Evidence for GABA as a neurotransmitter in the leech

Abstract

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.

Localization of GABA-like immunoreactivity in the central nervous system ofAplysia californica

“…Moreover, with the exception of a pair of fibers in the buccal nerves of Helisoma (Richmond et al, 1991), and a limited innervation of the lips in Helix (Hernadi, 1994), the GABAergic systems of molluscs appear to be restricted to the central nervous system. This organization contrasts sharply with those of other invertebrate phyla, including the arthropods (Usherwood, 1977;Atwood, 1982;Mulloney and Hall, 1990), nematodes (Johnson and Stretton, 1987), and annelids (Cline, 1986) where GABA acts as a major peripheral neurotransmitter.…”

“…Gamma-aminobutyric acid (GABA) is one neurotransmitter that is widely distributed in the mammalian central nervous system where it acts as a major mediator of synaptic inhibition (Roberts, 1960(Roberts, , 1986Florey, 1961). Substantial evidence suggests that GABA also acts as a neurotransmitter in a wide range of invertebrate phyla, including arthropods (Kuffler and Edwards, 1958;Kravitz et al, 1963;Otsuka et al, 1967), echinoderms (Newman and Thorndyke, 1994), annelids (Ito et al, 1969;Cline, 1983Cline, , 1986, nematodes (del Castillo et al, 1964;Johnson and Stretton, 1987;McIntire et al, 1993), and planarians (Eriksson and Panula, 1994).…”

γ-Aminobutyric acid modulation of acetylcholine-induced contractions of a smooth muscle from an echinoderm ( Sclerodactyla briareus )

“…Bicuculline immediately stimulated a large, phasic contraction followed by sustained tone in preparations that were either spontaneously contracting or quiescent. Cline (1986) found that bicuculline blocked the appearance of inhibitory junctional potentials and GABA-evoked responses in longitudinal muscles of the leech body wall, providing evidence for the presence of GABA A receptors on the muscle cell itself. The leech GABA A type receptors were sensitive to bicuculline but not picrotoxin.…”

Differential effects of GABA in modulating nociceptive vs. non-nociceptive synapses

“…Previous experiments that have used BIC in the leech have assumed that this drug was only acting on GABAgated Cl À channels (Cline, 1986;Baca et al, 2008;Higgins et al, 2013). However, an ACh-gated Cl À channel expressed in leech heart motor neurons has been described that can be blocked by BIC (Schmidt and Calabrese, 1992).…”