BACKGROUND AND PURPOSEHippocampus-dependent memory involves the activity of sharp wave ripples (SWRs), which are thought to participate in the process of memory consolidation. The hippocampus contains high levels of endogenous opioids and of m-opioid receptors (MORs). Here, we have assessed the role of MOR agonists in the modulation of SWRs.
EXPERIMENTAL APPROACHUsing recordings of extracellular potentials from the CA1 field of rat hippocampal slices, we examined the pharmacological actions of morphine, DAMGO and fentanyl on SWRs and on network excitability and paired-pulse inhibition.
KEY RESULTSAll three MOR agonists (1 nM-10 mM) significantly increased the amplitude of sharp waves and the occurrence of SWR sequences, but reduced the initiation of episodes of SWRs. Fentanyl was most potent in producing these effects and morphine the least. Interestingly, although SWRs were reduced by relatively high concentrations (Ն100 nM) of all agonists, they were significantly enhanced by very low concentrations of morphine (5-10 nM). Morphine and DAMGO at moderate-to-high concentrations increased network excitability and reduced inhibition. Furthermore, DAMGO suppressed inhibition more readily than it increased excitation, whereas morphine suppressed inhibition only at high concentrations. These drug effects were reversed by the MOR antagonists naloxone and CTOP.
CONCLUSIONS AND IMPLICATIONSWe found that the SWRs were significantly modulated by three MOR agonists and that the SWRs were very sensitive to subtle changes in the excitation/inhibition balance induced by MOR agonists. Such modulation might underlie the effects of these agonists on hippocampus-dependent memory.
AbbreviationsICI, intra-cluster interval; IEI, inter-event interval; IEpI, inter-episode interval; MOR, m-opioid receptor; PPI, paired-pulse inhibition; PS, population spike; SWR, sharp wave ripple BJP British Journal of Pharmacology DOI:10.1111DOI:10. /j.1476DOI:10. -5381.2012 1146 British Journal of Pharmacology (2013)
IntroductionEndogenous and clinically administered opioids act on many receptors (Waldhoer et al., 2004) and are involved in a wide variety of functions (Drolet et al., 2001) including hippocampus-dependent learning and memory (Wise, 1989;Meilandt et al., 2004;Bodnar, 2010;Kesner and Warthen, 2010;Dacher and Nugent, 2011). Nevertheless, the mechanisms by which opioids affect hippocampus-dependent memory appear to be highly complex and poorly understood.The critical involvement of the hippocampus in learning and memory (Eichenbaum, 2004;Squire et al., 2004) is underpinned by a number of different types of neuronal activity supporting distinct processes (Battaglia et al., 2011). For example, it is assumed that the encoding of new information that occurs during active behaviour is associated with q oscillation (Hasselmo, 2005) whereas memory consolidation takes place during inactivity (Alvarez and Squire, 1994;McGaugh, 2000). It is generally thought that memory consolidation entails the generation of irregular activity of sharp wave ripples...