Excitability changes induced in the striatal dopamine-containing terminals following frontal cortex stimulation

Effects of acute and chronic morphine on DOPAC and glutamate at subcortical DA terminals in awake rats

Effects of morphine and naloxone on the levels of 3,4-dihydroxy-phenylacetic acid (DOPAC) and glutamate in the striatum and nucleus accumbens of awake rats were studied with in vivo microdialysis. Acute morphine (50 mg/kg, IP) treatment increased the levels of DOPAC and glutamate in the striatum and nucleus accumbens, but both decreased from the elevated levels when naloxone (10 mg/kg, IP) was given 2 h later. Chronic morphine treatment, twice daily for 5 days in incremental doses (5, 10, 20, 40 and 50 mg/kg, IP), increased the level of DOPAC but decreased that of glutamate in the striatum and nucleus accumbens. When naloxone was given 2 h later, the reverse of the above phenomena are found. After given repeated morphine treatment and experiencing naloxone-precipitated withdrawal, the rats with an intact cortex and the rats with ibotenic acid (5 microg/0.5 microl/2.5 min) lesions on the medial prefrontal cortex and sulcal cortex have similar alternations in the levels of DOPAC and glutamate in the striatum. However, in the nucleus accumbens, the level of DOPAC dropped more and the level of glutamate increased more in the intact rats than the lesioned rats during the withdrawal stage. These data suggested that the intact cortex ordinarily exerted an inhibitory role to influence the level of DOPAC in the striatum and nucleus accumbens during chronic morphine treatment. In conclusion, morphine seems to activate different pathways in dependent and non-dependent rats.

Amphetamine-induced changes in nigrostriatal terminal excitability are modified following repeated amphetamine pretreatment

To investigate neural mechanisms associated with behavioral sensitization to amphetamine, we studied the effect of an intrastriatal infusion of amphetamine on nigrostriatal axon terminal electrical excitability in rats following withdrawal from repeated systemic treatment. Rats were injected with amphetamine 2.5 mg/kg s.c. or saline daily for 4 days. Either 24 h or 14 days after the last injection, extracellular recordings were obtained from dopaminergic neurons of the substantia nigra, in a blind design in which the experimenter did not know the pretreatment regime. In order to assess the electrical excitability of the nigrostriatal axonal field, neurons were activated antidromically by stimulating their terminal fields in the striatum. As previously reported, striatal infusion of amphetamine (1 microM/0.3 microliter) in control animals resulted in a significant reduction in excitability as indicated by an increase in striatal stimulus current necessary to evoke antidromic activity. In contrast, intrastriatal amphetamine administration to amphetamine-pretreated animals did not decrease excitability. Spontaneous firing rates and patterns of cell discharge did not differ between saline- and amphetamine-treated animals. The chronic amphetamine-induced change in the effect of an acute intrastriatal amphetamine infusion on nigrostriatal terminal excitability may be due to enduring alterations in the amphetamine-induced release of dopamine and other striatal neurotransmitters or to changes in the sensitivity of presynaptic hetero- and/or autoreceptors on the dopaminergic axons.

Merck, Sharp & Dohme Prize for Young Psychopharmacologists: A history of frontal and temporal lobe aspects of the neuropharmacology of schizophrenia

In the past two decades neurobiological research into schizophrenia has seen a shift of emphasis from the dopaminergic pharmacology of subcortical systems to areas of abnormality inferred from direct studies, i.e. frontal cortex and medial temporal lobe. Recent research has tried to integrate morphological abnormalities with neurochemical and pharmacological data. These integrated approaches have uncovered neurochemical systems other than dopamine which are of importance. This approach has led to greater understanding of neurochemical mechanisms associated with these abnormalities and may provide novel approaches for therapeutic intervention.

