Dopamine receptor blockade increases dopamine D2 receptor and glutamic acid decarboxylase mRNAs in mouse substantia nigra

Neurochemical and behavioral features in genetic absence epilepsy and in acutely induced absence seizures

The absence epilepsy typical electroencephalographic pattern of sharp spikes and slow waves (SWDs) is considered to be due to an interaction of an initiation site in the cortex and a resonant circuit in the thalamus. The hyperpolarization-activated cyclic nucleotide-gated cationic I h pacemaker channels (HCN) play an important role in the enhanced cortical excitability. The role of thalamic HCN in SWD occurrence is less clear. Absence epilepsy in the WAG/Rij strain is accompanied by deficiency of the activity of dopaminergic system, which weakens the formation of an emotional positive state, causes depression-like symptoms, and counteracts learning and memory processes. It also enhances GABAA receptor activity in the striatum, globus pallidus, and reticular thalamic nucleus, causing a rise of SWD activity in the cortico-thalamo-cortical networks. One of the reasons for the occurrence of absences is that several genes coding of GABAA receptors are mutated. The question arises: what the role of DA receptors is. Two mechanisms that cause an infringement of the function of DA receptors in this genetic absence epilepsy model are proposed.

Measurement of GABAergic parameters in the prefrontal cortex in schizophrenia: focus on GABA content, GABA(A) receptor alpha-1 subunit messenger RNA and human GABA transporter-1 (HGAT-1) messenger RNA expression

The hypothesis that the pathophysiology of schizophrenia may be associated with a dysfunction in GABA transmission in the human prefrontal cortex was investigated. Human post mortem brain tissue from 10 control cases and six cases of schizophrenia were processed for amino acid analysis and for radioactive in situ hybridization. Laminae III and V of three prefrontal cortical areas were examined in detail, namely Brodmann areas 9, 10 and 11. Of these three areas significant changes in GABAergic markers were found only in areas 9 and 10. Of note, a significant decrease in the tissue content of GABA was observed and this was accompanied by a marked increase in the cellular expression of the GABA(A) receptor alpha-1 subunit messenger RNA and a marked decrease in the expression of human GABA transporter-1, the messenger RNA encoding the neuronal GABA transporter protein. The amino acid analysis data provided in this study coupled with the detailed cellular study of several GABAergic markers in the human prefrontal cortex provide direct evidence in support of a disturbance in GABA transmission in the prefrontal cortex, which may be loosely termed "hypofrontality".

D3 receptors and the actions of neuroleptics in the ventral striatopallidal system of schizophrenics

The mesolimbic dopamine (DA) system and an important target receptor, the D3 receptor, have been implicated in schizophrenia. We have identified, using non-radioactive in situ hybridization histochemistry, that D3 mRNA-positive neurons are highly concentrated in the ventral striatum, efferents of the ventral striatum (globus pallidus internal, ventral palladium, substantia nigra pars reticulata), and in regions projecting to the ventral striatum (medial dorsal thalamus, nucleus basalis, extended amygdala). D3 receptors are also highly enriched in the "limbic" striatal-pallidal-thalamic loop, exhibiting segregation from the D2 receptor-enriched "motor loop." This supports data developed in rats showing that the D3 receptor is a target of the mesolimbic DA system that can modulate the limbic striatal-palladial-thalamic loop. However, D2 and D3 receptors and their mRNAs are co-localized in many sensory regions (lateral and medial geniculate nuclei, basolateral and basomedial amygdala, regions of thalamus), suggesting mechanisms of cross-talk. We have also demonstrated that there are 45% elevations in D3 receptor number in ventral striatal neurons and their striatopalladial targets in schizophrenics that is reduced by concurrent antipsychotic treatment. Chronic haloperidol treatment to rats for 6 months with a 2-month withdrawal does not result in elevated D3 receptor number. We hypothesize that antipsychotic treatment via D3 receptors returns balance to limbic efferents of the ventral striatum. We established that early neonatal damage to the nigrostriatal DA system in rats produces characteristic adaptations in the pre- and post-synaptic components of the mesolimbic DA system that can provide a model to explore regulation by antipsychotics. This includes elevated release of DA from the mesolimbic DA terminals, elevated D3 receptor mRNA in the Islands of Calleja and nucleus accumbens, and enhanced behavioral response to psychostimulants.

Quantification of vitamin D receptor mRNA in tissue sections demonstrates the relative limitations of in situ-reverse transcriptase-polymerase chain reaction

In situ-reverse transcriptase-polymerase chain reaction (IS-RT-PCR) is a recently described technique that is used to localize low levels of mRNA within cells and tissue sections. One of the major criticisms levelled at this technique is that positive results may be meaningless, as amplification is required to demonstrate the transcripts of interest. The use of IS-RT-PCR to demonstrate mRNA for receptors for 1,25-dihydroxyvitamin D3 (VDR) in sections of human kidney and bone has previously been described. To ascertain whether the levels of VDR mRNA detected following IS-RT-PCR were transcriptionally significant, computerized image analysis was used to determine the mean silver grain density in human kidney and bone cells following conventional in situ hybridization and after various cycles of IS-RT-PCR. Only a few cycles of PCR were needed to produce an optimum signal, but amplification of signal following IS-RT-PCR was found to be relatively inefficient. Following the optimum number of cycles of IS-RT-PCR in kidney sections, there was a less than four-fold increase in signal. Similarly, in bone, the optimum signal detected was only approximately five times greater than that found with conventional in situ hybridization. These results clearly demonstrate that the increase in signal following IS-RT-PCR follows a more linear pattern and is relatively inefficient, compared with the usual exponential increase with conventional solution phase RT-PCR.

