Prenatal tetrodotoxin infusion blocks segregation of retinogeniculate afferents

In the adult mammalian visual system, ganglion cell axons from the two eyes are segregated from each other into separate layers within their principal target, the lateral geniculate nucleus. The involvement of spontaneously generated action potential activity in the process of segregation was investigated during the fetal period in which segregation normally occurs in the cat, between embryonic day 45 (E45) and birth (E65). Tetrodotoxin, which blocks the voltage-sensitive sodium channel, was used to prevent action potentials. Fetuses received continuous intracranial infusions of tetrodotoxin from osmotic minipumps implanted in utero on E42. After a 2-week infusion, intraocular injections of anterograde tracers revealed that tetrodotoxin prevented segregation. The contralateral projection filled the lateral geniculate nucleus uniformly, and the ipsilateral projection expanded to occupy most of what would normally be contralaterally innervated layer A. Thus, in the fetus, long before the onset of vision, spontaneous action potential activity is likely to be present in the visual system and to contribute to the segregation of the retinogeniculate pathway.

Vocal experimentation in the juvenile songbird requires a basal ganglia circuit

Songbirds learn their songs by trial-and-error experimentation, producing highly variable vocal output as juveniles. By comparing their own sounds to the song of a tutor, young songbirds gradually converge to a stable song that can be a remarkably good copy of the tutor song. Here we show that vocal variability in the learning songbird is induced by a basal-ganglia-related circuit, the output of which projects to the motor pathway via the lateral magnocellular nucleus of the nidopallium (LMAN). We found that pharmacological inactivation of LMAN dramatically reduced acoustic and sequence variability in the songs of juvenile zebra finches, doing so in a rapid and reversible manner. In addition, recordings from LMAN neurons projecting to the motor pathway revealed highly variable spiking activity across song renditions, showing that LMAN may act as a source of variability. Lastly, pharmacological blockade of synaptic inputs from LMAN to its target premotor area also reduced song variability. Our results establish that, in the juvenile songbird, the exploratory motor behavior required to learn a complex motor sequence is dependent on a dedicated neural circuit homologous to cortico-basal ganglia circuits in mammals.

Monocular deprivation induces homosynaptic long-term depression in visual cortex

Brief monocular deprivation during early postnatal development can lead to a depression of synaptic transmission that renders visual cortical neurons unresponsive to subsequent visual stimulation through the deprived eye. The Bienenstock-Cooper-Munro (BCM) theory proposes that homosynaptic mechanisms of long-term depression (LTD) account for the deprivation effects. Homosynaptic depression, by definition, occurs only at active synapses. Thus, in contrast to the commonly held view that the synaptic depression caused by monocular deprivation is simply a result of retinal inactivity, this theoretical framework indicates that the synaptic depression may actually be driven by the residual activity in the visually deprived retina. Here we examine the validity of this idea by comparing the consequences of brief monocular deprivation by lid suture with those of monocular inactivation by intra-ocular treatment with tetrodotoxin. Lid suture leaves the retina spontaneously active, whereas tetrodotoxin eliminates all activity. In agreement with the BCM theory, our results show that monocular lid suture causes a significantly greater depression of deprived-eye responses in kitten visual cortex than does treatment with tetrodotoxin. These findings have important implications for mechanisms of experience-dependent plasticity in the neocortex.

The use of tetrodotoxin for the characterization of drug-enhanced dopamine release in conscious rats studied by brain dialysis

The effect of TTX (infused during brain dialysis of the striatum and nucleus accumbens) on the in vivo release of dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid, was investigated. In addition it was studied whether the increase in the release of dopamine, induced by various pharmacological treatments, was sensitive to TTX infusion. The following drugs were studied: haloperidol, amphetamine, haloperidol co-administered with GBR 12909, morphine and MPP+. Dialysis was carried out in the striatum, except for morphine, which was studied in the nucleus accumbens. The infusion of TTX revealed three different types of pharmacologically enhanced dopamine release in conscious rats. First, action potential dependent dopamine release (exocytosis), which was observed in untreated animals as well as in animals treated with haloperidol, haloperidol + GBR 12909, and morphine. Second, action potential independent release (carrier-mediated) was established in the case of amphetamine. Third, action potential independent DA release, probably caused by neurotoxic reactions was observed during MPP+ infusion.

