W.T. O'Connor

Release of gamma-aminobutyric acid in the dorsal horn and suppression of tactile allodynia by spinal cord stimulation in mononeuropathic rats.

OBJECTIVE The aim of the present study is to monitor the extracellular gamma-aminobutyric acid (GABA) levels in the lumbar dorsal horn of allodynic rats, which respond to spinal cord stimulation (SCS) with a normalization of the tactile withdrawal threshold. In addition, we monitored the GABA levels in nonresponding and sham-stimulated rats. METHODS Partial constriction injury of the sciatic nerve was performed, and a permanent electrode for SCS was inserted into the spinal canal. The response to SCS was assessed with von Frey hairs in awake animals. Later, microdialysis was performed in the dorsal horn of the spinal cord under halothane anesthesia. The concentration of GABA in the microdialysate was assessed by high-performance liquid chromatography. RESULTS Extracellular GABA levels in rats with sciatic nerve lesions and allodynia (2.3 +/- 0.5 nmol/L) were significantly lower (P < 0.001) than in control rats with intact sciatic nerves (8.1 +/- 1.0 nmol/L), whereas only slightly decreased GABA levels (5.7 +/- 1.1 nmol/L) were detected in nonallodynic rats with sciatic nerve lesions. In the allodynic rats, which respond to SCS by a normalization of the tactile withdrawal threshold, significantly (P < 0.001) increased GABA levels (6.7 +/- 2.3 nmol/L) were detected after SCS. In contrast, neither the allodynic rats, which did not respond to SCS, nor the sham-stimulated allodynic rats displayed increased GABA levels in response to stimulation. CONCLUSION Our results indicate that the development of allodynia, a common symptom in neuropathic pain states, may be linked to a decreased spinal release of GABA. We suggest that an SCS-induced release of GABA could be important for the suppression of allodynia observed in rats after SCS. Similar mechanisms could also be involved in the SCS-induced alleviation of pain in patients with peripheral neuropathy.

N‐methyl‐d‐Aspartic Acid Differentially Regulates Extracellular Dopamine, GABA, and Glutamate Levels in the Dorsolateral Neostriatum of the Halothane‐Anesthetized Rat: An In Vivo Microdialysis Study

Abstract: The effects of local perfusion with the glutamate receptor agonist NMDA and the noncompetitive NMDA receptor antagonist dizolcipine (MK‐801) on extracellular dopamine (DA), GABA, and glutamate (Glu) levels in the dorsolateral striatum were monitored using in vivo microdialysis in the halothane‐anesthetized rat. In addition, the sensitivity of both the basal and NMDA‐induced increases in levels of these neurotransmitter substances to perfusion with tetrodotoxin (TTX; 10−5 M) and a low Ca2+ concentration (0.1 mM) was studied. The results show that the local perfusion (10 min) with both the 10−3 and 10−4 M dose of NMDA increased striatal DA and GABA outflow, whereas only the (10−3 M) dose of NMDA was associated with a small and delayed increase in extracellular Glu levels. The NMDA‐induced effects were dose‐dependently counteracted by simultaneous perfusion with MK‐801 (10−6 and 10−5 M). Both the basal and NMDA (10−3 M)‐induced increase in extracellular striatal DA content was reduced in the presence of TTX and a low Ca2+ concentration, whereas both basal and NMDA‐stimulated GABA levels were unaffected by these treatments. Both the basal and NMDA‐stimulated Glu levels were enhanced following TTX treatment, whereas perfusion with a low Ca2+ concentration reduced basal Glu levels and enhanced and prolonged the NMDA‐induced stimulation. These data support the view that NMDA receptor stimulation plays a role in the regulation of extracellular DA, GABA, and Glu levels in the dorsolateral neostriatum and provide evidence for a differential effect of NMDA receptor stimulation on these three striatal neurotransmitter systems, possibly reflecting direct and indirect actions mediated via striatal NMDA receptors.

Differential effects of single and repeated ketamine administration on dopamine, serotonin and GABA transmission in rat medial prefrontal cortex.

