Superfusion of the hypothalamus with gamma-aminobutyric acid. Effect on release of noradrenaline and blood pressure

Inhibition of the dopamine system in rat amygdala attenuates the picrotoxin-induced locomoter hyperactivity and hypertension

The aim of the present study was to investigate whether picrotoxin-induced locomotor hyperactivity and hypertension can be inhibited by dopaminergic inhibition in rat amygdala. Locomotor activity was detected using a modularized infrared light matrix system in freely moving rats. In anaesthetized rats, blood pressure was measured while dopamine release was detected using in vivo voltammetry with carbon fibre electrodes. Systemic administration of picrotoxin (1-4 mg/kg) increased both locomotor activity (including horizontal motion, vertical motion and total distance travelled) and the number of turnings (both clockwise and anticlockwise), but inhibited postural freezing. The locomotor hyperactivity induced by systemic administration of picrotoxin was mimicked by direct injection of a small dose (1-3 micro g in 1.0 micro L) of picrotoxin into the amygdala. In vivo voltammetry data revealed that systemic administration of picrotoxin increased the release of dopamine in the amygdala of rat brain accompanied by hypertension. Local injection of kainic acid into the paramedian reticular nucleus (PRN) of the medulla oblongata decreased both the spontaneous release of dopamine in the amygdala and spontaneous levels of locomotor activity in rats. Furthermore, the picrotoxin-induced locomotor hyperactivity, hypertension and increased amygdaloid dopamine release were all suppressed following chemical stimulation of the PRN with kainic acid. Blockade of dopamine receptors with systemic or intra-amygdaloid injection of haloperidol (a dopamine receptor antagonist) significantly attenuated the picrotoxin-induced locomotor hyperactivity and hypertension. These results demonstrate that picrotoxin-induced hyperactivity and hypertension involve an increase in amygdaloid dopamine transmission that can be modulated by ascending projections from the PRN in the medulla oblongata.

Simultaneous comparison of cerebral dialysis and push-pull perfusion in the brain of rats: a critical review

Over the last 30 years, studies of the in vivo activity of neurotransmitters and other endogenous factors in the brain have comprised a major effort in the neurosciences. Historically, the technology of push-pull perfusion was utilized as a major approach to investigations in this field. In the last 10 years, cerebral dialysis has been used as an alternative method essentially for the same scientific purpose, since the perfusion technique was viewed as difficult and excessively damaging to tissue. This review considers the representative literature in which both systems have been used to study local neurochemical responses to a drug or other chemical factor, a physiological condition or other situation. In addition, new experiments have been undertaken to compare, in the same animal and at the same time, the utility and properties inherent in the techniques of push-pull perfusion and cerebral dialysis in terms of the profile of a neurotransmitter activity and their local histopathological effects. A miniaturized 33/26 ga push-pull needle and a 24 ga dialysis probe were implanted simultaneously in the left and right caudate nuclei, respectively, in the anesthetized rat. An artificial cerebrospinal fluid (CSF) was perfused simultaneously through both devices at a rate of 10 microliters/min in the push-pull cannula and at 1.0 or 2.0 microliters/min in the dialysis probe. Within a series of 8-10 successive perfusions, excess K+ ions in a concentration of either 30 or 60 mM were incorporated in the CSF and delivered simultaneously to both the push-pull cannula and dialysis probe. Samples of perfusate and dialysate were assayed chromatographically by coulometric HPLC detector and quantitated in terms of the pg/min efflux of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA). The results showed that the resting level of DA was almost undetectable in dialysate samples from either structure; in push-pull perfusates the recovery of DA ranged between 7.0 to 10.0 pg/min, which was increased threefold by excess K+ ions. The recovery of DA and the three metabolites in samples of push-pull perfusate was two to four times that in samples of dialysate during the condition of excess K+ ions. Post-mortem histological analysis of the sites of perfusion and dialysis revealed little or no differences in the cytological damage induced by either the perfusion needle or dialysis probe. Finally, the advantages and limitations of each of these two experimental approaches to in vivo analysis of neurotransmitter efflux are reviewed in relation to the selection of an open or closed system for the on-line study of in vivo neurochemical events.

Picrotoxin induces both hypertension and dopamine release in the rat amygdala

The effects of the epileptogenic agent, picrotoxin, on both the cardiovascular responses and the dopamine (DA) release in the amygdala were studied in anesthetized rats. In vivo voltammetry was used to measure change in extracellular concentrations of DA and its metabolites in the amygdala. Intravenous administration of picrotoxin produced hypertension, increased amygdaloid DA release and behavioral syndromes (such as increased masticatory movements, salivation, and forepaw tremors). Direct administration of picrotoxin into the amygdala also induced the same effects. The picrotoxin-induced effects were suppressed by activation of gamma-aminobutyric acid (GABA) receptors with diazepam or depleting brain DA with 6-hydroxydopamine. Blockade of central DA receptors with haloperidol also attenuated the picrotoxin-induced hypertension. These results indicate that picrotoxin affects interactions between GABA neurons and DA system in rat brain to induce hypertension during an epileptic attack.

beta-Alanine and taurine microinjected into the rat caudal ventrolateral medulla increase blood pressure

Unilateral microinjections of GABA, glycine, beta-alanine and taurine into the caudal ventrolateral medulla (CVLM) of the rat, led to an increase in blood pressure and heart rate. The responses to glycine, beta-alanine and taurine but not to GABA could be blocked by strychnine. The responses to taurine and beta-alanine but not to GABA and glycine could be blocked by 6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide (TAG), an antagonist of taurine. The taurine antagonist alone injected bilaterally into the CVLM produced a decrease in blood pressure. From CVLM areas microperfused with Krebs solution, spontaneous release of GABA, glycine, beta-alanine and taurine was detected and high K+ stimulation caused a calcium-dependent release of GABA, beta-alanine and taurine. These results suggest that beta-alanine and taurine as well as GABA may be involved in modulation of the cardiovascular control within the CVLM.