Effect of fluoxetine on serotonin and dopamine concentration in microdialysis fluid from rat striatum

Fluoxetine injected i.p. into rats at a dose of 10 mg/kg rapidly increased serotonin concentration in microdialysis fluid from the striatum by at least 4-fold, an increase that was maintained throughout the 3 hr observation period. Dopamine concentration in the microdialysis fluid did not change. The concentration of the two dopamine metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, was not changed in the microdialysis fluid, whereas the concentration of the serotonin metabolite, 5-hydroxyindoleacetic acid, was significantly decreased after fluoxetine injection. The increased extracellular concentration of serotonin no doubt resulted from inhibition of the serotonin uptake carrier by fluoxetine, and the lack of change in dopamine is evidence for the specificity of action of this uptake inhibitor.

Basolateral amygdala inactivation abolishes conditioned stimulus- and heroin-induced reinstatement of extinguished heroin-seeking behavior in rats

RATIONALE

Drug-paired stimuli elicit drug craving and relapse in addicts and drug-seeking behavior in rats. The functional integrity of the basolateral amygdala (BLA) is necessary for reinstatement of cocaine-seeking behavior elicited by cocaine-conditioned stimuli, but not by cocaine itself. It is unclear, however, whether the BLA plays a similar role in reinstatement of heroin-seeking behavior.

OBJECTIVES

To this end, we examined the effects of tetrodotoxin (TTX)-induced inactivation of the BLA on conditioned and heroin-primed reinstatement of extinguished heroin-seeking behavior.

METHODS

Rats were trained to press a lever for IV infusions of heroin (maintenance dose of 25 microg/infusion) paired with presentations of a light-tone stimulus complex during daily 3-h sessions. Responding was then allowed to extinguish prior to reinstatement testing. Reinstatement of extinguished heroin-seeking behavior (i.e. lever pressing in the absence of heroin reinforcement) was measured in the presence of response-contingent presentation of the heroin-paired stimulus complex alone and then following TTX (5 ng/0.5 microl per side) or vehicle infused into the BLA. In a separate group of rats, reinstatement was measured after saline injection (SC) and then following heroin priming (0.25 mg/kg, SC) with TTX or vehicle infused into the BLA.

RESULTS

Both response contingent presentation of the stimulus complex and heroin priming significantly reinstated extinguished heroin-seeking behavior, and BLA inactivation abolished the ability of the heroin-paired stimuli and of heroin priming to reinstate responding.

CONCLUSIONS

These findings suggests that the BLA is a critical component of the neural circuitry that mediates conditioned and heroin-induced reinstatement of heroin-seeking behavior. Furthermore, different neural substrates may mediate drug-primed relapse to cocaine versus heroin-seeking behavior.

Osmotic pump implant for chronic infusion of drugs into the inner ear

Continuous long-term delivery of experimental drugs to the cochlea of a small animal, such as a young guinea pig, presents several technical problems. A method of placing and securing a cannula-osmotic pump system is described in this paper. Guinea pigs (225-410 g) were unilaterally implanted with an Alzet micro-pump and cannula for delivery of 20 mM tetrodotoxin (TTX) (six animals) or saline (three animals) for three days (1 microliter/h). Auditory brainstem responses (ABRs) were recorded under light anesthesia on post-implant day 1 and day 3 and compared with pre-implant baseline values. In all six cochleas infused with TTX, most frequencies showed a 30-60dB decrease in sensitivity within 24 h. Saline control animals showed little or no change from baseline sensitivity for most frequencies. In three TTX-infused animals, the cannula-pump unit was removed on day 3, and ABRs were followed during recovery. Most frequencies returned to, or near, pre-implant levels after pump removal but recovery times varied. By day 6, all animals had recovered post-surgical weight loss and showed a gain of 10-40 g. Brains and cochleas were removed and processed for sectioning. Assessment of the cochlear nucleus of non-recovery TTX-treated animals showed a deafness-related flattening of auditory nerve active zones on the treated side.

