Endocrinol Metab.  2011 Sep;26(3):210-217. 10.3803/EnM.2011.26.3.210.

Somatostatin Inhibits Gonadotropin Releasing Hormone Neuronal Activities in Juvenile Mice

• Park SA
• Bhattarai
• Han SK

• KMID: 2169171
• DOI: http://doi.org/10.3803/EnM.2011.26.3.210

Abstract

BACKGROUND
The gonadotropin releasing hormone (GnRH) neurons perform a pivotal function in the central regulation of fertility. Somatostatin (SST) is an important neuromodulatory peptide in the central nervous system and alters neuronal activities via G protein- coupled SST receptors. A number of studies have shown that SST modulates the reproductive axis at the hypothalamic level. However, the precise action mechanisms of SST and related receptor subtypes have yet to be fully understood. In this study, we evaluated the direct effects of SST on GnRH neurons in juvenile mice.
METHODS
Juvenile (postnatal days, < PND 30) GnRH-GFP transgenic mice expressing green fluorescent protein were used in this study. Acute coronal brain slices containing the preoptic area were prepared and all identified GnRH neurons were recorded using the gramicidin perforated-patch clamp technique; type II SST receptor (SSTR2) mRNA expression was evaluated via single cell reverse transcription-polymerase chain reaction (RT-PCR).
RESULTS
SST caused membrane hyperpolarization, depolarization, no response, or membrane hyperpolarization with a reduction of action potential. Most (57.7%, 30/52) of the GnRH neurons tested were hyperpolarized by SST and this SST-induced hyperpolarization was found to be concentration-dependent. The percentage of responses, membrane potential changes (MPC), and resting membrane potential (RMP) by SST were not significantly different in juvenile male and female GnRH neurons. The SST-induced hyperpolarization was maintained in the presence of tetrodotoxin (TTX), a sodium channel blocker, and an amino acid blocking cocktail (AABC) containing AP-5 (NMDA receptor antagonist), CNQX (non-NMDA glutamate receptor antagonist), picrotoxin (GABAA receptor antagonist), and strychnine (glycine receptor antagonist). SSTR2 mRNA was expressed on 10 (38%) among 26 GnRH neurons. Seglitide, an SSTR2 agonist, mimicked this SST-induced hyperpolarization (11/23 47.8%) and this response was maintained in the presence of TTX and AABC.
CONCLUSION
Our data show that SST can exert potent inhibitory action against GnRH neuronal excitability via SSTR2 activation in juvenile mice.

Keyword

GnRH neuron; Somatostatin; Perforated-patch clamp

MeSH Terms

6-Cyano-7-nitroquinoxaline-2,3-dione
Action Potentials
Animals
Brain
Central Nervous System
Female
Fertility
Gonadotropin-Releasing Hormone
Gonadotropins
Gramicidin
Humans
Male
Membrane Potentials
Membranes
Mice
Mice, Transgenic
Neurons
Peptides, Cyclic
Picrotoxin
Preoptic Area
Receptors, Glutamate
RNA, Messenger
Sodium Channels
Somatostatin
Strychnine
Tetrodotoxin
Axis, Cervical Vertebra
6-Cyano-7-nitroquinoxaline-2,3-dione
Gonadotropin-Releasing Hormone
Gonadotropins
Gramicidin
Peptides, Cyclic
Picrotoxin
RNA, Messenger
Receptors, Glutamate
Sodium Channels
Somatostatin
Strychnine
Tetrodotoxin

Figure

• 

  Fig. 1. Effects of somatostatin (SST) on juvenile gonadotropin releasing hormone (GnRH) neurons in mice at gramicidin perforated current clamp mode. Representative traces showing membrane hyperpolarization (A. PND18, RMP = -72 mV), membrane depolarization (B. PND28, RMP = -63 mV), no response (C. PND28, RMP = -55 mV) and surcease of action potential followed by membrane hyper-polarization (D. PND14, RMP = -62 mV) by bath application of 300 nM SST. E. Stag col-umn showing percentage response by SST (H, hyperpolarization; N, no response; D, depolarization).

• 

  Fig. 2. Concentration-dependent hyperpolarizing effect of SST on juvenile GnRH neurons. A. Representative trace showing concentration dependent membrane hyperpolarization by application of 10, 30, 100, and 300 nM SST (PND28, RMP = -62.0 mV). B. Concentration-response relationship. ∗represents P < 0.05.

• 

  Fig. 3. A. Percentage response by 300 nM SST in juvenile males and females. R, responded; NR, not responded. B. Comparison of the mean membrane potential changes between males and females by 300 nM SST. Found no significant difference (P > 0.05; one way ANOVA).

• 

  Fig. 4. SST acts on the postsynaptic GnRH neurons directly. A. A representative trace showing hyperpolarization by successive application of 300 nM SST (PND27, RMP = -57.3 mV). Break bar depicts a time interval of 15-20 minutes. B. A representative trace showing hyperpolarization by SST and re-mains persisted in the presence of amino acid blocking cocktail (AABC) including AP-5 (20 µM, an NMDA receptor antagonist), CNQX (10 µM, a non-NMDA glutamate receptor antagonist), picrotoxin (50 µM, a GABA A receptor antag-onist), and strychnine (2 µM, a glycine receptor antagonist) with TTX (0.5 µM, a voltage-gated Na+ channel blocker) in a juvenile GnRH neuron resting at -59.6 mV. PND29. C. Relative potential changes of the 2nd response and response in the presence of AABC. ∗ and NS represent P < 0.05 and not significant, respectively (one sample t-test).

• 

  Fig. 5. A. An example of SSTR2 mRNA expression on GnRH neurons. GAPDH, a housekeeping gene was used to conform the harvested GnRH neurons (n = 8). L (100 bp ladder), 1-8 (harvested GnRH neurons), NC (negative control, harvested GnRH neurons reacted without reverse transcriptase), PC (positive control, cDNA of total brain extract). B. A representative trace showing hyperpolarization by seglitide (SEG) (1 µM), a SSTR2 agonist from a GnRH neuron resting at -70 mV depicting mimicry of SST-induced membrane hyperpolarization (PND28). C. A representative traces showing hyperpolarization by SEG in the presence of AABC. PND13, RMP = -72 mV. D. Bar graph showing the mean membrane hyperpolarization to SEG (1 µM) and with AABC (P > 0.05).

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