Inhibition of Nociceptive Neurons in the Intralaminar Nuclei by Electrical Stimulation of the Periaqueductal Gray of the Cat

Abstract

The present study was undertaken to assess whether nociceptive neurons in the intralaminar nuclei of the cat thalamus could be inhibited by electrical stimulation of either the ventromedial periaqueductal gray (PAG) or the nucleus raphe dorsalis (NRD).
Experiments were carried out on cats anesthetized with urethane and chloralose. Single unit activities were recorded from the thalamus using glass capillary microelectrodes filled with 2% pontamine sky blue in 0.5 M sodium acetate. Bilateral maxillary and mandibular canine dental pulps, the greater splanchnic nerve (SPL), the greater occipital nerve (C2) and the mesencephalic reticular formation (MRF) were prepared for electrical stimulation.
Nociceptive neurons were recorded from the nucleus centralis lateralis (CL) and the nucleus parafascicularis (Pf). The effects of stimulating either PAG or NRD on responses obtained from these neurons were investigated. Conditioning electrical stimulation of either PAG or NRD inhibited responses to electrical stimulation of the dental pulp, SPL and/or C2 in some of the CL and Pf neurons. The same conditioning stimulation also inhibited short latency responses to electrical stimulation of MRF. The stimulation had no effects on the majority of intralaminar nociceptive neurons.
These data suggest that PAG or NRD stimulation-produced inhibition of these neurons may be partially mediated by an ascending pathway, in addition to the well-known descending pathways. We found that electrical stimulation of either PAG or NRD was not effective in inhibiting the majority of the intralaminar nociceptive neurons. However, the same stimulation always inhibited resposes to electrical stimulation of SPL of nociceptive reurons in the nucleus ventralis posterolateralis.