Effects of gamma-vinyl GABA (vigabatrin) on blood pressure and body weight of hypertensive and normotensive rats

Inactivation of GABA was inhibited by gamma-vinyl GABA (GVG) and the effects of the increased GABA level in the brain on blood pressure and body weight of spontaneously hypertensive rats (SHR) and normotensive rats (WKY) were investigated. When started at the age of 8 weeks or 5 weeks, treatment of SHR and WKY with GVG (150 mg/kg, s.c.) for several weeks did not influence systolic blood pressure. In 1-week old SHR, treatment with GVG (up to 150 mg/kg, s.c.) abolished the rise in blood pressure until animals were 8 weeks old. Thereafter, arterial blood pressure started to increase but it remained distinctly lower than that in untreated animals. When started at the age of 1 week, treatment with GVG for 7 weeks did not influence arterial blood pressure in WKY. GVG delayed increase in body weight in SHR and WKY, irrespective of their age. GVG greatly increased GABA levels in the hypothalamus, frontal cortex, brainstem and rest of the brain in both WKY and SHR. It is concluded that an increase in the GABA level in the brain leads to a delay in the development of hypertension in young SHR. Hence, development of genetic hypertension seems to be susceptible to activation of the GABAergic system in a very early critical phase only.

Blood pressure changes modify the release rates of catecholamines in the intermediate nucleus of the solitary tract

In anaesthetized cats, the intermediate aspect of the nucleus of the solitary tract (NTS) was bilaterally superfused with artificial CSF through push-pull cannulae. The release of the endogenous catecholamines dopamine, noradrenaline and adrenaline was determined in the superfusates radioenzymatically. Blood pressure changes were elicited by intravenous injections of drugs (noradrenaline or chlorisondamine), or electrical stimulation of the intermediate NTS with the tip of the push-pull cannula. Intravenous injections of noradrenaline (3 or 10 micrograms/kg) elicited a rise in the arterial blood pressure which was associated with a decrease in the release rate of adrenaline in the intermediate NTS. The release rates of dopamine and noradrenaline were not influenced. The intravenous injection of chlorisondamine (3 mg/kg) lowered blood pressure and diminished the release rate of dopamine in the intermediate NTS. The release rate of noradrenaline was not modified by chlorisondamine. Electrical stimulation of the intermediate NTS contralateral to the superfused nucleus increased moderately the arterial blood pressure and decreased the release rate of noradrenaline and dopamine, while the release of adrenaline was not influenced. The findings suggest that experimentally induced changes in the arterial blood pressure by drugs injected intravenously modify the release rates of adrenaline and dopamine in the intermediate NTS so as to counteract the blood pressure change. In the intermediate NTS, release of adrenaline from adrenergic nerve terminals seems to act hypertensive. The results obtained with chlorisondamine point to a hypotensive function of endogenous dopamine in the intermediate NTS.

Age-dependent changes in cardiovascular responses induced by muscimol infused into the nucleus tractus solitarii and nucleus parabrachialis medialis in rats

1. The effects of muscimol and bicuculline on mean arterial blood pressure (MAP) and heart rate (HR) were studied after their microinfusion into the nucleus tractus solitarii (NTS) and into the nucleus parabrachialis medialis (NPBmed) in 3 and 24 month old rats. 2. In 3 month old rats a dose of 0.01 microgram of muscimol given into the NTS increased MAP, whereas higher doses (0.05, 0.1 and 0.25 microgram) produced dose-dependent bradycardia with either no change (0.05 and 0.1 microgram) or a decrease (0.25 microgram) in MAP. 3. On the other hand, in 24 month old rats, the same doses of muscimol given into the NTS failed to change MAP, whereas in comparison to 3 month old rats they produced a significantly lesser bradycardia. 4. The cardiovascular changes elicited by infusion of muscimol into the NTS in 3 and 24 month old rats were prevented by prior microinfusion of bicuculline into the same site. 5. Muscimol given into the NPBmed in doses from 0.05 to 0.25 microgram produced, in 3 month old rats dose-dependent decreases in MAP and HR which were prevented by prior administration of bicuculline. These effects were significantly reduced in 24 month old rats. 6. The present experiments show that with aging there is impairment of GABA-ergic mechanisms involved in the regulation of cardiovascular activity in the NTS and NPBmed.

Release of endogenous catecholamines in two different regions of the nucleus of the solitary tract as influenced by carotid occlusion

The effects of carotid occlusion on the release of catecholamines in the nucleus of the solitary tract (NTS) were investigated in anaesthetized cats. Two aspects of the nucleus (rostral or intermediate NTS) were superfused bilaterally through push-pull cannulae with artificial CSF and the release of the endogenous dopamine, noradrenaline and adrenaline was determined in the superfusate radioenzymatically. The superfusion rate was 150 microliters/min or 800 microliters/min. In some experiments, superfusion of the intermediate NTS was carried out after denervation of the aortic arch. In the rostral NTS superfused at a rate of 150 microliter/min, bilateral carotid occlusion led to a rise in blood pressure and decreased the release rate of dopamine. These changes continued after occlusion termination. The release rate of noradrenaline was transiently diminished during occlusion. The release of this amine was also decreased after occlusion termination. The release rate of adrenaline was not influenced during carotid occlusion, but it was found to be diminished after termination of the occlusion. Superfusion of the rostral NTS at a rate of 800 microliters/min also reduced the release rate of adrenaline after termination of carotid artery occlusion. In the intermediate NTS (superfusion rate 150 microliters/min) similar effects of the carotid occlusion on the release rates of dopamine and noradrenaline were observed. In this aspect of the NTS, denervation of the aortic arch abolished the decrease in the noradrenaline release during carotid occlusion, while the release rates of dopamine and adrenaline were decreased during and after termination of the carotid occlusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of catecholaminergic neurones of the nucleus of the solitary tract (NTS) in blood pressure regulation

Determination of the release of catecholamines in the rostral and intermediate aspects of the NTS before, during and after termination of a bilateral carotid occlusion revealed that increases in blood pressure elicited by the occlusion reduce the release rates of noradrenaline and adrenaline, while occlusion-induced decreases in blood pressure diminish the release rate of dopamine. These findings demonstrate that, in response to blood pressure changes elicited by carotid occlusion, in both aspects of the NTS noradrenaline and adrenaline act to increase blood pressure when released from their neurones, while the release of dopamine lowers blood pressure. Noradrenergic neurones of the NTS receive impulses from baroreceptors of carotid sinus and aortic arch.

