The influence of monoamine oxidase inhibition on catecholamine synthesis

On the mechanism of action of clozapine on the adrenergic neurone

1 The antipsychotic drug, clozapine, lowered noradrenaline and metaraminol (MA) concentrations in the rat heart. This action was blocked by the presence of a ganglionic blocking drug. 2 Other alpha-adrenoceptor blocking drugs (phenoxybenzamine, phentolamine) did not significantly lower heart amine concentrations. An inhibitor of neuronal amine uptake (desipramine) caused only a slight lowering. The combination of phentolamine and desipramine showed considerable activity, and this action was blocked by ganglionic blockade. 3 Clozapine had little or no action in blocking amine uptake, yet greatly potentiated amine release caused by the phentolamine-desipramine combination. 4 Other antipsychotic drugs (haloperidol, chlorpromazine, thioridazine) or other agents (propranolol, atropine) did not share this action of clozapine. 5 Ganglionic blockade markedly delayed amine release induced by reserpine administration. 6 It is suggested that clozapine may have an incomplete reserpine-like effect specifically on the adrenergic neurone, facilitating impulse-induced amine release.

On the role of storage granules in the functional utilization of newly synthesized dopamine

Previous studies have provided evidence for the prime importance of newly synthesized dopamine as compared with the large endogenous stores of amine in the functional mobilization of dopamine under conditions of a heavy demand on the neuron such as in the presence of a neuroleptic. The present study was designed to examine the role of storage granules in the utilization of newly synthesized dopamine and to examine the mechanism of the cataleptic action of reserpine-like drugs. It was found that the injection of a small dose of L-Dopa did not reverse the catalepsy produced by the short-acting dopamine depleter, Ro 4-1284, either when the depleter was given alone or was given after tyrosine hydroxylase blockade by alpha-methyltyrosine, provided that the L-Dopa was given at a time when Ro 4-1284 was present. However, the same dose of L-Dopa quickly reversed catalepsy when given at a time of dopamine depletion but in the absence of Ro 4-1284. Since alpha-methyltyrosine alone does not produce catalepsy until exhaustion of the large endogenous dopamine pool, but quickly potentiates neuroleptic-induced catalepsy, it is suggested that under normal conditions the slow transfer of stored dopamine to a releasable site is sufficiently rapid to maintain striatal function. However, the transfer rate appears to be too slow for adequate mobilization of the store under conditions of a heavy demand. It is further suggested that a prime role of storage granules is to channel newly synthesized dopamine into the synaptic cleft. Reserpine-like drugs appear to produce catalepsy, not by depletion per se of the main dopamine pool, but by interference with this granule channeling function.

Neuroendocrine effects of haloperidol therapy in chronic schizophrenia

The neuroendocrine effects of haloperidol therapy have been examined in 62 male chronic schizophrenic patients, aged 16-62 years. The duration of the disease varied between 2 and 29 years. The patients were divided into 48 hebephrenics with onset of the disease at puberty, or immediately after puberty, and 14 paranoids with onset of the disease in adulthood. They received 6 mg i.m.p.d. of haloperidol, for 30 days, up to a total dose of 180 mg. The following hormonal variables were examined before therapy and at 10-20 and 30 days of treatment: total urinary gonadotropins, serum FSH and LH, GH response to insulin stimulation, ACTH reserve (Metyrapone test), total urinary 17-ketosteroids and 17-hydroxycorticoids before and after an ACTH stimulation test, serum testosterone, insulin response to glucose load, plasma thyroxine before and after a TSH stimulation test. The basic hormonal values revealed decreased secretion of total gonadotropins, FSH, LH, ACTH and testosterone, and increased insulin secretion. The haloperidol therapy seemed to stimulate the secretion of FSH, LH, total gonadotropins, ACTH and testosterone, up to normal or low-normal levels. No modifications were observed in the other hormonal variables. The significance of these results is discussed.

The effect of blocking dopamine release on synthesis rate of dopamine in the striatum of the rat

Two experiments were carried out in rats to investigate the mechanisms by which dopamine (DA) synthesis is regulated. First, a unilateral lesion was made in the substantia nigra, thus interrupting the nervous impulse flow of the nigro-striatal pathway. Secondly, the release of DA in the striatum was blocked by means of 1-hydroxy-3-amino-pyrrolidone-2 (HA-966). In both experiments the synthesis rate of DA was accelerated as was shown by analysing the time course of the specific activity of striatal DA after an i.v. injection of 3,5-3H-tyrosine. Furthermore the influence of apomorphine on the rate of DA synthesis, accelerated by HA-966 or by lesion, was investigated. Apomorphine appeared to block the increase of DA synthesis. The results are discussed in the light of a transsynaptic feedback mechanism.

