Histamine H1 and H3 receptors in the rat thalamus and their modulation after systemic kainic acid administration

In rat thalamus, histamine H1 receptor and isoforms of H3 receptor were expressed predominantly in the midline and intralaminar areas. Correspondingly, higher H1 and H3 receptor binding was also detected in these areas. All isoforms of H3 receptor were expressed in several thalamic nuclei, but there were minor differences between their expression patterns. H1 mRNA expression was high in the ventral thalamus, but the H1 binding level was low in these areas. Since increased brain histamine appears to have an antiepileptic effect through the H1 receptor activity, kainic acid (KA)-induced status epilepticus in rat was used to study modulation of H1 and H3 receptors in the thalamus following seizures. After systemic KA administration, transient decreases in mRNA expression of H1 receptor and H3 receptor isoforms with full-length third intracellular loops were seen in the midline areas and the H1 receptor mRNA expression also decreased in the ventral thalamus. After 1 week, a robust increase in mRNA expression of H3 receptor isoforms with a full-length third intracellular loop was found in the ventral posterior, posterior, and geniculate nuclei. The changes indicate a modulatory role of H3 receptor in the sensory and motor relays, and might be involved in possible neuroprotective and compensatory mechanisms after KA administration. However, short-term increases in the H3 receptor binding appeared earlier (72 h) than the increases of H3 mRNA expression (1-4 w). The elevations in H3 binding were evident in the intralaminar area, laterodorsal, lateral posterior, posterior and geniculate nuclei, and were likely to be related to the cortical and subcortical inputs to thalamus.