A novel substance P binding site in rat brain regions modulates TRH receptor binding

Modulation of somatostatin receptors, somatostatin content and Gi proteins by substance P in the rat frontoparietal cortex and hippocampus

Substance P (SP) and somatostatin (SRIF) are widely spread throughout the CNS where they play a role as neurotransmitters and/or neuromodulators. A colocalization of both neuropeptides has been demonstrated in several rat brain areas and SP receptors have been detected in rat cortical and hippocampal somatostatinergic cells. The present study was thus undertaken to determine whether SP could modulate SRIF signaling pathways in the rat frontoparietal cortex and hippocampus. A single intraperitoneal injection of SP (50, 250 or 500 micro g/kg) induced an increase in the density of SRIF receptors in membranes from the rat frontoparietal cortex at 24 h of its administration, with no change in the hippocampus. The functionality of the SRIF receptors was next investigated. Western blot analysis of Gi proteins demonstrated a significant decrease in Gialpha1 levels in frontoparietal cortical membranes from rats treated acutely (24 h) with 250 micro g/kg of SP, which correlated with a decrease in functional Gi activity, as assessed by use of the non-hydrolyzable GTP analog 5'-guanylylimidodiphosphate. SRIF-mediated inhibition of basal or forskolin-stimulated adenylyl cyclase activity was also significantly lower in the frontoparietal cortex of the SP-treated group, with no alterations in the catalytic subunit of the enzyme. SRIF-like immunoreactivity content was increased in the frontoparietal cortex after acute (24 h) SP administration (250 or 500 micro g/kg) as well as in the hippocampus in response to 7 days of SP (250 micro g/kg) administration. All these SP-mediated effects were prevented by pretreatment with the NK1 receptor antagonist RP-67580. Although the physiologic significance of these results are unknown, the increase in SRIF receptor density together with the desensitization of the SRIF inhibitory signaling pathway might be a mechanism to potentiate the stimulatory pathway of SRIF, inducing a preferential coupling of the receptors to PLC.

Tachykinin receptors in the spinal cord

In summary, all three tachykinin receptors appear to be important modulators of physiological systems in the spinal cord. However, although there is a good deal of data concerning binding characteristics in peripheral tissues, work done in the spinal cord is scanty, leading to a number of unanswered questions. Firstly, Lui et al. (1993) have suggested a discrepancy between the location of SP binding sites and SP containing terminals. This might explain the conflicting evidence on the role of NK1 receptors in the dorsal horn. Furthermore, evidence that NK2 receptors are involved in nociception is increasing, however binding sites for these receptors in the spinal cord have not been demonstrated. This appears to be due to the difficulty in locating an ideal receptor specific ligand. The role of NK2 receptors in autonomic function is also unclear, perhaps for the same reason. Finally, there is evidence indicating that NK3 binding sites are increased following transection of the LIV-VI dorsal roots, however, studies on the effects of inflammation have not been done, as they have with the NK1 and NK2 receptors. All of these and many more unanswered questions require further investigation.

Serotonin modulates the levels of mRNAS coding for thyrotropin-releasing hormone and preprotachykinin by different mechanisms in medullary raphe neurons

The serotonergic neurons of the medullary raphe also contain the peptide neurotransmitters substance P (SP) and thyrotropin releasing hormone (TRH). In this study we asked whether the manipulation of serotonin levels alters the levels of mRNA coding for pre-proTRH. Just like the mRNA coding for the precursor of SP (preprotachykinin, PPT), levels of TRH mRNA are increased when serotonin synthesis is inhibited by para-chlorophenylalanine (pCPA) and decreased when serotonin reuptake is blocked by zimelidine. However, subtle differences suggest that the mechanisms behind these changes are different. Levels of TRH mRNA are still decreased after 14 days of zimelidine treatment, a time when levels of PPT mRNA have returned to control values. In addition, the serotonin reuptake blocker fluoxetine lowers levels of TRH but not PPT mRNA. Finally, the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) induces a transient decrease in levels of PPT mRNA similar to that induced by zimelidine, but does not decrease levels of TRH mRNA even when 10-fold higher doses are administered. These results suggest that while some pharmacological manipulations appear to alter TRH and PPT mRNA levels coordinately, the mechanisms regulating the synthesis of these two colocalized neurotransmitters are different.