Effects of GABAergic agents on anesthesia induced by halothane, isoflurane, and thiamylal in mice

Neurochemical Markers in the Mammalian Brain: Structure, Roles in Synaptic Communication, and Pharmacological Relevance

BACKGROUND

Knowledge of molecular marker (typically protein or mRNA) expression in neural systems can provide insight to the chemical blueprint of signal processing and transmission, assist in tracking developmental or pathological progressions, and yield key information regarding potential medicinal targets. These markers are particularly relevant in the mammalian brain in the light of its unsurpassed cellular diversity. Accordingly, molecular expression profiling is rapidly becoming a major approach to classify neuron types. Despite a profusion of research, however, the biological functions of molecular markers commonly used to distinguish neuron types remain incompletely understood. Furthermore, most molecular markers of mammalian neuron types are also present in other organs, therefore complicating considerations of their potential pharmacological interactions.

OBJECTIVE

Here, we survey 15 prominent neurochemical markers from five categories, namely membrane transporters, calcium-binding proteins, neuropeptides, receptors, and extracellular matrix proteins, explaining their relation and relevance to synaptic communication.

METHOD

For each marker, we summarize fundamental structural features, cellular functionality, distributions within and outside the brain, as well as known drug effectors and mechanisms of action.

CONCLUSION

This essential primer thus links together the cellular complexity of the brain, the chemical properties of key molecular players in neurotransmission, and possible biomedical opportunities.

Anesthetic management of a parturient with Stiff person syndrome for urgent cesarean delivery

Stiff person syndrome is a rare neurologic disorder with an estimated incidence of 1:1000000. The underlying pathophysiology is truncal and proximal limb muscle stiffness resulting from continuous co-contracture of agonist and antagonist muscle groups concomitant with superimposed episodic muscle spasms. Loss of gamma-aminobutyric acid-mediated inhibition creates chronic excitation manifested by tonic agonist-antagonist muscle contraction. To date, only three case reports referred indirectly to the anesthetic management of parturients with Stiff person syndrome. The authors describe their management of a parturient with Stiff person syndrome who underwent urgent cesarean delivery under epidural anesthesia.

Multiple anesthetics for a patient with stiff-person syndrome

Stiff-person syndrome is a progressive disease of muscle rigidity and spasticity due to a deficiency in the production of γ-aminobutyric acid. Because of the rarity of the condition, little is known about effects of anesthesia on patients with stiff-person syndrome. This report describes the clinical course for a single patient with stiff-person syndrome who received general anesthesia on 3 separate occasions. Her anesthetics included use of both neuromuscular blockade and volatile agents. Unlike several previous reports regarding anesthesia and stiff-person syndrome, the postoperative period for this patient did not require prolonged intubation or result in any residual weakness.

Age-dependent changes in the susceptibility to thiopental anesthesia in mice: analysis of the relationship to the functional expression of GABA transporter

The potency of anesthetics changes during development, probably due not only to pharmacokinetic factors such as differential distribution and/or metabolism, but also to pharmacodynamic factors such as changes to the GABAergic system in the brain. To explore the latter mechanism, we focused on the GABA transporter (GAT), the uptake system for GABA, which participates in the synaptic clearance of GABA. Thiopental-induced anesthesia, as assessed by the onset and duration of loss of the righting reflex, was more pronounced in 3-week-old mice than in 7-week-old mice. Both NO-711 and SKF89976A, selective GAT-1 inhibitors, significantly enhanced the anesthesia in the 7-week-old but not in the 3-week-old mice. In synaptosomes prepared from the cerebral cortex, the kinetics of GABA transport was similar between the two age groups, as assessed by [(3)H]GABA uptake assay. In addition, expression of GAT mRNA was similar between the two age groups, as assessed by quantitative RT-PCR. Thiopental reduced [(3)H]GABA uptake only at high concentrations in a similar manner at both ages. Conversely, the ability of SKF89976A to inhibit [(3)H]GABA uptake was greater in the 7-week-old mice than in the 3-week-old mice. Based on these results, GAT seems unlikely to contribute to the greater susceptibility to thiopental anesthesia in 3-week-old mice, while the increased ability of GABA uptake inhibitors to enhance thiopental-induced anesthesia in 7-week-old mice is at least partly due to higher sensitivity of GAT to the inhibitors.

