It has been suggested that serotonin (5-hydroxytryptamine) type-4 (5-HT4) receptors modulate the sensitivity of intrinsic afferents of the intestinal mucosa. We studied the involvement of 5-HT4receptors in the modulation of extrinsic afferent sensitivity of the intestinal wall. During distension ramps, mechanoreceptive rectal afferents in sacral dorsal roots were examined in decerebrate anaesthesia-free cats using the selective 5-HT4receptor partial agonist, tegaserod (HTF 919), and the 5-HT4receptor antagonist, SB 203186. The static discharge rate of the afferents evoked by rectal distension decreased after intravenous (i.v.) administration of tegaserod at intraluminal pressures above 30 mmHg, with the most effective reduction occurring at 50 mmHg. The effect was dose-dependent, with maximal reduction occurring at 1.2 mg kg-1 bodyweight, and could be partly reversed by i.v. administration of SB 203186. Tegaserod did not alter the pressure-volume relationship (compliance) of the rectum. It is tentatively concluded that 5-HT4receptor activation has an inhibitory effect on intramural mechanoreceptors in the cat's rectum. Our results are in line with the observation that tegaserod relieves the sensory symptoms of patients suffering from irritable bowel syndrome.

The afferent input from the rectum to the central nervous system (CNS) has yet to be thoroughly characterized. The characteristics of mechanoreceptive rectal afferents have been studied in unanaesthetized decerebrate cats. Following lumbo-sacral laminectomy, single-unit activity (occasionally multi-unit activity) was recorded from centrally cut filaments of the sacral dorsal roots (predominantly S2), while a balloon was inflated in the rectum. Starting from their background activities (mean 15.1 imp sec-1, SD 7.6 imp sec-1), afferent discharge rate increased with increasing balloon pressure (mean threshold 6.3 mmHg, SD 3.6 mmHg). The dependence of firing rate on intrarectal pressure began to flatten out at 25 mmHg (mean; SD 10 mmHg). For 22 out of 29 units (76%) complete saturation occurred at 35 mmHg (mean; SD 15 mmHg) with a maximum discharge rate of 31 imp sec-1 (mean; SD 12.6 imp sec-1). In a number of recording sessions, cyclical rectal contractions were observed. In these cases, changes in firing of the units were closely related to changes in intrarectal pressure. Pressure-related afferent activity could be enhanced by parasympathomimetic drugs which augmented rectal contractions. We conclude that sacral dorsal roots contain afferents from low-threshold mechanoreceptors located in the rectal wall, and that these afferents monitor the filling state and contraction level of the rectum.

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We studied the development of implicit and of verbally declared knowledge for normal human subjects who learned an unfamiliar motor task in one learning session. The exploratory nature of motor learning and a special period for optimizing skill were followed in real time. Subjects understood the goal for task success, but they had to learn a motor strategy of what pattern of serial movements to make and the tactics of bow much to scale their amplitudes and timing. We compared the time course for acquiring tactical skill with that for acquiring knowledge of strategy and of tactics, and their necessary cues. Implicit and declarative knowledge were distinguished from one another by correlating subjects' verbal self-reports with movement kinematics and their results. Implicit generation of the correct strategy and of the tactics developed in an exploratory manner from the beginning of the learning session. Implicit strategy learning soon gave way to conscious efforts, but tactical learn.ing remained implicit until its first unambiguous verbal declaration 5Corresponding author.(with one exception). First strategy declarations were voiced before those for tactics, during trial-and-error learning that did not require task success, and referred to reversing the direction of hand movements (one exception). In contrast, first declarations of tactics almost always required actual or imminent success, referred to when direction was to be reversed, and it was achieved near the top of a sigmoid learning curve that rose to tactical skill (with one exception). During the sigmoid rise, movement amplitudes and timing were optimized in a distinct manner, although tactics usually adapted thereafter to movements of more moderate speed that could still be successful.

The interaction between dopaminergic and cholinergic systems in the mammalian central nervous system, which is thought to have important implications in the pathophysiology of major extrapyramidal disorders, has never been adequately demonstrated in vivo. Renshaw cell burst responses to single electrical shocks to lumbar ventral roots in spinalized and decerebrated rats were studied. In this monosynaptic cholinergic pathway, apomorphine, a dopaminergic receptor agonist, inhibited whereas the D2-antagonist sulpiride facilitated the burst responses. The mutual antagonism of the two drugs and the depression coupled with the faster decay of post-tetanic potentiation of Renshaw cells by apomorphine demonstrate the involvement of presynaptic D2-receptors through which dopamine can modulate acetylcholine-mediated central synaptic transmission in vivo. The study also provides further evidence for the involvement of the spinal cord in extrapyramidal disorders.

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The static discharge rate of Renshaw cells (studied in deafferented, intercollicularly decerebrate cats) has a nonlinear dependence on the frequency of trains of stimulus impulses to alpha-motor axons in the ventral root. This dependence is well described by a rectangular hyperbola that approaches saturation with increasing stimulus frequency. The tendency to saturate is independent of the number of motor axons exciting a Renshaw cell. On average, the stimulus frequency at which the discharge rate reaches half its saturation value lies between 10 and 15 Hz. The effect of Renshaw cell activity -- measured as the antidromic inhibition of individual alpha-motoneurons -- reflects the forms of the static frequency characteristics. An electric circuit analog of the Renshaw cell membrane is presented which serves to explain the qualitative features of the static input-output relations; the nonlinearity is the result of synapses with linear properties acting together at the cell membrane.

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