Beta-alanine, like glycine, microinjected into the rat nucleus tractus solitarii increases blood pressure

Microinjections of beta-alanine and glycine into the nucleus tractus solitarii (NTS) of the rat, led to an increase in blood pressure and heart rate. The responses to glycine but not to beta-alanine could be blocked by strychnine. Both amino acids reduced the responses to aortic nerve stimulation. High K+ stimulation caused a calcium-dependent release of beta-alanine from tissues in the area of the NTS. These results suggest that beta-alanine and glycine may modulate the cardiovascular control within the NTS.

Sympathetic stimulation and hypertension in the pyridoxine-deficient adult rat

Pyridoxal phosphate is the coenzyme of various decarboxylases involved in the formation of monoamine neurotransmitters such as gamma-aminobutyric acid, serotonin, dopamine, and norepinephrine. Adult male Sprague-Dawley rats placed on a pyridoxine-deficient diet for 8 weeks showed significant hypertension compared with pyridoxine-supplemented controls. Hypothalamic contents of pyridoxal phosphate, gamma-aminobutyric acid, and serotonin in the pyridoxine-deficient rats were significantly lower than those in pyridoxine-supplemented controls. Hypertension was associated with sympathetic stimulation. Treatment of pyridoxine-deficient rats with a single dose of pyridoxine (10 mg/kg body weight) reversed the blood pressure to normal levels within 24 hours, with concomitant restorations of hypothalamic serotonin and gamma-aminobutyric acid as well as the return of plasma norepinephrine and epinephrine to normal levels. Also, pyridoxine treatment reversed the hypothalamic hypothyroidism observed in pyridoxine-deficient rats. These results indicate an association between pyridoxine deficiency and sympathetic stimulation leading to hypertension.

Involvement of limbic neuronal corticosterone in the regulation of arterial blood pressure

The effect of experimentally induced blood pressure changes on the in vivo release of neuronal corticosterone in limbic and hypothalamic areas was studied in anaesthetized rats. A fall of the arterial blood pressure (ABP) elicited by the intravenous (i.v.) injection of the vasodilatator nitroprusside or the ganglionic blocking agent chlorisondamine decreased the release of corticosterone in the central amygdala (AC) and the ventral hippocampus (VH) whereas an experimentally induced hypertension after i.v. administration of the alpha adrenoreceptoragonist tramazoline led to an enhanced release of the glucocorticoid in the limbic areas mentioned above. Alterations in ABP did not affect the rates of corticosterone release in the medial hypothalamus (MH). The results may indicate a functional role for neuronal limbic corticosterone in central blood pressure regulation.

Differential effects of experimentally induced blood pressure changes on the release of catecholamines in hypothalamic and limbic areas of rats

The effects of longer lasting blood pressure changes on the release of endogenous catecholamines (CA) in limbic and hypothalamic areas were studied in anaesthetized rats. For this purpose the central nucleus of the amygdala (AC), ventral hippocampus (VH) and medial hypothalamus (MH) were simultaneously superfused through push-pull cannulae with artificial cerebrospinal fluid and the release of the endogenous catecholamines dopamine (DA), noradrenaline (NA) and adrenaline (A) was determined before and after blood pressure manipulations. A fall in blood pressure elicited by the ganglionic blocking agent chlorisondamine resulted in different changes of the various CA release patterns in AC. Short lasting increased CA release rates as compared to prehypotension levels could be observed in the hippocampus. The activity of catecholaminergic neurons in MH remained unchanged. A rise in arterial blood pressure induced by intravenous injection of tramazoline did not change the release rates of DA in all 3 brain areas studied. In hippocampus, NA levels in the superfusates decreased initially during hypertension but returned to normal values 40 min after drug injection. In the late phase of hypertension increased rates of release of NA in the amygdala and of A in the hypothalamus could be observed. The different patterns in the release of CA suggest that DA, NA and A are differentially implicated in the regulation of experimentally induced blood pressure changes.

Effects of K+-stimulation and precursor loading on the in vivo release of dopamine, serotonin and their metabolites in the nucleus accumbens of the rat

The in vivo efflux of endogenous 3,4-dihydroxyphenylethylamine (DA 5-hydroxytryptamine (5-HT), 3,4-dihydroxyphenyl-acetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxy-indoleacetic acid (5-HIAA) in the nucleus accumbens of the anesthetized rat was studied using a push-pull cannula. Local perfusion for 10 minutes with 35 mM K+ significantly (P less than 0.01) increased the release of DA and 5-HT, but not their metabolites, from their respective control levels of 0.95 and 0.04 pmol/15 min to 2.5 and 0.23 pmol/15 min. Exposure to 35 mM K+ a second and third time resulted in a decrement in the amount of stimulated release for both DA and 5-HT. This decrease was prevented by local perfusion for 10 minutes with 50 uM L-tyrosine and -tryptophan starting 30 minutes before each episode of depolarization. The baseline amounts of DOPAC, HVA and 5-HIAA observed in the perfusates were several fold higher than the basal levels found for 5-HT and DA. In the absence of precursors, the efflux of DOPAC, HVA and 5-HIAA decreased approximately 60, 40 and 25%, respectively, from the first to the last baseline fraction collected. Addition of precursors prevented the decrease for DOPAC and 5-HIAA but not for HVA. The data indicated that (a) the in vivo release of DA and 5-HT, along with their metabolites, could be simultaneously measured with the present procedure, and (b) when using the push-pull cannula, local perfusion with precursors may be necessary following periods of sustained and/or repeated stimulation in order to replenish the monoamine transmitter pools.

