Low dose of morphine microinjected in the ventral periaqueductal gray matter of the rat depresses responses of nociceptive ventrobasal thalamic neurons.

The effect of low doses of morphine (2 micrograms in 0.2 microliter) microinjected in the periaqueductal gray matter (PAG) was studied on responses of 11 nociceptive (N) and 3 non-nociceptive (Nn) ventrobasal (VB) thalamic neurons in lightly anesthetized rats. Responses of Nn neurons were unmodified but their receptive field was consistently enlarged during 50-60 min. Responses of N neurons to calibrated pinches were strongly depressed when the microinjection site was located in the PAG or in the dorsal raphe (NDR) (7 cases), and not significantly changed when morphine was applied in the dorsal PAG. For the 7 depressed neurons, the mean value of the responses expressed as a percentage of the control was 20.12 +/- 5.17, 15 min after morphine application. This depressive effect was variably reversed by naloxone (0.1-0.5 mg/kg).