant increases in extracellular fluid concentrations of NE, its intraneuronal metabolite, dihydroxyphenylglycol (DHPG), and methoxyhydroxyphenylglycol (MHPG), a major neuronal and extraneuronal metabolite of NE. The increases in these levels were small or absent in the caudate/putamen, where dopamine is the primary catecholamine transmitter. During systemic infusion of tracer amounts of [3H]NE, little if any radioactive NE or DHPG appeared in the microdialysate, whereas substantial levels of [3H]MHPG were present and increased as plasma [3H]MHPG levels rose. The results support the view that alpha 2 adrenoceptor blockade in the brain increases hypothalamic and medullary release, reuptake, and metabolism of NE. The findings cannot be explained by disruption of the blood-brain barrier for catecholamines by insertion of the microdialysis probes. Enhanced sympathetic outflow and peripheral release of NE when alpha 2 adrenoceptors are blocked appears to be attended by enhanced central NE release, presumably as a result of presynaptic alpha 2 adrenoceptor blockade at noradrenergic terminals in the brain. This is consistent with the hypothesis that central noradrenergic NE release is regulated by presynaptic alpha 2 adrenoceptors.

Beta-endorphin responses to different serotonin agonists: involvement of corticotropin-releasing hormone, vasopressin and direct pituitary action.

Activation of serotonergic neurotransmission has been shown to increase plasma beta-endorphin-like immunoreactivity (beta-End-LI). To study the mechanism(s) of this action, we measured the effects of 3 potent serotonin (5-HT) agonists with different structures and 5-HT receptor binding profiles in conscious unrestrained Sprague-Dawley rats in vivo and in dispersed anterior pituicytes in vitro. The 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), the 5-HT1C agonist, m-chlorophenylpiperazine (m-CPP), and the 5-HT2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), all markedly increased beta-End-LI in plasma in vivo. All 3 responses were blocked by dexamethasone pretreatment. Pituitary stalk transection (PST), as well as pretreatment with rabbit serum hyperimmune against rat corticotropin-releasing hormone (CRH, TS-6) completely abolished beta-End-LI response to 8-OH-DPAT and attenuated the responses by about 60% to DOI. Responses to m-CPP were markedly attenuated in PST rats, but pretreatment with TS-6 had no significant effect. To examine whether vasopressin (AVP) might be involved in the CRH neutralizing antibody-resistant beta-End-LI responses after m-CPP and DOI, we measured AVP concentrations after each agonist, m-CPP, but not DOI or 8-OH-DPAT, significantly elevated circulating AVP levels. As a proof of direct pituitary effect, DOI markedly stimulated beta-End-LI release from the anterior pituitary cell culture preparation in vitro. It was approximately as potent as CRH in the picomolar range, m-CPP was much less effective than DOI, while 8-OH-DPAT did not stimulate beta-End-LI release in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Fate of tritiated 6-fluorodopamine in rats: a false neurotransmitter for positron emission tomographic imaging of sympathetic innervation and function.

In evaluating positron-emitting analogs of dopamine (DA) as imaging agents for visualizing tissue sympathetic innervation and function, we assessed the metabolic fate of systemically injected [3H]-6-fluorodopamine [( 3H]-6F-DA) in plasma, in sympathetically innervated tissues (left ventricle, spleen and salivary glands) and in excretory organs (liver and kidney) of rats. By 5 min after intravenous bolus injection of a physiologically inactive amount (450 ng, 10 microCi) of [3H]-6F-DA, 3H was concentrated in all the organs compared with that in blood or plasma. In the sympathetically innervated organs, most of the radioactivity was in [3H]-6F-DA and [3H]-6-fluoronorepinephrine [( 3H]-6F-NE), whereas in the blood, plasma and excretory organs most of the radioactivity was in noncatechol compounds such as O-methylated and conjugated metabolites. In sympathetically innervated organs, tissue/blood ratios exceeded 1.0 at all time points between 5 and 120 min after injection of [3H]-6F-DA and increased progressively (from 8 to 60 in myocardium), whereas the tissue/blood ratios in the kidney and liver increased by less than 2-fold during this interval. In all the studied tissues, the proportion of total tissue 3H that was due to [3H]F-NE increased progressively while that due to [3H]F-DA declined, consistent with conversion of [3H]F-DA to [3H]F-NE in vesicles in sympathetic nerve endings.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cortisol treatment on brain and adrenal corticotropin-releasing hormone (CRH) content and other parameters regulated by CRH.

