A glutamate receptor blocker blunted the inhibitory effects of nociceptive responses induced by HFS. A. Pre: Smaller unit discharges in response to mechanical stimulation. The second trace represents single unit responses selected by cluster analysis from multiple units in the top line. At 30 min after HFS, microinjection of MK-801 completely inhibited LLS of nociceptive responses. B. NMDA antagonists, APV and MK-801, suppressed the effects of HFS on nociceptive responses. The duration of control nociceptive responses was assessed as 100% and changes in duration after HFS to the BLA were converted into percentages of the control value. There were no significant changes from the pre-HFS control value. In the ACSF group, nociceptive responses were completely blocked at 10 and 30 min after HFS but had recovered to the pre-HFS control value by 60 min. *p < 0.05, versus the pre-HFS control value. Error bars represent S.E. n = 15. C. After microinjection of MCPG (an mGluR group I and II antagonist) into the PFC, unit discharges were recorded in the PFC. Pre: prior to HFS delivery to the BLA. Multiple unit discharges responded to peripheral noxious stimulation. At 30 min after HFS, microinjection of MCPG blocked induction of LLS of nociceptive responses. D. Effects of mGluR antagonists on LLS of nociceptive response. MCPG partially blocked LLS of nociceptive responses induced by HFS to the BLA. Inhibitory effects were observed only at 30 min after HFS. An mGluR group II antagonist, LY341495, completely blocked LLS of nociceptive responses. *p < 0.05, versus the pre-HFS control value. Error bars represent S.E. n = 12.