Regulation of the release of neuronally localized corticosterone in rat forebrain areas by catecholamines

The influence of intralimbic and intrahypothalamic application of catecholamine-related compounds on the release of neuronally localized corticosterone in the central amygdala, ventral hippocampus and medial hypothalamus was investigated under in vivo and in vitro conditions. It is reported that agonists and antagonists of the alpha-adrenergic system did not affect the release of corticosterone in the brain areas studied. In contrast, the beta-adrenoreceptor agonist isoprenaline markedly enhanced the release of corticosterone. This response is prevented in the presence of the beta-adrenoreceptor antagonist propranolol. The data reveal that limbic and hypothalamic beta-adrenoreceptors play a role in the regulation of corticosterone release and suggest a functional interaction between catecholaminergic and corticosterone releasing neurons in forebrain areas.

Evidence for the presence of GABAergic and glycine-like systems responsible for cardiovascular control in the nucleus tractus solitarii of the rat

Microinjections of gamma-aminobutyric acid (GABA) and glycine, into the medial area of the nucleus tractus solitarii (NTS) of the rat, led to an increase in arterial pressure and heart rate. The GABA receptor antagonist bicuculline and the glycine receptor antagonist strychnine decreased both of these cardiovascular parameters whereas the GABA uptake inhibitor nipecotic acid produced hypertension. High K+ stimulation caused a calcium-dependent release of GABA and glycine from tissues in the area of the NTS. Our results suggest that GABA and glycine may modulate the cardiovascular control within the NTS.

Effects of GABA natriuresis induced centrally by cholinergic stimulation

Sodium and potassium excretion and urine output have been studied in rats following water loading and intracerebroventricular (i.c.v.) injection of isotonic saline (NaCl-0.15M), gamma-amino butyric acid (GABA), picrotoxin, carbachol, GABA plus picrotoxin, GABA plus carbachol and GABA plus atropine. GABA injection decreased sodium and potassium excretion. Picrotoxin or carbachol injection elicited natriuresis and kaliuresis. GABA injection decreased the effects of the carbachol and atropine injection decreased the effects of the GABA on sodium and potassium excretion. These results suggest an interaction between gabaergic and cholinergic pathways in the control of sodium and potassium excretion.

Measurement of endogenous noradrenaline release in the rat cerebral cortex in vivo by transcortical dialysis: effects of drugs affecting noradrenergic transmission

The release of endogenous noradrenaline was measured in the cerebral cortex of the halothane-anesthetized rat by using the technique of brain dialysis coupled to a radioenzymatic assay. A thin dialysis tube was inserted transversally in the cerebral cortex (transcortical dialysis) and perfused with Ringer medium (2 microliter min-1). Under basal conditions, the cortical output of noradrenaline was stable over a period of at least 6 h and amounted to 8.7 pg/20 min (not corrected for recovery). Histological control of the perfused area revealed very little damage and normal morphology in the vicinity of the dialysis tube. Omission of calcium from the perfusion medium caused a marked drop in cortical noradrenaline output. Bilateral electrical stimulation (for 10 min) of the ascending noradrenergic pathways in the medial forebrain bundle caused a frequency-dependent increase in cortical noradrenaline output over the range 5-20 Hz. Stimulation at a higher frequency (50 Hz) resulted in a levelling off of the increase in cortical noradrenaline release. Systemic administration of the dopamine-beta-hydroxylase inhibitor bis-(4-methyl-1-homopiperazinylthiocarbonyl) disulfide (FLA 63) (25 mg/kg i.p.) markedly reduced, whereas injection of the monoamine oxidase inhibitor pargyline (75 mg/kg i.p.) resulted in a progressive increase in, cortical noradrenaline output. d-Amphetamine (2 mg/kg i.p.) provoked a sharp increase in cortical noradrenaline release (+450% over basal values within 40 min). Desmethylimipramine (10 mg/kg i.p.) produced a twofold increase of cortical noradrenaline release. Finally, idazoxan (20 mg/kg i.p.) and clonidine (0.3 mg/kg i.p.), respectively, increased and decreased the release of noradrenaline from the cerebral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo release of corticosterone in discrete brain areas of rats. A push-pull perfusion study

The spontaneous and depolarization-induced release of corticosterone was studied in the medial hypothalamus (MH) and the central amygdala (AC) in vivo, using a push-pull perfusion system. Significant variations in the spontaneous release of corticosterone could be observed over time (3-8 h). Corticosterone in the brain superfusates did not seem to stem from adrenocortical sites since a continuous release of the glucocorticoid could also be followed in bilateral adrenalectomized rats. Superfusion of MH and AC with depolarizing stimuli led to an enhanced release of corticosterone. Of greater general significance is the first demonstration that the release of a steroid hormone can be measured in discrete brain areas under in vivo conditions.

alpha-Adrenoceptor agonists applied in the area of the nucleus tractus solitarii in the rat: effect of anesthesia on cardiovascular responses

We investigated the blood pressure and heart rate responses to unilateral local application of norepinephrine or clonidine in the area of the nucleus tractus solitarii (NTS) of conscious rats and rats anesthetized by 3 general anesthetics commonly used in cardiovascular research. In the conscious state, microinjection of either substance produced an immediate, sharp increase in blood pressure and decrease in heart rate with a gradual return to baseline over a period of more than 30 min. However, light ether anesthesia, nembutal, or urethane drastically reduced, abolished or even reversed the pressor effect of local microinjection of the same substance. Our results suggest that in conscious rats, the initial effect of alpha agonists in the NTS area is to modulate baroreceptor reflexes towards higher blood pressure levels and that general anesthesia may significantly alter or mask the cardiovascular responses to various experimental manipulations.

