Yuen-Sum Lau

Receptor mechanisms of nicotine-induced locomotor hyperactivity in chronic nicotine-treated rats

Rats were pretreated with saline or nicotine (1.5 mg/kg per day) by subcutaneously implanting each animal with an Alzet osmotic mini-pump which continuously released saline or nicotine for 1, 5 and 14 days. At the end of each pretreatment period, animals were used for (i) determining their locomotor response to acutely injected nicotine (0.2 mg/kg, s.c.) and (ii) measuring the density of L-[3H]nicotine and [3H]spiperone binding sites in the striatum. We observed no changes in nicotine-induced locomotor response, striatal L-[3H]nicotine and [3H]spiperone binding in the animals pretreated with nicotine for 1 day. In rats which were pretreated with nicotine for 5 days, there was a significant increase in the nicotine-stimulated locomotor response which was associated with an increase in the number of L-[3H]nicotine binding sites and also with an elevated dopamine (DA) level in the striatum. The number of striatal [3H]spiperone binding sites was not affected. In animals pretreated with nicotine for 14 days, the nicotine-induced locomotor response remained to be potentiated. However, this response was correlated with an elevated number of striatal [3H]spiperone binding sites, whereas the number of striatal L-[3H]nicotine binding sites and the striatal DA level were normal. These results suggest that chronic nicotine-treated rats develop locomotor hyperactivity in response to nicotine initially due to increases of both the density of nicotinic receptors and DA concentration, followed by inducing DA receptor supersensitivity in the striatum.

Effects of probenecid on striatal dopamine depletion in acute and long-term 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice

1. The effect of probenecid on striatal dopamine depletion in acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice was examined. 2. Mice treated with a single dose of MPTP (15 mg/kg, s.c.) showed a significant depletion of striatal dopamine throughout a time-course of 7 days. Interestingly, this MPTP-induced striatal dopamine depletion was potentiated by a concomitant injection with a single dose of probenecid (250 mg/kg, i.p.). 3. However, this potentiation of dopamine depletion by probenecid was only a transient phenomenon seen at 4-5 days after the treatment. 4. In a long-term study, mice were treated with the same dosages of MPTP or probenecid plus MPTP twice a week for 5 weeks, we detected that probenecid plus MPTP caused a persistent depletion of striatal DA for 6 months. 5. During this period a partial recovery of DA levels was seen with MPTP alone-treated mice. 6. The detailed mechanisms by which probenecid causes acute potentiation and persistent long-term depletion of striatal dopamine by MPTP are still unclear. 7. With the evidence presented in this study, we determined that after the administration of MPTP in mice, the drug was rapidly metabolized in the periphery and excreted as MPTP N-oxide. 8. Probenecid was shown to inhibit the excretion of urine and urinary MPTP N-oxide shortly after MPTP administration, which may directly or indirectly increase the neurotoxic action of MPTP in mice.

Effect of nicotine pretreatment on striatal dopaminergic system in rats.

Rats were pretreated with saline or nicotine (1.5 mg/kg/day) by subcutaneously implanting each animal with an Alzet osmotic minipump for 1 or 14 days. Short-term (1-day) administration of nicotine to rats reduced the stimulatory effect of (+)-amphetamine on locomotor activity. This was correlated with an attenuation in the ability of (+)-amphetamine to stimulate [3H]dopamine formation from [3H]tyrosine in rat striatal slices of these nicotine-treated animals. In long-term (14-day) nicotine-pretreated animals, both the apomorphine- and (+)-amphetamine-induced locomotor activity were potentiated. This behavioral potentiation was associated with an increase in the total number of postsynaptic dopaminergic receptor binding sites in the striatum. The development of striatal dopamine receptor supersensitivity may be caused by a decrease in the rate of dopamine turnover in the striatum.

Lead attenuation of episodic growth hormone secretion in male rats.

The purpose of this study was to characterize the effects of chronic lead exposure on growth hormone and insulin-like growth factor-1 status in growing male rats. Pituitary growth hormone content, episodic growth hormone release, plasma insulin-like growth factor-1 levels, and growth hormone response to exogenous growth hormone-releasing factor were quantified in young rats given lead nitrate. Twenty male Sprague-Dawley weanling rats were given lead nitrate (1000 ppm lead) in drinking water for a period of 6 weeks. Lead treatment significantly reduced body weight gain. Pituitary growth hormone content was not altered by lead treatment. Mean plasma growth hormone levels were reduced 44.6% by lead treatment (46.41 +/- 6.2 ng/ml; p = .003) as compared to controls (83.82 +/- 10 ng/ml). Lead treatment reduced mean growth hormone peak amplitude by 37.5%, mean nadir concentration by 60%, and growth hormone peak area by 35%. These findings are consistent with decreased hypothalamic growth hormone-releasing factor secretion or reduced somatotrope responsiveness. Exogenous growth hormone-releasing factor increased plasma growth hormone in lead-treated and control rats. However, this response was blunted by the lead treatment (lead treated: 485.6 +/- 57.8 vs. controls: 870.2 +/- 127 ng/ml; p = .03). Plasma insulin-like growth factor-1 concentration was not significantly affected by lead treatment. These results demonstrate that lead intoxication attenuates growth hormone release without abolishing the hypothalamic endocrine mechanisms driving growth hormone pulsatility. This suggests that lead acts at the level of the pituitary somatotroph rather than at the level of the hypothalamus.

