Noradrenergic mechanism involved in the nociceptive modulation of nociceptive-related neurons in the caudate putamen.

Norepinephrine (NE) participates in pain modulation of the central nervous system. The caudate putamen (CPu) is one region of the basal ganglia that has been demonstrated to be involved in nociceptive perception. Our previous work has shown that microinjection of different doses of norepinephrine into the CPu produces opposing effects in the tail-flick latency (TFL) of rats. However, the mechanism of action of NE on the pain-related neurons in the CPu remains unclear. The present study examined the effects of NE and the alpha-adrenoceptor antagonist phentolamine on the pain-evoked response of pain-excitation neurons (PENs) and pain-inhibition neurons (PINs) in the CPu of rats. Trains of electric impulses were used for noxious stimulation, and were applied to the sciatic nerve. The electrical activities of pain-related neurons in the CPu were recorded by a glass microelectrode. The results revealed that intra-CPu microinjection of NE (8microg/2microl) increased evoked firing frequency of PEN and shortened the firing latency, but decreased the evoked firing frequency of PIN and prolonged the inhibitory duration (ID). Intra-CPu administration of phentolamine (4microg/2microl) showed the opposite effects. The above results suggest that NE in the CPu modulates nociception by affecting the baseline firing rates of PENs and PINs.