Further evidence for, and nature of, the facilitatory GABAergic influence on central noradrenergic transmission

In order to explore the nature of the facilitatory GABAergic control of cerebral noradrenergic neurons, we have studied the effect of a variety of GABA mimetics (given systemically or injected locally into brain areas containing noradrenergic cell bodies or terminals) on several indices of noradrenaline turnover in the rat brain. Systemic administration of both direct and indirect acting GABA mimetics enhanced; 1) the pargyline induced accumulation of normetanephrine in the hypothalamus; 2) total DOPEG levels in a number of brain regions innervated by noradrenergic neurons; 3) both DOPAC and MOPEG levels in noradrenergic cell body areas (A1, A2 and A6). These effects are probably mediated by GABAA receptors as specific GABAA or mixed GABAA/GABAB agonists but not the GABAB agonist baclofen enhanced noradrenaline turnover. Interruption of noradrenergic impulse flow (by local injection of tetrodotoxin or by hemitransection) blocked the ability of progabide to increase DOPEG concentrations in the hypothalamus and cerebral cortex. Similarly, the co-administration of clonidine with progabide antagonized the progabide-induced increase in hypothalamic total DOPEG levels. Co-administration of yohimbine with progabide provoked an additive effect on hypothalamic DOPEG levels at moderate but not at high doses of yohimbine. Thus, the acceleration of noradrenaline turnover induced by GABA mimetics appears to depend on ongoing activity in noradrenergic neurons and occurs via an increase in neuronal discharges. Local injection of muscimol into the nucleus accumbens or hypothalamus failed to affect DOPEG levels in these structures; similarly, local injection of muscimol into the locus coeruleus failed to modify DOPEG levels in corresponding noradrenergic projection areas. These data indicate that the GABAergic influence is not exerted via GABA receptors located on noradrenergic cell bodies or nerve endings. Furthermore, since systemically administered progabide still increased hypothalamic DOPEG levels after ibotenate-induced destruction of the hypothalamic neuronal cell bodies, a presynaptic modulation of noradrenergic neurons by local GABAergic interneurons is excluded. Chemical destruction of serotoninergic pathways or enhancement of 5-HT transmission by quipazine failed to alter the ability of progabide to increase cerebral DOPEG levels. Moreover, scopolamine or naloxone also failed to affect the progabide-induced increase in cerebral DOPEG levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition by yohimbine of the calcium-dependent evoked release of [3H]GABA in rat and mouse brain slices in vitro

Calcium-dependent release of [3H]GABA was elicited by electrical stimulation in slices of rat and mouse cerebral cortex or by potassium stimulation in the mouse brain-stem. The stimulation-evoked release of [3H]GABA was inhibited by yohimbine in a concentration-dependent manner. High concentrations of other alpha-adrenoceptor antagonists such as phentolamine, RS 21361, idazoxan, rauwolscine and corynanthine also inhibited [3H]GABA release. This effect was not observed with pseudoyohimbine or prazosin. [3H]GABA release was not affected by exposure to the alpha-adrenoceptor agonists clonidine, M7, noradrenaline or methoxamine. In addition, clonidine did not antagonize the yohimbine-induced inhibition of [3H]GABA release. The inhibitory effect of yohimbine did not result from an interaction with endogenously released noradrenaline since the inhibition was still observed in reserpine-pretreated animals. It is concluded that yohimbine and other alpha 2-adrenoceptor antagonists inhibit the stimulation-evoked release of [3H]GABA through a mechanism which appears to be independent of the blockade of alpha 2-adrenoceptors and which does not involve an interaction with endogenous noradrenaline. The present results indicate that yohimbine exerts non-specific actions on the release of [3H]GABA and that similar effects can be observed with other alpha-adrenoceptor blocking agents in high concentrations. Consequently, when studying the effects of yohimbine and other alpha 2-adrenoceptor antagonists on noradrenergic neurotransmission, the possibility of non-specific effects should be taken into consideration, particularly in the high concentrations range.

Temporal relationship between noradrenaline release in the central amygdala and plasma noradrenaline secretion in rats and tree shrews

In pentobarbital anesthetized Wistar rats and tree shrews (Tupaia belangeri) the hypothalamus, the hippocampus and the amygdala were simultaneously superfused through push-pull cannulae with artificial cerebrospinal fluid. Blood samples were withdrawn in order to make an attempt to correlate release rates of noradrenaline (NA), adrenaline and dopamine in the above-mentioned areas with plasma catecholamine levels. A strong, positive correlation was found between NA release in central amygdala and NA concentrations in peripheral blood suggesting a functional relationship between noradrenergic systems in discrete brain areas and the activity of the peripheral sympathetic nervous system.

In vivo release by histamine agonists and antagonists of endogenous catecholamines in the cat hypothalamus

The posterior hypothalamus of anaesthetized cats was superfused through a push-pull cannula with histamine agonists and antagonists and the release of endogenous catecholamines was determined in the superfusate. Hypothalamic superfusion with histamine, 2-methylhistamine (H1-agonist), dimaprit (H2-agonist) or metiamide (H2-antagonist) enhanced the release of the catecholamines dopamine, noradrenaline and adrenaline. The releasing effects of these substances depended on the presence of calcium ions. Superfusion with 2-pyridylethylamine (H1-agonist) was virtually ineffective, while superfusion with 2-thiazolethylamine (H1-agonist) enhanced the rates of release of noradrenaline and adrenaline without influencing the release of dopamine. Superfusion with mepyramine (H1-antagonist) inhibited the release of noradrenaline and adrenaline without affecting the release of dopamine. Hypothalamic superfusion with a concentration of procaine which was equi-anaesthetic to that of mepyramine was ineffective. Ranitidine (H2-antagonist) did not alter the rates of release of the catecholamines. The releasing effect of histamine was inhibited on hypothalamic superfusion with mepyramine and ranitidine. Ranitidine also inhibited the releasing effects of dimaprit and 2-methylhistamine thus indicating that the releasing action of the latter compound was mainly due to stimulation of H2-receptors. These data suggest that blockade of H1-receptors of the posterior hypothalamus reduces the release of noradrenaline and adrenaline, while stimulation of H1-receptors seems to increase the rates of release of these two catecholamines. Stimulation of H2-receptors enhances the release of all three catecholamines. Thus, dopaminergic neurones of the hypothalamus seem to possess H2-receptors, while noradrenergic and adrenergic neurones possess H1- and H2-receptors.