Malignant hypertension resulting from deoxycorticosterone acetate and salt excess: role of renin and sodium in vascular changes

The evolution of malignant hypertension was studied under metabolic balance conditions in 11 uninephrectomized rats given deoxycorticosterone acetate and 1% NaCl as drinking water. Changes in sodium and potassium balance were related to changes in blood pressure, plasma renin activity, hematocrit, and kidney histology. After 3-4 weeks of steadily positive sodium balance accompanied by continuously increasing blood pressure up to 185 plus or minus 19 (SE) mm Hg, periods of sodium loss accompanied by evidence of hemoconcentration were observed marking the onset of the malignant phase as defined by the development of fibrinoid necrosis in the kidney. Plasma renin activity remained markedly suppressed both at the fourth week (0.33 plus or minus 0.02 ng/ml hour-1) when the sodium balance was positive and the kidney biopsy negative and at the end of the experiment (0.35 plus or minus 0.36 ng/ml hour-1) when the sodium balance was negative and the kidney histology revealed malignant vasculitis. Infusion of the angiotensin II inhibitor 1-Sar-8-Ala-angiotensin II consistently failed to affect blood pressure, and the kidney tissue norepinephrine level was reduced (0.054 plus or minus 0.01 mug/g) compared with the control level (0.132 plus or minus 0.02 mug/g). We conclude that malignant vasculitis in this model is preceded by hypertension associated with sodium and water retention and is accompanied by negative sodium balance, decreases in body weight, falling blood pressure, and hemoconcentration without demonstrable participation of the renin-angiotensin system or the renal catecholamines.

On a prime role for newly synthesized dopamine in striatal function

Rats were given either the tyrosine hydroxylase inhibitor, alpha-methyltyrosine (alphaMT), in doses of 10 or 250 mg/kg or the neuroleptic, haloperidol (0.25 mg/kg). Other rats received both drugs (haloperidol 30 min after alphaMT). This dose of haloperidol alone caused only a slight, gradually developing catalepsy, while alphaMT alone caused none. The combination quickly caused a strong catalepsy. Striatal dopamine (DA) stores were only minimally depleted at the time of catalepsy potentiation. Th e marked elevation of striatal homovanilluc acid concentration seen after haloperidol administration was greatly inhibited by alphaMT pretreatment. It is concluded that the marked potentiation of haloperidol-induced catalepsy by alpha MT is related to the absence of newly synthesized DA rather than to an exhausted main DA pool and that newly synthesized DA has a greater role in striatal function than does DA of the main striatal storage pool.

In vivo rates of tyrosine and tryptophan hydroxylation in regions of rat brain at four times during the light-dark cycle

The in vivo rates of tyrosine and tryptophan hydroxylation were studied in three regions of young adult rat brains at 4 times the light-dard cycle. The procedure utilized was to analyze the accumulation of Dopa and 5-HTP after injection of a centrally effective L-amino acid decarboxylase inhibitor, NSD 1015. Monoamine levels were also determined in all control animals and some treated animals. The reate of tyrosine dydrosylation in the telencephalon was significantly higher 7 hrs after dard onset than at the other three times tested. Smaller variations in tyrosine and tryptophan hydroxylation rates as a function of time of day were also observed. 5-HT levels were significantly higher during the light phase than the dark in the telencephalon with the same trend occurring in the diencephalon and brainstem. NA was stable in the telencephalon but reached lower levels in the light and higher levels in the dark in the other two regions. In the telencephalon DA reached high levels early in the light and in the dark phases, showing a biphasic variation. Of particular interest was the apparent lack of carrelation between cyclic changes in the monoamine levels and the changes in hydroxylation rates. Rates of hydroxylation can be considered indicative of rates of monoamine synthesis. This observation is discussed in relation to feedback and other mechanisms regulating synthesis and release of monoamines.

The effects of acutely administered analgesics on the turnover of noradrenaline and dopamine in various regions of the rat brain

1 Noradrenaline and dopamine turnover rates were determined following blockade of synthesis by alpha-methyl-p-tyrosine. Morphine, pentazocine and methadone had no effect on steady state levels or on turnover of noradrenaline in whole brain and in the hypothalamus. Although morphine was without action on medulla-pons noradrenaline steady state levels, a drug-induced increase in turnover rate was observed which was antagonized by pretreatment with naloxone (5 mg/kg). Pentazocine and methadone failed to alter either the steady state level of noradrenaline in the medulla-pons or its turnover rate.2 Morphine accelerated the decline in striatal alpha-methyl-m-tyramine levels following subcutaneous injection of alpha-methyl-m-tyrosine 18 h previously.3 All three drugs increased the turnover of dopamine in whole brain and corpus striatum although the striatal effect was prevented by naloxone pretreatment. The minimum doses of morphine, pentazocine and methadone required to elicit a significant effect on striatal dopamine turnover were 10 mg/kg, 30 mg/kg and 10 mg/kg respectively.4 The possibility of a dopaminergic involvement in the antinociceptive effect of analgesics is discussed.

