By H. Torn. Clayton College and State University. 2018.
These clinical problems include postoperative ileus (bowel paral- ysis following its surgical manipulation) and urinary retention (bladder atony generic 120mg starlix free shipping, either postoperatively or secondary to spinal cord injury [the so-called neurogenic bladder]) discount starlix 120mg on-line. A heart attack (especially one affecting the inferior wall of the heart) may depress the electrical system of the heart purchase starlix 120 mg with amex, impairing cardiac output order starlix 120 mg with visa. The judicious parenteral use of atropine or some other antimuscarinic agent may be of value in increasing the heart rate. Antimuscarinics are widely used for gastrointestinal and genitourinary indications. In the treatment of simple traveler’s diarrhoea (or other mild, self-limited gastrointestinal hypermotility conditions) antimuscarinics provide rapid relief; frequently they are combined with an opioid drug (see chapter 5), which has an additive antiperistaltic effect on bowel motility. Atropine has been used to treat urinary urgency associated with bladder inflammation; oxybutynin (4. Finally, anticholinergics have been used in the treatment of Parkinson’s disease (section 4. The oldest anticholinergics are the tropane alkaloids of Atropa belladonna (night- shade). Atropine is the racemate of (−)hyosciamine, whereas scopolamine has an epoxide ring. In large doses, all of these anticholinergic agents have central excitatory and hallucinogenic effects and were prominent in medieval “witches’ brews. Although ganglia are, functionally, normal receptors, they probably differ structurally from the receptors at the neuromuscular endplate, and show different accessibility. Ganglionic and neuromuscular blocking agents are therefore two structurally different groups of anticholinergic drugs. However, because none of these compounds can dis- tinguish sympathetic from parasympathetic ganglia, they have numerous side effects. Consequently, they have largely been replaced by the more selective β-adrenergic blocking agents. They are capable of relax- ing the abdominal muscles without the use of deep anesthesia, and make surgery much easier for both the surgeon and the patient. These agents were developed through the study of curare, the arrow poison of South American Indians. Crude curare contains a number of isoquinoline and indole alkaloids, the best known of which is tubocurarine (3. A similarly rigid, large molecule is the syn- thetic steroid derivative pancuronium (4. These were discovered through mim- icking the N+–N+ distance described above with aliphatic compounds. It has a short, self-limiting action since it is easily hydrolyzed by serum cholinesterase. The structure–activity relationships of neuromuscular blocking agents are instruc- tive. The most interesting aspect of these correlations is that between the N+–N+ dis- tance and the receptor structure. As the number of atoms between the -onium groups is increased beyond 10, the activity decreases until a second peak is reached at around the 16-atom distance (hexacarbacholine (4. It is not necessarily the N+–N+ distance that is essential; any induced positive charge will be appropriate. The induced charge distance increases to about 2 nm, and the hexamethonium derivative is therefore inactive as a ganglionic blocker but becomes a very effective curarizing agent. It is interesting to note that lower invertebrates (cladocerans, annelid worms, rotifers) are more sensitive to compounds with an N+–N+ distance of 16 than those with an N+–N+ distance of 10, whereas in animals of phylogenetically higher taxa, such as mammals, this sensitivity is reversed. In addition, the neuromuscular site can accommodate not only compounds with an N+–N+ distance of 10 atoms but also those with an N−–N+ distance of 16 atoms. Clinically, Alzheimer’s is characterized by loss of short-term memory, impaired cognition, decline in intellectual function, and decreased ability to carry out the activities of daily life. At the cellular level, the histopathology of Alzheimer’s reveals “plaques and tangles,” where plaques are composed of β-amyloid peptide and tangles are composed primarily of phosphorylated tau protein. This degeneration subsequently leads to progressive regression of memory and learned functions and to the other symptoms of Alzheimer’s disease. Therapeutic approaches to Alzheimer’s disease initially targeted the cholinergic systems. In early work, drug treatment with either choline replacement or cholinergic agonists was of negligible value. By inhibiting the catabolic break- down of acetylcholine, these agents prolong the effective half-life of acetylcholine as a neurotransmitter, thereby alleviating some of the symptoms of the disease. About 70% of people show time-limited improvement in their memory when on these agents. Unfortunately, these agents are only symptomatic and do not treat the underlying cause, namely the accumulation of neurotoxic β-amyloid aggregates. Future research in Alzheimer’s disease is targeting other potential receptors, such as by blocking the synthesis of β-amyloid by inhibiting the secretase enzyme system involved in the con- version of amyloid precuror protein to β-amyloid, or by binding to β-amyloid and inhibiting its aggregation into a neurotoxic form.