Cognitive functions regulated by the prefrontal cortex are sensitive to changes in dopaminergic and serotoninergic transmission. The non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist ketamine influences dopaminergic transmission and induces psychotic symptoms in normal and schizophrenic individuals. This study examined the effect of single and repeated ketamine (25 mg/kg, i.p.) administration on extracellular levels of dopamine, GABA and the serotonin metabolite 5-hydroxyindoleacetic (5-HIAA) acid in the medial prefrontal cortex using in vivo microdialysis in conscious rat. In line with earlier studies, we observed a transient five-fold increase in dopamine release following single ketamine administration in drug naive animals. However, we also observed a two-fold increase in basal dopamine levels and an almost complete attenuation of the ketamine-induced increase in dopamine release in animals pre-treated with ketamine once daily for 7 days. Extracellular 5-HIAA levels were increased by ketamine in both drug naive and even more enhanced in ketamine-pre-treated animals but without a change in basal 5-HIAA levels. GABA levels were unaffected by either single or repeated ketamine administration. We demonstrate evidence for a differential effect of single and repeated ketamine administration on dopamine, serotonin and GABA transmission in the medial prefrontal cortex. We provide new evidence for a complex adaptation of neurotransmission following repeated NMDA receptor blockade whereby in the presence of increased basal dopamine levels the ketamine-induced increase in dopamine is attenuated and the increase in 5-HIAA is enhanced. It appears from our results that ketamine pre-treatment reduces the dynamics of dopaminergic transmission in the prefrontal cortex and may possibly alter the balance between dopamine and serotonin transmission.

An in vivo microdialysis characterization of extracellular dopamine and GABA in dorsolateral striatum of awake freely moving and halothane anaesthetised rats

This study describes the results of a systematic characterization of extracellular dopamine (DA) and gamma-aminobutyric acid (GABA) recovered from dorsolateral striatum using in vivo microdialysis in rats following acute (2.5 h) and chronic (1 day, 2 day and 4 day) implantation of the probe. The voltage and calcium dependence of DA and GABA overflow was characterised by perfusion with the sodium channel blocker tetrodotoxin (TTX 10-6M) and with Ca2(+)-free Ringers perfusion medium. In addition, the effect of halothane anaesthesia on the responsiveness of these neurotransmitter substances to TTX and Ca2(+)-free perfusion medium was investigated. Perfusion with TTX decreased basal DA levels by at least 60% in all groups. The TTX-induced decrease was most profound in halothane-anaesthetised rats, 24 h after implantation of the probe. Responsiveness of GABA to TTX infusion was different between the groups. In acutely implanted halothane-anaesthetised rats basal GABA levels were unaltered by perfusion with TTX while in the remaining groups at least a 35% reduction was observed. In awake rats 2 days following implantation of the probe removal and replacement of the Ca2+ from the perfusion medium resulted in a reversible reduction of basal DA by 87%. In addition, basal GABA levels were decreased by 52%. This decrease was delayed and was not reversed 1.5 h after the Ca2(+)-free perfusion medium was replaced with normal perfusion medium although basal GABA levels returned to pre-experimental levels by the following day.(ABSTRACT TRUNCATED AT 250 WORDS)

Facilitation of gaba release by neurotensin is associated with a reduction of dopamine release in rat nucleus accumbens

The main aim of the present study was to investigate the effects of local perfusion with the tridecapeptide neurotensin on extracellular GABA and dopamine levels in the nucleus accumbens of the halothane-anaesthetized rat, using in vivo microdialysis. In an initial set of characterization studies we examined the Na+ dependence of neurotransmitter release by local perfusion with ouabain, veratridine and tetrodotoxin. Local perfusion with the Na+ ATPase inhibitor ouabain (10 microM) or the Na+ channel agonist veratridine (20 microM) perfused into the nucleus accumbens increased both extracellular GABA and dopamine levels. The Na+ channel antagonist tetrodotoxin (1 microM) consistently decreased (24% of basal) dopamine levels, while even at 10 microM it did not affect GABA. However, tetrodotoxin (10 microM) abolished the veratridine-induced increase in both GABA and dopamine, demonstrating that Na(+)-dependent neuronal activity is involved in this release mechanism. In a second set of experiments a hypothesis for a functional link between neurotensin, dopamine and GABA in the medial nucleus accumbens was tested. Towards this aim, the effects of local perfusion with a high 1 microM concentration of neurotensin into the nucleus accumbens increased both GABA (210% of basal value) and dopamine (145% of basal) release. However, a low (10 nM) concentration of neurotensin again increased GABA release (160% of basal), but decreased that of dopamine (75% of basal value). Furthermore, the local perfusion with the GABAA receptor antagonist bicuculline abolished the neurotensin (10 nM) induced inhibition of dopamine release without affecting the increase in GABA release. These findings suggest that neurotensin modulates both GABA and dopamine neurotransmission in the nucleus accumbens.(ABSTRACT TRUNCATED AT 250 WORDS)

The striatonigral dynorphin pathway of the rat studied with in vivo microdialysis--II. Effects of dopamine D1 and D2 receptor agonists.

