Liisa Ahtee

Opiate-receptor mediated changes in monoamine synthesis in rat brain

The effects of morphine, β‐endorphin, naloxone and naltrexone on the rate of tyrosine and tryptophan hydroxylation were investigated in vivo by measuring the accumulation of dopa and 5‐hydroxytryptophan (5‐HTP) in different brain regions of rats after inhibition of the aromatic L‐amino acid decarboxylase. The cerebral concentrations of tyrosine and tryptophan were also measured. Morphine (3–30 mg kg−1) increased the accumulation of dopa dose‐dependently (25–50%) in the dopamine‐rich areas (limbic forebrain and corpus striatum). In the noradrenaline‐predominant parts of the brain (containing hemispheres, diencephalon and lower brain stem) only the highest dose of morphine (30 mg kg−1) significantly increased dopa formation (47%). Similarly to morphine, intracerebroventricularly injected β‐endorphin (5–10 βg per rat) increased the formation of dopa. This increase was doubled in limbic forebrain, corpus striatum and cerebral hemispheres. Doses of 10 to 20 μg of β‐endorphin were needed to increase dopa accumulation in the diencephalon and the lower brain stem. Naloxone antagonized the β‐endorphin‐induced increases in dopa. But naloxone and naltrexone (10–100 mg kg−1) decreased the dopa formation in the dopamine‐rich areas (about 20–25 %) but not in the noradrenaline‐predominant areas. Morphine (30 mg kg−1) and β‐endorphin (5 μg per rat) increased the accumulation of 5‐HTP whereas naloxone and naltrexone (10 mg kg−1) tended to decrease its formation. Morphine and β‐endorphine increased the concentrations of tyrosine and tryptophan, and naloxone decreased the cerebral tryptophan concentration. These results show that the effects of a narcotic agonist (morphine) and of pure narcotic antagonists (naloxone and naltrexone) on the synthesis of dopamine and 5‐HT are opposite to each other. Furthermore, the effects of β‐endorphine on brain monoamine synthesis are remarkably similar to those of morphine. Thus, it is probable that opiate receptors and their endogenous ligands are involved in the regulation of dopamine and 5‐HT synthesis.

The effect of narcotic analgesics on the homovanillic acid content of rat nucleus caudatus

L. AHTEE and I. KAARIAINEN, The effect of narcotic analgesics on the homovanillic acid content of rat nucleus candatus, European J. Pharmacol. 22 (1973) 206–208. The cataleptogenic effects of 10 narcotic analgesics and related drugs were compared to their effects on the content of homovanillic acid (HVA) in the nucleus caudatus of the rat. Generally the HVA increasing and the cataleptogenic properties of the drugs ran parallel. The results Indicate that the structural requirements for the cataleptogenic and HVA-increasing properties of the narcotic analgesics bear some relation to those required for the pain-relieving properties of these drugs.

Acid metabolites of monoamines in avian brain; effects of probenecid and reserpine

1 The concentration of the dopamine (DA) metabolite 3,4‐dihydroxyphenylacetic acid (DOPAC) in the anterior part of the nucleus basalis of pigeon brain was found to be 0.17 ± 0.01 μg/g, which is about one‐fifth of the concentration of homovanillic acid (HVA) in this region. In the chicken, the concentration of HVA in the (entire) nucleus basalis was 0.06 ± 0.006 μg/g, lower than in any other species examined, and giving a ratio of DA to HVA of about 50. The concentration of DOPAC in the 8 day old chick was 0.053 ± 0.002 μg/g. 2 Probenecid, 200 mg/kg intramuscularly, doubled the content of DOPAC in the nucleus basalis of the pigeon and increased the concentration of HVA in both the pigeon and the chicken by a factor of 4 to 5. These findings demonstrate the existence, in avian brain, of an active transport mechanism for the removal of acidic substances and explain the low concentrations of the acids found in bird brain. 3 A method is described for the estimation of 5‐hydroxytryptamine (5‐HT) and 5‐hydroxyindolylacetic acid (5‐HIAA) in the same tissue sample. Probenecid caused an increase in the 5‐HIAA content but produced no change in the 5‐HT content of the nucleus basalis of pigeon brain. 4 Reserpine caused a fall in the content of acidic DA metabolites in the nucleus basalis of the pigeon. The effect was more pronounced after raising the concentration of these acids with probenecid. 5 Treatment of pigeons with pargyline (100 mg/kg 17 hr before decapitation) did not significantly increase the DA content of the nucleus basalis, but it prevented to some extent the loss in DA caused by reserpine. 6 Pigeons and chickens were sedated by probenecid. The deepest sedation occurred at about the same time as the greatest increase in the acidic amine metabolites in the brain. 7 Intracisternal injection of HVA in the pigeon and intravenous injection of large amounts of HVA, DOPAC, 5‐HIAA or 3,4‐dimethoxyphenylacetic acid into newly hatched chicks did not produce any sedation or other effects on behaviour. In contrast, injection of sodium γ‐hydroxybutyrate caused paralysis followed by prostration and eye closure. 8 Estimation of the concentration of HVA in the brain of the young chick after intravenous injection of the acid (100 mg/kg) showed that the concentration was of the same order of magnitude as it is in animals given probenecid; this suggests that the sedation which follows probenecid is not related to the accumulation of acidic amine metabolites.

