Improving the quality of neurosteroid induced anaesthesia in rodents

Abstract

Laboratory rodents are widely used in a variety of experimental techniques throughout the world, which helps in advancing biomedical research. As this use is likely to continue in the future, it is imperative to prioritize animal welfare so that this research meets ethical standards, facilitates better science and increases the reproducibility of data.Laboratory rodents are regularly anaesthetised as part of biomedical research throughout the world. An anaesthetic agent that is predictable, safe and efficient enough to alleviate pain and distress throughout the duration of anaesthesia is considered ideal. Although there is no ideal anaesthetic available, a constant search for improving current anaesthetic strategies is important. Alfaxalone is a neuroactive steroid available as an injectable anaesthetic and used in large animals such as dogs, cats, horses and swine with safety. However, it often exhibits a significant motor excitation in the form of limb paddling and facial twitching when used in laboratory rodents. We sought to investigate if alfaxalone modulates synaptic neurotransmission to central motor neurons and if there are any underlying second messenger systems that are triggered leading to excitatory behaviour upon alfaxalone administration.We investigated the effects of alfaxalone on inhibitory glycinergic and excitatory glutamatergic synaptic transmission to rat hypoglossal motor neurons (HMNs). Whole-cell patch clamp recordings were made from neonatal (P7-14) rat HMNs within the hypoglossal nucleus in a 300μm thick transverse brainstem slice. Spontaneous or evoked excitatory post-synaptic currents (sEPSCs/eEPSCs) or miniature glycinergic inhibitory post-synaptic currents (mIPSCs) were recorded at a holding potential of -60mV using a CsCl-based internal solution. Similarly, action potential (AP) firing was recorded at a holding potential of -65mV with a K+ methyl sulfate-based internal solution. Our results show that alfaxalone significantly reduced mIPSC frequency (-39%) and amplitude (-54%) to HMNs, consistent with a reduction in inhibitory transmission to HMNs, possibly leading to neuromotor excitation exhibited as muscle twitching. There was also a positive shift (+35%) in the baseline holding current (Iholding) upon alfaxalone treatment. It was also noted that alfaxalone, even at higher concentrations (10nM-3μM), failed to significantly alter either spontaneous or evoked (25μM) EPSC frequency, amplitude, half-width, rise-time and Iholding. Similarly, repetitive action potential firing by HMNs was not altered by alfaxalone. However, the molecular mechanism behind the effect of alfaxalone in inhibiting glycinergic transmission remains unclear.Earlier reports have suggested that steroids and steroid-like molecules can bring about modulation of several TRP channels, which may be physiologically significant. Our data showed a shift in Iholding upon alfaxalone treatment, which could be due to an indirect modulation of transient receptor potential (TRP) channels. Hence, to investigate this further, the effects of capsaicin, a well-known TRPV1 agonist, on synaptic transmission to HMNs was examined. Interestingly, we found that capsaicin increased spontaneous excitatory post-synaptic current (sEPSC) frequency (+115%) and amplitude (+24%) recorded from neonatal rat HMNs. The frequency of miniature excitatory post-synaptic currents (mEPSCs), recorded in the presence of tetrodotoxin (TTX), was also increased by capsaicin, but capsaicin did not alter mEPSC amplitude, consistent with a pre-synaptic mechanism of action. A negative shift in Iholding was elicited by capsaicin under both recording conditions. The effect of capsaicin on excitatory synaptic transmission remained unchanged in the presence of the TRPV1 antagonists, capsazepine or SB366791, suggesting that capsaicin acts via a mechanism which does not require TRPV1 activation. Capsaicin, however, failed to show any effect on eEPSCs, and did not alter the paired- pulse ratio (PPR) of eEPSCs. Repetitive action potential (AP) firing in HMNs was also unaltered by capsaicin, indicating that capsaicin does not change intrinsic HMN excitability. Interestingly, the effect of capsaicin on spontaneous and miniature glycinergic inhibitory post-synaptic currents (sIPSC/mIPSC) was a significant decrease in current amplitude (-62% and 56% respectively) without altering frequency, indicating a post- synaptic mechanism of action. Interestingly, even this effect of capsaicin on inhibitory current amplitude was not blocked by TRPV1 antagonist, capsazepine. These results suggest that modulation of IPSCs by neuroactive steroid, alfaxalone, is unlikely to involve TRPV1 activation.Our results indicate that neuromotor excitation during alfaxalone anaesthesia are most likely to be mediated by decreases in inhibitory synaptic input, without alteration in spontaneous or evoked excitatory synaptic transmission, and that alfaxalone does not excite HMNs sufficiently to cause any changes in action potential firing. We also demonstrate that capsaicin exerts a modulatory effect on glutamatergic excitatory as well as glycinergic inhibitory, synaptic transmission in HMNs via an unknown mechanism independent of TRPV1 channels.