Effects of naftopidil on inhibitory transmission in substantia gelatinosa neurons of the rat spinal dorsal horn in vitro

Analyses of the Mode of Action of an Alpha-Adrenoceptor Blocker in Substantia Gelatinosa Neurons in Rats

To elucidate why naftopidil increases the frequency of spontaneous synaptic currents in only some substantia gelatinosa (SG) neurons, post-hoc analyses were performed. Blind patch-clamp recording was performed using slice preparations of SG neurons from the spinal cords of adult rats. Spontaneous inhibitory and excitatory postsynaptic currents (sIPSCs and sEPSCs, respectively) were recorded. The ratios of the frequency and amplitude of the sIPSCs and sEPSCs following the introduction of naftopidil compared with baseline, and after the application of naftopidil, serotonin (5-HT), and prazosin, compared with noradrenaline (NA) were evaluated. First, the sIPSC analysis indicated that SG neurons reached their full response ratio for NA at 50 μM. Second, they responded to 5-HT (50 μM) with a response ratio similar to that for NA, but prazosin (10 μM) did not change the sEPSCs and sIPSCs. Third, the highest concentration of naftopidil (100 μM) led to two types of response in the SG neurons, which corresponded with the reactions to 5-HT and prazosin. These results indicate that not all neurons were necessarily activated by naftopidil, and that the micturition reflex may be regulated in a sophisticated manner by inhibitory mechanisms in these interneurons.

Differential Effects of Alpha 1-Adrenoceptor Antagonists on the Postsynaptic Sensitivity: Using Slice Patch-Clamp Technique for Inhibitory Postsynaptic Current in Substantia Gelatinosa Neurons From Lumbosacral Spinal Cord in Rats

PURPOSE

Alpha1-adrenoceptors participate in improving storage symptoms of male lower urinary tract symptoms. However, the mechanism of action of these compounds remains unclear. The goal of the present study was to clarify the effect of α1- adrenoceptor antagonists on γ-aminobutyric acid (GABA)/glycine-mediated outward currents of the inhibitory postsynaptic current (IPSC) in substantia gelatinosa (SG) neurons from the lumbosacral spinal cord in rats.

METHODS

Male adult Sprague-Dawley rats were used. Blind whole-cell patch-clamp recordings were performed in SG neurons from isolated spinal cord slice preparations. IPSCs were recorded in individual SG neurons to which naftopidil (100μM), tamsulosin (100μM), silodosin (30μM), or prazosin (10μM) were applied sequentially with intervening washout periods. Strychnine (2μM), bicuculline (10μM), or tetrodotoxin (TTX)(1μM) were added before naftopidil. Individual outward currents were analyzed.

RESULTS

The bath application of naftopidil, yielded outward IPSCs in 13 of 52 SG neurons. The naftopidil response was unchanged in the presence of TTX. Regression analysis of the outward currents between the 1st and 2nd applications of naftopidil revealed a Pearson correlation coefficient of 0.996 with a line slope of 0.983. The naftopidil-induced outward current was attenuated in the presence of strychnine and/or bicuculline. The GABA/glycine-mediated outward currents induced by tamsulosin, silodosin, and prazosin were smaller than those obtained with naftopidil.

CONCLUSION

Naftopidil-induced GABA/glycine-mediated outward currents in a subset of SG neurons prepared from the L6- S1 level of rat spinal cord. The results indicated that α1-adrenoceptor antagonists, particularly naftopidil, induce neural suppression (in part) by mediating hyperpolarization. The response is associated with glycinergic and/or GABAergic neural transmission. Naftopidil may suppress the micturition reflex and improve urinary storage symptoms as a subsidiary effect resulting from hyperpolarization in SG neurons of the spinal cord.

Effect of Alpha 1-Adrnoceptor Antagonists on Postsynaptic Sensitivity in Substantia Gelatinosa Neurons From Lumbosacral Spinal Cord in Rats Using Slice Patch-Clamp Technique for mEPSC

Purpose

Alpha1-adrenoceptors participate in improving storage symptoms of male lower urinary tract symptoms (LUTS). However, the mechanism of action of these compounds remains unclear. To clarify the mechanism of the α1-adrenoceptor antagonists, the amplitude of miniature excitatory postsynaptic currents (mEPSCs) was analyzed in the lumbosacral spinal cord in rats.

Methods

Male adult Sprague-Dawley rats were used. Blind whole-cell patch-clamp recordings were performed on substantia gelatinosa (SG) neurons in spinal cord slice preparations. The amplitude of mEPSCs was recorded in individual SG neurons to which α1-adrenoceptors (100μM naftopidil, 100μM tamsulosin, and 30μM silodosin) were applied sequentially with intervening washout periods. Individual amplitudes were analyzed.

