Dopaminergic involvement in the improving effects of dynorphin A-(1-13) on scopolamine-induced impairment of alternation performance

The coumarin scopoletin potentiates acetylcholine release from synaptosomes, amplifies hippocampal long-term potentiation and ameliorates anticholinergic- and age-impaired memory

In a previous study the simple, naturally derived coumarin scopoletin (SCT) was identified as an inhibitor of acetylcholinesterase (AChE), using a pharmacophore-based virtual screening approach. In this study the potential of SCT as procholinergic and cognition-enhancing therapeutic was investigated in a more detailed way, using different experimental approaches like measuring newly synthesized acetylcholine (ACh) in synaptosomes, long-term potentiation (LTP) experiments in hippocampal slices, and behavior studies. SCT enhanced the K⁺-stimulated release of ACh from rat frontal cortex synaptosomes, showing a bell-shaped dose effect curve (E_max: 4 μM). This effect was blocked by the nicotinic ACh receptor (nAChR) antagonists mecamylamine (MEC) and dihydro-β-erythroidine (DHE). The nAChR agonist (and AChE inhibitor) galantamine induced a similar increase in ACh release (E_max: 1 μM). SCT potentiated LTP in hippocampal slices of rat brain. The high-frequency stimulation (HFS)-induced, N-methyl-D-aspartate (NMDA) receptor dependent LTP of field excitatory postsynaptic potentials at CA3-CA1 synapses was greatly enhanced by pre-HFS application of SCT (4 μM for 4 min). This effect was mimicked by nicotine (2 μM) and abolished by MEC, suggesting an effect on nAChRs. SCT did not restore the total inhibition of LTP by NMDA receptor antagonist d, l-2-amino-5-phosphonopentanoic acid (AP-5). SCT (2 μg, i.c.v.) increased T-maze alternation and ameliorated novel object recognition of mice with scopolamine-induced cholinergic deficit. It also reduced age-associated deficits in object memory of 15–18-month-old mice (2 mg/kg sc). Our findings suggest that SCT possesses memory-improving properties, which are based on its direct nAChR agonistic activity. Therefore, SCT might be able to rescue impaired cholinergic functions by enhancing nAChR-mediated release of neurotransmitters and promoting neural plasticity in hippocampus.

Big dynorphin, a prodynorphin-derived peptide produces NMDA receptor-mediated effects on memory, anxiolytic-like and locomotor behavior in mice

Effects of big dynorphin (Big Dyn), a prodynorphin-derived peptide consisting of dynorphin A (Dyn A) and dynorphin B (Dyn B) on memory function, anxiety, and locomotor activity were studied in mice and compared to those of Dyn A and Dyn B. All peptides administered i.c.v. increased step-through latency in the passive avoidance test with the maximum effective doses of 2.5, 0.005, and 0.7 nmol/animal, respectively. Effects of Big Dyn were inhibited by MK 801 (0.1 mg/kg), an NMDA ion-channel blocker whereas those of dynorphins A and B were blocked by the kappa-opioid antagonist nor-binaltorphimine (6 mg/kg). Big Dyn (2.5 nmol) enhanced locomotor activity in the open field test and induced anxiolytic-like behavior both effects blocked by MK 801. No changes in locomotor activity and no signs of anxiolytic-like behavior were produced by dynorphins A and B. Big Dyn (2.5 nmol) increased time spent in the open branches of the elevated plus maze apparatus with no changes in general locomotion. Whereas dynorphins A and B (i.c.v., 0.05 and 7 nmol/animal, respectively) produced analgesia in the hot-plate test Big Dyn did not. Thus, Big Dyn differs from its fragments dynorphins A and B in its unique pattern of memory enhancing, locomotor- and anxiolytic-like effects that are sensitive to the NMDA receptor blockade. The findings suggest that Big Dyn has its own function in the brain different from those of the prodynorphin-derived peptides acting through kappa-opioid receptors.

