Malcolm S. Reid

Differential modulation of striatal dopamine release by intranigral injection of γ-aminobutyric acid (GABA), dynorphin A and substance P

The effects of intranigral injection of gamma-aminobutyric acid (GABA) (dose range: 10.0-300.0 nmol), dynorphin A (0.005-0.5 nmol) and substance P (0.00007-7.0 nmol) on striatal dopamine (DA) release, and dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) formation were studied by microdialysis. It was found that both GABA and dynorphin A produced a dose-dependent decrease in the release of striatal DA following injection into the ipsilateral substantia nigra, the pars reticulata. In contrast, intranigral injection of substance P produced an increase in DA release. However, the dose-response curve for the substance P effect had a biphasic shape. The maximum effect was produced by 0.007 nmol, whereas higher doses (0.07-0.7 nmol) produced less pronounced effects. At the highest dose (7.0 nmol), substance P produced a strong decrease of DA release. Striatal levels of DOPAC and HVA were enhanced by GABA, dynorphin A and substance P. The present results support the concept that substance P, directly or indirectly, provides a positive feed-back regulation for the release of striatal DA, whereas GABA and dynorphin exert a negative feed-back regulation.

The effects of intranigral GABA and dynorphin A injections on striatal dopamine and GABA release: Evidence that dopamine provides inhibitory regulation of striatal GABA neurons via D2 receptors

The effects of injections of gamma-aminobutyric acid (GABA) and dynorphin A into the substantia nigra, pars reticulata on the levels of extracellular dopamine (DA) and GABA in the ipsilateral striatum of halothane-anaesthetized rats were studied using microdialysis. The effects of intranigral injections of substance P and neurokinin A were also studied. Intranigral GABA (300 nmol) or dynorphin A (0.5 nmol) injections produced a simultaneous decrease in DA and increase in GABA levels, while intranigral substance P (0.07 nmol) or neurokinin A (0.09 nmol) injections produced an increase in DA but had no effect on GABA levels. DA agonists, apomorphine (D1/D2), SKF 38393 (D1) and pergolide (D2) were applied locally by perfusing them through the microdialysis probe, each at a concentration of 10(-5) M. All 3 agonists decreased the levels of DA in the striatum. However, while apomorphine and SKF 38393 increased, pergolide decreased the levels of GABA in the striatum. The increase in striatal GABA produced by intranigral injections of GABA (300 nmol) was reversed by local perfusion with pergolide (10(-5) M), but was not reversed by local perfusion with SKF 38393 (10(-5) M). These findings suggest that D1 and D2 receptors differentially regulate striatal GABA release, and are stimulatory and inhibitory, respectively. Furthermore, it is suggested that nigrostriatal DA functions as an inhibitory modulator of striatal GABA neurons, acting via D2 receptors.

STRIATONIGRAL GABA, DYNORPHIN, SUBSTANCE P AND NEUROKININ A MODULATION OF NIGROSTRIATAL DOPAMINE RELEASE : EVIDENCE FOR DIRECT REGULATORY MECHANISMS

SummaryThe striatonigral pathway contains several neurotransmitters which may regulate the activity of the nigrostriatal dopamine projection in the rat. This was investigated by measuring extracellular dopamine levels in the striatum, using microdialysis, after injections of GABA (300 nmol/0.2 μl), dynorphin A (0.5 nmol/0.2 μl), substance P (0.07 mnol/0.2 μl) or neurokinin A (0.09 nmol/0.2 μl) into the ipsilateral substantia nigra, pars reticulata (SNR). Intranigral injections of GABA or dynorphin A inhibited, while intranigral injections of substance P or neurokinin A stimulated dopamine levels in the ipsilateral striatum. In rats with ibotenic acid lesions (2.5 μg/0.5 μl) in the SNR, intranigral injections of GABA or dynorphin A inhibited, while intranigral injections of substance P or neurokinin A stimulated dopamine levels in the ipsilateral striatum. These responses were not significantly different than those in unlesioned rats. Analysis of the intranigral lesion with in situ hybridization revealed a heavy loss of glutamic acid decarboxylase mRNA expression in the SNR and a significant loss of tyrosine hydroxylase (TH) mRNA expression in the SNC. Immunohistochemical analysis revealed a disappearance of TH-Like immunoreactivity (LI) im dendrites in the SNR, a considerable loss of TH-LI cell bodies in the SNC and a restricted loss of neuropeptide K-LI in the SNR around the tip of the injection cannula. Furthermore, lesioned rats rotated ipsilateral to the lesion after apomorphine (1 mg/kg, s.c.), indicating that the basal ganglia output mediated via the SNR GABA neurons was impaired on the lesioned side. Analysis of the striatum revealed that a dense TH-LI fiber network could still be seen on the lesioned side. Furthermore, basal and amphetamine stimulated extracellular dopamine levels in the striatum on the lesioned side were not significantly depleted. This indicates that the ascending nigrostriatal dopamine projection was functionally intact on the lesioned side. These findings indicate that intranigral GABA, dynorphin A, substance P and neurokinin A modulation of ipsilateral striatal dopamine release is mediated via direct action on the nigrostriatal projection. Thus, it is suggested that the striatonigral pathway, which contains GABA, dynorphin, substance P and neurokinin A, exerts a direct regulatory effect on the activity of the nigrostriatal dopamine projection.