Sodium channel beta2 subunits regulate tetrodotoxin-sensitive sodium channels in small dorsal root ganglion neurons and modulate the response to pain

Voltage-gated sodium channel (Na(v)1) beta2 subunits modulate channel gating, assembly, and cell-surface expression in CNS neurons in vitro and in vivo. beta2 expression increases in sensory neurons after nerve injury, and development of mechanical allodynia in the spared nerve injury model is attenuated in beta2-null mice. Thus, we hypothesized that beta2 modulates electrical excitability in dorsal root ganglion (DRG) neurons in vivo. We compared sodium currents (I(Na)) in small DRG neurons from beta2+/+ and beta2-/- mice to determine the effects of beta2 on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) Na(v)1 in vivo. Small-fast DRG neurons acutely isolated from beta2-/- mice showed significant decreases in TTX-S I(Na) compared with beta2+/+ neurons. This decrease included a 51% reduction in maximal sodium conductance with no detectable changes in the voltage dependence of activation or inactivation. TTX-S, but not TTX-R, I(Na) activation and inactivation kinetics in these cells were slower in beta2(-/-) mice compared with controls. The selective regulation of TTX-S I(Na) was supported by reductions in transcript and protein levels of TTX-S Na(v)1s, particularly Na(v)1.7. Low-threshold mechanical sensitivity was preserved in beta2-/- mice, but they were more sensitive to noxious thermal stimuli than wild type whereas their response during the late phase of the formalin test was attenuated. Our results suggest that beta2 modulates TTX-S Na(v)1 mRNA and protein expression resulting in increased TTX-S I(Na) and increases the rates of TTX-S Na(v)1 activation and inactivation in small-fast DRG neurons in vivo. TTX-R I(Na) were not significantly modulated by beta2.

A re-examination of tetrodotoxin for prolonged duration local anesthesia

BACKGROUND

Highly potent toxins such as tetrodotoxin that block sodium channels with great specificity have been studied for many years and can provide prolonged blockade when coadministered with vasoconstrictors or conventional local anesthetics. Their utility has been constrained, however, by systemic toxicity. The authors examined the efficacy of tetrodotoxin with and without epinephrine or bupivacaine for producing prolonged-duration sciatic nerve blockade in the rat, and they assessed the degree of concomitant toxicity.

METHODS

Rats received percutaneous sciatic nerve blockade using tetrodotoxin with and without epinephrine or bupivacaine. A subset received subcutaneous injections at the nuchal midline. Nociceptive, proprioceptive, and motor blockade were quantified using contralateral leg responses as controls for systemic effects.

RESULTS

Tetrodotoxin without epinephrine produced sciatic nerve blockade, but with considerable toxicity at most effective doses. Epinephrine reduced the median effective concentration of tetrodotoxin for nociception from 37.6 to 11.5 microM and prolonged its duration, such that reversible blocks lasting > 13 h were achieved. Epinephrine reduced measures of systemic distribution and increased the median lethal dose of tetrodotoxin from 40 to 53.6 nmole/kg, thus more than quadrupling the therapeutic index. Bupivacaine increased the local anesthetic potency of tetrodotoxin, reduced its systemic toxicity, and, when coinjected subcutaneously, increased the median lethal dose from 43.7 to 47.7 nmole/kg. The addition of epinephrine did not further improve the effectiveness of the bupivacaine-tetrodotoxin combination.

CONCLUSION

Combinations of epinephrine or bupivacaine with tetrodotoxin or with other high-potency toxins active on sodium channels should be examined for the potential to provide clinically useful, prolonged nerve blockade.

Extent of the tetrodotoxin induced blockade examined by pupillary paralysis elicited by intracerebral injection of the drug

Spatial extent and duration of the functional blockade elicited by intracerebral injection of tetrodotoxin (TTX) was examined in rats anesthetized with pentobarbital. Pupillary diameter was measured under dissecting microscope before and up to 24 h after injection of TTX (10 ng/l microliters saline) into or 1.0, 1.5 and 2.0 mm lateral from the Edinger-Westphal nucleus. TTX administration elicited mydriasis the latency of which was directly and amplitude indirectly proportional to the target-injection distance. The maximum mydriasis attained 3.4 mm, lasted 2 h and slowly decayed over subsequent 20 h. Impulse transmission and conduction was blocked in a spherical volume of tissue about 3 mm in diameter the development of which could be approximated by diffusion from an instantaneous point source. Completeness and full reversibility make the TTX block a convenient research tool.