Improvement of memory impairment by (+)- and (-)-pentazocine via sigma, but not kappa opioid receptors

(+/-)-Pentazocine is widely used clinically to treat mild to moderate pain as a racemic compound. Although it is known that (-)-pentazocine acts as a kappa opioid receptor agonist to exhibit analgesic actions and (+)-pentazocine acts as a sigma receptor agonist without analgesic effects, their combined effect on memory has not been investigated in detail. In this study, the effect of (+)- and/or (-)-pentazocine on scopolamine-induced memory impairment in mice was investigated using spontaneous alternation performance in a Y-maze. (+)-Pentazocine (0.35 micromol/kg, s.c.) administered 30 min before behavioral testing significantly improved the impairment of spontaneous alternation induced by scopolamine. A higher dose of (-)-pentazocine (3.50 micromol/kg, s.c.) also reversed the scopolamine-induced impairment of alternation performance. Interestingly, the ameliorating effects of not only (+)-pentazocine, but also (-)-pentazocine were antagonized by a selective sigma receptor antagonist, N,N-dipropyl-2-[4-methoxy-3-(2-phenylenoxy)-phenyl]-ethylamine monohydrochloride (NE-100) (2.6 micromol/kg, i.p.). However, those effects were not antagonized by a selective kappa opioid receptor antagonist, nor-binaltorphimine (4.9 nmol/mouse, i.c.v.). Coadministration of (+)- and (-)-pentazocine (0.35 or 3.50 micromol/kg each) did not have any additive or antagonizing effects on the percent alternation. An antinociceptive effect was observed only with (-)-pentazocine (3.50 micromol/kg, s.c.), and was antagonized by nor-binaltorphimine (4.9 nmol/mouse, i.c.v.), but not by NE-100 (2.6 micromol/kg, i.p.). These results suggest that although the analgesic effect of pentazocine was mediated via kappa opioid receptors, the ameliorating effect on scopolamine-induced impairment of spontaneous alternation was mediated via sigma receptors, not via kappa opioid receptors.

BIMU 1 and RS 67333, two 5-HT4 receptor agonists, modulate spontaneous alternation deficits induced by scopolamine in the mouse

The present study was conducted to determine the effects of two potent 5-HT4 receptor agonists, BIMU 1 (1 (3-ethyl-2,3-dihydro-N-[endo-8-methyl-8-azabicyclo (3.2.1)-oct-3-yl]-2-oxo-1H) benzimidazole-1-carboxamide hydrochloride; 1, 3, 10 mg/kg, i.p.) and RS 67333 (1-(4-amino-5-chloro-2-methoxyphenyl)-3-(1-n-butyl-4-piperidinyl)-1-propanone; 0.25, 0.5, 1 mg/kg, i.p.) on the learning impairment induced by the muscarinic acetylcholine receptor antagonist, scopolamine (1 mg/kg) in mice. Working memory was examined by observing spontaneous alternation behavior in the Y-maze test. Both BIMU 1 (10 mg/kg) and RS 67333 (1 mg/kg) prevented the scopolamine-induced alternation deficits, whereas no effect could be evidenced on locomotor or emotional indices. The reversal actions of BIMU 1 and RS 67333 on this cognitive dysfunction were abolished by the selective 5-HT4 receptor antagonist GR 125487 (1-[2-[(methyl sulfonyl)-amino]-ethyl]-4-piperidinyl-methyl-5-fluoro-2-methoxy-1H-indole-3-carboxylate; 10 mg/kg, i.p.). When given alone at the same doses, none of the three serotonergic agents had any measurable effect. These results demonstrate the ability of 5-HT4 receptor agonists to reverse spontaneous working memory deficits and further confirm the therapeutic potential of such ligands in the treatment of cognitive alterations that associate short-term working memory disorders and cholinergic hypofunction.

Endomorphins 1 and 2 induce amnesia via selective modulation of dopamine receptors in mice

The involvement of dopamine receptors in the amnesic effects of the endogenous micro-opioid receptor agonists endomorphins 1 and 2 was investigated by observing step-down type passive avoidance learning in mice. Although the dopamine D1 receptor agonist R(+)-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol hydrochloride (R(+)-SKF38393) (0.05 and 0.1 mg/kg), the dopamine D1 receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (R(+)-SCH23390) (2.5 and 5 microg/kg) or the dopamine D2 receptor agonist N-n-phenethyl-N-propylethyl-p-(3-hydroxyphenyl)-ethylamine (RU24213) (0.3 and 1 mg/kg) had no significant effects on the endomorphin-1 (10 microg)- or endomorphin-2 (10 microg)-induced decrease in step-down latency of passive avoidance learning, (-)-sulpiride (10 mg/kg), a dopamine D2 receptor antagonist, significantly reversed the decrease in step-down latency evoked by endomorphin-2 (10 microg), but not by endomorphin-1 (10 microg). Taken together, it is likely that stimulation of dopamine D2 receptors results in the endomorphin-2-but not endomorphin-1-induced impairment of passive avoidance learning.