Antisense strategies in dopamine receptor pharmacology

Recent advances in molecular biology have provided pharmacologists the opportunity of developing an entirely new type of agent for studying and treating a variety of biological disorders. These agents, termed antisense oligodeoxynucleotides, have as their target the messenger RNAs encoding specific proteins. They act by binding to selected portions of these mRNAs through complimentary interactions and thereby prevent the synthesis of these proteins. These novel pharmacological tools have the promise of being easier to design and being more selective and predictable in their actions. In addition, insofar as agents targeted to receptors for neurotransmitters are concerned, unlike the classical pharmacological agents, these new compounds may not lead to the upregulation of the very receptors the drugs are designed to inhibit. The present review summarizes briefly studies on the effect of oligodeoxynucleotides antisense to the mRNAs encoding the various subtypes of the dopamine receptor. The studies show that oligodeoxynucleotides antisense to the D2 dopamine receptor when intracerebroventricularly into brains of rodents are rapidly taken up into the brain tissue, distributed to brain cells, and produce effects characteristic of highly selective D2 dopamine antagonists. The compounds also produced specific reductions in the levels of D2 dopamine receptor mRNA and D2 dopamine receptors. Similarly, injecting an antisense oligodeoxynucleotide targeted to the D1 dopamine receptor mRNA produces effects characteristic of D1 dopamine receptor antagonists. Other studies using these agents has produced evidence that there is a small pool of receptors that turn over very rapidly and which constitute the functional pool of these receptors. The evidence suggests further that antisense oligodeoxynucleotides inhibit the synthesis of this small functional pool of dopamine receptors, thereby providing an explanation of why there is often a discordance between changes in dopaminergic function and changes in the levels of dopamine receptors. Studies of antisense oligodeoxynucleotides targeted to the other subtypes of dopamine receptor may help reveal the biological roles that these and other newly discovered subtypes of neurotransmitter receptors have. They may also provide an entirely new and potentially more selective therapeutic regimen for altering the functions of these receptors.

Intrastriatal administration of an oligodeoxynucleotide antisense to the D2 dopamine receptor mRNA inhibits D2 dopamine receptor-mediated behavior and D2 dopamine receptors in normal mice and in mice lesioned with 6-hydroxydopamine

Previous studies have shown that the intracerebroventricular injection of antisense oligodeoxynucleotides targeted to the mRNAs encoding the different subtypes of dopamine receptors inhibited behaviors mediated by these receptors. The present studies were designed to determine whether such antisense oligodeoxynucleotides could produce similar effects when injected into a discrete brain area. A D2 dopamine receptor antisense oligodeoxynucleotide (D2 antisense) was repeatedly injected into one corpus striatum of either normal mice or mice with unilateral lesions of the striatum induced by 6-hydroxydopamine. In the latter, intrastriatal injection of D2 antisense blocked the contralateral rotational behavior induced by the parenteral administration of the D2 dopamine receptor agonist quinpirole. The inhibitory effect of D2 antisense was dose- and time-related and was reversed upon cessation of D2 antisense treatment. This inhibitory effect was also selective in that D2 antisense treatment inhibited the rotational behavior induced by quinpirole but not that induced by the D1 dopamine receptor agonist SKF 38393 or by the muscarinic cholinergic agonist oxotremorine. Following repeated intrastriatal injections of D2 antisense into normal mice, parenteral administration of quinpirole caused rotational behavior ipsilateral to the side in which the D2 antisense was injected. No such rotational behavior was seen when similarly treated mice were challenged with SKF 38393 or oxotremorine. The quinpirole-induced rotational behavior in mice given intrastriatal injections of D2 antisense disappeared upon cessation of D2 antisense treatment. Repeated intrastriatal administration of D2 antisense also caused a significant reduction in the levels of D2, but not D1, dopamine receptors in striatum, as determined by receptor autoradiography. The levels of D2 dopamine receptors returned to normal upon cessation of D2 antisense treatment. Intrastriatal administration of an oligodeoxynucleotide with randomly placed nucleotides failed to alter the rotational response to quinpirole in either 6-hydroxydopamine-lesioned or normal mice and failed to alter the levels of D2 dopamine receptors in striatum. These results show that selective inhibition of behavioral responses mediated by D2 dopamine receptors can be achieved by the direct injection of a D2 antisense oligodeoxynucleotide into a discrete brain area.