Results

Pearson correlation coefficients (r) for the amplitudes of mEPSCs between the baseline and postadministration of α1-adrenoceptor antagonists indicated changes of the amplitude ranked in the order of naftopidil (r =0.393), tamsulosin (r=0.738), and silodosin (r=0.944). Together, the α1-adrenoceptor antagonists yielded significant increases in the amplitude of mEPSCs in SG neurons (n=108, P=0.012). However, the effects of each α1-adrenoceptor antagonist on the amplitude were as follows (relative to the baseline; n=36 each): naftopidil, P=0.129; tamsulosin, P=0.201; and silodosin, P=0.005. The rate of response to naftopidil for the outward current was relatively high among the α1-adrenoceptor blockers. An inward current was observed only with the naftopidil application.

Conclusions

Alpha1-adrenoceptor antagonists changed the amplitudes of mEPSCs in a subset of SG neurons in slices prepared from the L6–S1 levels of rat spine. Although the α1-adrenoceptor antagonists generated inward or outward currents in the SG neurons, different rates of response were observed with each antagonist. These results are important for understanding the mechanisms of action (at the spinal level) of α1-adrenoceptor antagonists for the storage symptoms of male LUTS.

Chronic pain models amplify transient receptor potential vanilloid 1 (TRPV1) receptor responses in adult rat spinal dorsal horn

Persistent pain is associated with negative affect originating from hypersensitivity and/or allodynia. The spinal cord is a key area for nociception as well as chronic pain processing. Specifically, the dorsal horn neurons in lamina II (substantia gelatinosa: SG) receive nociceptive inputs from primary afferents such as C fibers and/or Aδ fibers. Transient receptor potential vanilloid 1 (TRPV1) is a major receptor to sense heat as well as nociception. TRPV1 are expressed in the periphery and the central axon terminals of C fibers and/or Aδ fibers in the spinal cord. Activating TRPV1 enhances the release of glutamate in the spinal cord from naïve rodents. Here, we studied whether or not chronic pain could alter the response of TRPV1 channels to exogenous, capsaicin through study of synaptic transmission and neural activity in rat SG neurons. Using in vitro whole-cell patch-clamp recording, we found that bath application of capsaicin facilitated both the frequency and amplitude of miniature and spontaneous excitatory postsynaptic currents beyond a nerve injury and a complete Freund's adjuvant injection observed in the naïve group. Strikingly, capsaicin produced larger amplitudes of inward currents in pain models than compared to the naïve group. By contrast, the proportions of neurons that show capsaicin-induced inward currents were similar among naïve and pain groups. Importantly, the capsaicin-induced inward currents were conducted by TRPV1 and required calcium influx that was independent of voltage-gated calcium channels. Our study provides fundamental evidence that chronic inflammation and neuropathic pain models amplify the release of glutamate through the activation of TRPV1 in central axon terminals, and that facilitation of TRPV1 function in rat spinal SG neurons may contribute to enhanced capsaicin-induced inward currents.

Emotional Stress Facilitates Micturition Reflex: Possible Inhibition by an α1-Adrenoceptor Blocker in the Conscious and Anesthetized State

Purpose

To test the hypothesis that naftopidil prolongs intercontraction intervals in rats undergoing chronic stress as observed in previous animal models, voiding behavior and bladder function were measured and analyzed.

Methods

Female Sprague-Dawley rats weighing 200–230 g were exposed to repeated variate stress (RVS) for 1 week, chronic variable mild stress for 2 weeks, or simple mild stress for 1 week. Voiding behavior was assessed in metabolic cages. Voiding frequency and urine output were measured, and changes of these values were compared for the different types of stress. Micturition reflex was analyzed using unconscious cystometry. Naftopidil was administered orally at 30 mg/kg/day for 2 weeks.

Results

Unexpectedly, no stress-exposed rats exhibited increased micturition frequency compared to the normal nonstressed control. However, intercontraction intervals were shortened with each type of stress in the unconscious condition, especially by RVS (P<0.01). Naftopidil prolonged the shortened intervals.

Conclusions

Although voiding behavior appears approximately normal in rats chronically exposed to emotional stress, internal bladder function can be affected. With anesthesia, micturition intervals were moderately shortened by emotional stress and clearly improved by naftopidil. Therefore, naftopidil appears to act at the spinal level at least.

Characterization on Responsiveness of Excitatory Synaptic Transmissions to α1-Adrenoceptor Blockers in Substantia Gelatinosa Neurons Isolated From Lumbo-Sacral Level in Rat Spinal Cords

Purpose

The aim of this study was to characterize the responsiveness of miniature excitatory postsynaptic currents (mEPSCs) to α1-adrenoceptor blockers in substantia gelatinosa (SG) neurons from the spinal cord to develop an explanation for the efficacy of α1-adrenoceptor blockers in micturition dysfunction.