Different modulation of cholinergic neuronal systems by dynorphin A (1-13) in carbon monoxide-exposed mice

The effects of dynorphin A (1-13), a kappa-opioid receptor agonist, on the content of acetylcholine (ACh) and high K+-induced release of endogenous ACh were studied in mice exposed to carbon monoxide (CO). Mice were exposed to CO 3 times at 1-hr intervals and used 7 days after CO exposure. Administration of dynorphin A (1-13) (1.5 and 5.0 nmol/mouse, intracerebroventricularly) 15 min before killing significantly increased the ACh content in the striatum and hippocampus of control mice, but had no effect on the ACh content in CO-exposed mice. Dynorphin A (1-13) did not change the choline acetyltransferase (EC 2.3.1.6) activity in control or CO-exposed mice. The high K+-induced endogenous ACh release from hippocampal slices in CO-exposed mice was significantly lower than that of controls, although exposure to CO did not affect the basal release of endogenous ACh from hippocampal slices compared with controls. Dynorphin A (1-13) caused dose-dependent decreases in high K+-induced release of endogenous ACh from hippocampal slices in control mice. This inhibitory effect of dynorphin A (1-13) was blocked by co-perfusion with nor-binaltorphimine, a selective K-opioid receptor antagonist. On the other hand, dynorphin A (1-13) did not decrease high K+-induced release of endogenous ACh from hippocampal slices in CO-exposed mice. These results suggest that dysfunction of the cholinergic system occurred after exposure to CO, and as a result the inhibitory effects of dynorphin A (1-13) may be blocked in CO-exposed mice.

Involvement of dopamine receptors in beneficial effects of tachykinins on scopolamine-induced impairment of alternation performance in mice

The involvement of dopamine receptors in the beneficial effects of intracerebroventricular injection of substance P, neurokinin A and senktide on the scopolamine-induced impairment of spontaneous alternation performance was investigated in mice. Scopolamine (1 mg/kg) significantly impaired spontaneous alternation performance, while substance P (0.1 microg), neurokinin A (0.3 microg), senktide (0.003 microg) and S(-)-sulpiride (10 mg/kg), a dopamine D2 receptor antagonist, improved the scopolamine (1 mg/kg)-induced disturbance of spontaneous alternation performance. However, the dopamine D1 receptor antagonist SCH23390 (7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine maleate) did not influence the scopolamine-induced disturbance of spontaneous alternation performance. The dopamine D2 receptor agonist RU24213 (N-n-propyl-N-phenylethyl-p-(3-hydroxyphenyl)-ethylamine hydrochloride) (1 mg/kg) but not the dopamine D1 receptor agonist SKF38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1 H-3-benzazepine hydrochloride) (3 and 10 mg/kg) reversed the beneficial effects of substance P (0.1 microg) and neurokinin A (0.3 microg) on the scopolamine (1 mg/kg)-induced impairment of spontaneous alternation performance. In contrast, neither SKF38393 (3 and 10 mg/kg) nor RU24213 (0.3 and 1 mg/kg) significantly affected the beneficial effects of senktide (0.003 microg) on the scopolamine (1 mg/kg)-induced impairment of spontaneous alternation performance. Although RU24213 (1 mg/kg) and SCH23390 (0.03 mg/kg) markedly decreased total arm entries, SKF38393 (10 mg/kg), RU24213 (1 mg/kg), SCH23390 (0.03 mg/kg) or S(-)-sulpiride (10 mg/kg) had no significant effects on spontaneous alternation performance. These results suggest that stimulation of dopamine D2 but not D1 receptors reverses the ameliorative effects of substance P and neurokinin A, whereas neither dopamine D1 nor D2 receptors play an important role in the beneficial effects of senktide on the scopolamine-induced impairment of spontaneous alternation performance associated with spatial working memory.