Type B GABA receptors contribute to the restoration of balance during vestibular compensation in mice

Following unilateral vestibular damage (UVD), vestibular compensation restores both static and dynamic vestibular reflexes. The cerebellar cortex provides powerful GABAergic inhibitory input to the vestibular nuclei which is necessary for compensation. Metabotropic GABA type B (GABA(B)) receptors in the vestibular nuclei are thought to be involved. However, the contribution of GABA(B) receptors may differ between static and dynamic compensation. We tested static and dynamic postural reflexes and gait in young mice, while they compensated for UVD caused by injection of air into the vestibular labyrinth. The effects of an agonist (baclofen), an antagonist (CGP56433A) and a positive allosteric modulator (CGP7930) of the GABA(B) receptor were evaluated during compensation. Static postural reflexes recovered very rapidly in our model, and baclofen slightly accelerated recovery. However, CGP56433A significantly impaired static compensation. Dynamic reflexes were evaluated by balance-beam performance and by gait; both showed significant decrements following UVD and performance improved over the next 2 days. Both CGP56433A and baclofen temporarily impaired the ability to walk on a balance beam after UVD. Two days later, there were no longer any significant effects of drug treatments on balance-beam performance. Baclofen slightly accelerated the recovery of stride length on a flat surface, but CGP7930 worsened the gait impairment following UVD. Using immunohistochemistry, we confirmed that GABA(B) receptors are abundantly expressed on the vestibulospinal neurons of Deiters in mice. Our results suggest that GABA(B) receptors contribute to the compensation of static vestibular reflexes following unilateral peripheral damage. We also conclude that impairment of the first stage of compensation, static recovery, does not necessarily result in an impairment of dynamic recovery in the long term.

Anesthetic management of a patient with stiff person syndrome

We describe the successful anesthetic management of a patient with stiff-person syndrome (SPS) undergoing a right inguinal hernia repair, using a somatic paravertebral block supplemented with conscious sedation. We also present the implications of general anesthesia in patients with SPS. The use of regional anesthetic techniques in patients with SPS has the advantage of avoiding exposure to muscle relaxants. The use of general anesthesia in patients with SPS carries the risk of postoperative hypotonia due to enhancement of gamma-aminobutyric acid action on synaptic transmission by drugs that have a gamma-aminobutyric acid agonistic action.

Isoflurane exerts antinociceptive and hypnotic properties at all ages in Fischer rats

BACKGROUND

Some anaesthetic agents exhibit an age-dependent analgesic effect, for example nitrous oxide, which is ineffective in newborn rats. We investigated whether a similar time dependency existed for the responses to the volatile anaesthetic isoflurane.

METHODS

The analgesic and hypnotic properties of isoflurane at various ages was assessed using four cohorts of Fischer rats aged approximately 7, 16, and 28 days and adults (11-12 weeks old). Intraplantar administration of formalin mimicked inflammatory pain, and its effects were assessed using immunohistochemical (c-Fos staining) and behavioural paradigms. The hypnotic properties of isoflurane were assessed using loss of righting reflex.

RESULTS

Formalin administration produced a typical nociceptive response observed both behaviourally and immunohistochemically in all age groups; these nociceptive responses were significantly attenuated by isoflurane 0.5% at each age (P<0.05). Interestingly 7-day-old animals showed a significantly more potent hypnotic response than older animals (P<0.01): with adult rats being most resistant to isoflurane induced hypnosis (P<0.05).

CONCLUSION

In contrast to nitrous oxide, isoflurane is an effective antinociceptive agent in neonatal rats. If the data can be extrapolated to clinical scenarios these results suggest that isoflurane may be analgesic in newborns as well as adult humans. In addition, isoflurane is a potent hypnotic, especially in the very young, which is in contrast to the neonate's relative resistance to anaesthesia as assessed by minimum alveolar concentration.

Transporter-mediated effects of diclofenamic acid and its ascorbyl pro-drug in the in vivo neurotropic activity of ascorbyl nipecotic acid conjugate

Continuing our studies on SVCT2 ascorbic acid (AA) transporter-mediated drug delivery of neurotropic agents, we have now investigated the in vitro intracellular uptake of Diclofenac (Diclo) and its conjugate (AA-Diclo), both characterized by high affinity for the SVCT2 transporter. We have also investigated the in vivo uptake mechanism of AA-conjugate of Nipecotic acid (AA-Nipec) and the implication of the transporter-mediated effects of Diclo and AA-Diclo. Diclo resulted as a noncompetitive inhibitor of AA transport, but also showed a sodium-dependent and ascorbate-independent uptake, thus implying the possible involvement of specific transporters in the delivery to the brain of Diclo. This result opens a perspective in the discovery of new strategies in the targeting of this drug to the brain. Inhibitory effects of Diclo and AA-Diclo on the SVCT2 transporter were used to study anticonvulsant effects of AA-Nipec, confirming our hypothesis of an SVCT2-mediated transport in its neurotropic activity. AA-Diclo stability has been also investigated: it is hydrolyzed following a first-order kinetics in buffer, plasma (t(1/2) at about 10 h) and whole blood (t(1/2) at about 3 h), suggesting AA-Diclo as a potential candidate to enhance the short half-life of Diclo in vivo.

Stiff person syndrome and anesthesia: case report

IMPLICATIONS

This case report describes the successful perioperative management of a patient with a rare and disabling neurologic disorder, the stiff person syndrome. The patient had a delayed emergence despite apparent full reversal of neuromuscular blockade. We suggest an interaction between the GABAergic effects of baclofen and volatile anesthetics as a possible cause.