The role of serotonin in modulation of nociceptive reflexes

Serotonin uptake inhibitors: uses in clinical therapy and in laboratory research

Fluoxetine, zimelidine, sertraline, paroxetine, fluvoxamine, indalpine and citalopram are the selective inhibitors of serotonin uptake that have been most widely studied. Some of these compounds are or have been used clinically in the treatment of mental depression, obsessive-compulsive disorder and bulimia, and therapeutic benefit has been claimed in additional diseases as well. By blocking the membrane uptake carrier which transports serotonin from the extracellular space to inside the serotonin nerve terminals, these compounds increase extracellular concentrations of serotonin and amplify signals sent by serotonin neurons. Because serotonin neurons are widespread in the central nervous system, the functional consequences of blocking serotonin uptake are diverse, but are generally subtle. Animals treated with serotonin uptake inhibitors look normal in gross appearance, but effects such as reduced aggressive behavior, decreased food intake and altered food selection, analgesia, anticonvulsant activity, endocrine changes and neurochemical changes have been demonstrated and characterized. Serotonin uptake inhibitors have helped in revealing some dynamics of serotonin neurons; for example, when uptake is inhibited and extracellular serotonin concentration increases, presynaptic as well as postsynaptic receptors for serotonin are activated to a greater degree. A consequence of increased activation of autoreceptors on serotonin cell bodies and nerve terminals is a reduction in firing of serotonin neurons and a decrease in serotonin synthesis and release. The result is a limit on the degree to which extracellular serotonin and serotonergic neurotransmission are increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative autoradiographic mapping of serotonin 5-HT1 and 5-HT2 receptors and uptake sites in the neocortex of the rhesus monkey

The in vitro autoradiographic technique was used to characterize the distribution of serotonin 5-HT1 and 5-HT2 receptors and uptake sites in 11 cortical areas of frontal, parietal, and occipital lobes in the rhesus monkey; 5-HT1 receptors were labeled with [3H]5-HT; 5-HT2 receptors were labeled with [3H]ketanserin; and 5-HT uptake sites were labeled with [3H]citalopram. Five-HT1 and 5-HT2 receptors and 5-HT uptake sites were found in every cortical area examined with the absolute concentration of 5-HT1 receptors higher than that of 5-HT2 receptors in all areas. In eight regions of prefrontal and parietal as well as in prestriate cortex, 5-HT1 and 5-HT2 receptors had complementary distribution profiles: 5-HT1 receptors were concentrated in layers I and II and the upper strata of layer III, while 5-HT2 receptors had their highest concentration throughout layers III and IV. Only the primary motor and visual cortex had receptor distributions different from that described above. Thus, in the primary visual cortex, both 5-HT1 and 5-HT2 receptors were found in high concentration in sublayer IVc beta, though the density of 5-HT1 receptor was also high in other subdivisions of layer IV and in layers III, V, and VI. In the primary motor cortex, both receptor subtypes were concentrated in layers I and II and the upper strata of layer III. The pattern of distribution of serotonin uptake sites did not match the patterns of distribution of either 5-HT1 or 5-HT2 receptors alone; rather it approximated the combined patterns of distribution of both receptor subtypes. The complementary patterns of distribution of 5-HT1 and 5-HT2 receptors in most areas of the monkey cerebral cortex suggest that these two receptor subtypes may make differential contributions to cortical functions.

Serotonergic-catecholaminergic interactions and foot shock-induced jumping behavior in rats

The role of serotonergic neurons in foot shock-induced jumping behavior was evaluated by studying the effects of serotonin agonists and depletors under conditions of increasing activity of catecholaminergic neurons by combining different shock intensities with methamphetamine or clonidine pretreatment. Low shock (40 V) jumping was suppressed by three different kinds of serotonergic agonists: 5-hydroxytryptophan (50 and 100 mg/kg), 5-methoxydimethyltryptamine (1 and 5 mg/kg) and clomipramine (5 mg/kg) but these drugs did not suppress high shock (60 V) jumping. A depletor of brain serotonin, p-chlorophenylalanine (200 mg/kg), failed to affect the jumping induced by both shock intensities. The inhibitory effect of serotonergic agonists on low shock jumping was abolished by pretreatment with methamphetamine (1 mg/kg) or clonidine (100 micrograms/kg). Furthermore, 5-methoxydimethyltryptamine (5 mg/kg), a serotonergic postsynaptic receptor stimulant, potentiated high shock jumping in rats pretreated with methamphetamine (3 and 5 mg/kg). The results indicated that activation of serotonergic neurons has a differential effect on jumping behavior ranging from inhibitory to facilitatory, depending upon the increasing activity of catecholaminergic neurons.