U-50,488H, a selective kappa opioid receptor agonist, ameliorates memory impairments induced by muscarinic autoreceptor agonist, carbachol in mice

We investigated whether carbachol, a muscarinic receptor agonist, induces learning and memory impairments, and U-50,488H, a selective kappa opioid receptor agonist, ameliorates the impairments of learning and memory using a step-down type passive avoidance task in mice. Carbachol induced a dose-related dual response. Carbachol (3 nmol/mouse, i.c.v.) significantly shortened the step-down latency, while lower (1 nmol) and higher (10 nmol) doses of carbachol did not induce learning and memory impairments. U-50,488H (0.64 micromol/kg, s.c.) significantly improved carbachol-induced impairments of learning and memory. These findings suggest that kappa opioid receptor agonists ameliorate learning and memory impairments which may associate with dysfunction of presynaptic cholinergic neurons.

Effects of the kappa-opioid dynorphin A(1-13) on learning and memory in mice

The effects of intracerebroventricular administration of dynorphin A(1-13) on scopolamine- and pirenzepine-induced amnesia were investigated in mice by observing the step-down-type passive avoidance response and spontaneous alternation performance. The pre- or post-training, or preretention administration of dynorphin A(1-13) (0.3-10 micrograms) alone failed to affect the passive avoidance response, while scopolamine (1 mg/kg) significantly inhibited it. Dynorphin A(1-13) (1 microgram) given 15 min before training and retention tests, but not immediately after training, significantly improved the scopolamine (1 mg/kg)-induced impairment of passive avoidance response, indicating the anti-amnesic effects of dynorphin A(1-13). A lower dose (1 mg/kg) of the kappa-opioid receptor antagonist (-)-(1R,5R,9R)-5,9-diethyl-2-(3-furyl-methyl)-2'-hydroxy-6,7-benzomorpha n reversed the anti-amnesic effects of dynorphin A(1-13) (1 microgram). In contrast, although dynorphin A(1-13) (1, 3 and 10 micrograms) did not influence spontaneous alternation performance, scopolamine (1 mg/kg) and the muscarinic M1 receptor antagonist pirenzepine (3 micrograms) markedly decreased spontaneous alternation performance. Dynorphin A(1-13) (3, 5.6 and/or 10 micrograms) significantly improved the scopolamine (1 mg/kg)- and pirenzepine (3 micrograms)-induced impairment of spontaneous alternation performance. The improving effects of dynorphin A(1-13) (3 micrograms) were almost completely reversed by pretreatment with nor-binaltorphimine (4 micrograms), a kappa-selective opioid receptor antagonist. These results suggest that the stimulation of kappa-opioid receptors improves memory dysfunctions resulting from the blockade of muscarinic M1 receptors.

A synergistic interaction between dopamine D1 and D2 receptor subtypes in the memory impairments induced by concussive brain injury (CBI) in mice

Profound latent learning and memory deficits with increased monoamine levels in the brain following concussive brain injury (CBI) have been documented in our previous work. The purpose of the present study was to determine the role of dopamine (DA) receptor subtypes in the memory deficits associated with CBI. Profound latent learning and memory impairments were observed in the vehicle-treated CBI mice. SCH-23390 administered 15 min post-injury had no significant effects on the impairments of latent learning and memory in the CBI mice. Sulpiride significantly improved the impairments of latent learning and memory in a dose-dependent manner, indicating that activation of dopaminergic neuronal function is involved in the CBI-induced amnesia. Interestingly, co-administration of sulpiride and SCH-23390, at doses which alone has no significant effect, significantly ameliorated the impairments of latent learning and memory. These results strongly suggested that D1 and D2 receptor subtypes are synergistically involved in the dysfunction of learning and memory associated with CBI.

Effects of dynorphin A (1-13) on carbon monoxide-induced delayed amnesia in mice

The effects of dynorphin A (1-13) on carbon monoxide (CO)-induced amnesia in mice were investigated. Memory deficiency was apparent during Y-maze testing 5 days after CO exposure (delayed amnesia). Percent alternation in the CO-exposed group was significantly lower than that in the control group. Administration of dynorphin A (1-13) (1.5 nmol, i.c.v.) 15 min before the Y-maze test session reversed the impairment of spontaneous alternation performance in the CO-exposed group. To determine whether this effect was mediated via kappa opioid receptors, we attempted to block the effect of dynorphin A using the kappa opioid receptor antagonist nor-binaltorphimine. Nor-binaltorphimine (5.44 nmol, i.c.v.) blocked the effect of dynorphin A (1-13) on delayed amnesia. Dynorphin A (1-13) did not affect the impairment of alternation induced by the blockade of NMDA-receptors by dizocilpine (MK-801), but significantly prevented the impairment induced by mecamylamine. These results suggest that dynorphin A (1-13) modulates the kappa receptor-mediated opioid neuronal system, and reverses the impairment of spontaneous alternation performance induced by CO exposure.

