Secretory and structural effects of 6-hydroxy-dopamine on normal parotid glands of rats, and at different times after surgical sympathectomy.
The effects of i.v. 6-hydroxydopamine (6-OHDA), 100 mg/kg, have been studied on parotid glands of rats at 12, 24, 48, 72 hr and 3 weeks after avulsion of the right superior cervical sympathetic ganglion. The salivary flow from normal left control glands and from right glands 12 hr after ganglionectomy were similar, but at longer times after ganglionectomy the secretory response from the test glands was greatly reduced. Morphological assessment showed that 6-OHDA induced a massive depletion of secretory granules from all control glands and also at 12 hr after ganglionectomy but at 48 and 72 hr there was considerably less depletion of granules on the ganglionectomized side. It is thought that at the longer times after ganglionectomy the secretion from the test glands is caused by circulating catecholamines released by the action of 6-OHDS on adrenergic nerves elsewhere, plus a possible small direct secretogogue effect oomy are thought to be attributable to the release of catecholamines from adrenergic nerves within the gland. (+info)
Embryonic ventral mesencephalic grafts improve levodopa-induced dyskinesia in a rat model of Parkinson's disease.
We investigated the role of dopamine neurons in the manifestation of levodopa-induced dyskinesia in a rat model of Parkinson's disease. Daily treatment with a subthreshold dose of levodopa gradually induced abnormal involuntary movements (AIM) in 6-hydroxydopamine-lesioned rats, which included stereotypy and contraversive rotation. After 4 weeks of levodopa treatment, rats with mild and severe AIM were assigned to two treatment subgroups. The graft subgroup received embryonic ventral mesencephalic tissue into the striatum, whilst the sham-graft subgroup received vehicle only. Rats continued to receive levodopa treatment for 3 months post-graft. Brain sections at the level of the basal ganglia were processed for autoradiography using a ligand for dopamine transporter, and in situ hybridization histochemistry for mRNAs encoding postsynaptic markers. Levodopa-induced AIM significantly improved in grafted rats. The severity of AIM correlated inversely with the density of dopamine nerve terminals in the striatum (P < 0. 001), with almost no AIM when the density of dopamine nerve terminals was >10-20% of normal. Embryonic dopamine neuronal grafts normalized not only mRNA expression for preproenkephalin (PPE) in the indirect pathway, but also mRNA expression for prodynorphin (PDyn) in the direct pathway, which was upregulated by levodopa treatment. AIM scores correlated linearly with expression of PPE mRNA in the indirect pathway (P < 0.001) and also with PDyn mRNA in the direct pathway (P < 0.001). We conclude that embryonic dopamine neuronal grafts may improve levodopa-induced dyskinesia by restoring altered activities of postsynaptic neurons, resulting not only from dopamine denervation, but also from levodopa therapy, provided that the density of striatal dopaminergic nerve terminals is restored above a 'threshold' level. (+info)
Prolonged contraction duration in aged myocardium.
Isometric performance at 29degreesC was measured in left ventricular trabeculae carneae from young adult (6-mo) and aged (25-mo) rats (n equals 18 in each group). Active tension and maximal rate of tension development did not differ with age, but contraction duration was 255plus or minus6 ms in the young adult and 283plus or minus6 ms in the aged group (P less than0.001). Although catecholamine content per gram heart weight was less in the aged myocardium, additional experiments showed that neither 1 times 10-6 M propranolol nor pretreatment with 6-hydroxydopamine eliminated the age difference in contraction duration. To determine if this age difference resulted from a prolonged active state, electromechanical dissociation and the overshoot of contraction duration during recovery from hypoxia were measured. During paired stimulation greater mechanical refractoriness was found in aged muscles (P less than0.01), but intracellular action potential recordings showed no age difference in the electrical refractory period. On recovery from hypoxia, contraction duration overshoot was 117plus or minus 4percent of control in the young and 138plus or minus 4percent of control in the aged muscles (P less than0.01). The greater electromechanical dissociation and greater overshoot in contraction duration following hypoxia in aged myocardium suggests that prolonged contraction duration in aged myocardium results from a prolonged active state rather than changes in passive properties or myocardial catecholamine content. (+info)
Role of catecholamine degradative enzymes and the adrenergic innervation in determining the cerebrovascular response to infused norepinephrine.
Cerebral blood flow responses to intra-arterial infusion of norepinephrine (NE) at 0.55 microgram/kg/min and 1.1 microgram/kg/min were studied in 3 groups of baboons. The flow was measured by the intracarotid 133xenon clearance technique using a computer program to calculate flow (height over area--H/A) flow (initial slope--is) and cerebral metabolic utilization of oxygen (CMRO2). The normal response to NE was to increase flow without significant changes in CMRO2. Blockade of catechol-o-methyl transferase (COMT) produced vasoconstrictor responses to these same NE doses. Monoamine oxidase blockade abolished the normal vasodilation. Denervation of the cerebral circulation with intracisternal 6-hydroxydopamine produced vasoconstrictor responses with flow (H/A) but not with flow (is). It is concluded that the extra-neuronal COMT enzyme is important in limiting the access of blood-borne NE to cerebrovascular constrictor receptors. (+info)
Dopamine-dependent synaptic plasticity in striatum during in vivo development.
