1. The rostral ventromedial medulla (RVM) participates in the modulation of nociceptive transmission by spinal cord neurons. Previous anatomic studies have demonstrated that RVM neurons project to laminae I, II, and V of the dorsal horn; laminae VII and VIII of the intermediate and ventral horns; the intermediolateral column; and lamina X. The RVM contains at least three physiologically defined classes of neurons, two of which, the ON and the OFF cells, have been implicated in nociceptive modulation. Because these cell classes are intermingled in the RVM, it has not been possible to determine the spinal laminar projection targets of ON and OFF cells by anatomic methods. Therefore in the current study we employed antidromic microstimulation methods to determine the laminar projections of two of the three classes of RVM neurons, the ON and the OFF cells. 2. In lightly anesthetized (with methohexital sodium) rats, single-unit extracellular recordings were made from 48 RVM neurons that were physiologically characterized as ON (30) or OFF (18) cells. The recording locations of 45 of these neurons were recovered. Thirty-seven were found in the nucleus raphe magnus and eight were located near its dorsal and lateral borders. 3. Thirty-two physiologically identified RVM neurons (18 ON and 14 OFF cells) were antidromically activated from the cervical spinal cord using a monopolar stimulating electrode. The stimulating electrode was moved systematically in the white matter until antidromic activation could be produced with currents of less than or equal to 20 mu A (6.1 +/- 0.7 mu A, mean +/- SE). The points from which minimum currents were required to antidromically activate the neurons were located mainly in the ipsilateral dorsolateral funiculus (DLF) (27 of 32). In a few cases, lowest antidromic threshold currents were found near the border between the DLF and ventrolateral funiculus (VLF) or, rarely, in the VLF itself. In these cases, the cell recordings were found to be near the dorsal boundary of the RVM. 4. While one electrode was used to stimulate the parent axon in the lateral funiculus, a second was used to explore the gray matter for the presence of collateral branches. The identification of a branch was initially determined by an increase in antidromic latency. At the same rostrocaudal plane of the spinal cord, stimulation of the DLF induced an antidromic spike that invaded the neuron earlier than the antidromic spike elicited by stimulation in the gray matter. Collateral branches were confirmed by establishing that the location of the minimum threshold point for antidromic activation of the neurons from the second electrode was in the gray matter, that the minimum current required to antidromically activate the neuron from that point was too low to activate the parent axon in the DLF, and that a collision occurred between the spikes induced by the two stimulating electrodes. 5. In 17 cases, physiologically identified RVM neurons (10 ON and 7 OFF cells) were antidromically activated from the gray matter of the cervical spinal cord using a current of 8.4 +/- 2.1 (SE) mu A. Minimum threshold points for antidromic activation were found in laminae I-II (3 ON and 4 OFF cells), lamina V (5 ON and 6 OFF cells), and regions ventral to the lateral reticulated area (3 ON and 2 OFF cells) of the gray matter. As indicated by these numbers, some neurons were antidromically activated from more than one gray matter region. In general, all OFF cells and 9 of 10 ON cells were antidromically activated from low threshold points in either laminae I-II or lamina V. 6. In six cases, neurons were activated from separate points located in two or three different laminae of the gray matter. Three OFF cells were activated from laminae I-II and V, one OFF cell and one ON cell were activated from lamina V and from more ventral points, and one ON cell was activated from laminae I-II and from points ventral to lamina V. Finally, one of these neurons, an OFF cell, was activated from all three regions of the gray matter. 7. Although no systematic attempts were made to map the parent axons of RVM neurons at each segmental level, 25 parent axons were mapped in the rostral and caudal areas of the cervical enlargement. In all of these cases, neurons activated from points in the DLF at the level of C-5 were also activated from the DLF at the level of C-8-T-1. These data suggest that individual ON and OFF cells innervate neurons in several levels of the cervical spinal cord and in segments caudal to it. 8. Because mapping all branches of each neuron was not the focus of this work, we documented only one neuron that issued branches at two different rostrocaudal levels. However, the presence of branches at each examined cervical segment (segments C-5-C-8 and T-1 contained 1, 3, 7, 4, and 2 collateral branches, respectively) and the average rostrocaudal distance that was explored before finding a branch of an ON (1.2 mm) or an OFF (2.2 mm) cell suggests that individual RVM neurons issue collateral branches at more than one rostrocaudal level of the cervical spinal cord. 9. The mean conduction velocity of ON cell axons was 16.4 +/- 2.8 (SE) m/s in the DLF and 1.1 +/- 0.24 (SE) m/s in the gray matter. The mean conduction velocity of OFF cell axons was 11 +/- 2.7 m/s in the DLF and 0.6 +/- 0.15 m/s in the gray matter. Thus the conduction velocities of the branches were an order of magnitude slower than those of their parent axons. The differences found between the conduction velocities of ON and OFF axons were not significant in the DLF or the gray matter. 10. These findings indicate that RVM ON and OFF cells that project to the spinal cord through the DLF issue collateral branches to laminae I-II and V of the cervical dorsal horn. Because these laminae contain most of the nociceptive neurons that project to supraspinal sites, this result supports the hypothesis that RVM ON and OFF neurons modulate nociceptive transmission by acting directly on nociceptive neurons in the dorsal horn.
