Interactions of nigrostriate synaptic transmission, iontophoretic O-methylated phenethylamines, dopamine, apomorphine and acetylcholine

Modulatory action of acetylcholine on striatal neurons: microiontophoretic study in awake, unrestrained rats

Cholinergic interneurons innervate virtually all medium spiny striatal cells, but the relevance of this input in regulating the activity and afferent responsiveness of these cells remains unclear. Studies in anaesthetized animals and slice preparations have shown that iontophoretic acetylcholine (ACh) either weakly excites or inhibits striatal neurons. These differential responses may reflect cholinergic receptor heterogeneity but may be also related to the different activity states of recorded units and different afferent inputs specific in each preparation. Single-unit recording was combined with iontophoresis in awake, unrestrained rats to examine the effects of ACh and selective muscarinic (oxotremorine M or Oxo-M) and nicotinic agonists (nicotine or NIC) on dorsal and ventral striatal neurons. These effects were tested on naturally silent, spontaneously active and glutamate-stimulated units. We found that iontophoretic ACh primarily inhibited spontaneously active and glutamate-stimulated units; the direction of the ACh response, however, was dependent on the firing rate. The effects of ACh were generally mimicked by Oxo-M and, surprisingly, by NIC, which is known to excite units in most central structures, including striatal neurons in anaesthetized preparation. Given that NIC receptors are absent on striatal cells but located primarily on dopamine terminals, we assessed the effects of NIC after complete blockade of dopamine receptors induced by systemic administration of a mixture of D1 and D2 antagonists. During dopamine receptor blockade the number of NIC-induced inhibitions dramatically decreased and NIC had mainly excitatory effects on striatal neurons. Thus, our data suggest that under physiologically relevant conditions ACh acts as a state-dependent neuromodulator, and its action involves not only postsynaptic but also presynaptic cholinoreceptors located on dopamine- and glutamate-containing terminals.

Modulatory action of dopamine on acetylcholine-responsive striatal and accumbal neurons in awake, unrestrained rats

In ambulant rats, iontophoresis of low concentrations of dopamine (DA) enhances the response of neurons in striatum and nucleus accumbens to iontophoretic glutamate. In an extension of this line of investigation, we tested the effects of acetylcholine (ACh), a presumed modulator of neuronal function in these same brain regions, and assessed possible DA-ACh interactions. Data were obtained from spontaneously active neurons known to respond to ACh (5-30 nA) when the animals rested quietly with no overt movement. ACh iontophoresis either excited or inhibited striatal and accumbal activity but excitatory effects predominated in both areas. With multiple applications of ACh, especially at the lowest currents tested, either response often was interspersed with instances of no change in firing rate. Responsiveness to ACh also diminished during periods of spontaneous movement when basal firing showed phasic increases in activity. In fact, neurons with the highest rates of basal activity showed the smallest magnitude response to ACh. Prolonged applications (120-180 s) of DA attenuated basal firing as well as the iontophoretic effects of ACh both during the DA application itself and for up to 1 min after DA ejection offset. The result of these inhibitory effects was no net change in the relative magnitude of the ACh response. Thus, although ACh can modulate striatal and accumbal neuronal activity, DA does not regulate this effect in the same way that it regulates the neuronal responsiveness to glutamate.

Coexistence of inhibitory dopamine D-1 and excitatory D-2 receptors on the same caudate nucleus neurons

Microiontophoretic studies using cats anesthetized with alpha-chloralose were performed to determine whether or not dopamine D-1 and D-2 receptors co-exist in the same caudate nucleus (CN) neurons that receive inputs from the substantia nigra (SN), and in which spikes elicited by SN stimulation were blocked by domperidone, a selective D-2 antagonist. Iontophoretic application of dopamine produced a dose-dependent inhibition of spontaneous firing in 2 of 4 spontaneously active CN neurons and an increase in firing in the remaining 2 neurons. However, dopamine inhibited the glutamate-induced firing in 31 of 32 CN neurons that were not spontaneously active. Similar inhibition with iontophoretically applied SKF 38393, a selective D-1 agonist, was observed in 33 of 34 spontaneously inactive neurons tested. When the effects of dopamine, SKF 38393 and bromocriptine (D-2 agonist) were examined on the same CN neurons, the inhibitory effects of both dopamine and SKF 38393 were seen in 14 of 15 neurons, and both an inhibition by SKF 38393 and an excitation by bromocriptine were observed in 15 of 17 neurons. The inhibitory effects of dopamine and SKF 38393 were antagonized by haloperidol and SCH 23390 (D-1 antagonist) without being affected by domperidone. Furthermore, the dopamine-induced inhibition was converted to an excitation during simultaneous application of SCH 23390 in 6 of 10 CN neurons, and this excitation was antagonized by domperidone. These results strongly suggest that the inhibitory D-1 and excitatory D-2 receptors co-exist on the same CN neurons receiving inputs from the SN.