Differential nicotinic regulation of the nigrostriatal and mesolimbic dopaminergic pathways: implications for drug development.

Neuronal nicotinic acetylcholine receptors (nAChRs) modulate dopaminergic function. Discovery of their multiplicity has lead to the search for subtype-selective nAChR agonists that might be therapeutically beneficial in diseases linked to brain dopaminergic pathways. The regulation and responses of the nigrostriatal and mesolimbic dopaminergic pathways are often similar, but some differences do exist. The cerebral distribution and characteristics of various nAChR subtypes differ between nigrostriatal and mesolimbic dopaminergic pathways. Comparison of nicotine and epibatidine, two nAChR agonists whose relative affinities for various nAChR subtypes differ, revealed differences in the nAChR-mediated regulation of dopaminergic activation between these dopamine systems. Nicotine preferentially stimulates the mesolimbic pathway, whereas epibatidines stimulatory effect falls on the nigrostriatal pathway. Thus, it may be possible to stimulate the nigrostriatal pathway with selective nAChR agonists that do not significantly affect the mesolimbic pathway, and thus lack addictive properties. Furthermore, dopamine uptake inhibition revealed a novel inhibitory effect of epibatidine on accumbal dopamine release, which could form a basis for novel antipsychotics that could alleviate the elevated accumbal dopaminergic tone found in schizophrenia during the active psychotic state. Different regulation of nigrostriatal and mesolimbic dopaminergic pathways by nAChRs could be an important basis for developing novel drugs for treatment of Parkinsons disease and schizophrenia.

Regulation of nicotinic receptors in the brain of mice withdrawn from chronic oral nicotine treatment

The effect of nicotine withdrawal on regional regulation of brain nicotinic receptors was studied in mice after chronic administration of nicotine in the drinking water for 2, 4 or 7 weeks.Two weeks of chronic nicotine administration did not alter the binding of [3H]-nicotine in the midbrain, cortex or cerebellum of the mice, while after both 4-and 7-week treatments a significant increase in the specific [3H]-nicotine binding was observed in cortical and midbrain membranes. In the midbrain, the [3H]-nicotine binding was increased by about 40% in mice withdrawn for 48–72 h from the 4-week chronic nicotine treatment and in mice withdrawn for 48 h from the 7-week treatment. The [3H]-nicotine binding was significantly increased (by 55–65%) in the cortex at 48 h and 72 h after withdrawal from 4-week chronic nicotine and it was even somewhat more increased (by 72–66%) after 7-week treatment. The cortical [3H]-nicotine binding was not altered at 24 h after the 4-week treatment, but in mice withdrawn for 24 h from the 7-week treatment it was increased by 116%. The increases in [3H]-nicotine binding returned to control levels within 1 week after withdrawal. None of the studied treatments affected the [3H]-nicotine binding in the cerebellum. Tolerance towards nicotine-induced locomotor depression was only found in mice withdrawn for 24 h from the 7-week chronic nicotine administration. These findings suggest that at least 4-week chronic nicotine administration in the drinking water is needed before any upregulation of nicotinic receptors can be observed. Furthermore, in our experiments the increase in the [3H]-nicotine binding was seen before behavioural tolerance could be demonstrated. The differences between brain regions in the time course of nicotinic receptor upregulation may reflect variations in nicotinic receptor subunits and their sensitivity to chronic nicotine treatment.

Expression of fos protein in various rat brain areas following acute nicotine and diazepam

We studied the effects of an acute dose of (-)-nicotine (1 mg/kg) on Fos-like immunostaining (IS) in rat brain areas. Nicotine increased Fos IS significantly in the medial terminal nucleus of accessory optic tract (MT), and tended to increase it in the interpeduncular nucleus (i.p.), as well as in the stress-related areas, the paraventricular hypothalamic nucleus (PVN) and the supraoptic nucleus (SON). Previously nicotine was reported to increase Fos IS also in another stress-related area, the central nucleus of amygdala (ACe). This led us to study the interaction of nicotine with diazepam (10 mg/kg). Diazepam alone increased Fos IS in PVN and in SON as well as in ACe. In diazepam- and nicotine-treated rats Fos IS was increased in PVN and SON as well as in MT and i.p.. In MT and i.p. of diazepam and nicotine-treated rats Fos IS was similar to that induced by nicotine alone, and in PVN and SON of these rats Fos IS in ACe. Taken together, diazepam induced Fos IS in all stress-related areas studied (PVN, SON, ACe), but not in central visual structures, where nicotine induces Fos IS (MT, i.p.). No significant interactions on Fos expression were found between acute effects of diazepam and nicotine suggesting that these drugs activate different sets of neurons within the stress-related brain areas.