Corticotropin-releasing hormone (CRH) has been found in both hypothalamic and extrahypothalamic sites of the brain and also in the adrenal medulla. To study the timing and location of delayed glucocorticoid action in rats, we measured the effects of 2-day and 7-day cortisol treatment on immunoreactive CRH concentrations in hypothalamus, cerebral cortex, hippocampus, cerebellum, and adrenal gland. The activity of the hypothalamo-pituitary-adrenal (HPA) axis and the sympathoadrenal system were also measured. Studies were carried out both in the afternoon and/or in the morning, to get information about possible circadian changes. CRH contents were not changed in any brain areas studied, except there was a trend of decrease in the hypothalamus compared to vehicle in the afternoon due to the lack of circadian increase after 7-day cortisol treatment. Pituitary ACTH content decreased significantly after 7-day treatment, while beta-endorphin did not. Plasma levels of ACTH, corticosterone, norepinephrine and epinephrine and adrenal ACTH and beta-endorphin contents decreased after 2-day, adrenal CRH content after 7-day treatment with cortisol. Our findings suggest, that chronic cortisol treatment inhibits the circadian activation of the HPA axis at all levels but has variable effects on baseline measures because it causes different changes in release and synthesis at different sites.

Metabolic fate of injected radiolabelled dopamine and 2-fluorodopamine in rats.

In evaluating positron-emitting analogs of dopamine (DA) as possible imaging agents for visualizing tissue sympathetic innervation and function, the metabolic fate of systemically injected [3H]-DA or [14C]-DA was compared with that of [3H]-2-fluoroDA in plasma and in sympathetically innervated tissues (left ventricle, spleen and salivary glands) of rats. By 60 min after the injection of [3H]-DA or [3H]-2-fluoroDA, concentrations of [3H]-DA. [3H]-2-fluoroDA, [3H]-norepinephrine ([3H]-NE) and [3H]-2-fluoroNE in tissue exceeded concentrations in plasma by up to several thousand-fold. Whereas most of the radioactivity in tissue was in catechols, radioactivity in plasma was due to O-methylated metabolites of DA, including homovanillic acid (HVA) and of NE, including normetanephrine (NMN) and methoxyhydroxphenylglycol (MHPG). Estimated ratios of tissue: blood radioactivity at 60 min after injection of [3H]-2-fluoroDA were 4.10 for the heart, 1.91 for the spleen and 2.10 for the salivary glands. The patterns of levels of catechol metabolites and analogs of HVA in plasma and effects of blockade of neuronal uptake with desipramine suggested that [3H]-2-fluoroDA was not as efficiently removed by neuronal uptake and not as efficiently beta-hydroxylated as the non-fluorinated compound. Concurrent administration of [3H]-DA and large amounts of non-radioactive 2-fluoroDA did not substantially alter the pattern of metabolites of [3H]-DA in plasma. After injection of [18F]-fluoroDA, visualization of sympathetic innervation of tissue should be feasible by position emission tomography.

Mechanisms of serotonin receptor agonist-induced activation of the hypothalamic-pituitary-adrenal axis in the rat.

