Immunolocalization of NMDA receptors in the central nervous system of weakly electric fish: functional implications for the modulation of a neuronal oscillator

Abstract

Using a monoclonal antibody raised against the R1 subunit of the rat NMDA receptor, we mapped the distribution of NMDA receptors in the brains of three genera of electric fish. On Western blots, the antibody recognized a glycoprotein of approximately 105 kDa throughout the CNS. On tissue sections, it strongly labeled a number of neuronal somata and dendrites in the medulla, with weaker immunoreactivity in the forebrain and across much of the rest of the nervous system. At the ultrastructural level, reaction product was localized, though not exclusively, to the postsynaptic region of synapses. To study the role of NMDA receptors in a specific neural circuit, we focused on the medullary pacemaker nucleus. Neurons in this nucleus, which fire action potentials regularly and trigger each electric organ discharge (EOD), receive glutamatergic input from identified premotor areas. Activity in these areas can cause the pacemaker nucleus to produce outputs with distinct temporal dynamics, which are observed in the behaving animal as modulations of the EOD. The projection cells of the pacemaker nucleus, the relay cells, were heavily labeled with the anti-NMDA R1 antibody in all genera studied. These results are consistent with the previous finding that a particular EOD modulation mediated by the connection from one premotor area of the brain to the relay cells is blocked by application to the pacemaker nucleus of NMDA receptor blockers. Our results complement ongoing efforts to study this nucleus and provide additional evidence for the role of NMDA receptors in diverse neural circuits.

N-methyl-D-aspartate receptor 1 mRNA distribution in the central nervous system of the weakly electric fishApteronotus leptorhynchus

“…Therefore, it will be important to determine whether NMDA receptors play similar roles in low-level auditory and electrosensory processing. Spiro et al (1994), by using a monoclonal antibody generated against the mammalian NR1 subunit, have demonstrated a high density of NR1 protein within the gymotiform forebrain (Western blot), consistent with our results. However, their immunohistochemical results suggest a very limited distribution of NMDA receptors.…”

“…As Spiro et al suggest, it is likely that their antibody failed to recognize the antigenic sites of NR1 receptors in tissue sections through most gymnotiform brain regions. Spiro et al (1994) report that, within the pacemaker nucleus of Apteronotus, relay cells are intensely immunopositive for NR1, whereas pacemaker cells are negative; we find that both pacemaker and relay cells express similar moderate levels of NR1 mRNA. Physiological and pharmacological studies (Dye et al, 1989;Heiligenberg et al, 1996) have suggested that slow increases in the electric organ discharge frequency of Apteronotus are mediated by NMDA receptors on both relay cells and pacemaker cells.…”

A sensory brain map for each behavior?

“…They all converge at the level of the pacemaker nucleus, which controls the discharge rate of the electric organ. However, inputs modulating EOD rates in the context of communication behavior and jamming avoidance response, respectively, terminate on two different cell types and are mediated by different glutamate receptor subtypes (9,28,(30)(31)(32)(33)(34)(35)(36). Hence, the segregated information flow in the context of two different behaviors appears to be conserved from the ELL all through the central nervous system to the pacemaker nucleus.…”

Segregation of Behavior-Specific Synaptic Inputs to a Vertebrate Neuronal Oscillator

“…The second input is part of a pathway that generates increases in EOD frequency that occur during a particular behavior related to orientation and prey detection, the jamming avoidance response (JAR) (Bullock et al, 1972). It originates from the dorsal portion of the diencephalic nucleus electrosensorius Keller et al, 1990), sending excitatory connections to the medial portion of the diencephalic prepacemaker (PPnG), which projects to the pacemaker nucleus possibly via both NMDA and non-NMDAtype receptors (Metzner, 1993;Spiro et al, 1994;Juranek and Metzner, 1997). The third pathway controls frequency decreases during the JAR and originates in the ventral nucleus electrosensorius (nE2) Keller et al, 1990).…”

“…The SPPn is tonically active and also controls the EOD frequency, even in the absence of jamming signals. Its projection to the pacemaker nucleus is mediated by NMDA-type receptors (Metzner, 1993;Spiro et al, 1994;Juranek and Metzner, 1997).…”