dizocilpine-maleate and epibatidine

Mouse superficial superior colliculus (SuSC) contains dense GABAergic innervation and diverse nicotinic acetylcholine receptor subtypes. Pharmacological and genetic approaches were used to investigate the subunit compositions of nicotinic acetylcholine receptors (nAChR) expressed on mouse SuSC GABAergic terminals. [(125) I]-Epibatidine competition-binding studies revealed that the α3β2* and α6β2* nicotinic subtype-selective peptide α-conotoxin MII-blocked binding to 40 ± 5% of SuSC nAChRs. Acetylcholine-evoked [(3) H]-GABA release from SuSC crude synaptosomal preparations is calcium dependent, blocked by the voltage-sensitive calcium channel blocker, cadmium, and the nAChR antagonist mecamylamine, but is unaffected by muscarinic, glutamatergic, P2X and 5-HT3 receptor antagonists. Approximately 50% of nAChR-mediated SuSC [(3) H]-GABA release is inhibited by α-conotoxin MII. However, the highly α6β2*-subtype-selective α-conotoxin PIA did not affect [(3) H]-GABA release. Nicotinic subunit-null mutant mouse experiments revealed that ACh-stimulated SuSC [(3) H]-GABA release is entirely β2 subunit-dependent. α4 subunit deletion decreased total function by >90%, and eliminated α-conotoxin MII-resistant release. ACh-stimulated SuSC [(3) H]-GABA release was unaffected by β3, α5 or α6 nicotinic subunit deletions. Together, these data suggest that a significant proportion of mouse SuSC nicotinic agonist-evoked GABA-release is mediated by a novel, α-conotoxin MII-sensitive α3α4β2 nAChR. The remaining α-conotoxin MII-resistant, nAChR agonist-evoked SuSC GABA release appears to be mediated via α4β2* subtype nAChRs.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Acetylcholine; Adenosine Triphosphate; Animals; Bridged Bicyclo Compounds, Heterocyclic; Bungarotoxins; Conotoxins; Dizocilpine Maleate; Dose-Response Relationship, Drug; Drug Interactions; Excitatory Amino Acid Antagonists; Female; gamma-Aminobutyric Acid; In Vitro Techniques; Iodine Isotopes; Male; Mice; Mice, Inbred C57BL; Mice, Knockout; Nicotinic Agonists; Nicotinic Antagonists; Protein Binding; Protein Subunits; Pyridines; Receptors, Nicotinic; Serotonin Antagonists; Superior Colliculi; Synaptosomes; Tritium; Tropanes

This study assessed the effects of the nicotinic agonist (+/-)-epibatidine (EPIB) on the C-fiber flexor reflex in the anesthetized rat. Electrical stimulation of the hindpaw produces a long latency (> 150 ms) C-fiber mediated electromyographic (EMG) burst in hindlimb flexor muscles. EPIB (0.01, 0.03 micromol/kg, i.p.) significantly reduced (p < 0.05) C-fiber -related EMG activity by 46 and 64%, respectively. This effect was similar to that produced by the opioid morphine (21.0 micromol/kg, i.p.) and the NMDA receptor antagonist MK-801 (3.0 micromol/kg, i.p.). Nicotinic receptor blockade with the antagonists mecamylamine (5.0 micromol/kg, i.p.) and chlorisondamine (23.0 nmol/rat, intracerebroventricular) attenuated the effects of systemic EPIB on the C-fiber reflex. EPIB injection (0.04 nmol/rat) into the nucleus raphe magnus significantly decreased C-fiber EMG activity by 67%, suggesting a supraspinal site of action. In contrast, EPIB (0.6 nmol/rat) administered into the lumbar spinal cord significantly increased the C-fiber reflex by 117%. In summary, systemic and supraspinal EPIB exerted an inhibitory effect on central pain transmitting pathways, while a stimulatory effect is elicited in the spinal cord. The inhibitory effects are consistent with the reported analgesic properties of EPIB. The excitatory effect may be related to the reported algogenic responses when EPIB is administered intrathecally.