In vivo microdialysis was used to study the effect of intracerebral administration of dopamine agonists on dynorphin B release in the striatum and substantia nigra of rats. The release of dopamine and GABA was also investigated. Administration of the dopamine D1 agonist SKF 38393 (10-100 microM) into the striatum increased extracellular dynorphin B and GABA levels in the ipsilateral substantia nigra, in a concentration-dependent manner. After a short-lasting increase, nigral dopamine levels were significantly decreased after the highest concentration of striatal SKF 38393. An increase in striatal dynorphin B, GABA and dopamine levels was also observed. When SKF 38393 (10 microM) was administered into the substantia nigra, nigral dynorphin B and GABA, but not dopamine levels increased. No significant effects were observed on striatal levels. Administration of the dopamine D2 agonist, quinpirole (100 microM), into the striatum decreased dopamine levels in both striatum and substantia nigra, while no effect was observed on striatal or nigral dynorphin B and GABA levels. Quinpirole (10-100 microM) given into the substantia nigra, decreased striatal dopamine levels in a concentration manner. In the nigra, a short-lasting increase in dopamine levels was observed following the highest concentration of nigral quinpirole (100 microM). The effect was followed by a decrease in dopamine levels. No significant effects were observed on striatal or nigral dynorphin B and GABA levels. The results show that stimulation of D1 receptors in striatum and substantia nigra leads to activation of the striatonigral dynorphin pathway. A parallel effect could also be seen on nigral GABA release.(ABSTRACT TRUNCATED AT 250 WORDS)

Intramembrane Interactions between Neurotensin Receptors and Dopamine D2 Receptors as a Major Mechanism for the Neuroleptic‐like Action of Neurotensina

Evidence has been presented that behavioral actions of NT, inducing its neuroleptic-like action, can be explained on the basis of NT-D2 intramembrane receptor-receptor interactions in the basal ganglia, unrelated to the coexistence phenomenon, leading to reduced affinity and transduction of the D2 agonist binding site. By reducing selectively D2 receptor transduction at the pre- and postsynaptic level, the NT receptor appears capable of switching the DA synapses towards a D1 receptor-mediated transduction, illustrating how receptor-receptor interactions can increase the functional plasticity of central synapses (FIG. 12).

Regional specific effects of clozapine and haloperidol on GABA and dopamine release in rat basal ganglia

gamma-Aminobutyric acid (GABA) and dopamine release were measured concomitantly in rat dorsolateral striatum, fundus striati (a ventral region of striatum) and globus pallidus following s.c. administration of haloperidol or clozapine. Release was measured by microdialysis in halothane-anesthetized rats. Clozapine (5.0 mg/kg) increased GABA release in the fundus striati and haloperidol (0.5 mg/kg) increased GABA release in the globus pallidus. In contrast, clozapine (2.5-40 mg/kg) failed to increase GABA release in the globus pallidus and haloperidol (0.1-2.0 mg/kg) failed to increase GABA release in the fundus. Thus, haloperidol and clozapine are clearly distinguished by their effects on GABA release in the fundus striati and globus pallidus (both drugs increased GABA release in the dorsolateral striatum). Dopamine release was increased by haloperidol and clozapine in the two regions of the striatum. However, except in the fundus striati where clozapine-induced increases in dopamine and GABA occurred in parallel, both drugs were more potent in releasing dopamine than GABA. Drug-induced increases in GABA and dopamine release were reversed by addition of 1 microM tetrodotoxin to the perfusion medium. These data suggest that (1) regional differences in the effects of haloperidol and clozapine on GABA release in the basal ganglia may parallel the unique clinical profiles of these drugs; and (2) increases in dopamine release may occur independently of a GABAergic component in the dorsolateral striatum following low doses of haloperidol and clozapine and in the fundus striati following all effective doses of haloperidol.

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.

Characterization of gamma-aminobutyric acid and dopamine overflow following acute implantation of a microdialysis probe

The present study characterized the voltage and calcium dependence of gamma-aminobutyric acid and dopamine overflow after the acute implantation of a microdialysis probe. Probes were implanted in dorsolateral striatum and globus pallidus. Experiments were performed under light halothane anesthesia. Basal, extracellular levels of GABA were not affected by tetrodotoxin (TTX) and were increased to 140 percent of basal values by calcium free Ringer. Basal, extracellular levels of dopamine were reduced to 14 percent of basal values by the addition of TTX and to 30 percent of basal values by the removal of calcium from the Ringer solution. The results suggest that in this in vivo preparation basal extracellular dopamine is largely of vesicular origin while GABA is not.