Terminal excitability of the corticostriatal pathway. I. Regulation by dopamine receptor stimulation

Glutamatergic cortical and dopaminergic nigral afferents converge onto neurons of the neostriatum forming synapses in close proximity. Studies, mainly using pharmacological methods, suggest presynaptic interactions between these afferents. The influence of dopaminergic transmission on the cortical terminal fields in the striatum was assessed electrophysiologically using the terminal excitability method. Antidromic action potentials recorded from neurons in the prefrontal cortex were elicited by bipolar electrical stimulation (250 microns wire, 0.5 mm tip separation) of the cortical terminal field in the contralateral dorsomedial neostriatum. Threshold excitability was defined as the minimum current sufficient to elicit 95-100% antidromic response on non-collision trials. Under control conditions, the mean threshold current was 1.7 +/- 0.2 mA. Drugs were applied in a volume of 312 nl delivered over 5 min to the striatal stimulation site. Following local striatal administration of amphetamine (10 microM) or electrical stimulation of the nigrostriatal pathway (1-2 pulses, 1.5 mA/0.5 ms/1 Hz) an increase in striatal stimulating current was required in order to reinstate threshold levels of antidromic response. This decrease in the excitability of corticostriatal afferents could be reversed by local infusion of haloperidol (1 microM) or L-sulpiride (10 nM) and did not occur following depletion of dopamine stores with alpha-methylparatyrosine and reserpine. The possible participation of postsynaptic dopamine receptor stimulation was ruled out as these effects were still seen in animals with kainic acid induced lesions of the striatum. In addition, terminal excitability was not modified by the muscarinic agonist carbachol (10 microM). Striatal administration of apomorphine (10 microM) decreased terminal excitability similar to amphetamine. The specific D-2 agonist, quinpirole (10-20 microM) did not affect excitability. These results indicate that manipulations which have been shown to increase the release of endogenous dopamine decrease the excitability of prefrontal corticostriatal afferents by stimulation of presynaptic dopamine receptors which are insensitive to low doses of quinpirole but sensitive to L-sulpiride and apomorphine. The mechanisms underlying dopamine-induced changes in terminal excitability are likely to be similar to those which have been shown to alter conductance at postsynaptic sites.

Respective contributions of neuronal activity and presynaptic mechanisms in the control of the in vivo release of dopamine

Studies performed in several in vivo and in vitro conditions have demonstrated that the release of dopamine from nerve terminals of the nigrostriatal dopaminergic neurons depends not only on the activity of dopaminergic cells but also on presynaptic regulations by heterologous fibers. The presynaptic facilitation of dopamine release by the cortico-striatal glutamatergic neurons has been particularly investigated. A quisqualate/kainate receptor subtype is involved in the direct (tetrodotoxine-resistant) presynaptic regulation of dopamine release by glutamate. The respective roles of presynaptic events and nerve activity in the control of dopaminergic transmission are discussed.

Presynaptic regulation of dopaminergic transmission in the striatum

1. In vitro studies have indicated that several transmitters present in the striatum can regulate presynaptically the release of dopamine (DA) from nerve terminals of the nigrostriatal DA neurons. 2. The receptors involved in these local regulatory processes are located or not located on DA nerve terminals. 3. Recent in vivo investigations have demonstrated that the corticostriatal glutamatergic neurons facilitate presynaptically the release of DA and have allowed the analysis of the respective roles of presynaptic events and nerve activity in the control of DA transmission.

Regulation of dopamine function in the prefrontal cortex of the rat by the thalamic mediodorsal nucleus

Dopamine (DA) utilisation has been assessed in the medial bank of the prefrontal cortex (FCx) and the agranular insular cortex (AgCx) of the rat in response to unilateral manipulations of the thalamic mediodorsal nucleus (MD). The ratios of 3,4-dihydroxyphenylacetic acid (DOPAC):DA and 4-hydroxy-3-methoxyphenylacetic acid (homovanillic acid, HVA):DA are used as indices of DA utilisation and were shown to increase in the ipsilateral FCx following electrical stimulation of lateral MD. A similar response was observed 1 hr after an infusion of the excitotoxin sodium ibotenate into lateral MD, although in this case the increase in DA utilisation in FCx was bilateral. Longer periods of recovery after ibotenate treatment (2 day and 1 week) produced DA utilisation ratios that had returned to near control values and by 1 week a significant decrease was detected in HVA:DA of the contralateral FCx. All treatments had little effect on DA utilisation in AgCx, although there was a tendency towards enhanced ratios after electrical stimulation and short-term ibotenate injection. These findings suggest that stimulation of MD neurones may tend to activate the DA system in their convergent terminal regions of cortex. It is argued that these influences result from interactions at the level of the DA terminal rather than at the cell bodies of mesocortical DA neurones.