Plasma membrane density of GABA(B)-R1a, GABA(B)-R1b, GABA-R2 and trimeric G-proteins in the course of postnatal development of rat brain cortex

Mechanisms and Regulation of Neuronal GABAB Receptor-Dependent Signaling

γ-Aminobutyric acid B receptors (GABA_BRs) are broadly expressed throughout the central nervous system where they play an important role in regulating neuronal excitability and synaptic transmission. GABA_BRs are G protein-coupled receptors that mediate slow and sustained inhibitory actions via modulation of several downstream effector enzymes and ion channels. GABA_BRs are obligate heterodimers that associate with diverse arrays of proteins to form modular complexes that carry out distinct physiological functions. GABA_BR-dependent signaling is fine-tuned and regulated through a multitude of mechanisms that are relevant to physiological and pathophysiological states. This review summarizes the current knowledge on GABA_BR signal transduction and discusses key factors that influence the strength and sensitivity of GABA_BR-dependent signaling in neurons.

Participation of GABAB receptors in cortical postictal excitability in immature rats

Arrest of seizures is due to an active inhibition and is followed in mature brain by period of refractoriness markedly present one min after the end of seizures. To study changes in cortical excitability after epileptic seizures we used electrical stimulation of sensorimotor cortical area in immature rats - 25-day-old ones with mature postictal refractoriness and 12-day-old where postictal potentiation of afterdischarges (ADs) is present instead of refractoriness at one minute after the end of the conditioning AD. GABA_B receptor antagonist CGP35348 was found to partly suppress postictal refractoriness. In present study not only an antagonist CGP46381 (3 and 10mg/kg i.p.) but also positive allosteric modulator of GABA_B receptors CGP7930 (20 and 40mg/kg i.p.) were used to study the role of GABA_B receptors in both age groups. They were injected immediately after testing AD and 10min later the two stimulations were repeated. CGP46381 partly antagonized postictal refractoriness in 25-day-old rats but did not significantly affect ADs in 12-day-old animals. CGP7930 did not significantly change ADs duration in either age group. GABAB receptors participate in mechanism of postictal refractoriness but did not play an important role in 12-day-old rats where potentiation instead of refractoriness is present.

Determination of μ-, δ- and κ-opioid receptors in forebrain cortex of rats exposed to morphine for 10 days: Comparison with animals after 20 days of morphine withdrawal

BACKGROUND

Chronic exposure of mammalian organism to morphine results in adaption to persistent high opioid tone through homeostatic adjustments. Our previous results indicated that in the frontal brain cortex (FBC) of rats exposed to morphine for 10 days, such a compensatory adjustment was detected as large up-regulation of adenylylcyclases I (8-fold) and II (2.5-fold). The other isoforms of AC (III-IX) were unchanged. Importantly, the increase of ACI and ACII was reversible as it disappeared after 20 days of morphine withdrawal. Changes of down-stream signaling molecules such as G proteins and adenylylcyclases should respond to and be preceded by primary changes proceeding at receptor level. Therefore in our present work, we addressed the problem of reversibility of the long-term morphine effects on μ-, δ- and κ-OR protein levels in FBC.

METHODS

Rats were exposed to increasing doses of morphine (10-40 mg/kg) for 10 days and sacrificed either 24 h (group +M10) or 20 days (group +M10/-M20) after the last dose of morphine in parallel with control animals (groups -M10 and -M10/-M20). Post-nuclear supernatant (PNS) fraction was prepared from forebrain cortex, resolved by 1D-SDS-PAGE under non-dissociated (-DTT) and dissociated (+DTT) conditions, and analyzed for the content of μ-, δ- and κ-OR by immunoblotting with C- and N-terminus oriented antibodies.

RESULTS

Significant down-regulation of δ-OR form exhibiting Mw ≈ 60 kDa was detected in PNS prepared from both (+M10) and (+M10/-M20) rats. However, the total immunoblot signals of μ-, δ- and κ-OR, respectively, were unchanged. Plasma membrane marker Na, K-ATPase, actin and GAPDH were unaffected by morphine in both types of PNS. Membrane-domain marker caveolin-1 and cholesterol level increased in (+M10) rats and this increase was reversed back to control level in (+M10/-M20) rats.

CONCLUSIONS

In FBC, prolonged exposure of rats to morphine results in minor (δ-OR) or no change (μ- and κ-OR) of opioid receptor content. The reversible increases of caveolin-1 and cholesterol levels suggest participation of membrane domains in compensatory responses during opioid withdrawal.

GENERAL SIGNIFICANCE

Analysis of reversibility of morphine effect on mammalian brain.