Prolonged duration local anesthesia from tetrodotoxin-enhanced local anesthetic microspheres

There is interest in developing prolonged duration local anesthesics. Here we examine whether tetrodotoxin (TTX) can be used to prolong the block from bupivacaine microspheres with and without dexamethasone. Rats received sciatic nerve blocks with 75 mg of microspheres containing 0.05% (w/w) TTX, 50% (w/w) bupivacaine and/or 0.05% (w/w) dexamethasone. 0.1% (w/w) TTX microspheres were also tested. The carrier fluid contained 1:100,000 epinephrine. Nociceptive and motor blockade of the hindpaw were quantified. Nerves and adjacent muscles were harvested 2 weeks after injection for histological assessment by light microscopy. The median nociceptive block duration in hours from the microsphere groups were: bupivacaine=6.2, 0.05% TTX=0, bupivacaine+TTX=35.3, bupivacaine+dexamethasone=31.3, TTX+dexamethasone=8.1, TTX+bupivacaine+dexamethasone=221.7. Some animals receiving particles containing 0.05% TTX had deficits in the uninjected extremity; all animals receiving 0.1% (w/w) TTX particles died. Pockets of particles were associated with localized inflammation, and all samples showed some evidence of myotoxicity in the vicinity of the injection. The nerves themselves appeared intact. In summary, coencapsulation of TTX in controlled release devices containing bupivacaine and dexamethasone resulted in very prolonged nerve blocks. As formulated here, this preparation had a narrow margin of safety. While the myotoxicity appears consistent with the well-known reversible myotoxicity associated with local anesthetics, its long-term significance remains to be established.

Small interfering RNA-mediated selective knockdown of NaV1.8 tetrodotoxin-resistant sodium channel reverses mechanical allodynia in neuropathic rats

The biophysical properties of a tetrodotoxin resistant (TTXr) sodium channel, Na(V)1.8, and its restricted expression to the peripheral sensory neurons suggest that blocking this channel might have therapeutic potential in various pain states and may offer improved tolerability compared with existing sodium channel blockers. However, the role of Na(V)1.8 in nociception cannot be tested using a traditional pharmacological approach with small molecules because currently available sodium channel blockers do not distinguish between sodium channel subtypes. We sought to determine whether small interfering RNAs (siRNAs) might be capable of achieving the desired selectivity. Using Northern blot analysis and membrane potential measurement, several siRNAs were identified that were capable of a highly-selective attenuation of Na(V)1.8 message as well as functional expression in clonal ND7/23 cells which were stably transfected with the rat Na(V)1.8 gene. Functional knockdown of the channel was confirmed using whole-cell voltage-clamp electrophysiology. One of the siRNA probes showing a robust knockdown of Na(V)1.8 current was evaluated for in vivo efficacy in reversing an established tactile allodynia in the rat chronic constriction nerve-injury (CCI) model. The siRNA, which was delivered to lumbar dorsal root ganglia (DRG) via an indwelling epidural cannula, caused a significant reduction of Na(V)1.8 mRNA expression in lumbar 4 and 5 (L4-L5) DRG neurons and consequently reversed mechanical allodynia in CCI rats. We conclude that silencing of Na(V)1.8 channel using a siRNA approach is capable of producing pain relief in the CCI model and further support a role for Na(V)1.8 in pathological sensory dysfunction.

Reversible inactivation of the nucleus basalis magnocellularis induces disruption of cortical acetylcholine release and acquisition, but not retrieval, of aversive memories

The basal forebrain complex, which includes the nucleus basalis magnocellularis (NBM), provides widespread cholinergic and gamma-aminobutyric acid-containing projections throughout the brain, including the insular and pyriform cortices. A number of studies have implicated the cholinergic neurons in the mediation of learning and memory processes. However, the role of basal forebrain activity in information retrieval mechanisms is less known. The aim of the present study is to evaluate the effects of reversible inactivation of the NBM by tetrodotoxin (TTX, a voltage-sensitive sodium channel blocker) during the acquisition and retrieval of conditioned taste aversion (CTA) and to measure acetylcholine (ACh) release during TTX inactivation in the insular cortex, by means of the microdialysis technique in free-moving rats. Bilateral infusion of TTX in the NBM was performed 30 min before the presentation of gustative stimuli, in either the CTA acquisition trial or retrieval trial. At the same time, levels of extracellular ACh release were measured in the insular cortex. The behavioral results showed significant impairment in CTA acquisition when the TTX was infused in the NBM, whereas retrieval was not affected when the treatment was given during the test trial. Biochemical results showed that TTX infusion into the NBM produced a marked decrease in cortical ACh release as compared with the controls during consumption of saccharin in the acquisition trial. Depleted ACh levels were found during the test trial in all groups except in the group that received TTX during acquisition. These results suggest a cholinergic-dependent process during acquisition, but not during memory retrieval, and that NBM-mediated cholinergic cortical release may play an important role in early stages of learning, but not during recall of aversive memories.

