5-Hydroxytryptamine facilitates GABA-induced depolarization in bullfrog primary afferent neurons

Inhibition of peripheral anion exchanger 3 decreases formalin-induced pain

We determined the role of chloride-bicarbonate anion exchanger 3 in formalin-induced acute and chronic rat nociception. Formalin (1%) produced acute (first phase) and tonic (second phase) nociceptive behaviors (flinching and licking/lifting) followed by long-lasting evoked secondary mechanical allodynia and hyperalgesia in both paws. Local peripheral pre-treatment with the chloride-bicarbonate anion exchanger inhibitors 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothiocyanato-2,2'-stilbenedisulfonic acid prevented formalin-induced nociception mainly during phase 2. These drugs also prevented in a dose-dependent fashion long-lasting evoked secondary mechanical allodynia and hyperalgesia in both paws. Furthermore, post-treatment (on day 1 or 6) with 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid reversed established hypersensitivity. Anion exchanger 3 was expressed in dorsal root ganglion neurons and it co-localized with neuronal nuclei protein (NeuN), substance P and purinergic P2X3 receptors. Furthermore, Western blot analysis revealed a band of about 85 kDa indicative of anion exchanger 3 protein expression in dorsal root ganglia of naïve rats, which was enhanced at 1 and 6 days after 1% formalin injection. On the other hand, this rise failed to occur during 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid exposure. These results suggest that anion exchanger 3 is present in dorsal root ganglia and participates in the development and maintenance of short and long-lasting formalin-induced nociception.

Secondary mechanical allodynia and hyperalgesia depend on descending facilitation mediated by spinal 5-HT₄, 5-HT₆ and 5-HT₇ receptors

In the present study we determined the role of spinal 5-hydroxytriptamine (5-HT) and 5-HT(4/6/7) receptors in the long-term secondary mechanical allodynia and hyperalgesia induced by formalin in the rat. Formalin produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term secondary mechanical allodynia and hyperalgesia in both paws. In addition, formalin increased the tissue content of 5-HT in the ipsilateral, but not contralateral, dorsal part of the spinal cord compared to control animals. Intrathecal (i.t.) administration of 5,7-dihydroxytriptamine (5,7-DHT), a serotonergic neurotoxin, diminished tissue 5-HT content in the ipsilateral and contralateral dorsal parts of the spinal cord. Accordingly, i.t. 5,7-DHT prevented formalin-induced secondary allodynia and hyperalgesia in both paws. I.t. pre-treatment (-10 min) with ML-10302 (5-HT(4) agonist), EMD-386088 (5-HT(6) agonist) and LP-12 (5-HT(7) agonist) significantly increased secondary mechanical allodynia and hyperalgesia in both paws. In contrast, i.t. pre-treatment (-20 min) with GR-125487 (5-HT(4) antagonist), SB-258585 (5-HT(6) antagonist) and SB-269970 (5-HT(7) antagonist) significantly prevented formalin-induced long-term effects in both paws. In addition, these antagonists prevented the pro-nociceptive effect of ML-10302, EMD-386088 and LP-12, respectively. The i.t. post-treatment (6 days after formalin injection) with GR-125487, SB-258585 and SB-269970 reversed formalin-induced secondary allodynia and hyperalgesia in both paws. These results suggest that spinal 5-HT, released from the serotonergic projections in response to formalin injection, activates pre- or post-synaptic 5-HT(4/6/7) receptors at the dorsal root ganglion/spinal cord promoting the development and maintenance of secondary allodynia and hyperalgesia.

GABA(A) receptor beta 3 subunit gene and psychiatric morbidity in a post-traumatic stress disorder population

GABAergic systems have been implicated in the pathogenesis of anxiety, depression and insomnia. These symptoms are part of the core and comorbid psychiatric disturbances in post-traumatic stress disorder (PTSD). In a sample of Caucasian male PTSD patients, dinucleotide repeat polymorphisms of the GABA(A) receptor beta 3 subunit gene were compared to scores on the General Health Questionnaire-28 (GHQ). As the major allele at this gene locus (GABRB3) was G1, the alleles were divided into G1 and non-G1 groups. On the total score of the GHQ, which comprises the somatic symptoms, anxiety/insomnia, social dysfunction and depression subscales, patients with the G1 non-G1 genotype had a significantly higher score when compared to either the G1G1 genotype (alpha=0.01) or the non-G1 non-G1 genotype (alpha=0.05). No significant difference was found between the G1G1 and non-G1 non-G1 genotypes. When the G1 non-G1 heterozygotes were compared to the combined G1G1 and non-G1 non-G1 homozygotes, a significantly higher total GHQ score was found in the heterozygotes (P=0.002). These observations suggest a heterosis effect. Further analysis of GHQ subscale scores showed that heterozygotes compared to the combined homozygotes had higher scores on the somatic symptoms (P=0.006), anxiety/insomnia (P=0.003), social dysfunction (P=0.054) and depression (P=0.004) subscales. In conclusion, the present study indicates that in a population of PTSD patients, heterozygosity of the GABRB3 major (G1) allele confers higher levels of somatic symptoms, anxiety/insomnia, social dysfunction and depression than found in homozygosity.