Inhibition of glutamate release was thought to be the mode of action of lamotrigine discount 120mg starlix with mastercard. But it now seems likely that the actual block of sodium channels is its primary action (see later) buy starlix 120mg otc. Generally a reduction in monoamine function facilitates experimentally induced seizures (see Meldrum 1989) while increasing it reduces seizure susceptibility cheap 120 mg starlix fast delivery. The variability of the procedures used and results obtained do not justify more detailed analysis here buy starlix 120mg with visa. Some mention should perhaps be made of dopamine, considering its role in the control of motor function. How the drugs currently available for the treatment of epilepsy may utilise these mechanisms will now be considered. The decision on which drug to use depends not only on their proven efficacy in a particular type of epilepsy (some drugs are inactive in certain forms) but also what side-effects they have Ð many are sedative Ð how they interact with other drugs and how often they need to be taken. Compliance is a problem over a long period if dosing is required more than once a day. Only the latter have been developed chemically to modify the known synaptic function of the amino acids. It was largely replaced in 1932 by phenytoin for the management of tonic±clonic seizures and partial and secondary epilepsy. These remained, apart from the introduction of the benzodiazepines, the mainstay of therapy until the last decade. They were introduced solely on their ability to control experimentally induced seizures. Studies in cultured spinal cord neurons (Macdonald and McLean 1986) have shown that concentrations of phenytoin equivalent to those occurring clinically do not affect the resting membrane potential or the shape of a single-action potential but reduce the rapid discharge induced by depolarising the neuron, while leaving the first action potential intact (Fig. It is believed to block voltage-dependent sodium channels (not those mediating the synaptic currents) after their activation, i. Currently there are no clinically useful drugs that act as glutamate receptor antagonists seizures and clinically in focal and generalised epilepsy. Also, since they act only on the inactivated channel, they will not affect normal neuronal function, which is why in the experimental study, the first action potential remains unaltered. Neither compound is of any value against absence seizures and may exacerbate them. Experimentally it has no effect on the voltage-gated sodium channels affected by phenytoin but has been reported to suppress the transient T-type calcium currents in the thalamic neurons which are the origin of the 2±3 Hz spike and wave discharge characteristic of this form of epilepsy (see Mody 1998 for detail). Since these discharges are thought to arise from oscillations in excitability induced by changes in the T-type calcium current (see section above on the origin of absence seizures), this would obviously be a neat explanation of its efficacy in that condition. Unfortunately some workers have not been able to repeat this finding at clinically equivalent concentrations and consider ethosuximide to reduce a special persistent Na channel and a Ca2-activated K channel. Note that while the structures of phenytoin and ethosuximide are similar and also close to that of phenobarbitone, they are effective in different forms of epilepsy. Phenobarbitone may be as effective as phenytoin and carbamazepine in partial and generalised tonic±clonic seizures but its other central effects such as sedation, depression, listlessness and cognitive impairment mar its usefulness. Clonazepam, a typical 1:4 benzodiazepine, is effective in absence seizures, myoclonic jerks and tonic±clonic seizures and given intravenously it attenuates status epilepticus. Each column shows the response of a spinal cord neuron in culture to four increasing directly applied current pulses (amplitude in nA given at start of each sweep. Thus they do not affect the initial response but stop neurons from maintaining the abnormal sustained discharge that would be characteristic of epileptic activity. Resting membrane potentials (Em) are shown at the bottom of each column and amplitude (mV) and time (ms) at the bottom right with phenobarbitone, can precipitate seizures. Although still used in refractory myo- clonic epilepsy, when its depressant effect on the spinal cord may be significant, clonazepam, like phenobarbitone, is rarely used now, but the more recently introduced 1:5 benzodiazepine clobazam is quite often used as an adjunct (not in the United States). While there is some belief and evidence that clonazepam and clobazam are more effective than other benzodiazepines as anticonvulsants nothing is known specifically about their modes of action that supports this view. This is achieved, however, over a slower time-course than its anti-seizure effect, especially experimentally, which is now thought to be due to its phenytoin-like, use-dependent block of sodium channels. A programme of synthetic chemistry to manipulate the structure of the anti-folate compound pyri- methium to try to replace that property with anticonvulsant activity resulted in the synthesis of lamotrigine. It now appears that its primary effect is prolonging the inactivation of sodium channels in a use-dependent manner much like phenytoin, although in a recent study of intra- cellularly recorded activity of striatal neurons in the rat corticostriatal slice preparation some differences emerged. A worrying intramyelinic oedema in rat nerves has fortunately not been seen in humans or primates. Attaching nipecotic acid to a lipophilic component to increase brain penetration resulted in tiagabine. It does not appear to affect sodium or calcium channels even though experimentally chronic dosing blocks repetitive neuronal firing.