The effects of the administration of beta-phenylethylamine on tyramine metabolism

The concentrations of p-tyramine (p-TA), m-tyramine (m-TA), dopamine (DA) and their principal metabolites, p-hydroxyphenylacetic acid (p-HPAA), m-hydroxyphenylacetic acid (m-HPAA) and homovanillic acid (HVA) were determined in the corpus striatum of Swiss mice at various times after the subcutaneous administration of beta-phenylethylamine (PE) (50 mg/kg). Initially p-TA concentrations were reduced but rapid synthesis was apparent up to 2 h after PE administration. PE treatment increased m-TA and these increases reached significance at 1 and 8 h. PE caused a bimodal increase in p-HPAA and m-HPAA concentrations with the first peak observed at 0.5-1 h due to initial release of p-TA and m-TA. Rapid synthesis of p-TA and m-TA resulted in increased acid concentrations at 4 h. HVA concentrations were increased up to 1 h after PE administration. The synthesis of p-TA and m-TA is related to that of DA probably as a result of the activation of tyrosine hydroxylase. PE may serve as a precursor for p-TA synthesis when the endogenous PE concentration is greatly elevated.

Bilateral asymmetrical changes in the nigral release of [3H]GABA induced by unilateral application of acetylcholine in the cat caudate nucleus

In halothane anaesthetized cats, a push-pull cannula was implanted into the right caudate nucleus (CN) and in each substantia nigra (SN). The release of [3H]GABA continuously formed from [3H]glutamine was estimated in each structure. Acetylcholine (ACh, 5 x 10(-5) M) added in presence of eserine (5 x 10(-5) M) for 50 min in the right caudate nucleus 2 h after the onset of superfusion with [3H]glutamine, stimulated the [3H]GABA release locally. The effect was biphasic when ACh application was made in the median two-thirds of the structure and it was monophasic and transient when the ACh application was restricted to the lateral part. ACh application in the right caudate nucleus also induced changes in [3H]GABA released in the anterior (pars reticulata) and posterior (pars compacta) parts of both SN. While [3H]GABA release was enhanced in the ipsilateral anterior SN, it was reduced in the contralateral anterior SN. Respective opposite effects were observed in the posterior parts of the ipsi- and contralateral SN. These bilateral asymmetrical changes in [3H]GABA release were not dependent on the site of ACh application in the right caudate nucleus. These results indicate that the facilitation of cholinergic transmission in one caudate nucleus influences in an opposite way the striato-nigral GABA neurones on both sides of the brain.

Origins of postsynaptic potentials evoked in identified rat neostriatal neurons by stimulation in substantia nigra

Responses of striatal neurons to stimulation in substantia nigra were recorded intracellularly in intact rats and after acute or chronic unilateral lesions of cerebral cortex or after combined cortical lesions and unilateral thalamic transections. Spiny striatal efferent neurons were identified by intracellular injection of horseradish peroxidase. In intact animals substantia nigra stimulation evoked a complex response with both excitatory and inhibitory phases. Acute unilateral decortication abolished the inhibitory phase of the response and reduced the amplitude of the initial EPSP. Thus, part of the excitatory phase and most or all of the inhibitory phase of the response result from polysynaptic routes to striatum involving cerebral cortex. The remaining EPSP observed in acute decorticate animals exhibited two components distinguished on the basis of their time courses. The latter of these was abolished by thalamic transections. The earlier component was shown to be a monosynaptic EPSP evoked by axon collaterals of cortical efferent neurons projecting to brainstem and was not observed in animals subjected to chronic decortication. After removal of all of these non-nigral response components a small long latency EPSP could be evoked by nigral stimulation. The EPSP is probably due to activation of dopaminergic nigro-striatal axons.