Facilitation by 5-hydroxytryptamine of ATP-activated current in rat pheochromocytoma cells

The effects of 5-hydroxytryptamine (5-HT) on an inward current activated by extracellular ATP were investigated in rat pheochromocytoma PC12 cells. Under whole-cell voltage-clamp conditions 5-HT (10 microM) reversibly enhanced the amplitude of the current activated by 30 microM ATP. The enhancement may not be due to an increase in the number of functional channels because the current activated by 300 microM ATP was not remarkably augmented compared with the current activated by 30 microM ATP. The current enhancement by 100 microM 5-HT was less obvious than that by 10 microM 5-HT. When the current kinetics were compared, activation of the ATP-evoked current was accelerated to the same extent by either 10 or 100 microM 5-HT, whereas deactivation was largely more accelerated by 100 microM 5-HT. Propranolol (10 microM), a 5-HT1 receptor antagonist, or LY53857 (10 microM), a 5-HT2 receptor antagonist, exerted an agonistic effect: the ATP-activated current was facilitated by these compounds. Metoclopramide (10 microM), a 5-HT3 receptor antagonist, neither facilitated the ATP-activated current, nor blocked the current facilitation by 5-HT. Guanosine 5'-O-(2-thiodiphosphate) (GDP[beta S]) (2 mM), the non-hydrolysable analog of guanosine 5'-triphosphate (GTP), or K-252a (2 microM), a protein kinase inhibitor, did not affect the facilitation by 5-HT of the ATP-activated current when they were included in the intracellular solution. The ATP-activated current pre-facilitated by 10 microM dopamine was not enhanced by 10 microM 5-HT. Similarly, the pre-facilitation by 5-HT attenuated the current enhancement by dopamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction of GABAB inhibitory postsynaptic potentials by serotonin via pre- and postsynaptic mechanisms in CA3 pyramidal cells of rat hippocampus in vitro

The action of serotonin (5-HT) on GABAergic synaptic transmission was investigated with intracellular recordings in CA3 pyramidal cells of rat hippocampal slices. Local application of 5-HT (500 microM) hyperpolarized CA3 pyramidal cells, decreased cellular input resistance, and reduced slow afterhyperpolarizations. Serotonin attenuated the late (GABAB) component of polysynaptic inhibitory postsynaptic potentials (IPSPs; 47% of control) without affecting the early (GABAA) component. During bath application of the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (20 microM) and 2-amino-5-phosphonovalerate (AP-5) (40 microM), 5-HT similarly decreased the amplitude of the late (GABAB) component (17% of control) of monosynaptic IPSPs but did not affect the early (GABAA) component. The mean reversal potentials of poly- and monosynaptic IPSPs were unaffected by 5-HT. The conductance increases associated with the late component of poly- and monosynaptic IPSPs were reduced by 5-HT. Hyperpolarizing responses evoked in CA3 pyramidal cells by somatic applications of gamma-aminobutyric acid (GABA) were unaffected by 5-HT. During bath application of bicuculline (20-50 microM), hyperpolarizing responses elicited by dendritic GABA application were reduced by 5-HT (71% of control). The effect of 5-HT on these direct GABAB hyperpolarizations (29% decrease in response) does not appear sufficient to fully account for the effect of 5-HT on late GABAB IPSPs (53-83% decrease in response). Therefore, 5-HT may reduce GABAB IPSPs in CA3 pyramidal cells 1) by a postsynaptic action on pyramidal cells and 2) by a selective presynaptic action on GABAergic interneurons mediating the GABAB IPSP. This presynaptic action of 5-HT does not appear to involve excitatory afferents onto inhibitory interneurons.

Low plasma gamma-aminobutyric acid levels in male patients with depression

Plasma levels of gamma-aminobutyric acid (GABA) were significantly lower in males with primary unipolar major depressive disorder than in healthy controls. Although the difference in means between control and symptomatic depressed patient groups was small, the distribution of plasma GABA in the depressed patients was markedly different from controls. Forty percent of depressed patients had plasma GABA levels below those of controls. Plasma GABA levels correlated positively with duration of illness, and negatively with age at onset of the mood disorder and the total Endogenomorphic Symptom Score on the Hamilton Rating Scale. Plasma GABA levels may be a biochemical marker of vulnerability to depression, as opposed to a consequence of the illness. A low GABA condition in depression fits and complements the prevailing biogenic amine hypotheses of depression.

Serotonin and GABA-induced depolarizations of frog primary afferent fibers

The interaction of gamma-aminobutyric acid (GABA) and serotonin (5-HT) on primary afferent terminals of the isolated frog spinal cord was investigated by sucrose gap recordings from dorsal roots. Application of 5-HT (1.0-100 microM) to the Ringer's solution significantly reduced afferent terminal depolarizations elicited by concentrations of GABA ranging from 0.1 to 1.0 mM. The reductions of GABA-depolarizations which were produced by 1.0 microM 5-HT were mimicked by the 5-HT1A agonists 8-OH-DPAT (8-hydroxy-2-(n-dipropylamino)tetralin) and ipsapirone. The effects of ipsapirone were reversed by the 5-HT1A antagonist spiperone. The decreases of GABA-depolarizations produced by high doses of 5-HT were duplicated by application of alpha-methyl-5-HT, a 5-HT1C/2 agonist and reversed by superfusion of the cord with manserin, a 5-HT1C/2 antagonist. The presumptive 5-HT1A receptor-mediated effects of 1.0 microM 5-HT and 8-OH-DPAT appeared to result from a direct action on afferent terminals because the reduction of GABA responses was unchanged by addition of TTX to the Ringer's solution. In contrast, the putative 5-HT1C/2 receptor actions of 100 microM 5-HT and alpha-methyl-5-HT were substantially reduced by TTX and are presumably caused by activation of receptors located on interneurons. GABAB receptors did not appear to be affected by addition of 5-HT at low or high concentrations because baclofen-induced afferent terminal hyperpolarizations remained unchanged during exposure to 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)