Neurones in the dorsolateral periaqueductal grey matter in coronal slices of rat midbrain: electrophysiological and morphological characteristics

Intracellular recordings using the whole-cell patch-clamp configuration wer e made from 87 neurones in the dorsolateral "aversive" region of the periaq ueductal grey matter (PAG) in coronal slices of rat midbrain. Camera lucida reconstructions made of 28 of the cells, which had been filled with biocyt in, revealed round, triangular or oval cell bodies 11-40 mu m in diameter. Between two and seven primary dendrites were present, which branched furthe r, often becoming varicose. The dendritic tree was always contained within the dorsal half of the FAG. Biocytin-filled axons could be followed for 177 -2315 mu m from the soma. The axons typically travelled to the perimeter of the dorsal half of the ipsilateral FAG, before either turning ventrolatera lly to run parallel to the tecto-bulbopsinal fibres or continuing their tra jectory into the deep collicular layers or the mesencephalic reticular form ation. Electrophysiologically, two functional categories of cells could be distinguished: type-A cells (30%) showed inward rectification at membrane v oltages in excess of -77 mV and had short action potentials (1.6+/-0.07 ms) , which were followed by a biphasic afterpolarisation characterised by an i nitial fast and a later slow phase. Only 18.5% of the type-A cells were spo ntaneously active (<4 Hz). The second category of cell (type B, 70% of the recorded population) did not show rectification and had longer-duration act ion potentials (2.1+/-0.07 ms), which showed a smooth decay of the afterhyp erpolarisation phase. Approximately one third (37%) of the type-B cells fir ed spontaneously (<4 Hz). The gross morphology of the two types of cells wa s similar. However, in type-A cells (n=6), the axons could be seen to origi nate from the cell soma, whereas, in the majority of the type-B population (10/13), the axon arose from a primary dendrite. The results show that coro nal slices of midbrain contain a viable population of "output" or "projecti on" neurones, which are accessible In conscious animals, efferent output fr om this part of the FAG is concerned with mediating the autonomic and somat omotor changes which are characteristic components of aversive emotional be haviour. The output neurones should therefore reflect the net level of exci tability in the FAG in relation to its functional activity.