Safety signals mitigate the consequences of uncontrollable stress via a circuit involving the sensory insular cortex and bed nucleus of the stria terminalis

BACKGROUND

Safety signals exert a powerful buffering effect when provided during exposure to uncontrollable stressors. We evaluated the role of the sensory insular cortex (Si) and the extend amygdala in this "safety signal effect."

METHODS

Rats were implanted with microinjection cannula, exposed to inescapable tailshocks either with or without a safety signal, and later tested for anxiety-like behavior or neuronal Fos expression.

RESULTS

Exposure to the uncontrollable stressor reduced later social exploration but not when safety signals were present. Temporary inhibition of Si during stressor exposure but not during later behavioral testing blocked the safety signal effect on social exploration. The stressor induced Fos in all regions of the amygdala, but safety signals significantly reduced the number of Fos immunoreactive cells in the basolateral amygdala and ventrolateral region of the bed nucleus of the stria terminalis (BNSTlv). Inhibition of BNSTlv neuronal activity during uncontrollable stressor exposure prevented the later reduction in social exploration. Finally, safety signals reduced the time spent freezing during uncontrollable stress.

CONCLUSIONS

These data suggest that safety signals inhibit the neural fear or anxiety response that normally occurs during uncontrollable stressors and that inhibition of the BNSTlv is sufficient to prevent later anxiety. These data lend support to a growing body of evidence that chronic fear is mediated in the basolateral amygdala and BNSTlv and that environmental factors that modulate fear during stress will alter the long-term consequences of the stressor.

Two cases of adynamia episodica hereditaria: in vitro investigation of muscle cell membrane and contraction parameters

Membrane potentials, current-voltage relationships, and contractile parameters were studied in intact muscle cell bundles obtained from two patients with adynamia episodica hereditaria. In a normal extracellular medium, the cell membranes had resting potentials of about -80 mV and their current-voltage relationships were not significantly different from control curves. In contrast to normal muscles the afflicted cells were paralyzed in a medium having 6-10 mmol/liter potassium. The mechanisms of paralysis in the two specimens were different from each other. Many fibers from one patient were spontaneously active even in normal solution. In high potassium solution spontaneous activity was increased and the cells gradually depolarized to values at which excitatory sodium current is normally inactivated. This depolarization was connected with an increased sodium conductance and was reversed by the application of tetrodotoxin (TTX). The fibers from the other patient were not spontaneously active. In high potassium solution they were paralyzed at membrane potential values at which normal fibers would still contract. The reason for this paralysis was a reduced excitability.

Phototriggered Local Anesthesia

We report a phototriggerable formulation enabling in vivo repeated and on-demand anesthesia with minimal toxicity. Gold nanorods (GNRs) that can convert near-infrared (NIR) light into heat were attached to liposomes (Lip-GNRs), enabling light-triggered phase transition of their lipid bilayers with a consequent release of payload. Lip-GNRs containing the site 1 sodium channel blocker tetrodotoxin and the α2-adrenergic agonist dexmedetomidine (Lip-GNR-TD) were injected subcutaneously in the rat footpad. Irradiation with an 808 nm continuous wave NIR laser produced on-demand and repeated infiltration anesthesia in the rat footpad in proportion to the irradiance, with minimal toxicity. The ability to achieve on-demand and repeated local anesthesia could be very beneficial in the management of pain.

Neuroprotective effects of tetrodotoxin as a Na+ channel modulator and glutamate release inhibitor in cultured rat cerebellar neurons and in gerbil global brain ischemia

BACKGROUND AND PURPOSE

Studies examining the role of tetrodotoxin-sensitive ion channels in hypoxic-ischemic neuronal damage have concluded that sodium influx is an important initiating event. We examined the neuroprotectant effect of tetrodotoxin on both cultured cerebellar neurons and on CA1 hippocampal neurons of gerbils exposed to brain ischemia.

METHODS

We studied neuroprotective mechanisms using cultured rat cerebellar granule cells exposed to veratridine, which induced cytotoxicity, neurotransmitter release, and calcium influx. Survival of gerbil CA1 neurons was examined by direct neuron counts 7 days after 6 minutes of global ischemia with reperfusion.

