Modulation of Synaptic Reactions of the “Nociceptive” Neurons in the Cat Cortex upon Stimulation of the Periaqueductal Gray and Application of Some Pharmacological Agents

Abstract

We examined the effects of electrical stimulation of the periaqueductal gray (PAG) and systemic morphine injections on postsynaptic processes in neurons of the cat somatosensory cortex, which were activated by nociceptive influences. Stimulation of the PAG resulted in long-lasting suppression of synaptic reactions induced by stimulation of dental pulp nociceptors. There was certain parallelism within the conditioning effects of PAG stimulation and effects of systemic introduction of morphine. Ionophoretic application of strychnine on such pyramidal cortical neurons did not evoke paroxysmal depolarization shifts (PDSh) of the membrane potential in such neurons. At the same time, surface application of this agent on the cortex (which influenced extensive populations of cortical neurons) resulted in the development of considerable PDShs. This observation confirms the synaptic nature of the above shifts. Applications of strychnine using both techniques resulted in the blockade of, first of all, early IPSP components in pyramidal neurons but did not affect significantly late components of these potentials; this is indicative of different geneses of the above components. It is supposed that the early IPSP component is generated due to activation of axo-somatic inhibitory synapses, while the late component of such reactions is related to activation of inhibitory synapses localized in the dendrites. The mechanisms of modulation of postsynaptic reactions of somatosensory cortex neurons developing upon activation of high-threshold (nociceptive) afferent inputs are discussed. Such modulation is probably based on changes in both pre- and post-synaptic intracortical cell structures.