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 dorsal horn of the rat, the actions of the selective neurokinin-l receptor (NK1R) agonist [Sar(9),Met(O-2)(11)]substance P ([Sar(9),Met(O-2)(11)]SP), the neurokinin-2 receptor (NK2R) agonists [beta-Ala(8)]NKA(4-10) and GR64349 and the neurokinin-3 receptor (NK3R) agonist senktide were 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) associated 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 depolarizations (<2.0 mV) and no firing. Neurons were categorized as either 'tonic' or 'phasic' depending on their firing response to direct current step depolarizations. Tonic neurons, which, unlike phasic neurons, display no spike firing accommodation, generated a significantly larger depolarization to the NK1R and NK3R agonists. The putative contribution of these receptors to primary afferent-mediated synaptic transmission 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),beta-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 duration but not the amplitude of the DR-EPSP. MEN10,376 (1 mu M) had no effect 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 gelatinosa. The other three [Sar(9),Met(O-2)(11)]SP-sensitive neurons had dendrites within deeper laminae. These data support the existence of functional NK1Rs and NK3Rs in the deep dorsal horn which may be involved in mediating sensory afferent inputs from nociceptors.