Therapeutically relevant concentrations of neomycin selectively inhibit P-type Ca2+ channels in rat striatum

Inhibition of peripheral nociceptors by aminoglycosides produces analgesia in inflammatory pain models in the rat

Aminoglycosides (AGs) modulate nociceptors and ionic channels expressed in sensory neurons. The AG applied in situ could be useful to alleviate hyperalgesia in animal models of inflammatory pain. We tested streptomycin (ST) and neomycin (NEO) as analgesic agents applied in situ in rat paw inflammation caused by formalin or carrageenan administration. The action of ST and NEO on the action potential discharge produced by acidic stimuli in isolated dorsal root ganglion neurons was also studied in current-clamp recordings. In the formalin test, ST and NEO significantly reduced the nociceptive behaviour. ST reduced the N-(4-methyl-2-quinazolinyl)-guanidine (GMQ)-induced nociceptive behaviour, and NEO diminished the hyperalgesia to thermonociception and mechanonociception produced by CAR. In the current-clamp experiments, ST and NEO reduced the generation of action potentials when an acidic solution was applied. We conclude that ST and NEO produce analgesia to inflammatory pain, an effect that is due in part to the inhibition of ASIC activation in sensory neurons.

Antibiotic-induced neurotoxicity

Antibiotic neurotoxicity is rare but can cause significant morbidity when it occurs. The risk of antibiotic neurotoxicity appears to be highest in patients who are older, have impaired renal function, or have preexisting neurologic conditions. This review describes the clinical features of the most common antibiotic toxicities affecting the nervous system: seizures, encephalopathy, optic neuropathy, peripheral neuropathy, and exacerbation of myasthenia gravis.

Gentamicin blocks the ACh-induced BK current in guinea pig type II vestibular hair cells by competing with Ca²⁺ at the L-type calcium channel

Type II vestibular hair cells (VHCs II) contain big-conductance Ca²⁺-dependent K⁺ channels (BK) and l-type calcium channels. Our previous studies in guinea pig VHCs II indicated that acetylcholine (ACh) evoked the BK current by triggering the influx of Ca²⁺ ions through l-type Ca²⁺ channels, which was mediated by M2 muscarinic ACh receptor (mAChRs). Aminoglycoside antibiotics, such as gentamicin (GM), are known to have vestibulotoxicity, including damaging effects on the efferent nerve endings on VHCs II. This study used the whole-cell patch clamp technique to determine whether GM affects the vestibular efferent system at postsynaptic M2-mAChRs or the membrane ion channels. We found that GM could block the ACh-induced BK current and that inhibition was reversible, voltage-independent, and dose-dependent with an IC₅₀ value of 36.3 ± 7.8 μM. Increasing the ACh concentration had little influence on GM blocking effect, but increasing the extracellular Ca²⁺ concentration ([Ca²⁺]_o) could antagonize it. Moreover, 50 μM GM potently blocked Ca²⁺ currents activated by (−)-Bay-K8644, but did not block BK currents induced by NS1619. These observations indicate that GM most likely blocks the M2 mAChR-mediated response by competing with Ca²⁺ at the l-type calcium channel. These results provide insights into the vestibulotoxicity of aminoglycoside antibiotics on mammalian VHCs II.

Osteoblasts detect pericellular calcium concentration increase via neomycin-sensitive voltage gated calcium channels

The mechanisms underlying the detection of critically loaded or micro-damaged regions of bone by bone cells are still a matter of debate. Our previous studies showed that calcium efflux originates from pre-failure regions of bone matrix and MC3T3-E1 osteoblasts respond to such efflux by an increase in the intracellular calcium concentration. The mechanisms by which the intracellular calcium concentration increases in response to an increase in the pericellular calcium concentration are unknown. Elevation of the intracellular calcium may occur via release from the internal calcium stores of the cell and/or via the membrane bound channels. The current study applied a wide range of pharmaceutical inhibitors to identify the calcium entry pathways involved in the process: internal calcium release from endoplasmic reticulum (ER, inhibited by thapsigargin and TMB-8), calcium receptor (CaSR, inhibited by calhex), stretch-activated calcium channel (SACC, inhibited by gadolinium), voltage-gated calcium channels (VGCC, inhibited by nifedipine, verapamil, neomycin, and ω-conotoxin), and calcium-induced-calcium-release channel (CICRC, inhibited by ryanodine and dantrolene). These inhibitors were screened for their effectiveness to block intracellular calcium increase by using a concentration gradient induced calcium efflux model which mimics calcium diffusion from the basal aspect of cells. The inhibitor(s) which reduced the intracellular calcium response was further tested on osteoblasts seeded on mechanically loaded notched cortical bone wafers undergoing damage. The results showed that only neomycin reduced the intracellular calcium response in osteoblasts, by 27%, upon extracellular calcium stimulus induced by concentration gradient. The inhibitory effect of neomycin was more pronounced (75% reduction in maximum fluorescence) for osteoblasts seeded on notched cortical bone wafers loaded mechanically to damaging load levels. These results imply that the increase in intracellular calcium occurs by the entry of extracellular calcium ions through VGCCs which are sensitive to neomycin. N-type and P-type VGCCs are potential candidates because they are observed in osteoblasts and they are sensitive to neomycin. The calcium channels identified in this study provide new insight into mechanisms underlying the targeted repair process which is essential to bone adaptation.