Growth hormone secretion of the neonatal rat pituitaries is stimulated by gamma-aminobutyric acid in vitro

Growth hormone secretion from pituitaries of neonatal rats was stimulated by gamma-aminobutyric acid (GABA) and the GABA agonist muscimol in vitro. This response to GABA was absent after the 9th postnatal day. The stimulation of growth hormone secretion by GABA was antagonized by bicuculline-methiodide and by picrotoxin. Diazepam stimulated while baclophen had no effect on growth hormone secretion. This stimulatory GABA effect might be related to a certain developmental stage of the pituitary GABA receptors or to the lack of hypothalamic regulatory influence(s) in the newborn.

The regulation of cardiovascular functions by monoamine neurotransmitters in the brain

The complexity of the central nervous system's influence on cardiovascular integration is underscored by the extensive neural interconnections between the various higher and lower brain structures shown by electrophysiological studies to have an influence on cardiovascular function. Furthermore, complex neural connections exist within each level of cardiovascular integration (for instance, the medullary reticular formation, or the limbic system). The electrophysiological analysis of the integrative aspects of central cardiovascular regulation has been the subject of several reviews (Reis, 1972; Smith, 1974; Calaresu et al., 1975; Oberg, 1976). In recent years, the methodology of analyzing the anatomical neural pathways of central cardiovascular regulation by lesion-degeneration, evoked potential, and fluorescent histochemical procedures, has been improved with the introduction of anterograde and retrograde tracer techniques. The result is the delineation of previously undetected neural interconnections among various brain areas, long known as cardiovascular centres, such as the NTS, the parabrachial nucleus, the paraventricular nucleus of the hypothalamus, the central nucleus of the amygdala, and the bed nucleus of the stria terminalis (Loewy & McKellar, 1980). Undoubtedly the next few years will see major advances in the understanding of the neuroanatomical pathways involved in central cardiovascular control.

Release of endogenous histamine in the hypothalamus of anaesthetized cats and conscious, freely moving rabbits

The hypothalamus of anaesthetized cats and conscious, freely moving rabbits was superfused with CSF through double-walled, push-pull cannulae and the release of endogenous histamine was determined in the superfusates by a radioenzymatic assay. In the posterior hypothalamic area of the anaesthetized cat, the rate of release of endogenous histamine varied rhythmically; phases of high rate of release appeared at 60 min cycles. The release of histamine was increased by electrical stimulation of the superfused area, as well as by hypothalamic superfusion with potassium-rich CSF. In the conscious rabbit, the anterior hypothalamic area and the posterior hypothalamic nucleus were superfused simultaneously. In both regions, the resting release of histamine varied rhythmically at approximately 70 min cycles. Phases of high or low-rate of release in the anterior hypothalamic area coincided with the corresponding phases in the posterior hypothalamic nucleus. The rhythmic release of endogenous histamine in the hypothalamus, as well as the ability of depolarizing stimuli to enhance the release of the amine support the idea that histamine acts as a neurotransmitter in the central nervous system.

The release of endogenous catecholamines in the cat hypothalamus is affected by spinal transection and drugs which change the arterial blood pressure

1. In anesthetized cats, posterior and anterior hypothalamic areas were superfused with CSF through double-walled cannulae. The release of endogenous catecholamines (dopamine, noradrenaline and adrenaline) was determined in the superfusate by a radioenzymatic assay. 2. Transection of the brain caudal to the hypothalamus almost abolished the release of catecholamines in the posterior hypothalamic area, while that in the anterior hypothalamic area was moderately decreased. A circular transection around the hypothalamus greatly reduced the release of catecholamines in the anterior hypothalamic area. It is concluded that the catecholaminergic nerve endings of the anterior hypothalamic area do not originate entirely from cell bodies located in the brain stem. 3. Spinal transection elicited a pressor response which was followed by a fall of the arterial blood pressure. The pressor response was associated with increased rates of release of the catecholamines in the anterior hypothalamic area, while the release of catecholamines in the posterior hypothalamic area was reduced. The fall of blood pressure enhanced the rates of release of the catecholamines in the posterior hypothalamic area and reduced their release in the anterior hypothalamic area. 4. Chlorisondamine i.v. caused a fall of blood pressure which was associated with an increased release of catecholamines in the posterior hypothalamic area and a decrease in the rates of release in the anterior hypothalamic area whilst i.v. tramazoline elicited a pressor response and enhanced the rates of release of the catecholamines in the anterior hypothalamic area. 5. It is concluded that pronounced changes in the arterial blood pressure affect the release of catecholamine in opposite direction in the two hypothalamic areas to counteract the blood pressure change.

Hypothalamic superfusion with histamine agonists and antagonists modifies the pressor response to hypothalamic stimulation

The posterior hypothalamus of anaesthetized cats were superfused with CSF through a push-pull cannula and electrically stimulated with its tip. Hypothalamic stimulation elicited a pressor response. Superfusion of the hypothalamus with histamine-containing CSF elicited a concentration-dependent enhancement of the pressor response. The enhancing effect of histamine was abolished, when the hypothalamus was pre-superfused with metiamide, whereas pre-superfusion with mepyramine only slightly inhibited the effect of histamine. Hypothalamic superfusion with dimaprit caused a concentration-dependent increase in the pressor response. The effect of dimaprit was abolished by pre-superfusion of the hypothalamus with metiamide, but it was not affected by pre-superfusion with mepyramine. Superfusion of the hypothalamus with 2-methylhistamine did not affect the pressor response. It is concluded that histamine increases the pressor response to hypothalamic stimulation. The enhancing effect of histamine is mainly due to its action on H2-receptors of the posterior hypothalamus.