A substantial body of experimental evidence indicates that serotonin (5-HT) and several synthetic 5-HT receptor agonists activate the hypothalamic-pituitary-adrenal (HPA) axis. To explore the mechanism(s) by which 5-HT or 5-HT agonists enhance the activity of the HPA axis in vitro, we examined the stimulatory effects of the 5-HT1a agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), the 5-HT1c/5-HT1b agonist m-chlorophenylpiperazine (m-CPP), and the 5-HT2/5-HT1c agonist 1-(2,5-dimethoxy-4-iodophenyl)2-amino-propane (DOI) on plasma ACTH and corticosterone secretion in the rat. To test whether 8-OH-DPAT, m-CPP, or DOI increase plasma ACTH levels by stimulating the release of endogenous CRH, catheterized conscious male Sprague-Dawley rats were pretreated with hyperimmune CRH rabbit serum (TS-6) or normal rabbit serum and subsequently challenged with a maximally stimulatory dose of the above 5-HT agonists. Pretreatment with TS-6 completely suppressed the ACTH response to m-CPP and significantly blunted the responses to 8-OH-DPAT or DOI. To examine whether the remaining ACTH response to 8-OH-DPAT or DOI was also mediated by a pituitary site of action, we administered each of these agents to pituitary stalk-transected or sham-operated rats. The ACTH responses to 8-OH-DPAT and DOI in stalk-transected rats were preserved, although significantly blunted, compared to those in sham-operated rats. This suggested that both of these 5-HT agonists may also act at the pituitary level to stimulate ACTH release in vivo. Although the ACTH responses to 8-OH-DPAT, m-CPP, and DOI were blunted after both TS-6 pretreatment and pituitary stalk transection, corticosterone responses were only slightly affected, suggesting that some of these compounds may cause corticosterone release in the rat through another mechanism. To evaluate this hypothesis, ACTH and corticosterone responses to 8-OH-DPAT, m-CPP, and DOI were examined in rats whose HPA axis had been suppressed by a single high dose injection of dexamethasone. The corticosterone responses to 8-OH-DPAT and DOI were blunted compared to those of saline-pretreated rats, but were inappropriately high compared to the ACTH responses observed in these rats. On the other hand, both ACTH and corticosterone responses to m-CPP were completely abolished by dexamethasone.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonin agonists cause parallel activation of the sympathoadrenomedullary system and the hypothalamo-pituitary-adrenocortical axis in conscious rats.

The effects of three potent serotonin (5-HT) agonists with different structures and 5-HT receptor binding profiles on sympathoadrenomedullary and hypothalamo-pituitary-adrenocortical axis functions were assessed in conscious, freely moving male Sprague-Dawley rats. The 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), the 5-HT1C agonist, m-chlorophenylpiperazine (m-CPP), and the 5-HT2/5-HT1C agonist, 1-(2,5-dimethoxy-4-iodophenyl)2-amino-propane (DOI), all produced marked dose-dependent increases in plasma epinephrine and ACTH concentrations. Both epinephrine and ACTH responses peaked at 10 min and showed strong positive correlations across all drugs and doses studied. Corticosterone increases showed a saturable response pattern and were close to the maximum level with a relatively small (approximately 2.5-fold) increase in plasma ACTH concentrations. Norepinephrine levels showed small dose-dependent increases after 8-OH-DPAT and m-CPP and decreases after DOI treatment. These results suggest that both the sympathoadrenomedullary system and the hypothalamo-pituitary-adrenocortical axis can be activated via 5-HT1 and 5-HT2 receptors and that these two systems may have common or similar regulatory mechanisms triggered by these stimuli.

Effects of a peripherally acting alpha 2-adrenoceptor antagonist (L-659,066) on hemodynamics and plasma levels of catechols in conscious rats.

