Stimulation of the mesencephalic locomotor region elicits controlled swimming in semi-intact lampreys Mikhail G. Sirota, 2 Gonzalo Viana Di Prisco 2 and Re Âjean Dubuc 1,2 1 De Âpartement de Kinanthropologie, Universite  du Que Âbec a Á Montre Âal, C.P. 8888 succ. Centre-ville, Montre Âal, Que Âbec, Canada, H3C 3P8 2 Centre de Recherche en Sciences Neurologiques, Universite  de Montre Âal, C.P. 6128 succ. Centre-ville, Montre Âal, Que Âbec, Canada Keywords: brainstem, locomotion, mesencephalon, microstimulation, MLR, reticulospinal Abstract The role of the mesencephalic locomotor region (MLR) in initiating and controlling the power of swimming was studied in semi-intact preparations of larval and adult sea lampreys. The brain and the rostral portion of the spinal cord were exposed in vitro, while the intactcaudaltwo-thirdsofthebodyswamfreelyintheRinger's-containingchamber.Electricalmicrostimulation(2±10Hz;0.1±5.0 mA) withinasmallperiventricularregioninthecaudalmesencephalonelicitedwell-coordinatedandcontrolledswimmingthatbeganwithin a few seconds after the onset of stimulation and lasted throughout the stimulation period. Swimming stopped several seconds after the end of stimulation. The power of swimming, expressed by the strength of the muscle contractions and the frequency and the amplitude of the lateral displacement of the body or tail, increased as the intensity or frequency of the stimulating current were increased. Micro-injection of AMPA, an excitatory amino acid agonist, into the MLR also elicited active swimming. Electrical stimulation of the MLR elicited large EPSPs in reticulospinal neurons (RS) of the middle rhombencephalic reticular nucleus (MRRN), whichalsodisplayedrhythmicactivityduringswimming.Theretrogradetracercobalt-lysinewasinjectedintotheMRRNandneurons (dia. 10±20 mm) were labelled in the MLR, indicating that this region projects to the rhombencephalic reticular formation. Taken together, the present results indicate that, as higher vertebrates, lampreys possess a speci®c mesencephalic region that controls locomotion, and the effects onto the spinal cord are relayed by brainstem RS neurons. Introduction The mesencephalic locomotor region (MLR) was originally de®ned in the cat as a small region located in the caudal mesencephalon, where tonic electrical stimulation was able to: (i) activate the body postural system; (ii) initiate locomotor activity with kinematic parameters similar to the natural ones, and which ceased when the electrical stimulation stopped; (iii) control, according to the stimul- ation strength, the power of locomotion as expressed as the frequency of the locomotor movements and the force of muscle contractions (Orlovsky etal., 1966; Shik etal., 1966; Sirota & Shik, 1973; Grillner, 1976; Shik & Orlovsky, 1976; Grillner etal., 1997). The MLR has been described not only in mammalian species (Shik etal., 1966; Orlovsky etal., 1966; Garcia-Rill etal., 1986; Garcia-Rill & Skinner, 1987), but also in lower vertebrates. Kashin etal. (1974) showed that electrical stimulation of the MLR in a restrained carp evoked alternating lateral displacements of the tail that resembled swimming movements and that the same stimulation led to active swimming in an unrestrained carp. Moreover, stimulation of the midbrain region was shown to evoke rhythmic alternating swimming- like movements of the tail and body in the decerebrate gold®sh (Fetcho & Svoboda, 1993). The neural mechanisms that underlie locomotion and body orientation in lamprey are now understood in several aspects (see Grillner etal., 1995 for a review). Reticulospinal (RS) neurons in the brainstem constitute the main descending system that activates the spinal cord locomotor networks, and these cells display plateau properties that are involved in the initiation of swimming in response to sensory inputs (Viana Di Prisco etal., 1997). Electrical micro- stimulation of speci®c brainstem areas in the in vitro brain/spinal cord lamprey preparation elicits a co-ordinated `®ctive' swimming pattern in spinal ventral roots that lasts 5±10 s (McClellan & Grillner, 1984). These areas consist of a medially located strip of tissue extending from the mesencephalon to the caudal rhombencephalon (McClellan & Grillner, 1984; McClellan, 1986). More recently, chemical stimulation of several brainstem sites was shown to elicit ®ctive locomotion (Hagevik etal., 1996). In the present experiments, we have used a semi-intact lamprey preparation (see also Currie & Ayers, 1983; Margolin etal., 1985) to study the characteristics of the active motor behaviours that are elicited by stimulation of different sites in the brainstem, and to identify the MLR of lampreys. Speci®c questions were addressed: (i) Is there a speci®c MLR in the lamprey and where is it located? (ii) Can electrical stimulation of this region initiate active swimming that can be controlled by the stimulation parameters? (iii) Are the kinematics parameters of the active swimming evoked in the semi-intact preparation similar to that seen in intact animals? (iv) Do cells from MLR project to RS neurons? Our results indicate that lampreys possess an MLR that plays a role in initiating and controlling swimming. Preliminary results have been previously reported (Sirota etal., 1995; Viana Di Prisco etal., 1995b). Correspondence: Dr Re Âjean Dubuc, at 1 De Âpartement de Kinanthropologie, as above. E-mail: dubucr@physio.umontreal.ca Received 11 May 2000, revised 15 August 2000, accepted 21 August 2000 European Journal of Neuroscience, Vol. 12, pp. 4081±4092, 2000 ã Federation of European Neuroscience Societies