Search Results

Figure 1. Origin and distribution of somatic and autonomic nerves. ^1,2 Somatic (voluntary) neurons exit all levels of the brain and spinal cord (CNS). They release acetylcholine (ACh) to activate nicotinic receptors (N_m) on skeletal muscle. Preganglionic parasympathetic neurons exit the brain and sacral spinal cord, where they synapse with ganglia near or within smooth muscle and heart. Here ACh is released and activates nicotinic receptors (N_n) on postganglionic neurons. These neurons also release ACh to activate muscarinic receptors (M) on the target tissues. Preganglionic sympathetic neurons exit the thoracic and lumbar levels (thoracolumbar) of the spinal cord and synapse with ganglia near the cord and, like the preganglionic parasympathetic fibers, release ACh to activate N_n receptors on postganglionic neurons. The most abundant of these distribute to smooth muscle and heart where they release norepinephrine (NE) to activate alpha and beta receptors (α, β). The adrenal medulla is functionally a postganglionic neuron that secrets mostly epinephrine (∼80% epinephrine and 20% norepinephrine), which arrives at the target tissues via the circulation. Also noteworthy is that some postganglionic sympathetic fibers distribute to sweat glands and release ACh where it activates muscarinic receptors (M).

Figure 4. Catecholamine synthesis. The molecular structure of catecholamines includes a catechol and an amine. During their termination, each of these moieties is a target for a specific enzyme, ie, catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO). Catecholamine synthesis proceeds in a stepwise fashion, each step driven by a specific enzyme leading to a molecular change indicated by the asterisks. In the adrenal medulla the final step in the pathway results in the synthesis of epinephrine. Sympathetic neurons do not contain the methylating enzyme and therefore cannot synthesize epinephrine. All 3 neurotransmitters are found throughout the brain, but neurons within the basal ganglia release dopamine because they lack dopamine hydroxylase.

Influence of N₂O on descending inhibitory pathways. N₂O induces release of endogenous opioid peptides (EOP) that activate opioid receptors on γ-aminobutyric acid (GABA)-ergic pontine nuclei. This pathway, in turn, activates descending noradrenergic system in the dorsal horn of the spinal cord that directly inhibits or indirectly inhibits (through a GABA interneuron) nociceptive processing at the level of the primary afferent and second-order neurons that transmit sensory signals up the ascending nociceptive pathway.

Local anesthetic action. An injected local anesthetic exists in equilibrium as a quaternary salt (BH⁺) and tertiary base (B). The proportion of each is determined by the pKa of the anesthetic and the pH of the tissue. The lipid-soluble species (B) is essential for penetration of both the epineurium and neuronal membrane. Once the molecule reaches the axoplasm of the neuron, the amine gains a hydrogen ion, and this ionized, quaternary form (BH⁺) is responsible for the actual blockade of the sodium channel. Presumably, it binds within the sodium channel near the inner surface of the neuronal membrane.

Local anesthetic action. An injected local anesthetic exists in equilibrium as a quaternary salt (BH⁺) and tertiary base (B). The proportion of each is determined by the pKa of the anesthetic and the pH of the tissue. The lipid-soluble base (B) is essential for penetration of both the epineurium and neuronal membrane. Once the molecule reaches the axoplasm of the neuron, the amine gains a hydrogen ion, and this ionized, quaternary form (BH⁺) is responsible for the actual blockade of the sodium channel. The equilibrium between (BH⁺) and (B) is determined by the pH of the tissues and the pKa of the anesthetic (pH/pKa).

Mechanism of N₂O-induced anxiolysis. N₂O is thought to cause activation of the benzodiazepine (BZ) binding site as its effects are blocked by flumazenil. This action facilitates γ-aminobutyric acid (GABA) activation of its binding site, resulting in chloride ion influx. The increased chloride ion concentration in the neuron might cause activation of calmodulin (CaM), which then activates the enzyme nitric oxide synthase (NOS). NOS converts the amino acid L-arginine (L-Arg) to L-citrulline (L-Cit) and NO, which stimulates the enzyme soluble guanylyl cyclase producing the second messenger cyclic guanosine monophosphate (cyclic GMP). The cyclic GMP, in turn, stimulates a cyclic GMP-dependent protein kinase (PKG) that leads to the anxiolytic drug effect.

Effects of lidocaine and articaine on viability of SH-SY5Y cells. (A) Expression of Na(V) in SH-SY5Y cells. Polymerase chain reaction gel showing cells expressed mRNA for both Na(V)1.2 and Na(V)1.7. Gels show bands of expected size from 3 cell preparations. “-1.2” and “-1.7” indicate lanes where reverse transcriptase was omitted from the mix for Na(V)1.2 and Na(V)1.7, respectively. Bars are 100 base pairs. (B) Example of images of SH-SY5Y cells loaded with the Live/Dead assay in response to various conditions. Cells treated for 5 minutes with 4% articaine or 2% lidocaine (both from the cartridge), washed, then loaded with the Live/Dead dye. Positive control of cells treated with 70% ethanol are shown on the top left, while untreated cells are shown on the right. Green, calcein indicating healthy cells; red, ethidium homodimer indicating compromised cells. Bar = 100 μM. (C) Quantification of Live/Dead levels from SH-SY5Y cells treated with lidocaine + 1 : 100,000 epinephrine or articaine + 1 : 100000 epinephrine from the cartridges used clinically. The reduced viability observed using lidocaine at full strength was not significant (Kruskal-Wallis 1-way analysis on ranks with Dunn's post hoc test). Articaine did not lead to cell death at any strength. Numbers along the abscissa axis indicate the percentage of drug, with 2% lidocaine and 4% articaine the full strength from the cartridge. Numbers along the ordinate represent the ratio of light excited at 488 nm versus 544 nm, normalized to the mean control for each set. *p < .001 methanol versus saline; n = 10. (D) Quantification of the Live/Dead levels from SH-SY5Y cells treated with pure lidocaine or articaine. Lidocaine increased the number of dead cells when used in pure powdered form at the highest concentration, while pure articaine did not alter cell survival. Numbers along the abscissa indicate the concentration in mM, with the highest levels of both drugs equal to the maximum level with the cartridge. Numbers along the ordinate represent the Live/Dead ratio normalized as in C. *p < .001 (methanol and 74 mM lidocaine), n = 18.

Neuronal responsiveness impaired by previous lidocaine treatment. (A) Typical baseline cytoplasmic Ca²⁺ levels in SH-SY5Y cells. (B) Mean levels of Ca²⁺ under baseline conditions (B, 5 mM K⁺) and after exposure to 50 mM K⁺ (HK) in cells exposed to 2% lidocaine, 4% articaine or control solution 30 minutes before measurements were made. Baseline Ca²⁺ levels show no significant difference between the 3 treatment groups. While depolarization with the high K⁺ solution significantly raised cellular Ca²⁺ levels in the control cells (*p =0.004) and those previously exposed to articaine (**p = .031), the response in cells previously exposed to 2% lidocaine was not significant, Student's t test, n = 15.