Characterization of the effect of cholecystokinin (CCK) on neurons in the periaqueductal gray of the rat: immunocytochemical and in vivo and in vitro electrophysiological studies

The periaqueductal gray (PAG) is an important integration site for pain, autonomic functions, vocalization, fear and anxiety. Cholecystokinin (CCK) is a major neurotransmitter in the PAG and CCK receptors are heterogeneously distributed within the PAG. Since CCK antagonists are anxiolytic and potentiate morphine analgesia, it is possible that these effects of CCK are mediated through alteration of neuronal activities in the PAG. The goals of this study were to examine the anatomical and physiological properties of the PAG CCK containing systems. The distribution of CCK-containing axons and boutons in PAG was examined using immunohistochemical procedures. These studies show that CCK-like immunoreactive (CCK-LIR) fibers and terminals are present throughout PAG, but are particularly heavily concentrated in a focal column that runs longitudinally throughout the rostrocaudal axis of dorsolateral PAG and in nucleus cuneiformis which represents a caudolateral extension of PAG. The physiological effects of CCK on PAG neurons were examined in both in vivo and in vitro preparations. In the in vivo experiments multibarreled electrodes were used to record from PAG neurons and to apply CCK and the CCK antagonists, CR1409 and proglumide. Of 37 neurons recorded in vivo, CCK caused excitation in 25 cells, inhibited 7 cells and had no effect on 5 cells. The excitatory effect was blocked by CR1409 in 11/11 cells tested. Proglumide blocked the excitatory response of CCK in 12/14 cells. Proglumide blocked the inhibitory effect in 2 of 7 cells, but CR1409 had no effect on CCK-evoked inhibition in 7 cells tested. Extracellular, conventional intracellular and whole cell patch clamping procedures were used to study CCK actions in the in vitro slice preparation. In the extracellular recording experiments, responses of PAG cells to CCK were measured in slices that were maintained at 22°C (room temperature) and at 32°C. CCK excited 40/56, inhibited 7/56 and had no effect on 9/56 cells; excitatory responses were blocked by CR1409 in 32/36 cells and by proglumide in 25/27 cells tested. Inhibitory responses to CCK were unaffected by CR1409, but were blocked in 3/7 cells by proglumide. Conventional intracellular recordings were made from 13 cells. In 11 cells, CCK produced a membrane depolarization that ranged between 5 and 18 mV (mean = 9.0 ± 3.3 mV); the firing rates of these cells were increased by as much as 98% (from mean of 1.7 ± 1.0 spikes/s to 3.3 ± 1.6 spikes/s). The effects of CCK on 18 cells were examined in whole cell patch clamp recordings. In 13/18 cells CCK increased the membrane conductance; the reversal potential for CCK had a mean of -51.1 ± 5.7 mV. The remaining five neurons did not respond to CCK. From these studies it is concluded that: (1) CCK is a major excitatory transmitter in the PAG; (2) the excitatory effect of CCK is due to membrane depolarization most likely due to increase of Na⁺ and/or Ca²⁺ permeability; (3) the excitatory effect of CCK is mediated through postsynaptic mechanisms; and (4) the response to CCK is in part mediated through activation of CCK-A receptors.