GLUTAMATE-LIKE IMMUNOREACTIVITY IN ASCENDING SPINOFUGAL AFFERENTS TO THE RAT PERIAQUEDUCTAL GREY

The midbrain periaqueductal gray is a key structure for the mediation of an integrated defence behaviour. Although a prominent role for glut amate in FAG mechanisms is supported by both behavioural and morpholog ical studies, whether FAG afferents conveying somatosensory informatio n constitute a source of glutamatergic input to the FAG remains unknow n. Here, we have compared the projection pattern of orthogradely-label led spinoannular fibres with the distribution of glutamate-like immuno reactivity in the FAG at the Light microscopic level. Transaxonal labe lling was observed throughout the whole rostrocaudal axis of the FAG e xcept for the dorsolateral regions. Cell-processes and terminal-remini scent puncta were strongly immunoreactive in all FAG regions, includin g the dorsolateral areas. To ascertain whether glutamate-immunoreactiv e puncta observed at light microscopy indeed constituted axon terminal s of the spinoannular system, glutamate-like immunoreactivity was asse ssed in orthogradely-labelled synaptic terminals using a post-embeddin g immunogold procedure for electron microscopy. Quantitative analysis of gold particle densities revealed over twice as strong an immunoreac tivity in anatomically-identified spinoannular axon terminals as in de ndrites postsynaptic to them, perikarya and inhibitory Gray II synapse s, as well as an over 5-fold heavier immunolabelling than in glial pro files. These findings reveal that glutamate is accumulated in synaptic terminals of the spinoannular system, supporting a neurotransmitter r ole for this acidic amino acid in spinofugal afferents to the FAG. (C) 1998 Elsevier Science B.V.