Neuronal dependence of extracellular dopamine, acetylcholine, glutamate, aspartate and gamma-aminobutyric acid (GABA) measured simultaneously from rat neostriatum using in vivo microdialysis: reciprocal interactions

SummaryThe neuronal origin of extracellular levels of dopamine (DA), acetylcholine (ACh), glutamate (Glu), aspartate (Asp) and gamma-aminobutyric acid (GABA) simultaneously collected from the neostriatum of halothane anaesthetized rats with in vivo microdialysis was studied. The following criteria were applied (1) sensitivity to K+-depolarization; (2) sensitivity to inhibition of synaptic inactivation mechanisms; (3) sensitivity to extracellular Ca2+; (4) neuroanatomical regionality; sensitivity to selective lesions and (5) sensitivity to chemical stimulation of the characterized pathways.It was found that: (1) Extracellular DA levels found in perfusates collected from the neostriatum fulfills all the above criteria and therefore the changes in extracellular DA levels measured with microdialysis reflect actual release from functionally active nerve terminals, and so reflect ongoing synaptic transmission. (2) Changes in neostriatal ACh levels reflect neuronal activity, provided that a ACh-esterase inhibitor is present in the perfusion medium. (3) Extracellular Glu, Asp and GABA could be measured in different perfusion media in the rat neostriatum and probably reflect metabolic as well as synaptic release. However, (4) the majority of the extracellular GABA levels found in perfusates collected from the neostriatum may reflect neuronal release, since GABA levels were increased, in a Ca2+-dependent manner, by K+-depolarization, and could be selectively decreased by an intrinsic neostriatal lesion. (5) It was not possible to clearly distinguish between the neuronal and the metabolic pools of Glu and Asp, since neostriatal Glu and Asp levels were only slightly increased by K+-depolarization, and no changes were seen after decortication. A blocker of Glu re-uptake, DHKA, had to be included in the perfusion medium in order to monitor the effect of K+-depolarization on Glu and Asp levels. Under this condition, it was found (6) that neostriatal Glu and Asp levels were significantly increased by K+-depolarization, although only increases in the Glu levels were sensitive to Ca2+ in the perfusion medium, suggesting that Glu but not Asp is released from vesicular pools. (7) Evidence is provided that selective stimulations of nigral DA cell bodies may lead to changes in release patterns from DA terminals in the ipsilateral neostriatum, which are in turn followed by discrete changes in extracellular levels of GABA and Glu in the same region. Finally, some methodological considerations are presented to clarify the contribution of neuronal release to extracellular levels of amino acid neurotransmitters in the rat neostriatum.

The substance P(1–7) fragment is a potent modulator of substance P actions in the brain

The neuropeptide, substance P (SP), produces a spectrum of behavioural effects. When given locally into the substantia nigra, SP induces dopamine release in the ipsilateral striatum and produces contralateral rotation in a dose-dependent, but bell-shaped, manner. Similar dose-response relationships have been observed for SP and other peptides in different bioassays. To test whether SP fragmentation is responsible for this phenomenon, SP(1-7), which is the main SP fragment in rat CNS, was injected intranigrally. SP(1-7) was found to act as a very potent antagonist against the SP-induced responses and was formed locally in the nigra after SP injection. It is proposed that SP(1-7) is an endogenous modulator of SP actions. Generation of peptide fragments, which retain receptor affinity but not efficacy, may be a general mechanism for autoregulation in peptidergic systems.