Topics: Analysis of Variance; Animals; Bridged Bicyclo Compounds, Heterocyclic; Chlorisondamine; Dizocilpine Maleate; Drug Interactions; Electric Stimulation; Electromyography; Excitatory Amino Acid Antagonists; Injections, Intra-Arterial; Injections, Intraventricular; Injections, Spinal; Male; Morphine; Muscle, Skeletal; Narcotics; Nerve Fibers, Unmyelinated; Nicotinic Agonists; Nicotinic Antagonists; Pyridines; Raphe Nuclei; Rats; Rats, Sprague-Dawley; Reaction Time; Reflex

Alterations in neurotransmitter receptor expression in the central nervous system may contribute to physiological and behavioral deficits that follow traumatic brain injury (TBI). Previous studies from our laboratory have demonstrated significant and widespread deficits in alpha7* nicotinic cholinergic receptor (alpha7* nAChr) expression 2 days following cortical contusion brain injury. The purpose of this study was to evaluate changes in alpha7* nAChr expression over a wider range of post-TBI recovery intervals. Animals were anesthetized and subjected to a moderate cortical contusion brain injury (2 mm cortical compression). Animals were euthanatized at various post-TBI time intervals, ranging from 1 h to 21 days, and quantitative autoradiography was used to evaluate cholinergic receptor subtype expression in the cerebral cortex and hippocampus. As previously reported, the alpha7* nAChr was the most sensitive target of TBI-induced plasticity. Significant decreases in alpha-[(125)I]-bungarotoxin (BTX) binding occurred as early as 1 h post-TBI, and persisted in some brain regions for up to 21 days. A kinetic analysis of changes in BTX binding, performed 2 days following brain injury, indicated that the binding deficits are not due to significant changes in receptor affinity. TBI-induced changes in alpha3*/alpha4* nACh receptors, muscarinic cholinergic receptors, and NMDA-type glutamate receptor expression were lower in magnitude, restricted to fewer brain regions and more transient in nature. Persistent deficits in alpha7* nAChr expression following TBI may contribute to impaired functional outcome following brain injury.

Topics: Animals; Autoradiography; Brain; Bridged Bicyclo Compounds, Heterocyclic; Bungarotoxins; Disease Models, Animal; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Image Processing, Computer-Assisted; Kinetics; Male; Muscarinic Antagonists; Nicotinic Agonists; Protein Binding; Pyridines; Quinuclidinyl Benzilate; Radioligand Assay; Rats; Rats, Sprague-Dawley; Receptors, Cholinergic; Receptors, Muscarinic; Receptors, N-Methyl-D-Aspartate; Receptors, Nicotinic; Time

Epibatidine (exo-2-(6-chloro-3-pyridyl)-7-azabicyclo-[2.2.1]heptane), an extract of frog skin, is a novel and highly potent agonist for the nicotinic acetylcholine (ACh) receptor. The present study was undertaken to examine the expression of Fos protein in several rat brain regions following an acute administration of epibatidine. Furthermore, we also studied the role of the dopamine D1 and D2 receptors and the N-methyl-d-aspartate (NMDA) receptor, and nicotinic ACh receptor in the expression of Fos protein by epibatidine. A single administration of epibatidine (5, 10, 50 microgram/kg) caused a marked induction of Fos-immunoreactivity in the prefrontal cortex, medial striatum, nucleus accumbens, amygdala and superior colliculus of rat brain. In these regions, pretreatment with SCH 23390 (1.0 mg/kg), a dopamine D1 receptor antagonist, MK-801 (1.0 mg/kg), a NMDA receptor antagonist, and mecamylamine (5. 0 mg/kg), a nicotinic Ach receptor antagonist, inhibited the induction of Fos protein by epibatidine (10 microgram/kg). Pretreatment with sulpiride, a dopamine D2 receptor antagonist, blocked the induction of Fos protein in the prefrontal cortex and the core region of accumbens nucleus, but not in the medial striatum and the shell division of nucleus accumbens of rat brain. These results suggest that epibatidine induced the expression of Fos protein in several regions of rat brain, and that dopamine D1 receptor, NMDA receptor, and nicotinic ACh receptor may play a role in the expression of Fos protein by epibatidine in rat brain. Furthermore, dopamine D2 receptor may, in part, play a role in epibatidine induced expression of Fos protein in the prefrontal cortex and the core region of nucleus accumbens, but not in the medial striatum and the shell division of nucleus accumbens of rat brain.

Topics: Animals; Benzazepines; Brain Chemistry; Bridged Bicyclo Compounds, Heterocyclic; Dizocilpine Maleate; Dopamine Antagonists; Excitatory Amino Acid Antagonists; Male; Mecamylamine; Nicotinic Agonists; Nicotinic Antagonists; Proto-Oncogene Proteins c-fos; Pyridines; Rats; Rats, Sprague-Dawley; Receptors, Dopamine D1; Receptors, Dopamine D2; Receptors, N-Methyl-D-Aspartate; Receptors, Nicotinic; Sulpiride