Cholecystokinin-containing and nociceptive neurons in rat Edinger-Westphal nucleus

The putative peptide neurotransmitter cholecystokinin (CCK) is co-localized with substance P (SP) in a dense cluster of neurons located in the rat Edinger-Westphal (EW) nucleus. In an attempt to record electrophysiologically from these CCK-containing neurons, we have identified a group of nociceptive neurons located within the confines of the EW nucleus. The firing pattern of these nociceptive neurons is erratic, sometimes with a bursting pattern and at other times with fairly regular rates which vary generally from 1 to 7 Hz. These neurons respond to noxious stimuli, such as toe pinch, with an increase in rate and sometimes enter an apparent depolarization blockade (preceded by an increase in the duration and a decrease in the amplitude of the action potential). Systemically administered morphine suppresses both the spontaneous firing rate and the toe pinch-induced increase in firing rate. Naloxone is able to reverse the effects of morphine. Although we have identified in the EW area a moderate density of terminals containing enkephalin-like immunoreactivity, morphine locally applied via microiontophoresis is largely without effect on the firing of these neurons. We hypothesize that the opiate-induced suppression of these nociceptive neurons is not mediated directly on the EW cells, but rather indirectly through afferent systems.