Lidocaine suppresses the sodium current in Euhadra neurons which is mediated by cAMP-dependent protein phosphorylation

Sense and Insensibility - An Appraisal of the Effects of Clinical Anesthetics on Gastropod and Cephalopod Molluscs as a Step to Improved Welfare of Cephalopods

Recent progress in animal welfare legislation stresses the need to treat cephalopod molluscs, such as Octopus vulgaris, humanely, to have regard for their wellbeing and to reduce their pain and suffering resulting from experimental procedures. Thus, appropriate measures for their sedation and analgesia are being introduced. Clinical anesthetics are renowned for their ability to produce unconsciousness in vertebrate species, but their exact mechanisms of action still elude investigators. In vertebrates it can prove difficult to specify the differences of response of particular neuron types given the multiplicity of neurons in the CNS. However, gastropod molluscs such as Aplysia, Lymnaea, or Helix, with their large uniquely identifiable nerve cells, make studies on the cellular, subcellular, network and behavioral actions of anesthetics much more feasible, particularly as identified cells may also be studied in culture, isolated from the rest of the nervous system. To date, the sorts of study outlined above have never been performed on cephalopods in the same way as on gastropods. However, criteria previously applied to gastropods and vertebrates have proved successful in developing a method for humanely anesthetizing Octopus with clinical doses of isoflurane, i.e., changes in respiratory rate, color pattern and withdrawal responses. However, in the long term, further refinements will be needed, including recordings from the CNS of intact animals in the presence of a variety of different anesthetic agents and their adjuvants. Clues as to their likely responsiveness to other appropriate anesthetic agents and muscle relaxants can be gained from background studies on gastropods such as Lymnaea, given their evolutionary history.

Infusion of lidocaine into the dorsal hippocampus before or after the shock training phase impaired conditioned freezing in a two-phase training task of contextual fear conditioning

Learning in a contextual fear conditioning task involves forming a context representation and associating it with a shock. The dorsal hippocampus (DH) is implicated in representing the context, but whether it also has a role in associating the context and shock is unclear. To address this issue, male Wistar rats were trained on the task by a two-phase training paradigm, in which rats learned the context representation on day 1 and then reactivated it to associate with the shock on day 2; conditioned freezing was tested on day 3. Lidocaine was infused into the DH at various times in each of the two training sessions. Results showed that intra-DH infusion of lidocaine shortly before or after the context training session on day 1 impaired conditioned freezing, attesting to the DH involvement in context representation. Intra-DH infusion of lidocaine shortly before or after the shock training session on day 2 also impaired conditioned freezing. This deficit was reproduced by infusing lidocaine or APV (alpha-amino-5-phosphonovaleric acid) into the DH after activation of the context memory but before shock administration. The deficit was not due to drug-induced state-dependency, decreased shock sensitivity or reconsolidation failure of the contextual memory. These results suggest that in contextual fear conditioning integrity of the DH is required for memory processing of not only context representation but also context-shock association.

The amygdala modulates memory consolidation of fear-motivated inhibitory avoidance learning but not classical fear conditioning

Although the lateral and basal nuclei of the amygdala are believed to be essential for the acquisition of Pavlovian fear conditioning, studies using post-training manipulations of the amygdala in the inhibitory avoidance learning paradigm have recently called this view into question. We used the GABA(A) agonist muscimol to functionally inactivate these nuclei immediately after single-trial Pavlovian fear conditioning or single-trial inhibitory avoidance learning. Immediate post-training infusions of muscimol had no effect on Pavlovian conditioning but produced a dose-dependent effect on inhibitory avoidance. However, pre-training infusions dose-dependently disrupted Pavlovian conditioning. These findings indicate that the amygdala plays an essential role in the acquisition of Pavlovian fear conditioning and contributes to the modulation of memory consolidation of inhibitory avoidance but not of Pavlovian fear conditioning.

Lignocaine selectively reduces C fibre-evoked neuronal activity in rat spinal cord in vitro by decreasing N-methyl-D-aspartate and neurokinin receptor-mediated post-synaptic depolarizations; implications for the development of novel centrally acting analgesics

The action of lignocaine on nociceptive transmission in the spinal cord has been studied in vitro using ventral root potential (VRP) recordings from 10-12-day-old rat hemisected spinal cord preparations. Single-shock stimulation of a dorsal root at intensities sufficient to activate high-threshold C-primary afferent fibres elicited VRPs lasting for 15-20 sec in the corresponding ventral root. The VRP consisted of 3 distinct parts: the early, slow and prolonged components, as previously described (Thompson et al. 1992), where the early represents A beta fibre-evoked mono- and polysynaptic responses lasting for tens of milliseconds, the slow is a largely N-methyl-D-aspartic acid (NMDA) receptor-mediated small-calibre afferent-generated component, lasting for about 1.5 sec, and the prolonged is a neurokinin receptor-mediated long-lasting component generated by high-threshold fibres. Lignocaine superfusion (40-60 microM) significantly and reversibly reduced the slow and prolonged components of the C fibre-evoked VRP in a dose-dependent manner without any effect on the early or A beta fibre-mediated component of the VRP. The amplitude of the cumulative VRP generated by repetitive inputs (1 and 10 Hz) was also significantly reduced as was the depolarization produced by bath application of NMDA (100 microM) or substance P (SP, 1 microM) in the presence or absence of tetrodotoxin (TTX) (300 nM). At this dose range lignocaine had no effect on the compound action potential (CAP) elicited by stimulating the sciatic nerve and recorded on the dorsal root. The CAP was only significantly reduced with a 300 microM dose of lignocaine. Application of the opiate, glycine, GABAA and GABAB receptor antagonists, naloxone (1 microM), strychnine (100 microM), bicuculline (100 microM) and phaclofen (100 microM) did not alter the depressant effects of lignocaine on the VRP. Low concentrations of lignocaine have a selective action on nociceptive transmission in the spinal cord which is different and more potent than its local anaesthetic conduction blockade in the periphery. This includes a reduction of direct or synaptically driven NMDA- and NK receptor-mediated post-synaptic depolarizations indicating that this class of sodium channel blockers may be potentially useful as analgesic agents, possibly acting on TTX-resistant sodium ion channels.