Reticulospinal Neurons Receive Direct Spinobulbar Inputs During Locomotor Activity in Lamprey

Abstract

Reticulospinal neurons of the lamprey brain stem receive rhythmic input from the spinal cord during locomotor activity. The goal of the present study was to determine whether such spinal input has a direct component to reticulospinal neurons or depends on brain stem interneurons. To answer this question, an in vitro lamprey brain stem-spinal cord preparation was used with a diffusion barrier placed caudal to the obex, separating the experimental chamber into two baths. Locomotor activity was induced in the spinal cord by perfusion of d-glutamate or N-methyl-dl-aspartate into the spinal cord bath. The brain stem bath was first perfused with normal Ringer solution followed by a high-Ca(2+), -Mg(2+) solution, which reduced polysynaptic transmission. The amplitudes of membrane potential oscillations of reticulospinal neurons in the posterior and middle rhombencephalic reticular nuclei (PRRN and MRRN, respectively) recorded with sharp intracellular microelectrodes did not significantly change from normal to high-divalent solution. This finding suggests a large part of the spinal input creating the oscillations is direct to the reticulospinal neurons. Application of strychnine to the high-Ca(2+), -Mg(2+) solution decreased membrane potential oscillation amplitude, and injection of Cl(-) reversed presumed inhibitory postsynaptic potentials, indicating a role for direct spinal inhibitory inputs. Although reduced, the persistence of oscillations in strychnine suggests that spinal excitatory inputs also contribute to the oscillations. Thus it was concluded that both excitatory and inhibitory spinal neurons provide direct rhythmic inputs to reticulospinal cells of the PRRN and MRRN during locomotor activity. These inputs provide reticulospinal cells with information regarding the activity of the spinal locomotor networks.

The Contribution of Synaptic Inputs to Sustained Depolarizations in Reticulospinal Neurons

“…RS cells activate spinal interneurons and motoneurons (Rovainen, 1974;Buchanan, 1982;Buchanan and Cohen, 1982;Buchanan et al, 1987;Ohta and Grillner, 1989), and in turn, they receive inputs from the spinal cord locomotor networks Vinay and Grillner, 1993;Einum and Buchanan, 2004. Our results suggest that spinal inputs to RS cells are also involved to temporally amplify the sensory-evoked sustained depolarizations in RS cells.…”

“…We previously showed that intrinsic properties played a crucial role in the sustained depolarizations (Di Prisco et al, 1997. We now examined whether synaptic inputs from the spinal cord Vinay and Grillner, 1993;Einum and Buchanan, 2004) as well as glutamatergic excitatory inputs in general could add to the intrinsic properties to prolong the depolarizations of RS cells.…”

“…We investigated whether inhibitory amino acid transmission contributed to ending the sustained depolarizations. RS cells receive fast inhibitory glycinergic inputs from different sources such as the ascending dorsal and lateral columns in the spinal cord and from the trigeminal nerve (Dubuc et al, 1993a,b;Vinay et al, 1998a,b;Einum and Buchanan, 2004). Using the experimental paradigm described above, glycine (1 mM) was ejected on one of two homologous RS cells recorded simultaneously, during sustained depolarizations (n ϭ 19 pairs) induced by sensory stimulation.…”

Spinobulbar Neurons in Lamprey: Cellular Properties and Synaptic Interactions

“…In this experiment (n ϭ 3 cell pairs, enclosed in ellipses in Fig. 7B), the paired recordings were made while the brain stem was perfused in highdivalent cation solution (20 mM Ca 2ϩ and 5.8 mM Mg 2ϩ ), which was previously shown to reduce polysynaptic pathways (Einum and Buchanan 2004). Thus it seems unlikely that the SB neuron was exciting local inhibitory interneurons in the brain stem, although this possibility cannot be ruled out.…”

“…those PSPs tested persisted in the presence of high-divalent cation solution, which raises spike threshold (Frankenhaeuser and Hodgkin 1957) and reduces polysynaptic pathways (Einum and Buchanan 2004). However, strong excitatory synapses from SB neurons to local brain stem interneurons could potentially still fire interneurons in spite of this treatment.…”