RESULTS

Tetrodotoxin protected cultured neurons in a dose-dependent manner from veratridine-induced toxicity (protective concentration [PC50] = 22 nmol/L). Veratridine induced [3H]aspartate efflux that was sodium dependent, only 25% calcium dependent, and was inhibited by tetrodotoxin (inhibitory concentration [IC50] = 60 nmol/L). Veratridine initiated increases in intracellular calcium that were also reversed by tetrodotoxin (IC50 = 63 nmol/L); reversal was dependent on the sodium-calcium exchanger and the sodium-potassium pump. Neuroprotection of 90% (n = 10; P = .001 versus vehicle) of gerbil CA1 hippocampal neurons was achieved by pretreatment with 2 ng of tetrodotoxin delivered three times intracerebroventricularly, without causing hypothermia.

CONCLUSIONS

Sodium channel blockers like tetrodotoxin may have utility in treatment of ischemic neuronal injury by preventing excessive neuronal depolarizations, limiting excitotoxic glutamate release through reversal of the sodium-dependent glutamate transporter, preventing intracellular calcium overload, preserving cellular energy stores, and allowing recovery of ionic homeostasis through operation of the sodium-calcium exchanger.

Acquisition of conditioned taste aversion in rats is prevented by tetrodotoxin blockade of a small midbrain region centered around the parabrachial nuclei

A remarkable feature of conditioned taste aversion (CTA) is the resistance of the association between the gustatory trace and symptoms of poisoning against disruptive procedures. In an attempt to identify the neural substrate of this phase of CTA acquisition, thirsty rats were offered 0.1% saccharin for 15 min, were immediately afterwards anesthetized with pentobarbital, received stereotaxic injections of tetrodotoxin (TTX, 10 ng/microliters) into various brainstem regions and were poisoned with IP injection of LiCl (0.15 M, 2% body weight). In Experiment 1, TTX prevented CTA acquisition when injected into the parabrachial nuclei but was ineffective in the lower medulla. TTX alone did not elicit CTA even at brain sites in which it caused death in 30% of the animals. In Experiment 2, the brainstem was systematically explored by a grid of bilateral TTX injections. A spatial gradient of the CTA disruption pointed to the parabrachial nuclei as the brain region responsible for the amnesic effect observed. Experiment 3 showed that a single TTX injection into the parabrachial nucleus on one side did not prevent CTA acquisition and that similarly ineffective were TTX injections in the sagittal plane both at the mesencephalic and bulbar levels. It is concluded that the pivotal role of the parabrachial nuclei in the formation of the permanent CTA engram can only be revealed by functional blockade which is more radical than that achieved during general anesthesia.

The role of the basolateral amygdala in stimulus-reward memory and extinction memory consolidation and in subsequent conditioned cued reinstatement of cocaine seeking

The consolidation of cue-cocaine associations and extinction learning (i.e. cue-no cocaine associations) into long-term memory probably regulates the long-lasting control of conditioned stimuli (CS) over cocaine-seeking behaviour, and the basolateral amygdala (BLA) may play a role in this phenomenon. To test this hypothesis, rats previously trained to self-administer cocaine underwent a single classical conditioning (CC) session, during which they received passive pairings of cocaine infusions and a novel light + tone stimulus complex. After additional self-administration sessions in the absence of CS presentation and subsequent extinction training sessions, the ability of the CS to reinstate cocaine-seeking on five test days was assessed. Rats received intra-BLA microinfusions of vehicle or the Na+-channel blocker tetrodotoxin (TTX) immediately after CC (consolidation of CS-cocaine associations) or immediately after reinstatement testing (consolidation of extinction learning). TTX administered immediately after CC attenuated subsequent CS-induced reinstatement. In contrast, TTX administered after the first reinstatement test impaired the extinction of cocaine-seeking behaviour during a second reinstatement test by disrupting extinction memory. Overall, these findings suggest that Na+ channel-mediated mechanisms within the BLA mediate the consolidation of both cocaine-stimulus association and extinction learning, two processes that have opposite effects on subsequent cue-induced cocaine-seeking behaviour.

Antinociceptive effects of tetrodotoxin (TTX) in rodents

BACKGROUND

Tetrodotoxin (TTX) is a powerful sodium channel blocker extracted from the puffer fish. The analgesic effects of TTX were investigated in different animal pain models.