Evidence of a direct action of angiotensin II on neurones in the septum and in the medial preoptic area

Angiotensin II (AII) was microiontophoretically applied on neurones located in the septum and the medial preoptic area (MPOA). All the septal neurones sensitive to AII (15/37) responded by an inhibition to the peptide application. Of 44 units tested in the MPOA 21 cells (48%) were sensitive to AII and responded either by an increase (11/21) or decrease (10/21) in their firing. The specificity of these responses were ascertained by simultaneous application of the antagonist Sar-Ile-Angiotensin II. These data suggest that Angiotensin II acts directly on neurones of the septum and medial preoptic area, structures implicated in the control of drinking behaviour.

Neuropharmacological studies on the nigro-striatal and raphe-striatal system in the rat

The responses of single neostriatal neurones to substantia nigra (SN) and dorsal raphe nucleus (DRN) stimulation and iontophoretic administration of several drugs were studied in urethane-anaesthetised rats. Stimulation of the SN-evoked excitation followed by inhibition in striatal neurones. In some cells only inhibition of firing was evoked indicating that there may be separate nigrostriatal inhibitory and excitatory pathways. DRN stimulation evoked mainly inhibition of striatal cell firing. The activity of most neurones responding to SN and DRN stimulation was depressed by iontophoretically administered dopamine, 5-hydroxytryptamine and GABA and increased by acetylcholine. Studies with antagonist revealed that alpha-flupenthixol reduced responses to dopamine and 5-hydroxytryptamine and inhibition evoked by SN and DRN stimulation. Bicuculline methochloride only reduced responses to GABA. Methysergide selectively reduced responses to 5-hydroxytryptamine and also reduced DRN-but not SN-evoked inhibition. It was concluded that the SN-evoked inhibition was probably mediated by dopamine and DRN-evoked inhibition by 5-hydroxytryptamine.

Responses of central neurones to piribedil and 2-bromo-alpha-ergocryptine: comparison with dopamine and apomorphine

The effects of two putative dopamine receptor agonists, piribedil and 2-bromo-alpha-ergocryptine were studied by means of microiontophoresis. Their effects were compared with those of dopamine and apomorphine on neurones of the frontal cortex and of the caudate nucleus both of which are rich in dopaminergic terminals. Piribedil and S. 584, its dihydroxyphenyl derivative, displayed a potent dopamine-like inhibition in both these areas. 2-Bromo-alpha-ergocryptine exerted significant inhibition on cortical neurones which were spontaneously active but only weak or negligible inhibition on cortical and caudate glutamate-driven units. These observations raise the possibilities that 2-bromo-alpha-ergocryptine (1) mediates its inhibitory effect via a predominant presynaptic action; (2) acts preferentially on cortical neurones.

Dose-dependent biphasic alterations in the spontaneous activity of neurons in the rat neostriatum produced by d-amphetamine and methylphenidate

A dose-response analysis was performed on D-amphetamine- and methylphenidate-induced changes in neuronal activity in the neostriatum of immobilized, phenidate-induced changes in neuronal activity in the neostriatum of immobilized, locally anesthetized rats. Whereas a marked depression of firing rate characterized the response to intraperitoneal injections of 2.5 mg/kg D-amphetamine or 10 mg/kg methylphenidate, increasing the dose (5.0--7.5 mg/kg D-amphetamine and 20--25 mg/kg methylphenidate) shifted this response pattern to a prolonged increase in activity. Both stimulant-induced increases and decreases in neostriatal activity are reversed by 2.0 mg/kg haloperidol. In contrast to the response of neostriatal neurons, the firing rate in the substantia nigra pars compacta was inhibited by both stimulants, even at doses that increased the activity of neurons in the neostriatum. The possible mechanisms underlying these drug-induced changes in firing rate are discussed along with the behavioral implications of a stimulant-induced dose-dependent shift in neostriatal unit activity.

The effect of some neuroleptics on the interaction of cortically and nigrally evoked potentials in the rat striatum

The interaction of cortico- and nigrofugal inputs to the striatum of the rat was investigated using the technique of evoked potentials. Repetitive, unilateral stimulation of the substantia nigra inhibited potentials which were evoked from the ipsilateral rostral cortex and recorded from the ipsilateral striatum. The inhibition was antagonized by low doses of various intraperitoneally administered neuroleptics such as: pimozide (0.1 - 0.2 mg/kg), haloperidol (0.1 - 0.5 mg/kg), chlorpromazine (0.5 - 2.0 mg/kg) and thioridazine (0.5 - 4.0 mg/kg). These findings are discussed in view of the existing controversy regarding the mode of action of dopamine in the striatum.