Studies on the mechanism of the GABAergic inhibition of TSH secretion in male rats

The mechanism of the GABAergic inhibition of the TSH cold-response was studied in male rats. Picrotoxin, a putative GABA receptor antagonist (4 mg/kg but not 0.5 or 2 mg/kg intraperitoneally) significantly decreased the cold-stimulated TSH secretion but did not modify the TRH-induced (100 ng intraperitoneally) TSH-response. The inhibitory effects of two GABAergic drugs muscimol (1 mg/kg subcutaneously) and aminooxyacetic acid (10 mg/kg intraperitoneally) were not antagonized by picrotoxin (1-4 mg/kg intraperitoneally), by phenoxybenzamine (2.5 or 5 mg/kg intraperitoneally), by two 5-HT antagonists cyproheptadine (5 mg/kg intraperitoneally and R 41468 (2.5 mg/kg intraperitoneally) or by pimozide (0.5, 1.5 or 2.5 mg/kg subcutaneously). The results suggest that neither picrotoxin-sensitive GABA receptors nor noradrenaline, 5-HT or dopamine pathways seem to be involved in the inhibitory action of GABAergic drugs on the TSH cold-response in male rats.

Presynaptic regulation of the release of catecholamines in the cat hypothalamus

The posterior hypothalamus of cats was superfused through a push-pull cannula and the release of endogenous catecholamines was determined in the superfusate. Superfusion with yohimbine, isoprenaline, salbutamol or tazolol increased, while superfusion with propranolol decreased, the release of all three catecholamines. Transection of the brain caudal to the hypothalamus inhibited 'resting' and drug-induced release. It is concluded that alpha- and beta-adrenoceptors of the hypothalamus are involved in the regulation of the release of catecholamines.

Evidence for the involvement of a GABA-mediated inhibition in the hypovolaemia-induced vasopressin release

The influence of GABA and of drugs, known to alter GABA-metabolism, on the hypovolaemia-provoked vasopressin release was investigated in rats. Blood volume was decreased without altering plasma osmolality or arterial blood pressure by i.p. injection of polyethylene glycol and the resulting plasma vasopressin concentration was measured using a radioimmunoassay. I.c.v. injections of GABA (0.4-2 mg) markedly suppressed the hypovolaemia-induced vasopressin release. The central inhibitory effect of GABA could not be related to appropriate changes in peripheral parameters believed to regulate vasopressin release (arterial blood pressure, renin-angiotensin system). Aminooxyacetic acid (9-81 mg kg-1, i.m.) and gamma-vinyl-GABA (1.5 g kg-1, i.p.), two potent inhibitors of GABA aminotransferase and known to increase brain GABA content, reduced vasopressin release to a comparable degree as did GABA (i.c.v.). On the other hand, 3-mercaptopropionic acid (10-90 mg kg-1, i.p.), an inhibitor of the GABA synthetizing enzyme glutamic acid decarboxylase, promoted the release of vasopressin when the rats were killed prior to the onset of convulsions. These results, on the whole, intimate the existence of a GABA-mediated inhibition in the central control of vasopressin release.

Failure of isoprenaline and beta-receptor blocking drugs to modify depressor response and bradycardia induced by electrical stimulation of the anterior hypothalamus of cats

The role of the hypothalamic beta-adrenoceptors in the depressor response and bradycardia induced by stimulation of the anterior hypothalamus was studied in cats. In chloralose and urethane anaesthetized cats the anterior hypothalamus was superfused with artifical cerebrospinal fluid through a push-pull cannula. Electrical stimulation of the anterior hypothalamus with the tip of the cannula elicited a fall of systemic blood pressure and a decrease in heart rate. Superfusion of the anterior hypothalamus with isoprenaline did not change the depressor response and bradycardia induced by electrical stimulation of the anterior hypothalamus. Superfusion with atenolol or butoxamine also failed to modify the responses. Superfusion with (+/-)-propranolol significantly suppressed the responses. However, superfusion with (+)-propranolol suppressed the responses to the same extent. The resting systemic blood pressure and heart rate were not significantly changed by superfusion of the hypothalamus with these drugs. These results suggest that beta-adrenoceptors of the anterior hypothalamus are not involved in the depressor response and bradycardia elicited by hypothalamic stimulation.

Rise in blood pressure increases the release of endogenous catecholamines in the anterior hypothalamus of the cat

In anaesthetized cats, anterior and posterior hypothalamic areas were simultaneously superfused with artificial CSF using two push-pull cannulae. The rates of release of endogenous catecholamines were determined in the superfusates which were continuously collected in periods of 10 seconds. In both areas, the rate of release of dopamine was higher than the rates of release of noradrenaline and adrenaline. Electrical stimulation of the splanchnic nerve elicited a rise in the arterial blood pressure and increased the rates of release of noradrenaline, adrenaline and dopamine in the anterior hypothalamic area. The rates of release of the catecholamines in the posterior hypothalamic area were not influenced by the pressor response to stimulation of the splanchnic nerve. Transection of the brain caudal to the hypothalamus reduced the rates of release of the catecholamines in the anterior hypothalamus and abolished the releasing effect of the stimulation of the splanchnic nerve. The results indicate that acute rise in blood pressure activates catecholamine cell bodies in the lower brain stem, which in turn stimulate the anterior hypothalamus to counteract the pressor response.