Intranigral substance P modulation of striatal dopamine: interaction with N-terminal and C-terminal substance P fragments

The effects of unilateral injections of two substance P fragments, the N-terminal substance P (1-7) (SP1-7) and the C-terminal substance P (6-11) (SP6-11) into the substantia nigra, pars reticulata on dopamine (DA) release in the ipsilateral striatum of halothane-anaesthetized rats were studied using microdialysis. SP1-7 and SP6-11 were also tested for their ability to modify the DA stimulation produced by intranigral injections of SP or neurokinin A (NKA). In addition, the SP antagonist Spantide I was tested for its ability to modify the DA stimulation produced by an intranigral injection of SP1-7. Intranigral injections of SP1-7 (0.001-5.0 nmol) inhibited DA release after low doses (0.001-0.01 nmol), but stimulated DA release after high doses (0.1-5.0 nmol). Striatal dihydroxyphenylacetic acid (DOPAC) levels increased moderately after high doses of SP1-7 (1.0-5.0 nmol). Intranigral injections of SP6-11 (0.01-5.0 nmol) inhibited DA release, but enhanced striatal DOPAC levels, dose-dependently. SP1-7 (0.01-0.1 nmol), but not SP6-11 (0.1 nmol), blocked the stimulation of striatal DA release produced by intranigral SP (0.07 nmol). Neither SP1-7 (0.1 nmol) nor SP6-11 (0.1 nmol) could modify the stimulation of striatal DA release produced by intranigral NKA (0.09 nmol). The increase in DA release after a high dose of SP1-7 (1.0 nmol) was not modified by co-administration with Spantide I (0.07 nmol).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of intranigral substance P and neurokinin A on striatal dopamine release--II. Interactions with bicuculline and naloxone.

The functional roles of striatonigral neurokinins were studied by analysing the effects of intranigral injections of substance P and neurokinin A on the extracellular levels of dopamine and dihydroxyphenylacetic acid in the striatum, as measured by in vivo microdialysis in rats. An opioid antagonist, naloxone, and a GABAergic antagonist, bicuculline, were tested and analysed for their ability to modify the neurokinin effects. Unilateral injections of substance P (0.07 nmol) or neurokinin A (0.09 nmol) into the substantia nigra, pars reticulata of halothane anaesthetized rats produced long-lasting increases in ipsilateral striatal dopamine and dihydroxyphenylacetic acid levels. Intranigral injections of naloxone (30 and 300 nmol) produced short-lasting decreases in striatal dopamine, concomitant with an increase in dihydroxyphenylacetic acid. Intranigral injections of 7.0 nmol bicuculline produced an increase, while 70 nmol produced a decrease in striatal dopamine, however, both doses produced an increase in dihydroxyphenylacetic acid. When co-administered intranigrally, the high dose of naloxone (300 nmol) completely blocked the dopamine stimulation of substance P (0.07 nmol), but only moderately inhibited that of neurokinin A (0.09 nmol). The high dose of bicuculline (70 nmol) completely blocked the dopamine stimulation of neurokinin A, but only moderately inhibited that of substance P. Naloxone (30 and 300 nmol) enhanced the dihydroxyphenylacetic acid response to substance P, while bicuculline (70 nmol) inhibited the dihydroxyphenylacetic acid response to neurokinin A. These findings complement and extend the findings in the preceding paper, demonstrating that intranigral substance P and neurokinin A stimulate striatal dopamine via different neuronal mechanisms. We suggest that opioid drugs have a greater influence over substance P while GABAergic drugs have a greater influence over neurokinin A.

Effects of Intranigral Substance P and Neurokinin A Injections on Extracellular Dopamine Levels Measured with Microdialysis in the Striatum and Frontoparietal Cortex of Rats

Extracellular levels of dopamine (DA) and its metabolite, 3,4‐dihydroxyphenylacetic acid (DOPAC), in the striatum and frontoparietal (sensorimotor) cortex in halo‐ thane‐anesthetized rats were analyzed simultaneously using in vivo microdialysis. Basal DA levels, measured from the microdialysis perfusate, were 6.4 ± 0.8 nM (n = 15) in the striatum and 0.9 ± 0.1 nM (n = 15) in the frontoparietal cortex. Subcutaneous injections of d‐amphetamine (2 mg/kg) increased DA levels 10‐fold in the striatum and fivefold in the cortex. Injections of substance P (0.07 nmol/0.2 μl) into the substantia nigra pars reticulata (SNR) increased DA and DOPAC levels ∼30% in the ipsilateral striatum and 50% in the ipsilateral frontoparietal cortex. Injections of neurokinin A (0.09 nmol/0.2 μl) into the SNR increased DA and DOPAC levels ∼30% in the ipsilateral striatum but did not significantly affect DA levels in the ipsilateral frontoparietal cortex, although DOPAC levels were increased by ∼50%. It is suggested that striatal and cortical DA release is regulated differently by nigral substance P and neurokinin A terminals.