Dopaminergic dysfunction in neonatal hypothyroidism: differential effects of GM1 ganglioside.

The effects of daily treatment with GM1 ganglioside (30 mg/kg s.c.) from birth to day 30, on striatal pre- and postsynaptic markers of the dopaminergic system in euthyroid- and 32 day-old hypothyroid rats were studied. The purpose was to assess whether GM1 could prevent the extensive, hypothyroidism-provoked impairment of dopaminergic neurotransmission. Neonatal administration of GM1 well counteracted the hypothyroidism-related deficits in striatal synaptosomal uptake of [3H]dopamine and in membrane binding of [3H]tyramine, a putative marker for the vesicular carrier of dopamine. In the hypothyroid striatum, the decrease of concentrations of DOPAC and HVA, the loss of [3H]SCH-23,390-labelled D1-receptors and the decrease of basal- or dopamine-stimulated, D1-mediated activity of adenylate cyclase were not prevented by GM1. Although somatic and neurobehavioural aberrations of hypothyroids were not at all or only partially ameliorated, a slight improvement of the thyroid status was suggested by less decreased levels of serum thyroxine (T4) after treatment with GM1. The ganglioside-driven selective recovery of the transport and storage process of [3H]dopamine might result either from a chronically-exerted stimulation by GM1 on the NA/K- and Mg-ATPase activities, thus reflecting on the ATPase-dependent neuronal and vesicular transport processes of dopamine or from a GM1-promoted maturation of the otherwise retarded functionality of dopaminergic nerve endings in the neonatal hypothyroid striatum.

Neonatal hypothyroidism induces striatal dopaminergic dysfunction.

Oral administration of the antithyroid drug methimazole (50 mg/kg per day) to rats during the last six days of pregnancy, and subsequent daily s.c. injection of methimazole (20-30 mg/kg) to their pups from birth to postnatal day 30 provoked hormonal and somatic alterations resembling (with all caution to any association between rodent and human data) those of congenital hypothyroidism. The steady-state concentrations of striatal dopamine were similar in hypothyroid and euthyroid, 32-day-old rats, while the levels of the dopamine metabolites 3,4-dihydroxyphenylacetic and homovanillic acids were markedly decreased in hypothyroidism. The results of this and our earlier study [Vaccari A. and Gessa G. L. (1989) Neurochem. Res. 14, 949-955] show that the maximal synaptosomal uptake of [3H]dopamine, an index for the density of nigrostriatal dopaminergic terminals, and the maximum number of membrane [3H]tyramine binding sites, reflecting the concentration of the vesicular transporter for dopamine, were decreased in the hypothyroid striatum. There was also a loss of those D1-type dopaminergic receptors claimed to be located on neurons intrinsic to the striatum, and, consequently, dopamine-stimulated, D1-regulated adenylate cyclase activity was depressed. It is suggested that individual dopaminergic nerve endings in the neonatal hypothyroid striatum must contain more dopamine, owing to some loss of pertinent innervation and, therefore, to the presence of less vesicular transport sites for dopamine. Hypothyroidism-related decreases in the maximum number of striatal D1- and, reportedly, D2-receptors, plus the impairment of D1-coupled second messenger activity, may play a role in the derangement of those neurobehavioural patterns where a dopaminergic regulation is putatively implied.

Increase in D2 dopamine receptors in the substantia nigra after chronic (-)-sulpiride treatment.

Changes in D2 dopamine (DA) receptors in the substantia nigra (SN) and caudate nucleus (CN) homogenates were studied in rats following chronic treatment with the specific D2 receptor antagonist (-)-sulpiride (50 mg/kg s.c. twice daily for 8 days). Sixty hours after (-)-sulpiride withdrawal, the Bmax of [3H]spiroperidol binding (in the presence of 30 Nm ketanserin) was found to be increased by 76% and 38% in the SN and CN, respectively. No changes in the KdS were observed. The findings suggest that chronic (-)-sulpiride results in the upregulation of DA autoreceptors in the SN.

Sodium-dependent interaction of benzamides with dopamine receptors.

Sulpiride and other benzamide (BM)-displacing activity on [3H]-spiroperidol ([3H]SPIR) binding by rat striatal dopaminergic receptors was found to be uniquely sodium-dependent, while classical neuroleptic (NL) activity was not influenced by NaCl. These results suggest the existence of at least two populations of striatal dopaminergic receptors, sodium-dependent and sodium-independent, through which BM and NL respectively interact.

Interaction of flunarizine with dopamine D2 and D1 receptors.

Flunarizine dose dependently inhibits the binding of both [3H]spiperone and [3H]SCH 23390 with Ki values of 112 +/- 9 and 532 +/- 39 nM, respectively. The inhibition of [3H]spiperone binding by flunarizine was competitive as revealed by the Schild plot. The results indicate that flunarizine is a fairly potent dopamine D2 receptor antagonist and offer a possible explanation for the extrapyramidal side-effects observed in patients treated with the drug.

Flunarizine potentiates cocaine-induced dopamine release and motor stimulation in rats.