5-Hydroxytryptamine and 5-hydroxyindolylacetic acid content in brain of rat strains selected for their alcohol intake ☆

Abstract Brain 5-hydroxytryptamine (5HT) and 5-hydroxyindolylacetic acid (5HIAA) contents were studied in two strains of male albino rats of Wistar origin which had been selected for 16 to 17 generations for preferring either alcohol or water. Both 5HT and 5HIAA contents in the brain of the alcohol selecting rats were 15–20% higher those of the water selecting rats. After probenecid the difference in the 5HIAA content was statistically significant. When the rats were given a free choice of ethanol or water for a month the brain 5HT concentration of the alcohol addicted rats was 31% higher and 5HIAA content 10% higher than that of the non-addicted rats. No changes were found in the 5HT and 5HIAA content in the brain of non-alcoholic rats who had been forced to drink the same amount of ethanol as the alcoholic rats drank voluntarily.

Changes in brain monoamine metabolism during withdrawal from chronic oral self-administration of morphine and in response to a morphine challenge in the withdrawn state.

Although p.o. self-administration of morphine is a reliable and convenient means of inducing physical dependence, its effects on brain monoamine metabolism have not been determined. Accordingly, in the present experiment young Wistar rats drank increasing concentrations (0.1-0.5 mg/ml) of morphine in water, or water alone, for 37 days. Half the rats in each group were challenged with morphine (10 mg/kg s.c.) when 27 to 29 hr withdrawn, and half with saline. Rats were sacrificed 2 hr postinjection. Seven brain regions were analyzed for noradrenaline (NA), dopamine (DA), or 5-hydroxytryptamine (5-HT), and their respective metabolites. In all cases in which a comparison could be made with prior work utilizing repeated injections to produce dependence, the p.o. regimen produced the same effects. Thus, the mode of administration does not seem to modify the response of monoaminergic neurons to chronic morphine. In withdrawal, NA turnover increased but DA and 5-HT turnovers decreased. Acute morphine accelerated the turnover of all three monoamines. The NA response was attenuated in some brain regions of withdrawn rats, indicating the development of tolerance to the turnover-enhancing effect of acute morphine in noradrenergic neurons. In contrast, the effect of acute morphine on cerebral 5-HT turnover was not altered, and its effect on cerebral DA turnover was enhanced in withdrawn rats. Our results suggest that there are fundamental differences among the three monoaminergic systems in their capacities for adapting to chronic morphine treatment.

Effects of chronic oral nicotine treatment and its withdrawal on locomotor activity and brain monoamines in mice.

The effects of chronic nicotine and its withdrawal on locomotor activity and brain monoamines were studied using a new animal model of administering nicotine in the drinking water to male NMRI mice as the sole source of fluid. Locomotor activity as well as cerebral concentrations of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), noradrenaline (NA) and 3-methoxy-4-hydroxyphenylethyleneglycol (MOPEG) were measured post mortem on the 50th day of nicotine administration or at 12-14 or 23-25 h after withdrawal. On the 50th day of drug administration the chronically nicotine-treated mice were more active than the control mice drinking tap water and after withdrawal from nicotine the locomotor activity dropped to the level of the controls. In chronically nicotine-treated mice the striatal concentrations of DOPAC, HVA and 5-HIAA, hypothalamic 5-HIAA and NA as well as cortical NA were elevated. The concentrations of DOPAC, HVA and 5-HIAA reversed to control levels within 23-25 h after withdrawal from nicotine. The nicotine-induced elevation of the hypothalamic NA concentration was still significant at 23-25 h after withdrawal. At 12-14 h after withdrawal the hypothalamic concentration of MOPEG was increased. In conclusion, our findings on locomotor activity suggest that administration of nicotine in the drinking water to mice for several weeks seems to be a relevant method to study nicotine dependence. Furthermore, the alterations found in cerebral DA, NA and 5-HT metabolism during chronic nicotine administration indicate that all three cerebral transmitter monoamines might be involved in nicotine dependence and withdrawal.

Catalepsy and stereotypies in rats treated with methadone; Relation to striatal dopamine

Abstract To study methadone-induced catalepsy and stereotyped behaviour, rats were treated for 8 weeks with (±)-methadone. Acute administration of 10 mg/kg of methadone produced catalepsy but no stereotypies in control rats. After 5 weeks of chronic administration methadone still produced dose-dependent catalepsy, the degree of which gradually decreased with continuing treatment. However, all rats showed stereotypies which appeared after 4–5 days and were at their maximum after 5–6 weeks of chronic administration. Naloxone administered before methadone completely prevented the appearance of catalepsy and stereotypy; naloxone administered 0.5–1 hr after methadone abolished the catalepsy and stereotyped symptoms. Reserpine pretreatment prevented the appearance of stereotypies. 2 hr after methadone, 10 mg/kg, the striatal homovanillic acid (HVA) concentration of rats receiving methadone for 8 weeks was increased to about the same degree as in control (saline) rats receiving the same dose of methadone as a single injection; however, 19 hr after the last injection of methadone the striatal HVA concentrations of rats receiving methadone for 8 weeks was decreased to 55% of that of untreated control rats. The results suggest that the primary effect of methadone is catalepsy which causes increased dopamine production as a compensatory mechanism. The additional dopamine is a probable cause of stereotyped behaviour in rats which are partially tolerant to the cataleptic effect of methadone.