Protection Against Acute MPTP‐Induced Dopamine Depletion in Mice by Adenosine A1 Agonist

Abstract: The effects of the adenosine A1 agonist N6‐cyclohexyladenosine (CHA) on MPTP‐induced dopamine (DA) depletion in the striatum of C57BL/6 mice were studied. Twenty hours after a single injection of MPTP (30 mg/kg, s.c.), the toxin caused 62% depletion of striatal DA. CHA (0.2–3 mg/kg, s.c.), when given together with MPTP, prevented the toxin‐induced DA depletion in a dose‐dependent manner. This protective action was apparently mediated by the A1 receptors, because this effect was selectively antagonized by pretreating the animals with the A1 antagonist 8‐cyclopentyl‐1,3‐dipropylxanthine (25 mg/kg, i.p.) but not with the A2 antagonist 1,3‐dipropyl‐7‐methylxanthine (25 mg/kg, i.p.). When CHA (3 mg/kg) was injected 5 h after MPTP administration, at which point striatal DA levels were already reduced significantly, a rapid and complete recovery of the striatal DA levels occurred. These neurochemical data suggest that the A1 agonist CHA is potentially useful as a neuroprotective agent against MPTP‐induced toxicity.

Pharmacological effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on striatal dopamine receptor system

In mice, chronic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces an increase in the maximum number of [3H]spiperone binding sites in the striatum. The sensitivity of striatal protein phosphorylation to calcium plus calmodulin is also potentiated in MPTP-treated mice. These observations are associated with an enhancement of apomorphine-induced climbing behavior in the drug-treated animals. The results of this study suggest that in an animal model for Parkinsons disease, MPTP interrupts the dopamine (DA) transmission by chemically denervating the nigrostriatal neurons and through a compensatory mechanism, it increases the number of DA receptors as well as the sensitivity of protein phosphorylation to calcium plus calmodulin in mouse striatum. The latter two events may contribute to the development of DA receptor supersensitivity.

Effect of exposure to low level lead on growth and growth hormone release in rats

This study was undertaken to examine the effect of exposure to low level lead on growth and growth hormone (GH) release. Female pups exposed to lead beginning in utero were smaller than controls on postnatal day 7 (P = 0.06). There was no corresponding effect in males. No overall differences in body weights were detected in either sex with respect to treatment effect. No differences in food or water intake were observed at any time. Pituitaries from 49-day-old lead-treated pups responded to in vitro incubation with growth hormone releasing factor (GRF) with a smaller increase in GH release than those from control pups (P = 0.08). In the case of the dams, lead did not affect body weight, body length, food consumption or pituitary responsiveness; however, water consumption was significantly increased in the lactating dam (P < 0.05). Interestingly, blood lead content in 5-day-old pups (43.3 +/- 2.7 micrograms/dl) exposed to lead in utero was more than twice that of their 49-day-old litter-mates (18.9 +/- 0.7 micrograms/dl). At 49 days blood lead levels in female pups (19.94 +/- 0.8 micrograms/dl) were significantly higher than those of male pups (17.00 +/- 1.1 micrograms/dl). Maternal blood lead levels on the same day averaged 22.7 +/- 2.5 micrograms/dl. This study suggests that exposure to a low level of lead can reduce pituitary responsiveness to a hypothalamic stimulus. In addition, the data reinforce the importance of considering age and sex when evaluating the toxic effects of lead.

Effect of lead on TRH and GRF binding in rat anterior pituitary membranes

The effect of lead on binding of the hypothalamic peptides thyroid releasing hormone (TRH) and growth hormone releasing factor (GRF) to rat anterior pituitary receptors was examined in this study. Concentrations of lead ranging from 0.01 to 1 microM did not alter [3H]TRH binding; concentrations above 1 microM increased TRH association with pituitary receptors. A previously uncharacterized ligand, [125I]GRF (human 1-44 amide), was used to examine the binding of GRF to anterior pituitary receptors. A high affinity site (GRFH = 18.1%, KH = 11.5 pM) was displaced by human growth hormone releasing factor (hGRF) (1-44)-NH2 or hGRF (1-29)-NH2 but not by rat growth hormone releasing factor (rGRF) (1-29)-NH2. Use of this ligand also revealed a class of low affinity binding sites (GRFL = 81.9%, KL = 0.39 microM) which has not been previously described. The low affinity site could be displaced by hGRF (1-44)-NH2, hGRF (1-29)-NH2 and rGRF (1-29)-NH2. A synthetic growth hormone releasing peptide (GHRP) also interacted with the low affinity GRF binding site. Lead dose-dependently displaced the binding of [125I]GRF to its pituitary receptors. The IC50 of lead for inhibiting [125I]GRF binding was 0.195 mM added lead or 52 pM free lead. These data suggest that one mechanism by which lead may affect pituitary function is through inhibition of receptor binding.

CHRONIC ADMINISTRATION OF NICOTINE FAILS TO ALTER THE MPTP-INDUCED NEUROTOXICITY IN MICE

1. The effects of chronic (14 day) administration of nicotine on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (15 mg/kg, s.c.)-induced neurotoxicity in C57BL/6 mice were examined. 2. Nicotine pretreatment failed to alter the deficit in locomotor activity and the reduction in striatal levels of dopamine produced by MPTP. 3. Our results do not support a therapeutic action of nicotine in a Parkinsonian animal model.

Urinary excretion of MPTP and its primary metabolites in mice

Mice were injected with single doses of MPTP (15 mg/kg, s.c.) containing one microCi of [3H]methyl-MPTP. Approximately 42% of the total injected [3H] was detected in the urine within 3 hours after drug administration. The early urine samples were analyzed using high pressure liquid chromatography. MPTP N-oxide was identified as a major metabolite, with trace amounts of MPP+ and MPTP also detected. The urinary volume and excretion of MPTP metabolites were inhibited by pretreating the animals with probenecid (250 mg/kg, i.p.). These results indicate that large amounts of injected MPTP are rapidly metabolized in the periphery by liver enzymes to form MPTP N-oxide.