Calcium-sensing receptor: a high-affinity presynaptic target for aminoglycoside-induced weakness

Administration of aminoglycoside antibiotics can precipitate sudden, profound bouts of weakness that have been attributed to block of presynaptic voltage-activated calcium channels (VACCs) and failure of neuromuscular transmission. This serious adverse drug reaction is more likely in neuromuscular diseases such as myasthenia gravis. The relatively low affinity of VACC for aminoglycosides prompted us to explore alternative mechanisms. We hypothesized that the presynaptic Ca(2+)-sensing receptor (CaSR) may contribute to aminoglycoside-induced weakness due to its role in modulating synaptic transmission and its sensitivity to aminoglycosides in heterologous expression systems. We have previously shown that presynaptic CaSR controls a non-selective cation channel (NSCC) that regulates nerve terminal excitability and transmitter release. Using direct, electrophysiological recording, we report that neuronal VACCs are inhibited by neomycin (IC(50) 830 +/- 110 microM) at a much lower affinity than CaSR-modulated NSCC currents recorded from acutely isolated presynaptic terminals (synaptosomes; IC(50) 20 +/- 1 microM). Thus, at clinically relevant concentrations, aminoglycoside-induced weakness is likely precipitated by enhanced CaSR activation and subsequent decrease in terminal excitability rather than through direct inhibition of VACCs themselves.

Possible role for TRPV1 in neomycin-induced inhibition of visceral hypersensitivity in rat

Transient receptor ion channel 1 (TRPV1) is a nociceptor involved in visceral hypersensitivity. Aminoglycosides like neomycin are not only potent antibiotics but in vitro data suggest that neomycin also acts as a TRPV1-antagonist and alleviates somatic pain responses. To what extent neomycin reduces visceral hypersensitivity remains unknown. Therefore, we aimed to investigate whether neomycin can inhibit in vivo TRPV1-dependent hypersensitivity responses in two rat models of visceral pain. In the first model rats were pretreated with intraperitoneal (i.p.) capsazepine, the selective TRPV1 antagonist SB-705498, neomycin or vehicle alone and 30 min later instilled with intracolonic TRPV1-activating capsaicin. Likewise, rats were pretreated with 10 days oral neomycin and then subjected to intracolonic capsaicin. The visceromotor response (VMR) to distension was measured before and after capsaicin application. In addition, the VMR to distension was measured in adult maternal separated rats before and after acute stress. Before the 2nd distension protocol these rats were treated with i.p. neomycin, amoxycillin or vehicle alone. Our results showed that capsaicin administration induced an enhanced VMR to distension that was prevented by i.p. capsazepine, SB-705498 and neomycin. Oral neomycin treatment changed bacterial faecal content but could not inhibit capsaicin induced visceral hypersensitivity. In maternal separated rats acute stress induced an enhanced response to distension that was reversed by i.p. neomycin, but not amoxycillin. These data indicate that (i.p.) neomycin can inhibit visceral hypersensitivity to distension in a nonbactericidal manner and suggest that TRPV1-modulation may be involved.

Block of native and cloned vanilloid receptor 1 (TRPV1) by aminoglycoside antibiotics

Vanilloid receptor 1 (TRPV1) is a Ca2+ permeable non-specific cation channel located at the peripheral nerve terminals and functions as a polymodal nociceptor. Neomycin, an aminoglycoside antibiotic induces analgesia in various animal models. However, the mechanism of action of neomycin has not been fully understood. In this study, we have determined the effect of neomycin on native TRPV1 in cultured embryonic DRG neurons and cloned TRPV1 heterologously expressed in Xenopus oocytes using patch clamp, double electrode voltage clamp, and Ca2+ fluorescence imaging techniques. Here, we show that neomycin potently (IC50 approximately 400 nM) blocks TRPV1-mediated membrane currents in DRG neurons and the block is unrelated to capsaicin concentrations used to evoke currents, suggesting a non-competitive block. Similarly, capsaicin- and proton-induced currents are blocked in oocytes, but to a lesser extent. Increases in capsaicin-induced intracellular Ca2+ levels are also reduced by neomycin. Single-channel current analyses reveal that single-channel conductance is unaffected by neomycin and there is no indication of open channel block. The predominant effect is to lower, the open probability (Po) at both, negative and positive potentials. Kinetic analyses reveal that the number of exponential components required to fit the open time distributions remains the same or reduced, however, the longest open time constant and the area of distribution are shortened at negative and positive potentials, respectively. The area of distribution of longest closed-time constants were significantly prolonged at negative and positive potentials. We conclude that neomycin inhibits TRPV1 channel activity by allosteric binding and altering channel gating.