TACHYKININ ACTIONS ON DEEP DORSAL HORN NEURONS IN-VITRO - AN ELECTROPHYSIOLOGICAL AND MORPHOLOGICAL-STUDY IN THE IMMATURE RAT

To assess whether functional neurokinin receptors exist in the deep do rsal horn of the rat, the actions of the selective neurokinin-l recept or (NK1R) agonist [Sar(9),Met(O-2)(11)]substance P ([Sar(9),Met(O-2)(1 1)]SP), the neurokinin-2 receptor (NK2R) agonists [beta-Ala(8)]NKA(4-1 0) and GR64349 and the neurokinin-3 receptor (NK3R) agonist senktide w ere examined intracellularly in vitro. [Sar(9),Met(O-2)(11)]SP (1-4 mu M) and senktide (1-2 mu M) elicited slow depolarizations (<10 mV) ass ociated with increased synaptic activity and cell firing. [beta-Ala(8) ]NKA(4-10) (10-20 mu M) and GR64349 (0.25-10 mu M) caused small depola rizations (<2.0 mV) and no firing. Neurons were categorized as either 'tonic' or 'phasic' depending on their firing response to direct curre nt step depolarizations. Tonic neurons, which, unlike phasic neurons, display no spike firing accommodation, generated a significantly large r depolarization to the NK1R and NK3R agonists. The putative contribut ion of these receptors to primary afferent-mediated synaptic transmiss ion was assessed by testing the NK1R antagonist GR82334 (1 mu M), the NK2R antagonist MEN10,376 (1 mu M) and the NK3R antagonist [Trp(7),bet a-Ala(8)]NKA(4-10) (1 mu M) against the dorsal root-evoked excitatory postsynaptic potential (DR-EPSP). GR82334 and [Trp(7),beta-Ala(8)]NKA( 4-10) significantly reduced (P less than or equal to 0.05) the duratio n but not the amplitude of the DR-EPSP. MEN10,376 (1 mu M) had no effe ct on DR-EPSP amplitude or duration. Morphological detail was obtained for seven biocytin-filled deep dorsal horn neurons tested with [Sar(9 ),Met(O-2)(11)]SP. Five neurons responded to the NK1R agonist, and two of these had dorsally directed dendrites into the substantia gelatino sa. The other three [Sar(9),Met(O-2)(11)]SP-sensitive neurons had dend rites within deeper laminae. These data support the existence of funct ional NK1Rs and NK3Rs in the deep dorsal horn which may be involved in mediating sensory afferent inputs from nociceptors.