METHODS

Wistar rats were submitted to the formalin test and to partial ligation of the sciatic nerve (Seltzer's model). Swiss Webster mice were used in the writhing test. Rodents were divided into six groups receiving a s.c. injection of either 0.9% NaCl, TTX 0.3, 1, 3, or 6 microg kg(-1), or morphine (5 mg kg(-1)). Substances were injected 30 min before 2.5% formalin injection into the hind paw, acetic acid administration intraperitoneally or neuropathic pain testing consisting of mechanical allodynia (von Frey filament) and thermal hyperalgesia (Plantar test).

RESULTS

TTX decreased pain behaviour in the formalin test at the highest dose and in the writhing test at 3 and 6 microg kg(-1). It also diminished mechanical allodynia and thermal hyperalgesia with an ED(50) of 1.08 (0.89) and 0.62 (0.33) microg kg(-1), respectively. Observation of the rats after TTX injection did not show any motor deficit, respiratory distress or sedation. Morphine was also effective in relieving pain in all three tests but with signs of considerable sedation.

CONCLUSION

Systemic injections of TTX diminished pain behaviour in a dose-dependent manner in models of inflammatory, visceral and neuropathic pain without causing adverse events, whereas morphine analgesia was associated with heavy sedation. TTX is a very promising substance for the treatment of various types of pain but needs further evaluation.

Effect of thyrotropin releasing hormone (TRH) on acetylcholine release from different brain areas investigated by microdialysis

1. The effect of thyrotropin releasing hormone (TRH) administration upon acetylcholine (ACh) release in freely moving rats was investigated by means of transversal microdialysis coupled to h.p.l.c. TRH administered either s.c. or via local perfusion increased the ACh release from cortex and hippocampus but not from the striatum. The increase in ACh release was maintained after 7 days of s.c. administration of TRH. 2. After s.c. injection of the neuropeptide, the increase in ACh release was dose-dependent and reached a maximum at 40 min after administration. The maximal percentage increases were 18, 52, 66 and 89% at doses of 1, 2.5, 5 and 10 mg kg-1 and 35, 48 and 54% at doses of 2.5, 5 and 10 mg kg-1 in the cortex and hippocampus, respectively. The effect of TRH was dependent on neuronal activity since it was completely inhibited by perfusion with tetrodotoxin (TTX), 5 X 10(-7) M. 3. Perfusion with TRH, 2.5 micrograms microliters-1, caused 198% and 150% increase in ACh release 60 and 80 min after the beginning of the perfusion in the cortex and hippocampus, respectively. After this initial peak, a 100% increase in ACh release persisted throughout the perfusion. 4. Systemic TRH administration was followed by marked hyperactivity and stereotyped behaviour that showed a time course shorter than that of the increase in ACh release. 5. These findings demonstrate that TRH exerts a strong stimulant action on cortical and hippocampal cholinergic pathways.

An open-label, multi-dose efficacy and safety study of intramuscular tetrodotoxin in patients with severe cancer-related pain

Cancer pain is a prevalent and serious public health issue, and more effective treatments are needed. This study evaluates the analgesic activity of tetrodotoxin, a highly selective sodium channel blocker, in cancer pain. A Phase IIa, open-label, multicenter, dose-escalation study of intramuscular tetrodotoxin was conducted in patients with severe, unrelieved cancer pain. The study design called for six ascending dose levels of intramuscular tetrodotoxin, administered over a four-day treatment period in hospitalized patients, with six patients to be enrolled within each successive dose level. Twenty-four patients underwent 31 courses of treatment at doses ranging from 15 to 90 microg daily, administered in divided doses, over four days. Most patients described transient perioral tingling or other mild sensory phenomena within about an hour of each treatment. Nausea and other toxicities were generally mild, but two patients experienced a serious adverse event, truncal and gait ataxia, that resolved over days. Seventeen of 31 treatments resulted in clinically meaningful reductions in pain intensity, and relief of pain persisted for up to two weeks or longer. Two patients had opioids held due to narcosis concurrent with relief of pain. Somatic, visceral, or neuropathic pain could all respond, but it was not possible to predict which patients were more likely to have an analgesic effect. Tetrodotoxin was overall safe. It effectively relieved severe, treatment-resistant cancer pain in the majority of patients and often for prolonged periods after treatment. It may have a novel mechanism of analgesic effect. Further study is warranted.