Pharmacological Characterization of Tachykinin Receptors Controlling Acetylcholine Release from Rat Striatum: An In Vivo Microdialysis Study

Abstract: The regulation of striatal cholinergic function by tachykinins was examined in urethane-anesthetized rats by using microdialysis. Substance P (0.01–1 µ M ), [Sar⁹,Met(O₂)¹¹]substance P (1–10 µ M ), septide (0.1–3 µ M ), neurokinin (NK) A (0.1–10 µ M ), and senktide (0.1–10 µ M ) produced concentration-dependent increases in striatal acetylcholine (ACh) release. Septide was the most potent agonist for inducing release of ACh, whereas the stimulating effect of senktide was less pronounced and more progressive in onset. The response to septide was prevented by intraperitoneal administration of the nonpeptide NK₁ antagonist SR 140333 (1–3 mg/kg) but not by the nonpeptide NK₂ receptor antagonist SR 48968, indicating that the effect was mediated specifically by NK₁ receptors. ACh release caused by NKA was reduced by SR 48968 (1–3 mg/kg) and slightly affected by SR 140333, indicating a principal role for NK₂ receptors in the peptide response. The similar efficacy of SR 140333 and SR 48968 in blocking substance P-induced ACh release suggested that the effect of this peptide involves the stimulation of both NK₁ and NK₂ receptors. Finally, our results indicate that the increase in striatal ACh release induced by the D₁ agonist (+)-SKF-38393 (3 µ M ) may be mediated indirectly through local release of NKA or substance P acting at NK₂ receptors.     

Characterization of Basal and Morphine-Induced Histamine Release in the Rat Periaqueductal Gray

Abstract: Previous studies have shown that antinociceptive doses of systemic morphine increase extracellular histamine (HA) levels in the rat periaqueductal gray (PAG), although the cellular origin of basal and morphine-induced HA release in the PAG is unknown. Treatment with α-fluoromethylhistidine (FMH; 100 mg/kg, i.p.), the irreversible inhibitor of histidine decarboxylase, decreased basal HA release by a maximum of 80% and prevented morphine-induced HA release in the PAG. In addition, perfusion of this area with the sodium channel blocker tetrodotoxin (10⁻⁶ M ) decreased basal HA release by a maximum of 57% from baseline levels. When the perfusion medium was modified by substitution of magnesium for calcium, extracellular HA levels in the PAG decreased by a maximum of 72%, and morphine-induced HA release was prevented. Thioperamide (5 mg/kg, i.p.), an H₃ antagonist, increased HA release in the PAG to a maximum of 249% within the first 30–60-min period. Taken together, these results suggest that basal and morphine-induced HA release in the rat PAG have a neuronal origin.     

Effect of Voltage-Sensitive Calcium Channel Antagonists on the Release of 5-Hydroxytryptamine from Rat Hippocampus In Vivo

The effect of calcium channel antagonists on the release of 5-hydroxytryptamine from the hippocampus of the chloral hydrate-anaesthetised rat was studied using the technique of intracerebral microdialysis. As the basal concentration of 5-hydroxytryptamine was close to the limit of detection of the HPLC method (8 fmol), the 5-hydroxytryptamine reuptake inhibitor, fluoxetine (10 microM), was included in the perfusion fluid. The L-type voltage-sensitive calcium channel antagonists, PN200-110, diltiazem, and verapamil, all passed through the dialysis membrane, giving a recovery of 20-30%. The N-type voltage-sensitive calcium channel antagonist, omega-conotoxin, penetrated less readily (12% recovery). The dihydropyridine, PN200-110, adhered to the probe, resulting in an effective concentration at the membrane 30% of that in the perfusion fluid. The concentration of 5-hydroxytryptamine in the dialysate samples was reduced by 60% in the absence of calcium. The L channel antagonists had little effect on the release of 5-hydroxytryptamine, which was inhibited, in a dose-dependent manner, to a maximum of 40% by omega-conotoxin. It is concluded that, under physiological conditions, the release of 5-hydroxytryptamine from the rat hippocampus is dependent on the entry of calcium through N-type voltage-sensitive calcium channels, although another calcium channel may also be involved.     

Influence of Perfusate Glucose Concentration on Dialysate Lactate, Pyruvate, Aspartate, and Glutamate Levels Under Basal and Hypoxic Conditions: A Microdialysis Study in Rat Brain

Abstract: The aim of this study was to evaluate the influence of perfusion media with different glucose concentrations on dialysate levels of lactate, pyruvate, aspartate (Asp), and glutamate (Glu) under basal and hypoxic conditions in rat brain neocortex. Intracerebral microdialysis was performed with the rat under general anesthesia using bilateral probes (o.d. 0.3 mm; membrane length, 2 mm) perfused with artificial CSF containing 0.0 and 3.0 m M glucose, respectively. Basal dialysate levels were obtained 2 h after probe implantation in artificially ventilated animals. Dialysate levels of glucose were also measured for the two different perfusion fluids. The mean absolute extracellular concentration of glucose was estimated by a modification of the no-net-flux method to be 3.3 mmol/L, corresponding to an average in vivo recovery of 6% for glucose. Hypoxia was induced by lowering the inspired oxygen concentration to 3%. Hypoxia caused a disturbance of cortical electrical activity, evidenced by slower frequency and lower amplitudes on the electroencephalogram compared with prehypoxic conditions. This was associated with significant elevations of lactate, Asp, and Glu levels. There were no statistically significant differences in dialysate metabolite levels between the two perfusion fluids, during either normal or hypoxic conditions. We conclude that microdialysis with glucose-free perfusion fluid does not drain brain extracellular glucose in anesthetized rats to the extent that the dialysate lactate, pyruvate, Asp, and Glu levels during basal or hypoxic conditions are altered.     

The Ontogeny of Apomorphine-Induced Alterations of Neostriatal Dopamine Release: Effects on Spontaneous Release

The effects of apomorphine (0.05, 0.1, and 1.0 mg/kg, s.c.) on the extracellular levels of dopamine and the dopamine metabolite 3, 4-dihydroxyphenylacetic acid were studied through the use of in vivo microdialysis in the neostriatum of developing and adult rats. Fifteen-minute samples were collected from urethane-anesthetized rats 5, 10–11, 21–22, and 35–36 days old and adults and quantified by HPLC with electrochemical detection. Apomorphine attenuated extracellular levels of dopamine in all age groups, suggesting that the dopamine autoreceptor modulating release in the neostriatum is functional by 5 days of age. A dose-response effect of apomorphine on extracellular dopamine was observed in all age groups except at 10–11 days of age. Extracellular levels of 3, 4-dihydroxyphenylacetic acid were also significantly decreased in all age groups, consistent with the hypothesis that synthesis-modulating dopamine autoreceptors in the neostriatum are functional by 5 days of age. Apomorphine had a significantly greater effect on extracellular 3, 4-dihydrpxyphenylacetic acid levels at the 0.05 and 0.1 mg/kg doses in the 5- and 10–11-day-old age groups compared with the other ages. Absolute levels of extracellular dopamine were significantly attenuated at 5 days of age compared with the other ages, and absolute levels of extracellular 3, 4-dihydroxyphenylacetic acid monotonically increased with age.