In the leech, Hirudo medicinalis, the inhibitory motor neurons to the longitudinal muscles in the body wall, cells 1 and 2, are linked via central inhibitory synapses to the excitatory motor neurons innervating the same muscles. Examination of these synapses showed that the inhibitors are GABAergic according to several electrophysiological and pharmacological criteria. Presynaptic release of neurotransmitter during passage of depolarizing current into the inhibitors, as well as direct application of GABA to the excitor cell bodies, hyperpolarizes the postsynaptic excitor. Moreover, both synaptic and extrasynaptic GABA receptors of the excitors are specifically blocked by the GABA antagonist bicuculline methiodide. The inhibitors, dissected from the ganglion and grown in culture, synthesize GABA when exposed to the GABA precursor glutamate, whereas the excitors do not synthesize detectable levels of GABA under these same conditions. The innervation and neurotransmitter sensitivity of the longitudinal muscles in the body wall of the glossiphoniid leeches Haementeria ghilianii and H. oficinalis were examined in order to determine if the inhibitory neurotransmitter at the neuromuscular junction is GABA. Individual muscle fibers are innervated by both inhibitory and excitatory motor neurons in a manner such that the inhibitory and excitatory nerve terminals and neurotransmitter receptors are spatially and electrically separate. Intracellular recordings taken from the muscle fibers reveal a resting potential of about -70 mV. The amplitude of the spontaneous inhibitory junctional potentials (IJPs) falls to zero at a polarization of about -65 mV and reverses in sign at the normal resting potential. Application of GABA to inhibitory junctional sites results in an increased conductance of the muscle fiber membrane and a polarization with a reversal potential of about -65 mV. Bicuculline methiodide blocks reversibly the GABA response and all spontaneous IJPs, without blocking the spontaneous excitatory junctional potentials (EJPs) recorded from the excitatory junctional sites.

The activity-dependent mechanism that refines the topography of the retinotectal projection in frogs is mediated by the NMDA receptor. Earlier studies found that chronic treatment of the optic tectum with the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (DL-AP5) desegregated eye-specific stripes in three-eyed frogs, while chronic treatment with NMDA sharpened stripe borders (Cline et al., 1987; Cline and Constantine-Paton, 1990). We now report that this same chronic treatment with NMDA decreases the electro-physiologically measured sensitivity of the optic tectum to applied NMDA: acute application of a given concentration of NMDA decreased the evoked tectal potential to a lesser extent in animals chronically treated with NMDA than it did in normal and sham-treated animals. This is observed as a shift to the right in the NMDA dose-response curves for both the positive and negative postsynaptic components of the evoked tectal response. We believe that this decreased NMDA receptor effectiveness further restricts the intermingling of axon branches from the two eyes by limiting synapse stabilization to areas where afferent activity is most correlated. This would account for the anatomical sharpening of stripe borders (i.e., increased afferent segregation). Quantitative autoradiographic analysis of 3H-glutamate binding to NMDA receptors indicated that binding densities within the tectum did not differ between control groups and NMDA chronically treated groups. We suggest that in the experimental animals the response to NMDA may be decreased by a change in the effectiveness of individual NMDA receptors rather than by decreases in receptor number. This experimentally induced change may be analogous to naturally occurring decreases in receptor function that correlate with the end of some periods of visual plasticity in mammals.

We have investigated the development of neurotransmitter metabolism in embryos of the glossiphoniid leech Haementeria ghilianii. The neurotransmitter content of dissected embryonic tissues was measured by means of radioenzymatic assays, while the presence of neurotransmitters in individual identified neurons was detected by means of immunocytochemical and monoamine histofluorescence techniques. The capacity for synthesis of neurotransmitters was measured by incubating dissected embryonic tissues in radiolabeled neurotransmitter precursors. A specific neurotransmitter uptake system present in some neurons was detected by means of an autoradiographic technique. At an early stage of development of the nervous system, when most neurons are just beginning process outgrowth, the nerve cord acquires the capacity to synthesize ACh, 5-HT, and GABA from their immediate precursors, and contains ACh. Moreover, 5-HT-immunoreactive neurons and neurons that are capable of GABA uptake can be identified. Dopamine-containing neurons are first detected by their histofluorescence at a slightly later stage, after process outgrowth is under way. As development continues, the content of and capacity for synthesis of these neurotransmitters increase, as does the number of neurons capable of GABA uptake. During the earlier stages of development, ACh content exceeds 5-HT content, which in turn exceeds dopamine content. By the end of embryogenesis, however, 5-HT and dopamine contents have greatly increased relative to ACh content, with 5-HT content exceeding ACh content by a factor of 2. Of the neurotransmitters thus far studied, 5-HT is present in the highest amount in the juvenile and adult nerve cord. Our results indicate that in the development of the leech nervous system neurotransmitter metabolism is one of the first neuronal characters to differentiate and that the subsequent levels of the different neurotransmitters are differentially regulated.

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