Functional interrelations between nucleus raphé dorsalis and nucleus raphé medianus: a dual probe microdialysis study of glutamate-stimulated serotonin release

Abstract

Dual-probe in vivo microdialysis was used to explore the relationships between the two midbrain raphé nuclei, raphé dorsalis (DRN) and raphé medianus (MRN). Infusion of the excitatory neurotransmitter glutamate (10 mM) into the dorsal raphé nucleus produced a large increase in the extracellular 5-HT (5-HT(ext)) in the dorsal raphé (1400% of control values) that was limited to the time of infusion. This was followed by a significant decrease in extracellular 5-HT below baseline levels that continued for the duration of the experiment (3 h). Extracellular 5-HT (5-HT(ext)) was also increased to 500% of control values in the median raphé nucleus following infusion of 10 mM glutamate (GLU) into the dorsal raphé nucleus. Infusion of the competitive NMDA receptor antagonist AP5 prior to and during infusion of GLU into the DRN resulted in a decrease in the response to GLU in the DRN and an antagonism of the increase of 5-HT(ext) in the MRN. Infusion of 10mM GLU into the lateral midbrain tegmentum, an area of the brain just lateral to the DRN, also increased 5-HT(ext) in the probe in the lateral midbrain tegmentum (900% of control) but did not alter 5-HT(ext) in the MRN. When glutamate was infused into the MRN, 5-HT(ext) was also increased to 1400% of control in a time course similar to that seen with infusion of GLU into the DRN. Infusion of glutamate into the MRN, however, did not alter the 5-HT(ext) in the DRN. These data suggest a serotonergic innervation of the median raphé nucleus by the dorsal raphé nucleus. A reciprocal innervation from the median raphé to the dorsal raphé is not mediated by glutamate, does not appear to be serotonergic, and does not regulate extracellular serotonin in the dorsal raphé.

Figure 1

Schema representing the differential projection systems of the dorsal and median raphé nuclei. This diagram also shows the differential innervation of the forebrain by the rostral and caudal dorsal raphé nucleus (from [10]). Abbreviations (see [28]): AM – amygdaloid complex, EC – entorhinal cortex, HF – hippocampal formation, HYP - hypothalamus, IO – inferior olive nucleus, LMT – lateral midbrain tegmentum, MS – medial septum, NGC – nucleus gigantocellularis, NTS – nucleus tractus solitarius, PFC – prefrontal cortex, RM – nucleus raphé magnus, RO – nucleus raphé obscurus, RP – nucleus raphé pallidus, RPO – nucleus raphé pontis, ST – striatum.

Figure 2

Photomicrograph of probe placement into the median raphé nucleus (DRN). This shows the continuity of the guide cannula and microdialysis probe track. Also shown is part of the guide cannula track for the DRN (dorsal raphé nucleus) probe.

Figure 3

Effects of 1 mM glutamate infused into the dorsal raphé nucleus on extracellular 5-HT in the DRN and MRN. Points represent mean ± s.e.m. (n = 3) for 5-HT release expressed as a percent of the individual control periods (−40, −20, 0 min). Glutamate was infused by reverse dialysis through the DRN probe for 20min (GLU).

Figure 4

Effects of 10mM glutamate infused into the DRN on extracellular 5-HT in the DRN (n=5) and MRN (n=5). Points represent mean ± s.e.m. for 5-HT release expressed as a percent of the individual control dialysate concentrations (−40, −20, 0 min). Glutamate was infused for 20min through the probe (GLU). Note logarithmic scale on vertical axis. Infusion of GLU into the DRN produced a significant increase in 5-HT_ext in both brain areas, p < 0.05, two-way ANOVA. * = significantly different from time 0 min

Figure 5

Effects of AP5 and AP5 + GLU infused into the DRN on extracellular 5-HT in the DRN and MRN (n = 3). AP5 (1 mM) was infused for two 20 min periods prior to co-infusion of AP5 + GLU (10 mM). Note logarithmic scale on vertical axis.

Figure 6

Effects of reverse dialysis of 10mM glutamate into the midbrain tegmentum lateral to the DRN on extracellular 5-HT in the lateral midbrain tegmentum and the MRN. Glutamate was infused for 20 min through the probe (GLU). Note logarithmic scale on vertical axis. Points represent mean ± s.e.m. (n = 5) for 5-HT release expressed as a percent of the individual control periods (−40, −20, 0 min).

Figure 7

Effects of 10 mM glutamate infused into the MRN on extracellular 5-HT in the MRN (n = 6) and DRN (n = 6). Bars represent mean ± s.e.m. for 5-HT release expressed as a percent of the individual control concentrations (−40, −20, 0 min). Glutamate was infused for 20min through the probe (GLU). Note logarithmic scale on vertical axis.

Figure 8

Schema of the interactions of the dorsal (DRN) and median raphé nuclei (MRN). See text for discussion. LMT – lateral midbrain tegmentum, MFB – medial forebrain bundle, PFC –prefrontal cortex, RD – dorsal raphé nucleus, RM – median raphé nucleus