The neurotransmitters dopamine (DA) and glutamate in the striatum play key roles in movement and cognition, and they are implicated in disorders of the basal ganglia such as Parkinson's disease. Excitatory synapses in striatum undergo a form of developmental plasticity characterized by a decrease in glutamate release probability. Here we demonstrate that this form of synaptic plasticity is DA and DA D2 receptor dependent. Analysis of spontaneous synaptic responses indicates that a presynaptic mechanism involving inhibition of neurotransmitter release underlies the developmental plasticity. We suggest that a major role of DA in the striatum is to initiate mechanisms that regulate the efficacy of excitatory striatal synapses, producing a decrease in glutamate release. (+info)
Dinapsoline: characterization of a D1 dopamine receptor agonist in a rat model of Parkinson's disease.
Dinapsoline is a new potent, full agonist at D1 dopamine receptors with limited selectivity relative to D2 receptors. The efficacy of this compound was assessed in rats with unilateral 6-hydroxydopamine lesions of the medial forebrain bundle, a standard rat model of Parkinson's disease. Dinapsoline produced robust contralateral rotation after either subcutaneous or oral administration. This rotational behavior was attenuated markedly by the D1 receptor antagonist SCH-23390, but not by the D2 receptor antagonist raclopride. During a chronic 14-day treatment period in which rats received dinapsoline either once or twice a day, dinapsoline did not produce tolerance (in fact, some sensitization of the rotational response was observed in one experiment). Because dinapsoline shows less D1:D2 selectivity in vitro than other D1 agonists, the contribution of D2 activity to tolerance was assessed. Chronic daily cotreatment with dinapsoline and raclopride did not enable the development of tolerance to chronic dinapsoline treatment. In contrast, when dinapsoline was administered by osmotic minipump, rapid tolerance was observed. To explore further the contribution of D1 and D2 receptors to tolerance, experiments were performed with the selective D1 agonist A-77636. Daily dosing with A-77636 rapidly produced complete tolerance, as previously observed, whereas coadministration of the D2 agonist quinpirole plus A-77636 failed to either delay or prevent tolerance. Taken together, these results indicate that the development of tolerance to D1 receptor agonists is influenced by the pattern of drug exposure but not by the D1:D2 selectivity of the agonist. (+info)
Evidence that adrenergic nerves are responsible for the active uptake of noradrenaline in the guinea-pig isolated trachea.
1 6-Hydroxydopamine (50 mg/kg, i.p.) was given to guinea-pigs to destroy the adrenergic nerve terminals in the trachea. 2 The destruction was demonstrated by fluorescence histochemistry, which showed a marked loss of beaded fluorescent terminal fibres and by electrical transmural stimulation of the isolated atropinized trachea, which showed a marked reduction of dilator responses. 3 Such tracheae showed greatly reduced uptake-with-retention of (minus)-[3H]-noradrenaline in incubation experiments and the efflux curve of radioactive material showed a selective but incomplete reduction in the volume of the slowly exchanging compartment. 4 It is concluded that much, but perhaps not all, of the uptake-with-retention occurs into adrenergic nerves. (+info)
Dose-related neuroprotective effects of chronic nicotine in 6-hydroxydopamine treated rats, and loss of neuroprotection in alpha4 nicotinic receptor subunit knockout mice.
The present study examined the effect of a range of doses of chronic nicotine (0.75, 1.5, 3.0 and 30.0 mg kg(-1) day(-1), s.c., 14 days) upon striatal dopaminergic nerve terminal survival following 6-hydroxydopamine (6-OHDA; 10 microg intrastriatal unilaterally) in rats; and the effects of acute nicotine (1 mg kg(-1), s.c.) pretreatment upon striatal neurodegeneration induced by methamphetamine (5 mg kg(-1), i.p., three doses at 2 h intervals) in wild-type and alpha4 nicotinic receptor (nAChR) subunit knockout mice. In both models of Parkinsonian-like damage, loss of striatal dopaminergic nerve terminals was assessed by [(3)H]-mazindol autoradiography. In rats, chronic nicotine infusion delivered by osmotic minipump implanted subcutaneously 7 days prior to intrastriatal 6-OHDA injection produced significant and dose-related protection against 6-OHDA-induced neurodegeneration. Low (0.75 and 1.5 mg kg(-1) day(-1)) but not high (3.0 and 30.0 mg kg(-1) day(-1)) nicotine doses significantly inhibited 6-OHDA-induced degeneration. In wild-type mice, acute nicotine treatment produced significant inhibition of methamphetamine-induced neurodegeneration. In alpha4 nAChR subunit knockout mice, acute nicotine treatment failed to inhibit methamphetamine-induced neurodegeneration. Nicotine is capable of protecting dopaminergic neurons against Parkinsonian-like neurodegeneration in vivo. In rats, this neuroprotective effect is critically dependent upon nicotine dose and is consistent with the activation of nAChRs, as high, desensitizing doses of nicotine fail to be neuroprotective. Further, neuroprotection is absent in alpha4 nAChR subunit knockout mice. The current results therefore suggest that activation of alpha4 subunit containing nAChRs constitutes a major component of the neuroprotective effect of nicotine upon Parkinsonian-like damage in vivo. (+info)