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 glutamate in FAG mechanisms is supported by both behavioural and morphological studies, whether FAG afferents conveying somatosensory information constitute a source of glutamatergic input to the FAG remains unknown. Here, we have compared the projection pattern of orthogradely-labelled spinoannular fibres with the distribution of glutamate-like immunoreactivity in the FAG at the Light microscopic level. Transaxonal labelling was observed throughout the whole rostrocaudal axis of the FAG except for the dorsolateral regions. Cell-processes and terminal-reminiscent puncta were strongly immunoreactive in all FAG regions, including the dorsolateral areas. To ascertain whether glutamate-immunoreactive puncta observed at light microscopy indeed constituted axon terminals of the spinoannular system, glutamate-like immunoreactivity was assessed in orthogradely-labelled synaptic terminals using a post-embedding immunogold procedure for electron microscopy. Quantitative analysis of gold particle densities revealed over twice as strong an immunoreactivity in anatomically-identified spinoannular axon terminals as in dendrites postsynaptic to them, perikarya and inhibitory Gray II synapses, as well as an over 5-fold heavier immunolabelling than in glial profiles. These findings reveal that glutamate is accumulated in synaptic terminals of the spinoannular system, supporting a neurotransmitter role for this acidic amino acid in spinofugal afferents to the FAG. (C) 1998 Elsevier Science B.V.