L-659,066 is a new alpha 2-adrenoceptor antagonist which does not enter the central nervous system after systemic administration and therefore can be used to examine effects of blockade of peripheral alpha 2-receptors on hemodynamics and plasma levels of catechols. After i.v. administration to conscious rats, L-659,066 produced dose-related, small decreases in mean arterial pressure (MAP) and large increases in heart rate (HR), arterial plasma levels of norepinephrine (NE), and levels of the intraneuronal NE metabolite, dihydroxyphenylglycol (DHPG). After administration of L-659,066, HR, but not MAP, was strongly correlated with NE levels (r = 0.93, P less than 0.001). Levels of DHPG and dihydroxyphenylalanine (DOPA) also were strongly correlated with NE levels (r = 098 and r = 0.71). After comparison with responses during hypotension induced by the vasodilator, nitroprusside, the results indicated that L-659,066 increases sympathetically mediated NE release and catecholamine turnover due to inhibition of presynaptic alpha 2-receptors as well as due to reflexive sympathetic activation related to blockade of alpha 2-receptors on arterial smooth muscle cells.

Long-term cortisol treatment impairs behavioral and neuroendocrine responses to 5-HT1 agonists in the rat.

The effects of chronic cortisol treatment on neuroendocrine and behavioral responses to serotonin1 (5-HT1) receptor agonists were studied in conscious, freely moving rats. Seven-day cortisol treatment (25 mg/kg/day with osmotic minipumps) markedly suppressed basal plasma corticotropin (ACTH) and corticosterone concentrations, indicating a suppression of the hypothalamo-pituitary-adrenocortical axis. Cortisol also decreased body weight, food intake, plasma norepinephrine (NE), and epinephrine (E) levels. In the drug challenge studies, we used two 5-HT1 agonists, the 5-HT1B and 5-HT1C agonist, m-chlorophenylpiperazine (m-CPP), and the 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT), to examine the effect of cortisol on their behavioral and neuroendocrine effects. After 7-day cortisol treatment, plasma prolactin responses to both m-CPP and 8-OHDPAT were significantly decreased. While the plasma NE, E, and food intake responses to m-CPP were also significantly reduced by cortisol treatment, these same responses to 8-OHDPAT were unchanged. The effect of m-CPP on locomotor activity was also decreased. Since only the responses to m-CPP and 8-OHDPAT previously shown to be antagonized by pretreatment with the 5-HT1/5-HT2 antagonist, metergoline, were significantly attenuated after cortisol treatment, these changes may be specific to 5-HT receptors. These data indicate that chronic exposure to high glucocorticoid levels alters 5-HT1 receptor-mediated functions and provides additional evidence relevant to the contribution of glucocorticoid elevation to the symptoms of depression.

Different serotonin receptors mediate blood pressure, heart rate, plasma catecholamine and prolactin responses to m-chlorophenylpiperazine in conscious rats.

The serotonin (5-HT) agonist, m-chlorophenylpiperazine (m-CPP), has been shown to increase blood pressure (BP), heart rate (HR), plasma catecholamine and prolactin (PRL) concentrations and to cause hypoactivity in rodents. In the present study, we used selective 5-HT and adrenergic antagonists to study the involvement of different receptor subtypes on the effects of m-CPP in conscious, freely moving rats. Hypoactivity and PRL responses were blocked by pretreatment with metergoline but not by pretreatment with other antagonists. BP increases were antagonized by ritanserin (0.1, 0.63 and 2.0 mg/kg) and ketanserin; metergoline, xylamidine or prazosin pretreatment had only partial effects on BP responses. HR increases were antagonized by yohimbine, pindolol, ketanserin and by the two higher doses of ritanserin. After pretreatment with the two higher doses of ritanserin, m-CPP decreased markedly BP and HR. These decreases were prevented by metergoline pretreatment. Norepinephrine and epinephrine responses were dose-dependently attenuated by ritanserin; naloxone pretreatment attenuated epinephrine but not norepinephrine responses. These data suggest that hypoactivity, PRL responses, and cardiodepressive effects of m-CPP are mediated by 5-HT1 receptors. It is likely that the hypoactivity and PRL responses of m-CPP are mediated by 5-HT1B receptors, and the cardiodepressive effects by 5-HT1A receptors. Increases in BP appear to be primarily mediated by stimulation of 5-HT2 receptors. Both adrenergic and serotonergic mechanisms are involved in HR responses. The catecholamine responses to m-CPP appear to be partially mediated by 5-HT1C receptors and also by nonserotonergic mechanisms.