Methods

Male adult Sprague-Dawley rats were used. Blind whole-cell patch-clamp recordings were performed using SG neurons in spinal cord slices. Naftopidil (100μM), tamsulosin (100μM), or silodosin (30μM), α1-adrenoceptor blockers, was perfused. The frequency of mEPSCs was recorded in an SG neuron to which the 3 blockers were applied sequentially with wash-out periods. Individual frequencies in a pair before naftopidil and tamsulosin perfusion were plotted as baseline, and the correlation between them was confirmed by Spearman correlation coefficient; linear regression was then performed. The same procedure was performed before naftopidil and silodosin perfusion. Frequencies of pairs after naftopidil and tamsulosin perfusion and after naftopidil and silodosin perfusion were similarly analyzed. The ratios of the frequencies after treatment to before were then calculated.

Results

After the treatments, Spearman ρ and the slope were decreased to 0.682 from 0.899 at baseline and 0.469 from 1.004 at baseline, respectively, in the tamsulosin group relative to the naftopidil group. In the silodosin group, Spearman ρ and the slope were also decreased to 0.659 from 0.889 at baseline and 0.305 from 0.989 at baseline, respectively, relative to the naftopidil group. Naftopidil significantly increased the ratio of the frequency of mEPSCs compared to tamsulosin and silodosin (P=0.015 and P=0.004, respectively).

Conclusions

There was a difference in responsiveness in the frequency of mEPSCs to α1-adrenoceptor blockers, with the response to naftopidil being the greatest among the α1-adrenoceptor blockers. These data are helpful to understand the action mechanisms of α1-adrenoceptor blockers for male lower urinary tract symptoms in clinical usage.

Effects of High Concentrations of Naftopidil on Dorsal Root-Evoked Excitatory Synaptic Transmissions in Substantia Gelatinosa Neurons In Vitro

Purpose

Naftopidil ((±)-1-[4-(2-methoxyphenyl) piperazinyl]-3-(1-naphthyloxy) propan-2-ol) is prescribed in several Asian countries for lower urinary tract symptoms suggestive of benign prostatic hyperplasia. Previous animal experiments showed that intrathecal injection of naftopidil abolished rhythmic bladder contraction in vivo. Naftopidil facilitated spontaneous inhibitory postsynaptic currents in substantia gelatinosa (SG) neurons in spinal cord slices. These results suggest that naftopidil may suppress the micturition reflex at the spinal cord level. However, the effect of naftopidil on evoked excitatory postsynaptic currents (EPSCs) in SG neurons remains to be elucidated.

Methods

Male Sprague-Dawley rats at 6 to 8 weeks old were used. Whole-cell patch-clamp recordings were made using SG neurons in spinal cord slices isolated from adult rats. Evoked EPSCs were analyzed in Aδ or C fibers. Naftopidil or prazosin, an α1-adrenoceptor blocker, was perfused at 100 μM or 10 μM, respectively.

Results

Bath-applied 100 μM naftopidil significantly decreased the peak amplitudes of Aδ and C fiber-evoked EPSCs to 72.0%±7.1% (n=15) and 70.0%±5.5% (n=20), respectively, in a reversible and reproducible manner. Bath application of 10μM prazosin did not inhibit Aδ or C fiber-evoked EPSCs.

Conclusions

The present study suggests that a high concentration of naftopidil reduces the amplitude of evoked EPSCs via a mechanism that apparently does not involve α1-adrenoceptors. Inhibition of evoked EPSCs may also contribute to suppression of the micturition reflex, together with nociceptive stimulation.

Novel naftopidil derivatives containing methyl phenylacetate and their blocking effects on α1D/1A-adrenoreceptor subtypes

α₁-Adrenoceptor (α₁-AR) antagonists are considered to be the most effective monotherapy agents for lower urinary tract symptoms associated with benign prostatic hyperplasia (LUTS/BPH). In this study, we synthesized compounds 2-17, which are novel piperazine derivatives that contain methyl phenylacetate. We then evaluated the vasodilatory activities of these compounds. Among them, we found that compounds 2, 7, 12, which contain 2-OCH₃, 2-CH₃ or 2, 5-CH₃, respectively, exhibited potent α₁-blocking activity similar to protype drug naftopidil (1). The antagonistic effects of 2, 7, and 12 on the (-)-noradrenaline-induced contractile response of isolated rat prostatic vas deferens (α_1A), spleen (α_1B) and thoracic aorta (α_1D) were further characterized to assess the sub receptor selectivity. Compared with naftopidil (1) and terazosin, compound 12 showed the most desirable α_1D/1A subtype selectivity, especially improved α_1A subtype selectivity, and the ratios pA₂ (α_1D)/pA₂ (α_1B) and pA₂ (α_1A)/pA₂ (α_1B) were 17.0- and 19.5-fold, respectively, indicating less cardiovascular side effects when used to treat LUTS/BPH. Finally, we investigated the chiral pharmacology of 12. We found, however, that the activity of enantiomers (R)-12 and (S)-12 are not significantly different from that of rac-12.