Biogenic amines mediate many types of intercellular communication in multicellular organisms. Heretofore, little direct evidence has indicated that biogenic amines produce intracellular responses other than by triggering the enzymatic production of second messengers. Our electrophysiological studies of lobster olfactory receptor neurons now reveal that one biogenic amine, histamine, can directly gate an ion channel. The channel responds to histamine concentrations of 1 ,xM or more, is permeable primarily to Cl-, is more active at depolarized potentials, and has a conductance of 44 pS in the American lobster and 66 pS in the Caribbean spiny lobster. The expression of this ligand-gated channel in olfactory receptor neurons implies that these neurons are targets of a regulatory or feedback process.Histamine (HA) is a putative neurotransmitter and important neuroregulatory compound in diverse species (1-4). The intracellular effector mechanisms mediating HA responses investigated to date involve biochemical second messengers (5-9). We recently discovered that application of HA to the somata of spiny lobster olfactory receptor neurons suppresses both spontaneous and odor-evoking spiking in these cells by rapidly activating a Cl-current (10). The rapid time course of this effect, as well as recent evidence that HA mediates fast synaptic transmission in arthropods (11-15) and molluscs (1, 16), raises the possibility that HA can also act by directly gating ion channels. We now provide evidence that lobster olfactory receptor neurons express a chloride channel that is directly gated by HA. Evidence is also provided to support the conclusion that this channel mediates the suppressive action of HA on lobster olfactory receptor neurons.
MATERIALS AND METHODSAnimals. Adult specimens of the Caribbean spiny lobster Panulirus argus were collected in the Florida Keys and maintained in flowing seawater at 18°C-23°C. Similar specimens of the American lobster Homarus americanus were purchased from commercial suppliers in Woods Hole, MA, and were maintained in flowing seawater at 13°C-17°C.Patch-Clamp Recording. Olfactory receptor neurons were prepared for patch-clamp recording as described (17, 18). Briefly, olfactory organs were cut into hemicylindrical sections, exposing the clusters of receptor neuron somata. Sections from American lobsters were treated with collagenase (1 mg/ml; Sigma, type 1A) for 60 min followed by trypsin(1 mg/ml in Ca2+-free saline; Sigma, type IX) for another 20 min. Sections from spiny lobsters were treated with Lcysteine-activated papain (0.25 mg/ml; Sigma, type IV) for 20 min and then with trypsin for another 20 min. For whole-cell voltage-clamp recording, somata were isolated by drawing clusters of somata into a fire-polished glass pipette and expelling them into a recording dish, a process that severed the dendrite and any remaining axon from these bipolar neurons. Voltage-clamp and single-channel recording were performed by conventional patch-clamp methods with commercially available electron...