In vivo release of endogenous GABA in the cat hypothalamus

The posterior hypothalamus of anaesthetized cats was superfused with artificial cerebrospinal fluid through a push-pull cannula and the release of endogenous GABA from the hypothalamus into the superfusate was studied. The resting release of GABA varied rhythmically, since phases of high rate of release were separated from each other by phases of low rate of release. The time interval between two adjacent phases of high rate of release was about 70 min. Electrical stimulation of the posterior hypothalamus with the tip of the cannula enhanced the rate of release of GABA in a frequency-dependent way. Superfusion of the hypothalamus with CSF which contained a high concentration of potassium and a low concentration of sodium increased the rate of release of GABA; this effect was dependent on the presence of calcium ions in the superfusing fluid. Pretreatment of the cats with reserpine reduced the levels of GABA in hypothalamus and rest of brain and the concentration of GABA in the superfusate as well. Stimulation of the locus coeruleus with a bipolar electrode elicited an increased release of GABA in the hypothalamus.

In vivo release of endogenous catecholamines in the hypothalamus

The posterior hypothalamus of anaesthetized cats was superfused with a push-pull cannula and the release of the endogenous catecholamines noradrenaline, adrenaline and dopamine was determined in the superfusate. The rate of release of the three catecholamines followed an ultradian rhythm, the time interval between two adjacent phases of high rate of release being about 70 min. Pretreatment of the animals with reserpine decreased the levels of catecholamines in the hypothalamus and rest of the brain and reduced their rate of release into the superfusate. Hypothalamic superfusion with superfusing fluid of high concentration of potassium and low concentration of sodium enhanced the rate of release of noradrenaline and adrenaline; this effect was abolished when the hypothalamus was superfused with calcium-free solution. Electrical stimulation of the locus coeruleus ipsilateral to the superfused hypothalamus increased the release of noradrenaline and adrenaline, stimulation of the contralateral locus coeruleus enhanced the release of noradrenaline, adrenaline and dopamine. In both cases, the rate of release of adrenaline was enhanced to a lesser extent than the rate of release of noradrenaline. The release of noradrenaline and adrenaline was increased to a higher extent on stimulation of the ipsilateral locus coeruleus than on stimulation of the contralateral one.

Central adrenoceptors and cholinoceptors in cardiovascular control

1. In cats anaesthetized with chloralose, adrenoceptor and cholinoceptor agonists and antagonists were localized to the posterior hypothalamus (PH), lateral medullary pressor area (LMPA) and spinal autonomic loci to delineate the role of central cholinoceptors and adrenoceptors in cardiovascular control. 2 All along the neuroaxis, the alpha-adrenoceptors seem to subserve an inhibitory and the beta-adrenoceptors a facilitatory role in cardiovascular control. There appear to be a predominance of alpha-adrenoceptors at the medullary level and beta-adrenoceptors at the hypothalamic level. 3 The nicotinic cholinoceptors at the hypothalamic, medullary and spinal levels were facilitatory, whereas muscarinic cholinoceptors were inhibitory for cardiovascular control. However, muscarinic receptors were undetectable at the posterior hypothalamus. 4 The central cardiovascular effects of nicotine are attributed to nicotinic receptor activation and release of central catecholamines. 5 There appears to be a relationship between central cholinergic and adrenergic mechanisms in cardiovascular control.

The effect of intracerebroventricularly administered GABA on brain monoamine metabolism

Intracerebroventricular injection of gamma-aminobutyric acid (GABA) was performed in male rats and the brain monoamines, 5-hydroxyindoleacetic acid (5-HIAA), tyrosine and tryptophan levels were measured. GABA induced within 30 min a marked dose-dependent increase in the brain contents of dopamine (DA), serotonin (5-HT), tyrosine and tryptophan, while noradrenaline (NA) was lowered. Large doses of GABA, i.e. 1.5-3 mg/rat, were required for these effects. Aminooxyacetic acid (AOAA), an inhibitor of GABA-transaminase, when given alone in a dose of 25 mg/kg i.p. caused a significant rise of DA, 5-HT and tryptophan. The combination of GABA and AOAA raised these levels more than either agent alone. Picrotoxin (4 mg/kg, i.p.) a claimed GABA receptor antagonist partially counteracted the GABA-induced DA rise. Monoamine synthesis was studied in different parts of the brain by measuring the accumulated dopa and 5-hydroxytryptophan (5-HTP), 30 min after NSD 1015 (3-hydroxybenzylhydrazine HCl, 100 mg/kg) an inhibitor of aromatic L-amino-acid decarboxylase, given i.p. 5 min after GABA. GABA caused a marked rise in dopa formation both in DA- and NA-predominated brain regions. Also 5-HTP formation was enhanced. The effects on both dopa and 5-HTP formation showed marked regional differences. The data suggest that GABA, by activating specific receptors, causes inhibition of firing of dopaminergic neurones and the opposite effect on the noradrenergic neurones. The nature of the effect on 5-HT metabolism needs further investigation.

Presence of beta-adrenoreceptors in the hypothalamus; their importance for the pressor response to hypothalamic stimulation

The posterior hypothalamus of cats anaesthetized with pentobarbital sodium was superfused and electrically stimulated with a push-pull cannula. Superfusion of the hypothalamus with (+/-)-, (-)-propranolol, sotalol, practolol or metoprolol caused a concentration-dependent inhibiton of the pressor response to hypothalamic stimulation. (+/-)-Propranolol and a procaine concentration equi-anaesthetic to the concentration of (+/-)- and (-)-propranolol were ineffective. Lower concentrations of propranolol and metoprolol were needed to inhibit the pressor response than of sotalol or practolol. Superfusion with practolol and tolazoline impaired the pressor response to a greater extent than did superfusion with each of the drugs alone. Hypothalamic superfusion with isoproterenol elicited a concentration-dependent enhancement of the rise of blood pressure during electrical stimulation of the hypothalamus. It is concluded that beta-adrenorecptors are present in the posterior hypothalamus and that they are involved in the pressor response elicited by electrical stimulation of the hypothalamus. Propranolol and metoprolol seemed to possess a higher affinity to the beta-receptors of the hypothalamus than sotalol or practolol.