Pretreatment with flunarizine (20 mg/kg), a diphenylalkylamine calcium channel antagonist, markedly potentiated cocaine (10 mg/kg)-induced dopamine (DA) output from the ventral striatum, as measured by microdialysis in freely moving rats. Moreover, flunarizine enhanced cocaine-induced motor stimulation. Pretreatment with the D2 receptor antagonist (-)-sulpiride (25 mg/kg) potentiated cocaine-induced DA output but, unlike flunarizine, inhibited cocaine-induced motor stimulation. When added to striatal membrane preparations, flunarizine displaced [3H]spiperone binding with a Ki of about 100 nM. Since recent evidence indicates that the effects of cocaine are calcium-dependent and are prevented by dihydropyridine calcium antagonists, these findings suggest that the paradoxical potentiating effect of flunarizine is probably due to an interaction with the DA system and is not due to its calcium antagonist property.

Evidence for a direct action of amphetamine on dopamine metabolism in the rat substantia nigra in vivo.

The intraperitoneal administration of D-amphetamine (0.2--2.5 mg/kg) decreases dihydroxyphenylacetic acid (DOPAC) content not only in the caudate nucleus but also in the substantia nigra. This effect persists both in the substantia nigra and in the caudate nucleus after loss of dopamine-sensitive adenylate cyclase and presumably of dopamine receptors, induced by intrastriatal injection of kainic acid. These results indicate that the effect of amphetamine on DOPAC levels in the nigra is not mediated through a strionigral feedback loop.

Evidence for selective and long-lasting stimulation of "regulatory" dopamine-receptors by bromocriptine (CB 154).

Bromocriptine, an ergot-derivate with DA-receptor stimulating properties in vivo, produces long-lasting hypomotility in mice not accustomed to the motility cage and decreases brain DOPAC and HVA without affecting brain DA. These effects are obtained with doses 25 times lower than those which produce hypermotility. The decrease of brain DOPAC is correlated to the hypomotility both on a dose- and on a time-basis. Potent neuroleptics as pimozide, benzperidol and droperidol, which are considered to be fairly specific DA-receptor blockers, antagonize the hypomotility and the decrease of brain DOPAC produced by bromocriptine. These effects are obtained with very low doses (0.05--0.3 mg/kg) of neuroleptics which per se do not affect motility or brain DOPAC. The maximal decrease of brain DOPAC and HVA produced by bromocriptine is similar to that produced by apomorphine and the combination of these drugs does not result in a further decrease of brain DOPAC or HVA. On the basis of these results it is postulated that bromocriptine decreases brain DA-turnover and produces hypomotility by acting on "regulatory" DA-receptors different from the post-synaptic ones of the "terminal" dopaminergic areas.

Distribution of GABA(B) receptor mRNAs in the rat brain and peripheral organs.

GABA, the predominant inhibitory neurotransmitter present in the mammalian CNS, is also found in the periphery. GABA actions are mediated by the ionotropic GABA(A)/GABA(C) receptors, as well as the metabotropic GABA(B) receptor. The rat GABA(B) receptor has recently been cloned and two cDNA clones have been isolated encoding two isoforms of the receptor, GABA(B)R1a and R1b. Northern blot analysis revealed the presence of both transcripts in the rat brain using specific cDNA probes for GABA(B)R1a and R1b, respectively. However, Northern blot analysis, hybridized with a probe containing a sequence common to both isoforms, revealed specific RNAs in the rat brain and in testis, but not in other peripheral tissues. In the present study, by using the more sensitive reverse transcriptase-polymerase chain reaction with a specific set of primers for each isoform and Southern blot analysis, we found that both isoforms of the GABA(B) receptor are expressed not only throughout the brain but also in all peripheral organs examined, including heart, spleen, lung, liver, small intestine, large intestine, kidney, stomach, adrenal, testis, ovary and urinary bladder. The peripheral distribution of GABA(B)R1 mRNAs supports the notion of a physiological role for GABA in the control of a wide range of peripheral organs.

Low responsiveness to agents evoking 5-HT2 receptor-mediated behaviors in Sardinian alcohol-preferring rats.

The selective NK3 tachykinin agonist senktide evokes in rodents 5-HT mediated behaviors, including 5-HT2 receptor-mediated wet dog shakes (WDS) and head shakes (HS). It was observed previously that genetically selected Sardinian alcohol-preferring (sP) rats show a small number of WDS and HS following intracerebroventricular (ICV) injection of senktide. The present study was aimed at confirming these observations and at providing information on the reasons accounting for the anomalous response of sP rats. Senktide (500-2000 ng/rat, ICV) produced a much lower number of WDS and HS in sP rats than in nonselected Wistar (nsW) rats. Both behaviors were suppressed by the 5-HT2 antagonist ritanserin (1 mg/kg, subcutaneously), confirming that 5-HT2 receptors mediate the response. HS induced by the ICV injection of 5-HT agonists endowed with marked activity at 5-HT2 receptors, such as quipazine (1500-6000 ng/rat) or DOI (500-3500 ng/rat), were much less pronounced in sP rats than in nsW rats. Moreover, WDS following peripheral injection of 5-hydroxytryptophan, 25-100 mg/kg, and carbidopa, 12.5 mg/kg, were less intense in sP and in ethanol-naive sP rats than in nsW and in Sardinian alcohol-nonpreferring rats. These findings suggest that sP rats have an inherent different regulation of central 5-HT2 mechanisms.