Evidence for Glutamate as Neurotransmitter in Trigemino‐ and Spinothalamic Tract Terminals in the Nucleus Submedius of Cats

Abstract

The nucleus submedius in the medial thalamus of cats is an important termination site for lamina I trigemino-and spinothalamic tract (TSTT) neurons, many of which are nociceptive-specific, and the nucleus submedius has been proposed to be a dedicated nociceptive substrate involved in the affective aspect of pain. In the present study, the distribution of glutamate was examined by immunocytochemical methods in order to evaluate the possible role of this amino acid as a neurotransmitter in TSTT terminals in the nucleus submedius. TSTT terminals were identified by anterograde transport of horseradish peroxidase and wheatgerm agglutinin-horseradish peroxidase conjugate from the spinal cord or the medullary dorsal horn. Quantitative analysis of immunogold labelling revealed that TSTT terminals contain about twice the tissue average of glutamate-like immunoreactivity. A strong positive correlation was found between the density of synaptic vesicles and the density of gold particles in these terminals, whereas no relationship was seen between these variables in GABAergic presynaptic dendrites. Enrichment of glutamate-like immunoreactivity (approximately 250% of the tissue average) was also observed in terminals of presumed cortical origin. Presynaptic dendrites and neuron cell bodies in the nucleus submedius were found to contain relatively low levels of glutamate-like immunoreactivity, at or below the tissue average. These observations provide evidence that glutamate is a neurotransmitter in lamina I TSTT terminals in the nucleus submedius. The findings also suggest glutamatergic neurotransmission between cortical afferents and nucleus submedius neurons. Glutamate is therefore likely to be an important mediator of nociceptive processing in the medial thalamus.

PACAP and glutamate are co‐stored in the retinohypothalamic tract

“…Briefly, the anti-glutamate antisera were incubated with a cellulose acetate filter to which glutaraldehyde/formaldehyde fixated conjugates containing 200 M of glutamate, glycine, L-aspartate, GABA, glutamine, ␤-alanine, taurine, and L-homocysteic acid were spotted and then processed for immunocytochemistry by using biotinylated secondary antibodies and the avidin-biotin-peroxidase method (Hannibal et al, 1995). Furthermore, both anti-glutamate antisera were preabsorbed with a mixture of glutaraldehyde/ formaldehyde fixated complexes of glutamate or GABA (200 M) (Ericson et al, 1995) overnight and submitted to immunohistochemistry as described below. Finally, elimination of the primary antisera or incubation with nonimmune serum abolished glutamate staining.…”

“…Two polyclonal rabbit anti-glutamate antisera Code number G-6642 (Sigma Chemical Co., St. Louis, MO) and Code number Glu 607 (raised and characterised as described in Ericson et al, 1995) were used in parallel. Both antisera were characterised in a dot blot system as described (Ottersen and Storm-Mathisen, 1984;Ericson et al, 1995).…”

“…Two polyclonal rabbit anti-glutamate antisera Code number G-6642 (Sigma Chemical Co., St. Louis, MO) and Code number Glu 607 (raised and characterised as described in Ericson et al, 1995) were used in parallel. Both antisera were characterised in a dot blot system as described (Ottersen and Storm-Mathisen, 1984;Ericson et al, 1995). Briefly, the anti-glutamate antisera were incubated with a cellulose acetate filter to which glutaraldehyde/formaldehyde fixated conjugates containing 200 M of glutamate, glycine, L-aspartate, GABA, glutamine, ␤-alanine, taurine, and L-homocysteic acid were spotted and then processed for immunocytochemistry by using biotinylated secondary antibodies and the avidin-biotin-peroxidase method (Hannibal et al, 1995).…”

Metabotropic glutamate (mGlu) receptors and nociceptive processing

“…Of these, the ventrobasal complex has probably received the most attention, and the neurotransmitter pharmacology of this region has been extensively studied, indicating that ascending fibres are almost certainly glutamatergic De Biasi and Rustioni, 1990;Broman, 1994;, although ascending afferents to other thalamic nuclei, which may be important in nociception, are probably also glutamatergic [Ericson et al, 1995]. A role for NMDA receptors in thalamic nociceptive responses at the single-neurone and behavioural level is now well established [Eaton and Salt, 1990;Dougherty et al, 1996;Kolhekar et al, 1997;Bordi and Quartaroli, 2000].…”

Synaptology of trigemino‐ and spinothalamic lamina I terminations in the posterior ventral medial nucleus of the macaque

“…Standard electron micrographs (Kodak-4489 film; Kodak Kodabromide paper) were made of profiles of particular interest. Ultrastructural elements were identified as putative VMpo relay cell dendrites, PSDs, afferent (RL) terminals, corticothalamic (RS) terminals, reticular thalamic (F) terminals, and astrocytic processes, as in prior work (Jones, 1985;Ralston andRalston, 1992, 1994;Blomqvist et al, 1992Blomqvist et al, , 1996Ericson et al, 1995Ericson et al, , 1996Ericson et al, , 1997. Synaptic contacts were defined strictly by the identification of a distinct cleft, membrane specializations, and vesicle accumulation.…”