Responses of rat lateral hypothalamic neurons to periaqueductal gray stimulation and nociceptive stimuli

The effects of dorsal periaqueductal gray (D-PAG) stimulation and noxious stimuli on neural activity in the lateral hypothalamic area (LHA) were investigated in 56 adult male anesthetized rats. Strong tail pinch was used as noxious stimulation. We examined 234 extracellular and 75 intracellular recordings of LHA responses to electrical stimulation of D-PAG. To determine neurotransmitter candidates, the effects of the opioid agonist, morphine, and its antagonist, naloxone were investigated by systemic administration and microelectrophoresis. Of 234 spontaneously firing LHA neurons, 70 (30%) were inhibited by D-PAG stimulation. Of these 70 neurons, 26/40 tested (65%) were glucose-sensitive, 16/19 (84%) were inhibited by morphine and 12/18 (67%) were inhibited by tail pinch. Glucose-sensitive neurons were selectively inhibited by morphine and tail pinch. Naloxone attenuated inhibitory responses to D-PAG stimulation, tail pinch and electrophoretic morphine. From intracellular recordings these polysynaptic inhibitory responses to D-PAG stimulation were considered to be inhibitory postsynaptic potentials (IPSPs) with 6.1 +/- 3.2 ms (mean +/- S.D.) latency and reversal membrane potential of about -78 mV. Since LHA glucose-sensitive neurons receive, selectively, both inhibitory opioid inputs from the D-PAG and inhibitory inputs through noxious stimulation, we suggest that D-PAG might be an intermediate site for transmission of noxious stimuli to the LHA.