CARDIOVASCULAR EFFECTS OF MICROINJECTIONS OF OPIOID AGONISTS INTO THEDEPRESSOR REGION OF THE VENTROLATERAL PERIAQUEDUCTAL GRAY REGION

Microinjections of excitatory amino acids made into the ventrolateral midbrain periaqueductal gray of the rat have revealed that neurons in this region integrate a reaction characterised by quiescence, hyporeac tivity, hypotension and bradycardia. Microinjections of both excitator y amino acids and opioids into the ventrolateral periaqueductal gray h ave shown also that it is a key central site mediating analgesia. The effects of injections of opioids into the ventrolateral periaqueductal gray on arterial pressure and heart rate or behaviour are unknown. In this study we first mapped in the rat the extent of the ventrolateral periaqueductal gray hypotensive region as revealed by microinjections of excitatory amino acids. We found that ventrolateral periaqueductal gray depressor region extended more rostrally than previously thought into the tegmentum ventrolateral to the periaqueductal gray. Subseque ntly we studied for the first time, the effects of microinjections of mu-, delta-, and kappa-opioid agonists made into the ventrolateral per iaqueductal grey depressor region. In contrast to the effects of excit atory amino acid injections, microinjections of the mu-opioid agonist ([D-Ala(2),N-Me-Phe(4),Gly-ol(5)]enkephalin) evoked hypertension and t achycardia at approximately 50% of sites. Similar to excitatory amino acid injections, microinjections of both the delta-opioid agonist ([D- Pen(2),D-Pen(5)]enkephalin), and the kappa-opioid agonist idinyl)-1-ox aspiro[4,5]dec-8-yl]-benzeneacetamide) evoked either a hypotension and bradycardia, or had no effect. These results indicate that different opiate receptor subtypes are present on a distinct population of ventr olateral periaqueductal gray neurons, or at different ventrolateral pe riaqueductal gray synaptic locations (pre- or post-synaptic). (C) 1997 Elsevier Science B.V.