kappa-Opioid receptor agonists improve pirenzepine-induced disturbance of spontaneous alternation performance in the mouse

We investigated the effects of kappa-opioid receptor agonists such as dynorphin A-(1-13) and U-50,488H on the muscarinic M1-selective receptor antagonist pirenzepine (3 micrograms, i.c.v.)-induced impairment of spontaneous alternation performance in the mouse. Although dynorphin A-(1-13)(1-5.6 micrograms, i.c.v.) or U-50,488H ((+/-)trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide, methanesulfonate hydrate) (0.1-1 mg/kg, i.p.) alone did not influence either spontaneous alternation performance or total arm entries, pirenzepine (3 micrograms, i.c.v.) impaired spontaneous alternation performance without producing any significant change in total arm entries. In contrast, dynorphin A-(1-13) (3 and 5.6 micrograms, i.c.v.) and U-50,488H (0.3 and 1 mg/kg, i.p.) ameliorated the pirenzepine (3 micrograms, i.c.v.)-induced impairment of spontaneous alternation performance. The ameliorating effects of dynorphin A-(1-13)(3 micrograms, i.c.v.) and U-50,488H (0.3 mg/kg, i.p.) were almost completely reversed by pretreatment with nor-binaltorphimine (4 micrograms, i.c.v.), a kappa-opioid receptor antagonist. These results suggest that the stimulation of kappa-opioid receptors improves memory dysfunctions resulting from the blockade of muscarinic M1 receptors.

Substance P markedly ameliorates scopolamine-induced impairment of spontaneous alternation performance in the mouse

We investigated the effects of intracerebroventricular injection of substance P (SP) on the scopolamine (1 mg/kg)-induced impairment of spontaneous alternation performance in the mouse. SP (0.001-3 micrograms) alone did not influence either spontaneous alternation performance or total arm entries. Scopolamine (1 mg/kg) impaired spontaneous alternation performance accompanied by an increment in total arm entries. In contrast, SP (0.01-1 micrograms) significantly improved the scopolamine (1 mg/kg)-induced impairment of spontaneous alternation performance without influencing the scopolamine (1 mg/kg)-induced increase in total arm entries. The effects of SP (0.1 micrograms) on the scopolamine (1 mg/kg)-induced impairment of spontaneous alternation performance were almost completely reversed by pretreatment with WIN 62577 (1 mg/kg), a tachykinin NK-1 receptor antagonist. These results suggest that SP improves the scopolamine-induced impairment of spontaneous alternation performance through the mediation of tachykinin NK-1 receptors.

Dynorphin A-(1-13) potently improves scopolamine-induced impairment of passive avoidance response in mice

The effects of intracerebroventricular administration of dynorphin A-(1-13) on scopolamine-induced amnesia were investigated in mice by using a step-down type passive avoidance task. The pre- or post-training, or pre-retention administration of dynorphin A-(1-13)(0.3-10 micrograms) alone failed to affect step-down latency of the passive avoidance response, while scopolamine (1 mg/kg) significantly shortened step-down latency. Dynorphin A-(1-13)(1 microgram) given 15 min before training and retention tests but not immediately after training significantly improved the scopolamine (1 mg/kg)-induced shortening of step-down latency of the passive avoidance response, indicating antiamnesic effects of dynorphin A-(1-13) (1 microgram). A lower dose (1 mg/kg) of the kappa-opioid receptor antagonist, (-)-(1R,5R,9R)-5,9-diethyl-2-(3-furyl-methyl)- 2'-hydroxy-6,7-benzomorphan, reversed the anti-amnesic effects of dynorphin A-(1-13) (1 microgram). These results suggest that the antiamnesic effects of dynorphin A-(1-13) depend on the timing of drug treatments.