Potassium deficiency and cardiac catecholamine metabolism in the rat

Norepinephrine metabolism, arterial blood pressure, and electrolytes were studied in rats maintained on either potassium-deficient or control diets. After 3 weeks on the test diets, the arterial blood pressure was significantly lower and the ratio of [Na] to [K] in cardiac tissue was significantly higher in potassium-deficient rats as compared to controls. Ten minutes after intravenous injection of 3 H-norepinephrine the specific activity of cardiac norepinephrine was similar in the control and potassium-deficient groups despite a significant increase in endogenous norepinephrine concentration in the potassium-deficient hearts. But 24 hours after injection, there was a significant increase in 3 H-norepinephrine specific activity, concentration, and content in the potassium-deficient group. Cardiac tissue monoamine oxidase activity was similar in both groups. The increased 3 H-norepinephrine retention and concentration and the decreased arterial blood pressure of the potassium-deficient rate suggest an impairment in norepinephrine release by the sympathetic neurons.

Some effects of caffeine and aminophylline on the turnover of catecholamines in the brain

The effect of caffeine and aminophylline on the turnover of noradrenaline and dopamine in the mouse brain was studied by two different methods: estimation of the rate of disappearance of the transmitter after inhibition of synthesis and measurement of the accumulation of [3H]noradrenaline (3H-NA) and [3H]dopamine (3H-DA) after administration of [3H]tyrosine. After inhibition of the tyrosine hydroxylase the xanthines had little or no effect on the disappearance of both catecholamines. After inhibition of dopamine β-hydroxylase, both xanthines increased the rate of disappearance of noradrenaline. When given in sufficiently large doses both xanthines increased the yield of 3H-NA and 3H-DA from [3H]tyrosine. Pretreatment with a monoamine oxidase inhibitor caused a decrease in the yield of the tritiated catecholamines, which could be counteracted by the xanthines. Stimulation of the noradrenaline receptors by clonidine appeared to cause a decrease in the yield of 3H-NA and an increase in the amount of 3H-DA formed from [3H]tyrosine. Conversely, stimulation of the dopamine receptors by apomorphine caused a decrease in the yield of 3H-DA and an increase in that of 3H-NA. Also, in caffeine-treated animals, clonidine and apomorphine decreased the yield of 3H-NA and 3H-DA respectively. However clonidine could not increase 3H-DA concentrations, nor apomorphine the 3H-NA concentrations more than did caffeine alone. Thus, caffeine and aminophylline appear to increase the rate of turnover of both catecholamines in the brain.

Lesions of central norepinephrine terminals with 6-OH-dopamine: biochemistry and fine structure

Intracisternal injections of 6-hydroxydopamine produce rapid and long-lasting depletion of brain catecholamines without effects on serotonin concentrations. Depletion of norepinephrine is greatest in areas containing only nerve terminals and axons and least in areas containing monoamine cell bodies. The norepinephrine loss is accompanied by electron microscopic evidence of nerve terminal degeneration and decreased turnover. Dopamine loss is less marked and is not accompanied by degeneration or alteration of turnover rate.

Regulation of metaraminol efflux from rat heart and salivary gland

1. The turnover rate of noradrenaline (NA) in heart and submaxillary salivary gland was studied in rats exposed to 4 degrees C or maintained at room temperature (22 degrees C). Cold exposure increased the turnover of the NA store in heart but not in salivary gland.2. In another series of experiments the decline of metaraminol (M) from heart and submaxillary salivary gland was studied in rats exposed to 4 degrees C or maintained at room temperature. Cold exposure accelerated the efflux of M from heart, but not from salivary gland. It is concluded that the accelerated decline of M from heart is the consequence of selective activation of the sympathetic nerves that innervate the heart.3. The turnover of NA was studied in rat heart after the administration of M (100 mug/kg intravenously) or its precursor alpha-methyl-meta-tyrosine (200 mg/kg intraperitoneally). Turnover remained essentially normal after these drugs.4. The administration of desipramine (DMI, 20 mg/kg intraperitoneally) 1 hr after M (100 mug/kg intravenously) induced a rapid sustained efflux of M from heart and salivary gland. The results of this study suggest that the slow decline of M from heart is the result of the great affinity of the amine retrieval mechanism in sympathetic nerve endings for M. DMI inhibits the retrieval mechanism, thus accelerating the efflux of M.