Delayed effects of chronic cortisol treatment on brain and plasma concentrations of corticotropin (ACTH) and beta-endorphin.

The effects of 2-day and 7-day cortisol treatment on immunoreactive corticotropin (ACTH) and beta-endorphin concentrations were measured in the cerebral cortex, hippocampus, hypothalamus, and cerebellum in male rats. Plasma ACTH, beta-endorphin, corticosterone, and cortisol levels were also measured in parallel. Cortisol administration by osmotic minipumps (25 mg/kg/day) maintained a constant, moderately high concentration (23.0 +/- 2.7 micrograms/100 ml) of this glucocorticoid in plasma. Two-day cortisol treatment suppressed the plasma concentration of ACTH and corticosterone, and also decreased, to a lesser degree, concentrations of beta-endorphin. ACTH and beta-endorphin levels in the brain remained unchanged after 2 days of cortisol treatment. After 7-day treatment, however, plasma concentrations of ACTH and beta-endorphin further decreased, while ACTH and beta-endorphin concentrations in the cortex and beta-endorphin concentrations in the cerebellum were also significantly decreased. Peptide concentrations in other brain areas did not change significantly with either 2-day or 7-day cortisol treatment. These data suggest that there are delayed effects of glucocorticoids on pro-opiomelanocortin peptide secretion and/or metabolism in the central nervous system. These findings are consistent with the impaired cognitive functions of patients with diseases, such as Cushing's syndrome and depression, that have long-lasting elevated cortisol secretion.

Marked increases in plasma catecholamine concentrations precede hypotension and bradycardia caused by 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) in conscious rats.

Plasma noradrenaline and adrenaline responses to 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), a selective putative 5-HT1A receptor agonist, have been studied in conscious, freely moving rats. Intravenously administered 8-OH-DPAT caused dose-related and sustained increases in plasma noradrenaline (2-fold) and adrenaline (11-fold) concentrations. Neither metergoline pretreatment (0.5 mg kg-1 i.v.) nor splanchnicectomy had any effect on the noradrenaline and adrenaline elevation caused by 8-OH-DPAT (250 micrograms kg-1 i.v.). The catecholamine responses peaked early but were still present during nadirs in blood pressure and heart rate. The discrepancy between plasma catecholamine and cardiovascular changes raises further questions about the mechanism of action of 8-OH-DPAT and supports other evidence suggesting a role for vagus stimulation in the cardiovascular effects caused by this drug.

Opposite effects of chronic cortisol treatment on pre- and postsynaptic actions of clonidine in pithed rats.

1. Both clinical and experimental studies have shown that chronic elevation of plasma cortisol levels are attended by altered adrenergic receptor function. In the present study we examined the effects of chronic cortisol treatment (25 mg kg-1 day-1 for 7 days by minipumps) on the peripheral cardiovascular responses of pithed, adrenal demedullated vagotomized rats. 2. Chronic cortisol treated rats had higher basal diastolic blood pressures after being pithed, suggesting that sympathetic outflow is not required to sustain elevated peripheral resistance in glucocorticoid-induced hypertension. 3. Whereas alpha 1-adrenoreceptor-mediated diastolic blood pressure responses were unaltered, alpha 2-adrenoreceptor-mediated vasopressor responses were potentiated in pithed rats which were chronically treated with cortisol. 4. Elevation of plasma noradrenaline induced by the stimulation of the entire sympathetic outflow of pithed rats was not changed but the clonidine-induced presynaptic alpha 2-adrenoreceptor inhibition of noradrenaline release was attenuated by chronic cortisol treatment. 5. In conclusion, in adrenal demedullated pithed rats the responses mediated by peripheral pre- and postjunctional alpha 2-adrenoreceptors appeared to be affected oppositely after chronic cortisol treatment, suggesting that the pharmacologically homogeneous alpha 2-adrenoreceptor population may be modulated differently by chronic elevation of plasma glucocorticoids.