The HT 2157-induced increase in extracellular 5 HT LEVELS in the frontal cortex of freely moving rats is modulated by 5 HT1A receptors

HL Rowley1, SC Cheetham1, DJ Heal1& TP Blackburn2, 1RenaSci Consultancy Ltd, Nottingham, UK and 2Helicon Therapeutics Inc, Farmingdale, New York, USA.

HT-2157 , a selective Galanin-3 receptor antagonist, has been shown to increase extracellular levels of 5-hydroxytryptamine (5-HT) in various brain regions (Rowley et al., 2005). In this study the role of 5-HT1A receptors in the HT-2157-induced increase in 5-HT levels was examined in the rat frontal cortex using in vivo microdialysis. The serotonin selective reuptake inhibitor (SSRI), paroxetine, was used as a comparator. Male, Sprague-Dawley rats (250 – 350 g; Charles River, UK) were anaesthetised with isoflurane in an O2/N2O mixture and a concentric dialysis probe (manufactured in-house with 2 mm Hospal membrane tip) was stereotaxically implanted into the frontal cortex. During a recovery period of at least 16 h food and water were available ad libitum and probes were continuously perfused with an artificial cerebrospinal fluid (Harvard Apparatus, UK) at a flow rate of 1.2 μl/min. Four basal samples (20 min interval) were collected prior to oral (po) administration of HT-2157, paroxetine or vehicle. WAY 100 635 and 8-OH-DPAT were administered 2 h and 1 h, respectively, after HT-2157 or paroxetine, via the subcutaneous (sc) route. Dialysate 5-HT was quantified by reverse‑phase HPLC with electrochemical detection. Values are mean ± SEM (n = 6-8) and statistical comparisons were made between drug-treated groups by area under the curve analysis with one way ANCOVA.

HT-2157 (10 mg/kg po; 0.25% methylcellulose) evoked a significant increase in cortical 5-HT levels which was immediate and sustained for at least 4 h post-drug. A maximal increase of 746 ± 141% was observed, compared to vehicle-treated controls (P<0.001). The 5-HT1A antagonist, WAY 100 635 (0.3 mg/kg sc), administered 2 h after HT-2157, resulted in a small but significant potentiation of the HT-2157-induced increase in 5-HT levels (P<0.01) with a maximum rise in levels of 1113 ± 248%, compared to vehicle-treated controls. WAY 100 635 also potentiated the paroxetine-induced increase in cortical 5-HT levels. Maximal increases were 758 ± 119% (paroxetine only; 10 mg/kg po) and 930 ± 189% (paroxetine + WAY 100 635; 0.3 mg/kg sc; P<0.05), compared to vehicle-treated controls. Administration of the 5-HT1A agonist, 8-OH-DPAT (0.1 mg/kg sc), 1 h after HT-2157 (10 mg/kg po) resulted in a significant (P<0.01) attenuation in the HT-2157-induced increase in cortical 5-HT levels. Maximal increases were 672 ± 160% (HT-2157 only) and 472 ± 133% (HT-2157 + 8-OH-DPAT), compared to vehicle‑treated controls. Neither WAY100 635 nor 8-OH-DPAT alone had any effect on basal 5-HT levels.

These data show that the increases in cortical 5-HT levels evoked by HT-2157 and paroxetine were potentiated by the 5-HT1A receptor antagonist, WAY 100 635. The 5-HT1A agonist, 8-OH-DPAT, attenuated the increase in 5-HT evoked by HT-2157. Similar effects have been observed on paroxetine-induced increases in 5-HT levels (Gundlah etal., 1997). These data indicate that the rise in 5-HT evoked by HT-2157 was due, at least in part, to a mechanism dependent on neuronal firing.

Gundlah C et al. (1997) J Pharmacol Exp Ther, 283, 581-591.
Rowley HL et al. (2005) Br J Pharmacol, (This meeting).