This is a great advantage and has allowed electrophysiological techniques to be used to study ion channel activation and drug block of ion channels in great detail order 120 mg starlix amex. The first physically plausible mechanism for receptor activation was proposed by del Castillo and Katz (1957) buy 120mg starlix with amex. However generic starlix 120mg amex, recent results suggest that G- protein-coupled receptors (and potentially other receptors) can exist in the active state in the absence of agonist purchase starlix 120mg on-line. These constitutively active receptors give rise to interesting new predictions for the shape of the dose±response curve and an alternative interpretation for the difference between agonists, partial agonists and antagonists (Lefkowitz et al. If L combines with R* there will be an increase in active receptors and so L will behave as a conventional agonist. Where L has equal affinity for R and R*, it will not affect the fraction of receptors in the active state. However, it will reduce the binding of either a conventional or an inverse agonist, and so will behave as an antagonist. Del Castillo, J and Katz, B (1957) Interaction at endplate receptors between different choline derivatives. Current status of the nomenclature for nicotinic acetylcholine receptors and their subunits. Sautel, M and Milligan, G(2000) Molecular manipulation of G-protein-coupled receptors: a new avenue into drug discovery. Unwin, N (2000) Nicotinic acetylcholine receptor and the structural basis of fast synaptic transmission. Vernier, P, Cardinaud, B, Valdenaire, O, Philippe, H and Vincent, J-D (1995) An evolutionary view of drug±receptor interaction: the bioamine receptor family. This wave of excitation causes the opening of voltage-gated Ca2-channels or mobilisation of Ca2 from intracellular stores (e. As a result, there is a phasic increase in free intracellular Ca2, probably to a concentration of about 0. The subsequent fusion of neurotransmitter storage vesicles with the axolemma, together with the extrusion of their contents into the synapse, is thought to take about 100±200 ms; this cascade is therefore fast enough to effect rapid signalling between neurons. While this chapter is concerned primarily with the neurochemical mechanisms which bring about and control impulse-evoked release of neurotransmitter, some of the methods used to measure transmitter release are described first. This is because important findings have emerged from studies of the effects of nerve stimulation on gross changes in transmitter release and intraneuronal stores. The actual processes that link neuronal excitation and release of transmitter from nerve terminals have been studied only relatively recently. The neurochemical basis of this stimulus±secretion coupling, which is still not fully understood, is described next. The final sections will deal with evidence that, under certain conditions, appreciable amounts of transmitter can be released through Ca2-independent mechanisms which do not depend on neuronal activation. However, under resting conditions, the transmitter content of any given organ or brain region is remarkably constant. The store of classical transmitters (monoamines and acetylcho- line)in nerve terminals is rarely depleted by physiologically relevant rates of neuronal stimulation. Although this approach is rarely used nowadays, it is outlined here because it uncovered some important findings which are relevant to current studies of the regulation of transmitter release. Transmitter synthesis can be monitored by administering [3H]- or [14C]-labelled precursors in vivo; these are eventually taken up by neurons and converted into radiolabelled product (the transmitter). The rate of accumulation of the radiolabelled transmitter can be used to estimate its synthesis rate. One limitation of this method is that the specific activity of the radiolabel is progressively diluted as the radiolabelled transmitter is released from neurons and replaced by that derived from unlabelled substrate. This method also assumes that there is no compartmentalisation of the terminal stores, yet there is ample evidence that newly synthesised acetylcholine and monoamines are preferentially released. An alternative approach is to monitor the rate at which the store of neurotransmitter is depleted after inhibition of its synthesis (Fig. It is already evident that the turnover rate of a transmitter is only a crude measure of its release rate. Further limitations are that there is appreciable intraneuronal meta- bolism of some neurotransmitters: notably, the monoamines. Another problem, again affecting monoamines, is that some of the released neurotransmitter is taken back into the nerve terminals and recycled. Despite these drawbacks, studies of turnover rates uncovered some important features of transmitter release. This technique is still widely used to study drug-induced changes in noradrenaline release from sympathetic neurons and the adrenal medulla. Monoamines, for instance, are rapidly sequestered by uptake into neuronal and non-neuronal tissue whereas other transmitters, such as acetylcholine, are metabolised extensively within the synapse.